Investigation into the Noise Associated with Airbag Deployment: Part III – Sound Pressure Level and Auditory Risk as a Function of Inflatable Device

Abstract

Several criteria for assessing noise-induced hearing loss from automotive inflatable devices, such as airbags, were proposed in the past. However, their development was based on epidemiological studies of steady state noise and not impulsive noise. More recently, the US Army Research Laboratory (ARL) developed and validated a mathematical model of the ear, which may be used to assess noise induced hearing loss from impulsive noise sources. Previous studies have contributed to understanding the effects of impulse noise on occupants, but were performed on first generation frontal airbags and did not provide information on airbag and occupant safety systems in today’s fleet of vehicles. This study presents the results of a parametric investigation of current inflatable devices across a variety of vehicles and considers the size and seating location of the occupant in vehicles of varying volume. In addition, the study considers advanced airbag technologies such as dual stage frontal airbags, side airbags, inflatable curtains, and seat belt pretensioners.

The primary function of airbags and seat belt pretensioners is the reduction of injuries and fatalities associated with automotive collisions. However, a side effect of these safety countermeasures is a loud impulse noise due to the need to deploy the countermeasure before the occupant moves appreciably. Impulse noise is characterized by a large amplitude and short duration pressure wave. Since a typical crash (~145 dB) is over in less than 200 milliseconds, this necessitates extremely rapid filling of the airbag (frontal airbag system ~165 dB). The result of such rapid filling of an airbag is possible noise induced hearing loss (NIHL) characterized by a hearing threshold shift. A threshold shift is defined as a change in a person’s hearing sensitivity after exposure to a sound.

Research in the late 1960’s and early 1970’s was conducted to determine the source of the deployment noise and develop criteria to predict the risk of NIHL from a deploying airbag. The pressure wave is typically made up of a low frequency carrier wave with a high frequency wave superimposed. The low frequency wave at 3–5 Hz is due to the gas filling the airbag, which acts like a piston in the closed passenger compartment [Rouhana, Webb, Wooley et al 1994]. The bag surface dynamics contribute to the noise generated in the frequency range of 500–3500 Hz. For example, the “pleat popping” of the airbag folds produced frequencies of 1000 Hz and the unfurling of the airbag during filling produced lower frequencies [Posey and Hickling, 1973, Hickling 1976]. The high-speed gas flow through the manifold produced broadband frequencies up to 10 kHz [Hickling 1976].

In the same time period of these early studies, several attempts were made to develop a NIHL criterion [Coles, Garinther, Hodge et al 1968, Allen, Bruce, Dietrich, et al 1971, United States Department of Defense 1979]. These criteria originally drew on the criterion for steady state noise and correlated the measured hearing loss with the sound pressure level (SPL), duration and frequency of the noise. For a review of these criteria see Rouhana, SW. Dunn, VC. Webb, SR (1998).

An analysis tool developed at the US Army Research Laboratory (referred to as the Ear Model) shows much promise in the prediction of NIHL [Price and Kalb 1986, Kalb and Price 1987, Price and Kalb 1991, Rouhana et al 1998, Price and Kalb 1999]. The model is based on an electroacoustic analogue of the anatomic structures and physical properties of the human ear. The Ear Model accounts for the non-linearities associated with these anatomic structures as evidenced by the good correspondence between experimental measurements and predicted measurements of the model’s transfer functions. The Ear Model’s primary mode of failure of the hair cells on the basilar membrane in the cochlea is one of mechanical fatigue primarily due to the amplitude and frequency, in particular, the high frequency content [Rouhana et al 1998]. A discussion on the anatomy of the ear has previously been provided by Rouhana et al (1994).

Rouhana et al (1998) concluded that the Ear Model was repeatable and reasonably explained the predicted hearing loss when compared to the previous criteria. The Ear Model repeatedly predicted similar risks of NIHL for nominally identical airbags. When differences were found, the Ear Model predicted risks that could be explained based on the biomechanics of the ear. For example, in some circumstances the Ear Model predicted a greater risk of NIHL for the open compartment than for a closed compartment. The biomechanical explanation of this phenomenon is that the low frequency content is greatly reduced when the passenger compartment is open. But, the low frequency pressure wave acts to limit the amplitude of the higher frequency noise by pushing the stapes to its displacement limit and holding it in place until the higher frequency noise diminishes.

While these previous studies have advanced our understanding of the biomechanical response of the ear to impulse noise and have provided an important assessment tool to predict the risk of NIHL, they are limited to older generations of airbag technology. They are also limited by a small number of ear locations assessed within the passenger compartment and by the use of a single surrogate occupant size. Current airbag technologies have reduced pressure output and dual stage deployment delay times. Also, vehicles today are increasingly being equipped with side airbags, side inflatable curtains and seat belt pretensioners. Side airbags tend to be closer to the ear with the incidence angle of the incoming pressure wave more normal to the ear compared to frontal airbag systems. This may pose a greater risk of NIHL than frontal airbag systems. Seat belt pretensioners are additional noise sources within the passenger compartment that may increase the noise experienced by the occupant. Assessment of these factors can help to build upon the knowledge gained in the previous studies of impulse noise from airbags.

The purpose of the current study is to evaluate a comprehensive set of occupant seating positions, occupant sizes, vehicle sizes, and a more recent set of occupant safety systems. Based on past research the Ear Model has been shown to be an excellent airbag impulse noise assessment tool and the analyses in this study will use it exclusively.

METHODS

The current study took a systematic approach to assess the noise generated in the passenger compartment by safety countermeasures. This approach involved five test phases that began with a comparison of measurements made with a flexible mannequin, a Hybrid III dummy and in the free field. The study also includes a horizontal and vertical mapping of the passenger compartment, assessment of vehicle volume effects, and multiple safety device deployments. Generally, these tests employed a variety of human body sizes and a cross section of vehicle body sizes. The human body sizes approximated the 50^th percentile male, the 5^th percentile female and a 6-year-old child. The vehicle sizes ranged from a small car to full-size car, a mid-sized SUV to large SUV, a minivan to full-size van and a small pickup truck to full-size pickup truck. The test methodology used in each phase is individually discussed below, following a discussion of the instrumentation and data acquisition equipment used to support these tests.

The instrumentation employed piezoelectric pressure transducers (Model 2013V, Dytran Instruments Inc., Chatsworth, CA, USA) selected for their excellent low frequency response. A high-speed portable data acquisition system (Wavebook/516, IOtech Inc., Cleveland, OH, USA) was used including a dynamic signal-conditioning module (WBK14, IOtech Inc., Cleveland, OH, USA) that was capable of a 1 MHz aggregate sampling rate. Airbag deployment and data acquisition was triggered with a custom-built “firing box” powered by a 12-volt DC power supply. The pressure-time history was recorded at a sampling rate of 50 kHz for 1 sec. The recorded pressure wave contained a predeployment window from -100 to 0 msec and a deployment window from 0 to 900 msec to enable accurate identification of the initiation of the acoustic event.

The pressure transducers were placed in Hybrid III dummy head forms (FTSS, Plymouth, MI, USA) approximating the ear locations on the left and right side of the head form. The head forms had to be modified to accommodate the installation of the pressure transducers. This was accomplished by drilling a 25.4 mm (1.0 in) diameter hole in the head form at the anatomic ear location. Aluminum rods with a diameter of 25.4 mm (1.0 in) and length of 19.05 mm (0.75 in) were bored and tapped to accept mounting of the transducers per the manufacturer’s recommendation (Figure 1) and press fit into the holes previously drilled into the head forms (Figure 2A and 2B).

Fig. 1.

Installation diagram for Dytran pressure transducer model 2013V (Recreated from pressure transducer manufacturer’s installation specifications)

Fig. 2.

Photographs of the pressure transducer mounted to the Hybrid III Dummy head form without dummy skin (A), and with the dummy skin showing the hole cut out to expose the pressure transducer (B).

Each test phase incorporated several generic test methods. The tests were conducted in a series with 3 repeat tests for each test setup/condition in the five test phases. The mannequin and/or Hybrid III dummy was placed in the required seating position centered in the seat with their feet placed flat on the floor and their arms placed at their sides. Once seated, the ear locations were measured from the centerline of the pressure transducer to hard points on the vehicle interior to repeatedly locate the ear locations for subsequent tests. All deployments in the vehicle were performed with the windows open, except where noted otherwise.

All pressure-time histories were analyzed with the Ear Model. The pressure-time data files were not filtered or conditioned prior to being imported into the Ear Model for analysis. Once imported, the Ear Model requires the user to input the sampling rate and to condition the pressure-time history with the tools provided by the software. The Ear Model allows the user to specify a warned or unwarned situation for the analysis. The warned situations assume that the middle ear muscles (especially, the stapedius) have contracted prior to the arrival of the impulse. In contrast, the unwarned situation uses the middle ear muscle latency to delay the onset of contraction until sometime after the impulse has arrived. The Ear Model was run unwarned for all tests in this study. The Ear Model also allows the user to define the start of the pressure wave. The default start time was used in this study. Any DC offset present in the pressure-time history was zeroed and the signal was recalibrated by entering the known peak pressure. Lastly, the pre- and post-impulse background noise is averaged at the start and end of the pressure-time history by using the Taper Ends function. The output of the Ear model is an auditory risk value measured in auditory risk units (ARU). In this study, all of the predicted risks are presented as the normalized ARU. Normalization is defined as the resultant ARU for an airbag exposure divided by the maximum peak ARU of any test within that set of exposures.

Statistical analyses of the ARU responses were conducted using the general linear ANOVA model from a commercial statistical package (Minitab, Minitab Inc., State College, PA, USA). Statistical significance was set at 0.05. When main effects exhibited a statistical significance, all pair-wise post hoc comparisons were performed using the Bonferroni test method.

MANNEQUIN, HYBRID III DUMMY, AND FREE FIELD ASSESSMENT

This phase of testing determined if a low cost surrogate could be used in place of the Hybrid III dummy. Flexible department store mannequins (Northwestern Mannequin, Seattle, WA, USA), constructed of a foam body and bendable steel reinforced skeleton, were selected as the occupant surrogate to compare to the Hybrid III dummy. The mannequins satisfied the occupant size requirements of this study (Table 1). Large weight differences between the mannequins and the respective anthropometric weight were partially compensated by using lead medical aprons (EasyWrap Apron-65023 and Apronette-69098, HOSPEQ Inc., Miami, FL, USA). The mannequin head was removed and replaced with a Hybrid III dummy head form to support and mount the pressure transducers.

Table 1.

Comparison of mannequin sizes and weight compared to human anthropometry

Surrogate Size	Anthropometric Height cm (inch)	Actual Height cm (inch)	Anthropometric Weight kg (lb)	Actual Weight kg (lb)
50th Percentile Male	175.3 (69.0)	177.8 (70.0)	78.2 (172.3)	14.8 (32.6)
5th Percentile Female	151.1 (59.5)	154.9 (61.0)	49.4 (108.7)	13.1 (28.8)
6-Year-Old Child	114.0 (44.9)	124.5 (49.0)	23.4 (51.6)	10.5 (23.1)

These tests were conducted inside a mid-sized car to compare the 50^th percentile male mannequin, the Hybrid III 50^th percentile male dummy, and a free field measurement setup that located the pressure transducers at the mannequin and dummy ear locations. Mannequin and dummy tests were conducted simultaneously with the mannequin seated in the right front passenger seat and the dummy in the driver seat. The simultaneous mannequin and dummy tests were then repeated with the seating positions reversed. Before placing the mannequin and dummy centered in the seat, the seats were placed in the full down and full rear seat track position with the seat back set in the manufacturers design position. The free field measurements of the airbag deployment located the pressure transducers at the average ear locations determined from the mannequin and dummy ear location measurements. The pressure transducers were wrapped in 6.35 mm (0.25 in) thick neoprene and placed into a 19.05 mm (0.75 in) diameter hole bored in an aluminum block 38.1 × 38.1 × 50.8 mm (1.5 × 1.5 × 2.0 in). The aluminum blocks were mounted to a pipe-stand mechanically isolated from the vehicle and aligned so the sensitive axis of the pressure transducer was perpendicular to the longitudinal axis of the vehicle (Figure 3). Each surrogate was subjected to a dual frontal airbag system deployment with a driver and a passenger airbag. Both airbags were single stage airbags and were triggered to deploy at the same time.

Fig. 3.

Free field measurements made with pressure transducers mounted to a pipe-stand mechanically isolated from the vehicle

HORIZONTAL MAPPING OF THE PASSENGER COMPARTMENT

Horizontal mapping tests were performed to assess the influence of seating position on the risk of NIHL. These tests consisted of recording the noise generated by frontal airbag and side airbag systems at eight ear locations within the passenger compartment of nine vehicle types. The vehicles used in this set of tests were mid-sized to full-sized cars, mid-sized to large SUVs, minivan to full-sized passenger vans, and small pickup to full-sized pickup trucks. Five mannequins were placed in each of the vehicles except for the small pick up truck where the driver and right front passenger seat positions were the only two available locations. The anthropometric sizes of the five mannequins included two 50^th percentile males, two 5^th percentile females, and one 6-year-old child. For vehicles that had more than two rows of seating one additional test was performed with mannequins seated in the additional rows. The eight monitored ear locations and placement of the five mannequins were dependent on the airbag system deployed, frontal or side, and are summarized in figure 4. Frontal airbag system deployments used seating configurations 1 – 3. Seating configuration 4 was used for the side airbag system deployments on both the driver and passenger side.

Fig. 4.

Seating configurations for the different test phases showing the seat position of each mannequin within the vehicle.

Notes: All mannequins have transducers located in left and right ear locations except for:

- Seating Configuration 1, 2, and 3 – 5^th percentile females on left side of vehicle have transducer in left ear location only and 5^th percentile females on right side of vehicle have transducer in right ear location only

- Seating Configuration 4 – 5^th percentile females have transducer in right ear locations only

- Seating Configuration 5 and 6 – mannequins seated in the second row center position are not instrumented with pressure transducers

VERTICAL MAPPING OF THE PASSENGER COMPARTMENT

Vertical mapping was performed to assess the influence of occupant stature on the risk of NIHL. These tests consisted of recording the noise generated by a frontal airbag and side airbag system at eight ear locations within the passenger compartment of a mid-sized car. Five mannequins were placed in the vehicle with the eight monitored ear locations dependent on the airbag system deployed, frontal or side, as summarized in figure 4. Frontal airbag system deployments used seating configuration 1, 5 and 6. The side airbag system deployments used seating configuration 4 and 5 for the driver and passenger, side airbags.

VOLUME EFFECTS

Volume effects have been assessed in previous studies using closed and open passenger compartments [Rouhana et al 1994, Rouhana et al 1998]. The current study used this methodology as well as applying it to three vehicle sizes with different interior volumes. The vehicles used were a small car, a mid-sized car and a full-sized passenger van. In addition, nominally identical driver and passenger frontal airbags were deployed in each of the three vehicles instead of the their vehicle specific airbags so vehicle volume would be the only variable altered. Three tests were conducted in each vehicle with the windows full down (open) and another three tests for each vehicle with the windows full up (closed). Five mannequins were placed in the vehicles with the eight monitored ear locations detailed in figure 4. Seating configuration 1 was used for the frontal airbag system deployments.

MULTIPLE DEVICE DEPLOYMENTS

Current safety systems may include: dual stage frontal airbags, side curtain airbags, seat mounted side airbags and seat belt pretensioners. These systems may deploy in a variety of combinations and with staggered deployment initiation. The tests in this study were designed to look at hypothetical deployment strategies with these multiple devices deploying in concert (Table 2). The vehicle used in this test phase was a mid-sized sedan. The windows were full down (open), except in the case of Setup 2 where the windows were full up (closed). The mannequin seating positions and the monitored ear locations did not vary between airbag/pretensioner system deployments and used seating configuration 1 (Figure 4).

Table 2.

Safety system test setup for multiple device deployments

Test	Safety System	Dual Stage Airbag Delay Time	Pretensioner Delay Time
Setup 1	Frontal Airbag H/O and Seat Belt Pretensioners	Driver Stage 1/2 = 0/5 msec	0 msec
		Pass Stage 1/2 = 0/15 msec
Setup 2*	Frontal Airbag H/O and Seat Belt Pretensioners	Driver Stage 1/2 = 0/5 msec	0 msec
		Pass Stage 1/2 = 0/15 msec
Setup 3	Frontal Airbag L/O and Seat Belt Pretensioners	Driver Stage 1/2 = 5/25 msec	5 msec
		Pass Stage 1/2 = 5/25 msec
Setup 4	Seat Belt Pretensioners Only	N/A	0 msec
Setup 5	Frontal Airbag H/O and Seat Belt Pretensioners	Driver Stage 1/2 = 0/0 msec	0 msec
		Pass Stage 1/2 = 0/0 msec
Setup 6	Left and Right Side Inflatable Curtains	N/A	0 msec
Setup 7	Driver Side Seat Mounted Side Airbag and Curtain	N/A	0 msec

^*Only test conducted with the windows placed in the full up and closed position

H/O = high output stages; L/O = low output stages

RESULTS

MANNEQUIN, HYBRID III DUMMY AND FREE FIELD ASSESSMENT

The mannequin had higher auditory risks for all ear locations than the Hybrid III dummy (Table 3). However, while the dummy auditory risks were statistically less than those measured with the mannequin (p<0.05), the auditory risks were not statistically different when the mannequin was compared to free field measurements (p>0.05). Based on these results, it was concluded that the mannequin could be used as a surrogate to measure the noise produced by airbag deployments in the other phases of testing.

Table 3.

Results of the mannequin, Hybrid III dummy and free field noise measurement comparison

	Driver Left Ear Normalized ARU*	Driver Right Ear Normalized ARU*	Passenger Left Ear Normalized ARU*	Passenger Right Ear Normalized ARU*
Mannequin	0.71	1.00	0.56	0.74
Dummy	0.57	0.77	0.44	0.52
Free Field	0.43	0.81	0.54	0.74

^*ARU values normalized with respect to the peak ARU for the tests in this table

HORIZONTAL PASSENGER COMPARTMENT MAPPING

The highest predicted risk was the driver inboard ear with an average normalized ARU of 0.71 followed by the right front passenger outboard ear location with a normalized ARU of 0.54, the driver outboard ear with a normalized ARU of 0.51 and the right front passenger inboard ear with a normalized ARU of 0.35 (Figure 5). The four ear locations measured in the second row had an average normalized ARU of 0.16, which is approximately 1/3 the risk in the front row. The predicted risk in the third row (not shown) was measured in 4 vehicles and the risk in the fourth row (not shown) was measured in one vehicle. The average predicted risks for the third and fourth rows were a normalized ARU of 0.06 and 0.02, respectively. The risk in the third row was approximately 1/3 of the risk in the second row. The risk in the fourth row was approximately 1/3 of the risk in the third row.

Fig. 5.

Horizontal mapping of the normalized predicted risk across the tested vehicles and ear locations for frontal airbag systems

The ANOVA analysis for horizontal mapping tested the main effects of ear location and vehicle on the auditory risk. Main effects showed that ARU differences for ear location and vehicle were statistically significant (p<0.05). The post hoc comparisons showed that all four front row ear locations were statistically greater than all four of the second row ear locations (p<0.05). The predicted risk of the driver inboard ear location was significantly greater than all other front row ear locations (p<0.05). The risk at the driver outboard ear location was significantly greater than the right front passenger inboard ear location (p<0.05), but was not different from the right front passenger outboard ear location (p>0.05). Lastly, the predicted risk at the right front passenger inboard ear location was significantly less than all other front occupant ear locations (p<0.05).

The post hoc comparisons of the vehicle did not show any notable trends. The low output of the dual stage frontal airbags did not necessarily reduce the predicted risk. The predicted risk in the mid-sized sedan low output airbag deployment was statistically less than that in the high output deployment (p<0.05). However, the predicted risk in the minivan low output deployment was not statistically less than that in the high output deployment (p>0.05). The predicted risk in the mid-sized sedan low output deployment was statistically less than the full-sized van and mid-sized SUV (p<0.05), but it was not statistically different than that in the large and full-sized SUV, or the small and full-sized pickup trucks (p>0.05).

Horizontal mapping tests were also conducted with side airbag systems. There was a clear reduction of the predicted risk when the ear location was located farther away from the noise source (Figures 6 and 7). The rear row ear locations closest to the deploying side airbag had a lower ARU response than the front row ear location next to the side airbag system. The ANOVA analysis tested the main effects of the ear location, side airbag location (driver or passenger side) and the vehicle on the predicted risk. The main effects analysis showed that the side of the vehicle that the airbag was located on was not significant (p>0.05). However, the ear location and vehicle were significant (p<0.05). One of the four vehicles in this test series had an inflatable curtain airbag instead of a seat mounted side airbag. The post hoc comparison of the vehicles showed that the vehicle with the inflatable curtain produced a predicted risk that was statistically greater than the seat mounted side airbag systems (p<0.05). There were no statistical differences found between the three vehicles with the seat mounted side airbag systems (p>0.05). The front row outboard ear location closest to the side airbag was significantly greater than the other ear locations (p<0.05). The predicted risk in the rear row outboard ear location closest to the side airbag was less than that in the front row outboard ear location next to the airbag but not statistically different than any of the other ear locations (p>0.05).

Fig. 6.

Horizontal mapping of the predicted risk (ARU) for side airbag systems on the driver side

Fig. 7.

Horizontal mapping of the auditory risk (ARU) for side airbag systems on the passenger side

VERTICAL PASSENGER COMPARTMENT MAPPING

Frontal airbag deployments again resulted in higher predicted risks in the front row ear locations than the second row ear locations (Figures 8 and 9). The risk predicted for the outboard ear of the 50^th percentile male in the driver seat was a normalized ARU of 1.0 and was higher than the same ear of the 5^th percentile female with a normalized ARU of 0.41 (p<0.05). The other front row ear locations did not have statistically significant differences in average predicted risks between the mannequin sizes (p>0.05).

Fig. 8.

Vertical mapping of the predicted risk (ARU) in the front row ear locations for frontal airbag systems

Fig. 9.

Vertical mapping of the predicted risk (ARU) in the second row ear locations for frontal airbag systems

From the horizontal mapping phase a significant reduction in the predicted risk was noted for the second row ear locations. For this reason, the front and second row ear locations were analyzed separately in this phase. While the ANOVA main effects of the front row ear locations showed a statistical difference (p<0.05), the post hoc comparison between the front row ear locations did not indicate that there was a statistical difference between the mannequin sizes (p>0.05). The ANOVA analyses of the second row ear locations also showed a main effect statistical difference (p<0.05). The post hoc comparisons showed that there was a statistical difference between the risk at the inboard ear location of the 50^th percentile male in the left rear seat position and both of the ear locations of the 50^th percentile male in the right rear seat position (p<0.05). No other statistical differences were noted.

Stature was also studied for the side airbag system. Similar to the horizontal mapping phase of side airbags, there was a notable reduction of the ARU response with the maximum at the front row ear location closest to the deploying airbag and the minimum at the rear row ear locations (Figures 10 and 11). The predicted risk was lower in the ear locations on the opposite side of the vehicle from the deploying side airbag. The only statistical difference found was between the 50^th percentile male and the 6-year-old child mannequins (p<0.05).

Fig. 10.

Vertical mapping of the auditory risk (ARU) across the ear locations for side airbag systems (driver side)

Fig. 11.

Vertical mapping of the auditory risk (ARU) across the ear locations for side airbag systems (passenger side)

VOLUME EFFECTS

Tests with the windows in the full down position resulted in higher predicted risks than tests in the same vehicles with the windows in the full up position (Figures 12 and 13). In the tests with the windows down, the right front passenger outboard ear location had the highest predicted risk with an average normalized ARU for the three vehicles of 0.94 (Figure 12). This was followed by the driver inboard ear location with a normalized ARU of 0.83, the right front passenger inboard ear location with a normalized ARU of 0.46 and the driver outboard ear location with a normalized ARU of 0.41. The rear row predicted risks were approximately half those of the front row in the tests with the windows down. In the tests with the windows up, the predicted risk trend in the front row was different than the windows down condition and the predicted risks were lower (Figure 13). The right front passenger outboard ear location had the highest average normalized predicted risk of 0.28. This was followed by the driver inboard ear location with a normalized ARU of 0.14, the driver outboard ear location with a normalized ARU of 0.12 and finally the right front passenger inboard ear location with a normalized ARU of 0.09. The rear row predicted risks ranged from 0.01 to 0.02 normalized ARU. The pressure transducer in the left rear passenger outboard ear location was not included in this analysis because of difficulties in acquiring data for this particular transducer.

Fig. 12.

Predicted risk (ARU) by ear location, normalized across tests with windows down and up, for a nominally identical frontal airbag system in vehicles with different volumes and an open compartment

Fig. 13.

Predicted risk (ARU) by ear location, normalized across tests with windows down and up, for a nominally identical frontal airbag system in vehicles with different volumes and a closed compartment

The statistical analyses were conducted on the windows down and windows up conditions separately because previous studies indicated that there were significant differences between these two test conditions. When the windows were down, there was no statistical difference found for the predicted risks between vehicles (p>0.05), but there were differences in predicted risk for different ear locations. The risks predicted at both driver ear locations and the right front row right ear location were significantly greater than the risk predicted at the center rear ear locations, but all of the front row ear locations were different than the right rear row ear location (p<0.05). The driver outboard ear location predicted significantly less risk than the driver inboard ear location and the passenger outboard ear location (p<0.05), but not the passenger inboard ear location (p>0.05). The driver inboard ear location predicted significantly greater risk than the right front passenger inboard ear location (p<0.05), but no statistical difference was shown compared to the risk at the right front passenger outboard ear location (p>0.05). The right front passenger inboard ear location predicted significantly less risk than the outboard ear location (p<0.05).

When the windows were full up, the predicted risks in the vehicle with the largest interior volume, the full-sized van, were greater than those in the small and mid-sized car (p<0.05). The risk predicted at the right front passenger inboard ear location was not statistically different than that in the rear row ear locations, but the risks predicted at the remaining front row ear locations were. The risks predicted at the driver outboard and inboard ear locations were significantly less than that at the right front passenger outboard ear location (p<0.05), but were not statistically different than each other. The predicted risk at the passenger inboard ear location was significantly less than that predicted at the right front passenger outboard ear location (p<0.05).

MULTIPLE DEVICE DEPLOYMENTS

Unlike the mapping tests, there was no clear reduction in the auditory risks in the second row seating positions for the multiple device tests. When the frontal airbag system and seat belt pretensioners were deployed with windows down (Test Setup 1), the second row right passenger outboard ear location had the highest predicted risk (Figure 14). This ear location also had the highest predicted risk in the test with the low output frontal airbag system and seat belt pretensioners with the windows down (Test Setup 3) and the seat belt pretensioners (only) with windows down (Test Setup 4). The driver inboard ear location had the highest predicted risk for the high output frontal airbag system, seat belt pretensioners and the windows up (Test Setup 2). The driver inboard ear location also had the highest predicted risk when the frontal airbag system and pretensioners were deployed with no delay time and windows down (Test Setup 5).

Fig. 14.

Multiple device deployment comparison for a frontal airbag and pretensioner system simulating five hypothetical deployment scenarios

Test Setup 1 – 100% total output frontal airbags, seat belt pretensioners and widows down

Test Setup 2 – 100% total output frontal airbags, seat belt pretensioners and widows up

Test Setup 3 – 70% total output frontal airbags, seat belt pretensioners and widows down

Test Setup 4 – Seat belt pretensioners and windows down

Test Setup 5 – 100% total output (no delay time between stages) frontal airbags, seat belt pretensioners and widows down

The post hoc comparisons show that the predicted risks for Test Setups 1, 3, and 4 were not significantly different in magnitude (p>0.05). The predicted risk for Test Setup 5 was statistically greater than all other frontal systems (p<0.05). The predicted risk of Test 2 was significantly different than Test 3 and Test 4 (p<0.05). The comparisons also show statistical differences between both of the second row center ear locations and the second row right passenger outboard ear location (p<0.05). The risk predicted at the driver inboard ear location was statistically different than that at the right front passenger inboard ear location (p<0.05) and the risk predicted at both of the right front passenger ear locations were statistically different than the second row right passenger outboard ear location. Further review of the predicted risks for the second row ear locations shows that their values were similar to those for Test 4, which only deployed the seat belt pretensioners. In the test vehicle there were seat belt pretensioners present at each second row seating position.

The sixth and seventh tests in the multiple device deployment test series were side bag related deployments but they were not statistically compared to each other because they involved disparate deployment strategies (i.e. rollover and side impact). In Test Setup 6, with both side inflatable curtains deployed with the seat belt pretensioners, the outboard ear locations in the front row had higher predicted risks than the inboard ear locations of the same mannequins (Figure 15). This trend was not seen in the second row. Here the second row center ear locations had higher predicted risks than the second row outboard ear locations. The statistical analysis shows that the outboard ear locations in the front row were statistically greater than all other ear locations (p<0.05). In Test 7, the seat mounted side airbag and same side inflatable curtain airbag were deployed. Similar to the horizontal and vertical mapping test series, the predicted risk decreased as the distance of the ear location from the deploying airbags increased. The outboard ear location of the driver, which was closest to the side airbags, had the highest predicted risk in the front row (Figure 15). Likewise, the outboard ear in the second row left ear location had the highest predicted risk in the second row. The predicted risk at the driver outboard ear location was significantly greater than all other ear locations (p<0.05). However, the second row left outboard ear location was only significantly greater than the right front passenger outboard ear location (p<0.05).

Fig. 15.

Multiple device deployment comparison for a rollover airbag with pretensioner and driver side side airbag systems simulating the two hypothetical deployment scenarios

Test Setup 6 – Both side inflatable curtains, seat mounted airbags, seat belt pretensioners and windows down

Test Setup 7 - Driver side inflatable curtain, seat mounted airbag and windows down

DISCUSSION

This study has evaluated issues raised in the development of a standard test procedure for the assessment of noise in vehicles with inflatable restraints, including seating position, occupant stature, vehicle size, and varying safety systems. Since the US Army Research Laboratory Ear Model is an assessment tool based on the biomechanical behavior of the anatomical structures of the ear, these evaluations used the Ear Model exclusively. For more insight into the biomechanical criteria used to assess the risk of NIHL, which is beyond the scope of this study, the reader is referred to Rouhana et al. (1998).

The first phase of testing in this study was aimed at addressing whether or not a pressure transducer located in the head form and at the approximate location of the ear canal could be used as a surrogate for in-vehicle airbag noise assessment. The use of a head form attached to a flexible mannequin, ballasted with lead aprons, was compared to a Hybrid III dummy with the same head form and to the pressure transducers located in the free field. The risks predicted for the Hybrid III dummy were always less than those for the mannequin. This may be due to the large difference in weight between the two surrogates but is unknown. However, in these tests the predicted risk measured with the mannequins was similar to those measured in the free field. The free field measurements in previous studies were also analyzed with the Ear Model and it was concluded that they could reasonably predict the auditory risk of the airbag impulse noise [Rouhana et al 1998]. It should be noted that the pressure transducers in the previous study, while approximating the ear locations, were in the free field and oriented in a different direction than the canal opening. These results suggest that the Ear Model transfer functions can be used for both free field and ear canal entrance measurements. This also suggests that the mannequin can be used as a surrogate to assess the risk of NIHL in the other test phases of this study.

A major portion of this study dealt with the influence of seating position and occupant stature on the predicted risk of NIHL within the passenger compartment. The results of this study clearly showed that there were significant differences in predicted risk at different ear locations and that these were a function of the airbag system being deployed. In both the frontal and side airbag system deployments, the ear locations in the second row and beyond were at less risk of NIHL. In fact, there was approximately a 1/3 reduction in the predicted risk of NIHL for each seating row away from the frontal airbag system. Sommer (1973) saw an attenuation of the high frequency noise from the front to rear seating positions but Nixon (1969) concluded that the overall peak sound pressure level (SPL) was similar between the first and second rows. While these findings may appear to be at odds, they are actually very consistent given the fatigue mechanism of NIHL in the Ear Model. That is because attenuation of high frequency noise, even with the same peak SPL, should lead to less risk of NIHL.

The risk of NIHL also varied within a particular row of seating. The trend of predicted auditory risk for front row ear locations during a frontal airbag system deployment showed that the driver inboard ear location had the greatest predicted risk followed by the driver outboard and passenger outboard ear locations, which were similar to each other, and the passenger inboard ear location had the lowest predicted risk of NIHL.

The Ear Model predicted that the driver inboard ear location would have a greater risk of NIHL than the right front passenger ear locations. Rouhana et. al. (1994) suggested that the larger size of the passenger side airbag accounted for the higher peak pressure at the passenger ear locations in that study. This finding makes sense given that the peak pressure diminishes with the distance from the noise source. However, others have shown that a pressure wave with a normal incidence angle to the ear will have a greater effect on the risk of NIHL than a wave at grazing incidence [Rouhana et. al. 1998]. Therefore, it is reasonable that the driver inboard ear location would have a greater predicted risk of NIHL than other ear locations, because the passenger side frontal airbag is larger than the driver airbag, the passenger inflator typically has a higher output to fill the bigger bag and the driver inboard ear location has a greater component of the incident pressure wave normal to the ear canal opening. The results of this study corroborated this hypothesis since predicted risk was highest at the driver inboard ear location. It is unclear why the outboard ear locations of the driver and passenger are similar despite the differences in distance from the passenger side frontal airbag. This may be because the windows were down, reducing possible reflections of the pressure wave, which may be a much larger effect than the drop in pressure with the distance from the source.

The side airbag tests reinforce the concepts that distance from the noise source and shadowing will reduce the risk of NIHL and normal incidence angles will produce a higher risk of NIHL. In these tests, the ear location closest to the deploying side airbag had the highest predicted risk. The inboard ear location of the mannequin nearest to the airbag was shadowed from the deployment noise by the head. The predicted risk at this ear location was approximately half of the risk predicted for the outboard ear location of the same mannequin. The predicted risk also diminished with the distance away from the noise source. The results showed that ear locations on the far side of the vehicle had much lower predicted risk than the near side ear locations.

The previous impulsive noise studies did not conduct experiments to assess the risk to different sized occupants. This study looked at three different mannequin sizes that represented the stature of the 50^th percentile male, the 5^th percentile female and the 6-year-old child. It is important to note that the effect of age on the predicted risk cannot be assessed at this time due to a lack of biomechanical knowledge. The tests showed that there were differences in predicted auditory risk between the 50^th percentile male and 5^th percentile female at the driver inboard ear location during a frontal airbag system deployment. Also, there were differences seen between the 50^th percentile male and the 6-year-old child for a seat mounted side airbag system. The increased predicted risk for the 6-year-old child ear locations may be explained by the relative proximity of this mannequin’s ear to the seat mounted side airbag system that was used in these tests. This suggests that the airbag system location should be taken into consideration when assessing the risk of NIHL. This study also suggests that for a seat mounted side airbag system the 6-year-old child ear location may be a better location at which to assess the risk of NIHL than the adult ear locations. While the current study examined the noise from inflatable curtain airbags with respect to horizontal location of the ear within the passenger compartment, it did not examine the noise from this system with respect to mannequin stature. Based on the proximity of the ear and the incidence of the pressure wave of such an airbag system, the risk may be significantly influenced by the stature of the mannequin.

The effects of open and closed passenger compartments have been shown to influence the risk of NIHL [Rouhana et al 1994, Rouhana et al 1998]. This study took another look at this issue by assessing the predicted risk in vehicles with different interior volumes and both open and closed passenger compartments. The results corroborated the previous work. The closed compartment resulted in a reduction in the risk of NIHL compared to the open compartment for the same vehicle. When the three different vehicles were compared with the windows full down, the predicted risks of NIHL did not differ significantly. This supports the previous conclusions because with the windows down each vehicle can be considered to have an infinite effective volume. However, when the windows were full up, the interior volumes did influence the outcome of the predicted risk. The full-sized van having the largest volume of the three vehicles tested resulted in a higher predicted risk than the small and mid-sized sedans. While the predicted risk did increase with increases in vehicle volume when the windows were full up, the predicted risk was not greater than that in any of the vehicles with the windows full down. This implies that the window position has a greater impact on the predicted risk.

The desire to continuously improve automotive safety has caused the introduction of more noise generating devices in the passenger compartment. These devices include dual stage frontal airbags, seat mounted side airbags, inflatable curtain side airbags and seat belt pretensioners, among others. This test program evaluated several hypothetical combinations of devices that could be deployed together and the effects of delay times introduced between their deployment. In addition, the case where no delay time existed was evaluated. The results suggest that the delay time may influence the predicted risk. When no delay time was used the predicted risk was significantly greater. For dual stage frontal airbags, while the predicted risks for 100% and 70% total output deployments were not significantly different, the 70% output gave greater predicted risks in both driver ear locations and the right front passenger inboard ear location. This seemed to be counterintuitive because the output was depowered. However, two important findings were noted when compared to the 100% output deployment: first, the peak pressures observed were not reduced, and second, the pressure-time history showed that the acoustic event was longer in duration and had two distinct peak pressures associated with the two separate deployment stages. It is reasonable to assume that the duration can influence the risk of NIHL since a longer pulse will cause more oscillations of the basilar membrane in the cochlea. The number of cycles of vibration is part of the injury criterion assumed by the Ear Model.

The vehicle used in the multiple device part of this study had seat belt pretensioners for all five occupant seating positions. Differences in predicted risk at the rear row ear locations comparing the five pretensioners fired alone to both the 100% and 70% output deployments were not statistically significant. This suggests that the predicted risk at the rear ear locations in these tests were influenced to the greatest extent by the rear seat belt pretensioners.

While this study addressed a number of factors influencing the risk of NIHL, there are limitations with respect to age effects, other injuries and mannequin weight. Current biomechanical data has not concluded the effect of age on the predicted risk of NIHL. It is understood that there is an age related reduction in hearing acuity (presbycusis) but it is not known whether this predisposes or protects from further NIHL. Likewise, it is not known whether exposure of the developing ear of a child is more susceptible to injury. Secondly, while the Ear Model is appropriate for evaluating the risk of NIHL, it cannot, at present, evaluate risk of tinnitus (ringing in the ear) or recruitment, two other potential consequences of noise exposure. Lastly, the mannequin weights were not similar to the dummies or humans and may have had some effect on the predicted risks as noted in table 3.

CONCLUSIONS

While the significant life-saving benefits have been shown to result from airbags and other inflatable automotive devices, risk associated with the deployment noise has been addressed in this study. These devices generate impulsive noise that may pose a risk of NIHL. The Ear Model developed by the US Army Research Laboratory is an important tool to aid in the assessment of the predicted risk associated with these devices and was used exclusively in these analyses. The conclusions that can be drawn from this study are:

1. The mannequin tested in this study can be used as a surrogate to evaluate of the risk of NIHL due to the deployment of airbag systems and other inflatable devices in vehicles, but should probably have more ballasting to bring the weights closer to those of real occupants.

2. The driver inboard ear location had the highest predicted risk of auditory threshold shift due to a frontal airbag system deployment. The predicted risk of NIHL at the second row ear locations was approximately one third that at the front row ear locations for a frontal airbag system deployment. Similarly, the predicted risk for each subsequent row was one third that of the previous row. Therefore, a test procedure to assess frontal airbag risk of NIHL need only measure front row ear locations.

3. The ear locations closest to a deploying side airbag system had a higher predicted risk of NIHL than those farther away from the airbag. Therefore, a test procedure to assess side airbag risk of NIHL need only measure ipsilateral ear locations.

4. The ear location height only appeared to influence the predicted risk for frontal airbag systems in the driver outboard ear location. However, the proximity of the ear location to a side airbag system does suggest that the ear location height may influence the predicted risk at any nearside ear location. Therefore, ear location height should be considered for the assessment of side airbag system, but only at driver ear locations for frontal airbag systems.

5. The interior volume of the vehicle does influence the predicted risk of NIHL, but the effect of window position (closed or open) had a greater effect. For some vehicle/airbag combinations higher risk will be obtained with windows open. This may be a function of relative volumes of inflatable devices and vehicle interior, but needs more study. In the absence of this knowledge, a preliminary set of tests should be done with the windows up and windows down to determine the greatest risk. Subsequent tests can be conducted in the configuration that predicted higher risk.

6. Depowering the output of an airbag using dual stage designs with delay times may not reduce the predicted risk. This suggests that consideration of the dual stage airbag delay times may be useful if depowering is employed as an auditory risk mitigation strategy.