Frequency-tunable sound amplification in a conch-like cavity with graphene thermoacoustic resonance

Abstract

The two-dimensional (2D) thermoacoustic emitter excels in producing a flat sound spectrum above 5 kilohertz but struggles with reduced sound pressure at lower frequencies. To address this, we designed a wearable acoustic device that combines graphene with a 3D-printed cavity, enabling tunable resonant frequency and enhanced sound amplification based on thermoacoustic resonance. The design features laser-scribed graphene as a 2D flexible thermoacoustic source attached onto the cavity, with a specialized chamber above to facilitate air vibration through Joule heat release. The inversely proportional relationship between the operating resonant frequency and the path distance of sound propagation is verified, the sound pressure level increases from 32 to 71 decibels at 5.4 kilohertz when the cavity height increases from 0 to 10 millimeters. Last, a wearable conch-like spiral cavity with graphene is tested under a commercial artificial ear system, demonstrating an effective amplification at approximately 1 and 10 kilohertz, offering insights for developing flexible loudspeakers.

A graphene-integrated 3D cavity overcomes sound performance limitations, boosting SPL by 39 dB at 5.4 kilohertz.

INTRODUCTION

Over a hundred years ago, Arnold and Crandall (1) prepared a thermophone and established a corresponding theoretical model, providing a physical basis for the thermoacoustic effect. In recent years, the emerging two-dimensional (2D) materials that have high heat conductivity represent a possibility of a new type of thermoacoustic sound source (2, 3). It is advantageous over traditional loudspeakers for the high flexibility (4), ultrathin thickness (5), high electron mobility (6), and low heat capacity (7). Thermoacoustic effect (8) is observed in these 2D materials during the sound generation process, where Joule heat is produced on the surface of the conductive material. Under the action of the periodic voltage, the periodic Joule heat is released to the air, driving the periodic movements of air molecules to generate sound. Recently, some nanostructure materials such as carbon nanotube sheets (9, 10), nanowire (11, 12), tungsten nanobridges (13), and MXene (14) have attracted attention as the sound source for their low-cost, freestanding structure, and flexibility. Fan and colleagues (9) introduced the carbon nanotube thin film into thermoacoustic devices and measured that the sound pressure level (SPL) was one to two orders of magnitude higher than traditional metal–based thermoacoustic devices under equivalent input power, demonstrating the potential application of nanomaterials in thermoacoustic effect. Among them, the carbon-based materials, graphene (15–18), such as laser-induced graphene (17, 18) and monolayer and multilayer graphene, show their greatest potential as the thinnest 2D material in the world currently. Graphene also has high electrical conductivity, good mechanical strength, high heat conductivity (2, 19), and good biocompatibility (20, 21). Thus, these ultrathin and flexible sounding films have been researched in the area of speakers such as earphone (22, 23), loudspeaker (24, 25), and artificial throat (26, 27). The most important factors of a sound emission device are high SPL and wide spectrum. However, the existing thermoacoustic sound source still has some problems in performance.

Xie et al. (28) fabricated a point electrical thermoacoustic device based on an aluminum nanowire that has a flat power spectrum within 20-kHz range. The SPL reached as high as 67 dB with less than ±3-dB fluctuation, but its silicon wafer substrate used cannot meet the requirements of flexibility, and the released aluminum nanowires structure had low stability and normalization. The graphene-based thermoacoustic flexible sound source realizes a flat and wide spectrum. Tian et al. (29) made a graphene earphone that can be entertaining for both humans and animals. It can play sounds ranging to 50 kHz with ±10-dB fluctuation, three times lower than commercial earphones. The disadvantage is that the SP of the thermoacoustic device is positively correlated with frequency, so it has a relatively low SPL performance at low frequency. To improve it, Shin et al. (30) designed a multiway loudspeaker composed of three thermoacoustic resonators and one open-type thermoacoustic loudspeaker. It showed more than 10-dB SPL enhancement at the designated low frequency of 1.2 kHz. However, multiple resonators occupied a relatively large space, which was not conducive to the development of multifunction integration and miniaturization. Besides, different processes of the resonators increased the complexity of device preparation. Baughman and colleagues (31) quantified the energy loss mechanism of carbon nanotube devices systematically and identified that thermal conduction loss, thermal radiation loss, and acoustic impedance mismatch are the core factors limiting the efficiency of 2D thermoacoustic devices. Therefore, it is an urgent requirement to improve the thermoacoustic performance, in addition to optimizing the device itself, especially for the specific low-frequency range.

In this work, we demonstrate a conch-like sound cavity used for graphene thermoacoustic resonance. The laser-scribed graphene (LSG) (32) device is fabricated through laser-scribing technology. It can be attached onto the inlet of the cavity and used as a flexible 2D thermoacoustic sound source. The design of sound-amplified spiral cavity is inspired by the amplification function of the conch and fabricated through the 3D printing stereolithography appearance (SLA) technology for acoustic resonance enhancement. After the voltage signal applied to the LSG device, sound waves are generated by the LSG and propagate forward in the cavity. The value of the resonant frequency is tunable and basically determined by the height of cavity. Compared with the LSG device assembled by a conch-like Helmholtz resonant cavity (C-cavity), traditional commercial earphone cavity has a limited impact on acoustic resonance due to the small path distance of sound propagation in the cavity because it has a relatively large sounding unit with the magnet and coil and no spiral area.

RESULTS

Mechanism of a low-frequency sound-amplified resonant cavity

Figure 1A shows the photograph of a conch. The diagram presents the propagation and amplification of sound waves inside the conch chamber due to the acoustic resonance phenomena. Inspired by the amplification function of the conch, a wearable low-frequency sound-amplified C-cavity with a flexible ultrathin graphene device is designed, performing as an earphone, as shown in Fig. 1B. The graphene device is attached onto the bottom of the cavity inlet, used as a 2D thermoacoustic source. Compared with the commercial earphone cavity, a spiral chamber is added at the bottom of the cavity innovatively to enhance the acoustic resonance, mimicking a conch chamber (Fig. 1C). Sound waves are generated by the sound source at the inlet and are gained at the outlet after propagation and amplification in the cavity. Figure 1D illustrates the working mechanism of sound amplification in a simplified cavity based on thermoacoustic resonance. We divide it into four stages briefly. The first stage is the sound generation. The thermoacoustic graphene device is attached to the inlet of the cavity. It generates sound waves based on the thermoacoustic effect, and sound propagates forward in the cavity pipeline. The second stage is the acoustical coupling. Acoustic impedance changes with the cross-sectional area of the air column. After sound waves reach the outlet of the cavity during propagation, the cross-sectional area of the air column is enlarged from the cavity outlet to the open air. Thus, acoustic coupling occurs between the sound waves and the atmosphere at the cavity outlet. Parts of the sound waves are reflected backward, and other sound waves continue to propagate forward in the open air. The third stage is the acoustic resonance. The reflected sound waves resonate with the original sound waves in the cavity. The fourth stage is the propagation of amplified sound. Sound waves are amplified in the Helmholtz resonant cavity, propagate forward in the open air, and are heard by the human ear at last. Figure 1E presents each sound performance trend of graphene in the C-cavity, commercial cavity, and open air, respectively. In the audible frequency range, the SPL of the sound device in the open air is roughly flat and low, where it has one sound amplification at the resonant frequency peak in the commercial cavity and many frequency peaks in the C-cavity. Thus, it proves that the C-cavity has a better sound amplification effect than other conditions.

Fig. 1. The mechanism of the device in a low frequency–amplified conch-like Helmholtz resonant cavity and performance comparison.

(A) Photography of a conch. (B) Schematic diagram of wearing the C-cavity with a graphene device. (C) Propagation and amplification path of the sound waves in the C-cavity. (D) Mechanism of sound amplification in a cavity based on thermoacoustic resonance. (E) The sound performance of the sound device under different conditions.

The multilayer structure of the graphene-based sound source is shown in Fig. 2A, consisting of LSG, a piece of polyurethane (PU) film, and the paper substrate. The detailed fabrication process of the LSG device is shown in fig. S1, including the graphene oxide (GO) drop coating, laser reduction, and electrode connection. The special design of the device is the adoption of a type of thermal-transfer substrate, with a flexible PU film on the upper layer and a paper substrate on the lower layer. The paper substrate can be peeled off, and the PU film (~1 μm in thickness) with LSG remains. So, it can be attached onto any surface based on its flexible and ultrathin characteristics. Figure 2B displays the photograph of the prepared graphene device. it has an area of 3.5 mm in width by 3.5 mm in length, which is used as a sound generation device. The morphologies of LSG under scanning electron microscopy (SEM) are illustrated in Fig. 2 (C to E). Figure 2C shows the flat surface of LSG, which indicates that GO has been reduced uniformly. The curved cross section in Fig. 2D proves the flexibility of the device, its enlarged SEM image in Fig. 2E displays the three-layer structure of the LSG device, and the total thickness of the device is around 50 μm. Figure 2F shows the Raman spectra of the LSG, GO, and paper substrate. Compared with GO, the 2D peak at 2700 cm⁻¹ of LSG indicates the generation of multilayer graphene after laser-scribing processing.

Fig. 2. The photograph and morphologies of the LSG device and Roman spectra.

(A) The multilayer structure of the graphene-based thermoacoustic device. (B) Photograph of the LSG device with a 3.5 mm–by–3.5 mm area for sound emission. SEM images of (C) the surface and (D) the cross section of LSG. (E) Zooming in SEM of the cross-sectional image of (D). (F) Roman spectra of LSG, GO, and paper substrate. a.u., arbitrary units.

The sound emission performance test platform and results of the LSG sound device are shown in Fig. 3. The test platform contains a standard microphone and an LSG device in a straight cavity (Fig. 3A). The straight cavity is made of polylactic acid through a 3D printer. The microphone connected with an Agilent 35670A dynamic signal analyzer is used for sound detection, a 3.5 mm–by–3.5 mm LSG device put at the bottom of a straight cavity is connected with a signal generator, and the power supply is used for generating periodic Joule heat. The operation point bias voltage is set at 5 V, and the alternating voltage is set at 5 V. The Joule heat causes the periodic vibration of air molecules, releasing them into the ambient air. Thus, sound waves are generated. A standard microphone with a sensitivity of 14 mV Pa⁻¹ and a detection range of 20 Hz to 20 kHz is placed on the top of the device. The measuring distance is calculated by the distance between the microphone and the LSG device, consisting of the air column height and the straight cavity height. Then, the sound performance of the LSG device is verified through a series of tests in different sizes of cavities. First, the LSG device is tested in cavities of 10 mm in length by 10 mm in width, at a measuring distance of 10 mm (Fig. 3B). The inserted photograph is the fabricated straight cavities from 0 to 10 mm in height. Compared with the device in the open air (cavity height = 0 mm, air height = 10 mm), the SPL of the device in the cavity has been substantially enhanced at audible frequencies ranging from 20 Hz to 20 kHz. It can be found that there is no obvious peak of LSG in the open air, by increasing the heights of cavities from 0 to 10 mm. The resonant peaks of base and secondary frequencies (named $f_1$ to $f_n$ ) begin to appear. The base frequency $f_1$ shifts toward a lower frequency at 5.4 kHz in 10-mm cavity height. The corresponding SPL increases up to 71 dB, much higher than 39-dB SPL of the device in the open air. So, the operating resonant frequency of the LSG device in the cavity is tunable by adjusting the cavity height. Second, when cavity height further expands, the resonant peaks of frequencies keep moving left and decreasing the gap. Thus, more resonant peaks appear gradually (Fig. 3C). There are six obvious resonant frequency peaks at 1818, 4615, 7612, 10,809, 14,006, and 17,203 Hz when the straight cavity height is 50 mm and the measuring distance is set at 50 mm. According to the survey, the nth-order acoustic resonant frequency $f_n$ of the confined air cube with a rigid closed end and no pressure difference at the open end (33) can be expressed as

$$f_n=(2n-1)\frac{v_{\text{air}}}{4H}$$ (1)

Fig. 3. Test platform and results of sound emission performance of the LSG device in a straight cavity.

(A) Acoustic test platform of the LSG device in a straight cavity. Sound emission performance of the LSG device in the different heights of cavities ranging from (B) 0 to 10 mm and (C) 0 to 50 mm. Scale bar, 1 cm. (D) The output SP of LSG versus the cavity height. (E) The output SP of LSG in a cavity under different measuring distances. (F) Normalized SPL of the LSG device in this work and other thermoacoustic devices. CVD, chemical vapor deposition.

where $H$ is the height of the straight cavity and $v_{\text{air}}$ is the sound velocity in the air. The calculated theoretical values are close to the test data in Fig. 3 (B and C). To better represent the relationship between the resonant frequency and the height of the straight cavity, we select the peaks of the resonant frequencies of the device from 10- to 50-mm height cavities in Fig. 3D. For example, the measured $f_1$ of LSG decreases from 6613 to 1818 Hz as the cavity height increases from 10 to 50 mm, with values of 6613 Hz (10 mm), 4715 Hz (20 mm), 2817 Hz (30 mm), 2417 Hz (40 mm), and 1818 Hz (50 mm). It proves that the appeared resonant frequency peaks in the audible range from $f_1$ to $f_6$ are inversely proportional to the cavity height, which is consistent with Eq. 1 and the previous research (31, 33). Subsequently, the test of the LSG device in straight cavities with different bottom area sizes is shown in fig. S2. The cavity height is set at 10 mm, and the bottom area sizes are set at 8 cm by 8 cm, 10 cm by 10 cm, and 12 cm by 12 cm, respectively. It can be found that the impact of the bottom area of the cavity on sound performance is less than that of height. A smaller bottom area leads to a little higher SPL because of the Joule heat concentration. So, the value of the resonance frequency is mainly determined by the height of the cavity, that is, the propagation path distance of the sound waves. Figure 3E shows the output SP of LSG in a cavity under different measuring distances ranging from 6 to 20 mm at 0.5, 2, 3, 4, and 5 kHz. The SP value decreases as the measuring distance increases. It exhibits an inverse dependence on measuring distance due to the power loss in reduced acoustic attenuation at shorter ranges. At the same time, the SP increases in proportion to the input power in the effective area (within 3.2 W), as shown in fig. S3. The test is ineffective in the area above 3.2 W due to the maximum voltage of the input power equipment that can only reach 10 V, and the parameter of the equipment in the limit state is unstable, resulting in data failure. In the general test system, the operating voltage is around 3 V, so the normal operating area of the device and circuit system is within the effective area. In Fig. 3F, the normalized SPL of different thermoacoustic devices have been listed, such as graphene of different types, nanowire, carbon nanotube, porous silicon, and so on (9, 25, 34–42). The SPL values are normalized at the same measuring distance (1 cm) and input power (1 mW). Compared with the maximum SPL of other thermoacoustic devices during the audible frequency range (20 Hz to 20 kHz), the LSG device in a cavity in this work exhibits good sound performance (56 dB mW⁻¹ cm⁻¹ at 5.4 kHz), which is higher than that of the device without cavity in this work (27 dB mW⁻¹ cm⁻¹ at 5.4 kHz) due to the donation of thermoacoustic resonance.

Figure 4A shows the working mechanism of the thermoacoustic resonance of the LSG device in the cavity. The notable difference between this thermoacoustic resonance cavity and the commercial loudspeaker is the method of sound generation and the thermoacoustic resonance phenomenon. The commercial loudspeaker generates sound by the mechanical vibration of the diaphragm, while the LSG device generates sound based on the thermoacoustic effect. Therefore, the magnet, coil, and diaphragm take up the most space in the commercial cavity, but 100% room remains above the thermoacoustic device for releasing Joule heat, causing air vibration, and leading the thermoacoustic resonance. Figure 4B illustrates a physical model for the acoustic field in a straight cavity. The width of the cavity is represented as $2a$ , its effective straight height is $H$ , and $W$ denotes the thickness of the bottom substrate of the cavity. The nth-order resonant frequency of the cavity calculated by Eq. 1 assumes ideal boundary conditions: a rigid closed end and no pressure differential at the open end. However, practical applications require adjustments due to the following reasons. First, acoustic waves will penetrate into the bottom substrate at the closed end. Second, the open end exhibits a pressure differential due to radiation effect, which can be characterized by termination impedance represented as $\mathit{jk}\mathrm{\delta }{Z}_{\text{air}}$ (43).$k$ is the air’s propagation constant and determined by $k=2\mathrm{\pi }f/v_{\text{air}}$ , where $v_{\text{air}}$ represents the sound velocities in air, 𝑓 denotes the frequency of the acoustic wave, $\mathrm{\delta }/a$ is the end correction coefficient, and$Z_{\text{air}}$ is the air’s acoustic impedance. Considering these two conditions, we derive the following equations in the Supplementary Materials as

$$\begin{array}{c}\frac{2\mathrm{\pi }f}{v_{\text{air}}}\mathrm{\delta }=\text{cos}\left\{\frac{2\mathrm{\pi }f}{v_{\text{air}}},H,-,\text{acot},\left[\frac{Z_{\text{substrate}}}{Z_{\text{air}}}\text{tan}\left(\frac{2\mathrm{\pi }f}{v_{\text{substrate}}}W\right)\right]\right\}\hfill \end{array}$$ (2)

where $Z_{\text{air}}$ and $Z_{\text{substrate}}$ are the acoustic impedances in air and bottom substrate, respectively. $v_{\text{substrate}}$ denotes the sound velocity in the bottom substrate. The solution of Eq. 2 can be obtained graphically by observing the interception point of the frequency-dependent function $F\left(f\right)$ and $G\left(f\right)$ as

$$F\left(f\right)=\frac{2\mathrm{\pi }f}{v_{\text{air}}}\mathrm{\delta }$$ (3)

$$\begin{array}{c}G\left(f\right)=\text{cos}\left\{\frac{2\mathrm{\pi }f}{v_{\text{air}}},H,-,\text{acot},\left[\frac{Z_{\text{substrate}}}{Z_{\text{air}}}\text{tan}\left(\frac{2\mathrm{\pi }f}{v_{\text{substrate}}}W\right)\right]\right\}\hfill \end{array}$$ (4)

Fig. 4. Simulation and measurement analysis of thermoacoustic resonance of the LSG device in the cavity.

(A) The working mechanism diagram of thermoacoustic resonance in the cavity. (B) A physical model for the acoustic field in a straight cavity. (C) A physical model for the acoustic field in a C-cavity. (D) Simulation of thermoacoustic field distribution in the straight cavities for the first and fifth eigen resonating modes. (E) Resonant frequency peak calculation (cavity height = 50 mm) based on curve intercepting of function $F\left(f\right)$and $G\left(f\right)$ . (F) Simulated and measured SPL of LSG within 20-kHz range (cavity height = 50 mm). (G) Simulation of thermoacoustic field distribution in a C-cavity. (H) Changes in resonant frequencies according to the value of the maximum diameter $D_{\mathrm{C}}$ of the C-cavity. (I) Simulation and measurement of the resonant frequencies of the LSG device in a C-cavity.

Figure 4C illustrates a physical model for the acoustic field in a C-cavity. The width of the cavity is represented as $2a$ , and $W$ denotes the thickness of the bottom substrate of the C-cavity. It is worth mentioning that the propagation path of sound waves changes from a straight line in a straight cavity to a curve in the spiral pipeline of the C-cavity, so the length of the spiral curved pipeline of the C-cavity is equivalent to the height of the straight cavity $H$ . To keep consistent with the straight cavity height $H$ above, we name the length of the spiral curved pipeline of the C-cavity spiral height $H_{\mathrm{C}}$ . When the selected curved pipeline of the C-cavity is small enough, it can be equivalent to a straight pipeline of the straight cavity. Figure 4D shows the thermoacoustic field distribution for the first and fifth modes in a straight resonant cavity with an equivalent height of 52 mm. A 2-mm gap between the microphone diaphragm and the cavity in the test extends the equivalent height of the cavity by 2 mm. The eigen resonant frequencies for sound waves in two straight cavities with 10- and 50-mm height are shown in fig. S4A and Fig. 4E. The values of the resonant frequency peaks are calculated on the basis of the curve intercepting of function $F\left(f\right)$ and $G\left(f\right)$ . Figure S4B and Fig. 4F present the simulated and measured SPL of LSG in 10- and 50-mm cavities with a good coincidence, which validates the feasibility of the proposed model. The measurements of the damping coefficient and the corresponding values in this resonant system are shown in fig. S5. Figure 4G shows the simulated thermoacoustic field distribution in a C-cavity, and it can be seen that sound waves propagate within the C-cavity, and some waves are amplified at specific positions due to the acoustic resonance. To verify the relationship between the resonant frequency and the propagation path of sound waves within the C-cavity, we establish and analyze several C-cavity models with different maximum diameters $D_{\mathrm{C}}$ , as shown in fig. S6. As the $D_{\mathrm{C}}$ increases, the length of the spiral pipeline of the C-cavity increases equivalently, so the number of resonant frequency peaks in the audible range gradually increases. For convenience, the first six resonant frequency peaks are selected for analysis, as shown in Fig. 4H. The result shows that the values of the resonant frequency peaks in the audible range decrease gradually when $D_{\mathrm{C}}$ increases; among them, the resonant frequency $f_6$ decreases from 12.8 to 5.3 kHz, as $D_{\mathrm{C}}$ increases from 28 to 42 mm, which is consistent with that in the straight cavity. Figure 4I shows an agreement between the measured results and the simulation of the value of resonant frequency peaks of the device in a C-cavity ( $D_{\mathrm{C}}=35$ mm, $d=6.94$ mm, and $H_{\mathrm{C}}=20$ mm). For more simulation details about thermoacoustic resonant model analysis, please see the Supplementary Materials.

Figure 5 presents the application prospect of the frequency-tunable sound-amplified cavity with graphene thermoacoustic resonance. As shown in Fig. 5A, a wearable conch-like cavity is fabricated through SLA 3D printing technology. Figure S7 presents the detailed printing process of the C-cavity. The SLA technology works on the principle of photopolymerization of flexible liquid photosensitive resin. First, the 3D digital model is designed by 3D design software, and the discrete program is used to slice the model. Second, we irradiate the surface of the liquid photosensitive resin according to the designed scanning path. Last, the layered scanning and solidification are superimposed into a 3D structure. To obtain a suitable size for human wearing, we design three C-cavities, as shown in fig. S8 (A and B). The total side height of the C-cavity $H_{\mathrm{C}}$ is set at 20 mm, the wall thickness is 1 mm, the maximum diameter $D_C$ of the C-cavity is 35 mm, and the outer diameters of the spiral pipeline are set at $d_1=6.94$ mm, $d_2=6.24$ mm, and $d_3=5.54$ mm, respectively. The inset of fig. S8C shows the picture of three fabricated C-cavities. The LSG device is attached onto the bottom of the C-cavity inlet, generating sound waves. Then, sound signals are amplified and collected from the outlet of the C-cavity. Last, it leads sound propagation to the human ear, performing as an earphone (Fig. 5B). Figure S8C presents the values of the resonant frequency peaks of the LSG device in three C-cavities during the audible range. It can be seen that the C-cavity can amplify SPL at the resonant frequencies and smaller outer diameters $d_{\mathrm{C}}$ gain a larger spiral height, increasing the path distance of sound resonance in the C-cavity, thus leading to a lower resonant frequency. Figure S8D plots the responding SPL envelop lines of the LSG device at resonant frequencies of fig. S8C. The LSG device in three C-cavities has different resonant frequency peaks, with the main acoustic field ranging from 30 to 80 dB. To compare the sound performance of the LSG device under different conditions, we test the SPL of the LSG device in the commercial-shaped cavity, C-cavity, and open air, respectively (fig. S9). It is clear that the device in the C-cavity has the best sound performance during the total audible range, the commercial-shaped cavity has better sound amplification at a lower frequency with one resonant peak, and the LSG device in open air has a flatter spectrum at a higher frequency. The detailed values are shown in table S1. The LSG device in the C-cavity in this work can make the sound amplification of frequency as low as 1.2 kHz, with an ~6-dB average improvement in the overall audible frequency range. As we know, the sound performance detection equipment Agilent 35670A is usually used in the laboratory for scientific research, which is not popular in the market. So, to keep consistent with the sound performance data between the laboratory and market, we establish a commercial sound detection system based on the IEC711 artificial ear system. Figure 5C shows the circuit connection and performance test of the LSG device in a wearable C-cavity earphone by the commercial artificial ear system. Compared with the single LSG device, the sound waveform of the LSG device in the C-cavity has amplified substantially. Its frequency spectrum presents a brighter region at the resonant frequencies (Fig. 5D). Moreover, the SPL of the LSG device in a C-cavity is tested by the commercial artificial ear system in the audible range (Fig. 5E). There is an obvious SPL amplification in the C-cavity, especially around 1 and 10 kHz. It is noted that the SPL curve tested by the commercial artificial ear system is different from that by the Agilent 35670A because the artificial ear system adds a short straight channel at the sound detection unit to imitate the human ear canal.

Fig. 5. Wearable application of the frequency-tunable sound-amplified LSG earphone.

(A) The processing diagram of the SLA 3D-printed C-cavity. (B) Thermoacoustic resonance in a C-cavity earphone for sound emission. (C) Circuit connection and performance test of the wearable graphene-based C-cavity earphone under the commercial artificial ear system. (D) Sound waveforms and frequency spectra of the LSG device and device in the C-cavity. (E) SPL test of the LSG device by a commercial artificial ear system in the audible range.

DISCUSSION

In conclusion, we successfully developed a 3D-printed cavity for sound amplification using graphene-based thermoacoustic resonance. The graphene device, fabricated via one-step laser-scribing technology, generates sound signals through the thermoacoustic effect. This sound-emitting device is integrated at the base of the 3D-printed cavity, where specific frequency signals are amplified during sound wave propagation within the cavity pipeline. This amplification occurs because of the quarter-wavelength resonance effect, where it is verified that the amplified resonant frequency is inversely proportional to the propagation path distance within the cavity. Consequently, the operating resonant frequency can be tuned by adjusting the cavity height. In a straight cavity with a height of 10 mm, the frequency at 5.4 kHz is amplified substantially, with the SPL increasing from 32 to 71 dB. A physical model of the acoustic field in the straight cavity is established to elucidate the thermoacoustic resonance mechanism. For wearable applications, a conch-like spiral C-cavity is fabricated using SLA 3D printing technology, serving as an earphone shell. The increased spiral height enhances sound performance, achieving resonance as low as 1.2 kHz. Testing with a commercial artificial ear system confirmed the good sound amplification performance of the LSG device in the C-cavity, particularly around 1 and 10 kHz.

METHODS

Fabrication process of the LSG device in a C-cavity

The transparent and flexible C-cavity is printed through SLA technology by a 3D printer PreForm2, and the printer liquid material is Formlabs Form 3+ flexible 80A Resin. The 3D digital model of C-cavity is designed by a 3D design software first, and then the 3D printer irradiates the laser on the photosensitive resin according to the set path. At last, a designed C-cavity is processed in layers. The LSG device is fabricated through laser-scribing technology. A GO dispersion of 2 mg ml⁻¹ bought from Nanjing/Jiangsu XFNANO Materials Tech Co. Ltd. is mixed with tetrahydrofuran at a volume ratio of 1:5. Then, the mixture GO solution is dropped on the thermal-transfer paper consisting of a piece of PU film and paper substrate. After being put in a fume hood at room temperature for 12 hours, the dried GO is transformed to LSG by a laser with a 450-nm wavelength and a power density of 28.1 mW cm⁻².

Characterization

A ZEISS GeminiSEM 300# field-emission SEM is used to observe the surface and cross-sectional morphologies of the LSG. Raman spectroscopy is tested under a laser platform with a wavelength of 532 nm (HORIBA Inc.). The sound signal of LSG in the cavity is detected by the Agilent 35670A dynamic signal analyzer and the IEC711 commercial artificial ear system, respectively.

Supplementary Materials

This PDF file includes:

Figs. S1 to S9

Thermoacoustic resonant model analysis for the LSG device in a cavity

Table S1

References