Table 1.

Best geometry parameters of dual-tube spectrophones realized for four different QTFs operating at different frequencies: the prongs’ spacing (s), the internal diameter of the tube (ID), the sound half wavelength (λ/2), and the theoretical (l_th) and experimental (l_exp) optimal tube lengths.

QTF Frequency	Prong Spacing s (mm)	ID (mm)	λ/2 (mm)	lth (mm)	lexp (mm)	Ref.
32.7 kHz	0.3	0.60	5.25	4.7	4.4	[3]
7.2 kHz	0.8	1.30	23.9	22.8	23.0	[10]
25.4 kHz	1.0	1.52	6.8	5.5	5.3	[5]
12.4 kHz	0.8	1.59	13.8	12.4	12.4	[6]