The traditional high sound pressure calibrator, such as CA915 of BSWA, can only achieve the maximum sound pressure level of about 160 dB. The calibrator is very heavy and needs high power amplifier drive.

In 1940, Oberst proposed a method to obtain large amplitude pure sound wave in a closed tube, that is, two cylindrical tubes with different radius are connected. By using the resonance characteristics of the variable cross-section tube, the internal high-order harmonic vibration can be effectively suppressed, and the pressure ratio can be greatly improved. When the sound source is excited at the end of the thick pipe, the sound wave with high sound pressure and low distortion will be produced at the end of the thin pipe under resonance condition. As shown in the figure below:

If the end of the tube is assumed to be a rigid interface, the sound pressure level is:

Where Pe is the sound pressure level at the source end, S1 and S2 are the cross-sectional areas of the large and small tubes respectively, l2 is the length of the small tube, A, B and C are the quantities related to the sound frequency, tube length, tube radius and tube damping coefficient. Based on this theory, a variable cross-section tube can be designed for linearity and distortion calibrator of high sound pressure microphone.

Based on the principle of Oberst variable cross-section tube and the original SW4XX1 series impedance tube, BSWA has launched CA935 high sound pressure calibrator with built-in high sensitivity and low distortion loudspeaker, which can form a stable, pure and low distortion large amplitude sound field in the cavity, with a dynamic range of 94 dB ~ 180 dB. It also has a reference microphone installation position, which can be used for real-time monitoring of the sound pressure in the tube.

The internal acoustic coupling cavity of CA935 calibrator is designed to achieve the maximum sound pressure level at 500 Hz resonance frequency. The distortion of calibrator sound source within dynamic range ≤ 2.5 %. It is suitable for microphone distortion measurement and high pressure microphone calibration to avoid measurement error caused by overload. Due to the particularity of working environment of high sound pressure microphone, its diaphragm is easy to change or even damage due to the bad environment. Therefore, it is necessary to verify the linearity and total distortion of high sound pressure microphone regularly.

BSWA MPA473S high pressure microphone can be selected as the reference microphone to monitor the real sound pressure level in the calibrator cavity. In order not to affect the accuracy of the test, the MPA473S is specially selected to achieve a total distortion including the microphone and high pressure calibrator doesn’t exceed 2.5 % at 180 dB.

With BSWA high sound pressure linearity test software and data acquisition, a high sound pressure calibration system can be formed. The system can simultaneously measure the sound pressure level and total distortion of the reference microphone (Ref. MIC) and the microphone to be test (DUT). By comparing the difference of the sound pressure level of the reference microphone and the microphone to be test, the sound pressure linearity of the microphone to be test can be obtained.