Design of portable ultrasonic underwater sound pressure meter

With the development of underwater ultrasonic technology, the need to test its sound intensity has been raised in many application areas. We use CS-3 hydrophone to design a portable ultrasonic sound pressure meter.

System Design

Design goal requirements: To achieve the measurement of 15-45kHz ultrasonic sound pressure and sound intensity. The measurement range is 0-10 atmospheres (or sound pressure level range: 30-120dB). The measurement error is 3dB in the overall frequency range and less than 1dB for a single frequency.

The characteristic of CS-3 hydrophone is that the inconsistency of its M parameter is less than 3dB in 10-100kHz. M parameter refers to the output voltage generated by the hydrophone under the action of unit sound pressure, and the unit is V/Pa. The M parameter expressed in decibels is:

M(dB)=20log(M/Mo), where Mo is the reference sound pressure Mo=1V/礟a.

Sound intensity I=P2/(r*C), where P is the sound pressure, C is the sound speed, and r is the density.

In order to meet the design objectives, measurement error analysis is required.

Corresponding to a sound pressure level of 30-120dB, the sound intensity in the fluid is:

I=P*V*cosy

The sound intensity in a free field is:

I=P2/(r*C)

The expression of sound intensity level is:

I = 10log (I/Io)

Where Io is 10-12 (W/m2), and the sound pressure level is approximately equal to the sound intensity level in a free sound field.

It follows that (in approximate measurements) an inconsistency in the sound intensity level corresponds to an inconsistency in the M parameter of less than 3 dB.

It can be concluded that there is no need to perform frequency correction, and the inconsistency of sound pressure and sound intensity can meet the requirements.

Since the effective sound pressure is generally measured, which is the average value, the measurement accuracy is relatively easy to achieve. Following, amplification, filtering, peak and frequency detection, the errors are mainly caused by filtering, peak and frequency detection. The passband instability of the filter we designed is 1dB, and the error of peak detection is less than 1dB, but by performing frequency detection and time averaging, and performing software and hardware compensation, the error can better meet the requirements.

According to the above analysis and considering the characteristics of ultrasonic measurement, the system block diagram is shown in Figure 1. The pre-emphasis takes into account the DC isolation and system frequency characteristics.

Figure 1 System Block Diagram

Figure 2 High-pass filter circuit [page]

The follower is used to isolate the hydrophone and reduce the impact of the back-end circuit. The amplifier and the voltage divider are realized by controllable gain amplification, and the measurement range can also be switched by switches. Bandpass filtering is necessary for measuring the ultrasonic sound pressure of a specific frequency. Peak detection is for measuring and calculating the raw data of sound pressure, and frequency is for providing frequency data and frequency correction for sound pressure calculation.

Bandpass filter circuit

Considering that the system has a high requirement for the unevenness of the frequency range, Butterworth filter is used for design. The bandpass is realized by cascading high-pass and low-pass.

Considering the specific requirements, the cut-off frequency is set to 10kHz. The order of high-pass is 5, and the order of low-pass is 6. Figure 2 is a high-pass filter circuit.

Peak detection circuit

Figure 3 is a circuit that uses an integrated amplifier to implement peak detection.

Zero Crossing Detection Circuit

The zero-crossing detection circuit needs to consider the interface with MSP430 at the output, so an interface conversion circuit is added (Figure 4).

Figure 3 Peak detection circuit

Figure 4 Zero-crossing detection circuit

System Interface

This system uses the 12-bit A/D conversion of the P6 port of TI's MSP430 microcontroller to measure the peak value, and uses the timing capture/comparison of the P1 port to measure the frequency. The crystal oscillator, reset, clock circuit and JTAG are provided in the design of the MSP430 hardware to complete the overall design. According to the hardware, the relevant software is designed to measure the sound pressure.

Conclusion

The design of this sound pressure meter adopts the MSP430 microcontroller with extremely low power consumption, and uses simple and cheap integrated operational amplifiers and comparators to implement a relatively complex ultrasonic signal conditioning circuit design, which meets the design requirements.