Application Cases

Experiment Name: Self-Demodulation Signal Testing of Acoustic Frequency Directional Systems

Research Direction: Acoustic frequency directional systems are a new concept of sound sources that can produce highly directional audible sound. The introduction of parametric acoustic arrays provides a theoretical basis for generating directional sound beams using the nonlinear propagation effects of sound waves in air. This paper explores the basic theory and key technologies for using parametric acoustic arrays as audible sound sources to produce directional sound frequencies, in order to promote the fundamental research and practical application of acoustic frequency directional systems.

Experiment Purpose: So far, experimental research on acoustic frequency directional systems has been limited, leaving some key issues unresolved. The most representative of these issues are: 1. To what extent can the current theory of acoustic frequency directional systems guide the design of such systems; 2. How should the key performance parameters of acoustic frequency directional systems, such as directivity, harmonic distortion, and self-demodulation sound pressure, be tested to best reflect the overall performance of the system; 3. There is a lack of understanding of the parameters and patterns affecting the self-demodulation signals of acoustic frequency directional systems, making it impossible to establish an effective testing methodology. This experiment aims to explore the performance testing methods for acoustic frequency directional systems and find answers to these key questions through experimental research.

Testing Equipment: Signal generator, dynamic signal analyzer, 1 computer, high-voltage power amplifier, transducer: 1 monolithic PVDF membrane transducer measuring 300mm x 300mm, 1 microphone with a frequency response of 20Hz to 16.2kHz, 1 signal acquisition device with a maximum sampling rate of 192kHz.

Experiment Process: The general procedure for testing the self-demodulation signals of an acoustic frequency directional system is as follows: (1) A sine input signal or broadband audio signal is generated by the YB1639 signal generator, the Agilent35670A dynamic signal analyzer, or a computer; (2) The input signal is processed by the DSP system using various algorithms and then output to the power amplifier for amplification; (3) The power is amplified by the Aigtek ATA-4011B power amplifier; (4) The amplified signal is converted into an ultrasonic signal by the transducer and emitted into the air; (5) The ultrasonic wave self-demodulates in the air to produce highly directional audible sound; (6) The microphone converts the self-demodulated audible sound signal's sound pressure into an electrical signal and inputs it into the signal acquisition device; (7) The electrical signal converted from sound pressure is displayed, collected, and stored by the TDS1012 oscilloscope, the Agilent35670A dynamic signal analyzer, or a computer; (8) The signal stored in the TDS1012 oscilloscope or the Agilent35670A dynamic signal analyzer is imported into a computer for signal analysis using MATLAB, or directly analyzed using audio signal processing software.

Experimental Results: The test conditions for this experiment are as follows: (1) The test environment is an anechoic chamber with a background noise level of approximately 17-18 dBA; (2) The test system uses a dedicated acoustic testing system from B&K; (3) The test distance is 3 meters, with the microphone initially positioned on the axis of sound wave propagation for testing; (4) The signal processing method used by the acoustic frequency directional system is the Nth-order approximate square root method, with N=3 and m=0.6.

In this test, the directivity of the self-demodulated sound frequency waves generated in the air by input signals with frequencies of 500 Hz, 1 kHz, and 3 kHz was measured.

As shown in Figure 7-2(a), when the input signal is 500 Hz, the measured -3 dB directivity half-angle Θ-3dB/2 ≈ 2° and the -15 dB directivity half-angle Θ-15dB/2 ≈ 5°, indicating that the self-demodulated sound frequency signal has a strong directivity.

As shown in Figure 7-2(b), when the input signal is 1 kHz, the measured -3 dB directivity half-angle Θ-3dB/2 ≈ 1° and the -15 dB directivity half-angle Θ-15dB/2 ≈ 4.5°, showing that the directivity of the self-demodulated signal is slightly stronger than that of the 500 Hz self-demodulated signal.

As shown in Figure 7-2(c), when the input signal is 3 kHz, the measured -3 dB directivity half-angle Θ-3dB/2 ≈ 0.8° and the -15 dB directivity half-angle Θ-15dB/2 ≈ 4°, indicating that the directivity of the self-demodulated sound frequency signal is slightly stronger than that of the 1 kHz input signal.

Conclusion 7-1: (1) The acoustic frequency directional system using a monolithic PVDF membrane transducer can successfully produce sound frequency waves with strong directivity; (2) The directivity of the self-demodulated signal increases with the increase of the input signal frequency.