Application Cases

Experiment Name: Testing of Self-Demodulated Signals in Audio Frequency Directional Systems

Research Direction:
Audio frequency directional systems represent a novel type of acoustic source capable of generating highly directional audible sound. The concept of parametric acoustic arrays provides a theoretical basis for producing directional sound beams through the nonlinear propagation effects of acoustic waves in air. This study explores the fundamental theory and key implementation technologies for using parametric acoustic arrays as audible sound sources to generate directional audio frequencies, aiming to advance both fundamental research and practical applications of audio frequency directional systems.

Experiment Objective:
To date, limited experimental research has been conducted on audio frequency directional systems, leaving several critical issues unresolved. These include:

1. To what extent can current theoretical models effectively guide the design of audio frequency directional systems?

2. How can key performance parameters such as directivity, harmonic distortion, and self-demodulated sound pressure be tested to accurately reflect the overall performance of audio frequency directional systems?

3. There is a lack of understanding regarding the parameters and patterns influencing self-demodulated signals in audio frequency directional systems, hindering the establishment of an effective testing methodology. This study aims to explore performance testing methods for audio frequency directional systems and provide experimental answers to these critical questions.

Testing Equipment:

1. Signal source system: one YB1639 signal generator, one Agilent 35670A dynamic signal analyzer, and one computer.

2. Signal processing system: one set of DSP-based audio frequency directional systems.

3. Power amplifier: one Aigtek ATA-4011B power amplifier.

4. Transducer: one 300 mm × 300 mm integrated PVDF membrane transducer.

5. Microphone: one with a frequency response of 20 Hz–16.2 kHz.

6. Signal acquisition device: one with a maximum sampling frequency of 192 kHz.

7. Data display and storage devices: one TDS1012 oscilloscope, one Agilent 35670A dynamic signal analyzer, and one computer.

Experimental Procedure:
The general process for testing self-demodulated signals in audio frequency directional systems is as follows:

1. Generate sinusoidal or broadband audio input signals using the YB1639 signal generator, Agilent 35670A dynamic signal analyzer, or computer.

2. Process the input signals using various algorithms in the DSP system, and output the processed signals to the power amplifier for amplification.

3. Amplify the signals using the Aigtek ATA-4011B power amplifier.

4. Convert the amplified signals into ultrasonic waves via the transducer and transmit them into the air.

5. During propagation in air, the ultrasonic waves self-demodulate to produce highly directional audible sound.

6. Convert the self-demodulated audible sound signals into electrical signals using the microphone and input them into the signal acquisition device.

7. Display, acquire, and store the electrical signals converted from sound pressure using the TDS1012 oscilloscope, Agilent 35670A dynamic signal analyzer, or computer.

8. Import the signals stored in the TDS1012 oscilloscope or Agilent 35670A dynamic signal analyzer into a computer for analysis using MATLAB or directly analyze the test signals using audio signal processing software.

   
   

Experimental Results:
The testing conditions were as follows:

1. The test environment was a fully anechoic chamber with background noise levels of approximately 17–18 dB.

2. The testing system utilized a specialized acoustic testing system from B&K.

3. The testing distance was 3 m, with the microphone initially positioned along the acoustic propagation axis.

4. The signal processing method used in the audio frequency directional system was the N-th order approximate square root method, with N=3 and m=0.6.

The directivity of self-demodulated audio frequency waves generated by input signals at 500 Hz, 1 kHz, and 3 kHz was tested.

As shown in Figure 7-2(a), for a 500 Hz input signal, the measured -3 dB directivity angle Θ₋₃dB/2 ≈ 2°, and the -15 dB directivity angle Θ₋₁₅dB/2 ≈ 5°. This indicates that the self-demodulated audio frequency signal exhibits strong directivity.

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

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

Conclusion 7-1:

1. Audio frequency directional systems using integrated PVDF membrane transducers can successfully generate highly directional audio frequency waves.

2. The directivity of self-demodulated signals increases with the frequency of the input signal.

Recommended Power Amplifier: ATA-4011C

Figure: Specifications of the ATA-4011C High-Voltage Power Amplifier

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