Preface

VMU Zero is using the original piezo speaker in the VMU. Piezo actuator is essentially a capacitor and it requires higher voltage to drive.

Test Setup

Procedure

1. Set volume as 100%

   amixer cset numid=1 -- 100%

2. [Idle] Probe the GPIO18 directly at Pi output in idle state
3. [Frequency Response] Probe V_in at R21 and V_out at LT3469 output
4. Play pre-generated single tone wave file

   aplay sin_1khz_100.wav

5. Measure the sound level at 2cm proximity using SoundMetrer X on iPhone
6. Repeat step 3-5 with varying the frequency from 10Hz to 20kHz
7. [THD] Probe V_in at R21 and V_out at LT34469 output
8. Play pre-generated 1kHz wave file
9. Record the sound at 20cm proximity using WaveSpectra on PC
10. Repeat step 7-9 with varying the peak level of the wave from 10% to 100%

Results

Idle

Continuous 8.4kHz oscillation was observed at PWM output when no audio. Amplitude was about 1.6V and DC offset was around 1.60V.

Frequency Response - Individual

Peak volume at 5kHz.

Frequency Response - Sweep

Narrow peak frequency response from 3kHz to 6kHz. Anything below 2kHz is significantly attenuated with -20dB.

THD

THD was almost 100% when the volume is 50% level because the PWM output was clipped above 50%. This resulted 3rd harmonic tone surpassing the fundamental of 1kHz at the speaker output.