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  • Author Author: Jan Cumps
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Sound and Vibration Measurement: First Demo of the test jig

Jan Cumps
Sound and Vibration Measurement: First Demo of the test jig

For the Sound and Vibration Measurement Hat for Raspberry Pi road test, I'm reviewing Measurement Computing's IEPE Measurement DAQ HAT for Raspberry Pi.
Time to demonstrate the first setup of the Automated Vibration Test Jig.
In this video, the flow lets the vibration table shake from 20 to 40 Hz, with 2 Hz increment.
At each step, the MCC172 takes 32.768 samples, then calculates the RMS value of that result.
This gets written to a data file.

The video is real time. You will not hear the lowest frequency because my microphone (and your speaker) don't capture that. You can see the sample speed of the DAQ by looking at the LED blink time.
Excel output attached:

daqhatmcc172_0.xlsx

Link to all posts.

  • Hello Jan,

    You are hitting a problem I recall well from 48 years ago when I first tried to measure microphone frequency responses using a Bruel & Kjoer setup. There was a little speaker in a small enclosure with a reference microphone (known as an 'Artificial Mouth'). The sweep oscillator was mechanically controlled by the pen plotter (which used rolls of paper and real ink pens). The sweep oscillator had a compressor loop which took input from the reference microphone via a built in mic amp and a broadband rms responding rectifier ciruit. . The compressor loop would increase/decrease the drive to the speaker to keep the sound level constant. At low frequencies it would increase the drive level to the speaker until either the speaker or amplifier overloaded enough to make sufficient high freqency content to reach the correct soundlevel. if you set the system up to demand output that the speaker could not manage all frequency responses plotted looked suspiciously good !

    Eventually I built a fully analogue system with tracking filters to plot microphone and accelerometer responses.

    In your video I can hear lots of sound at an indicated 22Hz test frequency but it isn't 22Hz but much higher harmonics. The speaker or amplifier are overloading.

    It would be very instructive to look at the Fourier transform (or even scope like amplitude/time response)  of the captured data (I think Labview can do this.)

    MK

  • in reply to shabaz

    Yes, it's the subwoofer part of a Logitech 3 speaker system. It has a built-in amp for the sub and for the stereo sattelites. I'm using the subwoofer circuit only. It has a 30 Hz to a few 100 Hz bandwidth (3 dB). I chose it because it's strong, easy to drive from a generator. And because it was only €15 at the flea market.

  • Nice test Jan.

    I saw an application once where an engineer had glued mirrors to speakers and he then used them to scan an area using an IR sensor.

    The device made interesting images and produced very interesting sound at the same time.

  • Hi Jan,

    That's very cool : ) How are you driving the speaker, through an audio amplifier?