Sound and Vibration Measurement: First Demo of the test jig

RE: Sound and Vibration Measurement: First Demo of the test jig

dougw — Wed, 27 Apr 2022 13:36:12 GMT

Interesting setup. It looks like a large mass for the speaker to move, which will of course affect frequency response, but a great way to at least exercise some control of stimulus frequency/acceleration.

RE: Sound and Vibration Measurement: First Demo of the test jig

michaelkellett — Wed, 27 Apr 2022 09:23:41 GMT

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.)

RE: Sound and Vibration Measurement: First Demo of the test jig

DAB — Tue, 26 Apr 2022 20:38:06 GMT

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.

RE: Sound and Vibration Measurement: First Demo of the test jig

shabaz — Tue, 26 Apr 2022 20:21:31 GMT

Hi Jan,

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