We measured the frequency response(microphone out/signal to speaker) to see how well we can shield the outside acoustic. The test panel did help reducing the acoustic coupling, but there is still room for improvement.
Den lend me a blue bird microphone from 40m. So we setup a measurement to compare two panels we have. The first one was what we made yesterday (plastic with damping pad), the second one was the aluminum panel(1/8" thick) with soft foam on the inside and foam strip on the edge where the panel met the frame.
We measured the frequency response between the microphone signal and the speaker driving signal. The source was white noise (band limit) 100Hz - 6.5kHz, 1.4V. The output has a T so that one was sent to the speaker, another one was for chA. The SR785 chB input for microphone signal was floated since the mic gave differential output. This should prevent the pre-amp output to see "ground" at the output and break the opamp.The measurement was average over 5000 samples.
We measured with the speaker on and off (but the white noise ref to chA was still connected) to check we have a good SNR for every setup. Three setups were:
fig1: setup, with the panel remove.
fig2: two panels for testing. Left, a plastic piece with damping pad attached on (from yesterday). Right, an aluminum panel with soft foam
fig3: panel under test.
==conclusion + plan==
From the plot, it is not very clear if the aluminum panel (panel2) is better than the plastic one (panel1). It might be that noise coming from other panels(which we have not changed) is the dominating signal. We will put the mic in a smaller container surrounded by acoustic damping with an opening for the material/structure to be tested. Then we can test a sample easily without removing/installing the panel all the time.
For now, we are planing to use another kind of foam to put inside the box. We check by ears and found that it is better than the current foam we use with the aluminum panel.