My daughter has a high school physics extended research assignment to complete and the topic has to be something to do with sound waves. She has chosen to look into active noise cancellation.
So to assist her with her research and to ensure she has a good understanding of the technologies, I set up some demonstrations/experiments. One of these was to measure the actual noise reduction of active noise cancelling headphones. We did this by inserting a small microphone (Mipro MU-55HN removed from its mounting frame) under the ear cushions of my Bose QC15 headphones and measuring the audio level of a test signal being played through a speaker external to the headphones. We used a 100Hz sinusoidal test signal and measured the level picked up on the reference microphone with the Active Noise Cancellation switched off and then switches on, allowing the noise reduction to be calculated.
Here are the results. The red waveform is the original 100Hz noise source signal that was fed to am amplifier (Yamaha P5000) and speaker (EV ZX5). The yellow waveform is the output from the reference microphone (Mipro Mu-55HN) via an EV RE2 wireless transmitter connected to a Yamaha LS9 mixer (as see measured on an Omni Out via a mix-bus).
With the headphones power switch set to off, the following was observed:
The with the headphones switched on, the follwoing was observed:
Converting the measured yellow trace peak-to-peak voltages to RMS voltages (V[SUB]
RMS[/SUB] = V[SUB]
pk[/SUB]/(2 x √2), and the noise reduction is measured as:
20 x Log[SUB]
10[/SUB](V[SUB]2[/SUB]/V[SUB]1[/SUB])
= 20 x Log[SUB]
10[/SUB](12.73/178.2)
= -22.9dB, which I think is pretty darn impressive.
We did not optimise this result by testing with different noise source volume levels or seeking to find the frequency where maximum reduction occurs. We used 100Hz as that was the same frequency used for some other testing and measurements we did (not with the noise cancelling headphones) and wanted the scope outputs to be consistent.
