A listening test

Click to play in your browser, or right-click-download to play as you wish (24-bit .wav file):

multi-level sweeps.wav

I call this a listening test, because it involves your ears, equipment, and even environment. Any of these can influence your limits of hearing. There are practical limits to how quiet your environment is—a noisy street, nearby computer fans, air conditioning—we all strive to lower the background noise. Fortunately, electronic equipment it very low noise at affordable cost, but there is a minimum noise floor that can’t be eliminated at any cost, due to physical issues like Johnson-Nyquist noise and shot noise. You ears have limitations of the minimum energy required to deflect your eardrums, as well as your own human noise floor, which can include age and injury related issues such as ringing, in addition to the basics like breathing and blood flow.

The audio test file that repeats the same sweep signal, dropping in volume by one-bit increments. There are many type of test tones I could have used. It could have been a sinusoidal sweep or series of tones for each pass, dithered or not dithered, or noise bursts.

I choose to again create a purely digital signal (see Perspective on dither). The advantages are that it’s very easy to hear; it has higher harmonic content that sine waves, and it won’t compete directly with the noise of your electronics for your attention at low levels like noise test signal will. I think if it’s hearable by you in your environment and on your equipment, you’ll know with confidence whether you are hearing it or not. The fact it’s digital means there is no inherent distortion or aliasing—no quantization error, all changes line up exactly with the sample periods—and no need to add dither noise.

The audio file announces each sweep by effective sample size. The signal amplitude is two steps, peak to peak. That is, one lsb of the effective sample size positive, and one negative. That’s twice the minimum amplitude possible, but it’s a good choice because that matches things like the dither level for the sample size.

For instance, “5-bit” is announced, followed by a signal that is 2 steps in amplitude, peak to peak, out of 31 possible steps (there are 2^5 levels, or 2^5-1 steps). By the time it gets to “24-bit”, it’s 2 steps out of 16,777,215! A new sweep starts every five seconds. I used the Mac OS test-to-speech feature, unaltered from its default level. You may be annoyed that it keeps you from turning up the volume as it gets quieter, but that would be cheating the test anyway, and not give you the true idea of the relative levels. I could supply a file with only the sweeps, which is handy to find where your electronics noise floor overwhelms the signal, but I fear someone will damage their hearing or equipment, should something unexpected happen. Trust me, all electronics has a noise floor far above the 24-bit sweep, but it you want that minimum sweep, you can get it from the Perspective on dither article.

There is no quiz attached to this, it’s for you to explore you limits on your own. I hope this helps your perspective!

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