Exploring Human Hearing Range
Human Hearing Range
In this post, I'll look at an interesting aspect of Audacity – using it to explore the threshold of human hearing. In my book Digital Signal Processing: A Gentle Introduction with Audio Examples, I go into this topic and I include a side note on the amazing hearing range of our canine companions.
Creating a Test Audio File
Audacity allows for the generation of a variety of test signals. If you click the Generate->Tone menu, it looks something like this:
Select a frequency of interest, taking care to use one in the human hearing range, between 20 Hz and 20 kHz.
In the diagram, I've used a 9000Hz sine-based waveform. Click OK to generate the signal and you'll see the familiar Audacity blue block:
In passing, notice in the bottom left of the figure, the sample
rate (highlighted) is the default value of 44,100 Hz and the position
of the audio position (or signal cursor is zero), i.e., the leftmost
sample. As discussed in my earlier post, you can delve into the
signal by zooming in all the down to individual sample values.
Spectrum Analysis
So, we now have a basic single-tone (9000 Hz) audio signal. To verify that the signal contains the required 9000 Hz tone, let's do a spectrum analysis, by clicking Analyze->Plot Spectrum... to produce the following.
Notice the single peak of 9000 Hz generated using the default Audacity settings.
So, what does such a signal sound like? Well, Audacity provides a
playback option. As a precaution, make sure that your machine sound
volume is set low – single tone signals sound pretty horrible. As
soon as your volume is set low, hit the green play button:
You can then play around with generating other signals with higher/lower frequencies. Enjoy!
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A sine wave has no harmonics, as you probably know, and a square wave has lots. The first harmonic is at a frequency three times that of the base frequency, so an 1kHz square wave will have it's first harmonic at 3kHz.
Now comes the clever part. If you can tell the difference between a sine and a square waveform at the same frequency, it would indicate that you can at least hear the first harmonic of the square wave. So if you, for instance, compare an 8kHz sine wave with an 8kHz square wave and can tell the difference - that should indicate that you can hear up to 24kHz.
Of course, your ears isn't alone of having limits. Every unit in the signal chain can cap the frequency or distort the sound in any shape or form - so even if you don't hear any difference, that isn't proof that you are not able to. Only on equipment which can play above 20kHz, can you get a guaranteed result. One example can be the sample rate of your converter - at 44.1kHz it will hardly play over 20kHz depending on the LPF.
Interesting observation, thanks!
I worked Asana Integrations Firm so may be opinion is wroung, but the basilar membrane of the cochlea is the limiting element. The varied frequencies are mapped to distinct points along the length of the ear, which then communicates to the specific nerves that convey the signal to the brain. Each segment of the membrane functions as a resonator tuned to a distinct frequency to accomplish this.
The range of frequencies that the basilar membrane is tuned to is 20 Hz at one end and 20 kHz at the other, which is why human hearing is confined to the 20 Hz to 20 kHz range.
Thanks for the information. Would it be fair to say that it acts as a kind of waveguide? And that with age, fewer frequencies are transferred to the separate neurons?
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