Past Midway Ramblings on Business & Life

Robustness – a Lesson from Acoustics

It’s interesting what we can learn from just playing around in the lab. The thoughts below originate from messing with the equipment in the acoustics lab at Penn State many years ago.

If you generate an electronic pulse X times per second and run this signal through a speaker to hear it, so long as X is below 20 Hertz (beats per second), you hear the individual pulses. You can almost count them. But right about 20 beats per second, something strange happens – we stop hearing the individual pulses and we start hearing a tone, a very low frequency tone.

It’s almost as if this is the rate that exceeds our brain’s computing power (or processing speed), so instead of counting the individual sound pulses, our brain fashioned a new way to count… in the frequency domain instead of the time domain.

Essentially, our interpretation of tonal sounds is our inability to keep up with the individual pulses. We just can’t count that fast. To compensate, our brain developed a Fourier transform (long before Fourier was born) to convert the inbound pulses arriving as time series data, into the frequency domain. In other words, we hear a pitch.

This is a new way of counting the beats per second of the incoming pressure waves. With this nifty trick, we can keep track up to 16,000 beats per second (for people with really good hearing).1 It’s not that we are actually counting the pulses that fast, but yeah, kind of we are, just in a different way.

In this way, describing a pitch is the same as counting pressure waves really, really fast.

It’s easy to understand a tapping sound that you can count, say 4 or 8 or 10 beats per second. We can literally count that fast out loud. And, it’s easy for us to understand tones because they are just notes. We can’t count that fast, but we can hum with it. More specifically, we have discovered a way to make our vocal chords flap and vibrate at the same rate – the same number of beats per second – such that we can match the pitch. Some people are quite good at this and can match a pitch with exceptional accuracy. They are called singers.

But where it’s weird, is exactly at the transition point. At one rate, you can hear the beats and the next incremental step up, you hear a tone and it’s almost like there’s nothing in between. It’s almost one or the other… almost, but not quite. Because right there in between, you’re not quite sure what it is. I suppose it’s a little different rate for different people. But I remember it being right at 20 Hz for me as I was playing around with a spectrum analyzer late one evening in the acoustics lab. Anyway, the point is, the transition is what is strange. It messes with your mind a little. The brain is saying, “Which way should I count? The slow way or the fast way?”.

This is a common theme for many things in the world – that transitions are where odd things reside. In ecology, transitions between one biome and another produce plant and animal species that often can’t survive in either adjacent biome. But, it turns out that these transitions species give nature its robustness. The edge effects are beneficial.

This is also true in our personal lives. The strangest things happen in the transition periods of life. These can be the hardest times or the most rewarding, or both simultaneously (although often only in retrospect)… but they also provide the robustness we each need individually and collectively to survive and perhaps thrive.

Interestingly, in ecology, the transition area is called an ecotone, a hybrid word. “Eco” from “ecology” and “tone” from the Greek “tonos”, meaning “tension”. So, the transitional zones in nature are literally areas of “ecological tension”.

Similarly, in acoustics, when the vibrations outpace our ability to keep up with the slow-counting, we also hear a “tone”.

In life, when circumstances outpace our ability to keep up, we feel “tonos” – tension and stress. Perhaps there’s another way to count it.

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  1. Some people claim 20,000 Hz but I’m dubious. I turned it up really loud and I could only hear up to 16kHz.

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By Andy Jones
Past Midway Ramblings on Business & Life

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