# Do our ears (or brain) only perceive periodic disturbances?

Through a little time spent using sound editors for this and that, I've come to notice that if I play any musical note 440 times per second, I hear an A, whose frequency is 440Hz. Then I tried with very non-periodic sounds, even long ones like an entire song, sped them up and played them repeatedly at 440Hz and it worked. Not that surprising, but a little confusing. Confusing because first of all, any periodic wave has an infinite number of periods : possibly a smallest one, and then all its multiples. So which ones do we hear ? I'm guessing the notion of harmonics has something to do in there (which I am familiar with as a musician though I never had any true understanding of them). Confusing also since indeed, any non-periodic wave put together a few times becomes periodic, even a musical note followed by a silence, repeated, has a period equal to the duration of the note added to that of the silence.

Now finally here's my actual question : when we hear a note consisting of a random sound repeated at high frequency, what happens if we hear it during only one period, and if that one period itself is not made of a periodic signal ?

I can reformulate more mathematically : if the air pressure at the point my ear is is the function $\sin(2 \pi \cdot 440t)$ I hear an A, but what happens if I hear it for $\frac{1}{440}$ of a second ?

My own thought experiments incline me to say we don't perceive it, and formulate the theory that we only ever hear anything that has a frequency. In other words if I played a guitar string and could mute it the first time it came back to its resting position, then I wouldn't hear it. Otherwise it would make no sense not to hear something lower than the human threshold of 20Hz just because it has a frequency and hear something else that has no frequency.

• 1. What do you mean by "play a musical note 440 times per second"? 2. Do you know about the modulation property of the fourier transform? Apr 4 '17 at 21:42
• You can hear white noise. We have a noise generator at work, and I wish I couldn't hear it. Apr 5 '17 at 1:29
• @The Photon : what I mean by play a note 440 times per second means I hit the note 440 times per second so in one second we get 440 times the sequence "note/silence". Whichever note is played, it produces an A Apr 5 '17 at 5:47
• If you hear one cycle of a 440Hz note you'll hear a bang or a click. In general we perceive aperiodic sounds as bangs, thumps or clicks if they are not extended in time or as hisses or rumbles if they are. Obviously I have failed to define these terms (so this is a comment nit an answer) but the fact that we have these words -- and that drums exist as musical instruments -- tells you that we do indeed detect aperiodic sound.
– user107153
Apr 5 '17 at 6:39
• But even drum sounds and clicks have frequencies don't they ? Although we don't hear many of them due to short exposure. Isn't this true ? Apr 5 '17 at 7:10

What you perceive as sound is just your brain's interpretation of pressure changes in the inner ear behind the ear drum. When an air disturbance makes the ear drum move causing the pressure at the other side to change, your brain "hears" it.

Any motion of the ear drum is in this way "heard" as some "sound". Wave or not and periodic or not.

But you might not want to call it sound. You would rather call it noise. If you have ever been standing in a "silent" room (with all these pointy foam spikes on the walls), you'll realize that you in your daily life constantly hear noise. There is constantly some disturbance of your ear drum.

If you happen to hear a periodic disturbance, it will sound more ordered. It will sound as actual sound and not noise. If you hear just one period during one fourhundredandfourtieth of a second, then you can't in any way recognize it or distinguish it and you would call it noise along with all the other random disturbances in the air pressure.

Defining if what you hear is sound or noise depends on the person, and I don't think there is a specific official definition stating if a specific amount of period is required in order to call it sound. The degree of orderedness and enough periods to notice it as a periodic disturbance is subjective.

• I think the word "tone" might be more appropriate then "sound". You're making a distinction between noise and sound that seems completely arbitrary and not according to any real definition. en.wikipedia.org/wiki/Noise any distinction between sound and noise would be entirely subjective and not really in accordance with what you said. From a technical standpoint I'm not sure if it would even be noise or sound since it's not a real vibration (though that doesn't stop you from "hearing" it like you explained).
– JMac
Apr 4 '17 at 21:38
• I wish I could edit my question, I can see why and where it's not clear. Still you understood best what I meant to say, appreciate everyone's efforts and now I need to say : if ANY disturbance is perceived (even when it is not periodical) then how come we have a minimum threshold of 20 Hz ? I mean, if a disturbance is periodical with period less than 20 Hz, why should it be harder to hear than one single "unit" piece of that signal ? Apr 5 '17 at 6:02
• @JMac I understand that the word "sound" can be a bit off. The distinction between noise and sound or tone is indeed subjective, which is what I tried to explain Apr 5 '17 at 6:56
• @JamenWell Actually, you can edit your question; there should be an edit button just below it. When I say that any disturbance is perceived, then naturally there is some limitation still. Just like your ultrasensitive fingertips can't feel a roughness that is simply too small, and your eyes can see a hair a few micrometers thick but can't see molecules a few nanometers thick, then your brain can't detect pressure changes that are too small. All detection methods have their own limitations. Apr 5 '17 at 7:01
• "how come we have a minimum threshold of 20 Hz ? " -- because there are only cilia of certain lengths in the ears. The lengths determine which wavelengths stimulate the nerves attached to the cilia. You seem rather confused about sound wavelength vs. the frequency with which a sound is repeated. Apr 5 '17 at 10:05

Our hearing is primarily frequency based. In particular, our brains typically structure their concept of a "sound" around a so-called "fundamental frequency." Sounds can be decomposed into many frequencies which are often organized as harmonics. The lowest tone in this harmonic series is the "fundamental" frequency of that sound. Our brains then proscribe character to that sound based on the overtones.

There are a few places where this can break down. One great example is in Tuvan throat singing. In this art, one sings a drone and then manipulates the overtones with the shape of one's mouth to highlight a single overtone. When we hear this with our ears, we hear the normal harmonics of the done and think "that's one voice," but the highlighted overtone is too loud to fit in that distribution. It's much louder than the nearby harmonics. Thus, our brain chooses to treat it as "a second voice," and we get the illusion that Tuvan throat singers are actually singing two different notes at the same time!

We also capture some transient non-frequency information for purposes of spatial location. If I clap from one direction, you can tell where the clap came from because you're measuring the difference in timing between the signal reaching your right ear and the same signal reaching the left. We can show that this information is essential with what I consider to be a hilarious test setup. Put someone in a room with hard walls with two speakers, we'll call them A and B. You start playing a tone out of speaker A (any tone, midrange is best). Initially your ears will hear the transient effects of this speaker coming alive and you'll associate the sound to speaker A. However, with all those hard walls, pretty soon you'll loose track of those transients. All you'll have to work with is frequency data. At this point, we shift the sound from A to B, varying the power using an exponential curve. This transition makes it very hard for the ear to detect that speaker A is now silent and speaker B is now playing. Thus, you'll actually hear the sound come from speaker A. This effect can be so disconcerting that, even when you are told you are listening to speaker B it can be impossible to convince yourself. It is apparently quite disturbing to have someone tell you this, then walk up and unplug speaker A. You'll keep hearing the sound out of speaker A, even though it is unplugged! Magic trick!

So we do have tools in our wonderful squishyware brain to recognize facets of audio other than just frequency. However, evolutionarily speaking, the signals which could be well captured by frequencies have been very important, so the majority of our processing uses them.

Regarding the question in the title: Do we only hear sounds with frequency?

Audible sounds have (waveforms in) frequencies in approximately the range of 20 Hz to 20000 Hz. Sounds that are perceived to have a dominant frequency/pitch are called tones.

To quote Wikipedia: Pitch is a perceptual property of sounds that allows their ordering on a frequency-related scale.

We say that a sound has a certain pitch because it largely has the properties of a sine wave of the associated frequency. That is, there is a dominant frequency and it is the one we associate with that pitch.

if I play any musical note 440 times per second, I hear an A, whose frequency is 440Hz.

When you took a tone with a certain pitch and sped it up to play in the frequency of A you simply got an A tone (whose deviations from the ideal sine function were determined by the original tone). Because tone is determined by its frequency/pitch.

...what happens if we hear it during only one period, and if that one period itself is not made of a periodic signal ?

This confuses me, hopefully you could improve that part of your question now.

You would probably be interested in these:

I hope I did not say something incorrect ;)

• Thanks hey just before I read the harmonics link, I think I didn't make that clear : i didn't speed up the signal to get an A, I sped it up so that I would hear the note in question being struck 440 times per second. And even if you speed ANY sound up such that you hear it played 440 times per second you get an A. If you have software that does it, can record yourself reading a whole poem, speed it up to hear it 440 times per second and it produces a firm A note. :) Apr 5 '17 at 5:52
• Sorry, that is what i was trying to explain! Hz means "times per second", 440 Hz is the frequency of A, so if you produce a sound at that frequency you get an A tone. Apr 5 '17 at 6:18
• I thought you thought that I had stretched or compressed a periodic signal to change its period to 440 Hz, e.g. threefold stretching a 1320 Hz signal Apr 5 '17 at 7:14

if the air pressure at the point my ear is is the function $\sin⁡(2π⋅440t)$ I hear an A, but what happens if I hear it for $\frac{1}{440}$ of a second ?

If you have a "sufficiently long" sample, then the frequency becomes well-defined. You can imagine doing a Fourier transform on the sample and you'll get a nice peak at the dominant frequency ($440 \text{Hz}$ in your example).

As you shrink the sample, the uncertainty in the frequency increases. A single "pulse" doesn't have any periodic content. If you take the transform, you'll get a very wide frequency output. The harmonics created by going from zero output, to your signal, then back to zero in a short time can excite a range of different frequency sensors (such as the ones in your ears).

As an example, try taking a second or two of a pure tone, then make it smaller in an audio program like audacity. As you make it shorter and shorter, the actual tone of the sound is lost. The sudden start and stop overwhelms the tone and makes it sound like noise. This introduces a signal that is not present in your $20\text{Hz}$ tone and allows it to be perceived.