I'm interested in the Physical science of sound, I've read all the articles I can find on the internet, but I don't feel they explain it in a way that I really understand. I'm not currently in a situation that I can afford a textbook in, so don't suggest any. I know this probably doesn't technically go in MT, but of everywhere I've been on the site, you people are the smartest and most direct, so I came here.

So please, if you could spare some time, I want to know everything there is to know about the physics of sound, from the bottom up.

Any help is appreciated.

Quote by Zaphod_Beeblebr
Theory is descriptive, not prescriptive.

Quote by MiKe Hendryckz
theory states 1+1=2 sometimes in music 1+1=3.
Any specific questions? "Everything there is to know about the physics of sound, from the bottom up" is a pretty broad subject.
start with learning about waves
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Sound waves
Amplitude- Height of wave- Determines loudness
Frequency-How fast the cycles of the waves are (measured in cycles per second or HZ)-This determines pitch

A440 for example, what most tuners tune to, means that the note "A" you are tuning to is at 440Hz. The A an octave higher will be at 880Hz, and the A an octave lower will be at 220Hz. Then you get into overtones and harmonics and whatnot, but its too difficult to explain concisely. Read something!
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I know some very basics, like wave-length and pitch are related, but not much. One question, what exactly is a decibel, I know it's a measurement of volume, but I need more information than that.
Quote by Zaphod_Beeblebr
Theory is descriptive, not prescriptive.

Quote by MiKe Hendryckz
theory states 1+1=2 sometimes in music 1+1=3.
Quote by The_Sophist
I know some very basics, like wave-length and pitch are related, but not much. One question, what exactly is a decibel, I know it's a measurement of volume, but I need more information than that.

a decibel is a measure of sound intensity basically loudness, for every decibel increase the sound intensity level doubles
See also: Psychoacoustics.

We comprehend sound in the pitch domain as as an exponential curve.. That is, the space between notes widens out the higher we go.
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Anything is possible with music which is sooo awesome

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I just learn the formula, apply it to a key, and use said notes on fretboard. Why? Cuz I'm not a pussy.
Quote by The_Sophist
I know some very basics, like wave-length and pitch are related, but not much. One question, what exactly is a decibel, I know it's a measurement of volume, but I need more information than that.

A decibel is a measurement of sound pressure which converts a very large range of values into a smaller scale, making use of a logarithmic scale.
I'd strongly recommend you take an electronic music/production course. That will get you a good practical foundation to build upon for learning the meeting points of science and art.
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I'll tell you all I can remember in one post.

Each sound will have a different wave to it, as sound is actually made up of horizontal waves (ever noticed that a speaker pushes in and out?). A wave will be formed by a change in air pressure, as the vibrations of an object push and pull the air back and forth to create a change in pressure.

Open up audacity (download it), generate a sine wave and zoom into it to see what I mean. Although on audacity it displays it as a vertical wave (up and down), this actually represents a change in air pressure. At the peaks of the wave, it represents high pressure. At the troughs, it represents low pressure. In the middle it represents neutral pressure (normal air pressure)

The amplitude (how much difference there is between the highs and lows of the wave) will denote how loud the wave is.

The "fundamental frequency" will denote what pitch we label this wave, which will be very apparent in a sine wave as there is only one harmonic in a sine wave. In most other waves, there are a number of different harmonics (octaves) being played at once and the amplitude of each harmonic will be different so as to create a different wave. In effect, every sound you hear is actually not just one pitch, but many pitches playing at once, which combine and form a single wave.

Now, some simple sine wave maths. Most sinusoidal (meaning, of a sine wave) will look like this a*sine(b*x+h)+i. A denotes the amplitude, B denotes the wavelength of that particular wave inversely (so as B increases, the wave length decreases) and H and I denote the actual position of the wave. H and I aren't terribly important for us, but you'll need them for when you do year 11 and 12 maths methods (this subject is international, to my knowledge). So all you need to focus on is this:


When you add two sinusoidal waves together, they merge together to form a new wave. For instance, if two peaks of a wave happen simultaneously, the new peak of the new wave will be as high as both the old peaks added together. This concept is a little hard to understand without high school maths.

For instance, a saw tooth wave (ever listened to trance? it's the standard sound in trance) will have an equation similar to this: sine(x)+1/2*sine(2x)+1/4*sine(3x)+1/8*sine(4x)+1/16*sine(5x) and so on for potentially infinity, x denoting the fundamental frequency. As you can see, each new wave is higher than the fundamental frequency (sine(x)) by a multiple of whatever number, meaning each new wave is exactly an octave higher than the wave before it. Also, each new wave is half the volume of the wave before it. This creates a near perfect saw tooth wave (a perfect saw tooth waves requires an infinite number of harmonics).

With this knowledge, we can form any wave form known to man, as we can continually modify each harmonic so that the wave form changes. By knowing how to create any wave form, we can potentially achieve any type of sound timbre wise.

Anyway, this is mostly for electronic music composers and people who understand how to work synthesisers more complex than a 3xosc.
[U]        | |                     [/U]
[U]        |/     .-.              [/U]
[U]       /|_     `-’       |      [/U]
[U]      //| \      |       |      [/U]
[U]     | \|_ |     |     .-|      [/U]
      *-|-*    (_)     `-’
I hate to be the smartass for picking out the one little flaw in your otherwise excellent post, demon. Please, consider this more an extension to what you wrote.
Quote by demonofthenight
sound is actually made up of horizontal waves
Sound travels in every direction the medium allows it. A sound wave is really a four dimentional construct, time being the fourth dimention. A wave can be represented theoretically by a horizontal sinusoid, which is a two-dimentional graphic of a single point though which the sound wave travels. The sound wave's amplitude is the y-dimention, time is the x-dimention.

It's interesting that this powerful representation leaves such a strong mark on our imagination. A better way to think of sound, is as a filled, expanding sphere around the sound source, with concentrical variations in air density.
Last edited by Withakay at Sep 12, 2009,
Here's a basic summing up of sound from my knowledge. I'm only going to deal with sound in air here, it travels through any medium and all that is changed is the speed of the wave. If you want more about that i can post it later.

First let’s understand waves. “A wave is a transfer of energy (but not matter) over a distance”. There are 2 types "longitudinal" and "transverse". They both travel in different ways. Transverse waves have oscillations perpendicular to the direction of travel (up and down if the wave is going left to right), these do NOT need a medium to travel through and are typified by light waves and aren't really relevant here so i'll leave them. Longitudinal waves have oscillations parallel to the direction of travel. These must travel through a medium and their speed is defined by the medium they travel through. These are typified by sound waves so we’ll discuss them 

Firstly I fell it’s important to point out that a typical sound wave diagram you see on a computer screen (you know the type I mean) are not really longitudinal waves, they are transverse representations of longitudinal waves. The reason why sound waves are often shown in this “incorrect” way is purely because it is much easier to render a transverse wave than a longitudinal wave. (see http://www.antonine-education.co.uk/Physics%20A%20level/Unit_2/Waves/Progressive_Waves/wav_9.gif to realise it’s quite tricky to make much sense of them). Since it’s a lot more “obvious” to visualise waves in this way and to measure them (try judging the amplitude from that pic) I’m going to describe/think in a transverse diagramattic way despite the fact it’s technically wrong. It shouldn’t make much difference to what I actually have to say though.

A sound wave is created by repeated changes in air pressure, which are felt on your eardrum and interpreted (incredibly cleverly) by your brain to distinguish what you’re hearing. To make a sound: energy is put into the material from somewhere. This energy makes the molecules in the material move. Their repulsion from nearby air atoms causes those nearby atoms to move, which causes their nearby atoms to move and so on. This causes the initial energy input to move thus it is a “transfer of energy over distance” so is a wave. So all a sound wave really is, is the vibrations of air molecules in your ear

What you hear:
The 2 things your ear can detect, frequency and amplitude. Amplitude is just how loud something is, a bigger amplitude means each of the air molecules is pushed with more energy, so more sound energy reaches your ear, making the sound more intense (the sound energy per unit area is known as the intensity and is measured in dB). The frequency is the pitch of the note that you hear. 440Hz is A, 220Hz is A one octave lower – you want more detail on this? I guess you’ve stalked enough MT threads to get the basic idea of this anyway, if not I’ll post in more detail.

So how do we hear different instruments etc.?
A plain single frequency wave is boring, if you imagine the sound of an old radio tuning, or the sound of a signal generator, that is the sound of a “pure” sound wave, one with just a single frequency. Sound is much more wonderfully rich than that though as you well know. This is because there are ‘overtones’ to almost every sound you hear. You get the idea of these best from playing natural harmonics. All the natural harmonics you can play on the guitar are actually there in the sound you hear, their just quieter than the “main note” you are playing. Although you can only actually play a few natural harmonics on a guitar loud enough to hear, there are hundreds of them every time you pluck a string. What makes different instruments sound different is which ones are accentuated. Sound travels differently through different materials, which is why a metal flute sound very different to a wooden one as more/less overtones will be absorbed by the wood/metal. Different shaped instruments will resonate particular overtones more or less so different harmonics will be heard and a certain timbre will be achieved. You probably notice this most in guitars as you can play more different ones/different woods. A different grain pattern might emphasise some frequencies over others which gives each guitar it’s unique sound. So what’s important to how something sounds as opposed to the note it’s playing are the overtone frequencies and their amplitude.

this picture helps to show it quite well. You can see the space between the biggest crests is the same so there must be the same w/f => same pitch, but the overtones are different so the trumpet and clarinet will sound different.

This is a summary of basic (~GCSE) physics of sound, I’ve probably missed stuff but I hope I’ve explained it in a fairly graspable way. If you have any more specific questions I’d be happy to help out some more and try to explain/re-explain things.
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Last edited by doive at Sep 12, 2009,