MadSci Network: Physics Query:

### Re: What are beats?

Area: Physics
Posted By: David Barlow, Private individual, Grad education in Physics/Astrophysics and Comp. Support
Date: Tue Jul 29 06:47:11 1997
Area of science: Physics
ID: 868553170.Ph
Message:
```Hi Nauzad

First off I would point you to any school physicsbook  for a discussion on what

sound waves are and an explanation of the terms I am using. Basically a wave is

something that periodically changes amplitude. Typically waves are expressed as

mathematical sine functions. As a test for yourself plot out the equation

y=Sin(theta) for theta=0=360 degrees, this is an example of a wave that

oscillates once. The number of oscillations per second is the frequency of a

wave, measured in Hertz written as  Hz. So a wave of frequency 1 Hz oscillates

once a second. A wave of frequency 500 Hz oscillates 500 times a second. Normal

human hearing can hear around 30-20,000 Hz.

Now lets say you pluck the string on a  Sitar,  the length, thickness and stress

on the string will cause it to vibrate (oscillate). This causes the air to be

compressed and decompressed around the string, which is what you hear as the

note. The number of times the string vibrates is equal to the frequency of the

note you are hearing. Let's say you tune a string to play a note of 900 Htz and

another string to play a note of 902 Hz. The two waves created will interfere

with each other and create another wave of 902-900=2Hz. This is what a beat is.

I can't really describe it except that you hear a sort of WaaWaaWaa over the

other two notes. It is easy to set up a small experiment yourself by taking an

instrument, Sitar or two flutes, and doing this yourself. Alternatively, plot

two waves with slightly diffenerent frequencies on graph paper and then add them

together graphically to see what you get.

Another answer is that a beat is the musical term for how many notes you play,

in a minute,  in music. A slow march is played at 60 beats a minute, one a

second. A fast march is played at 120 beats a minute, two a second and so on.

I am not sure what context you are thinking of by Waxing and Waning. To Wax

generally refers to something becoming bigger or fuller, to Wane means something

getting smaller or lesser. A Waxing moon is one that goes from a New Moon to a

full Moon. The Waning Moon goes from a full Moon back to New. Gibbons famous

book "The rise and Fall of the Roman Empire" could be entitled "The Waxing and

Waning of.." These are not terms used in the physics of sound. You may be

thinking of an effect where if the source of a sound is coming towards you the

frequency of the sound increases (waxes I suppose) until it reaches you then

decreases (wanes) as it passes and goes away from you. Typically heard when a

fire engine is rushing down a street at you.

Hope this helps.

Dave Barlow
```

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