MadSci Network: Physics

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
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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