|MadSci Network: Physics|
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
Try the links in the MadSci Library for more information on Physics.