Understanding
sound wave graphs
Simple representations
of sound waves involve graphing the air pressure next to your ear drum
as a sound wave travels past you.
When you
listen to music, the sound waves from the source move past you, your ears
register the changing air pressures, and your brain interprets these as
sound. That's also what happens when you use a microphone to record sound.
The varying air pressures cause the microphone's diaphragm to move and
generate varying electrical signals at the output.
A cycle
| A
cycle is one complete rise and fall of pressure in the air
through which a sound is passing. |
|
Wavelength
|
Wavelength
is the physical distance between two points along a wave
that are at the same state with respect to their pressure. It
is measured in metres (or feet).
|
|
Frequency
| Frequency
is the number of cycles that occur in each second. The higher the
frequency, the higher the pitch of the musical note we hear. |
 |
 |
| Higher
frequency = more cycles/second |
Lower
frequency = fewer cycles/second |
|
A
complete cycle is the rise in normal air pressure (shown by
the horizontal line above) to the peak maximum, the fall through
normal atmospheric pressure to the minimum, and back to normal.
One complete cycle is shown in blue
for each of the two waves above.
Frequency
is still occasionally expressed as 'cycles per second', but it
is much more common to use the term Hertz, abbreviated
to Hz. For example: 440 cycles (or vibrations) per second
=440 Hertz or 440Hz.
Mind
you, a muso would be more likely to talk about the way they perceive
frequency - i.e. what we call pitch. A frequency of 440Hz
is perceived as a pitch of A above middle C.
|
Hearing range
|
Human
beings can only hear sound frequencies between about 25 Hz to
about 17,000 Hz (17 kHz) - and unfortunately the upper limit drops
as we get older.
Dogs
can hear frequencies up to about 35,000 Hz (35 kHz) and cave bats
up to about 120,000 Hz (120 kHz) - frequencies that are ultrasonic
for us. Elephants can hear frequencies as low as 15 Hz. These
are infrasonic for us and we feel them rather than hearing
them.
Prolonged
exposure to infrasonic sound may have interesting physical effects.
How do you feel after a few hours around the sub-woofers at your
favourite club?!
|
Pitch
| Only
simple sound waves have one frequency. More complex
sounds, such as those produced by musical instruments, have one
main frequency, the fundamental, plus higher frequencies,
called harmonics, that are mathematically related to the
fundamental. Our hearing system zeros in on the fundamental frequency
and from that we get a sense of the pitch of the note being played.
|
Frequency
Ranges and Pitch
Amplitude
|
Another
word for the energy carried by a wave is amplitude. The
greater the amplitude the higher the volume and vice-versa, as
shown below:
|
| The
GREATER the vibration of the source, |
The
SMALLER the vibration of the source, |
| The
GREATER the energy carried by the wave, |
The
LOWER the energy carried by the wave, |
| The
GREATER the amplitude of the wave, |
The
SMALLER the amplitude of the wave, |
| The
HIGHER the apparent volume. |
The
LOWER the apparent volume. |
|
|
|
Sound for Sound Engineers
