| ISLE | Chapter 10
| The Decibel Scale
Amplitude is usually measured in decibels.
A decibel (dB) is 1/10 of a bel, which is a unit
of sound intensity named in honor of Alexander
Graham Bell. The decibel scale is a logarithmic
scale. This means that the intensity (amplitude)
of a sound increases more quickly than the numbers
along the decibel scale. With every 6.02 dB, the
sound pressure actually doubles or with a -6.02 dB, the sound pressure actually halves.
A decibel level of 120 is nearly 1 million times greater in terms
of sound pressure than a decibel level of 10.
The formula to determine decibel level is given
by
dB = 20log(p/pr)
in which dB is the sound pressure level, p is the
measured sound pressure,
and pr is an reference sound pressure.
Sound measures of decibels are
measures of relative intensity, so it is important
to know what both sounds are.
The formula is a little different than what is in the text, but the both give the same numbers.
Since sound pressure is relative, if your pr is larger than your p, you will get negative values. You might see negative numbers on some audio equipment, that is using the loudest sound as the pr so all the volume settings are that loud (a dB of 0) or less. We will use this way of doing decibels in this illustration.
In this illustration, to help you experience decibels and get a feel for how they relate to sound pressure level, you can try different decebel steps and here how the loudness changes.
To see the illustration in full screen, which is recommended, press the Full Screen button, which appears at the top of the page.
Below is a list of the ways that you can alter the illustration. The settings include the following:
Play: start a series of tones starting from the loudest and decreasing in
constant decibel steps.
Decibel Step Size: the series will decrease by the decibel step selected here.
(Note: 6.02 will have the amplitude of the sound wave each time)
Amplitude or Intensity: this is more of a feedback showing you
the current intensity setting on the sound in relative amplitude of the sound wave.
Frequency: set the frequency of the sound.
Show Intensity: If checked, draw a vertical line from the 0 value to the first peak.
The height of this line is the intensity or amplitude of that wave.
Show Frequency: If checked, draw a line connecting the first two peaks of the wave.
The longer the line, the lower the frequency.
Pressing this button restores the settings to their default values.