Decibel Meters

How loud is it?


Loudness, L, is the human perception of the intensity of a sound. The intensity, I, of a sound is measured in watts per m^2. Perception of loudness depends on the pitch of a sound. The perception of sound is measured on a logarithmic scale. The loudness of a sound is measured with a decibel meter.


A decibel meter from Radio Shack.

To Do and Notice

Examine your decibel meter.

Set it to use the "A" scale rather than the "C" scale. The "A" scale gives readings of the loudness perceived by a human ear.

Set it to fast response time. (You can use slow response for continuous sounds and fast for short sounds.)

Measure the loudness of the room you are in. A quiet room may have such a low loudness that it cannot be measured on the meter.

Loudness, or sound level, is measured in decibels, dB.

Hold the sound level meter about 1 meter from your mouth. Say a continuous vowel like "aaaaa" and adjust your loudness to create a 70 dB loudness sound.

Now increase the loudness of your sound to 80 dB, then 90 dB, then 100 dB.

Optional, make a 110 dB sound.

Notice how the perception of loudness changes as the sound level increases by each 10 dB.

What's Going On?

The perception of sound is a logarithmic function of the sound intensity.

The relation between intensity and sound level is

L = 10 log I/I0

Where I0 is the reference intensity of 10^-12 W/m^2

so an intensity of 1 W/m^2 has a sound level of

L = 10 log 1/10^-12 = 120 dB

For each increase in intensity of an order of magnitude the sound level increases by 10 dB.

Said another way, when you multiply the intensity of a sound by ten you add ten to the loudness.

When you double the intensity of the sound you add 3 dB to the loudness.




Scientific Explorations with Paul Doherty


27 June 2006