Sound Intensity and Decibels

Sound Intensity

A sound not only has the property of frequency, it has volume, also known as amplitude or intensity. In pressure terms, intensity is a measure of the amount of compression and rarefaction of the sound pressure wave. It is measured in watts per square meter. Sound amplitude is a measure of the energy in a sound wave. The more energetic the speaker, between speaking and yelling, the more energy is present in the resulting wave. A microphone will measure the intensity of a sound and display the difference in the amplitude of the pressure plot.

If a sound is made in an open area, the intensity drops off quickly as you get farther from the source because the sound energy is spread in every direction. Note that if you make a sound in an enclosed area, especially one with hard walls that do not absorb energy, e.g., like a pipe, the intensity will not drop off with distance. You can still see pipes in old ships that the captain used to give commands to others in distant parts of the vessel.

Decibels

Human hearing is extremely versatile, being able to hear very soft sounds from 10-12 W/m2 to very loud sounds at 1 W/m2 where the sound starts to become painful. The loudest sound we can hear is about 1,000,000,000 times louder than the softest sound we can hear.

Since the range of hearing is so large, scientists invented a logarithmic scale which is based on multiples of 10. The decibel scale defines the threshold of hearing at 0 decibels and the threshold of pain at 130 decibels. Here's a list of common activities with an estimated sound intensity level. Each 10 dB is 10 times more amplitude. So a noise that is 30 db louder is 1000 times as loud.