Loudspeakers: Terms: Decibel

The human ear is capable of identifying a wide range of sounds. The amount of sound pressure in air that the ear would recognize as being barely audible (sometimes called the threshold of hearing) has been determined to be .0002 microbars. The microbar is a unit of measure of sound pressure just as the inch is a unit of measure of length. This small amount of pressure causes the ear membrane to move less than the diameter of a single atom! The loudest sound that just begins to create a sensation of pain (the threshold of pain) represents a sound pressure that is 3,162,300 times more pressure than the threshold of hearing! These are the two limits of our range of hearing. It is obvious that if we used absolute values of sound pressure to describe how loud sounds were, the numbers would be very cumbersome. In addition, it would be difficult to comprehend such a concept of comparison, not unlike trying to compare the length of a football field to the length of a mosquito wing using the inch as the unit of measure.

Early acousticians came up with a simple method of comparing two sounds. A sound that was perceived to be twice as loud as another sound was said to be one Bel greater in sound level. The Bel was used as a unit of COMPARISON, not a unit of measure. Its namesake was Alexander Graham Bell, a pioneer in the science of audiology (the study of human hearing). It soon became apparent that this unit of comparison was not very accurate in describing the difference between two sounds very similar in level. A smaller unit of comparison, the decibel, was established. One decibel is defined as being equal to one-tenth of a Bel; the prefix "deci-" is a French derivative that means "one-tenth", as in decimal, etc. Since a decibel is one tenth of a Bel, then 10 decibels would equal one Bel. In other words, a sound that is twice as loud as another sound could be described as being 10 decibels or (10 dB) louder. By definition, one decibel (or 1 dB) represents the smallest change in sound level that the human ear can perceive.

Take a look at Figure 1. This chart shows several common sounds and their RELATIVE sound pressure level (SPL). Notice that threshold of hearing is defined as 0 dB SPL, This does not mean that no sound exists. Remember that the dB (decibel) is a unit of comparison, not a unit of measure. For example, leaves rustling are given a relative value of 10 dB SPL. That means that the sound of leaves rustling is 10 dB louder than the threshold of hearing (0 dB SPL), or twice as loud. Or, for example, commercial background music is approximately 60 dB SPL and a very quiet radio at home is 40 dB SPL. A sound that is 50 dB SPL would be twice as loud as 40 dB SPL and 60 dB SPL would be twice again as loud as 50 dB SPL. It follows that commercial background music is 20 dB greater or 4 times as loud as a very quiet radio at home.

by Travis Ludwig © 1996 Internet Sound Institute (www.soundinstitute.com). This article is for personal use only. Any commercial reproduction is not permitted without permission. To obtain permission, contact ISI at hopi@soundinstitute.com

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