Noise, sound, decibels, hearing


A decibel–a measure of sound. Like all short answers, this one leaves out a lot. When we hear a sound, its because our brain has received a nerve impulse from the nerve cells in the Inner Ear, which consist of a bone, called the Cochlea because it resembles a snail shell. These cells have long “hairs” that project into fluid inside the cochlea. There is a hole in the cochlea which is covered with a flexible membrane. Attached to this is one of the three tiny bones that make up the Middle Ear, (hammer, anvil & stirrup because of their appearance). The middle bone is attached to the other two in such a way as to act like a pivot; when the outer bone pushes against the middle bone, the inner bone is pulled. The outer bone is also attached to a flexible membrane, located at the end of the ear canal. This membrane is called the Ear Drum.

Sound is produced by waves of alternating high and low air pressure hitting the ear drum, causing it to vibrate. Each vibration causes the bones in the middle ear to rock back and forth which, in turn causes the inner ear membrane to vibrate and thence causing the cochlear fluid to slosh back and forth and to make the “hairs” to wave back and forth as well. Each movement of a “hair cell” creates a nerve impulse which gets processed in the brain and is perceived as sound.

So, what’s a decibel? Well a decibel is the ratio of the air pressure caused by a sound source (like an engine or a stamping machine) and a reference pressure. The reference air pressure we use is the air pressure needed to cause a just barely perceptible sound in a normal, healthy 18-year-old ear! You may recall that air at sea level exerts a pressure of 14.7 pounds per square inch. Our reference air pressure, 2 x 10-5 Newtons per square meter, is equivalent to 2.9 x 10-9 pounds per square inch. Or, less than 3 one-billionths of a pound per square inch. More simply—extraordinary sensitivity.

OK-Now we can talk about decibels, the ratio of air pressures. Because numbers like 2.9 x 10-9 and 2 x 10-5 involve lots of zeros, logarithms are used make handling these numbers easier. By definition, a decibel is: 20 log (P/2 x 10-5 N/m2) where “P” is the pressure caused by the sound source. Remember, logarithms are actually powers or exponents (of “10,” usually). So if near total silence is “0” decibels then a sound that is 10 times more powerful is 10 dB and a sound that is 100 times more powerful than near total silence is 20 decibels and a sound 1000 times more powerful is 30 dB and so on. Here are some common sounds and their dB ratings:

  • Near total silence: 0 dB
  • A whisper: 15 dB
  • Normal conversation: 60 dB
  • Lawn mower (gasoline) 90 dB
  • Automobile horn 110 dB
  • Jet engine 120 dB
  • Gunshot 140 dB

Over time, our hearing loses some of its sensitivity. This “aging” hearing loss is due to less flexibility in the bones in our middle ear. Our hearing can also be adversely affected by excessive, repeated high noise levels. This damages those hair cells. Either way, 60 decibels to a 12-year-old seems a lot louder than 60 decibels to a 50-year-old. A hearing aid can help aging hearing loss but not noise induced hearing loss.

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