The decibel scale: how loud is loud?



The decibel scale is what we use to measure how loud a sound is. It's a logarithmic scale, so it's a little tricky to understand sometimes.


There's a lot more to it than I'm presenting here, but this should be enough for you to understand what you need to know about the decibel scale in relation to hearing loss. Unless you want to be an audiologist, that is. If you want to be an audiologist, then instead of reading my web site, you need to enroll in a graduate program in audiology.

Sound travels in waves. The human auditory system is well-adapted to detect sound waves in air, but to understand how sound waves work, it may be helpful to think of wave action in water, since that's something most of us have had a chance to watch with our eyes. Ready to visualize? Here goes!

You are standing on the bank of a calm pond. You toss a pebble into the middle of the pond and watch the waves ripple outward from the place the pebble strikes the surface. One of the things you might notice is that the distance between the waves is pretty regular, and that they're all traveling at the same speed, so the time from the arrival of one wave on the bank where you're standing to the arrival of the next is fairly predictable. The number of waves that hit a fixed point (like the bank) within a given time period is called the frequency. In the world of sound, frequency corresponds with pitch; we perceive a high frequency sound as having a high pitch, and a low frequency sound as having a low pitch.

Another thing you'll notice is that the waves are fairly small. The size of a wave is called its amplitude. The bigger a wave is, the more force it exerts on whatever it hits. If you put your finger into the water as the ripples from the pebble arrive at the bank, you'll barely feel them.

Now, instead of tossing a pebble, visualize yourself setting up a large, medieval-style trebuchet on the shore and tossing a Volkswagen into the middle of the pond (visualize removing the battery first, and draining all the gasoline, oil, and brake fluid--we don't want to visualize poisoning the fish). These waves will be quite a bit bigger, and if you dangle your hand in the water as they arrive, you'll definitely feel them, and they may even push your hand back and forth.

If you stand on the North Shore of Hawai'i in the winter, you are likely to witness waves large enough, and strong enough, to push a person on a surfboard--or to slam a clueless graduate student from Indiana onto the beach so hard that he spends what feels like an eternity writhing on the sand, convinced he'll never breathe again, while nearby teenage girls in bikinis laugh at him. I don't want to talk about it.

Sound waves of higher amplitude exert more force on your ear drum, and are perceived as louder, while low amplitude sound waves exert less force and are perceived as quiet sounds. If the amplitude is low enough, it does not move the ear drum enough for us to notice it.

The amplitude of ocean waves is measured in the distance from peak to trough. However, since sound waves are not visible, we measure and describe them in terms of the force they exert rather than their physical size. This is where the decibel scale comes in.

We can also measure sound pressure (the force exerted by a sound wave) in micropascals, but that gets tricky. For one thing, the number of micropascals at which a sound is audible differs according to frequency. In addition, doubling the strength of a sound in micropascals results in a sound that we perceive as being noticeably louder, but not twice as loud. For some reason, that's just not the way our auditory system works. Finally, the range from the quietest audible sound we can hear to the kind of really super-loud sound that would destroy your hearing instantly is so huge as to be incomprehensible if expressed in linear terms (like number of micropascals). For example, a jet airliner taking off is around 145 dB; that's more than one hundred trillion times as loud as the quietest sound an average young person with no hearing loss can hear. Once numbers get that big, they pretty much lose their meaning to most of us.

The solution to these problems is the decibel scale. Here's how the decibel scale works. The level at which a sound becomes audible to a young, average, healthy ear is designated as zero decibels (0dB). After that, there's way more math than I'm interested in writing about (you can find it quite easily with a Google search), but you'll be fine as long as you remember a few basic ideas:

1. Every increase of 3 dB is a doubling of the sound pressure level (twice as many micropascals). A sound of 23 dB is twice as much energy as a sound of 20 dB.

2. Every increase of 10 dB is a tenfold increase. A sound of 30 dB is ten times as loud as 20 dB, and 100 times as loud as 10 dB.

3. Conversational speech is at about 60-70 dB; a whisper is around 20 dB

4. The decibel scale is actually a pretty accurate representation of how the human auditory system perceives sound. In reality, a 40 dB sound is 100 times as loud as a 20 dB sound, but it seems twice as loud. Twice as loud as 20 dB is actuallly 23 dB, but it doesn't really seem that way.

It's important to remember, though, about the doubling of sound energy when you are dealing with sounds loud enough to cause hearing loss. Exposure to 100 dB noise for more than 15 minutes can cause permanent hearing damage; 103 dB is twice as loud, so noise at that level can cause damage after half as long, or seven and a half minutes. Chances are you can't even hear the difference between 100 dB and 103 dB, so when you're dealing with noise at that level, it's safest just to wear hearing protection no matter how long you're going to be exposed.

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