What Are Decibels, and How Are They Measured?

Decibels (abbreviated dB) are the units used to measure the relative intensity of a sound. The decibel unit of measurement is a little odd because the human ear is incredibly sensitive.

The human ear can hear everything from your fingertip brushing lightly over your skin to a loud jet engine. In terms of power, the sound of the jet engine is about 1,000,000,000,000 times more powerful than the smallest audible sound. That's a big difference!

It would be difficult to account for the large range of sound levels in human hearing using a linear scale, so the decibel is a logarithmic unit. Some people describe one decibel as the "just noticeable difference" of intensity between two sounds, but this only holds for quieter sounds. The difference in sound pressure level between loud sounds is more noticeable and might be half a decibel.

Technically, the units that capture the human perception of a broad range of sounds are A-weighted decibels (dBA).

There are other decibel scales, like decibel-milliwatts (dBM), which measure electric power and have a reference level of one milliwatt, and decibel-watts (dBV), which measure output voltage level. These scales use different units but are calculated the same way: by comparing two signals in a logarithmic way.

Because A-weighted decibels are the only kind used outside highly technical applications, most people just use "decibel" to mean sound level.

On the decibel scale, the smallest audible sound (near total silence) is 0 dB. It's a logarithmic scale, so a sound 10 times more powerful is 10 dB. A sound 100 times more powerful than near total silence is 20 dB. A sound 1,000 times more powerful than near total silence is 30 dB.

Here are some common sounds from everyday life and their decibel ratings:

You know from your own experience that distance affects the intensity of sound — if you are far away, the power is greatly diminished. All of the ratings above are taken while standing near the sound.

A person with "normal hearing" should be able to detect sounds around 10 dB, while someone with moderate hearing loss may not be able to hear decibel levels lower than 50.

According to the Centers for Disease Control and Prevention, 17 percent of adults have permanent damage to their hearing due to noise.

Any sound above 85 dB can cause hearing loss, and the loss is related both to the power of the sound as well as the length of exposure. You know you are listening to an 85-dB sound if you have to raise your voice to be heard by somebody else.

Eight hours of 90-dB sound intensities can cause damage to your ears; any exposure to 140-dB sound causes immediate hearing damage (and causes actual pain).

The National Institute for Occupational Safety and Health (NIOSH) recommends workers not be exposed to dB levels over 85 for more than 8 hours. For every 3-dB increase in loudness, the maximum noise exposure time is halved. So, workers should only be exposed to an 88-dB sound for up to 4 hours and 100-dB sounds for 15 minutes.

In addition to limiting exposure to loud sound sources, NIOSH recommends wearing hearing protection (such as ear plugs or ear muffs) during even short exposure to loud noises (anything over 85 dB). Not sure how loud things are? NIOSH has an app that turns your phone into a sound level meter.

This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.