What Is A Decibel?

The first goal of this site is to provide (hopefully) good explanations of acoustical concepts in a community noise. In this post I will try to explain the most basic concepts of measuring sound, since that is where people typically go wrong. A good grasp of the following terms are vitally important to being able to discuss noise in a meaningful way.

Admittedly, these definitions tend to have more detail than necessary. You don’t need to understand every little bit of what is written below to be able to discuss acoustics intelligently. If you only have the time or patience to learn one term, please read about A-weighted decibels (dBA).

Pages and pages could be written about what sound is, but the thing you really need to understand is that sound is, at its most basic level, a change in air pressure. A very, very small change in air pressure that fluctuates with time. That is why sound is measured in Pascals, the same units used to measure water pressure or atmospheric pressure.

This might be the most important yet least understood concept in acoustics. A decibel is a ratio. It’s a numerical tool that comes in handy when measuring things that don’t act in a linear way. Sound, earthquakes and electrical signals are all items that it is useful to use decibels to describe. Although it appears to be used that way, it is not a unit of loudness.

Decibels are used in acoustics in a variety of ways, leading to further confusion among people trying to understand it. The most common uses are measurements of sound power and sound pressure. In talking about community noise, sound pressure is what we are most concerned with. You might see the term SPL, which stands for Sound Pressure Level. Decibels of sound pressure are calculated by dividing the absolute sound pressure at a location by a universal reference quantity (20 micro-Pascals, it turns out), taking the log of that, and then multiplying the resulting quantity by 20. Like this:

“Lp” is just shorthand for Level – Pressure, which is equivalent to SPL.

This is where people tend to start getting glassy looks in their eyes when they’re trying to wrap their heads around acoustics in one sitting. But it’s very, very important, so it’s worth the effort to try to understand it.

Sound is a vibration in air. Vibrations happen in frequencies. Frequency describes how many times per unit of time a vibrating object goes back and forth. The most common unit used for frequencies is Hertz. 1 Hertz just means “one time per second.” So if you have a sound with a frequency of 100 Hz, the air pressure is changing 100 times every second.

Sound is very rarely only a single frequency. “A sound” is usually vibrating at a whole bunch of frequencies. How much of each frequency determines what something “sounds like.” Sounds with lots of low frequencies are low and rumbly. Sounds with lots of high frequencies are hissy.

Each of the frequencies of a sound is completely independent. So, in a sense, a sound that you hear is really many individual sounds put together. When acoustical engineers do their work, they typically divide a sound up into groups of frequencies called bands. The most common bands are octave and 1/3-octave. You’ve probably seen bands like these on your stereo’s equalizer. An equalizer is a device that adjusts the volume of each frequency band independently.

While treating frequencies individually is the most accurate way of talking about acoustics, it’s not very convenient. This brings us to our next term…

A-weighted Decibels (dBA)
When people who are discussing community noise say “decibels” or “dB,” chances are what they really mean is dBA. A-weighted decibels is a convenient way of adding up all the frequencies in a sound into a single number. The vast majority of noise ordinances and enivronmental acoustical criteria use dBA.

The way dBA is calculated is by first weighting individual frequency bands based on their relative audibility to humans. Then each of the weighted bands are added up into a single number which approximately describes the total loudness of that sound to people.

dBA places high importance on higher frequencies and low emphasis on lower frequencies. It is possible to make significant changes to the low frequency content of a sound without having much, if any, effect on the overall dBA level.

There are plenty of worthy guides on the internet that can teach you the basics of acoustics that probably do a better job than this post. If you have a very real interest in acoustics and noise, I recommend spending some time acquainting yourself with the concepts of sound and its measurement. If anyone reading this has any favorite websites, please let me know and I’ll link to them.

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