What Is a Decibel?


What Is a Decibel?

Open an acoustical engineering report and flip to the glossary. Under “decibel” you might see something like this: “10 times the common logarithm of a ratio of two quantities linearly related to power.”

For many laypeople, definitions like the one above can muddy key takeaways in a report prepared by their friendly acoustical consultant. And yes, as friendly as your acoustical consultant may be, some of the language we use is anything but.

Most folks know that a decibel is a measure of sound, but there’s much more to it than that. This post outlines a few acoustical concepts that might help clarify some of the technical language we use in our field.

Describing Sound

Let’s start by understanding what sound is. For an acoustician, sound is fluctuations in pressure, particle displacement, or particle velocity propagated in any medium, or the auditory sensation that may be produced by it.

For everyone else, sound is something we hear. Sound travels through the air or through objects and, when it reaches our ears, we hear it.

We generally describe sound in frequency and magnitude, which is analogous to pitch and loudness. A sound’s magnitude decreases with distance. From far away, the rumble of a large diesel engine is quieter than it would be if you were standing next to it. Your ears can attest to this.

We quantify frequency in hertz (Hz). Low-frequency sounds (20 to 500 Hz) rumble (like that diesel). High-frequency sounds (1,000 to 20,000 Hz) whistle or hiss. Our ears are most sensitive to mid-frequency sounds (500 to 4,000 Hz), so we use frequency-weighted metrics to account for this.

Sound Power Level and Sound Pressure Level

When it comes to magnitude, we can understand it two ways: sound power level (Lw) and sound pressure level (Lp).

Sound power level is a fixed theoretical value. It’s the sound energy being emitted by the source. Whether that diesel is running next to you or across the field, its sound power level is the same. But the sound power level doesn’t tell us how loud the diesel is in the real world. That’s because sound is affected by physical objects and distance.

That’s where sound pressure level comes in. Pressure is what excites our ear drums. And it changes with location and the acoustical environment. It’s the energy that we measure with our sound level meters.

Remember our technical definition of sound: “fluctuations in pressure, particle displacement, or particle velocity”? Those fluctuations—or sound waves—dissipate with distance. They are also affected by the acoustical environment. In indoor locations with smooth, hard surfaces like concrete or bare walls, sound waves lose little energy as they reflect around. This can result in an increase in magnitude. Therefore sound pressure levels depend on where the measurement is taken.

Both sound pressure level and sound power level are normally expressed in decibels (dB), which is a logarithmic ratio. Sound power is measured in Watts—hence the abbreviation Lw. Sound power level is measured in decibels referenced to 10-12 Watts. Sound pressure is measured in pascals; sound pressure level is measured in decibels referenced to the hearing of a healthy human.

What Is a Decibel?

A decibel represents what we hear. It’s a number on a scale that comprises the sound pressure levels within the range of human hearing. Basically, the quietest sound we can hear is 0 dBA and pain starts around 120 dBA.

The decibel scale is logarithmic, which means it compresses a wide range of values into a manageable set of numbers. Without the decibel scale, we would be using a lot more numbers. That’s because 0 dB represents 20 micropascals (µPa) and 120 dBA represents 20,000,000 µPa.

Each proportional change on the decibel scale has equal value. If a sound source is doubled, the level increases by 3 dBA, which is noticeable, but just barely.

Moreover, an increase of 10 dBA sounds twice as loud and a 20 dBA change sounds four times louder. If typical human conversations occur at 60 dBA, a lawn mower at 90 dBA could seem six times louder. At the other end of the scale, if that lawn mower noise changed by 1 or 2 dBA,  the change would be essentially inaudible.

Common Noise Levels and Typical Reactions

noise thermometer decibel

Frequency-Weighted Metrics

You might have noticed that our examples above use dBA. Since decibels quantify sound pressure levels referenced to the thresholds of human hearing, it makes sense to assess sound levels in the frequencies we can actually hear. That is, unless someone needs us to determine the effects of rail squeal on their Bichon Frise.

Sound level meters are equipped with filters that alter the instrument’s sensitivity. The most common of these is an A-weighting filter, which de-emphasizes sound pressure levels at low frequencies and, to a lesser extent, high frequencies to simulate the frequency response of human hearing. Because we are most sensitive to mid-frequency sounds, A-weighted sound levels or dBA better represents human hearing.

BKL and Our Clients: All on the Same Page

As consulting engineers, we often rely on technical language to communicate complicated ideas and processes. It’s simply more precise than plain language.

We also realize that we are specialists in a small field, and while using technical language is fun for us, and an efficient way to communicate with other acousticians, it can sometimes obfuscate important messages for others.

At BKL, we think about how we communicate with clients. We make an effort to ensure our deliverables are clear and easy to read while also containing all the relevant technical details. By sharing our knowledge using language that’s as friendly as we are, we can be confident that we’re all on the same page.

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