No. 2 in a series on Myths, Misunderstandings, and Magical Thinking about Noise
It’s not uncommon for the “decibel” or “dB” to be discussed as if it’s a physical unit with a fixed meaning, like a kilogram. This is not correct. In truth it is an artificial shorthand for expressing energy ratios using a tidy textbook equation. In reality, measuring or interpreting actual sounds using decibels can actually be quite complex. Without the proper training, it can be downright confusing.
The misuse of decibels in a noise control context results in regulations that are unenforceable, specifications that yield no benefit, and measurement results that are all over the map. Each failure causes folks to be less hopeful about noise control, even if no noise control engineer was actually involved. In this way, decibel abuse leads to a noisier world, or at least to one that doesn’t get quieter anytime soon.
Noise control engineers are trained to navigate the alphabet soup of parameters, sift data that has been provided by others, and make measurements that are meaningful. The sooner noise control engineers are involved, the sooner the world starts getting quieter.
Decibels are used in acoustic measurements in a variety of ways, to describe both
- sound (power, intensity, and pressure) and
- vibration (acceleration, velocity, and displacement).
In particular, decibels express ratios of sound energy:
- here vs there,
- before vs after,
- incident vs transmitted,
- measured vs just audible or feelable, etc.
In order for measured decibel values to be meaningful, the form of energy and the components of the ratio must be explicit.
Human perception and noise control effectiveness vary strongly with frequency. So the energy in particular frequency ranges (bands) is often of interest. At other times it’s convenient to evaluate the overall energy in a sound, usually applying an appropriate frequency-weighting. The most common are
In order for measured decibel values to be meaningful, the frequency range and frequency weighting must be explicit.
Finally, most sounds fluctuate in both amplitude and spectral content due to Impulses (e.g., firearms, snare drum), rapid variations (e.g., music, speech), and slower variations (e.g., vehicle passby, moving leafblower, change in wind direction) . Sound level meters use time constants like
- SLOW (for human perception)
- FAST (for human perception)
- PEAK (for blast damage to ears or structures)
Averaging or statistics are often used to highlight particular characteristics of the sound, such as
- LEQ (equivalent sound level over a period of time)
- LNN (statistical sound levels, such as L50 median)
- LMAX and LMIN
And of course the distance from the source, location in an interior volume, or weather conditions outdoors, affect measured sound levels.
In order for measured decibel values to be meaningful, the time constants, details of averaging and statistics, and location and conditions must be explicit.
If all this information is confusing or overwhelming to you, congratulations, you are perfectly normal. You just need help sorting it out, and that’s part of what noise control engineers do every day.
Copyright 2020 Nelson Acoustics
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