The unit used most often to measure and describe sound levels — the decibel (dB) — can be complicated and confusing for several reasons. It is a logarithmic scale rather than a linear one, and is often also “weighted” to highlight certain frequencies (see below). Accurate measurements require equipment that must be calibrated and used correctly.
For these and other reasons, some noise-reduction advocates recommend that noise-related public policies use perceived sound levels — i.e., the real-world experience of those (including enforcement officials) actually exposed to it — rather than a metric such as decibels. And in fact, Providence’s sound ordinance includes such a standard, which is “audible at 200 feet from the source.”
Logarithmic scale
As noted above, decibels are measured on a logarithmic rather than linear scale. A linear scale increases at a constant rate, so if sound-measurement “X” is twice as high as measurement Y on a linear scale, it would also be twice as loud. Graphs of linear measurements depict such increases with straight lines that rise steadily, making their relative volume levels easy to compare.
By contrast, logarithmic scale is exponential, which means as sound-measurement values increase, the rate of increase also gets higher — i.e., they increase at an increasing rate — which means a sound-measurement number that’s twice as high isn’t double the volume, but several times louder. When graphed, logarithmic functions use curved lines that get steeper as they increase, rather than rising steadily on a straight line from one value to the next.
Weighting
To further complicate matters, most noise-level measurements also use what are known as “weighted” decibels that (de-)emphasize parts of the sound spectrum, rather than the entire audible range. These include A-weighted decibels, less common C-weighting, and rarely used Z-weighting (which is actually an absence of weighting — i.e., “raw” or unweighted data).
Most modern public-policy applications such as noise regulations and enforcement measures use the “A-weighted” or dB(A) scale — the blue line on the graph below — which focuses on the middle and upper frequencies that comprise the human voice (not the human hearing range), and was originally devised for use in industrial settings such as factories and other workplaces.
The idea behind A-weighting was that if exposure to excessive noise while working damaged an employee’s hearing to the point that it was difficult for them to understand other people’s voices (e.g., co-workers, supervisors, etc), they would be arguably less employable and could sue the company for compensation for future lost wages. So for reasons of legal liability, it was important to measure noise in the range of human speech.
As a result, A-weighting deliberately excludes lower-frequency “bass” tones that actually comprise a disproportionate percentage of urban noise sources — such as mufflers, subwoofers, leafblowers, and other vibration-centric sounds — that travel farther and more easily penetrate buildings than higher-frequency sounds. Thus, A-weighting is uniquely ill-suited for measuring urban noise.
As you can see in the graph above, C-weighting is far more accurate in capturing the low-frequency sounds that A-weighting omits, and is closest to Z-weighting (i.e., no weighting). For that reason, the Noise Project uses C-weighting on its graphs as a more accurate depiction of the sound levels that Providence residents are actually exposed to on a recurrent basis every day and night.