A very important concept that’s usually missing from people’s understanding of measuring sound is the necessity to include the distance from the source that the measurement was taken. There is no single sound pressure level (SPL, what people usually mean when they say “decibels” or “decibel level”) for any given sound source; SPL varies considerably with distance.

In this video about the shut-down of Shady Grove, you can see David Scott taking measurements of various sound sources in an effort to put “70 decibels” into perspective.

Without holding his sound level meter in a meaningful location, his measurements are just about worthless. “Our truck exhaust pushed near 80” he says, while holding the meter 3 feet from the tail pipe. If he were to move away another 3 feet from that tailpipe, his sound level meter would read 74 dBA. 6 feet more and it would read 68 dBA. (In general, a doubling of distance from the source means a decrease of 6 dB).

This is why the Austin noise ordinance needs to be changed to be based on sound levels at the receiver, not at the source. Limiting a restaurant with live music to 70 dBA “at the property line” does not take into account the distance from the stage to the nearest sound sensitive receptor, nor does it take into account any noise barriers, such as hills or other buildings, that would help to decrease sound where it is not wanted.

An outside music venue located thousands of feet away from the nearest residence should be able to have much louder music than a venue located across the street from an apartment building. Currently, the ordinance makes no distinction between the two.

An outside music venue with an enclosed stage and sound walls protecting the nearest neighbors should be able to have louder music than a venue with an open stage. Again, the noise ordinance as it stands makes no distinction.

To illustrate the importance of the position of the receiver relative to the source, I’ve used Shady Grove as an example and done some very simple calculations to show how sound pressure levels driven by the same source can be very different depending on distance. The only factor I’ve taken into account here is distance; I’ve neglected any ground effects, atmospheric effects, shielding from buildings, background noise, or diffraction from trees. Including additional effects in the calculations would cause the SPLs to vary even more.

At Shady Grove, the stage is only about 14 feet from the property line. Even though the property line is next to a busy street and the nearest residences are some 150 feet away, sound levels at the stage must be kept low in order to maintain the limit at a point only 14 feet away.

But, by simply moving the stage away from the property line an additional 45 feet (2 doublings of distance), Shady Grove would suddenly be able to turn the music up by 12 dB while still maintaining the property line limits. As a result, the SPL at Meghan Lane would be 9 dB higher.

If the ordinance were receiver based rather than source based, outdoor venues would have a measurable incentive to design their facility with the goal of keeping SPLs at nearby properties within the applicable limits. Sound barriers and speaker directionality could be used to the maximum benefit of the venue and the surrounding community.

Another consideration is that, under the current system, someone can call in a noise complaint from anywhere, regardless of what the noise level at their home is. Since the limits are set only for the property line of the source, the distance between the source and the receiver is irrelevant, as is the actual measurable SPL at the location of the complainant. This is not a fair arrangement.