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Originally Posted by Rob
I'd like to ask you for clarification on few points.
How do you calculate the reduction in dB level over distance? Also, I would think that some consideration is made for the 'size' of the volume source. I read that a telephone dial tone is 80 dB. I can believe that, when it's up against one's ear, it's certainly loud enough. But move that telephone receiver 5 feet away, and one would barely be able to hear it. On the other hand, 5 feet would do nearly nothing to dissipate the volume of say, a power saw or a boat engine.
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Of the states with noise laws, there are several different standards, but they fall into a few specific groups. After people have a chance to look at the link, I would be interested to hear what people think of New Hampshire’s dB level and how it compares to other states. In other words, is the law acceptable as it’s written and is enforcement the problem, is it neither, or both? I should acknowledge that Mee-n-Mac previously brought this up in an attempt to get the thread back on track, but I thought I would restate it here.
Well, I suppose that's enough for now. I hope no one fell asleep reading this  .
Rob |
Let me step in and see if I can help. Most noise measurements I'm aware of use the Sound Pressure Level (SPL) measured via decibels (dB) using the "A scale" (vs the B or C scales). This logarithmic
measurement tends to mimic the tonal (frequency) response of human hearing with some caveats I'll add in a bit. The math is that X dB SPL means that X = 20 x log(pressure/0.02 mPa). The 0.02 milli Pascals is a somewhat arbitrary reference level (idealized minimum detectable sound level) but the math explains the following. For every 6 dB increase or decrease the SPL doubles or halves and sound pressure (with another caveat) follows the usual inverse square law, halving for each doubling of distance. Said another way if some noise measures 60 dB SPL at 100 ft then at 200 ft it's 54 dB and at 400 ft it's 48 dB. The caveat is that this doesn't account for reflections or refraction of the sound waves. The water surface will reflect (depends hugely on the sea state/calmness) sound which will reinforce certain frequencies and cancel others depending on the distance. Add in echos (reflections) from other surfaces and a hard scientific way of accurately estimating SPL vs distance
over the lake eludes me. It may be better or worse that the simple inverse square law everyone likes to mention. I don't know of any good emperical methods to account for lake effects (there may be some rule of thumb, I just don't know it).
The first caveat I mentioned has to do with the A scale. As LRSLA mentions it tends to discount the lower (<500Hz) and higher (>10,000Hz) frequency sounds but it does so because human hearing does likewise, at least at lower SPLs. Human hearing is non-linear in it's response to increasing SPL. We poorly hear low frequency sounds at low levels but as they increase in level we tend to hear them disproportionately louder than we would a similar increase in mid-tones. At loud SPLs perhaps the flatter (frequency response-wise) C scale would be a better match. I'm not sure which should be applied at ... say ... an 80 dBa SPL.
Lastly I would emphasize the non-linear nature of human hearing once again. We don't "hear" a doubling of sound power or sound level that same way a microphone would measure it. I've always heard
that a 3 dB increase is just noticeable and a 10 dB is perceived as a doubling in loudness, 20 dB as a quadrupling. A low 60's dBA SPL is normal conversation, mowing the lawn produces mid 80's, a chainsaw something around 100-110 and a Foghat concert (from personal experience) at least 120
So getting back to the topic, I'll opine from my experience in Alton Bay there are a few boats that unlawful and more that are probably at the legal limit. The latter don't stop conversation but are noticable over the general din of the background level. These don't bother me but do annoy "Mee". As I said earlier this is a topic that will get many responses depending on what someone thinks is "too" loud. I brought my trusty RS sound meter up to the lake to get some objective data during bike week, perhaps next season (or this one when possible) I can collect some dockside data to better frame the situation. I don't think it's as bad as some say but I do agree it's a valid point to debate and there are some boats which are too loud even for me. While I can understand trying to wring the best possible performance from the boat, this has to be balanced against the desire of the rest of us to not be disturbed. Especially when 50 extra HP won't make that big a difference in top speed.