October 18, 2008


There seems to be a fair bit of confusion regarding the new city noise ordinance.  What is a decibel?  How is it measured?  How does 87 dB compare to other sounds and noises in every day life.  What are reasonable standards?  No group of people seems to be more confused than the poor Mayor and City Council of Myrtle Beach.  Politicians really should leave science to scientists.  In their headlong rush to get rid of the bike rallies, the council may well have outlawed lawn mowers and blenders, not to mention their own city vehicles.  But, we are jumping the gun a bit here, so let's start with the basics.  We'll start off with some science content and then put it into perspective for you.


The first thing to know is that the decibel intensity scale is logarithmic.  The decibel is defined as one tenth of a bel, where one bel is the difference in level between two intensities I1 and I0 such that one is 10 times the other.  When it comes to sound, I0 is defined as the bare minimum sound that the human ear can perceive.  So, we have the following equation:


                                      Intensity Level = 10 LOG10(I1/I0) (dB)


There are three things that you need to know from this relationship:


1. An increase of 10 dB is a factor of 10 times increase.  So, 20 dB is 10 times higher intensity than 10 dB, and 30 dB is 100 times higher intensity than 10 dB.

2. An increase of 10 dB is perceived by the human ear as twice as loud.  So, by our last example, 20 dB is twice is loud as 10 dB, and 30 dB is 4 times as loud as 10 dB.

3. When adding together two separate sound sources of equal level, the combined level is 3 dB more than either of the individual sources.  So, for example, two motorcycles each at 88 dB combine to a level of 91 dB.  Four such motorcycles combine to a level of 94 dB, and so forth.


Now that you have the basics, you are already way smarter than the Myrtle Beach City Council, as we will demonstrate.  The first question is "What is the sound level of some typical noise sources in the environment?"  Let's take a look at the following table of typical average sound levels.

There are a couple of noteworthy observations from this chart that are immediately apparent.  First, a number of common sound sources are significantly higher in sound intensity than the average motorcycle at 30 feet.  In fact, when we look at this chart you might be led to suspect that in setting a noise standard of 87 dB, the council may have been making a ham-handed attempt to pretty much eliminate all motorcycles.  And, it is even worse than this surface appearance since another thing you need to know about sound intensity is that it falls off as the square of the distance.  That is, double the distance and you get 1/4th the level.  Double it again, and you get 1/16th the level, and so forth.  This relationship will be key when we consider federal noise standards and how exactly sound level is supposed to be measured in the next section.  For the time being, we will just stick with this chart and determine just how many motorcycles at 88 dB from 30 feet away equal one ordinary power mower at 3 feet.  By the way, the average stride length for men is 2.5 feet, so 30 feet is a mammoth 12 steps away.


Remember that 2 sources of equal sound level add up together to be 3 dB higher than either one separately.  How many motorcycles would add up to a single lawn mower using this relationship?  We can get our answer from the following chart:

hells bells it's decibels!

Yes, you are reading that right.  64 motorcycles, each at 88 dB sound level from 30 feet away, are approximately the same as a single lawn mower at 3 feet away, and 16 such bikes are the same as a single diesel truck measured at the same distance of 30 feet.  We could make all sorts of similar comparisons, but you get the point.  Your neighbor's lawnmower in the yard right next to you is a bigger threat to your sonic tranquility than hundreds of bikes a block away.  We know the mayor and the city council may be a bit challenged with numbers since they seem to think (and said exactly this in their letter of a couple weeks ago) that chasing away 500,000 tourists is actually good for the local economy.  So, to make this relationship clear in a way that even they can understand, you can consider the chart below:


Now here is where things get really interesting.  The federal EPA noise standards for motorcycles are found in 40CFR205 sub parts D, E, and Appendix I-1.  You will find that the noise level limit for motorcycles is either 80 or 83 dB depending on the model year.  On the surface this might lead you to believe that the council has been generous with an 87 dB standard, but hold on.  Appendix I specifies the procedure for actually measuring that noise level, and while it is long and complex as most federal standards are, the key is that the level is to be measured at 49.2 feet with the bike under acceleration and deceleration.  That, of course, is NOT the kind of measurement or standard that the city council is attempting to enforce.  Their standard is for a stationary bike exhaust level.  The measurement standard that applies to this condition is SAE J1287.  In this case, the noise level is measured at a distance of 20 inches from the exhaust pipe at an angle of 45 degrees off to the side.  So, can we relate an EPA standard at 49.2 feet to what the equivalent level would be at a distance of 20 inches?  Sure.  Recall that sound level drops off as the square of the distance away from the source.  Using this relationship, we can easily calculate what the equivalent level at 20 inches is compared to 83 dB at 49.2 feet.


49.2 feet is 29.5 times farther away than 20 inches.  1/(29.5^2) = 0.0011.  Loosely speaking, this is one one-thousandth the sound intensity, or considering it the other way, the sound intensity at 20 inches would be 1000 times that measured at 49.2 feet.  Recall that a 10 dB increase is 10X the sound level, 20 dB is 100X, and 30 dB is 1000X.  So, a roughly equivalent noise standard to the EPA standard of 80 to 83 dB at 49.2 feet is 110 to 113 dB measured at 20 inches.  Even if we allow that the stationary test does not fully probe the maximum sound level of the bike, we still don't get anywhere near as low as 87 dB.  When you consider these numbers, you can appreciate just how completely absurd the 87 dB ordinance is, and we are assuming that the MBPD are following the recommended measurement procedure (SAE J1287) in the first place.  While we are considering that measurement standard, there are a few important items in there that can dramatically affect the results, and which we wonder if they are being adhered to.  For example:


1. There can be no nearby reflecting surfaces such as billboards, signs, parked cars, etc, within 16 feet of the measurement.

2. The ambient background noise from other sources must be at least 10 dB less than the bike being measured.

3. During the test, no more than one person (other than the rider) can be within 10 feet of the bike being tested.

4. Wind speed must be less than 20 mph and must be measured by an anemometer as part of the test.

5. The measurement meter must be calibrated on-site with special equipment for this purpose at least once every hour of operation.


We can speculate if all of these procedures and conditions were adhere to at the random roadblocks the city setup, or if they are even following this standard at all, but given the history of this whole thing we would not count on it.


The bottom line on all this is:


1. Neither the Mayor nor the City Council appear to have the slightest idea what sound is, how it is measured and perceived by the human ear, or how proper noise ordinances should be developed and applied.

2. They do not seem to recognize that their stationary close-up standard is ridiculously restrictive compared to Federal EPA standards and if applied equally to other vehicles besides bikes will end up citing almost anything that moves or mows (or in the case of blenders, anything that makes mixed drinks).

3. Those who are using "noise" as a means to try and eliminate the bike rallies may want to rethink their strategy and come up with a better fake rationale lest they have to hire sheep to replace their lawnmowers.

4. As we said at the beginning, politicians need to leave science to scientists.  After all, if they were any good at it, that's what they would be doing instead of buying votes with other people's money.


But, maybe that's just me.