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this point is overlooked )ecattse the people involved are not
<br />aware of the situation, l- owever, an attorney who hires an
<br />acoustical expert may re' cai this discrepancy and get a
<br />client's violation dismissed on that basis.
<br /> Another hindrance to ~nforcement is that octave band
<br />analy-zers are expensive (~picall), more than $4,000 each). For
<br />this reason, man), municipalities do not even have the
<br />equipment required to
<br />enforce their own
<br />ordinances. In such
<br />instances, only high- Antiquated Octave
<br />visibilir)' cases are Band Frequency
<br />investigated because Designation (Hz)
<br />an acoustical
<br />consultant must be 0- 75-
<br />hired to perform the 75 - 150
<br /> 150 - 300
<br />measurements.
<br />Another limitation 300 - 600
<br />on the effectiveness of 600 - 1,200
<br /> 1,200 - 2,400
<br />these ordinances is the
<br /> . 2,400- 4,800
<br />setting of absolute above 4,800
<br />limits for noise levels ....... "
<br />
<br />These limits are
<br />typically in terms cfA-weighted SPLs (denoted in units of dBA,
<br />or A-weighted decibels). A.-weighting attempts to compensate
<br />for the human hearing m4chanism's differing sensitivities to
<br />different pitches. The dec!bel scale is logarithmic in nature,
<br />similar to the Richter scal~ used in earthquake analysis. Because
<br />of this mathematical basis~ a doublin~ of sound power equals an
<br />addition of three decibels ~o the soun~d level and a ] 0-fold
<br />increase tn sound power e~uals an addmon of 10 decibels. Table
<br />2 shows SPLs in terms of~ommon
<br />associated sources.
<br /> A common example of using absolute
<br />limits in noise ordinances ~is statin~ that
<br />no,se levels cannot exceedfX dBA ourmg
<br />the da.,,, and Y dBA at nigfit. For example,
<br />Houston's noise ordinanc{ (promulgated in
<br />January 1993) sets X at 6~and ¥ at 58 for
<br />residential properties. If _~e background
<br />noise level without the off~nding sound
<br />source operating were 45 ~BA, the
<br />ordinance states that it wC~Uld be acceptable
<br />for a noise source to raise fl~e backuround
<br />level by up to 20 dBA dur!ng the day and
<br />13 dBA at night. Table 3 thews typical
<br />human perception of soun!d level increases.
<br />The hypothetical increase~ discussed above
<br />would create a clear intrusion for any
<br />person with normal hearir~g but wot;Id not
<br />violate the ordinance.
<br /> Yet another common limiting criterion
<br />is the use of vague qualific4tions on noise,
<br />such as nuisance or disturbJ~nce, without any
<br />ouantificarion. We can int0ude in this category the commonly
<br />~ed qualification of a sou~. d being audibl~ These are purely
<br />subjective criteria and ofteh cause the dismissal of cases
<br />
<br /> TABLE 1
<br />
<br />Current Octave
<br />Band Frequency
<br />Rsnge (Hz)
<br />
<br />invoiving clear violations, depending more on the sav~, of the
<br />legal representation than on the case at hand.
<br />
<br />Effective Sections to Include
<br />The most important thing to remember when drafting a noise
<br />ordinance is that it will be enforced by people with little or no
<br />background in acoustics. ]t should therefore be as simple as
<br /> possible. A single number can be read
<br /> from a sound level meter having no or
<br /> few buttons. Such sound level meters
<br /> can fit in an officer's pocket. They are
<br /> Current Octave also the least expensive of all nrpes of
<br /> Band Frequency ,
<br /> Designation (Hz) meters, usual}), costing under $500 for
<br /> a complete set including a calibrator
<br /> and al} necessaD' associated equipment.
<br /> Be sure to note that all
<br /> monitoring equipment should
<br /> comply with ANSi standard S1.4-
<br /> 1983, or the latest version thereof.
<br /> This standard divides sound level
<br /> meters into categories called types,
<br /> labeled by the numbers 0, 1, or 2.
<br /> Type 2 meters have the minimum
<br />sensitivity required by the standard, and type 0 have the best
<br />sensitivity. Type 2 meters are usually good enough for the
<br />purposes of ordinance enforcement while type 0 meters are
<br />used for laboratory-precision measurements.
<br /> All an enforcing officer would have to check is that the
<br />sound }ex, el meter she is using complies with either g, pe 0, l, or
<br />2. This information should be printed directly on the sound
<br />level meter, but if not, it would be listed in the user's manual.
<br /> The reliability, of the
<br /> -.. monitored data
<br /> TABLE 2 cannot be assured
<br /> without this
<br /> Sound Pressure k~vel (dBA) information. Sound
<br /> level meters costing
<br /> tess than $200
<br /> probably do not meet
<br /> the tolerance of these
<br /> standards and are
<br /> therefore nor
<br /> recommended for any
<br /> enforcement use.
<br /> The most
<br /> practical criteria for
<br /> noise ordinance
<br /> compliance are
<br /> relative criteria in
<br /> which the difference
<br /> in sound level
<br /> between the cases
<br /> with and without the
<br /> sound source
<br />operating ' 4s limited. For clarification, a clear distinction
<br />must be made. bet~,een ambient and background sound
<br />levels. These' ierms' are used interchangeably in man),
<br />ordinances, but the), are quite different.
<br /> The ambient sound level includes all sound sources in an
<br />area. The background sound level is the level of sound
<br />monitored, including all sound sources exce?r the specific source
<br />in question that may be violating the ordinance. When using
<br />relative criteria, we are then assessing the difference between the
<br />
<br />· 45 - 8B 63
<br />88 -177 . 125
<br />177 - 354 250
<br />354 - 7O7 500
<br />707- 1,414 1,000
<br />1,414 - 2,828 2,000
<br />2,828 - 5,657 4,000
<br />5,657- 11,314 8,000
<br />
<br />Sound Environment
<br />
<br />i 'Threshold.of:hearing 0 -:? -,.
<br />
<br />: ..Broadcast~studio.imerior.: ':- · '.- .:lO . . -.. -~,.-..
<br />
<br />! '.Quiet:house interior .... ;:20 .. ':::. '=
<br />...... ' .... " ..... .." ':'-."5_i':":'-:;- - '. ..... ' .'
<br /> 'tau~et~'' omce.~ntenor.,-' ._ . -:..: _: :....:. :-: ,. :30 - . ..::,.....
<br />
<br /> ..~Ouiet:rural..area . -'Z 'ii- '-' '~ ".'. '.-- .40. .
<br />; '..C~Jie! ~uburbamarea ' "- "' -'7 ~:" ~:';:;:'(:750 '!-')
<br /> Office interior . .60
<br />
<br />Human voice at 1011. '"70
<br />;...Passing :ca[at~?0~ It....: ... =7; '.-. ,....:.:,-:; ',.:80.!: ....-.
<br />:';Passing bus orlmck.at .1011:~::,. :"-.'.:5'5:::}i':~:~ z:90 .(
<br /> _ %;:2 ;' '-' < 7 .' - _' ::%~,g¢~i'~;~,; · ,"7-L"':~' -:'-'.;7:T/ZU;'-'g~' ';k:;
<br />i Passlng:subwa. y,trmn:atT~O~tt.:~.:&:='!' :.-._-; :~.~ O0 :}Z"::.:.
<br />! ::. Night club With ;ban d .playing :;.~}:':' )'~'..:.:)i-:.'-: '. _/110: ~.!-i~.':':
<br /> %hreshold or,pain :Z' '-;;~(~,?.;(i.;_~;.:.;~;!:7~-). *i'--' 7'120 (;!...:. !'.
<br />
<br />J,,mes ?. Cowan is the rr~na~er of acoustical analysis for McCormick,
<br />Tailor & Axsocime~ in ?3ilal~lphia. He is a cerfifiM nobe-conrrg
<br />engineer ~nd th~ ~urhor of l'~andbook of Environmental Acoustic,
<br />just published b), Van Nosrra~d Reinhold.
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