CHAPTER 31B. NOISE CONTROL

B. Decibel-A-Weighted (dBA). The sound level, in decibels, measured with a sound level meter using the A-weighting network or scale as specified in ANSI S1.4-1983 (Specification for Sound Level Meters).

C. External Calibration - The testing under controlled conditions of the sound level monitoring system, by use of a calibration device, whose sound level output and frequency is accurately known, for the purpose of ascertaining the accuracy of the system within the meaning of Section 31B-7(d) of the Code. (The system includes a microphone, microphone cable, sound level meter and its internal components.)

D. Field Crest Factor - A field crest factor, for the purposes of making a determination of impulsive noise, must be accomplished using an impulse precision sound level meter and will be calculated using:

Field Crest Factor - (Max IP (dBA)) - (Max SR (dBA))

Where Max IP = Maximum impulse peak sound level

Max SR = Maximum slow response sound level

E. Impulsive Noise.

(1) Noise characterized by short bursts of acoustical energy, having:

(a) A rapid rise to a maximum pressure followed by a somewhat slower decay;

(b) A duration not greater than 1 second; and

(2) Examples of impulsive noise sources are weapons fire, pile drivers, and some rock drills.

F. Intermittent Noise - A noise which goes on and off but which is steady while it is on.

G. Large Reflecting Surface - Surfaces which may cause an interference with the noise measurements being taken (e.g., wal1s, floors, etc.)

H. Noise. Any steady-state or impulsive sound occurring on either a continuous or intermittent basis that annoys or disturbs humans or that causes or tends to cause an adverse psychological or physiological effect on humans.

I. Noise disturbance. Any sound that is:

(1) Unpleasant, annoying, offensive, loud, or obnoxious;

(2) Intermittent or continuous;

(3) Abnormal for the time of day or location where it is produced;

(4) Produced or correctable by human agency; and

(5) Prejudicial to the reasonable enjoyment of health, comfort, or safety of any individual or causes injury or damage to persons, property, or the conduct of business.

J. Periodic noise. Noise characterized by repetitive fluctuations in level of 10 dBA or more that occurs at a repetition rate between 0.5 repetitions per minute and 2.0 repetitions per second.

K. Qualification - A laboratory type calibration including a series of tests using the full frequency sweep capability of microphone plus meter, plus A-weighting network demonstrating compliance with ANSI specification S1.4. This is not a required calibration within the meaning of Section 31B-7 of the Noise Control Ordinance which discusses the field calibration of the sound level meter.

L. Qualified - Calibrated by an authorized acoustical standards laboratory or an authorized factory representative.

M. Quasi-Steady Noise - A rapid series of impulses that have the same effect on the meter as a steady noise.

N. Rapidly Fluctuating - Continuous noise whose level varies rapidly over a range greater than 3 dBA on the fast scale during the course of the measurements.

O. Steady Noise - Noise whose level varies less than 3 dBA read on the slow scale during the course of the measurements.

P. Sound Level. The level observed on an approved sound level meter, as specified in Section 31B-7 of the Code.

Q. Source. Any activity, occupation, business or operation conducted on land or water or in or upon a building or other structure, including streets and thoroughfares.

R. Source of Violation - Consists of a single source, or of several distinct sources if all are being operated on the property of a single owner.

S. Tonal noise. Any sound that is composed of one or more pure tones. A pure tone means any sound that can be distinctly heard as a single pitch. For the purposes of this Regulation, a pure tone exists if the one-third octave band sound pressure level in the band with the tone exceeds the arithmetic average of the sound pressure levels of the two contiguous one-third octave bands by 5 dB for center frequencies of 500 hertz and above and by 8 dB for center frequencies between 160 and 500 hertz and by 15 dB for center frequencies less than or equal to 125 hertz.

T. Violator. Any person actually causing, operating, responsible for or otherwise having direct or indirect control over any use which is proved to violate any provision, standard, regulation or rule of this Regulation.

Note: Definitions B, D, E, H, I, J, P, Q, S, and T are quoted from the ordinance for ready reference here.

31B.00.01.02 Measurements

The intent of the following measurement section is to specify straightforward procedures for measuring sound levels for compliance with the Montgomery County Noise Control Ordinance, applicable to the large majority of expected situations, and to specify the limits of applicability of these procedures, i.e., situations in which acoustical specialists and/or specialized equipment may be required to supplement these procedures.

The following procedures shall be reviewed periodically with reference to changes in the practice of noise measurement, instrumentation, County Code, and legal precedent.

A. Equipment

The following equipment shall be used:

(1) Sound-level meter and microphone meeting Type II specifications per the current ANSI S1.4. For measurements of tonal noise and impulsive noise, the meter and microphone shall meet Type I specifications. The meter shall be qualified annually with equipment traceable to the Bureau of Standards.

(2) External calibration device with a known sound level output at a single frequency between 200 Hz and 1000 Hz.

(3) Windscreen with insertion loss less than 0.5 dB below 8 kHz.

(4) Stopwatch capable of cumulative operation and accurate to 2% or better.

(5) Wind speed indicator with at least 20% accuracy at 12 miles per hour.

(6) Earphones with impedance conforming to the sound level meter manufacturer's specifications.

B. Sound Level Meter Calibration

(1) The sound level meter shall be externally calibrated immediately before a series of measurements at each test site and if necessary, adjusted to read within 0.2 dBA of the level specified by the calibration unit. Measurements may be assumed to be valid if calibration checks before and after a series of measurements agree to within 0.5 dBA, otherwise these measurements shall not be used. Such calibration checks shall also be made immediately after the series of measurements and at least once per hour during the measurement series to ensure continuing accuracy of the equipment.

(2) A battery check shall be performed immediately before and after a series of measurements at each test site. If either battery check is unsatisfactory, the measurements shall not be used for enforcement purposes.

C. Equipment Settings

(1) Steady (including Intermittent, Slowly Fluctuating or Quasi-Steady) Noise.

a. A windscreen shall be used for all outdoor measurements.

b. The "A" scale shall be used on the sound level meter.

c. The attenuator shall be set so that maximum levels fall between the zero mark and full scale mark on the meter scale. (Wait at least five (5) seconds after changing the attenuator to take measurements.)

d. The slow meter setting shall be used on the sound level meter.

(2) Tonal Noise.

a. A Type I sound level meter shall be used.

b. Measurements shall be made in one-third octave bands.

c. A wind screen shall be used for all outdoor measurements.

d. 'The flat or linear scale shall be used on the sound level meter.

(3) Impulsive Noise.

a. Two Type I sound level meters shall be used.

b. A wind screen shall be used on both meters for outdoor measurements.

c. The impulse hold capability shall be used for measuring the peak sound level.

d. The A scale and slow meter setting shall be used to measure the slow response.

e. Two persons, one using the impulse hold and one using the slow response, will take measurements simultaneously.

D. Measurements Locations

(1) Measurements shall be taken at any location known to be on the property line of the alleged violator or outside the property of the alleged violator. Any doubt regarding the actual location of the property line must be resolved in favor of the operator of the source being investigated. (One exception is where the law specifies otherwise - e.g., at least 50 feet from construction equipment.)

(2) The microphone shall be at least 3 feet above the ground while measurements are being taken.

(3) The microphone orientation shall be as specified by the manufacturer.

(4) No measurement shall be made closer than 3 feet to any large reflecting surface.

(5) Measurements conducted on private property must be done with permission of the owner or his representative, or under the authority of a search warrant or a court order.

(6) Outdoor measurements shall not be conducted in the presence of wind speeds greater than 12 miles per hour, nor in the presence of precipitation or fog.

(7) When measurements are made at the boundary of the property of an alleged violator, they shall be made at a minimum of two locations. The observer should walk the site boundary and note the trend of sound levels. The locations should be selected to meet one or more of the following criteria:

a. Local maximum; the location where the highest A-weighted sound level is measured.

b. Local minimum; the location where the lowest A-weighted sound level is measured.

c. Sensitive locations; considering sound sources and receivers, including upper floors of nearby structures.

d. Locations nearest to habitations.

E. Measurement Procedures

(1) General

a. Unless impracticable, earphones should be used while noise measurements are being made to ensure that the meter is responding properly to the noise source being monitored.

b. The sound level meter shall be held in front of the observer. The observer shall be oriented with respect to the principal sound source so that the sound energy arrives at the microphone at an orientation specified by the equipment manufacturer.

c. Survey measurements should initially be taken using a Type II sound-level meter, unless there has been a complaint or other reason to believe that a Tonal Noise or Impulsive Noise is present. If the operator believes, after the initial survey measurements, that a Tonal Noise or an Impulsive Noise exists, further measurements should be made using a Type I sound-level meter.

(2) Steady (including Intermittent, Slowly Fluctuating or Quasi-Steady) Noise.

a. A measurement of the ambient noise level shall be required. If the ambient noise level is fluctuating, the procedure in 31B.00.01.03E.(2)f shall be used. If the ambient is within 10 dBA of the source sound level, apply the correction found in the following table to determine the noise due solely to the source being investigated.

CORRECTION FOR MEASURED
AMBIENT SOUND PRESSURE LEVELS (in dBA)

Difference (in Decibels Between Correction (in Decibels) to be

Sound Pressure Level Measured Subtracted from Sound

with Sound Source Operating and Pressure Level Measured with

Ambient South Pressure Level Sound Source Operating to

Alone Obtain Sound Pressure Level

Due to Sound Source Alone

less than 4 See 31B.00.01.02H(2)

4 2.2

5 1.7

6 1.3

7 1.0

8 0.8

9 0.6

10 0.4

more than 10 0

b. Where extraneous noise, lack of space, or inaccessibility of a desired measuring location makes measurement of a specific source difficult, and the property owner agrees or where otherwise authorized by law, the measurement may be taken at a point on a line between the desired location and the nearest edge of the source. When this procedure is followed, a correction for spreading loss must be subtracted from the readings.

For a point source, this correction in dBA is 20 log (Dd/Da), where Dd is the desired distance and Da is the actual distance from the nearest edge of the source to the point of measurement. The result will be a conservative (i.e., possibly low) value for the level at the desired location. Actual readings as well as corrected results must be reported.

c. In the presence of an apparent pure tone, the microphone shall be moved continuously along an arc at least 3 feet long. Record the maximum and minimum readings. If the minimum is within 3 dBA of the maximum, average the two, if not, use the maximum minus 3 dBA as the basis for further action. If the reading is within 5 dBA of the allowable limit, make further measurements as in 31B.00.01.02.E.4 to determine whether a tonal noise exists.

d. Before citing a violation of a noise limit which may not be exceeded regardless of duration, allow a 2.5 dBA grace to account for all possible inaccuracies. It is recommended that several measurements showing such violations be taken with at least 20 seconds between successive measurements.

e. When the fluctuations of the indicating pointer on the sound level meter are less than 3 dBA on the slow scale, the noise is considered to be steady and the level is taken to be the numerical average of the maximum and minimum levels during the period of observation.

f. If the level fluctuates constantly over a range greater than 3 dBA on the slow scale, the-noise is considered to be fluctuating. A set of measurements should be taken by reading the sound-level meter indications to the nearest decibel at approximately equal time intervals of between 5 seconds and 20 seconds.

The operator should avoid visually averaging the indications of the sound-level meter. The reading should be obtained by glancing at the indicator and noting the sound level at that instant. Recordings at the same interval should be continued until the number of observations is at least ten times the range of the readings in decibels. The range is the difference between the maximum and minimum levels.

g. If the noise is intermittent, the average level during the “on” period is the noise level of interest. When the level is steady for five seconds or more, the slow scale should be used; when the level is steady for one to five seconds, the fast scale should be used. For shorter bursts, the noise should be treated as impulsive, and the field crest factor should be measured as in 31B.00.01.02.E(3) below.

If the field crest factor is less than 10 dBA, a noise containing bursts shorter than one second shall be considered quasi-steady. If the level during the “on” period exceeds the maximum allowed level, then duration measurements should be taken in accordance with 31B.00.01.02F.

(3) Impulsive Noise.

a. A 5 dBA penalty for impulsive noise may be applied without additional instrumentation if the source of the noise is clearly impulsive.

b. Several distinct readings indicating a violation of a noise limit must be obtained before issuing a citation.

c. For purposes of making a determination of impulsive noise, the field crest factor must be measured. Using a sound level meter with impulse-hold capability, the maximum impulse peak A-weighted sound level must be measured. The maximum slow response A-weighted sound level must also be measured. Then the

Field crest factor - (Max IP (dBA)) - (Max SR (dBA))

Where

Max IP = Maximum impulse peak sound level

Max SR = Maximum slow response sound level

d. If the noise has a field crest factor of 10 dBA or greater, has a duration not greater than one second, and displays a rapid rise to a maximum level followed by a somewhat slower decay, then it is an impulsive noise.

(4) Tonal Noise.

a. A sound composed of one or more pure tones is a tonal noise. The maximum permissible sound levels established by Subsections 31B-5b and 31B-8a of the Montgomery County Code shall be reduced by 5 dBA for a tonal noise.

b. To establish the existence of a pure tone, one-third octave band sound pressure levels must be measured using a Type I sound level meter without weighting. Levels of the ambient noise and of the tone with the ambient must be measured, and corrections specified by 31B.00.01.02.E(2)a must be applied to determine the level of the tonal noise in each band.

Measurements must be made for a series of contiguous bands, including the band containing the tone and the two contiguous one-third octave bands. Measurements should be made in a sufficient number of bands to allow for the possibility that more than one pure tone exists.

c. A pure tone exists if the one-third octave band sound pressure level in the band with the tone exceeds the arithmetic average of the sound pressure levels of the two contiguous bands by 5 dBA for center frequencies of 500 hertz and above, by 8 dB for center frequencies between 160 and 500 hertz, or by 15 dB for center frequencies less than or equal to 125 hertz.

F. Duration Measurements

(1) The County ordinance specifies maximum levels which may not be exceeded even momentarily, and levels which may be exceeded for a certain period of time, i.e., 30 seconds per hour, 3 minutes per hour, and 12 minutes per hour.

(2) Measurement of the time-above levels may be made using an accumulating stopwatch and using the noise measurement techniques described in 31B.00.01.02C, D, and E. The operator shall start the stopwatch when the meter reading exceeds the level being monitored and stop it when the reading falls below that level.

(3) The level being monitored for duration measurements shall be the applicable one found in the Montgomery County Code, Chapter 31B, plus a 2.5 dBA grace to account for possible meter inaccuracy. If measurements are taken at a point between the property line and the source, (31B.00.01.02 E(2)b), then a correction for spreading loss must be added to the time-above level in the ordinance to insure a valid simulation of the time above at the property line.

(4) An additional grace on duration measurements shall be allowed to account for possible inaccuracy in stopwatch duration measurements. Durations of 60 seconds per hour, 4 minutes per hour, or 15 minutes per hour must be recorded before issuance of citations for 30 seconds, 3 minute, or 12 minute violations, respectively.

(5) This procedure may be applied to Steady (including Intermittent, Slowly Fluctuating, or Quasi-Steady) or Tonal Noise Noise. This procedure may not be applied to Impulsive or Rapidly Fluctuating or Periodic Noise.

G. Data and Reports Required

The operator shall provide all pertinent information on the noise data form for field measurements.

For each set of readings, the following information shall be required. Date, time, location, air temperature, operator’s name; wind speed and direction; all instrumentation models and identifying numbers; a dimensioned sketch of site, including the north direction, measurement points, noise sources, and buildings; measurement of noise peaks and steady levels to the nearest decibel; duration; description of special techniques employed; description of noise, noting especially if there is an apparent pure tone or impulsive content; meter damping used (fast or slow); measurement of ambient with source off, description of ambient if obvious (highway, dogs, etc.); if the ambient level is 4-10 dBA below the level found with the source operating, corrections are required to obtain the source noise level. These corrections must be shown.

H. Special Measurements

(1) The measurement of noise produced by motor vehicles in transit is prescribed by State and Federal Laws. Enforcement by police officers is provided for by State Law.

(2) Any situation in which the offending noise source cannot be demonstrated to increase the total noise at the measurement point by at least 4 dBA over the ambient level shall be considered unmeasurable by the techniques described in these procedures.

(3) Any situation in which readings exceed the statutory limits but are less than the statutory limit plus the grace limit must be measured with more specialized techniques and/or more accurate equipment prior to deciding whether the source is in compliance with the County Ordinance. In such case, consultation with qualified professional personnel shall be required.

(4) No sound level meter readings are necessary in order to establish the existence of a noise disturbance. However it must be shown that a sound has all five of the elements in the definition of a noise disturbance in order to be so established.

Disclaimer: This Code of Ordinances and/or any other documents that appear on this site may not reflect the most current legislation adopted by the Municipality. American Legal Publishing provides these documents for informational purposes only. These documents should not be relied upon as the definitive authority for local legislation. Additionally, the formatting and pagination of the posted documents varies from the formatting and pagination of the official copy. The official printed copy of a Code of Ordinances should be consulted prior to any action being taken. For further information regarding the official version of any of this Code of Ordinances or other documents posted on this site, please contact the Municipality directly or contact American Legal Publishing toll-free at 800-445-5588.

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