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HomeMy WebLinkAboutOrd 1126A Noise ControlDATE g Li CONTINUED TO ORDINANCE NO. 112�, ay.o( PASSED TO 2ND READING AN ORDINANCE OF THE CITY COUNCIL"' OF THE CITY OF PALM DESERT, CALIFORNIA, MODIFYING SECTION 9.24.030 OF THE PALM DESERT MUNICIPAL CODE - NOISE CONTROL. The City Council of the City of Palm Desert, California, does hereby ordain as follows: Section 1: That Section 9.24.030 of the Code of the City of Palm Desert, California, be and the same is hereby amended to read aa'follows: "Section 9.24.030 Sound level limits. A. The following 10-minute average sound level limits, unless otherwise specifically indicated, shall apply as indicated in the follow table: Applicable 10-Minute Zone Time Average Decibel Limit Residential — All 7 a.m. to 10 p.m. zones 10 p.m. to 7 a.m. Commercial Zone 7 a.m. to 10 p.m. 10 p.m. to 7 a.m. 7 a.m. to 10 p.m. Manufacturing Industrial Agricultural Zone 10 p.m. to 7 a.m. 55 58 (A -Weighted) 55 45 65 55 70 Applicable 10-Minute Average Decibel Limit (C-Weighted) 58 48 68 58 73 B. If the measured ambient noise level exceeds the applicable limit as noted in the table in subsection (A) of this section, the allowable average sound level shall be the ambient noise level. The ambient noise level shall be measured when the alleged noise violation sources is not operating. C. The sound level limit between two zoning districts shall be measured at the higher allowable district." Section 2: That Section 9.24.110 of the Code of the City of Palm Desert, California, be and the same is hereby amended to read as follows: "Section 9.24.110 Noise level measurement. A. The location selected for measuring exterior noise levels between residential properties shall be at the property line of the affected residential property. Affected residential property shall be the address from which the complaint was received. Interior noise measurement shall be made within the affected residential unit. The measurement shall be made at a point at least four feet from the wall, ceiling or floor nearest the noise source. RM PUB\DERW IN\271863.3 ORDINANCE NO. 1126A The location selected for measuring exterior noise levels between non- residential properties shall be at the property line of the affected property. B. The location selected for measuring exterior noise levels between two zoning districts shall be at the boundary of the two districts." Section 3: The City Clerk shall certify to the passage and adoption of this ordinance and shall cause the same to be published once in the Desert Sun, a newspaper of general circulation, printed and published within the County of Riverside, and circulated within said City. PASSED, APPROVED, AND ADOPTED by the City Council of the City of Palm Desert, California, at its regular meeting held this day of , 2006, by the following vote, to wit: AYES: NOES: ABSENT: ABSTAIN: JIM FERGUSON, MAYOR ATTEST: RACHELLE D. KLASSEN, CITY CLERK CITY OF PALM DESERT, CALIFORNIA RMPUB\DERW IN1271863.3 2 9.24.030 Sound level limits. Page 1 of 1 Palm Desert Municipal Code Up Previous Next Main Search Print No Frames Title 9 PUBLIC PEACE MORALS AND WELFARE IV. OFFENSES AGAINST PUBLIC PEACE Chapter 9.24 NOISE CONTROL 9.24.030 Sound level limits. 1 CodeAlert: This item has been affected by 1125. Please refer to the CodeAlert Ordinance List for the most current provisions. A. The following one -hour average sound level limits, unless otherwise specifically indicated, shall apply as indicated in the following table: Applicable Limit One -Hour Average Sound Level Zone Time Decibels Residential —All zones 7 a.m. to 10 p.m. 55 10 p.m. to 7 a.m. 45 Commercial zone 7 a.m. to 10 p.m. 65 10 p.m. to 7 a.m. 55 Manufacturing Industrial 7 a.m. to 10 p.m. 70 Agricultural zone 10 p.m. to 7 a.m. 55 B. If the measured ambient noise level exceeds the applicable limit as noted in the table in subsection (A) of this section, the allowable average sound level shall be the ambient noise level. The ambient noise level shall be measured when the alleged noise violation source is not operating. C. The sound level limit between two zoning districts shall be measured at the higher allowable district. (Ord. 691 § 2, 1992: Ord. 647 § 1, 1991; Ord. 420 (part), 1985) http://gcode.us/codes/palmdesert/view.php?topic=9-iv-9_24-9_24_030&frames=on 8/14/2006 9.24.110 Noise level measurement. Page 1 of 1 Palm Desert Municipal Code Up Previous Next Main Search Print No Frames Title 9 PUBLIC PEACE, MORALS AND WELFARE IV. OFFENSES AGAINST PUBLIC PEACE Chapter 9.24 NOISE CONTROL 9.24.110 Noise level measurement. 11 CodeAlert: This item has been affected by 1125. Please refer to the CodeAlert Ordinance List for the most current provisions. The location selected for measuring exterior noise levels shall be at the property line of the affected residential property. Affected residential property shall be the address from which the complaint was received. Interior noise measurement shall be made within the affected residential unit. The measurement shall be made at a point at least four feet from the wall, ceiling or floor nearest the noise source. (Ord. 420 (part), 1985) http://gcode.us/codes/palmdesertiview.php?topic=9-iv-9_24-9_24_110&frames=on 8/14/2006 � b6/566 � \ � O GORDON BRICKEN & ASSOCIATES ACOUSTICAL and ENERGY ENGINEERS RECEIVEI� July a6, 2006 JUL 3 1 200� Development Service5 City of Palm Desert D E V E L O P M E N T C I T Y M O D I F I E D O F A N O I S E F 0 R T H E O R D I N A N C E D E S E R T Prepared by : _ �s � � � � �.', _-. i . , , - , � f,��-�/J y✓�vy'.. y.�_� � �- Gordon'�Bricken President /mrnb � O F P A L M Prepared for: MR. MARTIN ALVAREZ CITY OF PALM DESERT 73-310 Fred Waring Palm Desert, California 9226G 1621 East Seventeenth Street, Suite K Santa Ana, California 92705-8518 Phone (714) 835-0249 FAX (714) 835-1957 i � 0&/556 \ GORDON BRICKEN & ASSOCtATES ACOUSTICAL and ENERGY ENGINEERS S U M M A R Y This repor� addresses the issue of low frequency noise. The analysis addresses the current I�oise Ordir_a�ce, various th2ories of regulating noise, practical limits in noise measu�e- m�ent, r.oise nropagation issues, and the role of ambient noise. The study concludes that the least complicated method of regulating low frequency emissions in a manner Lhat is relativ�ly easy tc apply is to use the C-Weighted sound level. Sect�on 9.24.030 shoaid be modi�ied to read: D_ For noise sources consisting of music or music in combirLa�ion with other noise, tne C-Weig�ted levei after 1�:00 P.M. shall not be more than three (3) decibe�s higher than the allow2d A-�rleighted level over any ten (-�) minute period. 0 1621 East Sevenieenth Street, Suite K Sania Ana. California 92705-8518 � Phone (714) 835-0249 FAX (714) 835-1957 / � E 06/566 / \ � C� GORDON BRICKEN & ASSOCIATES ACOUSTICAL and ENERGY ENGINEERS 1.0 PURPOSE The purpose of this study is to develap a metr�od to account for low frequency noise in the Noise Ordinance. The issue has arisen as a result of several complaints about music from some commercial operations. Modern amplified music will o�ten contain strong low frequency energy accompanied by a periodic beat. These characteristics of music have become more prevalent with the introduction of equipment with extended low frequency performance capabilities and disco type musical formats. Also, rnany nightclub dance formats are based on the production of high level sounds as a part of the ambience of the facility and what is often called the "energy" of the venue. Naturally, the higher the source sound level, the higher the probability of a disturbance at nearby locat?ons. 2 . v BACitGROUND The present City IVoise Ordinance was developed based on the theory that community noise impact was related p-rimarily to the A-Weighted Sound Level, the duration of the noise event, the land use or zoning and Che time of day at which the noise occurred. The primary form is given in Table 1 on the following page. 1621 East Seventeenth Street, Suite K Santa Ana, California 92705-8518 � Phone (714) 835-0249 FAX (714) 835-1957 / % 06/566 ZONE TABLE i CITY NOISE ORDINANCE LIMITS�I� Resid2nt�a1 -- all zones Commercial zone Manufacturina Industrial Agricultural zone TIME OF DAY 7 A.P�[. to 10 P.M. 10 �.M. to 7 A.M. 7 A.M. to i0 P.M. 10 P.M. to 7 A.N. 7 A.M. to 10 P.M. 10 P.M. to 7 A.M. ON� HOUR AVER�GE SOUND LEV�L SO 45 65 55 70 55 (1} a. If the ambient exceeds the applicable noise limit above, the allowable noise limit shall be the ambient leve'. b. The sound limit at the boundary of two zoning districts will be the higher of the two zones. Tne average noise level measure was adopted to make it zasier for code enforcement officers to enforce with standard measurement equipment. Tne present Ordinance recognizes that objective measurement may not always define the extent of the noise impact. Thus, Sectior. 9.24.040 of the Ordinance lists cr.a�acteristics of the situatior. that might be used in evaluating whether a particular condition produces a noise impact. These conditions are as foliows: 1. 2. 3. 4. S. 6. 7. 8. 9. iv 1? Th� level of noise. Whether tre nature of the noise is usual or unusual. Whether the origin of the noise is ratural or ' unnatural. I'he level of background noise. The proximity of the roise tc slee�ing �acilities. The nature of the zonir_g of the areas in which the noise emanates. Tne density of the habitation of tne area in which the ::oise emanates. The time o� day or night the r.oise occurs. Tr:e duration o� the noise. Whethe�r the r.oise is recur�er_t or constant . Whether the noise is produced by a comr.lerci�i or non=commercial acti.vity. n 06/566 There are no guidelines on how to interpret these conditions, so the interpretation in each case is left to the assigned City staff for that case. This uncertainty is mitigated to some extent in Section 9.24.050 which states that a prima facie violation occurs when any instrument, television set, machine, loudspeaker or similar device is operated between the hours of 10:00 P.M, and 8:00 A.M. in such a manner as to be plainly audible froM �he property line. The present version of the Noise Ordinance was adopted ir� 1992. Prior to 1992, the Ordinance had the form given ir. Table 2 TABLE 2 PRE-1992 NOISE ORDINANCE LIMITS FOR RESIDENTIAL PROPERTY �1' ALLOWED LIMIT EXTERIOR INTERIOR ALLOWED DURATION DAY NIGHT DAY NIGHT SYNBOL 30 minuCes in hour 55 50 15 minutes in hour 60 55 5 minutes in hour 65 60 1 minute in hour 70 65 Anytime in hour 75 70 il} Day = 7:00 A.M. to 1G:00 P.M. Night = lO:OQ P.M. to 7:00 A.M. 55 45 60 50 65 55 L50 L25 L8 L2 Lmax The pre-1992 version had five different conditions that had to be met simultaneously in order to have compliance. The f�ve conditions were based on duration sensitive paramete-rs. Measure- ment required bcth a speciai inst-rument and a highly trained technician to interpret the measurement data, especially when caking the ambient level into account. The complexity of the practical application resulted in the present single measure type ' of Ordinance. In theory, the present Noise Ordinance offers the subjective tools by which to address disturbances produced by low frequency sound. However, it is always better to craft objective limits whenever possible. The objective ir. tris study is to reta�n t�e cor_cepts of an Ordinance that is enforceable without special equipmen� or complex training. 5 06/566 3.0 BASIC MEASUREMENT THE�RX Wi�r �he exception of pure tones,�a11 sound contains multip'e frequency componer_ts. This �s se�etimes referred to as the sound signature. For example, a passing car �enerates signi`icant sound pressure levels in a frequency range oF 250 Hertz to 8,0�0 H�rtz. Some forms of music genera�e sigr.ificant sou�d pressure levels i� the range oi 40 to 4,000 Hertz. Certain practices have arisen to classiiy the sound s�gnacure. Tne main methods are as follows: l. Subdivide the frequency spectrum. The conventional practice is to group the freauencies by ectaves or sub-octaves. The conventional groups are the octaves centered at 16, 3I.5, 63, 125, 250, 500, 1,000, 2,000, �,000, �,QOo and 16,000 nertz. Each band sums all the energy zn the upper and lower ends of the band. For example, the 125 Hertz octave band spans the frequency range 88 Hertz to 1;�7 Hertz for a rangQ of 89 Hertz. The higher the band center frequency, the wider the band limits. ror example, the 1,000 Hertz octave band ii�its are 710 to 1,42o Hertz for a range of 710 Hertz. The level in any octave or sub-cctave band is Cermed the "sound pressure ievei" and expressed in decibels. In octave formattir�g, any soun� is expressed by eleven (11; values witn each value presenting an octave band. 2. T'�e concept of combining rar.ges of frequencies can be extended to tre point that aII sound ene-rgy is combined into a sirgle number representina tre total sound eneYgy present. This single number is called �he "sound level" and is expz-essed in decibels. 3. It was recognized early in the development o� measurement instruments that tne sound entering the human ear is not quite wnat is perceived in the brain. This is because `he human ear's mechanism fi�ters the sound. The ear filter tenas to suppress the low and high zrequency contributions. However, the deg-ree o� suppression varies with tn2 total energy level of tne arriving sound. 4. It was aiso recognized early, that som? way was needed to classify sound �r a simple wa�•_ Therefore, the idea was a�opted trat favored measurement o� the total ene-rgy present so as to reduce the charac�erization of the sound to a single number representing the magnitude of the sound. 0 06/So6 5. In the early development of ineasurement, the instrument makers knew that they could not electronically simulate the time and level varying sophistication of the human ear. So, they arrived at a compromise by creating static filters that presented the human ear's frequency response under certain conditions. There were originally three filters. They were called the "A", "B" and "C" filters. The "C" iilter is almost the same as having no filter. Each of the filters represents the frequency response of the human ear for different magnitudes of sound. Over many years, the "A" filter has come to be the cne most closely identified with the response of the human ear, in part, to simplify the process of adoptir.g noise regulations. Today most sound level measuring equipment has both an "A" filter and a"C" filter. Measurements using the filters are termed C-Weighted or A-Weighted and given the symbols dBC or dBA. The practica•1 effect of the two filters can be seen on Exhibit l. This Exhibit is an octave band frequency plot of souncl pressure levels from SO to 16,000 Hertz for a typical music source at 100 feet from the source. The C-Weighteci sound level, the sum of the sound pressure level in 10 octave bar.ds is 99 dBC. The A-Weighted sound level is 90 dBA, or 9 dB lower. The difference lies in the fact that the A-Weighted curve suppresses the contributions of the octave band below 1,000 Hertz with significant discounting oi the sour.d pressure levels below 250 Hertz. 4.0 PROPAGATION OF SOUND Certain facts about sound propagation have some bearing on the approach to addressing the issue of low frequency sound. They are as follows: 1. The rate of decay of sound energy varies with distance from the sound source. Exhibit 2 shows the curve for the 125 Hertz and the 1,000 Hertz octave band sound pressure level_ At about S00 feet, the i,000 Hertz sound pressure ievel begins to decay raster tnan the 125 Hertz sound pressure level. Tris has the practical effect of mak�ng the low frequency contributio:� in the total sound more and more do;ninant. However, most City noise issues occur at distances under 1,000 feet, so the differenC decay rates are not especially an issue relative to the type of ineasuremer�t being taken in a particular study. /1 Oo'/566 2. The weather is known to produce some effects ir. the desert that are not aiways pr�sent elsewnere. Comparison of the decay rate for a combination of 70 deg�ees and 50 nercent relative humidity versus 105 degrees and ten percent relative humidity for A-4deighted and C-Weigh�ed measures are srowr. on Exhibits 3 and 4. There is little difference in the A-�deighted carves_ This is because there is very l�ttle low rrequency content sound level. The C-Weignted curves show differences starting around 2,000 Hertz. This is because the rate o� decay of the higher frequencies tend to be greater than the low frequencies as was pointed out on Exhibit 2. However, these differences are not especially important at tre distances that the City is likely to occur between the sound source and the receiver of sound. 3. The most significant weathe� factor affecting sound propagation is wind. There is very little difference in the effect for A-weighted and C-Weighted measures. In both cases, measure�ents snould be conducted when the sustained wind speeds are nc higher tnan five (5) miles per hour. 4. Sound barriers are any s�ructure or element t�at impedes sound propagation. In gen2ral, these wiil be physical structures since trees and other �andscaping do little to reduce sound. Exhib�t 4 shows the effect or, the decay rate of an eight foot (8') high sound barrier placed �0 feet from tne music source. The differences =� the A-Weighted a�d C-�veighted show up at distances over �O,COC feet. T'r�us, the d�fferences have litcle effect at the distances between tne source and receiver typical of City issues. 5_0 COMMUNITY NOISE ORDINANCES A noise is a complex combination of frequencies, 1e.�els, duratio� a�d rate of occurrence. Human reac�ion to noise is based on a wide variety of conditions includi�a distance from tne source, weather, time of day, �ersonal characteristics of the listener, expectatior_s of the listener, and che heal�h of the listene�. Commun�sies desiring to adopt some type of noise cor_trol m�asures must a�rive at regulations that t�e community be�ieves balance the interests of the listener, the needs of the commur.ity and `he interests of the producer of tne potentially of�ensi�re soand. r� 06; 566 Some typical approaches contained in Community Noise Ordinances are as follows: 1. General Nuisance Most communities have some version of a general nuisance provision in their municipal code. Tne language in its simplest form reads, "�o not make noise that bothers other people." Another variation of tnis general nuisance language is to declare certain noise sources as in violation on their face value (prima facie}. 2. Specific Sources Many communities have specific limits on the amount of noise that can be produced by a specific noise source. Frequently, the limit is expressed in language such as, "No person within X feet of the source shall be able to hear the sound." This form of the limit is used mainly to contro� noise from boom-boxes and auto sound systems. Specific limits will often have defined noise limits expressed in decibels measured at some distance. The limit is .usually in A-Weighted decibels. 3. Hours of Operation Many Ordinances attempt to control noise by limiting the hours of operation. The limit is usually in the form of, "The source (insert actual name) shall operate between the nours of X and Y," or tre reverse, "The source (insert actual name) shall not operate between the hours of X and Y." Sometimes the a1.lowed hours will be varied by the day of the week or time of the year. 4. Land Use Restrictions Ordinances tend to allow difierent leveis of sound for different land uses. Residential uses are considered the most sensitive so they have the lower allowed limits. Some communities use the actual zoning rather than the actual land use to set limits. Some communities make a distinctior. between single family and multiple family residential uses. � 05/566 5. Location The usual practice is to apply the Iimits tc the location receiving the noise. However, some communities set the iimits at the property line of the sound source. 6. Single Limits Some communities establis'�1 a single allowed 1?mit of sound that can be p-roduced by a source. This can be a one time, anytime measu-re, o-r �he average of a set or readings over some time in:.erval. 7. Time Weighted Limits The most common types of time weighted limits are those shown in Tables 1 and 2 that nave been used at times in Paim Desert. Tabie 1 is the average noise level over and hour, although some communities wili shorten the sampling time to 10 or 15 r�inutes. Tab1e 2�s the multiple listings form. In th�s form, the listed sampling time is usualiy one no�ar. The t�me weighted limits use the A-Weighted decibel as the metr� c . 8. Octave Formats Ear1y in the development of local noise ordinances w�th specific 1im,�ts, it was common for commu�ities to s2t up lim,its in th� octave band form. This form sets sound pressure lir�its in each octave bard, usual�y from 3i.5 Hertz to 8,000 riertz. This �yoe of Ordinance accounts for the presence of low frequer_�y noise. As more and more communities aaopted Noise Ordinarces, t!:e use of the rlaltiple octave bands was dropped since measurement required special instruments and training that were not practical for most local jurisdictions. The equipment and trainir.g problems gave rise to the use of the single value composite sound level as rhe measurement metr�c of choice. 9. Adjustments It has been recognized�tY:at tne impact of sound will change witr, varying conditicns =n the community and the nature of the souna source. Thus, limits 'na��e been made lower in many cases �oY noise containing ;masic, speech, impacts and a variety of other source contents. Adjustm`nts have been made for tim.e of day, t�me of the year, the existing ambient, previous noise exposure, rate of fluct�zation and type of sourc�. 10 06/566 Each of the various adjustments have some merit. IL was found that trying to accommodate the various parameters was too complicated for local noise ordinances. Therefore, in most ordinances today, only the time of day adjustment and a single content adjustment are included as an adjustment on the noise 1eve1_ The time of day is usually split into two time periods called day and night. The content adjustment is usually a five decibel reduction in the allowed sound 1eve1. 10. Ambient Any new noise source will add its contribution to the existing ambient noise. Technically, in conducting a measurement, the measured values include the new noise as well as the existing noise. To determine the contribution of the new source by itself, it is necessary to subt-ract out the contribution of the existing noise from the overall noise_ While the technique is valid in theory, it has been found to be very difficult to imp�ement. Communities have addressed the ambient noise issue in several ways. One way is to conduct a measurement with the subject source on and the subject source off, then subtract the two readings to get the source contribution alone. This method has proven to be impractical in most applications. A variation is to go some place where conditions approximate the conditions at the place of ineasurement and record the ambient separately. The lower the frequency, the higher tre ir_fluence of the ambient. Community noise usually has a large low frequency content since, as noted, the �ow frequencies tend to be attenuated less than high frequencies over long distances. Since the A-Weighted measure suppresses the low trequency contribution, it is more useful as a measure when trying to account for the ambie�� contribuLion to a measurement. 11. Dyriamic Response The measurement of sound levels is affected by the rate at which the measurement instrument can process the data. This is also crue of the hum�an hearing system. Therefore, early in the development of sound measurement equipment, it was decided to incorporate a time delay that tended to accommodate human hearing. Initially, there were two delay curves. One was called "FAST" and the other "SLOW". The feeling was that the "SLOW" delay was closer to the way a sound is 11 0�/566 heard by tne human ear. The main differ�nces in tha two delays lie in how they report maximum and minimum sound levels in time t,�arying sounds. The "SLOW" deiay wzll read lower maximum lzvels and higher m�nimum levels than the "FAST" delay. The differences do not make mucn diffe-rence in actual -readings �or slowiy varying sounds or average sound level measur�r�ents for periods exceed�ng five minutes. The two measures will pro�uce significantly different results when �he sourds are rapi�ly var�ing or contain impu�sive sound components such a drum beats. 12. Rate of Change Some souncis have rapidly varying levels w�th time. There is some theory that suggests t��is may nave a bearing on community response. Tre most common example of a time varying sound is vehicle traffic. The rev�.ew of the various approaches to no�se control at the local level tend toward the following characteristics for Ordinances with specific numerica� limits: 1. The use of the A-Weighced sound level. 2. Some cype of accommodatio�-: �.o che length of a noise event. 3. A simple adjustment to accou:�� for certain source noise character_stics, but not tne iow frequer_cy cor.ten� directly. Specific seurces are oiten listed ir the Ordinance because t�ey tend to nave sig:�ificant 1ow frequency content. 4, A recognition of t!�e time of day in whic'rl a sound occurs. The usual adjustment is to divide the day into two time periods called "Day" a:�d '�I�ight . `� S. Some type of exemption Lormat. The r:zost common ex2r�ption irom the applica�ion of the I�Toise Ordinance are extended to schoo's and chu�cnes. 5. Some type of gen�ral nuisance provision. Not a�l Ordinances incorporate all cf t'ne s�x features listed. The former and cu-rrent Palm Deserc Noise Ordinance contains all six items i:� tre list. 12 06/566 6.0 ADDRESSING LOW FREQUENCY NOISE The discussion in Section l.o pointed out that the nature of the basic A-Weighted single number measurement metric used in Noise Ordinances mitigates against any significant consideration of the low frequency content. The reasons for this are as follows: 1. There are no noise standards that address low frequency noise impact. 2. Measurement or low frequencies usually requires equipment that measures 1ow frequency in octave or sub-octave bands. This equipment at one time was very expensive, although that is not so much the case today. Also, most octave equipment will not simultaneously measure the time weighted duration parameters. 3. The presence of a large amount of low frequency content in ambient noise even with low A-Weighted levels complicates the measurement process and the interpretation of the measurement results. 4. The equipment issue and the interpreting of the measurements require special training that usually requires dedicated noise enforcement personnel. Most communities do not have the resources to commit to such p2rsonnel. Issues that require special analysis are farmed out to private noise consultants. S. Most community noise studies have tended to conclude that the A-Weighted sound level is a good predictor of community response. This tends to be quite convenient since the single A-Weighted sound le�Tel is easy to measure. It is useful to examine how the A-Weighted and C-Weighted' sound level measures address various sound sources. Previous discussions have shown that the music sample frequency curve shown on Exhibit 1 results ir: a C-Weighted �evel nine (9) dBA higher than the A-Weighted version. The highest octave band at 63 Hertz was 94 dB, or 29 dB higher than the A-Weighted numbers and five dB less than the C-Weighted number. Other noise sources would exhibit different spectrums. Three typical noise source examples have been selected fo-r discussion. They are a local'street, a barking dog, and a cooling tower. The octave band frequency distribuLions are shown on Exhibit 5. The C-Weighted, A-Weighted and h�ghest octave levels are giver in Table 3 or the fo�lowing page. 13 06/S56 TABLE 3 DECIBEL VALUES FOR THREE TYPICAL COMMUNITY NOISE SOURCES SOURCE dBC dBA HIGHEST OCTAVE LEVEL Coo�ir.g Tower 87 77 85 at 125 Hertz Local Street 81 74 7o a� 63 Hertz Dogs Barking 87 86 84 at 1,000 Hertz The results reveal the following three factors: l. Where t'ne highest octave band is at mid-range, as is the case for the barking dogs, the dBC and dBA values will be similar since tnere is so little low frequency energy compared to the mid-range bar.ds. 2. Where there is a strong 1ow •_`requer.cy content, as �s the case for the coolir.g tower, the highest octave band leve? is a�most the same as the dBC r_umber, which is 10 dB hig'r�er than the dBA r.umber. 3. Where ther� tends to be equal sound pressure levels irl the low to mid range bands, as is tne case for the loca� street, tne dominant octave band sound pressure level is similar numerically to the dBA value, but the aBC number is seven (7) dB higher thar. the dBA number. 4. In all three examples, the �BC value is a fair predictor of the low freque:�cy content, although tnere will be some variation in the di�=erence between tne C-Weighted and A-4�7eigrted readings. 'T'he examples p-rovide a hinr as to wny A?oise Ordinances tend to account for certain sources expected to nave some low freque:�cy centent such as music or impulses by simply lowering the allowed ievel. In mosL cases, the allowed level is f�ve dBA lower t'r�an that fo= other noise sources. The same intent could be accounted for by saying the C-Weighted 1eve1 cannot be more than a cercain dB higher than the A-Weighted level, or the level �r. some octave band cannot be more than a certain dB higner than the A- Weignted level. The otner way o� accounting ior the �ow �requency content is te set an allowed leve� fo� each octave band, or an allowed C-Weighte� sound le�rel. Very lit�le exis�s �r. the litera�ure or in actual practice to use as guidance. Rs noted, most studies tend to arrive at the conclusior_ that the A-Weighted measu�e is a reasonable predictor of community response. HoweveY, tha� is no� al.ways true as evidenced by the experience tha� prompted the I�edlin ]. 4 06/566 study. The Medlin study focused on �he low frequency content, but also the fact that this content varied rapidly, what they referred to as the "Bump - Bump." Some studies suggest that sleep disturbance in some form is related to rapid changes in the noise levels. Some communities will limit sound emissions from auto sound systems, which are known to exhibit the "boomy" tyne of sound. Such sound is almost always accompanied by a"beat" type oz rhythm. Rap music is often cited as the best example. It is that characteristic that precipitates the regulations. However, the limits are always based on such phrases as, "the sound shall not be heard within X feet of the vehicle." The phenomena of rapidly varying sound has been studied, but rarely is it addressed in any formal method of noise control or noise metric. Traffic noise analysts will sometimes use a term cailed the Tratfic Noise Index which is based on a formula that addresses the difference between the maximum and minimum noise level for a time varying sound. However, r.o standards have been developed based on this index. Moreover, attempts to adopt a similar idea to other community sounds have not proven fruitful. Part of the problem is that of trying to devise a way to add-ress a rapidly changing source signal in the presence of a rapidly changing ambient condition such as produced by traffic. A measurement protocol can be set up in a given case, but a general protocol that is easily applied without complicated equipment, in a1i cases does not seem possible. 7.0 CHANGES IN SOUND LEVEL While differences between A-Weighted and C-Weighted sound levels will exist in many cases, there is the quesci�n of what amount of change is significant. The current practice is to state that a three (3} to five (S) dB change in the A-Weighted level is significant. The three (3} dB change is often also considered to be the minimum difference in two readings that can be reliably supported when accounting for equipment resolution. The mir_imum detectable change between two sounds is a change of three (3) decibels at any frequency. This means that a person can often detect a change in the sound even when the A-Weighted does not change. However, this usually occurs in the �requencies below 250 Hertz in the bands that are so heavily discounted by the A-Weighting filter. The effect on the C-Weighting depends on the frequency at which a change occurs. For example, it can take as much as seven (7) dB change ir� the levels below 250 Hertz to produce a net three (3) dB difference in the C-Weighted and A-Weighted levels. In genera�, for tnere to develop a three (3; dB difference between the C-Weighted ar.d A-Weighted readings of a spectrum, the spectrum must 15 06/�65 rave octave band Ievels below 250 Hertz aoproximately equal to or greater than the 500 and 1,000 Hertz bands. �.0 THE FRAMEWORK OF CR�TERIA Iaeally, the source noise shoula not increase the ambiert noise level by more than three (3) dB in any octave band. however, considering all the known information, the motivation and the practical limitations that are imposed on the enLorcement process, a standard which accounts for low frequency content woulc have �he following characteristics: l. The metric must be measurable with readily available equipment that does not require special expert�se for interpreting the measurement. This requirement tends to exclude complicated equipment and interpretation techniques. This limits the choice of the metric to the C-Weighted soand leve� or the Unweighted sound level. 2. Tne motivaCion for this study was a problem with an electronically amplified music source. This type of source wili be the most common source that presents the problem with low frequency noise and the one where the C-Weighted and A-Weighted Ievels are most likely to dive-rge. Other sources or noise, such as roadways and equipment, do not present the pattern of diff2rence between C-Weighted and A-weignted sounds, and in some cases, do not present the same problem as music sources. 3. Rapidly varying sound is not necessarily a problem. It seems to be more of a problem with" music sources. However, some of the reactions may be tied to acceptance of a type of music as opposed to some ��niversally ur.acceptable character of the sound pattern. 4. The problem which precipitated this stuay occurred in the period after 10:00 P.M. Therefore, it might be best to limit application of any low frequency standard to the period be�ween 10:00 P.�. and 7:00 A.M., which is the neriod callea night in the Noise Ordinar_ce. S. To mir�imize tne impact of addir�g a new component to the Noise Ordinance, the present use of the time average noise 1eve1, Leq, should i�e employed. However, it is desirable to reduce the sampling tir�e to ten minutes. This w�ll insure that 16 06/566 the music contribution is captured in a sample. 6. Finally, the C-Weighted level should not exceed the A-Weighted level by�more than three !3) decibels. This will insure that the low frequency contributions wi11 not exceed 10 dB in any octave band. 9.0 AN RPPLICATION EXAMPLE I� the music curve is given by the C-Weighted curve on Exhibit 1, then the A-Weighted and C-Weighted levels would differ by nine (9) decibels. For the two to be different by only �hree (3) decibels, the levels below 250 Hertz would need to be reduced by 12 dB. The result is graphed on Exhibit 6. 10.0 RECOMMENDED MODIFICATION Section 9.24.030 should be modified to read: D. For noise sources consisting of masic or �usic in combination with other noise, the C-Weighted level after 10:00 P.M. shall not be more than three !37 decibels higrer than the allowed A-Weighted ievel over any ten (10) minute period. 0 1% EXHIBIT 1 � � w m U W � 115 >>o io5 100 95 90 85 80 75 70 65 60 55 � 50 �' 45 40 35 30 25 20 15 10 5 0 OCTAVE MUSIC SOURCE SOUND LEVELS AT 100 FEET FROM THE SOURCE (Crosshatch represents the frequncy components supressed by A-Weighting) , UNWEIGHTED = 99 DBC � A-WEIGHTED = 90 DBA `2''��- _--- -t�, ' �,, , ,*�\ .., �. �. 63 125 250 500 1k 2k THIRD OCTAVE BANDS 4k 8k 16k EXHIBIT 2 � J w m U t�{ � i o0 95 — 90 85 80 75 70 65 60 55 50 45 40 35 30 - 25 - 20 : 15 10 5 0 iCr7 RATE OF DECAY OF SOUND PRESSURE LEVEL FOR A MUSIC SAMPLE � �:; ., � ♦ � �� �„\ �. �� � :� `' � ''`� °a'. .� � � �. � �� . • . � � 100 1000 DISTANCE 125 HERTZ 1000 HERTZ � � 10000 100000 EXHiBIT 3 � � W m U w � RATE OF DECAY OF UNWEIGHTED SOUND LEVEL FOR A MUSIC SAMPLE FOR 70 DEGREES AND 50 PERCENT RELATfVE HUMIDiTY COMPARED TO 105 DEGREES AND 10 PERCENT RELATIVE HUMIDITY t:. 70 DEGREES. 50% RELATIVE HUMIOITY � ♦- 105 DEGREES. 10°.6 REIATIVE NUMlDITY � �. • • �.� % �. • �, k� � F. 'l iio 1�5 100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 10 10� 1000 DISTANCE 10000 100000 EXHIBIT 4 � J U� m U w 0 110 105 100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 RATE OF DECAY OF A-WEIGHTED SOUND LEVEL FOR A MUSIC SAMPLE FOR 70 DEGREES AND 50 PERCENT RELATIVE HUMIDITY COMPARED TO 105 DEGREES AND 10 PRECENT RELATIVE HUMIDITY ; 70 DEGREES, 50°/a RELATIVE HUMIDITY ♦ 105 DEGREES. l0% RELATIVE HUMIDITY � • �� �\�'�`� ��� � �. � �C : .R,� �' 10 100 1000 DISTANCE 10000 100000 � EXHIBtT 5 RATE OF DECAY FOR EIGHT FOOT HlGH BARRIER AT 40 FEET FOR A-wEIGHTED AND C-WEIGHTED MEASURES o • 5 ! 10 � 15 Z 2Q O F= � 25 � W 30 � J m 35 U u' ae 0 45 50 55 60 10 � - C-WEIGHTED ♦ A-WEIGHTED � � � �� �,,, �`. . :k Y. � �� 1000 DISTANCE 10000 100000 EXHIBIT 6 (/? J LS_1 m U w � 115 110 � 105 100 95 90 - 85 �.. 80 75 !' � ' - 70 65 ,� 60 - = 55 � 50 45 40 35 30 25 - 2� _ 15 _ 10 5 0 63 125 OCTAVE BAND SOUND LEVELS FOR THREE COMMUNITY NOISE SOURCES ♦ t • �. . , . v. � ♦ ., �- - •-� _ r �- � R, ' r �., �. �, ,: '� • ♦ COOLING TOWER AT 50 FEET = 87 DBCl77DBA � LOCAL STREET = 81 DBA174 08A -�- DOGS BARKING = 87 DBAl86 DBA 250 500 1k 2k 4k 8k 16k THIRD OCTAVE BANOS EXHIBiT 7 OCTAVE MUSIC SOURCE SOUND LEVELS AT 100 FEET FROM THE SOURCE WITH BASS ROLLOFF 115 {Crosshatch represents the frequncy components ��o supressed by the rollof� 105 - 100 95 90 �� 85 � /�� �� �' �� •. 80 = . 75 ' �. 70 � 65 � J m 60 U 55 _ w 50 - � 45 4O ., ORIGINAL CURVE = 99 DBC/90 DBA � 35 ♦ CORRECTEO CURVE= 92 DBe/89 DBA 30 _ _ _ 25 20 15 ' 10 5 0 63 125 250 500 1k 2k 4k 8k 16k THIRD OCTAVE BANDS . - .