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HomeMy WebLinkAboutSD-22-10 - Supplemental - 0500 Old Farm Road (39) MEMO RSG 55 Railroad Row, White River Junction, Vermont 05001 www.rsginc.com TO: Andrew Gill FROM: Dana Lodico, PE, INCE Bd. Cert. DATE: March 10, 2022 SUBJECT: Hillside Development Phase 2 Quarry Construction Sound Study This memo describes the results of ambient sound measurements and sound modeling of drilling and crushing operations at the quarry site during construction for the O’Brien Brothers Hillside Development Phase 2 in South Burlington, Vermont. An acoustics primer is attached as Appendix A. Executive Summary Temporary drilling and crushing are proposed as part of the construction of Phase 2 of the O’Brien Brothers Hillside Development in South Burlington, Vermont. Drilling is anticipated to occur at the quarry site on 73 days. Drilling activities would occur periodically on these days. The estimated total cumulative drill time for the Project quarry site is 10.5 full construction days. Construction would occur during daytime hours (7 am to 7 am) only. Based on sound propagation modeling, the maximum sound levels from Project drilling and crushing activities at the quarry site are expected to range from 35 to 57 dBA Lmax at the nearest existing homes when drilling is located adjacent to residences. Sound levels inside these homes with windows closed would be about 9 to 30 dBA Lmax, assuming a 27 dBA reduction from outside to inside. In comparison, the existing average daytime exterior sound levels in the vicinity of the project, generated primarily from aircraft and vehicular traffic, are in the range of 55 to 72 dBA Leq, with maximum sound levels from aircraft in the range of 70 to 80 dBA Lmax. There are no State or local statutes or regulations that establish quantitative noise standards applicable to construction of this Project. For informational purposes, Project construction sounds are given with respect to the Vermont Act 250 commonly applied noise limit for operational sound sources of 55 dBA Lmax. Maximum sound levels are modeled to be as high as 57 dBA Lmax when drilling is closest to residences for seven cumulative days or fewer at each individual location. Sound levels from the crusher are not modeled to exceed 55 dBA Lmax at any existing residences. 2 Given the short duration of construction sounds at any individual receptor, exceedance of the operational limit would not cause an adverse impact on aesthetics with regard to noise. Project construction could exceed 55 dBA Lmax for periods of up to seven days at individual receptors and would typically be similar in level or below background sound levels generated by aircraft and vehicular traffic. Given the short duration of these sounds at any individual receptor, Project construction would not cause an undue adverse impact on aesthetics with regard to noise. Project Description O’Brien Brothers is proposing to construct the second phase of the Hillside Development in South Burlington, Vermont. The project would construct approximately 152 residences to the east and west of Old Farm Road and southwest of Kimball Avenue. Project operations are not anticipated to include any notable sound sources that would negatively affect the community. Project construction is anticipated to involve land clearing, drilling, and blasting. To reduce construction truck trips, a portion of the blasted material would be crushed in the eastern portion of the site and the crushed stone would be used on-site. Construction will take place during daytime hours only. Drilling is anticipated to occur intermittently at the quarry site on a total of 73 days. Sound Level Standards There are no State or local statutes or regulations that establish quantitative noise standards applicable to construction of this Project. The noise limit commonly applied to operational sounds generated by Projects in the State of Vermont through Act 250 is 55 dBA Lmax at homes and areas of frequent human use during the day. For informational purposes, Project construction sounds are given with respect to the operational 55 dBA Lmax limit. However, given the short duration of these sounds at any individual receptor, exceedance of the operational limit would not cause an adverse impact on aesthetics with regard to noise. Background Sound Levels The existing sound environment is comprised primarily of aircraft arriving and departing from Burlington Airport, vehicular traffic from Interstate 89 (I-89) and Kimball Avenue, and other local natural, agricultural, and traffic sounds. The northern extent of the project area is about 1,000 feet south of the 65 dBA DNL1 contour shown in the Burlington International Airport noise exposure maps and within the 50 to 55 dBA 24-hr equivalent A-weighted sound level (24-hr Leq) on the National Transportation Noise Map. The Chittenden County Noise Map, developed by RSG, estimates daytime traffic equivalent average sound levels of 50 to 60 dBA Leq. 1 The DNL is a day-night equivalent average sound level, with 10 dB added to the nighttime noise. It is the standard metric used in airport noise mapping. 3 To supplement this information, RSG conducted sound monitoring at three locations in the vicinity of the site from Wednesday, July 5th, 2021 to Wednesday, July 14th, 2021. The monitoring locations included a site along the southern property line (Monitor A), a site near Kimball Avenue (Monitor B), and a site located at 150 Old Farm Road (Monitor C). The sound monitoring locations are shown in Figure 1. FIGURE 1: SOUND MONITORING LOCATIONS Sound level data from each monitor were averaged into 10-minute periods and summarized over the entire monitoring period. Data occurring during undesirable meteorological conditions or periods with extraneous sounds such as equipment interactions by RSG staff were excluded from averaging. Sound monitoring results are presented in Figures 2, 3, 4 for Monitors A, B, and C, respectively. Data is presented as weekly graphs of sound level as a function of time throughout the monitoring period. Each point on the graph represents data summarized for a single 10-minute interval. Equivalent continuous sound levels (Leq) are the energy- average level over 10 minutes. Tenth-percentile sound levels (L90) are the statistical value above which 90% of the sound levels occurred during the 10-minutes. The data from periods which were excluded from processing are included in the graphs but shown in lighter colors. The bands at the top of the graph indicates that data were excluded in the particular 10-minute period; the color designates the reason that data were excluded. 4 FIGURE 2: SOUND PRESSURE LEVELS OVER TIME - MONITOR A, 7/5/2021 TO 7/14/2021 FIGURE 3: SOUND PRESSURE LEVELS OVER TIME - MONITOR B, 7/7/2021 TO 7/14/2021 5 FIGURE 4: SOUND PRESSURE LEVELS OVER TIME - MONITOR C, 7/5/2021 TO 7/14/2021 As shown in the data, aircraft flyovers occur regularly, resulting in elevated noise levels during the flyover event. Sound levels at all three monitoring locations exhibit a diurnal pattern due to traffic and aircraft being greater during the day than at night. An overall summary of the monitor results is provided in Table 1. Sound levels are summarized over the entire monitoring period (overall) and by daytime and nighttime periods. TABLE 1: SUMMARY OF BACKGROUND SOUND LEVELS BY MONITOR LOCATION Location Sound Levels (dBA) Overall Day Night Leq L90 L50 L10 Leq L90 L50 L10 Leq L90 L50 L10 A-South Property Line 56 38 45 51 58 41 46 52 45 37 41 47 B-Kimball Avenue 57 37 46 54 59 42 49 55 47 35 40 49 C-150 Old Farm Road 54 39 45 51 55 41 47 52 45 36 42 48 Average 56 38 45 52 57 41 47 53 46 36 41 48 Project construction will occur during the daytime only. As shown in Table 1, daytime sound levels at the monitoring locations ranged from 55 to 59 dBA Leq. All three monitor locations include aircraft flyover sounds, which are calculated to result in an average daytime sound level of about 55 dBA Leq throughout the site. The lowest sound levels occur at Monitor C, which is not located adjacent to any major traffic sound sources. 6 Vehicular traffic on Kimball Avenue and I-89 result in elevated levels at Monitors A and C, respectively. The arithmetic average of the Leq across all sites is 57 dBA during the day, 46 dBA during the night, and 56 dBA overall. Sound Propagation Modeling Sound propagation modeling was conducted to determine the maximum sound levels expected by drilling and crushing operations at nearby dwellings. METHODOLOGY Modeling for Project construction was in accordance with the standard ISO 9613-2, “Acoustics – Attenuation of sound during propagation outdoors, Part 2: General Method of Calculation.” The ISO standard states, This part of ISO 9613 specifies an engineering method for calculating the attenuation of sound during propagation outdoors in order to predict the levels of environmental noise at a distance from a variety of sources. The method predicts the equivalent continuous A-weighted sound pressure level … under meteorological conditions favorable to propagation from sources of known sound emissions. These conditions are for downwind propagation … or, equivalently, propagation under a well-developed moderate ground-based temperature inversion, such as commonly occurs at night. The model takes into account source sound power levels, surface reflection and absorption, atmospheric absorption, geometric divergence, meteorological conditions, walls, barriers, berms, and terrain. The acoustical modeling software used here was CadnaA, from Datakustik GmbH. CadnaA is a widely accepted acoustical propagation modeling tool, used by many noise control professionals in the United States and internationally. Modeling of background traffic noise levels was conducted using the Federal Highway Administration’s (FHWA) Traffic Noise Model (TNM), as implemented in CadnaA. A 20-meter by 20-meter grid of receivers was set up at a height of 1.5 m (5 ft) in the model, covering approximately 1,000 hectares (2,500 acres) around the Project. The cumulative number of construction days of exposure of sound levels exceeding 55 dBA at surrounding residential locations was also determined. ASSUMPTIONS The following assumptions were used in the sound modeling, 1) Construction will occur during daytime hours only; 7am to 7pm Monday through Saturday. 2) Tree removal will be phased to keep as much cover as possible during the drilling/crushing phase. By keeping trees in place, off-site noise will be reduced. For a conservative analysis, foliage was not included in the sound propagation model. 7 3) A 15- to 20-foot-high topsoil stockpile will be located to the northeast of the quarry location. This stockpile is modeled as a 15-foot-high embankment in the sound model. 4) For the development of the sound contour mapping, the drill and crusher were each represented by point sources. The drill had a sound power rating of 128 dBA when drilling into exposed ledge and 125 dBA when drilling directly into overburden, and the crusher had a sound power rating of 121 dBA. The crusher sound power includes the sound from loading and material conveying. 5) Area sources were used to represent the area in which drilling would take place in order to calculate the duration of sound level exposure at specific receptors. The number of days of drilling was estimated based on the size of the drilling area divided by the drilling rate provided by the blaster of about 465 m2 (5,000 ft2) per day. 6) Residences owned by the company or by shareholders in the development entity are not included in the analysis. This includes 100, 150, 200, and 205 Old Farm Road. MODELING RESULTS Sound contour mapping runs were conducted for three scenarios, representing existing background sound levels and two phases of construction at the quarry site: drilling and crushing. The results are shown as contours of equal sound pressure level overlaid on ortho-imagery of the area surrounding the proposed Project. The cumulative number of days of exposure of sound levels exceeding 55 dBA at representative locations was also modeled. Note that background sound levels are given in terms of an hourly average sound level (Leq), while the sound levels for the Project drilling and crushing operations are given in terms of maximum sound levels (Lmax). 8 Background Sound Levels Modeling of existing daytime background sound levels, including aircraft and vehicular traffic sound sources, was validated to sound levels measured during the sound monitoring survey. The model includes vehicular traffic along I-89, Kimball Avenue, and Kennedy Drive. The aircraft exposure was assumed to be 55 dBA Leq throughout the site and surrounding area. As shown in Figure 5, existing background levels at receptors in the vicinity of the Project range from 55 to 72 dBA Leq during daytime hours. Maximum sound levels from aircraft flyovers (not shown) are in the range of 70 to 80 dBA Lmax throughout the area. FIGURE 5: MODELED BACKGROUND (LEQ) SOUND LEVELS FROM VEHICULAR AND AIRCRAFT TRAFFIC 9 Drilling at the Quarry Site The first Project scenario includes drilling at the quarry site only. Drilling at the quarry site would be into exposed ledge and would not be conducted simultaneously to crushing operations. Crushing would only begin once drill at the quarry site was completed. As shown in Figure 6, the maximum sound level at the closest home, located along Millham Court, is 57 dBA Lmax. Sound levels inside the home with windows closed would be about 30 dBA Lmax, assuming a 27 dBA reduction from outside to inside (Source EPA 1974). Background sound levels at this location are modeled to be 57 dBA Leq (see Figure 5). Therefore, while drilling at the quarry site may be audible at these locations, it would be similar in level to sound generated by local traffic along Kimball Avenue and below maximum levels generated by vehicular traffic and aircraft flyovers. FIGURE 6: MODELED MAXIMUM EXTERIOR SOUND LEVELS FROM DRILLING AT QUARRY SITE 10 Crushing at the Quarry Site The second Project scenario includes crushing on a pad in the quarry area. As shown in Figure 7, the maximum sound level at the closest home, located along Millham Court, is 53 dBA Lmax. Sound levels inside the home with windows closed would be about 27 dBA lower, or 26 dBA Lmax. Maximum sound levels are not anticipated to exceed 55 dBA Lmax at any receptors during this scenario and would generally be similar to or below background sound levels. FIGURE 7: MODELED MAXIMUM EXTERIOR SOUND LEVELS FROM CRUSHING AT QUARRY SITE 11 Duration of Sound Level Exposure The cumulative number of days of exposure of sound levels exceeding 55 dBA at surrounding residential locations was determined by modeling the areas in which drilling would take place at the quarry. The areas slated for drilling at the quarry, totaling about 0.5 hectares (1.2 acres), are represented by area sources in the model. Although drilling is anticipated to occur on 73 days at the quarry site, drilling would occur only periodically on these days. The number of cumulative (full) days of drilling was estimated based on the size of the individual drilling area divided by the drilling rate provided by the blaster of 465 m2 (5,000 ft2) per day. Sound levels generated at five representative receptors, shown in Figure 8, were tracked as the drilling moved between the drilling areas. Throughout the two-year construction period, the estimated cumulative drill time at the quarry site is 10.5 full (12-hour) construction days. FIGURE 8: REPRESENTATIVE RECEPTORS AND DRILLING AREA SOURCES Results of this analysis are shown in Figure 9. The sound levels represent the average of the maximum sound level from drilling during operation of the drill; the levels are not sustained or daily averages. That is, for each hole drilled in an area, there is a maximum sound level. The figures in the graphic represent the equivalent average of these 12 maximum sound levels as the drill operates. The overall average sound levels would be 3 to 8 dB lower, depending on several factors. The highest sound levels from quarry construction activity occur at 42 Millham Road. Table 2 shows the number of days for which the maximum drilling sound levels will be in specified sound levels ranges and Table 3 lists the sound levels at the representative receivers from the crusher operation only. FIGURE 9: AVERAGE MAXIMUM INSTANTANEOUS DRILLING SOUND LEVELS OVER TIME AT REPRESENTATIVE RESIDENCES TABLE 2: NUMBER OF DAYS MAXIMUM DRILLING SOUND LEVELS ARE EXCEEDED Maximum Sound Level, dBA Receptor Location 80 Old Farm Road 103 Old Farm Road 175 Kennedy Drive 91 Bayberry Lane 42 Millham Court Less than 55 11 11 11 11 4 55 to 60 0 0 0 0 7 60 to 65 0 0 0 0 0 65 to 70 0 0 0 0 0 Greater than 70 0 0 0 0 0 Total days greater than 55 dBA 0 0 0 0 7 13 TABLE 3: MAXIMUM SOUND LEVEL OF CRUSHING OPERATION S Sound Source Receptor Location 80 Old Farm Road 103 Old Farm Road 175 Kennedy Drive 91 Bayberry Lane 42 Millham Court Crusher 50 44 39 40 53 CONCLUSIONS Temporary drilling and crushing are proposed as part of the construction of Phase 2 of the O’Brien Brothers Hillside Development in South Burlington, Vermont. On-site crushing reduces the number of truck trips required to bring material to the site, resulting in reduced sound and traffic impacts along roadways serving the site. Drilling is anticipated to occur intermittently on a total of 73 days at the quarry site. The maximum sound levels from Project drilling and crushing activities are expected to range from 35 to 57 dBA Lmax at the nearest existing homes. Sound levels inside residences would be about 27 dBA lower with windows closed. Maximum sound levels that exceed 55 dBA Lmax will occur when drilling is closest to residences but will last seven full construction days or fewer at each individual location as drilling moves around the site. In comparison, the existing average daytime sound levels in the vicinity of the project, generated primarily from aircraft and vehicular traffic, are in the range of 55 to 72 dBA Leq, with maximum sound levels from aircraft in the range of 70 to 80 dBA Lmax. There are no State or local statutes or regulations that establish quantitative noise standards applicable to construction of this Project. The noise limit commonly applied to operational sounds generated by Projects in the State of Vermont through Act 250 is 55 dBA Lmax at homes and areas of frequent human use during the day. Project construction could exceed 55 dBA Lmax for periods of up to seven full construction days at individual receptors and would typically be similar in level or below background sound levels generated by aircraft and vehicular traffic. Given the short duration of these sounds at any individual receptor, Project construction would not cause an undue adverse impact on aesthetics with regard to noise. 14 APPENDIX A. ACOUSTICS PRIMER Sound consists of tiny, repeating fluctuations in ambient air pressure. The strength, or amplitude, of these fluctuations determines the sound pressure level (SPL). “Noise” can be defined as “a sound of any kind, especially when loud, confused, indistinct, or disagreeable.” Expressing Sound in Decibel Levels The varying air pressure that constitutes sound can be characterized in many different ways. The human ear is the basis for the metrics that are used in acoustics. Normal human hearing is sensitive to sound fluctuations over an enormous range of pressures, from about 20 micropascals (the “threshold of audibility”) to about 20 Pascals (the “threshold of pain”).2 This factor of one million in sound pressure difference is challenging to convey in engineering units. Instead, sound pressure is converted to sound “levels” in units of “decibels” (dB, named after Alexander Graham Bell). Once a measured sound is converted to dB, it is denoted as a level with the letter “L”. The conversion from sound pressure in pascals to sound level in dB is a four-step process. First, the sound wave’s measured amplitude is squared and the mean is taken. Second, a ratio is taken between the mean square sound pressure and the square of the threshold of audibility (20 micropascals). Third, using the logarithm function, the ratio is converted to factors of 10. The final result is multiplied by 10 to give the decibel level. By this decibel scale, sound levels range from 0 dB at the threshold of audibility to 120 dB at the threshold of pain. Typical sources of noise, and their sound pressure levels, are listed on the scale in Figure 10. Human Response to Sound Levels: Apparent Loudness For every 20 dB increase in sound level, the sound pressure increases by a factor of 10; the sound level range from 0 dB to 120 dB covers 6 factors of 10, or one million, in sound pressure. However, for an increase of 10 dB in sound level as measured by a meter, humans perceive an approximate doubling of apparent loudness: to the human ear, a sound level of 70 dB sounds about “twice as loud” as a sound level of 60 dB. Smaller changes in sound level, less than 3 dB up or down, are generally not perceptible. 2 The pascal is a measure of pressure in the metric system. In Imperial units, they are themselves very small: one pascal is only 145 millionths of a pound per square inch (psi). The sound pressure at the threshold of audibility is only 3 one-billionths of one psi: at the threshold of pain, it is about 3 one-thousandths of one psi. 15 FIGURE 10: A SCALE OF SOUND PRESSURE LEVELS FOR TYPICAL NOISE SOURCES 16 Frequency Spectrum of Sound The “frequency” of a sound is the rate at which it fluctuates in time, expressed in Hertz (Hz), or cycles per second. Very few sounds occur at only one frequency: most sound contains energy at many different frequencies, and it can be broken down into different frequency divisions, or bands. These bands are similar to musical pitches, from low tones to high tones. The most common division is the standard octave band. An octave is the range of frequencies whose upper frequency limit is twice its lower frequency limit, exactly like an octave in music. An octave band is identified by its center frequency: each successive band’s center frequency is twice as high (one octave) as the previous band. For example, the 500 Hz octave band includes all sound whose frequencies range between 354 Hz (Hertz, or cycles per second) and 707 Hz. The next band is centered at 1,000 Hz with a range between 707 Hz and 1,414 Hz. The range of human hearing is divided into 10 standard octave bands: 31.5 Hz, 63 Hz, 125 Hz, 250 Hz, 500 Hz, 1,000 Hz, 2,000 Hz, 4,000 Hz, 8,000 Hz, and 16,000 Hz. For analyses that require finer frequency detail, each octave-band can be subdivided. A commonly-used subdivision creates three smaller bands within each octave band, or so-called 1/3-octave bands. Human Response to Frequency: Weighting of Sound Levels The human ear is not equally sensitive to sounds of all frequencies. Sounds at some frequencies seem louder than others, despite having the same decibel level as measured by a sound level meter. In particular, human hearing is much more sensitive to medium pitches (from about 500 Hz to about 4,000 Hz) than to very low or very high pitches. For example, a tone measuring 80 dB at 500 Hz (a medium pitch) sounds quite a bit louder than a tone measuring 80 dB at 60 Hz (a very low pitch). The frequency response of normal human hearing ranges from 20 Hz to 20,000 Hz. Below 20 Hz, sound pressure fluctuations are not “heard”, but sometimes can be “felt”. This is known as “infrasound”. Likewise, above 20,000 Hz, sound can no longer be heard by humans; this is known as “ultrasound”. As humans age, they tend to lose the ability to hear higher frequencies first; many adults do not hear very well above about 16,000 Hz. Most natural and man-made sound occurs in the range from about 40 Hz to about 4,000 Hz. Some insects and birdsongs reach to about 8,000 Hz. To adjust measured sound pressure levels so that they mimic human hearing response, sound level meters apply filters, known as “frequency weightings”, to the signals. There are several defined weighting scales, including “A”, “B”, “C”, “D”, “G”, and “Z”. The most common weighting scale used in environmental noise analysis and regulation is A- weighting. This weighting represents the sensitivity of the human ear to sounds of low to moderate level. It attenuates sounds with frequencies below 1000 Hz and above 4000 Hz; it amplifies very slightly sounds between 1000 Hz and 4000 Hz, where the human ear is particularly sensitive. The C-weighting scale is sometimes used to describe louder sounds. The B- and D- scales are seldom used. All of these frequency weighting scales are normalized to the average human hearing response at 1000 Hz: at this frequency, the filters neither attenuate nor amplify. When a reported sound level has been filtered 17 using a frequency weighting, the letter is appended to “dB”. For example, sound with A- weighting is usually denoted “dBA”. When no filtering is applied, the level is denoted “dB” or “dBZ”. The letter is also appended as a subscript to the level indicator “L”, for example “LA” for A-weighted levels. Time Response of Sound Level Meters Because sound levels can vary greatly from one moment to the next, the time over which sound is measured can influence the value of the levels reported. Often, sound is measured in real time, as it fluctuates. In this case, acousticians apply a so-called “time response” to the sound level meter, and this time response is often part of regulations for measuring noise. If the sound level is varying slowly, over a few seconds, “Slow” time response is applied, with a time constant of one second. If the sound level is varying quickly (for example, if brief events are mixed into the overall sound), “Fast” time response can be applied, with a time constant of one-eighth of a second.3 The time response setting for a sound level measurement is indicated with the subscript “S” for Slow and “F” for Fast: LS or LF. A sound level meter set to Fast time response will indicate higher sound levels than one set to Slow time response when brief events are mixed into the overall sound, because it can respond more quickly. In some cases, the maximum sound level that can be generated by a source is of concern. Likewise, the minimum sound level occurring during a monitoring period may be required. To measure these, the sound level meter can be set to capture and hold the highest and lowest levels measured during a given monitoring period. This is represented by the subscript “max”, denoted as “Lmax”. One can define a “max” level with Fast response LFmax (1/8-second time constant), Slow time response LSmax (1-second time constant), or Continuous Equivalent level over a specified time period LEQmax. Note that, in the Act 250 precedents set by the former Environmental Board and the Vermont Superior Court Environmental Division utilizes LSmax. Accounting for Changes in Sound over Time A sound level meter’s time response settings are useful for continuous monitoring. However, they are less useful in summarizing sound levels over longer periods. To do so, acousticians apply simple statistics to the measured sound levels, resulting in a set of defined types of sound level related to averages over time. An example is shown in Figure 11. The sound level at each instant of time is the grey trace going from left to right. Over the total time it was measured, the sound energy spends certain fractions of time near various levels, ranging from the minimum (about 28 dB in the figure) to the maximum (about 65 dB in the figure). The simplest descriptor is the average sound level, known as the Equivalent Continuous Sound Level. Statistical levels are used to 3 There is a third time response defined by standards, the “Impulse” response. This response was defined to enable use of older, analog meters when measuring very brief noises; it is no longer in common use. 18 determine for what percentage of time the sound is louder than any given level. These levels are described in the following sections. Equivalent Continuous Sound Level - LEQ One straightforward, common way of describing sound levels is in terms of the Continuous Equivalent Sound Level, or Leq. The Leq is the average sound pressure level over a defined period of time, such as one hour or one day. Leq is the most commonly used descriptor in noise standards and regulations. Leq is representative of the overall sound to which a person is exposed. Because of the logarithmic calculation of decibels, Leq tends to favor higher sound levels: loud and infrequent sources have a larger impact on the resulting average sound level than quieter but more frequent noises. For example, in Figure 11, even though the sound levels spends most of the time near about 34 dBA, the Leq is 41 dBA, having been “inflated” by the maximum level of 65 dBA. FIGURE 11: EXAMPLE OF DESCRIPTIVE TERMS OF SOUND MEASUREMENT OVER TIME Percentile Sound Levels – LN Percentile sound levels describe the statistical distribution of sound levels over time. “LN” is the level above which the sound spends “N” percent of the time. For example, L90 (sometimes called the “residual base level”) is the sound level exceeded 90% of the time: the sound is louder than L90 most of the time. L10 is the sound level that is 19 exceeded only 10% of the time. L50 (the “median level”) is exceeded 50% of the time: half of the time the sound is louder than L50, and half the time it is quieter than L50. Note that L50 (median) and Leq (mean) are not always the same, for reasons described in the previous section. L90 is often a good representation of the “ambient sound” in an area. This is the sound that persists for longer periods, and below which the overall sound level seldom falls. It tends to filter out other short-term environmental sounds that aren’t part of the source being investigated. L10 represents the higher, but less frequent, sound levels. These could include such events as barking dogs, vehicles driving by and aircraft flying overhead, gusts of wind, and work operations. L90 represents the background sound that is present when these event noises are excluded. Note that if one sound source is very constant and dominates the noise in an area, all of the descriptive sound levels mentioned here tend toward the same value. It is when the sound is varying widely from one moment to the next that the statistical descriptors are useful. Sound Levels from Multiple Sources: Adding Decibels Because of the way that sound levels in decibels are calculated, the sounds from more than one source do not add arithmetically. Instead, two sound sources that are the same decibel level increase the total sound level by 3 dB. For example, suppose the sound from an industrial blower registers 80 dB at a distance of 2 meters (6.6 feet). If a second industrial blower is operated next to the first one, the sound level from both machines will be 83 dB, not 160 dB. Adding two more blowers (a total of four) raises the sound level another 3 dB to 86 dB. Finally, adding four more blowers (a total of eight) raises the sound level to 89 dB. It would take eight total blowers, running together, for a person to judge the sound as having “doubled in loudness”. Recall from the explanation of sound levels that a difference of 10 decibels is a factor of 20 in sound pressure and a factor of 10 in sound power. (The difference between sound pressure and sound power is described in the next Section.) If two sources of sound differ individually by 10 decibels, the louder of the two is generating ten times more sound. This means that the loudest source(s) in any situation always dominates the total sound level. Looking again at the industrial blower running at 80 decibels, if a small ventilator fan whose level alone is 70 decibels were operated next to the industrial blower, the total sound level increases by only 0.4 decibels, to 80.4 decibels. The small fan is only 10% as loud as the industrial blower, so the larger blower completely dominates the total sound level. The Difference between Sound Pressure and Sound Power The human ear and microphones respond to variations in sound pressure. However, in characterizing the sound emitted by a specific source, it is proper to refer to sound power. While sound pressure induced by a source can vary with distance and 20 conditions, the power is the same for the source under all conditions, regardless of the surroundings or the distance to the nearest listener. In this way, sound power levels are used to characterize noise sources because they act like a “fingerprint” of the source. An analogy can be made to light bulbs. The bulb emits a constant amount of light under all conditions, but its perceived brightness diminishes as one moves away from it. Both sound power and sound pressure levels are described in terms of decibels, but they are not the same thing. Decibels of sound pressure are related to 20 micropascals, as explained at the beginning of this primer. Sound power is a measure of the acoustic power emitted or radiated by a source; its decibels are relative to one picowatt. Sound Propagation Outdoors As a listener moves away from a source of sound, the sound level decreases due to “geometrical divergence”: the sound waves spread outward like ripples in a pond and lose energy. For a sound source that is compact in size, the received sound level diminishes or attenuates by 6 dB for every doubling of distance: a sound whose level is measured as 70 dBA at 100 feet from a source will have a measured level of 64 dBA at 200 feet from the source and 58 dBA at 400 feet. Other factors, such as walls, berms, buildings, terrain, atmospheric absorption, and intervening vegetation will also further reduce the sound level reaching the listener. The type of ground over which sound is propagating can have a strong influence on sound levels. Harder ground, pavement, and open water are very reflective, while soft ground, snow cover, or grass is more absorptive. In general, sounds of higher frequency will attenuate more over a given distance than sounds of lower frequency: the “boom” of thunder can be heard much further away than the initial “crack”. Atmospheric and meteorological conditions can enhance or attenuate sound from a source in the direction of the listener. Wind blowing from the source toward the listener tends to enhance sound levels; wind blowing away from the listener toward the source tends to attenuate sound levels. Normal temperature profiles (typical of a sunny day, where the air is warmer near the ground and gets colder with increasing altitude) tend to attenuate sound levels; inverted profiles (typical of nighttime and some overcast conditions) tend to enhance sound levels.