HomeMy WebLinkAboutARCH-0161-2019 Acoustic Report
D avid Lord, PhD dl@45dB.com
S arah Taubitz , MSME st@45dB.com
California | Colorado
www.45dB.com
This report (including any enclosures and attachments) has been prepared for the exclusive use and benefit of the addressee(s)
and solely for the purpose for which it is provided. No part of this report shall be reproduced, distributed or communicated to
any third party without written permission. We do not accept any liability if this report is used for an alternative purpose from
which it is intended, nor to any third party.
April 9, 2020
Project# 18013
Revision B
Acoustic Assessment: French
Hospital Expansion, Proposed
Helistop
1911 Johnson Avenue
San Luis Obispo, CA 93401
Requested by:
Brian Starr, NCARB
Studio Design Group
San Luis Obispo, CA 93401
805.541.3848
brian@sdgarchitects.com
Owner:
Dignity Health
French Hospital Medical Center
1911 Johnson Avenue
San Luis Obispo, CA 93401
1 Executive Summary
This report provides an evaluation of the potential noise impacts from the proposed French
Hospital Expansion Helistop located at the above address. The revised location of the helistop as
of 3/17/20 has been incorporated into this analysis and report.
The California Environmental Quality Act guidelines consider that a significant noise impact
would occur if the French Hospital Helistop would result in a substantial increase in ambient
noise levels in the project vicinity above the sound levels existing without the project.
This analysis indicates that the French Hospital Helistop hourly noise levels at the
nearest noise sensitive receptor to the west of the project are expected to range between 53 dBA
and 62 dBA over short periods of time. These noise levels would not result in a substantial
increase of the existing ambient noise levels monitored in the project vicinity.
The proposal is for helicopter activity occurring 4.2 times per month and the helicopter take-offs,
approaches, and overflights would be of short duration, less than five minutes each.
There will be short-term, audible noise increase during arrivals and departures. Our study
concludes that the proposed French Hospital Helistop will result in a Community Noise
Equivalent Level increase of less than 1 dBA (i.e., 2 dB or less). Therefore, the increase in
sound level would be less than significant.
for 45dB Acoustics, LLC, a California Limited Liability Company
Sarah Taubitz, MSME David Lord, Ph.D.
45dB Acoustics, LLC French Hospital Helistop, Rev. B
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Contents
1 Executive Summary ............................................................................................................................................ i
Contents ....................................................................................................................................................................... ii
List of Figures ............................................................................................................................................................. ii
List of Tables .............................................................................................................................................................. iii
2 Description .......................................................................................................................................................... 4
2.1 Acoustical Criteria .......................................................................................................................................... 6
3 Existing Noise Environment, Measured and Modeled .................................................................................... 7
3.1 Measured Levels .............................................................................................................................................. 7
3.2 Modeled Sound Levels ..................................................................................................................................... 9
4 Predicted Noise Levels with proposed Helistop ............................................................................................. 12
4.1 Helicopter Noise Source Definition ............................................................................................................... 12
4.2 Modeled Sound Levels ................................................................................................................................... 14
5 References ......................................................................................................................................................... 17
7 Appendix ........................................................................................................................................................... 19
7.1 Characteristics of Sound ............................................................................................................................... 19
7.2 Terminology/Glossary ................................................................................................................................... 20
7.3 Traffic Noise Model ....................................................................................................................................... 22
7.4 SoundPLAN® Acoustics Software ................................................................................................................. 23
List of Figures
Figure 1: Proposed Helistop Site Plan, in red (SDG drawing) ....................................................... 4
Figure 2: Location of 24-hour Noise Level Measurements ............................................................ 7
Figure 3: Hourly LAeq and CNEL-adjusted levels ........................................................................ 8
Figure 4: Example traffic data published by the City of San Luis Obispo ..................................... 9
Figure 5: Road traffic-only noise contours with measured 24-hour CNEL sound levels overlaid
in four locations ............................................................................................................................ 10
Figure 6: Daytime hourly Leq contours from UPRR pass-by plus hourly road traffic, with
measured maximum hourly Leq at four measurement locations overlaid .................................... 11
Figure 7: ICAO approach sound level measurement, constant deceleration ................................ 14
Figure 8: CNEL levels for existing road + rail noise .................................................................... 15
Figure 9: CNEL levels with new project buildings and helicopter noise added ........................... 16
45dB Acoustics, LLC French Hospital Helistop, Rev. B
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List of Tables
Table 1: Maximum Noise Expoure for Noise-Sensitive Uses Due to Transportation Sources ...... 6
Table 2: Measured Levels, 16:00 Dec 13 to 15:00 Dec 14, 2018 ................................................... 8
Table 3: Sound Level Measurements of Eurocopter EC135 Helicopter ....................................... 13
Table 4: Predicted Helicopter Noise Levels (dBA) ...................................................................... 17
Table 5: Sound Level Change Relative Loudness/Acoustic Energy Loss .................................... 19
45dB Acoustics, LLC French Hospital Helistop, Rev. B
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2 Description
This report provides an environmental noise impact analysis for the proposed helipad
operations at the French Hospital Center in the city of San Luis Obispo, California. A helistop is
planned on top of a proposed new parking structure located at the southwest corner of the
existing hospital. The helistop is indicated by an “H” outlined in red in Figure 1.
The purpose of this study is to quantify the existing noise environment around the hospital,
calculate the future CNEL and peak-hour Leq noise contours from helicopter activity, calculate
the change in CNEL noise level due to the addition of a helistop, and evaluate potentially
significant noise impacts with respect to City noise standards.
Figure 1: Site Plan with Proposed Helistop, at “E” (SDG drawing)
45dB Acoustics, LLC French Hospital Helistop Noise Analysis, Rev B
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Figure 2: Helistop plan, with nearby houses in gray hatch
Figure 3: North elevation view of helistop
45dB Acoustics, LLC French Hospital Helistop Noise Analysis, Rev B
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2.1 Acoustical Criteria
Noise regulations are addressed by federal, state, and local government agencies. Local policies
are generally adaptations of federal and state guidelines, adjusted to prevailing local condition.
The City of San Luis Obispo’s Noise Element of the General Plan states the following
policy applicable to this project:
“1.4 Noise created by new transportation noise sources, including road, railroad, and airport expansion
projects, shall be mitigated to not exceed the levels specified in Table 1 for outdoor activity areas and
indoor spaces of noise-sensitive land uses which were established before the new transportation noise
source.”
Table 1 is reproduced below.
Table 1: Maximum Noise Expoure for Noise-Sensitive Uses Due to Transportation Sources
City of San Luis Obispo
The City does not specify an impact threshold for a maximum allowable change in CNEL/LDN
by a proposed project. In this analysis, we consider any increase in CNEL/ LDN greater than 3 dB
as potentially significant impacts, which is consistent with California Environmental Quality Act
(CEQA) and other similar environmental noise analysis criteria.
The City of San Luis Obispo uses both the LDN and the CNEL descriptors for describing 24-hour
noise metrics. (See the Appendix for further explanation of these terms.) Both metrics include a
10dB ‘penalty’ added to nighttime measured values; CNEL values also include an additional
5dB penalty during even hours of 7pm to 10pm. CNEL usage is becoming more common, and
there is usually less than a 1 dB difference between Ldn and CNEL values; therefore; CNEL and
not LDN values will be presented in this report. All values reported are A-weighted levels.
45dB Acoustics, LLC French Hospital Helistop Noise Analysis, Rev B
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3 Existing Noise Environment, Measured and Modeled
3.1 Measured Levels
To quantify the existing noise environment around the planned helistop, continuous long-term
(i.e., 24 hour) noise measurements were conducted by 45dB from 13 December through 14
December 2018 at nearby residential sensitive receivers. Measurements were conducted using
four ‘Piccolo’ Type 2 sound level meters, field-calibrated with a Brüel&Kjær 4231 Type 1
calibrator. All measurements were made at the standard receiver height of 1.3m Above Ground
Level (AGL).
Figure 4 shows the location of the hospital (“+”), and of the measurements, numbered “02”,
“05”, “06”, and “07.” Figure 5 is a graph of the hourly Leq levels for a 24-hour period, along
with the same levels with the CNEL penalty adjustments of +5dB added for evening hours and
+10dB added for nighttime hours.
Figure 4: Location of 24-hour Noise Level Measurements
45dB Acoustics, LLC French Hospital Helistop Noise Analysis, Rev B
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Figure 5: Hourly LAeq and CNEL-adjusted levels
Table 2 summarizes the resulting measured data into CNEL and 24-hour LAeq for reference,
along with the range of Leq for the period. The City’s Noise Element only stipulates hourly Leq
criteria for facilities that are not typically used in the evenings (e.g., churches, meeting halls,
office buildings, schools, libraries and museums). The City stipulates hourly Leq criterion for
performing arts facilities; however, we are not aware of any performing arts facilities located in
the vicinity of the helistop.
Table 2: Measured Levels, 16:00 Dec 13 to 15:00 Dec 14, 2018
Measurement
Location
Major Noise
Sources
Existing 24-
hour Leq Level
24-hour
CNEL
Level
Daytime
“Ld”
Level
Leq
Range
02: Street side of
parking lot at
SLCUSD at Fixlini
St., north of Lizzie St.
Residential Traffic;
San Luis Unified
School District
parking lot
55.1 56.3 57.5 45 to 67
05: Westernmost
parking lot boundary
of French Hospital,
just east of UPRR
tracks
French Hospital
parking lot traffic,
distant Johnson Ave.
traffic, and occasional
train pass-bys
50.8 53.9 52.6 47 to 67
06: Empty, treed lot
SW of Fairview St. at
Breck St. just east of
UPRR tracks
Johnson Avenue
traffic and occasional
train pass-bys
56.7 57.8 59.1 46 to 69
07: Front
entrance/yard of
1545 Lizzie St.
Residential Traffic 55.4 56.2 57.8 46 to 69
45dB Acoustics, LLC French Hospital Helistop Noise Analysis, Rev B
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3.2 Modeled Sound Levels
To ensure that the acoustic model is starting from an accurate quantification of noise—a
baseline—prior to adding any additional future noise sources, such as the helicopter flights
proposed for the hospital expansion, the measured data are used to compare to the modeled noise
levels based on current Annual Average Daily Traffic (AADT) traffic counts published by the
City of San Luis Obispo. An example of traffic data utilized for one intersection is shown in
Figure 6.
Figure 6: Example traffic data published by the City of San Luis Obispo
Sound level contours due to road traffic alone are shown in Figure 7. The locations of the four
measurement sound level meters are shown. This enables comparison between the noise
propagation model based on published traffic counts to the actual measurements from Dec 13-14,
2018. Measured levels for locations 02 and 07, in the residential neighborhood east of Johnson
Avenue, were approximately 2-3 dB higher than the noise model. ‘Spurious’ noises from people,
motorcycles, dogs barking, etc. can easily account for this relatively small difference.
45dB Acoustics, LLC French Hospital Helistop Noise Analysis, Rev B
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Figure 7: Road traffic-only noise contours with measured 24-hour CNEL sound levels
overlaid in four locations
Measurement locations 05 and 06 are next to UPRR tracks, therefore measured levels should be
compared to the sound level contours which include train pass-by as a second noise source. The
calculated hourly daytime Leq noise contours of traffic and UPRR train pass-by noise are shown
in Figure 8. The measured maximum hourly Leq and the hour of the day it was measured are
overlaid in red text in this figure, for comparison. Predictably, the maximum hourly levels at
positions 05 and 06 near the UPRR tracks were measured at the 3:00pm hour, when several
trains pass by daily. At positions 02 and 07, the maximum hourly level was measured during
morning rush-hour (9:00am and 8:00am, respectively), for this residential area with only local
road traffic.
45dB Acoustics, LLC French Hospital Helistop Noise Analysis, Rev B
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Comparing Figure 7 to Figure 8 underscores how short-duration events that create relatively high
Leq levels, yet do not significantly impact 24-hour averaged (CNEL) levels.
Though it has a relatively high hourly maximum of 69 dBA at 3:00p.m., Location 05 is
significantly lower in CNEL and 24-hour average Leq than the other locations. We imagine this
is because the cumulative effect of noise is lessened here due to its distance away from road
noise. However, the range of hourly Leq noise levels for all locations is quite similar.
Figure 8: Daytime hourly Leq contours (“Ld”) from UPRR pass-by and hourly road
traffic, with the max measured hourly Leq at four measurement locations noted
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4 Predicted Noise Levels with proposed Helistop
The sound levels from the helicopter trip noise can be added to road traffic and rail noise levels
in order to see the effect of helicopter noise on the surrounding areas at specified locations.
4.1 Helicopter Noise Source Definition
Helicopters produce a unique sound that is easily recognizable. While modern light- and
medium-weight civil helicopters are much quieter than older helicopters and much quieter than
heavy military helicopters they are often the focus of much community concern.
A SoundPLAN® noise model was prepared, utilizing Federal Highway Administration’s
(FHWA) Traffic Noise Model (TNM) calculation method for road noise, the Federal
Transportation Administration / Federal Railway Administration (FTA/FRA) for railway noise,
and ISO 9613-2 for outdoor noise propagation from point and line sources (See Sections 7.3 and
7.4) along with a published library of aircraft levels from European Union Aviation Safety
Agency (EASA) Noise Level Documentation.
Aircraft noise-specific models merely provide a compiled library/database of aircraft noise
source emission values. In this case, where only one aircraft and flight path are needed, we have
“calibrated” a line source to match the measured Sound Exposure Level (SEL) limit in
accordance with ICAO as defined in the published database. We conservatively utilized the
overflight limit, i.e. SEL, of 87.4 dBA, for our initial calibration of the source level line. We then
utilized the published increases from overflight for takeoff and approach for a reasonably
conservative emission value for takeoff and approach—so, we increased the takeoff level by 4.6
dB to 92 dBA, and the approach level by 8.7 dB to 96.1 dBA on the ground. The line source
length and duty cycles (i.e. operational time percentage in daytime, evening and nighttime hours)
combine to create 24-hour CNEL contours.
The input data for the model is summarized below:
Helistop location and preliminary plans provided by the architect, Studio Design Group
(undated)
Expected flight volumes of the Airbus H135, as provided by the Heliplanners “Project
Memo 1”, consists of 4.2 flights per month, or 13.8% of any 24-hour day will have a
flight. Assuming a 10-minute total flight event, this is 0.096% of the total time in a given
year.
Published measurements for the Eurocopter EC135 (now rebranded as Airbus H135)
helicopter were utilized to define the sound level inputs for the noise propagation model
Table 3. Published measurements for the EC135 with the maximum weight were utilized
for takeoff, flyover, and approach, as presented in Table 3. The International Civil
Aviation Organization (ICAO) defines the aircraft approach to be utilized for the
measurements, shown in Figure 9 for reference.
45dB Acoustics, LLC French Hospital Helistop Noise Analysis, Rev B
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Published noise levels for the Eurocopter EC135 (now the Airbus H135) were taken from
the European Union Aviation Safety Agency’s aircraft type certification library
(http://easa.europa.eu/certification/type-certificates/docs/tcdns-
databases/MAdB%20Rotorcraft-14-28112013.xlsx).
A line source was developed for the Approach and Departure flight path, which was
“calibrated” to affect the published overflight level of 81.4 dBA, and a Sound Exposure
Level (SEL). The published increases of takeoff and approach from overflight (in
EPNdB) in Table 3—i.e. 4.6 and 8.7 dB, respectively—were added to the overflight level
to arrive at the takeoff and approach flight path sound power levels per unit distance, 86
dBA and 90.1 dBA, respectively). The resulting takeoff and approach values represent
the typical noise levels and not the peak levels, which may never occur.
The helicopter flight path was provided by the revised Heliplanners’ memo (Reference
1), dated July 6, 2019. The flight path is as follows:
o Primary approach flightpath from 143 degrees true heading
o Primary departure flightpath towards 301 degrees true heading
70% of flights will occur during daytime hours (i.e. 7:00 a.m. to 7:00 p.m.); 15% of
flights will occur during evening hours (i.e. 7:00 p.m. to 10:00 p.m.); and 15% of flights
will occur during nighttime hours (i.e. 10:00 p.m. to 7:00 a.m.). An “average annual day”
here consists of different probability of flights during daytime, evening, and nighttime
periods. With 50.4 flights per year and assuming a 10-second total flight event at each
location along the line when a flight occurs, this is 0.08 seconds/hour during daytime
hours, 0.07 sec/hr during evening hours, and 0.022 sec/hr of nighttime hours, which will
contain flight path noise.
Table 3: Sound Level Measurements of Eurocopter EC135 Helicopter
EASA Type Certification data
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Figure 9: ICAO approach sound level measurement, constant deceleration
ICAO annex 16 volume 1 chapter 8
4.2 Modeled Sound Levels
Figure 10 shows sound level contours for the existing situation with road traffic and rail noise, in
plan view. The sound power level of the flight paths for approach and takeoff match the
maximum ICAO/FAR levels defined in Table 3. Figure 11 is the same situation, with helicopter
noise added, as defined in Section 4.1. Locations 01, near the helistop, and location 08, at the
northeast corner, replaced locations 01 and 07 that were further afield.
45dB Acoustics, LLC French Hospital Helistop Noise Analysis, Rev B
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Figure 10: CNEL levels for existing road + rail noise
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Figure 11: CNEL levels with new project buildings and helicopter noise added
Table 4 is a tabulation of differences in the modeled levels of the existing CNEL and Daytime
(“Ld”) levels and with the helicopter noise added, at four locations around the heliport. Locations
05—almost directly under the flight path—and 06 are identical to the measurement locations.
Location 01, across the street from the heliport at what is believed to be a residential home, and
location 08 to the northeast have been added to give a comprehensive picture of the changes in
noise levels due to the helistop.
As stated in Section 4.1, with only 4.2 flights per month and a 10-second flight event at every
location along the flight path, this is 0.0016% of the total time in a given year, or a given “annual
average day”, that will have helicopter flight noise in it. Such a low frequency is what renders
the increases in sound levels to be negligible at locations 06 and 08, and less than significant (i.e.
<3dB) at locations 01, which is next to the hospital, and 05, which is under the flight path.
At 1.5 dB, the CNEL increase is greatest at Location 01 nearest the hospital, which is a less-than-
significant increase.
45dB Acoustics, LLC French Hospital Helistop Noise Analysis, Rev B
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It bears noting that helicopter noise may (unexpectedly) occur during night and evening hours.
Keeping notice and annoyance of helicopter noise to a minimum can be accomplished by
keeping flight paths as vertical as possible and descending to the helistop slowly. Residential
areas surrounding the hospital and further away from road noise may notice or pay more
attention to this new noise source, however infrequent and short in duration it may be.
Table 4: Predicted Helicopter Noise Levels (dBA)
Location Existing
CNEL
Modeled
(Measured)
Cumulative
with
Helicopter
CNEL
Increase
in
CNEL
Existing
Hourly
Daytime*
Leq
Modeled
(Measured),
and Range
Calculated
Cumulative
Daytime
Leq
Increase in
Average or
Range of
Daytime
Leq
01 (next to
hospital)
56 55.8 56 - 54.9* 55.0 -
05 54 (54) 55 1 52.9* (53.9)
47 to 58
54.0 1
06 58 (58) 58 - 53.9* (57.8)
46 to 69
54.1 -
08 57 57 - 55.3* 54.6 55 -
* Single values given are modeled average level across all daytime hours; where available, measured
range is also given.
Our study concludes that the proposed French Hospital Helistop will result in a Community
Noise Equivalent Level increase of less than 1 dBA (i.e., 2dB or less). Therefore, the increase in
sound level is less than significant.
5 References
1. Jeff Wright of Heliplanners. July 6, 2019. FREN-2 PM2: French Hospital Medical
Center, San Luis Obispo; Revised Info for Acoustics Consultant.
2. May 7, 1996. City of San Luis Obispo Noise Element.
3. Malcolm Hunt Associates. June 16, 2017. Assessment of Environmental Noise Effects:
Helicopter Landing Area and Ground Noise Impact Report, State Highway 6 Fox
Glacier.
4. American National Standards Institute, Inc. 2004. ANSI 1994 American National
Standard Acoustical Terminology. ANSI S.1.-1994, (R2004), New York, NY.
5. American Society for Testing and Materials. 2004. ASTM E 1014 - 84 (Reapproved
2000) Standard Guide for Measurement of Outdoor A-Weighted Sound Levels.
6. Bolt, Beranek and Newman. 1973. Fundamentals and Abatement of Highway Traffic
Noise, Report No. PB-222-703. Prepared for Federal Highway Administration.
7. California Department of Transportation (Caltrans). 1982. Caltrans Transportation
Laboratory Manual.
8. ______. 1998. Caltrans Traffic Noise Analysis Protocol for New Highway Construction
and Highway Reconstruction Projects.
45dB Acoustics, LLC French Hospital Helistop Noise Analysis, Rev B
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9. Federal Highway Administration. 2006. FHWA Roadway Construction Noise Model
User’s Guide Final Report. FHWA-HEP-05-054 DOT-VNTSC-FHWA-05-01
10. Harris, Cyril M., editor. 1979. Handbook of Noise Control.
11. Computer Modeling of STC – Options and Accuracy. Horan, Daniel. 2014. Cavanaugh
Tocci Associates, Sudbury, MA. December 2014, p. 8 ff.
12. National Research Council Canada. 1998. Gypsum Board Walls: Transmission Loss
Data. NRC-NCRC Internal Report IRC-IR-761.
13. Office of Noise Control, California Department of Health Services. 1981. Catalog of STC
and IIC Rating for Wall and Floor/Ceiling Assemblies.
14. Rotorcraft Noise Database: http://easa.europa.eu/certification/type-
certificates/docs/tcdns-databases/MAdB%20Rotorcraft-14-28112013.xlsx
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7 Appendix
7.1 Characteristics of Sound
When an object vibrates, it radiates part of its energy as acoustical pressure in the form of a
sound wave. Sound can be described in terms of amplitude (loudness), frequency (pitch), or
duration (time). The human hearing system is not equally sensitive to sound at all frequencies.
Therefore, to approximate this human, frequency-dependent response, the A-weighted filter
system is used to adjust measured sound levels. The normal range of human hearing extends
from approximately 0 to 140 dBA. Unlike linear units such as inches or pounds, decibels are
measured on a logarithmic scale, representing points on a sharply rising curve. Because of the
physical characteristics of noise transmission and of noise perception, the relative loudness of
sound does not closely match the actual amounts of sound energy. Table 5 below presents the
subjective effect of changes in sound pressure levels.
Table 5: Sound Level Change Relative Loudness/Acoustic Energy Loss
0 dBA Reference 0%
-3 dBA Barely Perceptible Change 50%
-5 dBA Readily Perceptible Change 67%
-10 dBA Half as Loud 90%
-20 dBA 1/4 as Loud 99%
-30 dBA 1/8 as Loud 99.9%
Source: Highway Traffic Noise Analysis and Abatement Policy and Guidance, U.S. Department of Transportation,
Federal Highway Administration, Office of Environment and Planning, Noise and Air Quality Branch, June 1995.
Sound levels are generated from a source and their decibel level decreases as the distance from
that source increases. Sound dissipates exponentially with distance from the noise source. This
phenomenon is known as spreading loss. Generally, sound levels from a point source will
decrease by 6 dBA for each doubling of distance. Sound levels for a highway line source vary
differently with distance because sound pressure waves propagate along the line and overlap at
the point of measurement. A closely spaced, continuous line of vehicles along a roadway
becomes a line source and produces a 3 dBA decrease in sound level for each doubling of
distance. However, experimental evidence has shown that where sound from a highway
propagates close to “soft” ground (e.g., plowed farmland, grass, crops, etc.), a more suitable
drop-off rate to use is not 3.0 dBA but rather 4.5 dBA per distance doubling (FHWA 2010).
When sound is measured for distinct time intervals, the statistical distribution of the overall
sound level during that period can be obtained. The Leq is the most common parameter
associated with such measurements. The Leq metric is a single-number noise descriptor that
represents the average sound level over a given period of time. For example, the L50 noise level
is the level that is exceeded 50 percent of the time. This level is also the level that is exceeded 30
minutes in an hour. Similarly, the L02, L08 and L25 values are the noise levels that are exceeded
2, 8, and 25 percent of the time or 1, 5, and 15 minutes per hour. Other values typically noted
during a noise survey are the Lmin and Lmax. These values represent the minimum and
maximum root-mean-square noise levels obtained over the measurement period.
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Because community receptors are more sensitive to unwanted noise intrusion during the evening
and at night, State law requires that, for planning purposes, an artificial dB increment be added to
quiet-time noise levels in a 24-hour noise descriptor called the CNEL or Ldn. This increment is
incorporated in the calculation of CNEL or Ldn, described earlier.
7.2 Terminology/Glossary
A-Weighted Sound Level (dBA)
The sound pressure level in decibels as measured on a sound level meter using the internationally
standardized A-weighting filter or as computed from sound spectral data to which A-weighting
adjustments have been made. A-weighting de-emphasizes the low and very high frequency
components of the sound in a manner similar to the response of the average human ear. A-
weighted sound levels correlate well with subjective reactions of people to noise and are
universally used for community noise evaluations. An A-weighted Leq is designated as “LAeq”.
Air-borne Sound
Sound that travels through the air, differentiated from structure-borne sound.
Ambient Sound Level
The prevailing general sound level existing at a location or in a space, which usually consists of a
composite of sounds from many sources near and far. The ambient level is typically defined by
the Leq level.
Background Sound Level
The underlying, ever-present lower level noise that remains in the absence of intrusive or
intermittent sounds. Distant sources, such as Traffic, typically make up the background. The
background level is generally defined by the L90 percentile noise level.
Community Noise Equivalent Level (CNEL)
The Leq of the A-weighted noise level over a 24-hour period with a 5-dB penalty applied to
noise levels between 7 p.m. and 10 p.m. and a 10-dB penalty applied to noise levels between 10
p.m. and 7 a.m. CNEL is similar to Ldn.
Day-Night Average Sound Level (Ldn or DNL)
Day-Night Average Sound Level (Ldn or DNL) – A descriptor established by the U.S.
Environmental Protection Agency to represent a 24-hour average noise level with a 10dB penalty
applied to noise occurring during the nighttime hours (10 p.m. to 7 a.m.) to account for the
increased sensitivity of people during sleeping hours.
Decibel (dB)
The decibel is a measure on a logarithmic scale of the magnitude of a particular quantity (such as
sound pressure, sound power, sound intensity) with respect to a reference quantity.
DBA or dB(A)
A-weighted sound level. The ear does not respond equally to all frequencies and is less sensitive
at low and high frequencies than it is at medium or speech range frequencies. Thus, to obtain a
single number representing the sound level of a noise containing a wide range of frequencies in a
manner representative of the ear’s response, it is necessary to reduce the effects of the low and
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high frequencies with respect to the medium frequencies. The resultant sound level is said to be
A-weighted, and the units are dBA. The A-weighted sound level is also called the noise level.
Energy Equivalent Level (Leq) or Leq
Because sound levels can vary markedly in intensity over a short period of time, some method
for describing either the average character of the sound or the statistical behavior of the
variations must be utilized. Most commonly, one describes ambient sounds in terms of an
average level that has the same acoustical energy as the summation of all the time-varying
events. This energy-equivalent sound/noise descriptor is called Leq. In this report, an hourly
period is used. Therefore, Leq is the equivalent steady-state sound level (in decibels) that, in a
stated period of time, would contain the same acoustic energy as the time-varying sound level
during the same period of time.
Effective perceived noise in decibels (EPNdB)
A measure of the relative loudness of an individual aircraft pass-by event. Separate ratings are
stated for takeoff, overflight and landing phases, and represent the integrated sum of loudness
over the period within which the noise from the aircraft is within 10 dB of the maximum noise
(usually at the point of closest approach.) It is defined in Annex 16 of the Convention on
International Civil Aviation and in Part 36 of the US Federal Aviation Regulations. The scaling
is such that the EPNdB rating represents the integrated loudness over a ten-second period;
EPNdB of 100 dB means that the event has the same integrated loudness as a 100 dB sound
lasting ten seconds. The EPNdB rating of an aircraft is used to estimate how much contribution a
given aircraft operation will make to the noise impact of an airport in a community, which is
estimated using the day-night average sound level metric. Detailed information on measurement
of aircraft acoustic signature to meet the requirements of Annex 16 is found in ICAO Document
9501and IEC 61265. Data acquisition in one-third-octave bands is required, followed by
processing to yield a logarithmically-scaled value in decibels relative to a sound pressure of 20
micropascals, approximately the threshold of hearing.
Apparent Sound Transmission Class (ASTC)
A single number rating similar to STC, except that the transmission loss values used to derive the
ASTC are measured in the field. All sound transmitted from the source room to the receiving
room is assumed to be through the separating wall or floor-ceiling assembly.
Outdoor-Indoor Transmission Class (OITC)
A single number classification, specified by the American Society for Testing and Materials
(ASTM E 1332 issued 1994), that establishes the A-weighted sound level reduction provided by
building facade components (walls, doors, windows, and combinations thereof), based upon a
reference sound spectrum that is an average of typical air, road, and rail transportation sources.
The OITC is the preferred rating when exterior façade components are exposed to a noise
environment dominated by transportation sources. Apparent OITC (AOITC) is the field-
measured OITC.
Percentile Sound Level, Ln
The noise level exceeded during n percent of the measurement period, where n is a number
between 0 and 100 (e.g., L10 or L90)
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Sound Transmission Class (STC)
STC is a single number rating, specified by the American Society for Testing and Materials,
which can be used to measure the sound insulation properties for comparing the sound
transmission capability, in decibels, of interior building partitions for noise sources such as
speech, radio, and television. It is used extensively for rating sound insulation characteristics of
building materials and products.
Structure-Borne Sound
Sound propagating through building structure. Rapidly fluctuating elastic waves in gypsum
board, joists, studs, etc.
Sound Exposure Level (SEL), or Sound Exposure Noise Equivalent Level (SENEL)
SEL is the sound exposure level, defined as a single number rating indicating the total energy of
a discrete noise-generating event (e.g., an aircraft flyover) compressed into a 1-second time
duration. This level is handy as a consistent rating method that may be combined with other SEL
and Leq readings to provide a complete noise scenario for measurements and predictions.
However, care must be taken in the use of these values since they may be misleading because
their numeric value is higher than any sound level which existed during the measurement period.
Sound Pressure Level (p or SPL)
The acoustic pressure level, typically in units of decibels relative to 20 micropascals (µPa), at
any given receiver location due to all noise sources affecting that location. It is a property of the
field at a point in space.
Sound Power Level (P, LWA, or SWL)
The level, typically in units of decibels relative to 1 Watt, at which sound energy is emitted by a
source. For a sound source, unlike sound pressure, sound power is neither room-dependent nor
distance-dependent. Sound power is a property of a sound source, equal to the total power
emitted by that source in all directions.
Subjective Loudness Level
In addition to precision measurement of sound level changes, there is a subjective characteristic
which describes how most people respond to sound:
A change in sound level of 3 dBA is barely perceptible by most listeners.
A change in level of 6 dBA is clearly perceptible.
A change of 10 dBA is perceived by most people as being twice (or half) as loud.
7.3 Traffic Noise Model
The Federal Highway Administration Traffic Noise Model (TNM) used within SoundPLAN®
software for the sound level analysis in this study, contains the following components:
1. Modeling of five standard vehicle types, including automobiles, medium trucks, heavy
trucks, buses, and motorcycles, as well as user-defined vehicles.
2. Modeling both constant- and interrupted-flow traffic using a field-measured data base.
3. Modeling effects of different pavement types, as well as the effects of graded roadways.
4. Sound level computations based on a one-third octave-band data base and algorithms.
5. Graphically-interactive noise barrier design and optimization.
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6. Attenuation over/through rows of buildings and dense vegetation.
7. Multiple diffraction analysis.
8. Parallel barrier analysis.
9. Contour analysis, including sound level contours, barrier insertion loss contours, and
sound-level difference contours.
These components are supported by a scientifically founded and experimentally calibrated
acoustic computation methodology, as well as a flexible data base, made up of over 6000
individual pass-by events measured at forty sites across the country.
7.4 SoundPLAN® Acoustics Software
SoundPLAN, the software used for this acoustic analysis, is an acoustic ray-tracing program
dedicated to the prediction of noise in the environment. Noise emitted by various sources
propagates and disperses over a given terrain in accordance with the laws of physics. Worldwide,
governments and engineering associations have created algorithms to calculate acoustical
phenomena to standardize the assessment of physical scenarios. Accuracy has been validated in
published studies to be + / - 2.7 dBA with an 85% confidence level.
The software calculates sound attenuation of environmental noise, even over complex terrain,
uneven ground conditions, and with complex obstacles.
The modeling software calculates the sound field in accordance with many optional standards
depending on the noise source type, including the FHWA’s TNM described in the previous
subsection, and ISO 9613-2 “Acoustics - Attenuation of sound during propagation outdoors, Part
2: General Method of Calculation.” This standard states that “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 under a well-developed moderate ground-based
temperature inversion, such as commonly occurs at night.”