HomeMy WebLinkAboutSDP 05-18; ECR CORPORATE CENTER; INTERIOR NOISE STUDY; 2006-01-09IWAll Medlin & AN Associates, Inc.
Acoustical Consultants
Interior Noise Study
La Costa Greens Medical Offices
Prepared for:
John White
Carltas Company
RECEIVED
JAN: 09 2006
CITY OF CARLSBAD
PLANNING DEPT
January 9, 2006
760-930-6515
P0 Box 130941 Carlsbad. CA 92013-0941
mail@medlin-acoustics.com
Executive Summary
Noise from Palomar Airport air traffic is not included in this study.
Future year (2020) noise at the site will be produced mainly by traffic on El Camino
Real (ignoring aircraft noise).
Future year traffic noise levels impinging on the project buildings will approach
69 dBA during morning peak hours.
The City of Carlsbad maximum acceptable level of interior noise in a commercial
office space is 55 dBA (one-hour average).
Interior noise levels based on future year traffic noise are expected to be between
4045 dBA for representative rooms with little absorptive surfaces, and between
3540 dBA for rooms with moderate to high absorption (within the Carlsbad limit).
Representative size rooms were modeled as the actual room configurations will be
determined by future tenants.
Modeling assumed 1/4" monolithic glass in fixed windows, thoroughly sealed around
the edges (window frame noise-control characteristics are assumed to be equal to or
greater than those of the glass).
Framed portions of the buildings will be 6-1/2" concrete; interior noise levels will
therefore be determined by window selection.
Steps must be taken during design and construction to ensure that no alternate paths of
noise entry exists (vents, chimneys, gaps, etc.)
Mechanical ventilation is required.
Medlin & Associates, Inc. SUMMARY La Costa Greens
Contents
INTRODUCTION ...................................................................................................................................... 1
PROJECT DESCRIPTION .1
tt.PPJ4IC_BI1E REGULATIONS ............................................................................................................... 3
BRIEF OVERVIEW OF HIGHWAY NOISE...........................................................................................5
EXISTING TRAFFIC NOISE MEASUREMENTS..................................................................................6
PROJECTED (2020) TRAFFIC NOISE LEVELS....................................................................................9
INTERIOR NOISE LEVELS...................................................................................................................11
CONCLUSIONS AND RECOMMENDATIONS ...................................................................................14
APPENDIX 1: FUNDAMENTALS OF NOISE ...........................15
APPENDIX 2: INTERIOR NOISE LEVEL COMPUTATIONS .......................................................... 17
APPENDIX 3: 2020 TRAFFIC VOLUME PROJECTIONS 19
Medlin & Associates, Inc. CONTENTS La Costa Gieens
Introduction
Caritas Development Company proposes to build two medical office buildings on the east
side of El Camino Real near the intersection of Camino Vida Roble in Carlsbad, CA.
Medlin & Associates, Inc. was tasked to perform a study of the potential traffic noise
impacts on the interior of these buildings. This report documents the findings of that
study.
This study focused only on potential highway traffic noise impacts within the building
interiors. No consideration was given to any potential impacts due to aircraft
overflights from Palomar Airport. No consideration was given to any other potential
noise source, existing or future.
Project Description
The project site is a 7.68 acre lot adjoining the east side of El Camino Real just north of
Camino Vida Roble in Carlsbad (Figure 1). It lies between the CNEL-60 and CNEL-65
contours on the City of Carlsbad Future Noise Exposure Contours map (part of the
General Plan).'
The site is currently being graded to approximately the same elevation as El Camino Real.
Two 42,500 square-foot two-story medical office buildings will be situated on the east
side of the project, diagonal to El Camino Real. Building "A" will lie about 230 feet away
from this road at its nearest point, while building "B" will lie at least 400 feet away. Both
buildings will be surrounded by parking lots.
The site rises well above properties to the north, south, and east. A proposed day-care
center (not part of this study) lies at the southwest corner of the project, between building
"B" and El Camino Real. A cul-de-sac (Metropolitan Street) will lie about 150 feet away
from building "A", with its connecting street (Town Garden Lane) lying 300 feet away at
the closest point.
'CNEL - Community Noise Equivalent Level; see Appendix I for definition of this and other terms
Medlin & Associates, Inc. I La Costa Greens
Applicable Regulations
California Building Code (Title 24) does not provide limitations on acceptable noise
levels interior or exterior to a commercial office space. The City of Carlsbad Noise
Guidelines Manual, however, specifies an acceptable interior noise level limit of 55 dBA
(one-hour average) for uses including general office, general commercial, heavy
commercial, etc.2
It further provides guidelines for maximum exterior noise levels for various land uses, as
shown in Figure 2. Exterior noise levels up to CNEL-65 are considered "normally
acceptable" for land uses including professional office buildings. Levels between CNEL-
65 and CNEL-70 are considered "conditionally acceptable", meaning that an analysis
should be conducted to ensure that interior noise levels will be within the acceptable level
of 55 dBA as stated above.
Since the interior noise limit is specified as a one-hour average, and because the buildings
will presumably be occupied only during daytime hours, the exterior level stated in terms
of CNEL may be ignored. The limit of concern for this study, therefore, is the one-hour
average interior noise level of 55 dBA. Particular attention was paid to those hours which
experience highest traffic volumes on El Camino Real.
'From Table - i" on page Summazy ii of the Noise Guidelines Manual
Mcdliii & ASSOCiateS, Inc. 3 La Costa Greens
Brief Overview of Highway Noise
Motor vehicles produce noise due to engine and exhaust operation, tire/pavement
interaction, airflow, and other ancillary sources. In general, vehicle noise increases with
increasing vehicle speed. At higher speeds (above 40 miles per hour), tire/pavement
interaction ("pavement noise") becomes a substantial, if not the dominant, noise source
for most automobiles. Pavement selection can therefore have a marked effect on the noise
emissions from a freeway or major arterial.
Vehicle type plays a role in determining noise emissions. A large truck traveling at 35
miles per hour produces noise equivalent to 30 passenger automobiles at the same speed.
Unmuffled motorcycles, of course, also produce inordinately high levels of noise. Vehicle
type also determines the nature of the noise emissions in addition to their levels. Large
trucks engines produce heavy "rumble" while pavement noise is more of a "rushing"
sound. The frequency content of noise is a major factor in the effectiveness of noise
mitigation. It is therefore necessary to take into account the expected traffic mix when
projecting highway noise levels.
Transportation noise is generally held to be exempt from any local noise ordinances, such
as those which control acceptable property-line levels due to commercial operations,
music, etc. In cases where a local government controls the project, the local general plan
may set forth goals and guidelines to determine acceptable noise impacts on sensitive land
uses, however, these are normally not considered enforceable limits like those in a noise
ordinance.
Abatement of traffic noise usually takes the form of sound walls, or in some cases earthen
berms. Sound wall design can be rather involved, however, the two primary factors are
the wall's height and location with respect to the highway and impacted receivers. The
taller a wall is, the more effective it will be in blocking highway noise. Similarly, a wall
is more effective if it lies close to the highway or the affected receivers; a sound wall
situated halfway between these offers the least benefit. Pavement type also substantially
affects highway noise emissions. Differences in noise-level emissions in the speech band
may exceed ten decibels for similar freeways using different pavement types.
Future traffic noise levels are usually modeled with appropriate computer software. An
application currently in wide use is the Federal Highway Administration's Traffic Noise
Model (7WM). This software takes into account such features as road geography and
pavement type, vehicle speed and type mix, and terrain features and ground cover. Noise
barriers may be modeled to determine their effectiveness.
Mcdliii & Associates, Inc. 5 La Costa Greens
Existing Traffic Noise Measurements
Existing traffic noise measurements were conducted on the project site around midday on
17 December 2005. Measurements were conducted on a Saturday in order to avoid
current grading operations at the site. Traffic volumes on El Camino Real appeared
somewhat less than normal weekday traffic, however, sufficient volume flowed in order to
obtain reliable noise measurements.
Measurements were conducted at the four locations shown in Figure 1. Locations 3 and 4
were selected to represent that noise which will impinge on buildings "A" and "B"
respectively. As these buildings will sit at an angle to El Camino Real, measurement
points were selected on the ends closest to this road. Location 4 was atop a hill which had
not yet been razed. As such, it had a less obstructed view of El Camino Real, with a
consequent increase in noise level, and thus represents traffic noise impinging on second-
floor windows in this building. Measurement locations 1 and 2 were chosen closer to El
Camino Real in order to provide a more complete description of the noise environment at
this site. The closest, location 1, lay approximately 100 feet from the road.
Noise at this site is driven primarily by traffic on El Camino Real. Very little traffic from
Palomar Airport flew over the site during measurements, and no other noise sources of
significance were noted. Ground cover comprised unpacked, freshly graded soil, thus
presenting a more absorptive surface than will exist with the future parking lots.
Noise measurement results are presented in Table 1. These levels are somewhat low for a
site which lies next to a busy highway, and it is likely that the loose, freshly graded soil
was providing a substantial amount of ground absorption. Additionally, portions of the
road were obscured from view due to small mounds of soil near the road resulting from
grading operations. These existing conditions were therefore taken into account in
projecting future traffic noise levels at this site.
Table 1: Noise measurement results
Location Start
Time Duration Level
(Leq)
Level During Traffic
Counts (Leq)
1 11:53 am 44 minutes 58.5 dBA -
2 11:50 am 27 minutes 57.9 dBA -
3 12:40 pm 24 minutes 55.7 dBA 56.9 dBA
4 12:18 pm 46 minutes 57.2 dBA 57.5 dBA
Traffic was counted for an eighteen-minute interval during noise measurements at
locations 3 and 4, starting at 12:43 p.m. Measured noise levels during these counts are
presented in the last column of Table 1, while recorded volumes are shown in Table 2.
Traffic comprised mostly automobiles, with small percentages of trucks and motorcycles.
3Mcasurements conducted with Larson Davis 824 Type 1 integrating sound level meters and spectrum analyzers, fitted with windscreens
and calibration-checked before and after measurements; microphone heights were at least five feet above ground; weather cloudy, 60
degrees, 35% relative humidity, winds 9-10 mph.
Medlin & Associates, Inc. 6 La Costa Greens
Normalizing these counts to hourly traffic volumes produces the values shown in the last
row of Table 2, for a total hourly existing daytime volume of 1660 vehicles per hour.
Table 2: Counted traffic volumes (El Camino Real -
both directions, 18 minutes)
Total Autos Medium
Trucks
Heavy
Trucks
Motor-
cycles
Counted 498 484 8 3 3
Percentage 100% 97.2% 1.6% 0.6% 0.6%
1-hour normalized 1660 1613 1 27 1 10 10
Applying these volumes to the traffic noise model described below and allowing for
vehicle speeds of 60 miles per hour, loose soil ground cover, and a one-foot high terrain
line along. the eastern edge of El Camino Real produced good agreement with actual
measured noise levels (within one decibel).
The following additional items are of note:
Traffic volumes were split evenly between northbound and southbound lanes in the
model; actual traffic counts did not separate lanes.
Traffic signals were modeled on El Camino Real at the intersections with Camino
Vida Roble (south of the project) and an existing driveway north of the project (the
future Town Garden Lane). Signals were modeled as stopping 20% of traffic on El
Camino Real.
A pavement type of "average" was used.
The posted speed limit on El Camino Real at this location is 55 miles per hour. (The
modeled speed was 60 miles per hour.)
Spectral measurements of traffic noise were conducted concurrently with the above
measurements. Figure 3 shows noise levels as a function of frequency measured at
location 1. This curve is typical of traffic noise. As location 1 was least affected by
ground absorption, this curve was used to project interior noise levels as described below.
Medlin& Associates, Inc. 7 . La Costa Greens
Projected (2020) Traffic Noise Levels
Projected peak-hour traffic volumes for the year 2020 were provided by Linscott Law and
Greenspan (LLG) and shown in Table 3 (also see Appendix 3). They are subdivided by
morning and evening peak hours, and further separated by lane direction. The exception
was Metropolitan Street (the cul-de-sac north of the project), for which the values in Table
3 are estimated based upon the average daily traffic volume reported by LLG.
For the purposes of this study, Table 3 values were further subdivided by. vehicle type in
accordance with the same percentages observed during on-site measurements.
Table 3: 2020 traffic volumes (peak hour)4
Total Autos Medium
Trucks
Heavy
Trucks
Motor-
cycles
El Camino Real northbound (am.) 2400 2333 39 14 14
El Camino Real southbound (a.m.) 1288 1252 21 8 8
El Camino Real northbound (p.m.) 1069 1039 17 6 6
El Camino Real southbound (p.m.) 2468 2399 40 15 15
Town Garden Lane eastbound (a.m.) 622 605 10 4 4
Town Garden Lane westbound (a.m.) 470 457 8 3 3
Town Garden Lane eastbound (p.m.) 661 642 11 4 4
Town Garden Lane westbound (p.m.) 610 593 10 4 4
Metropolitan Street (estimated) 202 196 3 1 1
Traffic volumes in Table 3 were used to project future (year 2020) traffic noise levels at
the project site us4 the Federal Highway Administration's INM version 2.5 traffic noise
modeling software. Because a.m. peak-hour traffic volumes on El Camino Real are
higher than p.m. peak-hour volumes, a.m. peak-hour volumes for all roads were used in
modeling future traffic noise levels.
The same parameters applied to the existing-condition model were used in the future-
condition model except for the following changes:
Ground cover was changed from "loose soil" to "pavement" in order to account for the
future parking lot which will cover the site.
The terrain line along the edge of El Camino Real was removed.
Vehicle speeds on Town Garden Lane and Metropolitan Street were modeled as
40 mph and 25 mph, respectively. (Volumes and speeds on these roads were zero in
the existing-condition model.)
4Source: Linscott Law & Gieenspan 30 December 05 letter to John White at Carltas Company
Medlin & Associates, Inc. has not verified, and does not guarantee, the accuracy of TNM ver2.5 results.
Medlin & Associates. Inc. 9. La Costa Greens
Based on these parameters, the projected noise levels for the year 2020 are provided in
Table 4. These levels are one-hour average levels (Leq) for morning peak-hour traffic,
stated in A-weighted decibels (dBA). Two receiver heights were modeled for locations 3
and 4, five feet above ground to represent ground-floor noise levels, and fifteen feet above
ground to represent second-floor levels. As stated above, location 3 represents the front of
Building "A" closest to El Camino Real, while location 4 represents the same for
Building "B".
Table 4: Projected 2020 noise levels
(a.m. peak-hour)
Location Building Level
I - 72.4 dBA
2 - 69.7 dBA
3 (lower) A -downstairs 68.6 dBA
3 (upper) A - upstairs 68.0 dBA
4 (lower) B - downstairs 65.3 dBA
4 (upper) B - upstairs 64.8 dBA
The combination of increased traffic volumes and a paved surface result in future noise
levels being well over those measured under existing conditions. It was noted that traffic
on Metropolitan Street and Town Garden Lane made insignificant contributions to the
levels in Table 4.
As the downstairs location-3 noise level is the highest, this value was used to project
interior noise levels as described below.
Medlin & Associates, Inc. 10 La Costa Greens
Nine-foot high ceilings were assumed in all cases. For each room, three different levels of
absorptive finishes were modeled. From lowest to highest, these were:
low: hard-tile floors, gypsum-board walls, gypsum-board ceiling
medium: thin carpet (indoor/outdoor), gypsum board walls, gypsum-board ceiling
high: heavy carpet on a foam rubber pad, gypsum board walls, suspended ceiling of
3/4" thick absorptive ("acoustical") tile with an NRC value of 0.94.
In all of the above cases it was assumed that only one wall faced the exterior (i.e. noise
only entered from one wall). To account for the possibility of corner offices with two
exterior walls, the standard office, executive office, and conference room were modeled
again using the low-absorption scenario and twice the exterior exposure.
Interior noise levels were estimated using the standard formula:
Lint -':Lod _TL +l0log(---' A)
where "Lint" is the interior noise level, "Le," the exterior (traffic) noise level, "U is the
composite transmission loss of the exterior wall, "S" is the size of the exterior wall in
square feet, and "A" is the room absorption in Sabins. The exterior-to-interior noise
transmission was analyzed on a full-octave basis in the 125 Hz to 4000 Hz bands, then
adjusted and summed to provide A-weighted interior noise levels. (See Appendix 2 for
details.)
Table 5 shows the projected average interior noise levels. In all cases, these levels fall
well within the Carlsbad requirement of 55 dBA.
Table 5: Projected average interior noise levels
Interior Level
Room Dimensions Exterior
Level
low
absorption
medium
absorption
high
absorption
standard office lOx lOx 9 69 dBA 43.0 dBA 41.5 dBA 37.3 dBA
executive office 12 x 12 x 9 69 dBA 43.1 dBA 41.5 dBA 37.0 dBA
vice-president office 15 x 15 x 9 69 dBA 43.0 dBA 41.3 dBA 36.6 dBA
conference room 20 x 20 x 9 69 dBA 42.2 dBA 40.2 dBA 35.0 dBA
standard office - corner lOx lOx 9 69 dBA 45.5 dBA -- -
executive office - corner 12 x 12 x 9 69 dBA 44.7 dBA - -
conference room - corner 20 x 20 x 9 69 dBA 1 44.7 dBA - -
The results in Table 5 are based on the assumption that the windows are closed, fixed, and
thoroughly sealed around their edges. In other words, the noise-control properties of the
glass alone were modeled without consideration to the window frame. This was necessary
Medlin & Associates, Inc. 12 La Costa Greens
as no window manufacturer or model has yet been specified. (It is implicitly assumed that
the windows remain closed.)
The results in Table 5 are further based on the assumption that no alternate path for
exterior noise penetration exists, such as vents, chimneys, uncaulked gaps, etc. Where
such penetrations are necessary, they should be shielded or faced away from significant
exterior noise sources (i.e. El Camino Real).
Medlin & Associates, Inc. 13 La Costa Greens
Conclusions and Recommendations
As shown in Table 5, average interior noise levels in the two buildings will lie well within
the Carlsbad requirement of 55 CIBA (one-hour average), based upon year 2020 traffic
noise projections. The levels in Table 5 are derived upon the assumed use of 1/4"
monolithic glass in the windows. Because the framed portions of the exterior walls will
comprise 6-1/2" concrete (with very low sound-transmission characteristics), the choice of
windows will largely drive the interior noise levels within these buildings. Tenant
selection of room dimensions and surface treatments will have a secondary effect on
interior levels.
Because the Carlsbad interior noise limit is met, no special mitigation measures are
required, either exterior to the building or in its construction. However, because projected
interior levels were based upon the assumption of doors and windows closed and no
alternative noise paths, the following measures are required in the design/construction of
these buildings:
No vents, chimneys, gaps, etc. shall be allowed which would introduce and alternative
path for exterior noise to enter these buildings. Where vents/chimneys are required,
they should be shielded or faced away from significant exterior noise sources.
Alternatively, they should be adequately isolated inside the building so as not to allow
a sound path into occupied spaces.
Window glass shall be mounted in frames with noise-control properties approaching
or exceeding the glass. Hollow frames should be avoided unless filled with a sound-
damping material. Frames shall be thoroughly sealed (caulked) and contain no air
gaps to the exterior. Window glass should preferably be mounted using a resilient
gasket.
The following is a requirement of the City of Carlsbad: "Where windows are required
to be unopenable or kept closed in order to meet the interior noise standards,
mechanical ventilation and cooling, if necessary, shall be provided to maintain a
habitable environment. The system shall supply two air changes per hour to each
habitable room including 20% (one-fifth) fresh make-up air obtained directly from the
outdoors. The fresh air inlet duct shall be of sound attenuation construction and shall
consist of a minimum often feet of straight or curved sound attenuating duct or six
feet plus one sharp 90 degree bend."
Ventilation systems shall be designed and installed so as not to create noise levels
exceeding 55 dBA in any occupied space.
Finally, all prospective tenants or purchasers of these buildings shall be advised that the
property is subject to overflight, sight, and sound of aircraft operating from McClellan
Palomar Airport.
Medlin & Associates, Inc. 14 La Costa Greens
Appendix 1: Fundamentals of Noise
Rapid variations in ambient air pressure are perceived as sound by the human ear when
they occur within certain limits. Specifically, the ear is sensitive to variations which
occur at the rate of twenty times per second (20 Hertz) to twenty-thousand times per
second, and at pressure differentials of at least twenty millionths of a Pascal (20
micropascals).
These are extreme limits for healthy ears. Most human hearing takes place in the
frequency range of 100 Hz to 10,000 Hz, with the highest sensitivity at about 4,000 Hz.
The human voice contains most of its energy in the frequency range between 125 Hertz
and 8,000 Hertz.
The pressure variation of 20 micropascals is the lower limit of perceptibility. Human
hearing extends from this limit up to the threshold of discomfort where pressure variations
approach 20 pascals—a range of one million to one. Because of this large range of
values, sound pressure is usually measured in terms of "decibels":
L =20 log( L
P.
L is the value of sound pressure level in decibels, P is the mean pressure variation, and P
is the lower limit described above. Sound pressure levels are referenced to the lower limit
of hearing, meaning a level of zero decibels corresponds to that limit whereas a level of
one-hundred decibels represents a pressure variation one-hundred thousand times greater
than that limit. The logarithmic conversion provides a compression effect. Thus, sound
pressure level is a method of expressing the wide range of human hearing in a manageable
range of numerical values.
Because of the logarithmic conversion, decibel arithmetic dBA
works differently than ordinary arithmetic. Doubling the 120 rock concert
sound power in a measured environment results in only a three 110
decibel addition to the measured values, not a doubling of the 100 lawn mower number of decibels; a ten-fold increase in the sound power
results in an addition often decibels to the measured value. 90
Similarly, averaging sound levels involves taking the anti- 80 school cafeteria
logarithms of measured sound levels. A simple arithmetic 70
average of sound levels produces meaningless results, 60
particularly if the two levels are widely divergent. (Note, normal conversation
however, that local ordinances often use a simple arithmetic 50
average of sound levels when setting statutory thresholds on 40
property-line limits involving two different zoning areas.) 30 whisper
20 —4--
Conveniently, human perception of "loudness" is also 10± approximately logarithmic. A three decibel change in sound
level is just noticeable to most people. A five decibel change 0± hearing threshold
is readily noticeable, whereas a change often decibels is Figure A usually perceived as a doubling of the "volume".
Mcdliii & Associates, Inc. 15 La Costa Greens
Because human hearing is not equally sensitive at all frequencies, various weighting
schemes have been developed to account for these variations. The most commonly used
is the "A" weighting. It heavily discounts measured levels at lower frequencies, while
providing slight emphasis around 2500 Hertz. The abbreviation for decibels is "dB".
When levels have been A-weighted, they are expressed as "dBA" or "M(A)". Figure A
depicts several representative noise sources and the A-weighted sound levels they produce
at a typical receiver location.
Objects in the environment rarely produce steady levels of noise. Fluctuating levels
produce fluctuating measurements, thus requiring a method of describing the noise
environment in a meaningful way. The common method in use is the equivalent-
continuous sound level, abbreviated L, which expresses the energy-average noise level
over a specified interval of time (typically one hour). It is important to note that, like other
averaging methods, L does not indicate the range of noise level measurements. Two
identical values of L may represent two widely different ranges of actual noise
measurements. Because of the logarithmic nature of expressing sound level, however,
very loud sounds of any significant duration will tend to "swamp" quieter sounds of
longer duration, thus biasing measurements in favor of the louder sounds.
Because quieter conditions are normally preferred during sleeping hours, various
measures have been developed which account for additional annoyance produced by
noises occurring at night. In California, the Community Noise Equivalent Level (CNEL)
is standard in most statutes and requirements. CNEL is a twenty-four hour "equivalent"
noise level. It accounts for the additional annoyance above by adding a 5 decibel penalty
to noises measured between 7 p.m. and 10 p.m., and a 10 decibel penalty to noises
between 10 p.m. and 7 a.m.. An alternative measure, the Day-Night Level (DNL or L)
is similar to CNEL but does not assess a penalty from 7 p.m. to 10 p.m.
DNL and CNEL are average values only. Because a noise source produces a DNL or
CNEL value below a specified threshold does not mean that the noise will be inaudible.
Rather, DNL and CNEL thresholds are normally set so that the occurrence of a disturbing
noise is not so frequent that it causes substantial annoyance to people or other receivers in
the affected area.
Medlin& Associates, Inc. 16 La Costa Greens
Appendix 2: Interior Noise Level Computations
Interior noise levels in this report were derived from the Medlin & Associates, Inc.
proprietary architectural acoustics model (M/Arch ver 2.4). Computation details are
described below.
In order to determine an average interior noise level, three quantities must be known:
the exterior noise level
the sound-attenuating properties ("transmission loss") of the facade separating the
interior and exterior
the amount of sound-absorption in the room
These values are then applied to a standard formula to determine the interior noise level.
For a high degree of accuracy, these quantities should be known on a spectral basis, rather
than simple A-weighted values, as they tend to be highly frequency-depàndent.
The amount of sound-absorption in a room can be computed by measuring the dimensions
of the surface areas and applying known absorption coefficients for the various finishes
(carpet, drywall, etc.). Room absorption will vary depending upon the presence and kinds
of furniture, wall hangings, etc. For ease of calculation and conservative results, however,
a room is assumed to be empty of all but floor and ceiling treatments when calculating its
absorption.
Similarly, the transmission loss of a facade is determined by identifying test data obtained
for similar constructions. Transmission loss is based upon the size of the facade and its
construction. Where a facade consists of two or more different type of materials (such as
glass and stucco), a composite transmission loss is derived and applied to the formula.
Attenuation and absorption are typically calculated for each octave band from 125 Hz to
4000 Hz, as these are the bands for which test data are readily available. (They are also
the bands in which most sounds of interest occur, including speech.)
The general formula for determining interior noise levels is:
L1 —TL +10logf'--' A)
where:
L, is the exterior noise level
Lmt is the desired interior noise level
T[ is the transmission loss of the facade
S is the surface area of the facade
A is the total sound absorption of the interior
The above levels are averages, as the formula assumes a reasonably diffuse sound field.
This is the case in rooms which are evenly proportioned and do not have excessive levels
of, or grossly unbalanced, absorption.
Medlin& Associates, Inc. 17 . La Costa Greens
In order to compute an overall A-weighted interior noise level, the above formula is
computed on a band-by-band basis, with the results summed on an energy basis.
Within reasonable limits, the difference between interior and exterior levels will remain
constant regardless of the actual exterior level. In the event that the exterior noise level is
not known, or is not known on a spectral basis, the interior/exterior difference may be
computed using the above formula and an assumed exterior level applied. The assumed
level may be based on exterior noise measured at a similar location under similar
conditions. Conversely, a maximum allowable interior noise level may be assumed, and
from this a maximum allowable exterior noise found. In both cases, however, relative
spectral values are required in order to make an accurate determination.
Medlin & Associates, Inc. 18 La Costa Greens