Acoustical Noise Control 69if an NC-25 were met in an urban residence, it is likely
that the occupants would perceive it as being quiet. That
same NC-25 represents the upper limit of what is
acceptable for a recording studio. Most recording
studios, especially those that record material with wide
dynamic content, will require NC-20 or even 15. Levels
lower than NC-15 require expensive construction and
are difficult to achieve in urban settings.
4.2 Site SelectionPart of meeting a noise goal is the careful selection of a
building site, a site that is appropriate for the application
and where the NC is achievable and affordable. It is one
thing to build a room meeting an NC-15 in a cornfield
in central Iowa. It is another thing altogether to build an
NC-15 room in downtown Manhattan. When surveying
a site, watch for busy roads, especially freeways;
elevated, ground level, or underground railroads, busy
intersections, airports, and fire stations. When economic
or other factors make such a location imperative, allow-
ance must be made for the extra cost of the structure to
provide the requisite protection from such noise. When
considering space in an existing building, inspect all
neighboring spaces and be wary of adjacent spaces that
are vacant unless the owner of the sound-sensitive space
also controls the vacant space.
Remember that buildings can be very noisy spaces.
Sources of noise include elevator doors and motors,
heating, ventilating, and air-conditioning equipment;
heel taps on hard floors, plumbing, and business
machines.
If selecting a plot of land, a limited amount of
protection can be achieved by erecting earthen embank-
ments or masonry walls between the structure and the
noise source. These are reasonably effective at high
frequencies, but low-frequency components of noise
whose wavelengths are large relative to the size of the
embankment tend to diffract over the top. A stand of
dense shrubbery might yield as much as 10 dB of
overall attenuation. Physical separation of the proposed
structure from the noise source is helpful but limited by
the inverse-square law. The 6 dB per distance double
rule applies only to point sources in free-field condi-
tions but it is useful for rough estimation. Going from
50 ft to 100 ft (a change of 50 ft) from the source yields
the same reduction of noise level as going from 100 ft to
200 ft (a change of 100 ft). Clearly, increasing separa-
tion counts most when close to the source. At any given
location sites, locating the sound-sensitive rooms on the
face of the building away from a troublesome noisesource is favorable, especially if no reflective structures
are there to reduce the barrier effect.
There are two ways that noise travels from the
source to the observer. It is either transmitted through
the air—airborne noise—or carried through the struc-
ture or the earth—structure borne noise. A highway
carrying heavy truck traffic or an overhead or subway
railroad, may literally shake the earth to such an extent
that large amplitude, low-frequency vibrations of the
ground may be conducted to the foundation of the struc-
ture and carried to all points within that structure. Even
if such vibrations are subsonic, they have been known
to shake microphones with good low-frequency
response so as to overload low level electronic circuits.
Vibration, both subsonic and sonic, is carried with
amazing efficiency throughout a reinforced concrete
structure. The speed of sound in air is 344 m/s whereas
the speed of sound in reinforced concrete, for example,
is on the order of 3700 m/s.^5 A large-area masonry wall
within a structure, when vibrated at high amplitude, can
radiate significant levels of sound into the air by
diaphragmatic action. It is possible by using a combina-
tion of vibration-measuring equipment and calculations
(outside the scope of this treatment) to estimate the
sound pressure level radiated into a room via such a
structure-borne path. In most cases noise is transmitted
to the observer by both air and structure.4.2.1 Site Noise SurveyA site survey gives the designer a good idea of the noise
levels present at the proposed building site. It is impor-
tant to know how much noise exists in the immediate
environment so that appropriate measures can be taken
to reduce it to acceptable levels.
Ambient noise is very complex, a fluctuating
mixture of traffic and other noises produced by a variety
of human and natural sources. The site noise should be
documented with the appropriate test equipment.
Subjective approaches are unsatisfactory. Even a
modest investment in a studio suite or a listening room
justifies the effort and expense of a noise survey of the
site which provides the basis for designing walls, floor,
and ceiling to achieve the low background noise goals.
One approach to a noise survey of the immediate
vicinity of a proposed sound room is to contract with an
acoustical consultant to do the work and submit a
report. If technically oriented persons are available, they
may be able to turn in a credible job if supplied with the
right equipment and given some guidance.
The easy way to survey a proposed site is to use one
of the more sophisticated microprocessor-based