Four acoustic terms you need to be familiar with:
- Noise Reduction Coefficient (NRC)
Sound Transmission Class (STC)
In an enclosed
space, when a sound source stops emitting energy, it takes some time for the sound
to become inaudible. This prolongation of the sound in the room caused by continued
multiple reflections is called reverberation.
time plays a crucial role in the quality of music and the ability to understand
speech in a given space. When room surfaces are highly reflective, sound continues
to reflect or reverberate. The effect of this condition is described as a live
space with a long reverberation time. A high reverberation time will cause a build-up
of the noise level in a space. The effects of reverberation time on a given space
are crucial to musical conditions and understanding speech. It is difficult to
choose an optimum reverberation time in a multi-function space, as different uses
require different reverberation times. A reverberation time that is optimum for
a music program could be disastrous to the intelligibility of the spoken word.
Conversely, a reverberation time that is excellent for speech can cause music
to sound dry and flat.
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Reflected sound strikes a surface
or several surfaces before reaching the receiver. These reflections can have unwanted
or even disastrous consequences. Although reverberation is due to continued multiple
reflections, controlling the Reverberation Time in a space does not ensure the
space will be free from problems from reflections.
or peaked ceilings can create a megaphone effect potentially causing
annoying reflections and loud spaces. Reflective parallel surfaces lend themselves
to a unique acoustical problem called standing waves, creating a fluttering
of sound between the two surfaces.
Reflections can be attributed to the
shape of the space as well as the material on the surfaces. Domes and concave
surfaces cause reflections to be focused rather than dispersed which can cause
annoying sound reflections. Absorptive surface treatments can help to eliminate
both reverberation and reflection problems.
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The Noise Reduction
Coefficient (NRC) is a single-number index for rating how absorptive a particular
material is. Although the standard is often abused, it is simply the average of
the mid-frequency sound absorption coefficients (250, 500, 1000 and 2000 Hertz
rounded to the nearest 5%). The NRC gives no information as to how absorptive
a material is in the low and high frequencies, nor does it have anything to do
with the materials barrier effect (STC).
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Transmission Class (STC):
Sound Transmission Class (STC) is a single-number
rating of a materials or assemblys barrier effect. Higher STC values
are more efficient for reducing sound transmission. For example, loud speech can
be understood fairly well through an STC 30 wall but should not be audible through
an STC 60 wall. The rating assesses the airborne sound transmission performance
at a range of frequencies from 125 Hertz to 4000 Hertz. This range is consistent
with the frequency range of speech. The STC rating does not assess the low frequency
sound transfer. Special consideration must be given to spaces where the noise
transfer concern is other than speech, such as mechanical equipment or music.
Even with a high STC rating, any penetration, air-gap, or flanking
path can seriously degrade the isolation quality of a wall. Flanking paths are
the means for sound to transfer from one space to another other than through the
wall. Sound can flank over, under, or around a wall. Sound can also travel through
common ductwork, plumbing or corridors.
For more information on Sound
Transmission Class, visit STCratings.com.