367
Primary absorption mechanisms (viscous loss and intrinsic
damping) are described in Section 4.8. The amount of absorption
that occurs generally depends on both the material characteristics
and on the frequency or pitch of the sound. Absorption coefficients
that are measures of the sound-absorbing capabilities of various
materials can be experimentally determined. This coefficient is the
fractional part of incident diffuse sound energy that is absorbed
and not reflected by a surface. Values vary widely. The higher the
value, the more efficient the material is as a sound absorber. Some
metals and stones are as low as 0.01. Highly absorbing materials
used for special purposes can be as high as 0.96 and almost up to
the maximum possible value of 1.0. Values, however, are dependent on the frequency of the impinging sound waves. For a complex
assembly consisting of several materials, a measure called the sound
transmission class (STC) is sometimes used as a single figure rating
system. STC is intended to provide a measure that estimates the
sound insulation properties of an assembly. It can be used to rankorder a series of different assemblies. Sound transmission loss (STL)
is a related measure.
The part that these parameters play in the actual analysis of sound
in a complex spatial environment involves many more considerations concerning the configuration and dimensions of the space
and distribution of materials used, as we will soon see.
general noise-Control approaches
Sounds reaching a point or hearer may be directly airborne from
a source within the space (e.g., from a machine or TV) or external
to it, airborne sounds that result from structure-borne vibrations,
or sounds from impacts. The intent of a sound-control strategy
varies very widely according to the design context and whether
the sounds are wanted or unwanted. In some cases the intent is
simply to reduce unwanted noise. Here the normal intent is to
reduce or eliminate noise in order for a space to have an overall
sound environment conducive to normal work and life activities.
Another intent could be to eliminate unwanted sounds because
of the vibrations that they might induce in delicate instruments
or manufacturing facilities. The latter is particularly important in
the context of this book—for example, vis-à-vis issues in manufacturing or characterizing nanomaterials. Conversely, in other
situations, an intent is to provide particular types of sound to
a space for achieving certain kinds of ambient conditions (the
notion of a totally quiet room is known to be antithetical to
Sound and Acoustical Environments
Primary absorption mechanisms (viscous loss and intrinsic
damping) are described in Section 4.8. The amount of absorption
that occurs generally depends on both the material characteristics
and on the frequency or pitch of the sound. Absorption coefficients
that are measures of the sound-absorbing capabilities of various
materials can be experimentally determined. This coefficient is the
fractional part of incident diffuse sound energy that is absorbed
and not reflected by a surface. Values vary widely. The higher the
value, the more efficient the material is as a sound absorber. Some
metals and stones are as low as 0.01. Highly absorbing materials
used for special purposes can be as high as 0.96 and almost up to
the maximum possible value of 1.0. Values, however, are dependent on the frequency of the impinging sound waves. For a complex
assembly consisting of several materials, a measure called the sound
transmission class (STC) is sometimes used as a single figure rating
system. STC is intended to provide a measure that estimates the
sound insulation properties of an assembly. It can be used to rankorder a series of different assemblies. Sound transmission loss (STL)
is a related measure.
The part that these parameters play in the actual analysis of sound
in a complex spatial environment involves many more considerations concerning the configuration and dimensions of the space
and distribution of materials used, as we will soon see.
general noise-Control approaches
Sounds reaching a point or hearer may be directly airborne from
a source within the space (e.g., from a machine or TV) or external
to it, airborne sounds that result from structure-borne vibrations,
or sounds from impacts. The intent of a sound-control strategy
varies very widely according to the design context and whether
the sounds are wanted or unwanted. In some cases the intent is
simply to reduce unwanted noise. Here the normal intent is to
reduce or eliminate noise in order for a space to have an overall
sound environment conducive to normal work and life activities.
Another intent could be to eliminate unwanted sounds because
of the vibrations that they might induce in delicate instruments
or manufacturing facilities. The latter is particularly important in
the context of this book—for example, vis-à-vis issues in manufacturing or characterizing nanomaterials. Conversely, in other
situations, an intent is to provide particular types of sound to
a space for achieving certain kinds of ambient conditions (the
notion of a totally quiet room is known to be antithetical to
Sound and Acoustical Environments