C hapter 4 Material Classes, structure, and properties
142
comes from. If it is generated within the room, one seeks to absorb
the sound. If it is airborne and comes from outside, one seeks to
insulate the space to keep the sound out. And if it is transmitted
through the frame of the structure itself, one seeks to isolate the
structure from the source of vibration. Cellular, porous solids are
good for absorbing sound, and, in combination with other materials, they can help in isolation. They are very poor at providing
insulation.
Soft, porous materials absorb incident, airborne sound waves, converting them into heat. Sound power, even for very loud noise, is
small, so the temperature rise is negligible. Porous or highly flexible materials—for example, low-density polymeric foams such as
plaster and fiberglass—absorb well; so do woven polymers in the
form of carpets and curtains. Several mechanisms of absorption are
at work here. First, there is the viscous loss as air is pumped into
and out of the open, porous structures; sealing the surface (with
a paint film, for instance) greatly reduces the absorption. Second,
there is the intrinsic damping in the material. It measures the fractional loss of energy of a wave, per cycle, as it propagates within
the material itself. Intrinsic damping in most metals and ceramics
is low, typically 10
−6 to 10
−4 ; in polymers and foams made from
them it is high, in the range of 0.01 to 0.2. The proportion of sound
absorbed by a surface is called the sound-absorption coefficient. A
material with a coefficient of 0.8 absorbs 80% of the sound that is
incident on it; material of coefficient 0.03 absorbs only 3% of the
sound, reflecting 97%.
Sound insulation requires materials of a completely different sort.
The degree of insulation is proportional to the mass of the wall,
floor, or roof through which sound has to pass. This is known as
the mass law: the heavier the material, the better it insulates. The
lightweight walls of modern buildings, designed for good thermal
insulation, do not in general provide good sound insulation. Foam
plaster or fiberglass, for obvious reasons, are not a good choice,
either; instead, the practice is to add mass to the wall or floor with
an extra layer of brick, concrete, or a lead cladding.
Impact noise is transmitted directly into the structure or fabric of
a building. Elastic materials, particularly a steel frame if there is
one, transmit vibrations throughout the building. Unlike airborne
sound, noise of this sort is not attenuated by additional mass. Since
it is transmitted by the continuous solid part of the structure, it
can be reduced by interrupting the sound path by floating the floor
or building foundation on a resilient material. Here low modulus
142
comes from. If it is generated within the room, one seeks to absorb
the sound. If it is airborne and comes from outside, one seeks to
insulate the space to keep the sound out. And if it is transmitted
through the frame of the structure itself, one seeks to isolate the
structure from the source of vibration. Cellular, porous solids are
good for absorbing sound, and, in combination with other materials, they can help in isolation. They are very poor at providing
insulation.
Soft, porous materials absorb incident, airborne sound waves, converting them into heat. Sound power, even for very loud noise, is
small, so the temperature rise is negligible. Porous or highly flexible materials—for example, low-density polymeric foams such as
plaster and fiberglass—absorb well; so do woven polymers in the
form of carpets and curtains. Several mechanisms of absorption are
at work here. First, there is the viscous loss as air is pumped into
and out of the open, porous structures; sealing the surface (with
a paint film, for instance) greatly reduces the absorption. Second,
there is the intrinsic damping in the material. It measures the fractional loss of energy of a wave, per cycle, as it propagates within
the material itself. Intrinsic damping in most metals and ceramics
is low, typically 10
−6 to 10
−4 ; in polymers and foams made from
them it is high, in the range of 0.01 to 0.2. The proportion of sound
absorbed by a surface is called the sound-absorption coefficient. A
material with a coefficient of 0.8 absorbs 80% of the sound that is
incident on it; material of coefficient 0.03 absorbs only 3% of the
sound, reflecting 97%.
Sound insulation requires materials of a completely different sort.
The degree of insulation is proportional to the mass of the wall,
floor, or roof through which sound has to pass. This is known as
the mass law: the heavier the material, the better it insulates. The
lightweight walls of modern buildings, designed for good thermal
insulation, do not in general provide good sound insulation. Foam
plaster or fiberglass, for obvious reasons, are not a good choice,
either; instead, the practice is to add mass to the wall or floor with
an extra layer of brick, concrete, or a lead cladding.
Impact noise is transmitted directly into the structure or fabric of
a building. Elastic materials, particularly a steel frame if there is
one, transmit vibrations throughout the building. Unlike airborne
sound, noise of this sort is not attenuated by additional mass. Since
it is transmitted by the continuous solid part of the structure, it
can be reduced by interrupting the sound path by floating the floor
or building foundation on a resilient material. Here low modulus