Acoustics
Absorption of Sound
Sound Energy
The law of the conservation of energy states
that energy can neither be created or destroyed,
but it can be changed from one form to another.
Sound is the vibratory energy of air particles
and it can be dissipated in the form of heat.
It takes the sound energy of a million people
talking to brew a cup of tea.
Dissipation of Sound Energy
S = sound wave
A, B, & C =
reflections from
material boundaries
E - K = absorption
from heat loss
D = paths of
refraction
Refraction changes
the direction of
travel of a sound
wave by differences
in the velocity of
propagation.
Evaluation of Sound Absorption
The absorption coefficient is a measure of the efficiency
of a surface or material in absorbing sound.
If 55% of the incident sound energy is absorbed, the
absorption coefficient is said to be 0.55.
One square foot of this material gives 0.55 absorption
units (sabins).
An open window is considered a perfect absorber
because sound passing through it never returns to the
room. It would have an absorption coefficient of 1.0.
Ten square feet of open window would give 10 sabins of
absorbance.
Mounting of Absorbents
The absorption of porous material is greater with an
airspace between the material and the wall.
Fibrous Materials
Porous absorptive materials most commonly
used as sound absorbers are usually fuzzy,
fibrous materials in the form of boards, foams,
fabrics, carpets, cushions, etc.
Great quantities of glass fiber materials are used
in the treatment of recording studios, control
rooms, and public gathering spaces.
Glass Fiber
Building Insulation: In wood or steel stud singleframe walls, double walls, and staggered-stud
walls thermal insulation batts are commonly
used. Such material is often identified as R-11,
R-19, or other such numbers.
Boards: used in the acoustical treatment of
audio rooms, in the form of semi-rigid boards of
greater density than building insulation. Two
common types are Owens-Corning Type 703
Fiberglass and Johns-Manville 1000 Series
Spin-Glass, both having 3 lb/cu ft density.
Acoustical Tile
One of the
problems of using
acoustical tile in
critical situations
is that absorption
coefficients are
rarely available.
The diagram
shows average
absorption
coefficients of
eight different
brands.
Effect of Thickness of Absorbent
It is logical to expect
greater sound
absorption from thicker
materials, but this logic
holds primarily for the
lower frequencies.
A 4” thickness of glass
fiber material of 3 lb/cu
ft density has
essentially perfect
absorption over the
125 Hz to 4 kHz region.
Effect of Airspace Behind Absorbent
Spacing 1-inch
material out 3 inches
makes its absorption
approach that of the
2-inch material
mounted directly on
the wall.
Effect of Density of Absorbent
There is relatively
little difference in
absorbance between
the flimsy thermal
insulation and the
rigid boards used
widely in the industry.
Open-Cell Foams
Open-cell foams typically don’t have
as much absorption as glass fiber
materials.
Carpet as Sound Absorber
The high absorbance of
carpet is only at the
higher audio frequencies.
This is a major problem
encountered in many
acoustical treatment
jobs.
The unbalanced
absorption of the carpet
can be compensated in
other ways, principally
with resonant-type, lowfrequency absorbers.
Drapes as Sound Absorbers
The distance a drape is
hung from a reflecting
surface can have a great
effect on its absorption
efficiency.
Maximum absorption for
any frequency is
achieved at distances of
¼-wavelength and odd
multiples of ¼wavelength.
Absorption of Sound in Air
A church seating 2,000 has a volume of about
500,000 cu ft. Using the above chart, we can
calculate that the absorption at 4 kHz is about
3600 sabins (500 x 7.2 = 3600). This is
equivalent to 3600 sq ft of perfect absorber.
This could be 20% to 25% of the total absorption
in the space.
Low-Frequency Absorption
Bass Traps are
widely used in
recording studios
and control rooms
to absorb low
frequencies.
It’s designed with
a depth of ¼-λ at
the frequency at
which maximum
absorption is
desired.
Diaphragmatic Absorbers
Diaphragmatic
(Resonant Panel)
Absorbers utilize a
diaphragm vibrating in
response to sound and
absorb some of that
sound by frictional
heat losses in the
fibers as it flexes.
A piece of plywood
mounted on 2-by-4s is
one example.
The diagram shows
absorption coefficients
for three different
panels.
Resonant Panel Absorbers
Polycylindrical Absorbers
The absorption
coefficient of
polycylindrical
absorbers is greatest
at low frequencies.
Filling the cavity with
mineral wool
increases the
absorption even
more.
Poly Construction
Membrane Absorbers
Building insulation
commonly comes with a
kraft paper backing.
If the paper side is facing
the room, the high
frequency absorption of
the insulation is reduced
considerably.
This can be used to your
advantage if you are
looking for more
absorption in the 250 –
500 Hz range.
Helmholtz Resonators
The Helmholtz type of resonator is widely used
to achieve adequate absorption at lower
frequencies.
Sound is absorbed at the resonant frequency
and at nearby frequencies.
Inserting an absorbent material in the mouth or
neck increases the bandwidth of absorption.
Sound impinging on a Helmholtz resonator that
is not absorbed is reradiated in a hemispherical
pattern, or diffused, which is a very desirable
thing in a studio or listening room.
Helmholtz Resonators
Perforated Panel Absorbers
Perforated hardboard or plywood panels spaced from the
wall constitute a resonant type of sound absorber.
The frequency of resonance of perforated panel
absorbers backed by a subdivided air space is given
approximately by:
A Graphic Presentation
The graph shows the
resonant frequency
with various
perforation
percentages and
depths of air space.
The Effect of Depth
The bandwidth of absorption
is increased with greater
depth of air space and
mineral fiber.
Slat Absorbers
Another type of resonant absorber is that utilizing closely
spaced slats over a cavity.
The narrower the slots and the deeper the cavity, the lower
the frequency of maximum absorption.
Placement of Materials
The application of sound-absorbing materials in
random patches is an important contribution to
diffusion.
If several types of absorbers are used, it is
desirable to place some of each type on ends,
sides, and ceiling so that all three axial modes
will come under their influence.
Material applied to the lower portions of high
walls can be as much as twice as effective as
the same material placed elsewhere.
Untreated surfaces should never face each
other.
Reverberation Time of Helmholtz Resonators
The Q-factor (quality
factor) describes the
sharpness of tuning of
the Helmholtz
resonator.
Q-Factors
Absorbers
made of wood
with glass fiber
to broaden the
absorption
curve have Qs
so low that their
sound dies
away much
faster than the
studio or
listening room.
Taming Room Modes
The diagram
shows the lowfrequency
modal structure
of the sound
field of a small
room before
introduction of
a tuned
Helmholtz
resonator
absorber.
Taming Room Modes
The same
room after the
introduction of
the 47 Hz
Helmholtz
resonator
absorber.
Helmholtz Resonator Design
This resonator
is made from a
concreteforming tube.
Laminated
wood covers
are tightly
fitted into both
ends. The
length of PVC
pipe is varied
to tune the
resonator to a
specific
frequency.
An absorbent
partially fills
the resonator.
Modules
The BBC has
pioneered a
modular
approach to the
acoustical
treatment of
their numerous
small voice
studios.
All modules can
be made to
appear
identical, but
the similarity is
only skin deep.
Modules
This diagram
shows the
absorption
coefficient of
the four
different
modules.