Acoustics

1
Acoustics

• Absorption of Sound

2
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.

3
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.

4
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.

5
Mounting of Absorbents

• The absorption of porous material is greater with
  an airspace between the material and the wall.

6
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.

7
Glass Fiber

• Building Insulation In wood or steel stud
  single-frame 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.

8
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.

9
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.

10
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.

11
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.

12
Open-Cell Foams
Open-cell foams typically dont have as much
absorption as glass fiber materials.
13
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, low-frequency absorbers.

14
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
  ¼-wavelength.

15
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.

16
Low-Frequency Absorption

• Bass Traps are widely used in recording studios
  and control rooms to absorb low frequencies.
• Its designed with a depth of ¼-? at the
  frequency at which maximum absorption is desired.

17
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.

18
Resonant Panel Absorbers
19
Polycylindrical Absorbers

• The absorption coefficient of polycylindrical
  absorbers is greatest at low frequencies.
• Filling the cavity with mineral wool increases
  the absorption even more.

20
Poly Construction
21
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.

22
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.

23
Helmholtz Resonators
24
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

25
A Graphic Presentation

• The graph shows the resonant frequency with
  various perforation percentages and depths of air
  space.

26
The Effect of Depth

• The bandwidth of absorption is increased with
  greater depth of air space and mineral fiber.

27
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.

28
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.

29
Reverberation Time of Helmholtz Resonators

• The Q-factor (quality factor) describes the
  sharpness of tuning of the Helmholtz resonator.

30
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.

31
Taming Room Modes

• The diagram shows the low-frequency modal
  structure of the sound field of a small room
  before introduction of a tuned Helmholtz
  resonator absorber.

32
Taming Room Modes

• The same room after the introduction of the 47 Hz
  Helmholtz resonator absorber.

33
Helmholtz Resonator Design

• This resonator is made from a concrete-forming
  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.

34
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.

35
Modules

• This diagram shows the absorption coefficient of
  the four different modules.