Unit 2 - Absorption

Question 1

what happens When a sound wave strikes a solid surface

Answer 1

When a sound wave strikes a solid surface, to extents that depend on the surface material,

sound energy will

• pass through the surface (be transmitted)
• be reflected
• be converted into heat (absorbed)

Question 2

What are we interested in for acoustic insulation,

Answer 2

Question 3

Types of Absorbers

Answer 3

• Porous absorbers,
• Resonant absorbers (such as panel absorbers or Helmholtz resonators);
• A combination of both.

Question 4

Porous Absorbers

Answer 4

The absorption from porous materials is due to air movements penetrating into the body of the material, where energy is dissipated by viscous drag and,

• depends on the shape and the interconnections between the pores.
• The absorption co-efficient of porous materials can be estimated if the flow resistance and porosity of the materials are known.

The greatest absorption occurs when the particle velocity is highest.

• maximum air movement takes place 1/4 wavelength from the room surface

Question 5

type and characteristics

Resonant Absorbers

Answer 5

Resonant absorbers can be classified into panel (or membrane) absorbers and Helmholtz (or cavity) absorbers.

A characteristic of both types of resonant absorbers is that their absorption is selective, being highest at, or close to, a resonance frequency.

Question 6

A characteristic of both types of resonant absorbers

Answer 6

Both types of absorbers can be modelled as mass-on-a-spring systems,

• consist of three essential elements,
• mass (or inertia), stiffness (or resilience, provided by the spring) and some damping or frictional mechanism, which converts vibrational energy into heat

Question 7

natural freq of res absorbers

Answer 7

The system has a natural frequency which depends mainly on the mass and the spring stiffness.

If the system is forced to vibrate,

• the greatest amplitude of vibration occurs at resonance. This occurs when the driving or forcing frequency equals the natural frequency of the system
• the maximum absorption occurs at the resonant frequency

Question 8

Panel Absorbers

Answer 8

Panel absorbers consist of a flexible panel or membrane (e.g. plywood or plasterboard)

mounted over an air space.

frequency response will depend on the mass and stiffness of the panel, and the size of the air gap

Question 9

Helmholtz (Resonant) Absorbers

Answer 9

In a mass-on-a-spring representation of a Helmholtz resonator, the “mass” is the mass of the air in the neck of the cavity, which depends on the diameter and length of the cavity.

“stiffness” is the stiffness of the air in the cavity, which depends on the volume of the cavity

(the greater the volume, the lower the stiffness).

Question 10

Practical porous absorbers include:

Answer 10

• Mineral wool,
• Expanded Foams,
• Mineral fibre, and
• Carpet.

Question 11

issues with Modern building design utilising the thermal mass of concrete

Answer 11

Modern building design can utilise the thermal mass of concrete constructions to reduce the

heating and cooling loads on the buildings.

• For this to be effective the concrete soffits of the occupied spaces are often left exposed.
• As concrete is a poor acoustic absorber, additional absorption is require, often at the perimeter of the ceiling at high level on the wall

Question 12

Practical resonant absorbers include:

Answer 12

• Perforated plasterboard systems,

• Wide freq range performance
  
  • Can have wool behind and come in different perf patterns

• Perforated or slotted timber

• Timer planks mounted on battons with a void behind

• Hollow concrete blocks.

Question 13

Scattering Coefficien

Answer 13

The scattering coefficient represents the ratio of sound energy scattered in a non-specular

manner to the total reflected sound energy.

Scattering occurs when waves incident upon a surface are reflected back into the room in a non-specular manner (e.g. angle of incidence is not equal to the angle of reflection).

The coefficient has a range of values from 0 (no scattering, specular reflection) to 1 (all reflections are randomly scattered).

As with absorption, scattering and diffusion coefficient will vary with frequency and angle of incidence.

Question 14

Diffusion Co-efficient

Answer 14

The diffusion coefficient is a measure of the uniformity of the scattered sound. The purpose of this coefficient is enable comparisons between diffusers

Question 15

Quadratic Residue Diffuser (QRD)

Answer 15

A quadratic residue diffuser (QRD) is a surface feature which can be applied to a flat structure to achieve a dramatic increase in its diffusivity

Question 16

Binary Amplitude Diffsorber

Answer 16

Binary Amplitude Diffsorbers (BAD) use a surface interface known as a reflection phase grating.

To provide diffusion at higher frequencies, the size of the reflective and absorptive areas must be smaller than the wavelength.

Question 17

Shaped Panels

Answer 17

Many shaped wall panels, with curved or angular surfaces are marketed as diffuser panels

Question 18

Measurement of absorption coefficient

Answer 18

Question 19

when is it more relevant to measure the total absorption area

Answer 19

• Audience seating is normally treated as an area with a measurable coefficient,
• whereas performers, large pieces of furniture, etc. are normally treated by measuring their total absorption area

Question 20

The weighted absorption coefficient

Answer 20

• Absorption coeefs are plotted against a reference curve from BS EN ISO 11654
• the measured values are compared with a reference curve which is then shifted until the sum of the unfavourable deviations is no greater than 0.1.
• The frequency weighted value is then that corresponding to the shifted reference curve

Question 21

Optimum Reverberation Times

Answer 21

There is an optimum reverberation time value for every space, depending on its function (i.e. for performance of speech or for music)

suitable value for speech is about one second (varying from about 0.6 seconds for small rooms to 1.2 seconds or more for large spaces), with values for music being up to double these value

Question 22

Optimum Reverberation Times in atria

Answer 22

Large Atria can often have reverberation time greater than 3 seconds, yet occupants find the acoustics suitable.

Where RT is too high but aids in masking the use of localised areas of absorption, i.e. close to occupied areas of the atria has the greatest benefit

Question 23

What is an echo

Answer 23

An echo occurs when a pulse of sound (e.g. a syllable in speech) can be heard twice

Echoes can arise from a reflective surface on the back wall of a room

Question 24

treating back walls in auditoria for echoes

Answer 24

• treated with absorbing or diffusing material to suppress the echo, or
• shaped so as to disperse the sound in directions other than straight back towards the sound source

Question 25

Flutter echo and solution

Answer 25

Flutter echoes arise from parallel surfaces with low absorption coefficients. An impulsive sound such as a hand-clap is heard as a whole series of impulses

Solutions are to;

• avoid parallel surfaces
• use absorption on the side walls (assuming this is possible while still ensuring the total absorption of the room is sufficient)
• use diffusing structures on parallel wall

Question 26

Reasons for focussing

Answer 26

Focusing can result from concave surfaces and concentrate sound energy at one particular point so that a small group of listeners may find the sound uncomfortably

Question 27

Solutions to focussing

Answer 27

• avoid concave surfaces, particularly those exposed to a direct sound field
• make sure any unavoidable concave surfaces are lined with sound absorbing materials
• arrange for the focus of the curve to be well away from the audience (for example a domed ceiling well above the heads of the audience

Question 28

how best to direct sound to the audience

Answer 28

It is often desirable to direct a high proportion of the sound energy towards the audience, and this can be helped by;

• suitably angled reflectors behind, above and to the sides of the performers
• using reflective surfaces at the front part of the hall
• raking the seating to give the whole audience line of sight to the source

Question 29

benefit of balconies in auditoria

Answer 29

Balconies reduce the distance from the performers to the audience but shading effects can occur for the section of the audience beneath the balcony