Unit 4 tranmisison of sound

Question 1

tranmission coefficient

Answer 1

When a sound wave strikes a solid surface there is very little transfer of acoustic energy and most of the wave energy is reflected back from the surface.

The fraction of sound energy transmitted by the wall is termed the transmission coefficient, τ

For building materials, τ can have values between 0.01 and 0.000001.

transmission coefficient varies with frequency

Question 2

Why is transmission coefficient is so small?

Answer 2

We need to consider the physical nature of waves in air compared with the elastic waves in the partition to understand why the transmission coefficient is so small

• Air has low density and elasticity with vibration forces being small and their displacement amplitude large.
• The high elastic modulus and density of the partition result in large vibration forces and small displacement amplitudes.
  
  • This creates a mismatch between the two vibration at the interface. The result is poor ‘coupling’ and low energy transfer.
    
    • This is termed an impedance mismatch

Question 3

Impedance of air

Answer 3

The characteristic impedance of air at room temperature is about 420 Pa·s/m

Question 4

impedance differences

Answer 4

Since sound speed and density of concrete are much higher, the result is an impedance of 8.16 MPa·s/m, about 19,000 times higher than that of air.

• This large impedance mismatch leads to a very low transmission coefficient for an air/concrete interface.
  
  • And therefore a more reflections

Question 5

Mass law derivation

Answer 5

Question 6

What happens at resonance frequency in a panel

Answer 6

At the resonance frequencies,

• the partition couples efficiently with the incident sound waves and the Sound Reduction Index is reduced and shows a dip in R these panel resonances usually occur at low frequencies

Question 7

What happens below the resonance frequency in a panel

Answer 7

At frequencies below the panel resonances, sound transmission is governed by the stiffness of the panel

Question 8

What happens Below and above the coincidence frequency

Answer 8

Below the coincidence frequency, the panel wave velocity is lower than the sound speed.

At and above the coincidence frequency, sound waves become very efficient at exciting flexural panel waves

Question 9

heavy partitions critical Frequency

Answer 9

For heavy partitions the critical Frequency is quite low, probably in the region where elasticity and inertial effects cancel.

• For plywood, plasterboard, window glazing etc., the Critical Frequency can be in the 1 to 4 kHz region
  
  • thus quite important for the design of partitions with high noise reduction

Question 10

The relationship between critical frequency and panel stiffness

Answer 10

The speed of bending waves in panels depends on their bending stiffness and frequency,

At any given frequency the wave speed is higher for a stiffer panel than for one with lower stiffness.

A stiffer panel means the critical frequency reduces as panel stiffness increases.

Question 11

Increasing the Sound Reduction Index of a simple partition

Increasing the Surface Density

Answer 11

The overall acoustical performance of many partitions is mass controlled.

Doubling the mass can give a 6 dB increase in R in the mass region.

To obtain a larger increase in R of 30 dB, in the mass controlled frequency range an increase of the surface density of 32 times is required.

• This is expensive way,
• reduces the available floor area
• would require a substantial increase in structural supports

The increase in thickness would increase stiffness and raise the fundamental resonant frequency, while reducing the critical frequency.

• significantly decreasing the frequency range over which the mass law applied.

Question 12

Increasing the Sound Reduction Index of a simple partition

Complex Partitions

Answer 12

Complex partitions can be used to increase the Sound Reduction Index of a structure. Complex partitions can be constructed in several ways:

1. Adding an absorbent layer to the partition surface.
   
   • Reduce local sound pressure over surface and reduce tranmisted sound
2. Adding a resilient layer to the partition surface.
   
   • Bitumen can be applied to add damping at resonant frequency and improve the dip and therefore increase R
3. Using multiple layer construction.
   
   • Two ply laminates with one material being led will increase surface desnity but add little stiffness and would increase critical frequency and extend mass law attenuation
4. Using double or triple wall construction with or without absorbent infill between the cavity
   
   • Increase surface density without any stiffness penalty
     
     • Two routes for sound to travel
       
       • Radiation from each later into airspace that exictes other layer
         
         • Reduce by adding absorption
         • Widening gap to lower res freq below important Hz
       • Energy transmitted by structural support framing partition linking the partition panels creating flanking path
         
         • Reduce by isolating each leaf on isolation pads and flexible edge sealant

Question 13

SI Prediction Methods

Answer 13

Predicting the sound insulation of multi-wall constructions is complicated and increases with each additional component

Several commercially available software prediction tools

There is also a large amount of data on different wall constructions available online.

However, most data and prediction methods relate to either laboratory conditions or well-constructed walls.

• real-world performance of a partition may be adversely affected by
  
  • the nature of the building construction around the partition
  • poor installation
  • service penetrations or building techniques.