1. NOISE CONTROL TECHNIQUES (ENCLOSURES and ABSORPTION)

2. Noise Enclosures
The three requirements for an effective noise enclosure are:-
Insulation
Prevent sound energy from getting through the enclosure walls (transmission loss, R)
Absorption
Convert sound energy into heat energy.
Isolation
Prevent vibration (which can be re-radiated as sound) from being transmitted.

3. INSULATION
An enclosure must be constructed from material with sufficient mass to prevent sound transmission and reflect the sound energy back.
Sheet steel
Timber
Plasterboard
Masonry
Transmission loss, R (octave bands)

4. ABSORPTION
Insulation alone is ineffective since the sound intensity inside the enclosure will continue to increase until it eventually breaks out.
Noise level inside enclosure = 125dB
Enclosure transmission loss = 30 dB
Noise level close to machine without an enclosure = 95dB
Noise level close to enclosure = 95dB

5. LINING MATERIAL
Any absorptive material will reduce the internal sound intensity.
Mineral fibre, glass fibre, acoustic foam, fibre board.
Most of these materials are mechanically weak and usually have a facing of perforated steel or vinyl, sometimes backed with,
Tissue, foil, or Melenex (thin plastic)
to prevent the break-up of
the fibrous infill and/or
contamination by oil etc.

6. ISOLATION
Small amounts of vibration energy will become important if the enclosure is effective.
A 30 dB reduction in noise level means that only 1/1000 of the energy is transmitted through the walls of the enclosure
If only a small amount escapes by vibration of the floor the effectiveness will be reduced.

7. VIBRATION ISOLATOR
Steel springs, rubber, cork, etc. prevent vibration from being transmitted to the floor (we will cover vibration isolation later)

8. ENCLOSURE INSERTION LOSS
Reverberant level in a room due to a noise source with sound power level, Lw
Lp = Lw + 10lg(4/A)
where A is the absorption area in the room
Noise Source Sound Power, Lw

9. Reverberant sound level inside an enclosure containing a noise source, Lw
Lp = Lw + 10lg (4/Aencl)
where Aencl is tha absorption area inside the enclosure

10. Reverberant sound level inside a room due to noise in another room (enclosure)
L2 = L1 – R + 10 lg(Se/A)
where R is transmission loss of enclosure panels
Se is area of panels
A is absorption area in the room L1

11. EQUATING Lp in the room without an enclosure
The insertion loss, IL is the difference between the sound level in the room before and after enclosure is installed
Lp in the room without an enclosure
Lp (Before) = Lw + 10lg(4/A)
Lp in the room with an enclosure
Lp (After) = L1 – R + 10lg(Se/A)
L1 is the level in the enclosure which is,
Lp (inside enclosure) = Lw + 10lg(4/ Aencl)

12. Insertion Loss, IL = R – 10lg(Se/ Aencl)
Therefore, by substitution
Lp (After) = Lw + 10lg(4/ Aencl) – R + 10lg(Se/A)
IL = Lw + 10lg(4/A) – [Lw + 10lg(4/ Aencl) – R + 10lg(Se/A)]
After cancelling and tidying we get,
Insertion Loss, IL = R – 10lg(Se/ Aencl)

13. Effect of Gaps
Any gaps at panel joints or access doors will reduce the overall Insertion Loss of the enclosure.
Note that even small gaps have a large effect

14. Typical Enclosure (diesel engine powered generator)

15. DUCTS AND PIPES
Pipes and ducts carrying turbulent flow or vibration along their walls can be lagged. (enclosed?)
The vinyl is loaded (heavy)
The de-coupler is a resilient layer
Performance not as good as enclosure
Or, if more attenuation
is required they can
be enclosed.
Duct and Enclosure must
Be on anti-vibration hangers

16. REDUCING VIBRATION OF PANELS
Increase the mass (this lowers the resonant frequency and decreases the amplitude of vibration)
Make panel profiled (increases stiffness and hence reduces amplitude (it also increases the resonant freq.))
Free layer damping
(bitumen impregnated card)
stretching only
Constrained Layer
(Metal / damping / metal)
Shearing is much more effective

17. VIBRATION ISOLATION Hangers for pipes and ducts
Pipe supports (with rubber insert)
Spring isolator for a machine

18. ENCLOSURE PERFORMANCE
Rigid Sealed 20 – 25 dB
Enclosure
Rigid Sealed dB
Enclosure plus
Vibration Isolation
Enclosure, 40 – 45 dB
Absorption and
Isolation
Double Walled 60 – 80 dB
Enclosure,

19. ROOM ABSORPTION
In large reflective spaces (factories) it is possible to reduce the reverberant noise level by increasing the amount of sound absorption.
Lp(rev) = Lw + 10lg(4/A)
Where A is the absorption area in the room
This give a reduction in the reverberant level of 6 dB / doubling of absorption area.