NOISE CONTROL TECHNIQUES (ENCLOSURES and ABSORPTION)

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.

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)

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

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.

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.

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

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

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

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

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)

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)

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

Typical Enclosure (diesel engine powered generator)

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

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

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

ENCLOSURE PERFORMANCE Rigid Sealed 20 – 25 dB Enclosure Rigid Sealed 30 - 35 dB Enclosure plus Vibration Isolation Enclosure, 40 – 45 dB Absorption and Isolation Double Walled 60 – 80 dB Enclosure,

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.