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1.8 ENCLOSURES, BARRIERS AND WALLS
À pPrevious notes concentrated on sound from local sources
But, another class of problems involve walls, and barriers between sound sources, and listeners.
The effects of walls are measured as a Transmission Loss (TL)
Barrier Transmission Loss is a function of frequency
Wall stiffness region - hard to predict exactly, the stiffer the wall, the greater the TL. Resonances in this region are a function of structure.
Coincidence Region - (not common) Sound waves hit wall at angle - effectively changing frequency. Because of the lowering of the high frequency to a lower value, the wall can now be excited.
A table of TL values for walls are, [source ????????]
Another list of transmission loss figures is given below for typical building materials, [source ???????]
A table of typical NR values for walls are, [source ??????????]
The Sound Transmission Class (STC) is the Transmission Loss (TL) at 500Hz
1.8.1 Noise Reduction Through Walls
(|We can use the transmission loss of the wall to estimate sounds heard in adjoining rooms.
1.8.2 Walls with composite layers
(|Sometimes wall are built up of layers to improve sound absorption
Most walls have areas covered by different materials. When this is the case we can combine the coefficients
Holes in the wall will increase the sound that is transmitted, but these are often needed for wires, airways, etc. Offsetting holes can help maintain sound isolation properties.
Another problem with sound isolation is that other paths may be ignored. For example a barrier may be built between two rooms, but raised floor is ignored, and sound continues to travel through this route. Other well known sound paths are ventilation ducts.
1.8.3 Enclosures
(|Hoods are a common method of dampening noises
The NR and TL are for reduction of the new reverberant field Lp1, but we are concerned with the reduction of Lp0. This is the Insertion Loss (IL).
The noise reduction for hoods is very good at higher frequencies Isolation pads can be used to reduce the noise at lower frequencies. When used together the total noise levels will be decreased.
1.8.4 Acoustic Barriers
(|Sometimes barriers (or free standing walls) can be placed in the direct path of the sound.
1.8.5 Panel Absorbers
(|In effect we can increase absorption in a room by adding suspended additional surface area.
suspended panels can flex, best absorption characteristics at low frequencies. typical values for alpha are .2 to .5 for 125Hz.
These can be tuned for specific frequencies
Locating these absorbers next to a wall with absorbing material will increase high frequency absorption.
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