We know the situation where the source of noise and the recipients are in the same room. In addition to this, however, there is another class of enclosure problem which is frequently encountered in practice.
In this case, the noisy machinery is in one room, and the people affected by noise are in adjacent rooms, or even outside the building. The following empirical relations can be used to estimate the sound pressure level in the receiving space in various cases. All the sound pressure levels in these relations are expressed in db.
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(a) Room to room:
L2 = L1 – Rav + 10 log10 (Sp / A2). … (9)
(b) Room to outside:
L2 = L1-R-6,…(10)
(observer near wall)
L2= L1– Rav + 10 log10 SP – 20 log10 r – 14. … (11)
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(observer some distance away from wall)
In Eq (9) – (11), the symbols used are as follows:
L2 = Sound pressure level (in dB) at the position of observer (either in the adjacent room or in the space outside);
L1= reverberant sound pressure level (in dB) in the source room; R = average sound reduction index (in dB) of the intervening partition;
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Sp = area of partition (in m2);
A2 = total absorption in the receiving room (in m units); and
r = distance of an outside observer (in m) from the wall of the source room.
It is evident from Eq. (9) – (11) that these equations are different from the original relation for noise reduction in the same room due to enclosure (Eq (1)) In spite of this difference, the requirement to maximise the sound reduction index still remains. Moreover, both in the case of room to room transmission and that of outside observer some distance away from the wall, average sound reduction index must be used as before.
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On the other hand, noise reaching an outside observer immediately next to the source room will be determined only by the sound reduction index of the material immediately next to the observation position.
It may be safely assumed that the partition are SP, absorption in the receiving room, and the distance r from the outside wall are all fixed by architectural or operational considerations and may, therefore, be regarded as constant Thus the only parameter left to the acoustic engineer to control noise in this case is the sound pressure level inside the source room.
To reduce this, the following options are available:
(1) Reducing the strength of the source machine;
(2) Providing a close-fitting enclosure over the machine; and
(3) Providing the maximum amount of acoustic absorption inside the source room.
Design of Acoustic Rooms:
Designing the walls of rooms within buildings to obtain maximum sound reduction index offers much more flexibility than does the designing of close-fitting enclosures for noisy machines.
For example, no one expects rooms to be demountable in the sense that prefabricated enclosures are. It follows, therefore, that the designer can take advantage of heavier masonry construction offering a much higher range of attenuation than lighter materials.
The mass law can be taken to apply for single-leaf masonry or concrete partitions, and for the construction of conventional cavities. In this connection, ceiling and floor slabs may be regarded as “partitions” for the application of mass law.
It is an obvious implication of the mass law in the present context that the use of light-weight blocks for internal lining (or for the complete wall) will produce less noise insulation than the use of more dense brick or concrete.
The conventional double-leaf construction may not give sufficiently large sound reduction index in some situations. When this is the case, improvements of 5-10 dB across the frequency spectrum can be achieved by incorporating a degree of mechanical isolation between the leaves.
In masonry partitions, this involves breaking the foundation slab between the two leaves, building each on a rubber pad and using flexible tiles between leaves. In the case of lighter-weight partitions, one or both skins may be fixed to studding using spring clip systems.
Ceiling and floor slabs present some problem, since the range of double- leaf construction as well as the overall weight are limited. In this case, the ideal solution is a “floating floor” (an internal floor slab or screed, separated from the structural floor by a layer of resilient material, forming an impedance barrier to transmission of acoustic energy to the slab forming the ceiling of the room below).
In some cases (for example, with an existing installation), it may not be practicable to lay a floating floor. The only solution then is to provide a resiliently suspended mass barrier as a false ceiling in the room below. However, a heavy ceiling and a good seal all round are essential in this case. Ordinary suspended lightweight ceilings hardly give any improvement.
Using the methods mentioned above, it is possible to achieve average sound reduction indices of up to 55 dB, in the frequency range 1-3.2 kHz, for all the partitions surrounding the source room.
As in the case of a close fitting enclosure, sound absorption inside the source room must be as large as possible for maximum overall performance of the room as an enclosure. In the case of a room, however, there is more flexibility with the materials that can be used, and also with the methods of their fixing.
If sound attenuations of 60 dB and upwards are to be achieved, a “double- room” construction has to be used, and the designer has to provide the twin essentials of complete mechanical isolation and maximum possible airspace between these rooms.
The starting point for this is an internal floor slab separated from the structural floor by an impedance barrier. On this slab are built up the internal walls, and the inner room is completed by a separately cast roof necessary to lay a thick absorptive slab in the airspace (between the rooms) all round to prevent resonance of the cavity.
Double acoustic windows will be required, and two acoustic doors (one in each wall leaf) will also be necessary.
Services in the case of a double-room construction should be provided with a flexible section across the airspace. With such construction, it should be possible to reach an overall sound reduction index of 70 dB or more.
Acoustic Refuge:
The case where the source of noise is enclosed to prevent the acoustic energy radiated by it reaching the observer. Under certain circumstances, however, enclosure of the noisy machine in a separate room or in its own close-fitting enclosure may not be feasible operationally and/or financially.
If the operators have to spend appreciable periods of time in the same room where the source of noise is located, they have to be shielded from the noise. One way to do this is to provide the operators with individual hearing protection. These devices, however, are uncomfortable to wear for longer periods of time.
An alternative technical solution is to provide the operators with their own enclosure (or refuge). If control panels, workloads, desks, etc., are located inside this refuge, it should be possible for an operator to spend most of his time subjected to a noise level considerably less than that in the source room. When the operator has to leave the refuge, he may use some form of personal ear protector.
From the point of view of design, the only difference between the acoustic refuge and a machine enclosure is that in the case of a refuge, noise is transmitted into it, while it is transmitted out of the enclosure in the latter case.
One significant difference between the designs of an acoustic refuge and a machine enclosure is that a refuge will almost always require a large area of glazing. Special care has, therefore, to be taken with the acoustic design of the panels (taking into account the generally lower sound reduction index of glass). This may well require an especially thick panel.