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A WORD ABOUT PRIVACY AND SECURITY
Everyone has been subjected to the acoustics of offices or dwellings with varying degrees of sound insulation performance. One has probably noticed that when the background noise level value is especially low, it becomes easier to hear what is going on next door. One may also have found that in flats during the daytime, one merely feels a presence in the next flat, while at nighttime it may be possible to recognize voices.
Actually, the ability to hear and understand what is being said in a room depends on the intelligibility of the message (more about that will be covered in Section 6.5). This means that on the receiving side it will mainly depend on the sound insulation between the rooms of interest and on the background noise level at the receiving end.
As a rule of thumb, the minimal value of the sound insulation DnTw needed to ensure privacy is 45 dB.
Privacy and Security
Privacy means that under normal circumstances (i.e., a normal spoken voice on the emitting side, and a background noise on the receiving side featuring the usual mechanical services and small chatter), one will not understand what is being said at a normal voice level on the emitting side. This will entail a minimum 45 dB sound insulation. But should the voice level be raised or the background noise level drop, this will be an entirely different matter.
If one wants to avoid such a situation, such contingencies have to be taken into account, and a sound insulation of 55 dB is a minimum.
But there is worse to come: Should somebody be intent on eavesdropping, there are quite a few tricks much more sophisticated than the old ear on the partition, that is, a vibration captor on the wall or floor (cf. Section 3.12.17), or even measuring the vibration of such a surface using laser equipment.
Water-filled ducts usually are poor acoustic conductive paths. But empty or air-filed ducts are another matter, and it may be necessary for security purposes to fit them with a vibrator too.
Masking sound comes handy in such circumstances. For simple cases, one will merely have a sound system adjusted so as to ensure that the background noise levels around the protected room are more than 10 dB above the transmitted speech sound. For more complicated cases, one will also introduce vibration generators on the walls and floors in order to disrupt any attempt at measuring the vibrations of the envelope of the room.
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