- Background noise (dB): for example, speech, scraping chairs, humming ventilation, traffic, machinery and equipment, sound from corridors, adjoining rooms, playgrounds. Increased background noise can have long-term negative effects, such as illness, fatigue, decreased productivity and efficiency. Therefore it is not recommended to attempt to obtain better daily speech privacy by increasing the ventilation noise, or use other sound masking systems. Another thing to bear in mind is that people are differently sensitive to sound and noise in general. Privacy and seclusion in open plan spaces can only be solved satisfactory by creating separate rooms for confidential discussions and work tasks needing higher concentration.
- Reverberation time: the time it takes for the sound pressure level to fall by 60 dB after the sound has been turned off. Measuring the reverberation time allows us to calculate the total sound absorption. The reverberation time varies according to the frequency.
- Sound insulation: the ability of a building element or building structure to reduce the sound transmission through it. The sound insulation is measured at different frequencies, normally 100-3150 Hz. Airborne sound insulation is expressed by a single value,Dn,f,w, Rw or R'w. Impact sound insulation is expressed by a single value Ln,w or L' n,w .
- Speech intelligibility: speech intelligibility is directly dependent on the level of background noise, reverberation time and the shape of the room. Different methods are used to evaluate speech intelligibility, the most common ones are RASTI, STI and %-Alcons.
- Room acoustic descriptors: as regards acoustic design, it is an advantage if different designs and procedures can be evaluated objectively. For this purpose, a number of measurable room acoustic descriptors have been defined. These descriptors can be used to formulate room acoustic specifications and to check the effect of different procedures. It would, of course, have been an advantage to have only one descriptor that works in all rooms. But hearing is multidimensional, so several descriptors are required.
- Reverberance: is linked to the speed at which sound energy disappears in a room. An unfurnished room with hard surfaces, such as a church, is perceived as being more reverberant than a well-furnished living room.
- Different acoustic room types: different types of room will create such different sound fields that this in itself requires different descriptors if a meaningful evaluation is to be made. The list of actual types of rooms can of course be made very long but, if we restrict ourselves to the most common ones, three different basic acoustic types can be identified.
- Choosing the right sound absorption: a material's sound absorbing properties are expressed by the sound absorption coefficient, α, (alpha), as a function of the frequency. α ranges from 0 (total reflection) to 1.00 (total absorption).
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