Reverberation Time Reverberation time is defined as the time required for a steady state sound pressure level in an enclosed space to decay by 60 dB, measured from the moment the sound source
is switched off. By averaging multiple decays within the space it can be accurately measured and is one of the most important criteria for rooms involving speech and music.
We often get asked about how to reduce the noise levels and echo in restaurants, offices, village halls etc. so speech can be heard more clearly without having to shout.
It is possible to calculate likely reverberation times in spaces given the volume and surface types and although we sometimes to this for clients its much better to actually
measure the space in the first place as this allows far more accurate calculations to be carried out to formulate a solution. Calculation without measurement can often produce results that are
often not truly representative of the space especially when only simple formula are used.
Estimates from simple calculations are subject to the absorbent materials being placed evenly within the space and also assume a fully diffuse field within the room.
Generally this is not the case as windows, doors, wall hangins, electric conduits, lights etc prevent even spacing of absorbent materials and in these situations the ceiling is often the only
space that can be treated. Calculations like the Sabine or Eyring formulas break down under these conditions and do not produce accurate results.
If you have a problem and want it resolved you need to know what your dealing with. Its not cost effective to simple pop in a few acoustic panels only to find that the problem still exists.
We gain many customers who ask us to provide a solution after they have tried other companies solutions that simply have not worked and it is usually because the predictions were way
out or, the acoustic material was placed in the wrong places, there was not enough coverage or the panels used were not efficient at the frequencies that were causing the problem in the first place.
Recently we were asked to review a proposal put forward to the customer for reducing the level of noise within an industrial applications. The costs of adding acoustic material ran into tens
of thousands of pounds. Our more complex calculations demonstrated that if this proposal was implemented the customer would only have gained a reduction of approx. 2dB so the money would
have been wasted and the problem would still have been present. We formed an alternative solution to the problem, that cost considerably less, and reduced the problem by 12dB.
It is essential for architects and building designers to get the reverberation time of a space suitable for the intended purpose.
If the reverberation time is too long, the communication
of speech in spaces such as meeting rooms can often be hard to understand.
If it is too short then vocies may have to be raised or electo-acoustic amplification may be required.
The reverberation time of a space is essentially determined by its volume and the types of absorbent or reflective surfaces within that space.
Hard and reflective surfaces such as concrete, glass and timber floors lead to long reverberation times that may create an unpleasant, 'echoey' acoustic environment.
For this type of acoustic solution to be effective it is very important that the optimum amount of acoustic material is applied to the most appropriate areas.
This can require a delicate balancing act. If too little acoustic material is installed, or if it is installed in the
wrong place, then the reflected noise will still be too high. This would lead to the background noise remaining too high and communication will still be difficult.
If too much acoustic absorbent
material is installed there will be too little reflected sound and the teacher will need to shout to be heard at the back of the class room. So, obtaining specialist
advice from an independent expert can be invaluable in ensuring a satisfactory acoustic solution. |
