Reverberation Time Measurement
Reverberation Time Measurements and Analysis BS EN ISO 3382
It is essential for architects and building designers to ensure that the reverberation time of a space is suitable for the intended purpose.
Reverberation time is defined as the time required for a steady state sound pressure level in an enclosed space to decay by 60dB, 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 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 do this for clients it is 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 produce results that are often not truly representative of the space especially when only simple formulas 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 hangings, 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 you are dealing with. It’s not cost effective to simply 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 off, or the acoustic material was placed in the wrong places, or 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 application.
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 know if the reverberation time of a space is 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 voices may have to be raised or electro-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 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.
Measuring the reverberation time of a room requires the measurement of the decay of a signal of suitable amplitude above the background noise.
This is usually done using an omni-directional dodecahedron loudspeaker although we sometimes use swept sine or MLS signals.
Our dodecahedron loudspeaker meets the following Standards:
BS EN ISO 140-3 Annex C (Laboratory measurements)
BS EN ISO 140-4 Annex A (Field measurements) and the
BS EN ISO 3382 Annex A (Reverberation Time measurements).
New requirements contained in Building Bulletin 93 Acoustic conditions in schools and Approved Document E of the Building Regulations demand the calculation of reverberation time to ensure stringent limits are met.
For further information about measuring or predicting reverberation time then please do not hesitate to contact us, either by email or phone 01730 269572.
Custom Audio Designs Ltd has the equipment and experience to measure and predict reverberation times in schools, colleges, offices and meeting rooms as well as auditoriums and cinemas and other large internal spaces.
The reverberation time is the major factor in determining the internal acoustic performance of a room or space. In some spaces where intelligible speech is mandatory the specification of the internal room acoustics can be critical.
It should be noted that good acoustic design from the outset should always be undertaken. Trying to resolve reverberation and speech intelligibility issues after a building is completed can be very expensive and is sometimes not practical.
We have had to deal with many brand new halls and rooms that, once completed, were found to be completely unsuitable for their intended use due to poor, or no, acoustic design.