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Room Acoustics - The Problem

Room Acoustics - Research Room Acoustics - Solutions Room Acoustics - How it Worked Room Acoustics - Evaluation Calculation Table for Different Absorption Coefficients/Sabines
Instrument Acoustics- The Problem Instrument Acoustics -  Research Instrument Acoustics - Solutions Instrument Acoustics - Practical Designs Instrument Acoustics - How it Worked Instrument Acoustics - Evaluation

Dealing with Acoustic Room Treatment (Part 1)

The Problem

We began our task by entering the studio and clapping. The clap sound took well over 1 second to die away. This is because the original sound does not exist in isolation but is reflected by all of the surfaces around it. These reflections take a longer time to reach the ear because of the greater distance they have travelled. This reflections as well as being heard by us are also themselves reflected from all the surrounding surfaces. These reflections take even longer to be heard. Also, because a certain amount of sound is absorbed by each of the surfaces it reflects from these reflected sounds get quieter with each bounce. The reflections will continue to bounce until all of the sound energy has been absorbed. The time taken for these sounds to die away to nothing is known as the reverberation time.

The sound appeared to be predominantly high frequencies by the end. This is because different surfaces, depending on the materials from which they are constructed and their size and weight, absorb different frequencies in different amounts. We took this to mean that the low frequencies in the room were being absorbed better than the higher ones

We set about measuring the room. We entered the room dimensions and dimensions of various surfaces within the room made from differing materials into a spreadsheet which we designed (see over).

The room itself if 7.2 metres long, 9.03 metres wide and 3.06 metres high. The wall at one end is solid brick with a plaster covering. Another side is an external wall of a similar make up but has 4 large windows which make up roughly two thirds of itís area. A third wall is made from plasterboard with a large single glass pane. The final wall is again plaster covered brick and contains three small windows near roof level, the door and a white board. The ceiling is broken up, approximately a third of the way across, by a large beam running from one side to the other. The floor is flat linoleum. There are also a number of desks containing computers along one wall and a number of TV cameras and monitors scattered throughout the room. The ceiling also has large numbers of studio lights attached to metal frames.

                           Reverberation time = 0.16 x volume

Using the formula                                    Total Sabines* we calculated the reverberation time at different frequencies. To our surprise the high frequencies were actually shorter than the low ones. This we put down to the objects which had not been measured and taken into account (desks, computers, TV cameras, etc.). It could also be because handclaps contain little low frequency energy to start with.

* A Sabine is a unit of total absorption equivalent to one metre square of open window (an open window will reflect no sound). Anything which gives less than total 100% absorption will have a Sabine rating of less than 1. See calculation table for different absorption coefficients/sabines

Most of the surfaces in the studio are parallel which means standing waves could be a problem. A standing wave is the frequency at which a wavelength is equal to the distance between 2 surfaces. At this frequency there is potential for the signal to be bounced back and for the between these two surfaces and to be emphasised more than other frequencies.

However, realigning the walls of the studio is beyond our remit and while such standing waves can be tackled using sound diffusers it was decided these waves were not a significant problem.

Because of the nature of its use such long reverberation times are not acceptable in the TV studio. While sound diffusers may have been useful in some respects they would not reduce the reverberation time significantly. It was decided that absorbing the sound was more important. The greater the amount of absorption at each bounce, the shorter the reverberation time. We therefore undertook the building of sound absorbers.

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