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Evaluation of the Acoustic Properties a Performance Area.

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Evaluation of the acoustic properties a performance area.

Sound is described as a "mechanical compression and rarefaction or a longitudinal displacement wave that propagates through a medium (solid, liquid or gas)" (source Put into simpler terms this means that sound is caused when an object vibrates, causing changes in pressure within a medium. For example when a speaker cone vibrates in a room the movement of the cone causes the air particles close to it to move in the same way. These moving air particles now bump into the air molecules next to them transferring energy and making them move in correspondence with the speaker cone, this process repeats again and again and the sound wave moves further away from the source. The further away the sound wave moves the less energy there is to be transferred between air particles due to absorption or the dissipation of energy hence the sound is quieter the further away the listener is from the original sound source.

Sound is frequently seen as having two parts, frequency and amplitude. Frequency refers to the speed at which the air molecules vibrate and is directly linked to the pitch of a sound and is measured in hertz. Where as amplitude refers to the amount that the particles move and is related to the volume of the sound. The graph below shows two full cycles of the waveform of a simple sound. The frequency and amplitude can be seen clearly. The frequency is shown by the length of time taken for the sound wave to complete one full cycle, it takes 5msec for this waveform to complete a cycle therefore this will be repeated 200 times in one second giving this sound wave a frequency of 200hertz. The amplitude is represented by the height of the waveform (graph taken from lecture notes on blackboard).

Many acoustic factors have to be taken into account when designing a performance space three of these are reverberation, the production of standing waves and the fact that bass frequencies need at least one quarter of their cycle to fully form these are all related to the dimensions of the area. Reverberation is caused when sound is created within an enclosed space, the sound waves that are produced reflect off any reflective surfaces e.g. walls, ceilings, floors etc. these reflections create echoes of the original sound which can be a blessing or a curse in different circumstances. After the original sound has stopped the echoes continue for a period of time afterwards gradually decreasing in amplitude until they are no longer audible. The time that the sound takes to decrease by 60dB of the original sound pressure level is known as the reverberation time or RT60.

Standing waves occur when one cycle of a sound wave is the same length as the distance between two parallel walls in a room. The sound wave bounces off one wall and is reflected back, but because it has the same length as the room the reflected wave comes back on itself thus reinforcing the original wave coming from the sound source. This can lead to problems as the frequencies at which these will occur will become more prominent than the rest of the frequencies present, this can ruin the sound of an acoustic space because the room will resonate around this specific frequency and the harmonics related to it, however standing waves can be combated by having the parallel walls and the ceiling and floor offset by a few degrees.

The formulae for room modes are used to calculate how sound moves and reflects from wall to wall. There are a few modes that can be calculated, axial modes are concerned with two parallel surfaces, two walls or the ceiling and floor. Next there are tangential modes which deal with two sets of parallel walls. This could be either the four walls in the room or one set of walls and the floor and the ceiling. These are roughly half as strong as the axial modes. Finally there are oblique modes these involve all six surfaces of the room, the four walls and the floor and ceiling. These are about one quarter the strength of the axial modes and half as strong as the tangential modes. When all three formulae are combined this gives the first real approximation of how a room will behave. The formula used is;

F = (c / 2) x sq. root of (p / length) 2 + (q / width) 2 + (r / height) 2

Where c is equal to the speed of sound, p, q and r values run from 1, 0, 0 up as far as possible to build up as full a picture as possible. This allows clusters of frequencies to be seen clearly and also spaces in the frequency range can be identified, and rectified.

The ambience of a particular space plays a huge part in how well a piece or style of music sounds. Just enough reverb can increase the warmth of a room and the overall sound of the music being played, where as too much reverb can muddy the sound and affect the intelligibility. This is why many performance halls are extremely expensive to design and build, as they are acoustically designed to accommodate either one specific style of sound or as many as possible. Now some performance spaces are built with moveable walls and ceilings so that the acoustics of the venue can be altered to suit different types of performances. These problems



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