Reverberation Time

A reverberant sound in any closed space like a room or concert hall diminishes as the sound energy is absorbed by multiple interactions with the surfaces of the room. Though the speed it takes to diminish depends on the variable of the rooms surfaces, for instance a reflective  or "live" room will take longer than a very absorbent or "dead" room. The time for the sound to completely die away will depend how loud the sound is at the start and how well the observer hears. 


The standard reverberation time has be established as the time it takes for a sound source to diminish by 60 dB below its original level. This is established at 60dB from testing in auditoriums where the loudest crescendo in an orchestra is 100dB and typical background noise for a music-making area being around 40dB. 60dB is also roughly the dynamic range for orchestral music.


In terms of time it depends on the rooms use. In a classroom you want the room to have low reverberation time to keep articulation clear and reduce build up from people talking. In an auditorium you probably want a bit more so you can really feel the music. As a general rule you wont get long reverberation time in a small room because of how fast the energy is bouncing off the surfaces and being absorbed a bit each time.


Reverberation time is just one part in creating pleasing environments for sound, you also need to consider the absorption in terms of frequencies.


Thanks,


Matt Price

Inverse Square Law Of Sound

Hello All,


This blog is on the inverse square law of sound which is a very important law that not many really know about (we will get onto that later) Sound waves propagate spherically and so distributed over and ever increasing surface of diameter at the front surface of the wave. The inverse square law tells us that every doubling of the distance from the sound source in a free field situation the sound pressure level will diminish by 6 decibels.


Why many may not know this law is that it is hard to get a free field situation such as an explosion of a bomb in mid air. Though not perfect for real life situations it is a very important guide to sound sources and how they diminish over distance. 


As an example lets say a person shouts at 70dB at 3 feet away from you in a room, at 6 feet it will be 62dB and at 12 feet 56dB and at 24 feet 50dB. This example on works as a guide not taking into account reflections from any walls of a room and any other sound in the room at the time of shouting.


Thanks,


Matt Price

Sound Pressure Level and Decibels

Sound Pressure Level is the amount of air pressure fluctuation a sound source creates. We perceive this pressure in terms of loudness. A simple example is a drum, when you hit it gently the surface doesn't move very far and so because the of this the pressure is lower than hitting it hard.
The pressure is also effected by where the listener is from the source and the environment they are in. A drum hit hard in a small bathroom will sound a lot louder than a drum in a cathedral because many of the discrete reflections will never reach the listener due to the expanse of the space. This is obviously even less in an open field where there is nothing to reflect off in the sky.


Sound pressure is usually expressed as pascals (Pa). Usual conversation pressure is 0.02 Pa and a petrol lawn mower is around 1 Pa and sound becomes painful at around 20 Pa. So a healthy range is around 0.00002 to 20 Pa. 


Pressure ranges obviously offer such a wide range of scale that we use the decibel scale. This is because it compresses the scale into a manageable range. This conversion from sound pressure to the decibel scale is called sound pressure level.


0dB is 0.00002 Pa and this is the start of the scale.

This is redrawn on paper due to copyright and is still an accurate reference.

Thanks,


- Matt Price