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

Q+A: 6 Miscellaneous Sound Questions... Part 1

Compilation of general and random questions...

1. What does 94.1 mean on a radio dial?

This mean the station is emitting at 94.1 million Hertz or 94,100,000 waves per second.

2. If sound becomes louder, what wave characteristics increase? frequency, wavelength, amplitude or speed?

Only amplitude.

3. How much bigger is a sound at 40dB compared to 0dB? 

10,000 times.

4. How much bigger is a sound at 110dB compared to 50dB?

1,000,000 times.

5. What does supersonic mean? 

Faster than the speed of sound

6. General speeds according to (this site)

WATER:
Distilled water = 1489 m/sec
Sea Water = 1531 m/sec (higher because it is denser)
WOOD:
Ash, along the fiber = 4670 m/sec
Ash, across the rings = 1390 m/sec, about 3 times slower, and a little slower
than the speed of water!
Beech, along the fiber = 3340 m/sec
Elm, along the fiber = 4120 m/sec
Maple, along the fiber = 4110 m/sec
Oak, along the fiber = 3950 m/sec.


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

Q+A: Sample rates explained....

Sample rate is the number of samples per unit of time taken from a continuous signal to make a discrete signal, the unit of sample rates are Hertz. We record a discrete signal because we only need lots of little samples of the signal to track how loud it is etc... So the higher the samples the more of these measurements are taken per second. The standard for recording dialogue is 48kHz (48,000 hz) or 44.1kHz and unless your playing on really nice speakers you really wont be able to tell the difference.

For some sound effects recording people record at 192kHz which has advantages for slowing effects down and manipulating them. Also higher sample rates help prevent aliasing, which refers to an effect that causes different signals to become indistinguishable 


also see: Bit Depth

Thanks,

- Matt Price

Q+A: Basic Sound Questions...

This is going back to basics with what is sound anyway?...


What is sound and how is it created?
Sound is a vibration and/or a wave of air molecules caused by motion of an object. This wave is a compression wave where the molecules are bunched up like a wave in the sea, A sound wave is created when a series of these pressure changes/waves move through the air. This wave or waves speed depends on the temperature, e.g. at 15 degrees Celsius the speed is 340.276 m/s but at 25C the speed is 346.13 m/s. (Calculator here).


Through solids the wave will bounce back or echo/reverberate and the energy can change from acoustical to electrical energy which is how microphones and telephones are able to work.


What is frequency?
When drawing a sound wave, like with any wave there is a peak and a valley which vary in distance from each other. Sound sources vibrate at different rates or frequencies as they move through the air. We measure frequency in cycles per second or Hertz. This is named after Heinrich Rudolf Hertz (February 22, 1857 – January 1, 1894) who was a German physicist who was the first to satisfactorily demonstrate the existence of electromagnetic waves by building an apparatus to produce and detect VHF or UHF radio waves.
The faster an object vibrates the higher the frequency and thus higher the pitch of the sound. a common example is a tuning fork for A above middle C will vibrate at 440 times per second and so has a frequency of 440 Hertz.




What is amplitude? 
We already understand by now that sound is made by pushing air molecules together in varying degrees of strength, this is amplitude. for example if something is struck really hard the wave will carry some of that energy through the air and it will sound louder than if it is struck gently. Sound waves with the same frequency can have different amplitudes.


Feel free to add any comments if i haven't covered anything in enough detail.


Thanks,


- Matt Price