Sound Reinforcement

Audio System design is probably the most interesting and challenging of all the systems that MeyCom is involved with as the acoustic environment within which the system must operates contains so many variable factors.

The design of the systems is a balance between power to overcome the background noise and the clarity of speech and music.

Golden Rule
The golden rule in design is to remember that the sound pressure level halves (-3dBA) every time you double the distance from the speaker, this is a logarithmic relationship.

3dB may not sound like much, some years ago a company installing a sound system in a sports stadium in Johannesburg under designed the amplifiers by 3dB. The reality is they had to double the number of amplifiers to meet the specifications at their own expense.

Speech Clarity
The reproduction of the person voice is critical, from the microphone through the amplifier, down the cable network to the loudspeaker.

On an evacuation system a compressor limiter installed directly after the microphone to limit distortion when a person is stressed is a good idea. In and emergency a person usually raises their voice and shouts into the microphone, which causes distortion.

Measuring speech clarity one uses Alcons means Percentage Articulation Loss of Consonants.
Regarding speech, it is the consonants that provide the intelligibility not the vowels.

Wikipedia provides further interesting information on this topic.
link http://en.wikipedia.org/wiki/Alcons

Equalization ~ the cost saver
Equalizing an area or a room often results in the need for less acoustic power = less amplifiers.

The equalization of room to remove the effect of feedback, today this is very easy to achieve, ¹/₃ Octave Equalizers perform the task dynamically in real time.

Back in the 80's when we started to use ¹/₃ Octave Equalizers the task had to be performed using an oscilloscope and audio generator, beating the feedback frequency against a known frequency generated by an audio generator.

This created Lissajous patterns related, those single and multiple circles on the display tube, from which you calculated the frequency of the feedback and then notched it out using the equalizer.
To equalize a room could take 4 - 8 hours, how technology has made life easier.

Lissajous patterns on YouTube

Acoustics of the environment ~ where the costly mistakes are made
The starting point in the design of system.

At the end of the day it is really experience that counts, often acquired through mistakes which are always costly. Standing waves are often over looked in the design of background music systems where speakers are installed in ceilings and the sound is fired at a hard floor and bounces back.

A book by Sabine which is now unfortunately out of print on Acoustics provides a very good guideline. Sabine established the physics of acoustics and went onto found a very successful company in the USA.
Sometimes the book is available on Amazon ~ Acoustics and Architecture Book by Paul E. Sabine; 1932.

By designing the system smartly one can make considerable savings.

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