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Live PA - Basic Sound Engineering Overview
Live Public Address (Live Live P.A.) systems
come in many different shapes and sizes and can
often confuse the newbie into not knowing even
just the basics. This article is aimed at giving
a basic overview of non-specific equipment
configurations in an attempt to de-mystify some
of the typical errors a newbie can make with
Live P.A. systems.
The function of a Live P.A.
Two types of Live P.A.
Reinforcement
This is for speech or music which would sound
good in a small room without artificial
assistance.
In the case of a classical guitar which is a
very quiet instrument, this natural sound is
only good for an audience of around 200 - 300
depending on room size.
For an audience of 500 - 600 it is possible to
reinforce the sound so that everyone can hear
clearly and to most people it will still sound
natural.
Amplification
This is where the original sound is
insignificant in comparison with the amount of
sound coming from the Live P.A.
The aims of public address:
1.) To provide adequate volume ( not necessarily
loud ).
2.) To provide adequate clarity.
A Thought on Acoustics
A discussion on sound reinforcement is
impossible without a mention of acoustics.
The free field
If the venue is an outdoor event then the
engineer need not concern himself/herself a
great deal with acoustics as this is the ideal
situation.
Sound in the open air travels away from the
source and keeps going until it's energy is used
up ( inverse square law ). There are no walls
for the sound to bounce off and return to
interfere with the next wavefront.
Indoor Live P.A.
Sound behaves in much the same way as any other
wave, it bounces off walls (reflects), and bends
around them (diffracts), and cannot pass
directly through materials. Therefore speaker
placement becomes important as does speaker
coverage.
Consider Figure 1.0.
The sound waves being ommited from the cabs have
a coverage of 120 degrees therefore it can be
seen that there will be obvious 'blind spots' in
the coverage.
Figure 1.0 Shows a small venue and a typical
coverage angle of a driver.
It should be pointed out that the directionality
of sound waves is somewhat frequency dependent
and that the above diagram shows a potential
problem for high frequencies.
High frequencies have a smaller wavelength than
low frequencies hence they are very directional
( λ = v/ƒ ), λ = wavelength, v = velocity, ƒ =
frequency .
Objects placed in the Live path of HF block
them, whereas LF tend to bend (diffract) around
them. Therefore a subject positioned behind the
wall in Figure 1.0. will hear an attenuation in
HF hence a dull sound.
Golden rule number 1.
Always ensure nothing is in the line of sight of
a HF driver otherwise you may encounter loss of
HF.
Reflection and phase cancellation (comb
filtering )
Consider Figure 1.1.
Figure 1.1 Shows the Live path of a sound wave,
the venue is assumed to have reflective surfaces
and after a period of time the wave can be seen
to have travelled around the room bouncing off
the surfaces and crossing over other sound
waves.
This situation can create what is know as 'comb
filtering'. As you can imagine the sound takes
time to travel around the room (340 metres/sec)
and if the reflected wave (having being time
delayed) coincides with another wave whose
polarity is the inverse or a fraction of, then
cancellation will occur.
Conversely, if the combination of merging waves
have the same polarity then addition will take
place.
The name comb filtering is adopted because
looking at the frequency response of the
product, the shape of the teeth on a comb can be
seen. Showing areas of addition and subtraction.
Figure 1.2 The peaks and troughs of comb
filtering can be seen in this frequency response
plot.
Golden rule number two.
Ensure the minimum amount of reflection by
pointing the speakers in a suitable direction.
The main thing is to keep comb filtering to a
minimum this is sometimes easier said than done
as most venues have reflective surfaces. There
will always be pockets of 'bad sound' and
pockets of 'good sound'.
If you wonder round a venue and listen to the
mix you will find these spots, it is your job as
an engineer to keep the 'bad spots' to a minimum
through speaker positioning, coverage and
equalisation.
In professional venues architectural
acousticians get paid lots of money to design
environments which produce 'good spots'
throughout the venue by such methods as
absorption paneling.
Standing waves
Standing waves are a result of sound being
reflected back and forth between two parallel
surfaces.
As the first wave reflects it meets a newly
arriving wave and the result can be that a
stationary wave is produced which resonates at a
frequency dependant on the transmitted waves and
the distance between the Live parallel surfaces.
The wavelength of the transmitted waves in
relation to the distances between the Live
parallel surfaces is important for consideration
then.
If this distance equals the wavelength or a
ratio of it then a standing wave could be be
made to oscillate.
Example
The wavelength of a 20 Hz wave is 17 metres, if
this wave was transmitted between two Live
parallel surfaces whose distance was 17 metres
an oscillation could occur.
Standing waves can be a problem in venues where
the dimensions of the venue coincide with Live
particular wavelengths.
At LF, standing waves can 'creep up' on the
engineer as they gather energy and appear to
'feedback', which could of course occur if the
standing wave was picked up by the microphones
on stage and amplified.
Careful use of room equalisation and speaker
positioning can combat standing waves to a
degree.
What's wrong with a lot of Live P.A.'s
Low efficiency speaker systems
cure - ensure you have efficient speakers.
Not enough amplifier power
cure - ensure you have plenty of amp power
Poor frequency response
cure - ensure all components in the chain have a
'flat' response
Miss half your audience
cure - ensure you have enough speakers that are
angled to cover everyone
Room reverberation swamps the sound
cure - choose speakers with suitable directional
and dispersion qualities, thus avoiding
reflective surfaces
Basic systems for two different sized rooms
Example 1
A small sized room having the dimensions of
around 30 by 30 by 10 feet.
Figure 1.3
The system in block diagram form.
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