Live PA - Basic Sound Engineering Overview

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.

Figure 1.4

This would be a suitable setup giving adequate coverage.

The power amp would be rated at around 150/200 watts per channel and the speakers would be full range.

Example two

A medium sized room having the dimensions of around 50 by 40 by 15 feet.

Figure 1.5

Figure 1.6

This system is known as a two-way system, and for this room a total power of around 1KHZ would be adequate. The audio spectrum is split in two at around 250 Hz. Thus two power amplifiers are nessessary. Percentage wise the Low end would have around 65%. Leaving a further 35% for the mid and top end.

Large Live P.A.

Large Live P.A. systems can often be anything from two-way to five way and can contain massive amounts of drive untis for each seperate bandwidth.

A large Live P.A. system would have two engineers. One for the front of house one for the monitirs mix.

Often delayed loudspeakers are needed.

In a large open air concert say, a person standing 340 meters from the stage will not hear the emitted sound wave until one second has elapsed if he/she is standing 640 meters then two seconds will have elapsed before the sound can be heard.

This is due to the speed with wich sound travels through the air. i.e. 340m/s.

By the time the sound has travelled this distance it has suffered great losses. Therefore, further speakers will be needed for the audience to the rear of a concert.

The sound from these drivers need to delayed in order for the sound emitted from the main drivers to be of the same phase.

Figure 1.7 The basic configuration of delayed loudspeakers.

The delayed signal could come from groups, from the main mix or aux's etc.

Further points to note:

A rock Live P.A. should be as intelligable as a West end musical.

It should cover the audience evenly allthough this is not always possible.

The system should be visibly in tune with the type of work and surroundings.

References:

Electro voice, The Live P.A. bible.