Designing your Sound System.

A public address system is a means of sound reinforcement for relating speech and audible information to a group of people. The set up may vary from a single portable amplifier in a small room to an intricate multi-room zoned system with multiple speaker arrays.

Since each public address venue will have its own characteristics, there are many different speaker types and installation methods depending upon the architecture of the building, ambient noise levels and coverage requirements. The key demands from any public speaker are efficient speech delivery and wide area coverage.

 

Identifying your requirements .

 

The simplest sound system might be a microphone connected to an amplifier and speaker to deliver speech at a volume loud enough for a group of people to hear. More sophisticated systems may need to be able to deliver high quality stereo background music and voice announcements with different sound sources delivered to different areas or groups of listeners.

Background music is a term used for pleasant and unobtrusive playback of recorded material. The use of cabinets fitted with either a 2-Way or 3-Way drive units are preferred. High quality is imperative with any background music system.

Systems used mainly for the reproduction of speech can use speakers which have single full range drive unit.

Selecting your Speakers.

Choosing the right speaker for your application is the first requirement. You have a choice of Wall Mounted, Re-entrant Horns, Ceiling or Pendant.

Wall Mounted.

The most popular speaker cabinet, very versatile and can be used for all sound reinforcement applications. The most basic wall speaker is fitted with a single drive unit. For background music, cabinets are available with 2 and 3-way crossovers. Most of our ranges of Wall speakers are supplied with a universal mounting clamp. These brackets allow for the rotating and tilting the cabinet into the optimum position.

Re-Entrant Horn Speakers.

When high sound pressures are required, horn speakers may be the perfect option. They can produce high quality, loud, directional sound. Horn speakers are perfect for use in large areas such as warehouses, sports arena, patios, auction sites, theme parks, marinas and swimming pools.

Our range of horn speakers have high voice intelligibility, as well as a range of dedicated music horns. Our music horns use state of the art technology to produce amazing sound quality that conventional horns cannot muster.

Ceiling Speakers.

Ceiling speakers are suitable for use in rooms or corridors which have suspended ceilings. Ceiling speaker which are fitted with a single cone are most suited to the reproduction of speech. Ceiling speakers with 2-way crossovers are ideal for music. Some ceiling speakers have dual coils and are best used in low impedance stereo applications.

 Pendant Speakers.

For large open areas, pendant speakers produce an omni directional sound. With similar characteristics to ceiling speakers they are both suitable for the reproduction of background music or speech.

Ceiling and Pendant speakers are capable of effectively providing excellent coverage to an area. To calculate the spacing required between speakers use the following formula.

 

Assume a speaker has a dispersion angle of 90º. Installed in a 2.8m high room and assuming that an average person is around 1.5m tall.

= (2.8 – 1.5) × TAN (45°) × 2
= 1.3 x 1 x 2 = 2.6

Therefore, speakers should be spaced 2.6m apart.

Sound Systems.

The preferred method for multi-speaker installations involves using a technique known as ‘100 Volt Line’. This method allows a large number of speakers to be driven by one amplifier, so long as the overall power being tapped by the speakers is within the amplifier’s output capacity.100 Volt Line.

For most commercial sound installations it is necessary to drive many speakers with a single amplifier, giving greater coverage to multiple rooms or over a given area. Constant voltage systems (usually known as 100 Volt line) employ a transformer in the amplifier to convert the output to high impedance and a corresponding transformer in each speaker to tap of the required power from the amplifier.

In addition, the transformers in each speaker may have tappings at different points along the primary coil, which effectively allow the speaker to draw different Wattages from the amplifier. A speaker rated at 15 Watts may also have tappings for 10, 5 or 2.5 Watts.

 Example

A 100 Volt Line Amplifier with a rated power of 100 Watts allows a large number of speakers to be driven by one amplifier, so long as the overall power being tapped by the speakers is within the amplifier’s output capacity. (Usually 90% of the amplifier’s maximum rated output for speech and 70% of the amplifiers rated output for high level background music).

Selecting your Amplifier.

Amplifiers come in all shapes and sizes. The main categories are a basic amplifier (Slave) or a Mixer amplifier. They are available in Mono (Single Channel), Stereo Two (Channels) or multiple channel models. Each amplifier will normally have 100 Volt and 8 ohm outputs.

The first characteristic to identifier is how many inputs you will require and for what purpose each input will be used. Mixer amplifiers can have many inputs, Microphones, CD, MP3, Radio, Telephone or Line level. The more advanced models now come with a built-in MP3 Player, USB socket and SD card reader.

Second when you have established the number and type of speakers needed add up their total wattage, this will then give you the minimum wattage of the amplifier you require.

Amplifier (Watts)	70% Music (Watts)	90% Speech (Watts)
25	17	20
30	20	25
40	25	35
60	40	50
90	60	80
120	80	100
240	158	210
360	250	320
480	330	430

The table above gives a guide to the total load that may be presented to an amplifier. If your installation is mainly used for loud background music the total wattage of the speakers should kept at 70%. If you are installing a public address system the load should be limited to 90% of amplifier power. For general, multi purpose set up keep the loudspeaker load between the two values.

Remember when calculating the speaker power (Watts) it is the tapping you have selected on the line transformer not the power of the driver.

Example

Two microphone inputs, input for MP3 source and input for a CD Player. Three 100 Volt Line Ceiling speaker at 8 Watts each, four wall mounted background speakers, 30 Watts each and one Pendant speaker rated at 15 Watts.

8 + 8 + 8 +30 + 30 + 15 = 99 Watts

Therefore a 120 Watt mixer amplifier with 4 inputs would be the minimum required.

Connecting your system.

Inputs to your amplifier should be made using screened cable. Often these are provided with the appliance. Microphone inputs (if possible) should be made with balanced screened leads. This type of lead is fitted with XLR connectors.

For longer runs a balanced connection is better because the screen is not connected to the signal path, so reducing the chance of interference on the signal.

Connection to your speakers should be made with double insulated speaker cable. The overall size of the conductors will depend on the length of the cable run and the total wattage of the speakers connected to it.

The following table provides an overview for maximum speaker lengths for a given conductor size with a given load.

Cross Sectional Area of Cable
	0.75mm²	1.0mm²	1.5mm²	2.5mm²
30 Watts	800m	1066m	1600m	2666m
60 Watts	400m	533m	800m	1333m
120 Watts	200m	266m	400m	666m
240 Watts	100m	133m	200m	333m

Induction loop cable installation

Prior to installation, it is important to check if there will be any equipment in the vicinity that may interfere with the loop’s magnetic field, such as large transformers, high power cables or substations. It is also important to ensure that no equipment will be adversely affected by the loop field, such as sensitive data or signal cables. In these cases, it may not be possible to cover some or any of the required area with an induction loop.

The cable gauge will depend upon the total length of the induction loop. Usually, the loop will be installed around the perimeter of the listening area. However, there are various techniques which can be employed to build arrays to vary the shape and strength of the field. In general, calculations outlined here are based upon a perimeter installation.

It is recommended to use good quality insulated pure copper cable for the induction loop. The cable gauge used will need to be determined by the total length of the cable run. The LA-series loop amplifiers are designed to operate with a load of between 0.2Ω and 2Ω. The following equation can be used to calculate the total cable resistance, which will show if the gauge is correct.

R = Cable length (m) x 0.01786Ω*mm²/m (specific resistance of Copper)
Cable CSA (mm²)

So, for a cable 80m long with a Cross-Sectional Area (CSA) of 1.5mm²... R = 80/1.5 x 0.01786 = 0.9525Ω

So, this gauge of wire (1.5mm²) would be OK for the 80m run because it is between 0.2Ω and 2Ω

As a quick reference, refer to the table below. 

 
The wire may be run in plastic conduit but not in any metal containment so as not to impair the magnetic flux. The ideal height to install the cable depends upon the width of the loop (i.e. the narrowest dimension relative to the listener). The optimum output will be achieved if the cable is installed 14% of the loop width higher or lower than the plane of listening (ear height).

In practical terms, the loop is usually installed onto skirting boards, under flooring or at ceiling height so that it is unobtrusive. It is recommended to avoid varying height levels as much as possible to avoid anomalies in field strength.

For irregular shaped areas or larger than the loop amplifier’s capacity, multiple induction loops may be required for coverage. Connecting further loop amplifiers using the Slave connection will enable multiple loops to be used together.

 

 Cancellation Loops

If there are areas adjacent to the loop where the magnetic field would cause problems, it is possible to avoid this by use of a “cancellation loop”, which is a narrow loop parallel to the main loop at the problem area. This loop is the opposite polarity to the main loop and causes the magnetic field to narrow along the adjacent edge of the main loop to control the spill of the loop field.

Finally.

Once you have decided on your system, you might like to consider the following points before you commence.

For larger installations your systems could be divided into Zones. A Zone can be a room or a group of rooms. Equally, a zone could be part of a room or even an outside space. This makes the system more flexible and provides control of the sound that is delivered to different areas. Essentially, wherever separate control over the amplified sound is needed would be considered as a separate zone.

The individual output of a 100 Volt line speaker may be varied using remote volume controls, known as attenuators. They can be wall mounted beside the speaker cabinet.

When switching 100 volt line loudspeakers "on" or "off", it is necessary to use the circuit indicated below. This circuit will ensure that the loudspeaker is not affected by inductive pick-up in the disconnected cable, this can happen when speakers are switched at the amplifier and leave long open ended cables. The loudspeaker is not only isolated from the line but also shorted to itself in the "off" position this ensures that there is no possibility of signal breakthrough.

 

 

Installation considerations.

1. Do not run microphone cables near mains, data, and telephone or Speaker cables.
2. Do not run 100V line cables near data, telephone or other low voltage cables.
3. Do not exceed 90% of the amplifiers output power when using 100V line (speech only)
4. Do not exceed 70% of the amplifiers output power when using 100V line (high level background music)
5. Do not use re-entrant horn loudspeakers for background music unless the loudspeaker has been specifically designed for this purpose.
6. Always use a balanced or floating low impedance microphone terminating into a balanced input on long microphone cable runs.
7. Ensure that all loudspeakers are in-phase.
8. Ensure that there are no short circuits on the loudspeaker line before connection to the amplifier.
9. For outdoor, humid or dusty environments, ensure that the speaker units used have an adequate IP rating for protection, and that all amplifiers and rack components are completely sheltered from ingress.
10. Secure amplifiers and other rack components in an appropriate case or cabinet capable of supporting the weight and giving adequate cooling ventilation.
11. Ensure correct AC voltage and adequate current rating at the mains supply for all amplifiers and rack components.
12. Only use the mains leads supplied with equipment or of equal compliance and current capacity.
13. Fix all speaker components securely to walls and ceilings using appropriate fixings for the substrate, and employ safety wires wherever possible.
14. Ceiling speakers fitted in suspended ceiling should be fitted with fife domes to maintain building fire regulations.