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Features any Well-Designed System Should Have
The sound from a good public address system should be:
Some of these terms overlap or are interdependent, but our aim in specifying and setting up a PA system - whether it is a permanent installation or a touring one - is that it can meet all these criteria. Other artefacts - intentional effects, or dynamic control - may be added later (and may be requirements for some specific applications), but the design of the core system needs to address its core function, which is to make things louder without changing them in any other way.
Clean sound is free from distortion and noise.
Distortion is what happens when the output signal differs in form from the input signal. Ideally the relationship between the sound source and what the audience hears should be linear: the PA system's output at the listening position should vary in direct proportion to the sound source (the only permissible difference is in amplitude). Any non-linearity in that relationship is distortion.
Non-linearity can be introduced by:
Any component in the signal path - from microphone to loudspeaker - can introduce distortion in the form of non-linearity between input and output. Generally (unless tolerance-limits are exceeded), components whose effect is purely electronic (e.g. preamplifiers, mixer circuits and power amplifiers) maintain the closest relationship between input and output, and their distortion characteristics will be included in manufacturer specitifcations (e.g. Distortion [typical], Full rated power, 20Hz - 20kHz, <0.02%). Components introducing the greatest non-linearity are usually transducers (things that convert the signal from one form to another, like microphones and loudspeakers). Of these, loudspeakers are the most critical component, so loudspeaker systems are one of the most important elements of system design. Variations in output over a loudspeaker's frequency range are one kind of distortion, and what looks like a fairly good frequency response (e.g. 65Hz-18kHz ± 3dB) represents a 6dB range of error, which is a 50% difference in sound pressure. Even the best loudspeakers will show much higher non-linearity over their frequency range than typical electronic components.
Although there are other forms of distortion (see non-linearity, above), the most recognisable kind is characterised by the ‘fuzziness’ and/or ‘roughness’ that is created when amplifier circuits are overloaded. Although this is desirable in some situations (an ‘overdriven’ electric guitar sound, for example), it is very undesirable in a live sound reinforcement system: our aim is to reproduce the ‘overdriven’ electric guitar sound accurately, not to add distortion that wasn't already there.
The cleanest-sounding system in the world will still produce audible distortion if mixer channel gain settings are too high (so that input channels are clipping), if mix busses are overloaded, or if you drive a +4dBu amplifier channel with the +20dBu peak output of your DJ mixer.
Although components may perform well on their own, they may perform less well when poorly matched with other components. For example, a high-impedance microphone connected to a low-impedance mixer input may sound ‘thin’, while a pair of 4Ω loudspeakers driven by a single under-powered amplifier channel may sound very different to how they did in the shop.
Similarly (also see User Settings, above), it is relatively easy, regardless the of gain setting, to overload microphone preamplifiers - usually designed to accept signals in the order of −50 to −80dBV (a few millivolts) - if you connect line-level signals to XLR inputs and your mixer doesn't have Pad buttons.
To produce a clean sound you will need to select high-quality components, ensure their impedance and signal levels are appropriate for other components in the system, and ensure the system is correctly configured at every stage of it operation. You can find some information about components and their specifications in our Equipment Guide section, but some system choices - like using same-brand controllers, amplifiers and loudspeakers - can eliminate a few of the main compatibility and set-up issues.
The main sources of noise in PA systems are electronic noise and interference (which includes ground loops).
In any electronic device there will be some random electron movement, and a fundamental design-requirement in audio equipment is that this should be very low in level compared with audio signals: noise itself is inescapable, but if it is very much lower in level than the audio signal it will not be noticeable while the band is playing. Most manufacturers will give at least some nominal indication of how well their equipment meets this criterion, but with some equipment - mixers, in particular - you may need to look at more than one specification (e.g. microphone Equivalent Input Noise, channel gain, nominal channel input level, and residual output noise). In practice, system output levels will generally be appropriate for programme styles and audience capacity (see Adequate Sound Level, below), but an amplifier/loudspeaker system capable of very high output levels (e.g. 130dB SPL) at full gain may make a mixer's -90dBu Residual Output Noise audible in a very quiet auditorium. Note that some types of equipment (e.g. microphones with very low output that require high gain settings, or compressors that reduce overall dynamic range) will increase relative noise levels.
Interference occurs when any part of the system acts as an antenna for extraneous signals. Ground loops occur when signal grounds form a loop, and act as an antenna for 50Hz mains-borne interference. The best solution to this is to use balanced equipment and cables throughout, which is a basic requirement in any professional sound system. Interference can arise in other ways: it can be picked up through instrument pickups and other transducers, through mains electricity supplies, through radio systems, and through connection faults (dry solder joints, for example, can act as antennae). Sources of interference include unshielded electrical devices (including mains drills, generators, blenders and washing machines), induction loops, and radio transmissions (including local taxi, police and ambulance services).
Another word for clarity is transparency: we want the sound system to have the same effect on sound as a high quality optical lens has on light; it can make what we are looking at appear larger or smaller, but in every other respect it should behave as if it wasn't there.
What this means, in practice, is that its reproduction should be detailed and accurate, and a key component in this is the high-frequency delivery of the loudspeaker system: in general, better high-frequency performance (greater high-frequency range and lower distortion) will result in a more transparent-sounding system. However (see ‘Naturalness’, below), a system that over-emphasises high frequencies will tend to sound ‘thin’ or ‘harsh’, while a system that over-emphasises lower frequencies will sound less-transparent, no matter how good the HF drivers are.
Sound is intelligible when all its components are clearly recognisable and comprehensible (also see Intelligibility in the Glossary). If someone is speaking clearly into a microphone, they should sound like someone speaking clearly (they should be intelligible) in all audience positions. Where poor room acoustics limit the range of intelligibility (see critical distance in the section on Speaker Position), a good system properly set up should nevertheless succeed in producing sound that is intelligible for most of the audience.
If some frequencies are predominant or lacking, the resulting sound will be unnatural (in Hi-Fi Speak, it will be "coloured"). For some special effects this may be desirable, but in general we don't want the singer to sound as if s/he is using a cheap megaphone, or the kick drum to sound as if someone is banging on a wooden door with a rubber hammer. If the singer is actually using a cheap megaphone, however, we want that to be reasonably obvious to anyone who has their eyes shut. Note that ‘natural’ is not necessarily the same thing as ‘pleasant’: although equalisation can compensate for some instrument qualities, a budget plywood third-world guitar naturally sounds like a budget plywood third-world guitar, and there are limits to what even the best sound system in the world can do with it, especially if the pickup is a budget third-world transducer. Similarly, very expensive microphones, mixer, amplifiers and speakers will make a badly-tuned undamped drum-kit sound exactly like a badly-tuned undamped drum-kit.
Balance is achieved when all the separate elements contribute in the right proportion to the overall sound. If the guitar is so loud you can't hear the rest of the band clearly, or if the snare drum is obscuring the vocals, the audience probably won't get the best night of their lives. Balance is important for the audience, but can be less useful for individual musicians, who need to concentrate on their own or other specific instruments (which is why even small systems often need monitors). Guitarists (even guitarists in the audience) often think the guitar isn't loud enough. The same applies to bassists and basses, fiddle players and fiddles, and you-name-it players with you-name-its. Although there is a great deal of personal taste and subjectivity involved, the aim is balance, not even more of the musician who makes the most noise or is going out with your sister.
To this end the PA system must be capable of applying sufficient acoustic gain to the weakest element in the overall sound (usually vocals, or low-volume acoustic instruments like acoustic guitars) to balance it with high-volume elements (like drums, electric guitars, or brass). While output capability is a component of this (see ‘Adequacy’, below), available acoustic gain may be limited by the onset of feedback, so the capability of the system as a whole is also dependent on:
Uneven frequency response - in any system component, but particularly in loudspeakers - will make the system more prone to feedback.
If direct or reflected sound from the loudspeakers can be picked up by microphones, the system will be more prone to feedback. Loudspeaker dispersion should be selected for the space(s) the system will normally be used in, and loudspeakers should be positioned and angled to minimise reflection. Microphones should not be positioned within the arc of loudspeaker high-frequency output, and should be far enough away from loudspeakers to avoid feedback at less directional (lower) frequencies.
We don't just want clean, clear, intelligible, natural, balanced sound in the place where the sound engineer is sitting: we want everyone to get it. We want even coverage in as much of the audience area as possible. Radical changes in tone as you move through the audience area are undesirable.
Two key factors here are loudspeakers and loudspeaker placement: loudspeakers need to have even dispersion (the level and frequency balance should remain reasonably constant within the area the speakers are designed to cover), and good pattern control (sound must not spread to areas where it isn't wanted: sound reflecting from walls and ceilings interferes with evenness, causing peaks and troughs - AKA comb filtering - at different frequencies); they also need to be positioned to minimise interaction with the room (see the section on Speaker Position for further details).
Although very high volume can seem impressive, loudness can interfere with all the desired characteristics of a good PA system (and, indeed, sound that isn't clean, clear, intelligible, natural, balanced, or evenly dispersed - or even, for that matter, adequate in level - can often be described as ‘loud’). There may also be legal implications if the sound is too loud. It does, however, need to be loud enough.
As well as being loud enough a system needs headroom (we want at least a few decibels spare, to stay well clear of distortion). For that reason, it is better to have more power than you need. The downside of this is the cost involved: all else being equal, you need to double the system size for every 3dB of extra output. If you want 3dB of headroom, you need a system that is twice as powerful as one that has no headroom at all.
The level at which a sound is adequate will depend on the main purpose of the system.
For speech, system requirements are relatively modest (a system capable of producing a peak level of around 85dBA at the listener's position is generally adequate). Headroom requirements are also modest, and output below 100 Hz - or even much below 200 Hz - is unnecessary (and can even be a hindrance).
For recorded music (e.g. typical disco/dance music), higher output and extended bass range are needed. However, there is no need for lots of extra headroom, as the peak level of recorded music is predictable, and the dynamic range of typical disco/dance music is limited.
For live music applications, system requirements are much more demanding. A snare drum can produce levels above 120dB SPL at the microphone. Other acoustic instruments (particularly brass) produce impressive amounts of sound without a public address system in sight. Amplified instruments (especially where the sound is compressed by the amplifier, producing very high average levels) are even more of a challenge. A loud rock band will need a system that is capable both of exceeding the level of the backline, and of delivering it to the audience at much higher than safe listening levels (typically peaking at around 110-115dB SPL at the ears of the audience). Also, dynamic range is typically much greater (with a difference of around 12dB between average and peak levels), which places a corresponding demand on the system's headroom even where compressors and/or limiters are used.
Our own systems are generally adequate for audiences in the size ranges stated on each system's page. However, very ‘loud’ styles (e.g. heavy metal) will generally require much higher output capability.
It is important to remember that a PA system is always used for a purpose in a location, and while its design should always address where and why it is needed, some environmental conditions cannot be solved by audio equipment. Among other things, the PA system cannot:
• Disperse sound evenly round corners
Where there isn't a sight-line from audience to loudspeaker (behind pillars, alcoves, partitions, balconies), or where the audience is dispersed over a wider angle than the speaker system is designed to cover, some unevenness (or worse) is inevitable. Covering the whole target area may mean you need extra loudspeakers, as well as additional processing so that time-alignment between separate speakers or speaker clusters is as close as possible.
• Compensate for room reflections or resonance
Rooms can have an adverse effect on sound quality, and the more sound the PA system makes the worse it will get. There is only so far you can go with loudspeaker placement and EQ, and a room with a 10-second reverb-time is going to present problems with intelligibility and frequency balance whatever you do with the sound system. If you have to produce an event in a difficult space then you will obviously achieve some improvement using the best system and layout for the job, but you may achieve more by treating the space (with acoustic tiles, curtains, carpets, and anything else that reduces or disperses reflection) than by upgrading the PA system.
• Make a mumbling talker or incoherent singer intelligible
Poor enunciation and bad microphone technique can be fixed: take elocution lessons, and learn how to use a microphone. Where you will be dealing with inexperienced users (guest speakers at a meeting or conference, or the bride's father), giving some brief pointers beforehand can be more effective than any system changes.
• Make a cheap piezo pickup sound natural
A good EQ-section can do quite a lot with the most prominent colouration, but don't expect miracles. Similarly, using your own microphone can be a good idea, but the better the PA system, the more you will notice the difference between your £20 bargain and the other singer's Neumann. Both will sound their best if you have the best PA possible, but there is a lot of distance between them.
• Clean up distortion in source instruments or backline amplifiers
If it was distorted before it reached the multicore it will still be distorted when it gets to the mixer, and at every subsequent stage.
• Silence a noisy effects pedal or stop your guitar from buzzing near the dimmers
Single-coil pickups can pick up almost anything (including dimmers, induction loops, and the local taxi service). There isn't a ‘subtraction’ button on the desk, and although notch-filters and high-pass switches can reduce the effect of instrument-borne 50Hz mains hum, they can't remove it.
• Make a balanced sound out of a ragged unrehearsed noise
Although the mixer offers some control, it can't completely compensate for poorly-managed dynamics, and certainly can't stop the harmonica player from soloing over the lyrics. A watchful sound engineer can limit the damage, but can't make the overdriven lead solo happen at the right time. In general, the better the system, the more clearly the audience will hear what the band is doing, so practice and rehearsal are at least as important as equipment or technicians.
• Overcome poor planning or inadequate budgets
Covering big and/or difficult spaces optimally isn't easy or cheap (and combining adequate levels with high intelligibility for every seat in every row may not be realistically or economically possible).