The term “ live sound equipment” here refers to the equipment used to make a performance audible to it’s audience and also to the performers.  This sound system is traditionally referred to as a “PA” or “PA system” PA being an abbreviation of “public address”.

While there is a wide variation in size and complexity of PA systems depending on their application the component parts are constant they are, working from the performer to the audience;

1.The means of converting the sound produced by the performers into an electrical signal that can be amplified and so be heard by the audience. This is usually done by Microphones which are transducers converting sound into electricity.  The output of electronic instruments can be directly plugged into the PA system

2 .The electrical signal produced by microphones and instrument outputs is very small and needs to be amplified in order to make useable. First the signal will go through a pre-amplifier, if there is only 1 microphone or a mixer, if there are 2 or more. Here the volume and other parameters can be set . The mixed signal is then made larger by a power amplifier or amplifiers to a sufficient level so that it can operate…

3. Loudspeakers, which are transducers, which work on the opposite way to microphones and convert electrical energy into sound, these are directed at the audience and also in many cases at the performers.

First let’s look in more detail at microphones and other input devices of the types used in PA systems.

Microphones can be categorised by their operating systems and acoustic characteristics. The most common type is the dynamic microphone. This consists of a plastic cone with a coil wound round it. The coil is held in a magnetic field. Movement of the cone caused by sound induces an electrical current to flow in the coil. This current varies with the volume and frequency of the sound waves applied to it.

Moving coil microphones tend to be robust and have good fidelity although they have a resonant peak in their frequency response at around 2.5khz. This can be used to advantage in good designs to increase intelligibility of speech but can be nasty in cheap microphones.

Moving coils are used for most purposes in live sound with particular models being tailored for specfic uses. The best known models being the Shure SM58, for live vocals and the Shure SM57 for instrument micing. These particular microphones exemplify the best in moving coil microphone design. They are extremely robust, utilise the natural response peak well, have good sonic characteristics and reasonable price.  Acoustically the good designs have good feedback rejection, particularly important when dealing with loud rock acts.

The one area where moving coils are not the best choice for live use is where lots of very high frequencies are needed to be captured such as miking cymbals.  In this case a capacitor or condenser microphone would be preferred.

The capacitor microphone works when sound waves strike a plastic aluminium coated diaphragm stretched over a metal cavity. This creates a capacitor. The sound causes the diaphragm to vibrate creating a fluctuating capacitance and so a charging current flows. This can be then amplified.  A DC charging current is needed for the capacitor and because the output impedance is very high a preamplifier needs to be built in to the microphone otherwise severe high frequency loss would occur.  The diaphragm is very light and so can respond very quickly to changes in sound and also respond to very high frequencies.  A good example of this type much used in PA applications is the AKG C451 which is often used on hi-hats and cymbals.

The complexities of construction mean that capacitor microphones are expensive. They are also less tough than moving coil types and are susceptible to moisture, which can cause flashovers across the capacitor. The polarising voltage is often supplied from the mixing desk in the form of “phantom power” and it is supplied to the microphone via the audio connector.

Because of the cost of true capacitor microphones a type called the electret has been developed, this has a permanently charged capacitor and a built in preamplifier usually powered by a 1.5 volt battery. The C1000, electret, also by AKG has proved very popular and is a good choice if you can only afford 1 capacitor mic.

The electret microphone offers many of the advantages of the true capacitor microphone at a much lower cost albeit with some compromises in performance mainly due to the thicker diaphragm of this type.

These are the 2 main types of microphone used in live sound, other types such as ribbons are mainly used for studio work. However it is worth mentioning one other type that can be useful in live sound; that is the pressure or boundary microphone. The operating principle of which uses the reflected sound wave from an object such as a wall very close to the microphone to reinforce it’s output.

This type if used correctly is very good for picking up large groups such as choirs although care must be taken in placement if good results are to be obtained and feedback avoided.

A major factor governing microphone choice for live applications is the polar response pattern of the microphone, that is how it’s sensitivity varies with the direction of the sound, obviously one usually wants it to pick up just the intended sound source.

With this in mind, designers make microphones with predictable response patterns.  These can be categorised as follows;

1 Omni directional, that is, picks up sound equally from all around

2 Figure of 8, picks up sound equally fro front and rear but less from sides

3 Cardioid, this is a heart shaped response pattern, and is the most used type in live sound applications. The supercardioid or hypercardioid type has an even narrower response plot than that of the cardioid.

For live sound applications there are then several factors that govern microphone choice and use.

1 Cost, as a rule the more expensive microphones work better than cheap ones but while it is true that there are very few budget microphones that work well in a live context, in recording though sometimes a cheap unit will give a particular sound which works well for a specific purpose, there is no point in using very expensive microphones if the rest of the system isn’t of equal quality.

2 Robust construction, Capacitor microphones in particular are fragile and need careful handling. Moving coil types tend to be most rugged

3 Response pattern and resistance to feedback, microphones with tight polar response patterns and high resistance to feedback will generally be preferred

4 Sound quality and client preference, a sound system can only be as good as its weakest component and microphones do have a sonic footprint with models being tailored for particular uses such as rock vocals or use in bass drums.

Client preference is also a factor, microphones become fashionable because well know performers use them and so suppliers will be asked to provide certain models.

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