Guide to getting and playing better gigs
Speakers
There are few guitarists who are totally happy with their sound. Some people will spend thousands on gear trying to find that elusive perfect tone, but all too frequently the most important element in the sound chain is neglected. Speakers are a great leveller, you can massage your guitar sound with the finest hand built point-to-point valve exotica, but just like the most basic practice amps, everything ends up as vibrations on a paper cone, so it stands to reason that the speaker can make or break a guitar sound.
Your guitar pickup is a transducer, it converts the kinetic energy of a moving string into a small electrical current. At the other end of the signal chain, the speaker changes that electrical current back into kinetic energy, moving a paper cone which in turn moves air molecules to create sound.
At the heart of your speaker is a coil of wire known as the voice coil, passing the amplifier's output signal through this coil generates a small magnetic field, which is acted on by the much bigger static field of the loudspeaker's magnet, and the end result is that the coil moves forwards and backwards in relation to the audio signal being passed through it.
Attach a diaphragm or cone to that voice coil and you get sound. In fact, a speaker is a dynamic microphone in reverse. The voice coil, magnet and cone are three of the speaker's four main components, number four is the basket, a steel or alloy pressing which supports the cone and magnet and maintains the close tolerances necessary for the speaker to work.
Ohms and impedance:
The most commonly misunderstood thing about loudspeakers is impedance - the loudspeaker's voice coil resistance. Wiring up one driver is straightforward, but when you tackle multiple loudspeakers in, say a 4x12 cab, things get a little more complex. But there's no real secret - just like any other resistor, there are only three ways to wire up multiple drivers, series, parallel and the series/parallel combination. Let's deal with series wiring first in a hypothetical 4x12.The 'hot' wire carrying the amp signal goes to the positive tag on driver 1 - the negative tag of driver 1 is wired to the positive tag of driver 2 and so on, with the negative tag of driver 4 going back to the amp. The sum total resistance is D1 + D2 + D3 + D4. If all the speakers were 4 ohm, the total impedance would be 16 ohms. Now for parallel wiring - this time the hot signal wire from the amp connects to all the positive terminals, the return wire connects to all the negative terminals.
The sum total resistance is calculated using reciprocals - if all the speakers were 16 ohm then you add 1/16 + 1/16 + 1/16 + 1/16 = 4/16 or 1/4, so the total load is 4 ohms. Series/parallel wiring is when you take two pairs of speakers each wired in series, and connect the two pairs in parallel - or vice versa - and the calculations are done just the same, so a 4x12 with four 8 ohm drivers wired in series/parallel will have a total load of 8 ohms. I just used the words 'impedance' and 'load' - in practice they mean the same thing.
A speaker's impedance 'loads' the amplifier's output stage, governing the amount of power available. Unlike a simple resistor, this impedance varies with frequency and heat - the ohm ratings are an indication, not a true figure. If an amp delivers 100 watts into an 8 ohm speaker, substituting a 4 ohm unit will make the amp try to deliver more power - the load has been reduced. If the power supply and output devices can handle it you could expect about 140 watts into 4 ohms, but if the amp was only designed for 100 watts into 8 ohms minimum, then something will give, you will either blow a fuse or an output device.
Valve amps are even less forgiving and much more dangerous, due to the high voltages present. A valve amp must never run below its minimum impedance, and never, ever run without a speaker connected. The output transformer on a valve amp not only matches the speakers to the valves, it also isolates the speaker/chassis from high voltage DC - typically 400 to 500 volts - so you can see why cooking it is not a good thing! Increasing the load is less hazardous - both valve and solid state amps will simply deliver less power.
If you buy a speaker you have to check its spec, looking for the right impedance and power handling. It's wrong to suppose that a 100 watt speaker won't get fried by a 100 watt amp - the RMS (Root, Mean, Square) method is the most realistic way of measuring amp power outputs, but doesn't take peak distortion levels into account - a 100 watt Marshall head flat out can hit peaks of 200 watts and more.
A speaker is it's own tone control - the size of the cone determines the upper and lower limits of frequency it will reproduce. The bigger the cone, the lower the range and vice-versa. A typical 12" driver is most efficient between 100Hz and 6KHz, which coincidentally is perfect for electric guitars although a 15" is sometimes used for extra low end thump.
A 10" driver on it's own lacks bottom end, but put four of them into one cab and you enhance the low end response - hence the success of the legendary Fender Bassman combo. But it's the harmonics above this region that give a driver its tone - distinguishing the silky-smooth midrange of an Eminence from the 'boxier' tonality of a Celestion Vintage 30 for example. And here there's no easy tip for success - your ears have to decide what works for you.