7 Speakers and amps In order to reproduce sound or project it above its normal acoustic levels we need to amplify it and force it in the desired direction. The basics of this involve a speaker (or loudspeaker to give it its full name) and an amplifier, or amp for short. These pieces of equipment are designed to reproduce the audio signals that they receive and project them at a higher level, and/or provide a greater spread of sound in a way as faithful to the input as possible plus any enhancements that are required to make the sound more pleasing. Although we're mainly interested in PA systems here, the use of speakers and amps obviously extends to the amplification of instruments as well. While speakers are similar for PA and instruments the amplifiers used are not quite the same. As the signal is being adjusted by a mixing console and other processing equipment there is no need for the amplifiers to have controls such as EQ, reverb or extra gain stages; we usually call them `power amps' as that is their main purpose, to power the system. Without going into a physics lesson we'll take a quick look at the principles behind what we're trying to achieve with our speaker and amp. Sound radiates from its source as vibrations, which are often likened to ripples on water when a stone has been thrown in. These `waves' have differing frequencies, which, very simply put, would be the frequency of the peaks of our ripples in the water, counted over a mea- sured time such as a second. Low frequencies could be 50 times a second, for instance, and high frequencies could be 10 000 times a second. Without jumping the gun the low would be the lower end of the audio range that we call `bass' and the high would be nearer the high end that we call `treble', just like on your home music system. Most