chapter  7
19 Pages

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 measured 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 lowwould 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

sound sources produce a range of frequencies and so they are likely to consist of a very complex pattern of waves. We use a measurement unit called hertz to express

the frequency of sound, which translates as ‘cycles per second’ and is abbreviated to ‘Hz’. In true human fashion it is named after the scientist Heinrich Rudolf Hertz (1847-1894), who was the first to transmit and receive radio waves (useful to know for quizzes). When we reach

into the thousands of hertzwe add the prefix kilo (meaning thousand), so 10 000Hz would equal 10 kHz, much easier to write and remember. As examples, middle C (C4) on a piano is 262Hz, A4

is 440Hz, low E string on a bass (E1) is 41Hz and instruments such as cymbals may work in the high kilohertz up to around 16 kHz and above. You could also go to megahertz (millions of cycles per

second) or gigahertz (billions of cycles per second), but

we can’t actually hear above around 20 kHz and so we don’t need to go there for live sound! The lower end of human hearing is around 20Hz but low frequencies can often be felt physically as well. Hearing range will weaken over time, so as we grow older the higher figure usually drops to maybe 12-14 kHz. Bear this in mind and do not accelerate it by exposing yourself to high levels for long periods of time.