Wikijunior:How Things Work/Audio Speakers
When we hear things, our ears are responding to tiny vibrations in the air, and converting them to brain signals. These air vibrations are called audio or sonic frequencies. They are a bit like waves on a pond - the air is compressed and stretched many times a second. How many times a second? The range is typically considered to be between twenty and twenty-thousand times a second. We write that as 20Hz - 20kHz in honor of a German physicist, Heinrich Hertz. The k is for kilo, meaning "multiply this by one-thousand". Hertz is one of the International Standards or SI Units.
When we speak, we make the air in our larynx (plural larynges) vibrate at an audio frequency. The larynx is also known as the voicebox in our throats. Electronic devices such as telephones and radios need "speakers" too. They make the air vibrate using a disk of stiff material called a diaphragm, which is vibrated by an electro-magnetic device called a transducer. Trans means to transfer, and ducto means to lead, so in this case, the phrase "electro-mechanical transducer" implies that electrical signals lead to mechanical movement. If audio-speakers are very small and do not make much noise we usually call them earphones or headphones. Loudspeakers tend to be rather bigger and, er, well louder!
Traditional earphones and speakers relied on a transducer made by an electro-magnetic coil suspended in a strong magnetic field. Today there are other types of transducers which use crystals, but for most speakers, the traditional design is common (although modern materials have greatly enhanced their performance). Actually, speakers and microphones are really quite similar, except that speakers generally have to move lots of air, and are therefore bigger and heavier than microphones, which have to detect rather weak sound-waves.
Speakers and microphones can not follow the whole range of sounds we can hear, which is why in high-fidelity systems, there are tiny "tweeter" speakers to reproduce the high frequencies and big "woofers" for the low bass notes, as well as other mid-range speakers. If you hold your open hand near a woofer and turn up the volume, probably you will feel the low frequency sound waves.