A moving-coil speaker consists of a wire coil suspended inside a magnet. When an audio signal passes through the coil a varying magnetic field is set up which interacts with the magnet and causes the coil to move back and forth. A cone (or in the case of high frequency speakers, a dome) is attached to the coil, which compresses and rarifies the air generating sound waves.
Flexible
Surround
Voice Coil
Pole Piece
Coil Former
Speaker Cone
Rear Suspension
Chassis (or Basket)
Because large speaker drive-units cannot reproduce high frequencies well, and small ones cannot reproduce low frequencies at all, practical hi-fi loudspeakers use two or more drive- units connected via a ‘crossover network’ which divides the frequencies between the bass and the treble units. Additionally, because even a large bass- unit is very small compared to the length of a low-frequency sound wave, some form of box is needed to prevent the waves from the back of the cone cancelling the sound from the front.
Moving-coil speakers have undeniable advantages of cost and ease of manufacture, and the cabinets can be designed in many different shapes, sizes and finish. However, the cabinet is also a prime source of colouration in the bass region. Additionally, because the cone of a bass-unit is large and heavy, it cannot start and stop instantaneously. Some of the energy is stored and releases over time, causing smearing and ‘boom’ in the bass. In the cross- over region, where not only are the masses between the drive units hugely dissimilar but so are their dispersion patterns, this shows up as unevenness of response and a confused image, precisely in the critical area where the ear is at its most sensitive.