LUC & SAM Electronics: Speaker Construction

The construction of speakers is approached in the same way as musical instrument making. Fine tolerances and attention to detail make large differences to performance. Large musical instruments and speakers suit low frequencies and vice versa. Each speaker and instrument can only function efficiently with linearity, within 3 octaves (octave is ratio 1:2). Theoretically a single speaker would have to change diameter from (1in - 24ft) (20mm - 8m) to maintain similar level and dispersion over the frequency spectrum.

The majority consist of paper or plastic moulded into a cone shape, loosely suspended in a frame so as to easily move back and forth to vibrate the air. Glued to the back of the cone is a coil of wire (voice coil) within a strong magnet field. Passing electricity through wire causes a magnetic field around the wire, which attracts or repels, causing the cone to move back and forth. The larger the magnet and voice coil (BL) the greater the power and efficiency, if well made. Externally vibrating the cone will cause the voice coil to generate electricity. A speaker can work well as a microphone especially for bass drums.

The energy of the magnet is conducted through the mild steel pole plates and pole piece, and concentrated (north - south) across the gap. Hopefully the voice coil has been perfectly centred in the gap. The clearances are very very small, less than half a bees dick. The smaller the gap - the more intense the magnetic field - the greater the efficiency. The slightest variations in alignment, during manufacture, cause large variations in performance. No two speakers or musical instruments can be identical.

Voice Coils. Passing electricity through wire causes a magnetic field around the wire. Changing polarity of the electric current through the wire, also changes the polarity of the magnetic field created around the wire. The interaction of the two magnetic fields, causes the voice coil, to be pushed out of the gap, forward or backward, depending on the polarity of the electricity through the voice coil.