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What It All Means

Loudspeakers

This month, we tell you (very softly) exactly what the loudspeaker jargon really means.


CHASSIS: The metal frame holding the magnet and cone in place, and supplying the holes on the rim to fix the speaker.

COLORATION: Bad speakers can change the sound from the original, emphasising or squashing certain frequencies. That's coloration.

CONE: The bit which actually shifts the air. It's shaped to gather air efficiently and throw it at you. Must be light but fairly stiff (often a treated paper).

CONE EDGE: The termination is the flexible but resilient material which attaches the edge of the cone to the outside of the metal chassis. Often a rubber coated cloth.

CROSSOVER: Divides the full, incoming signal into separate frequency bands for the speakers which cover those areas — bass, mid-range etc. Can be passive (needs no external power, and is usually connected between amp and speaker), or active (needs power, found between pre-amp and power amp so each band is also individually amplified).

DISPERSION: How far the speaker projects the sound, and at what angle. Long throwers pitch the sound to the back of the hall, but in a narrow beam. A broad beam dispersion may cover the front row from one edge of the hall to the other, but penetrate no further than the 10th row of seats. Generally, the higher the frequency range of the speaker, the more directional (narrower beam) it becomes. For example a bullet tweeter featuring a short horn with a circular mouth projects the sound well, but with a very narrow beam. Slot tweeters (again the shape of the mouth) give the opposite.

DUST CAP: The dome at the centre of the cone, made of PVC or paper, which protects the voice coil.

HORN LOADED: See Infinite Baffle. Far more complex than reflex. Extra interior struts, curves and vents are added, and the speaker often pointed backwards in the cab to fool the air into doing better things for you.

IMPEDANCE: The degree of opposition a speaker will put up to an incoming signal. As always simple matching is more important than the theory but is easier to ensure on speakers than on recorder line inputs, mikes, etc. Most speakers are eight ohms, and that's a perfect match for the majority of amps. Transistor amps can usually run at four ohms (giving a little more power) and 16 ohms (a little less). Much lower than four and it's like trying to shout loud enough to deafen someone in St Paul's. Your throat won't take it. Valve amps are a little fussier and often have different sockets for eight or 16 ohm speakers.

INFINITE BAFFLE: Each time a speaker pushes air forward at the front, it obviously sucks it in at the back. If these two air currents mix they spoil the sound, so we mount the speaker in a wall (a baffle) to cut the currents off from each other. So no air from the rear can creep round the edges, we make the wall infinitely high and infinitely wide. Not so difficult: you simply bend the edges backwards to form a box and this is the infinite baffle.

SENSITIVITY: One measure of how loud (not a technical term) one speaker will sound against another. The greater the sensitivity (expressed in dB) the more sound you will get from one watt of input (the result usually being measured one metre from the speaker).

SOUND PRESSURE LEVEL: You may not see the words on the spec sheet, but the figures are there. The SPL is what's actually being recorded at that one metre distance. It's the true measure of loudness. Watts only describe the amount of electrical energy the speaker can withstand, not how well the speaker converts that power to volume. A 50 Watt combo may sound louder than a 100 Watt combo because the speakers are more efficient and sensitive. The SPL is measured in dB and a difference of 3dB represents a doubling or halving (-3dB) of the sound output. A loudspeaker with an average SPL of 99dB will be twice as loud as one with 96dB.

POWER HANDLING: (Read Sensitivity first.) A guide to how loud the speaker will ultimately sound when you're turned up full. A speaker with 100dB sensitivity and a 40 watt rating won't sound as powerful as a 98dB 80 watt alternative when both are up full. But look out for the distortion at rated power. When comparing speakers of equivalent power rating, the one with the lower distortion will squeeze out more before the speaker cracks up.

REFLEX: See Infinite Baffle. Low bass notes move a lot of air, but the restricted amount of 'breathing space' within a closed cabinet can stifle that movement. A hole (port or vent) of carefully calculated size is cut in the front of the baffle as a windpipe to improve the bass response.

VOICE COIL: Ploughing through your Physics notes you'll remember that a coil of wire placed within the field of a permanent magnet will move when a voltage is passed through it. The voice coil is such a beast. It's wound on the circular former (treated paper, glass fibre, but always non conductive) and is fixed to the speaker cone. When the signal passes through the coil, it and the cone are moved. We have sound. Much heat is generated; formers have to be tough.



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Making Music - Copyright: Track Record Publishing Ltd, Nexus Media Ltd.

 

Making Music - Jul 1986

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