A loudspeaker (or ‘speaker’) is an electro-acoustic device that converts electrical signals into sound. Speakers pulse in accordance with the variations of an electrical signal and sound waves propagate through air.
We will give a brief description of how electrodynamic speakers (the most commonly used type of speaker) work to reproduce as faithfully as possible the various sounds that nature and musical instruments produce.
Electrodynamic speakers are the most popular speakers. They come in various shapes, sizes and price brackets. Our familiar cones and domes characterize electrodynamic speakers. They are the diaphragms that generate the sound and usually the only visible parts of a loudspeaker. The electrodynamic speakers’ operation is based on the principles of electromagnetic induction. That is, when a conductor moves in a magnetic field it experiences forces that result in the generation of an internal electric field and potential differences at its ends.
At the heart of the electrodynamic speakers there is a strong permanent magnet, cylindrical in shape, with a cylindrical shaft (the pole) at its centre. Between the pole and the magnet there is a space of a few millimeters, in which a very strong and homogeneous magnetic field exists. The space between the magnet and the pole holds the voice coil. The voice coil is free to move in the magnetic field and supported by an elastic suspension, which makes sure that the coil does not touch the pole, and behaves as if it is floating. When the audio signal in the form of alternating current is conducted through the coil, forces are generated that cause it to move back and forth. On the outside of the coil a diaphragm is attached, the size of which determines the lowest frequency that can be reproduced. The diaphragm moves and sound is generated.
Larger diameter cones require larger voice coils and magnets. This creates speaker drives with large mass and inertia, which require more power to drive. In an effort to minimize the mass of the diaphragm while keeping the required rigidity, synthetic and sometimes exotic materials are used, such as polypropylene, Kevlar, titanium etc.
When the cone of a loudspeaker moves forward to impart pressure on the air layers in front of it, then an equal and opposite-directed decompression is created behind it. The low frequencies generated in front of the cone are non-directional and move to cover the area in front and behind the cone. This causes their cancellation, since they interfere destructively with the equal, yet out of phase, low frequencies generated behind the cone.
The ideal way to avoid this phenomenon is to place the speaker in the middle of a large surface. This is known as an ‘infinite baffle’. Of course, this solution is totally impractical so manufacturers resort to surrounding the speaker with a cabinet. Cabinets are therefore used to support the speaker drives and nullify unwanted cancellations.
The cabinet design contributes greatly to the response of the loudspeaker and either mimics the free loudspeaker behavior (infinite diaphragm design) or uses the enclosed air to improve performance (bass reflex and acoustic suspension design). The ideal speaker cabinet is rigid so that it does not vibrate from the internal air pressure variations. In addition the cabinet must have significant damping behavior to minimize unwanted sound radiation.