The Basics in Loudspeaker Driver Mechanics – It’s important to understand how a basic loudspeaker works so we’ve included a basic diagram of a typical woofer with all the major components labeled.
The loudspeaker is broken into several parts:
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The Basics in Loudspeaker Driver Mechanics
Loudspeaker Driver
The Diaphragm (aka cone or dome) – the actual physical surface area of the driver that moves up and down from the voice coil interaction with the permanent magnet, thus producing sound by compressing and rarefying air, thus creating sound waves. (sound is not “moving air” it is propagating pressure fluctuations.)
The Dustcap (optional) – located at the center of the cone and covers the voice coil and can increase rigidity of the diaphragm and most importantly in seals the diaphragm.
The Basket (aka. Frame)– the physical structure that holds the entire driver together
The Voice Coil – basically a wire coiled around a bobbin (electromagnet) at the center of the speaker and attached to the spider. When current flows through the coil supplied by the amplifier. it produces a temporary magnetic field when a signal is applied to interact with the permanent magnet (++ oppose ; +- attract). Where extra bobbin strength is needed, a collar can be added to that portion of the bobbin not covered by the coil.
The Spider (aka. Damper) – Two prime functions of the spider, in conjunction with the surround, are to center the diaphragm, and force the forward and back movements of the voice coil to be linear, so that the coil does not scrape the narrow magnetic gap. The Spider provides the primary restorative force in the suspension being ideally placed close to the motive force, the voice coil. Linearity of a drivers suspension (and hence compliance) is critical and a lot of research has gone into designing spiders with the necessary stiffness and linearity, and designed not to add structural resonances of their own. The compliance of the spider & surround helps determine the low frequency limit of the drivers useful bandwidth, specified by the well-known free-air resonance (fs) parameter
The Suspension (aka. Surround) – typically made of cloth, butyl rubber or foam, attaches the diaphragm to the driver basket and in conjunction with the Spider, helps to control and dampen cone movement. In some cases there may be more than one spider and in some cases (tweeters) there is no spider. In woofers, the surround is necessary to simply hold the edge of the cone in place and, along with the spider, to ensure linear in and out movement of the voice coil. It also functions as a mechanical termination for the radial traveling waves in the cone (cones do flex!) where it can dampen standing waves that will occur. The stiffness of the cone material is a factor in determining the frequencies at which resonances occur – higher being better, but at least outside the intended operating frequency range.
The Permanent Magnet – a fixed DC magnet which is part of the motor structure affixed to the basket of the driver creating a stable magnetic flux across the annular gap which the voice coil sits in.
The Phase Plug (optional) – similar to the cone but typically looks like a bullet. Its purpose is three-fold, to reduce moving mass and on-axis beaming and also serve as a heat sink and to a smaller extent offering venting.
The Vented Pole Piece (optional) – this is a hole located in the center at the back of the permanent magnet to help reduce air pressure under the dust cap and cool the voice coil.
To generalize a bit more, the magnet assembly of a loudspeaker consisting of a top plate, bottom plate and pole piece, voice coil and magnet can be considered the “motor structure”.
This Polk Loudspeaker Driver example appears to be quite a heavy duty driver as evident by how much venting and free air-flow they are incorporating into the design. This is the type of bass or midrange driver you typically see in more expensive speaker systems that are able to sustain high output levels with low distortion and low compression. More budget-oriented designs typically won’t have vented pole pieces, which will limit how much thermal energy they are able to dissipate at high output levels, thus potentially causing more compression.[/fusion_builder_column][/fusion_builder_row][/fusion_builder_container]
