Electrodynamic speaker driver

Anelectrodynamic speaker driver,often called simply aspeaker driverwhen the type is implicit, is an individualtransducerthat converts an electricalaudio signaltosound waves.While the term is sometimes used interchangeably with the termspeaker(loudspeaker), it is usually applied to specialized transducers that reproduce only a portion of theaudible frequencyrange. Forhigh fidelityreproduction of sound, multiple loudspeakers are often mounted in the sameenclosure,each reproducing a different part of the audible frequency range. In this case the individual speakers are referred to asdriversand the entire unit is called aloudspeaker.Drivers made for reproducing high audio frequencies are calledtweeters,those for middle frequencies are calledmid-rangedrivers and those for low frequencies are calledwoofers,while those for very low bass range aresubwoofers.Less common types of drivers aresupertweetersandrotary woofers.

Theelectroacousticmechanism most widely used in speakers to convert the electric current to sound waves is thedynamicorelectrodynamicdriver, invented in 1925 byEdward W. KelloggandChester W. Rice,which creates sound with a coil of wire called avoice coilsuspended between the poles of amagnet.There are others that are far less widely used:electrostatic drivers,piezoelectric drivers,planar magnetic drivers,Heil air motion drivers,andionic drivers,amongother speaker designs.^[1]

Design

Cutaway view of a dynamic loudspeaker for the bass register.

Magnet
Voicecoil
Suspension
Diaphragm

Cutaway view of a dynamic midrange speaker.

Magnet
Cooler (sometimes present)
Voicecoil
Suspension
Diaphragm

Cutaway view of a dynamic tweeter with acoustic lens and a dome-shaped membrane.

Magnet
Voicecoil
Diaphragm
Suspension

The most common type of driver, commonly called adynamic loudspeaker,uses a lightweightdiaphragm,orcone,connected to a rigidbasket,orframe,via a flexible suspension, commonly called aspider,that constrains avoice coilto moveaxiallythrough a cylindrical magnetic gap. A protectivedust capglued in the cone's center prevents dust, most importantlyferromagneticdebris, from entering the gap.

When an electrical signal is applied to the voice coil, amagnetic fieldis created by the electriccurrentin the voice coil, making it a variable electromagnet. The coil and the driver's magnetic system interact in a manner similar to asolenoid,generating a mechanical force that moves the coil (and thus, the attached cone). Application of alternating current moves the cone back and forth, accelerating and reproducing sound under the control of the applied electrical signal coming from the amplifier.

The following is a description of the individual components of this type of loudspeaker.

Diaphragm

The diaphragm is usually manufactured with a cone- or dome-shaped profile. A variety of different materials may be used, but the most common are paper, plastic, and metal. The ideal material is rigid, to prevent uncontrolled cone motions, has low mass to minimize starting force requirements and energy storage issues and is welldampedto reduce vibrations continuing after the signal has stopped with little or no audible ringing due to itsresonancefrequency as determined by its usage. In practice, all three of these criteria cannot be met simultaneously using existing materials; thus, driver design involvestrade-offs.For example, paper is light and typically well-damped, but is not stiff; metal may be stiff and light, but it usually has poor damping; plastic can be light, but typically, the stiffer it is made, the poorer the damping. As a result, many cones are made of some sort of composite material. For example, a cone might be made of cellulose paper, into which somecarbon fiber,Kevlar,glass,hemporbamboofibers have been added; or it might use a honeycomb sandwich construction; or a coating might be applied to it so as to provide additional stiffening or damping.

Basket

The chassis, frame, or basket, is designed to be rigid, preventing deformation that could change critical alignments with the magnet gap, perhaps allowing the voice coil to rub against the magnet around the gap. Chassis are typicallycastfrom aluminum alloy, in heavier magnet-structure speakers; orstampedfrom thin sheet steel in lighter-structure drivers.^[2]Other materials such as molded plastic and damped plastic compound baskets are becoming common, especially for inexpensive, low-mass drivers. A metallic chassis can play an important role in conducting heat away from the voice coil; heating during operation changes resistance, causes physical dimensional changes, and if extreme, broils the varnish on the voice coil; it may even demagnetize permanent magnets.

Suspension

The suspension system keeps the coil centered in the gap and provides a restoring (centering) force that returns the cone to a neutral position after moving. A typical suspension system consists of two parts: thespider,which connects the diaphragm or voice coil to the lower frame and provides the majority of the restoring force, and thesurround,which helps center the coil/cone assembly and allows free pistonic motion aligned with the magnetic gap. The spider is usually made of acorrugatedfabric disk, impregnated with a stiffening resin. The name comes from the shape of early suspensions, which were two concentric rings ofBakelitematerial, joined by six or eight curvedlegs.Variations of this topology included the addition of a felt disc to provide a barrier to particles that might otherwise cause the voice coil to rub.

The cone surround can berubberor polyesterfoam,treated paper or a ring of corrugated, resin-coated fabric; it is attached to both the outer cone circumference and to the upper frame. These diverse surround materials, their shape and treatment can dramatically affect the acoustic output of a driver; each implementation has advantages and disadvantages. Polyester foam, for example, is lightweight and economical, though usually leaks air to some degree and is degraded by time, exposure to ozone, UV light, humidity and elevated temperatures, limiting useful life before failure.

Voice coil

The wire in a voice coil is usually made ofcopper,thoughaluminum—and, rarely,silver—may be used. The advantage of aluminum is its light weight, which reduces the moving mass compared to copper. This raises the resonant frequency of the speaker and increases its efficiency. A disadvantage of aluminum is that it is not easily soldered, and so connections must be robustly crimped together and sealed. Voice-coil wire cross sections can be circular, rectangular, or hexagonal, giving varying amounts of wire volume coverage in the magnetic gap space. The coil is oriented co-axially inside the gap; it moves back and forth within a small circular volume (a hole, slot, or groove) in the magnetic structure. The gap establishes a concentrated magnetic field between the two poles of a permanent magnet; the outside ring of the gap is one pole, and the center post (called the pole piece) is the other. The pole piece and backplate are often made as a single piece, called the poleplate or yoke.

Magnet

The size and type of magnet and details of the magnetic circuit differ, depending on design goals. For instance, the shape of the pole piece affects the magnetic interaction between the voice coil and the magnetic field, and is sometimes used to modify a driver's behavior. Ashorting ring,orFaraday loop,may be included as a thin copper cap fitted over the pole tip or as a heavy ring situated within the magnet-pole cavity. The benefits of this complication is reduced impedance at high frequencies, providing extended treble output, reduced harmonic distortion, and a reduction in the inductance modulation that typically accompanies large voice coil excursions. On the other hand, the copper cap requires a wider voice-coil gap, with increased magnetic reluctance; this reduces available flux, requiring a larger magnet for equivalent performance.

Electromagnets were often used inmusical instrument amplifierscabinets well into the 1950s; there were economic savings in those using tube amplifiers as the field coil could, and usually did, do double duty as a power supply choke. Very few manufacturers still produceelectrodynamic loudspeakers with electrically powered field coils,as was common in the earliest designs.

Alnico,an alloy of aluminum, nickel, and cobalt became popular after WWII, since it dispensed with the problems of field-coil drivers. Alnico was commonly used until the 1960s, despite the problem of alnico magnets being partiallydemagnetized.^[3]In the 1960s, most driver manufacturers switched from alnico toferrite magnets,which are made from a mix of ceramic clay and fine particles of barium or strontium ferrite. Although the energy per kilogram of these ceramic magnets is lower than alnico, it is substantially less expensive, allowing designers to use larger yet more economical magnets to achieve a given performance. Due to increases in transportation costs and a desire for smaller, lighter devices, there is a trend toward the use of more compactrare-earth magnetsmade from materials such asneodymiumandsamarium cobalt.^[4]

Components

Cut-away view of a dynamic loudspeaker

Speaker drivers include adiaphragmthat moves back and forth to createpressure wavesin the air column in front, and depending on the application, at some angle to the sides. The diaphragm is typically in the shape of a cone for low and mid frequencies or a dome for higher frequencies, or less commonly, a ribbon, and is usually made of coated or uncoated paper or polypropylene plastic.^[5]More exotic materials are used on some drivers, such as wovenfiberglass,carbon fiber,aluminum,titanium,pure cross carbon and a very few use PEI, polyimide,PET filmplastic film as the cone, dome or radiator.

All speaker drivers have a means of electricallyinducingback-and-forth motion. Typically there is a tightly wound coil of insulated wire (known as avoice coil) attached to the neck of the driver's cone. In a ribbon speaker, the voice coil may be printed or bonded onto a sheet of very thin paper, aluminum, fiberglass or plastic. This cone, dome or other radiator is mounted at its outer edge by a flexible surround to a rigid frame which supports a permanentmagnetin close proximity to the voice coil. For the sake of efficiency, the relatively lightweight voice coil and cone are the moving parts of the driver, whereas the much heavier magnet remains stationary. Other typical components are a spider or damper, used as the rear suspension element, simple terminals orbinding poststo connect the audio signal, and possibly a compliantgasketto seal the joint between the chassis and enclosure.

Enclosures and acoustic isolation

Drivers are almost universally mounted into a rigid enclosure of wood, plastic, or occasionally metal. Thisloudspeaker enclosureor speaker box isolates the acoustic energy from the front of the cone from that of the back of the cone. Ahornmay be employed to increase efficiency and directionality. Agrille,fabricmesh,or other acoustically neutral screen is generally provided to cosmetically conceal the drivers and hardware, and to protect the driver from physical damage.

Operation

In operation, a signal is delivered to the voice coil by means of electrical wires, from the amplifier throughspeaker cable,then through flexibletinsel wireto the moving coil. The current creates amagnetic fieldthat causes the diaphragm to be alternately forced one way or the other, by the magnetic field produced by current flowing in the voice coil, against the field established in the magnetic gap by the fixed magnet structure as the electrical signal varies. The resulting back-and-forth motion drives the air in front of the diaphragm, resulting in pressure differentials that travel away assound waves.

The spider and surround act as a spring-restoring mechanism for motion away from the balanced position established when the driver was assembled at the factory. In addition, each contributes to centering the voice coil and cone, both concentrically within the magnet assembly, and front-to-back, restoring the voice coil to a critical position within the magnetic gap, neither toward one end nor the other.

The voice coil and magnet essentially form alinear motorworking against the centering "spring tension" of the spider and surround. If there were no restriction on travel distance imposed by the spider and surround, the voice coil could be ejected from the magnet assembly at high power levels, or travel inward deep enough to collide with the back of the magnet assembly. The majority of speaker drivers work only against the centering forces of the spider and surround and do not actively monitor the position of the driver element or attempt to precisely position it. Some speaker driver designs have provisions to do so (typically termedservomechanisms); these are generally used only in woofers and especially subwoofers, due to the greatly increased cone excursions required at those frequencies in a driver whose cone size is well under the wavelength of some of the sounds it is made to reproduce (ie, bass frequencies below perhaps 100 Hz or so).

Performance characteristics

Speaker drivers may be designed to operate within a broad or narrowfrequencyrange. Small diaphragms are not well suited to moving the large volume of air that is required for good low-frequency response. Conversely, large drivers may have heavy voice coils and cones that limit their ability to move at very high frequencies. Drivers pressed beyond their design limits may have highdistortion.In a multi-way loudspeaker system, specialized drivers are provided to produce specific frequency ranges, and the incoming signal is split by acrossover.Drivers can be sub-categorized into several types:full-range,tweeters,super tweeters,mid-rangedrivers,woofers,andsubwoofers.

Applications

Speaker drivers are the primary means for sound reproduction. They are used among other places in audio applications such as loudspeakers,headphones,telephones,megaphones,instrument amplifiers,television andmonitorspeakers,public addresssystems,portable radios,toys,and in many electronics devices that are designed to emit sound.

References

^"The development of the loudspeaker Prof. Dr.–Ing. Dietmar Rudolph March 23, 2013"(PDF).
^Rumsey, Francis; McCormick, Tim (2009).Sound and Recording(6th ed.). Oxford, UK: Focal Press. p. 81.ISBN 978-0-240-52163-3.
^"Speaker Geeks: Alnico or Ceramic... What Gives?".RetrievedApril 26,2024.
^Self, Douglas (2012).Audio Engineering Explained.Taylor & Francis. p. 361.ISBN 9781136121258.
^"Electrodynamic loudspeaker patent US7676053B2".

[1] "The development of the loudspeaker Prof. Dr.–Ing. Dietmar Rudolph March 23, 2013"(PDF).

[2] Rumsey, Francis; McCormick, Tim (2009).Sound and Recording(6th ed.). Oxford, UK: Focal Press. p. 81.ISBN 978-0-240-52163-3.

[3] "Speaker Geeks: Alnico or Ceramic... What Gives?".RetrievedApril 26,2024.

[4] Self, Douglas (2012).Audio Engineering Explained.Taylor & Francis. p. 361.ISBN 9781136121258.

[5] "Electrodynamic loudspeaker patent US7676053B2".

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