Editing Loudspeaker (section)

==Other speaker designs==
While dynamic cone speakers remain the most popular choice, many other speaker technologies exist.<ref name="Ballou 2008" />{{Rp|705–714}}

===With a diaphragm===

====Moving-iron loudspeakers====
{{Main|Moving iron speaker}}
[[File:Moving-iron cone speaker 1929.png|thumb|upright|Moving iron speaker]]

The original loudspeaker design was the moving iron. Unlike the newer dynamic (moving coil) design, a moving-iron speaker uses a stationary coil to vibrate a magnetized piece of metal (called the iron, reed, or armature).  The metal is either attached to the diaphragm or is the diaphragm itself.  This design originally appeared in the early telephone. 

Moving iron drivers are inefficient and can only produce a small band of sound.  They require large magnets and coils to increase force.<ref>{{cite book |last=Crowhurst |first=Norman H. |date=1959 |title=Basic Audio |url=http://www.tubebooks.org/Books/crowhurst_basic_1.pdf |via=Tubebooks.org<!--old public domain book; somebody republished a scanned copy on their website--> |volume=1 |publisher=John F. Rider Publisher |page=63 |access-date=April 30, 2024 }}</ref>

Balanced armature drivers (a type of moving iron driver) use an armature that moves like a see-saw or diving board.  Since they are not damped, they are highly efficient, but they also produce strong resonances.  They are still used today for high-end [[earphones]] and hearing aids, where small size and high efficiency are important.<ref>{{cite web |last=Hertsens |first=Tyll |title=How Balanced Armature Receivers/Drivers Work |url=http://www.innerfidelity.com/content/how-balanced-armature-receiversdrivers-work#IboPCt5E0YtmLTAL.97 |website=InnerFidelity.com |date=December 16, 2014 |archive-url=https://web.archive.org/web/20150914044126/http://www.innerfidelity.com/content/how-balanced-armature-receiversdrivers-work#IboPCt5E0YtmLTAL.97 |archive-date=September 14, 2015 |access-date=September 5, 2015 }}</ref>

====Piezoelectric speakers====
{{Main|Piezoelectric speaker}}
{{See also|Piezo tweeter}}
[[File:2007-07-24 Piezoelectric buzzer.jpg|thumb|A piezoelectric buzzer. The white ceramic piezoelectric material can be seen fixed to a metal diaphragm.]]

Piezoelectric speakers are frequently used as beepers in [[watch]]es and other electronic devices, and are sometimes used as tweeters in less-expensive speaker systems, such as computer speakers and portable radios. Piezoelectric speakers have several advantages over conventional loudspeakers: they are resistant to overloads that would normally destroy most high-frequency drivers, and they can be used without a crossover due to their electrical properties. There are also disadvantages: some amplifiers can oscillate when driving capacitive loads like most piezoelectrics, which results in distortion or damage to the amplifier. Additionally, their frequency response, in most cases, is inferior to that of other technologies. This is why they are generally used in single-frequency (beeper) or non-critical applications.

Piezoelectric speakers can have extended high-frequency output, and this is useful in some specialized circumstances; for instance, [[sonar]] applications in which piezoelectric variants are used as both output devices (generating underwater sound) and as input devices (acting as the sensing components of [[underwater microphone]]s). They have advantages in these applications, not the least of which is simple and solid-state construction that resists seawater better than a ribbon or cone-based device would.

In 2013, [[Kyocera]] introduced piezoelectric ultra-thin medium-size film speakers with only one millimeter of thickness and seven grams of weight for their 55" [[OLED]] televisions and they hope the speakers will also be used in PCs and tablets. Besides medium-size, there are also large and small sizes which can all produce relatively the same quality of sound and volume within 180 degrees. The highly responsive speaker material provides better clarity than traditional TV speakers.<ref>{{cite web |last=Fingas |first=Jon |url=https://www.engadget.com/2013/08/29/kyocera-piezoelectric-film-speaker/#continued |title=Kyocera Piezoelectric Film Speaker Delivers 180-Degree Sound to Thin TVs and Tablets (Update: Live Photos) |website=Engadget |date=August 29, 2013 |access-date=April 30, 2024 }}</ref>

====Magnetostatic loudspeakers====
{{Main|Magnetostatic loudspeaker}}
[[File:Magnepan MG1.jpg|thumb|upright|Magnetostatic loudspeaker]]

Instead of a voice coil driving a speaker cone, a magnetostatic speaker uses an array of metal strips bonded to a large film membrane. The magnetic field produced by signal current flowing through the strips interacts with the field of permanent bar magnets mounted behind them. The force produced moves the membrane and so the air in front of it.  Typically, these designs are less efficient than conventional moving-coil speakers.

====Magnetostrictive speakers====
Magnetostrictive transducers, based on [[magnetostriction]], have been predominantly used as [[sonar]] ultrasonic sound wave radiators, but their use has spread also to audio speaker systems. Magnetostrictive speaker drivers have some special advantages: they can provide greater force (with smaller excursions) than other technologies; low excursion can avoid distortions from large excursion as in other designs; the magnetizing coil is stationary and therefore more easily cooled; they are robust because delicate suspensions and voice coils are not required. Magnetostrictive speaker modules have been produced by Fostex<ref>{{cite web|first=Takeyoshi |last=Yamada |title=Fostex Prototypes Tabletop Vibration Speaker System Using Super Magnetostrictor |url=http://techon.nikkeibp.co.jp/english/NEWS_EN/20051117/110823/ |publisher=Tech-On! |date=November 2005 |quote=The cone-shaped speaker system is 95 mm in diameter and 90 mm high. It features an actuator using a magnetostrictor that extends and shrinks in line with magnetic field changes. The actuator converts input sound into the vibration and conveys it to the tabletop thus rendering sound. |access-date=October 5, 2009 |ref=Fostex1 }}</ref><ref>{{cite web |first=Hirofumi |last=Onohara |title=(WO/2006/118205) Giant-Magnetostrictive Speaker |url=http://www.wipo.int/pctdb/en/wo.jsp?wo=2006118205 |archive-url=https://archive.today/20120805230459/http://www.wipo.int/pctdb/en/wo.jsp?wo=2006118205 |archive-date=August 5, 2012 |publisher=World Intellectual Property Organization |date=November 2006 |access-date=October 5, 2009 |quote=A giant-magnetostrictive speaker exhibiting good acoustic characteristics when it is used while being placed on a horizontal surface. |ref=Fostex2}}</ref><ref>{{Cite patent |country=JP |number=WO/2006/118205 }}</ref> and [[FeONIC]]<ref>{{cite web|url=http://media.feonic.com/downloads/specs/Retail_Whispering_Windows_-_FeONIC_F1.pdf|archive-url=https://www.webcitation.org/5nhuXaouq?url=http://media.feonic.com/downloads/specs/Retail_Whispering_Windows_-_FeONIC_F1.pdf |archive-date=February 21, 2010 |title=Whispering Windows |publisher=FeONIC |access-date=October 5, 2009 |ref=feonicf1 }}</ref><ref>{{cite web|title=FeONIC D2 Audio Drive |url=http://media.feonic.com/downloads/specs/D2SpecsheetNov2008.pdf |archive-url=https://web.archive.org/web/20091229011920/http://media.feonic.com/downloads/specs/D2SpecsheetNov2008.pdf |archive-date=December 29, 2009 |publisher=FeONIC |page=1 |access-date=October 5, 2009 |quote=The D2 is unlike traditional speaker technology because it uses a very high powered magnetostrictive smart material as the driver instead of a moving coil. The material was originally developed by the US military for sonar applications and is now de-restricted for commercial use. |ref=feonicd2}}</ref><ref>{{cite web |first=Florin |last=Tibu |title=Terfenol-D: No Speakers = Great Sound! |date=February 26, 2008 |url=http://news.softpedia.com/news/Terfenol-D-No-Speakers-Great-Sound-79569.shtml |publisher=Softpedia |access-date=October 5, 2009 |ref=Softpedia |archive-date=September 17, 2011 |archive-url=https://web.archive.org/web/20110917201318/http://news.softpedia.com/news/Terfenol-D-No-Speakers-Great-Sound-79569.shtml }}</ref><ref>{{cite web |title=MINDCo launches FeONIC Invisible & Green audio |url=http://news.ezw.com/News.aspx?newsID=15 |publisher=Economic Zones World |date=January 2010 |ref=MINDCo |access-date=January 19, 2010 |archive-url=https://web.archive.org/web/20110710215008/http://news.ezw.com/News.aspx?newsID=15 |archive-date=July 10, 2011 }}</ref> and subwoofer drivers have also been produced.<ref>{{cite web |title=FeONIC S–Drive Bass Sounder |url=http://media.feonic.com/downloads/specs/SDriveSpecsheetNov2008.pdf |archive-url=https://www.webcitation.org/5nhuXaqk0?url=http://media.feonic.com/downloads/specs/SDriveSpecsheetNov2008.pdf |archive-date=February 21, 2010 |publisher=FeONIC |date=November 2008 |access-date=October 5, 2009 |ref=feonicsub }}</ref>

====Electrostatic loudspeakers====
{{Main|Electrostatic loudspeaker}}
[[File:Es spk.gif|frame|right|Schematic showing an electrostatic speaker's construction and its connections. The thickness of the diaphragm and grids has been exaggerated for the purpose of illustration.]]

Electrostatic loudspeakers use a high-voltage electric field (rather than a magnetic field) to drive a thin statically charged membrane. Because they are driven over the entire membrane surface rather than from a small voice coil, they ordinarily provide a more linear and lower-distortion motion than dynamic drivers. They also have a relatively narrow dispersion pattern that can make for precise sound-field positioning. However, their optimum listening area is small and they are not very efficient speakers.  They have the disadvantage that the diaphragm excursion is severely limited because of practical construction limitations—the further apart the stators are positioned, the higher the voltage must be to achieve acceptable efficiency.  This increases the tendency for electrical arcs as well as increasing the speaker's attraction of dust particles. Arcing remains a potential problem with current technologies, especially when the panels are allowed to collect dust or dirt and are driven with high signal levels.

Electrostatics are inherently dipole radiators and due to the thin flexible membrane are less suited for use in enclosures to reduce low-frequency cancellation as with common cone drivers. Due to this and the low excursion capability, full-range electrostatic loudspeakers are large by nature, and the bass [[roll-off|rolls off]] at a frequency corresponding to a quarter wavelength of the narrowest panel dimension. To reduce the size of commercial products, they are sometimes used as a high-frequency driver in combination with a conventional dynamic driver that handles the bass frequencies effectively.

Electrostatics are usually driven through a step-up transformer that multiplies the voltage swings produced by the power amplifier. This transformer also multiplies the capacitive load that is inherent in electrostatic transducers, which means the effective impedance presented to the power amplifiers varies widely by frequency. A speaker that is nominally 8 ohms may actually present a load of 1 ohm at higher frequencies, which is challenging to some amplifier designs.

====Ribbon and planar magnetic loudspeakers====
A '''ribbon speaker''' consists of a thin metal-film ribbon suspended in a magnetic field. The electrical signal is applied to the ribbon, which moves with it to create the sound. The advantage of a ribbon driver is that the ribbon has very little [[mass]]; thus, it can accelerate very quickly, yielding a very good high-frequency response. Ribbon loudspeakers are often very fragile. Most ribbon tweeters emit sound in a dipole pattern. A few have backings that limit the dipole radiation pattern. Above and below the ends of the more or less rectangular ribbon, there is less audible output due to phase cancellation, but the precise amount of directivity depends on the ribbon length. Ribbon designs generally require exceptionally powerful magnets, which makes them costly to manufacture. Ribbons have a very low resistance that most amplifiers cannot drive directly. As a result, a step down transformer is typically used to increase the current through the ribbon. The amplifier ''sees'' a load that is the ribbon's resistance times the transformer turns ratio squared. The transformer must be carefully designed so that its frequency response and parasitic losses do not degrade the sound, further increasing cost and complication relative to conventional designs.

Planar magnetic speakers (having printed or embedded conductors on a flat diaphragm) are sometimes described as ribbons, but are not truly ribbon speakers. The term planar is generally reserved for speakers with roughly rectangular flat surfaces that radiate in a bipolar (i.e. front and back) manner. Planar magnetic speakers consist of a flexible membrane with a voice coil printed or mounted on it. The [[Laplace force|current flowing through the coil interacts with the magnetic field]] of carefully placed magnets on either side of the diaphragm, causing the membrane to vibrate more or less uniformly and without much bending or wrinkling. The driving force covers a large percentage of the membrane surface and reduces resonance problems inherent in coil-driven flat diaphragms.

====Bending wave loudspeakers====
Bending wave transducers use a diaphragm that is intentionally flexible. The rigidity of the material increases from the center to the outside. Short wavelengths radiate primarily from the inner area, while longer waves reach the edge of the speaker. To prevent reflections from the outside back into the center, long waves are absorbed by a surrounding damper. Such transducers can cover a wide frequency range and have been promoted as being close to an ideal point sound source.<ref>{{cite web |url=http://www.stereophile.com/floorloudspeakers/687ohm/ |title=Stereophile magazine. ''Ohm Walsh 5 loudspeaker'' (review by Dick Olsher, June 1987) |date=January 17, 2008 }}</ref> This uncommon approach is being taken by only a very few manufacturers, in very different arrangements.

The Ohm Walsh loudspeakers use a unique driver designed by [[Lincoln Walsh]], who had been a radar development engineer in WWII. He became interested in audio equipment design and his last project was a unique, one-way speaker using a single driver. The cone faced down into a sealed, airtight enclosure. Rather than move back and forth as conventional speakers do, the cone rippled and created sound in a manner known in RF electronics as a "transmission line". The new speaker created a cylindrical sound field. Lincoln Walsh died before his speaker was released to the public. The Ohm Acoustics firm has produced several loudspeaker models using the Walsh driver design since then. German Physiks, an audio equipment firm in Germany, also produces speakers using this approach.

The German firm Manger has designed and produced a bending wave driver that at first glance appears conventional. In fact, the round panel attached to the voice coil bends in a carefully controlled way to produce full-range sound.<ref>{{cite web |last=Manger |first=Josef W. |url=http://issuu.com/manger-msw/docs/acoustical_reality-1-?mode=window&viewMode=singlePage |title=Acoustical Reality |work=issuu }}</ref> Josef W. Manger was awarded with the [[Rudolf-Diesel-Medaille]] for extraordinary developments and inventions by the German institute of inventions.

====Flat panel loudspeakers====
There have been many attempts to reduce the size of speaker systems, or alternatively to make them less obvious. One such attempt was the development of ''exciter'' transducer coils mounted to flat panels to act as sound sources, most accurately called exciter/panel drivers.<ref>{{Cite book |last=Lee |first=Roger |url=https://books.google.com/books?id=Z_pmDwAAQBAJ&pg=PA86|title=Computational Science/Intelligence & Applied Informatics |date=July 31, 2018 |publisher=Springer |isbn=978-3-319-96806-3 }}</ref> These can then be made in a neutral color and hung on walls where they are less noticeable than many speakers, or can be deliberately painted with patterns, in which case they can function decoratively. There are two related problems with flat panel techniques: first, a flat panel is necessarily more flexible than a cone shape in the same material, and therefore moves as a single unit even less, and second, resonances in the panel are difficult to control, leading to considerable distortions. Some progress has been made using such lightweight, rigid, materials such as [[Styrofoam]], and there have been several flat panel systems commercially produced in recent years.<ref>{{cite web |url=https://www.desireeasy.com/2018/11/abuzhen-mini-portable-wireless.html |title=Abuzhen Mini Portable Wireless Bluetooth Speaker |website=DesireEasy.com |access-date=April 14, 2018 |archive-url=https://web.archive.org/web/20181206102224/https://www.desireeasy.com/2018/11/abuzhen-mini-portable-wireless.html |archive-date=December 6, 2018 |last1=Speakers |first1=Compare }}</ref>

====Heil air motion transducers====
{{Main|Air Motion Transformer}}
[[File:AirMotionTransformer.png|thumb|In Heil's air motion transducer, current through the membrane 2 causes it to move left and right in magnetic field 6, moving air in and out along directions 8; barriers 4 prevent air from moving in unintended directions.]]

[[Oskar Heil]] invented the air motion transducer in the 1960s. In this approach, a pleated diaphragm is mounted in a magnetic field and forced to close and open under control of a music signal. Air is forced from between the pleats in accordance with the imposed signal, generating sound. The drivers are less fragile than ribbons and considerably more efficient (and able to produce higher absolute output levels) than ribbon, electrostatic, or planar magnetic tweeter designs. ESS, a California manufacturer, licensed the design, employed Heil, and produced a range of speaker systems using his tweeters during the 1970s and 1980s. [[Lafayette Radio]], a large US retail store chain, also sold speaker systems using such tweeters for a time. There are several manufacturers of these drivers (at least two in Germany—one of which produces a range of high-end professional speakers using tweeters and mid-range drivers based on the technology) and the drivers are increasingly used in professional audio. Martin Logan produces several AMT speakers in the US and GoldenEar Technologies incorporates them in its entire speaker line.

====Transparent ionic conduction speaker====
In 2013, a research team introduced a transparent ionic conduction speaker which has two sheets of transparent conductive gel and a layer of transparent rubber in between to make high voltage and high actuation work to reproduce good sound quality. The speaker is suitable for robotics, mobile computing and adaptive optics fields.<ref>{{cite web |last=Grey |first=Melissa |date=August 30, 2013 |url=https://www.engadget.com/2013-08-30-transparent-gel-speaker-ionic-conduction.html#continued |title=Transparent Gel Speaker Plays Music through the Magic of Ionic Conduction (Video) |website=Engadget |access-date=May 12, 2024 }}</ref>

===Digital speakers===
{{Main|Digital speaker}}

[[Digital speakers]] have been the subject of experiments performed by [[Bell Labs]] as far back as the 1920s.<ref>{{cite web |title=Speaker Exchange |date=April 11, 2010 |url=http://reconingspeakers.com/2010/04/digital-speakers/ |access-date=May 1, 2012 }}</ref> The design is simple; each [[bit]] controls a driver, which is either fully 'on' or 'off'. Problems with this design have led manufacturers to abandon it as impractical for the present. First, for a reasonable number of bits (required for adequate [[Sound recording and reproduction|sound reproduction]] quality), the physical size of a speaker system becomes very large. Secondly, due to inherent [[analog-to-digital conversion]] problems, the effect of [[aliasing]] is unavoidable, so that the audio output is ''reflected'' at equal amplitude in the frequency domain, on the other side of the [[Nyquist limit]] (half the sampling frequency), causing an unacceptably high level of [[Ultrasound|ultrasonics]] to accompany the desired output. No workable scheme has been found to adequately deal with this.

===Without a diaphragm===

====Plasma arc speakers====
{{Main|Plasma speaker}}
[[File:Moeller Plasma.jpg|thumb|upright|Plasma speaker]]

[[Plasma arc loudspeaker]]s use electrical [[Plasma (physics)|plasma]] as a radiating element. Since plasma has minimal mass, but is charged and therefore can be manipulated by an [[electric field]], the result is a very linear output at frequencies far higher than the audible range. Problems of maintenance and reliability for this approach tend to make it unsuitable for mass market use. In 1978 Alan E. Hill of the Air Force Weapons Laboratory in Albuquerque, NM, designed the [[Plasmatronics]] Hill Type I, a tweeter whose plasma was generated from [[helium]] gas.<ref name="nutshellhifi">[http://www.nutshellhifi.com/library/speaker-design1.html Hill Plasmatronics] described. Retrieved March 26, 2007.</ref> This avoided the [[ozone]] and [[NOx]]<ref>{{cite web| url=https://massless.info/images/NOxinplasmareactors.pdf| title=NOx production in plasma reactors by pulsed spark discharges, J.Phys, 2020}}</ref> produced by [[Radio frequency|RF]] decomposition of air in an earlier generation of plasma tweeters made by the pioneering DuKane Corporation, who produced the Ionovac (marketed as the Ionofane in the UK) during the 1950s.<ref>{{cite web|url=https://www.radiomuseum.org/r/dukanecorp_ionovac_plasma_tweeter_14a435a.html |title=Ionovac Plasma Tweeter |website=RadioMuseum.org |access-date=October 12, 2021 }}</ref>

A less expensive variation on this theme is the use of a flame for the driver, as flames contain ionized (electrically charged) gases.<ref>{{cite magazine |url=https://deramp.com/swtpc.com/PopularElectronics/May1968/Flame_Amplification.htm |title=Flame Amplification and a Better Hi-Fi Loudspeaker? |via=DerAmp.com |magazine=Popular Electronics |publication-date=May 1968 |access-date=October 12, 2021 }}</ref>

====Thermoacoustic speakers====
In 2008, researchers of Tsinghua University demonstrated a [[thermophone|thermoacoustic loudspeaker]] (or ''thermophone'') of [[carbon nanotube]] thin film,<ref>{{cite journal |last1=Xiao |first1=Lin |author2=Kaili Jiang |title=Flexible, Stretchable, Transparent Carbon Nanotube Thin Film Loudspeakers |journal=Nano Letters |date=2008 |volume=8 |issue=12 |pages=4539–4545 |doi=10.1021/nl802750z |pmid=19367976 |bibcode=2008NanoL...8.4539X }}</ref>  whose working mechanism is a thermoacoustic effect. Sound frequency electric currents are used to periodically heat the CNT and thus result in sound generation in the surrounding air. The CNT thin film loudspeaker is transparent, stretchable and flexible.
In 2013, researchers of Tsinghua University further present a thermoacoustic earphone of carbon nanotube thin yarn and a thermoacoustic surface-mounted device.<ref>{{cite journal |last1=Wei |first1=Yang |author2=Xiaoyang Lin |title=Thermoacoustic Chips with Carbon Nanotube Thin Yarn Arrays |journal=Nano Letters |date=2013 |doi=10.1021/nl402408j |pmid=24041369 |volume=13 |issue=10 |pages=4795–801 |bibcode=2013NanoL..13.4795W }}</ref> They are both fully integrated devices and compatible with Si-based semiconducting technology.

====Rotary woofers====
A [[rotary woofer]] is essentially a fan with blades that constantly change their pitch, allowing them to easily push the air back and forth.  Rotary woofers are able to efficiently reproduce [[Infrasound|subsonic]] frequencies, which are difficult to impossible to achieve on a traditional speaker with a diaphragm.  They are often employed in movie theaters to recreate rumbling bass effects, such as explosions.<ref>{{cite web |url=http://www.iar-80.com/page142.html |title=Eminent Technology TRW-17 Subwoofer Part I: The Only Subwoofer |website=International Audio/Video Review |access-date=April 29, 2024 }}</ref><ref>{{cite web |last=Guttenberg |first=Steve |date=August 29, 2010 |title=World's Most Amazing Subwoofer Has No Woofer |url=http://www.cnet.com/news/worlds-most-amazing-subwoofer-has-no-woofer/ |website=CNET |access-date=April 29, 2024 }}</ref>