Panphonics AudioTransducer Technology

Panphonics Oy manufactures electrostatic audio transducers having a patented structure consistingof porous surface layers and a diaphragm in between them. As an electric voltage is applied tothe stator plates, an electric field is produced between them. As the electric field moves thediaphragm in same phase over the transducer area, the element transmits acoustical plane waveswith dipole radiation characteristics. Conversely, the element may also be used as audio receiver,as diaphragm movement in relation to the stators caused by incident sound can be detected aselectric signal.

Radiation efficiency of a piston dependson its size and the size of the baffle aroundit [Morse & Ingard 1968]. As seen in Fig.1, even an infinite baffle has little effectwhen piston sizes comparable to those oftypical Panphonics audio panels are con-sidered. The theoretical limit for low fre-quency cutoff is also seen from these plots.If maximal efficiency is aimed for at thehigh frequencies, -6 dB relative attenuationwill be perceived around 100...200 Hz.

The the acoustic load developed by abaffled piston depends on the relationbetween frequency and piston dimensions.The acoustic resistive load exerted on avibrating diaphragm is constant in thepiston band, which consists of all frequen-cies above the piston band cutoff frequen-cy:

The one-piece construction of the trans-ducer enables high-volume manufacturingof elements of variable sizes and shapes.As the distances between the diaphragmand the stators are small, low voltage isused compared to traditional electrostaticloudspeakers. Due to their light weightand high directivity, the Panphonics audiopanels are used in growing numbers inlocations where traditional cone speakerscan not serve the audio reproduction needs.

In the following, the basic theory behindthe Panphonics transducer is briefly re-viewed, after which the specific character-istics of the transducer are discussed inmore detail.

Sound Radiation from a PistonSound radiated by a loudspeaker elementof any kind, set in a plane baffle, can beestimated by a plane piston set in an infiniteplane baffle [Kinsler et al. 2000]. Thisestimation is especially useful for panelloudspeakers such as the Panphonics audiotransducer, as its radiating surface is effec-tively flat.

An important measure in this estimationis the source's capacity to convert thepiston motion into acoustic sound power,or the acoustic radiation resistance.It is the ratio of pressure [N] to the volumevelocity of the piston [m3/s]. At high fre-quencies, all pistons have the same abilityto produce real acoustic power per unitsurface area.

where c is the sound velocity [m/s] andis the piston diameter [m]. Below the pistonband, as the wavelength of produced soundapproaches the size of the piston, theradiation resistance starts to decrease atthe rate of 12 dB / octave or the square ofthe decreasing frequency. The value of theacoustical load can be calculated by

where RD is the piston band diaphragmresistance [Ns/m], is the characteristicresistance of air [Ns/m3], and SD is thediaphragm area [m2] [Kuttruff 2000]. Theforce exerted by the diaphragm is calculatedby

where vD is the diaphragm movementvelocity [m/s], related to the frequency f

and displacement d by

Figure 1: Radiation efficiency of a piston: baffeled(solid line) and unbaffeled (dashed line, lowfrequency asymptote).

Directivity of a piston source depends ofits size and shape. At frequencies abovethe piston band cutoff frequency the radi-ation pattern narrows and produces sidelobes. In the piston band, the directivityindex and the on-axis gain increase by 6dB/octave, ie. the beam angle is approxi-mately halved for each octave increase infrequency.

Acoustical Plane Waves

For a plane wave, the acoustic variableshave constant amplitudes and phases onany plane perpendicular to the directionof propagation. Any acoustical wavefrontresembles a plane wave, when observedat a location far from the source.

Figure 2: Point, line and area sources and theirradiation patterns. [Peltonen 2003]

A traditional loudspeaker typically hascharacteristics varying from a line sourceto a point source depending of the con-struction and the radiated frequency. Incontrast, due to the large diaphragm arearadiating in same phase, the Panphonicsaudio element functions as area sourceproducing plane waves at high frequencies,gradually changing to a point source atlow frequencies as the wavelength reachesthe element dimensions.

Acoustical plane wave as a physical phe-nomenon has been known for a long time.In contrary to spherical waves, ideal planewaves have no geometrical attenuation,as the wavefront does not spread to areaslarger than the source area. Thus, the pres-sure amplitude is independent of distanceif losses in the propagation media areneglected.

In practice, some geometrical spreading isobserved due to boundary effects andunideal accuracy of the movement of thesound source. Also, some frequency de-pendent attenuation is always present insound propagation in air.

Figure 3: Basic structure of a push-pull electro-static transducer.

To make the diaphragm react to the chang-es of the electric field caused by the signalvoltage, it needs to be charged. In electro-satic loudspeakers, this is most often solvedby applying a constant bias voltage to thediaphragm. This enables the use of inex-pensive materials. The bias voltage causesthe electrical force to be non-linear at veryhigh amplitudes, but this has no noticeableeffects when the transducer is designedproperly to keep the diaphragm movementin linear range.

Panphonics audio elementcharacteristics

Panphonics’ proprietary electrostatic trans-ducer technology provides large scale man-ufacturing of thin, light weight loudspeakersand microphones [Kirjavainen 1997, Kir-javainen 2002]. They are easy to installand to use due to their exceptional one-piece structure. The high directivity andthe plane wave characteristics of the pro-duced sound field enable many novel audiousages.

Mechanical Structure

The elements consist of porous stator platesand a metallized diaphragm. Each stator isless than 2 mm thick and has one metallizedsurface with 160 m grooves to allow thediaphragm movement. At the same time,the construction is kept stable and relativelyrigid by joining the layers together at multiplelocations in and around the element. Porousplastic plates enclose the charged surfacesto provide perfectly safe operation.

Audio Source Usage

When the Panphonics audio transducer isused as a loudspeaker, the diaphragm isconnected to DC bias voltage, while signalvoltage is applied to the stators.

Directivity Characteristics

The directivity of the transducer panel de-pends of its dimensions and of the outputfrequency. For example, the isobar plot ofa square shaped element is shown in Fig.4. At low frequencies, the radiation patternis a wide figure-of-eight. At frequenciesbetween 500 Hz and 2 kHz the main lobegets narrower and noticeable side lobesexist. At higher frequencies also the sidelobes diminish remarkably.

The directivity pattern can be affected forexample with surface treatments or withelectric means – by changing the localmagnitude or phase output characteristicsover the element area. Both narrower andwider main lobes can be achieved depend-ing of the customer needs.

Electrostatic LoudspeakerPrinciple

An electrostatic loudspeaker is basically aparallel plate capacitor, one plate being aflexible diaphragm. A push-pull operation,essentially linearizing the operation of thetransducer, includes two air-permeablestator plates between which the diaphragmis moving, as depicted in Fig. 3. Applyinga signal voltage to the two plates producesa uniform electric field in the space betweenthem. A charge placed anywhere in thisspace will experience a force causing it toaccelerate towards the stator with oppositesign voltage.

The electrostatic principle enables theconstruction of loudpeakers with lowercoloration, better transient response, lowerdistortion and higher directivity in compar-ison to other techniques. The advantagesof the electrostatic loudspeakers have beenknown for decades. The first commercialproducts appeared in the 1920s and 1930sand development of the materials made itpossible to produce full-frequency-rangeelectrostatic loudspeakers in the 1950s[Walker 1979, Borwick 2001].

Dr. Antti Kelloniemi, Technology ManagerR & D, Panphonics Oy

Olarinluoma 16, 02200 ESPOO,FINLAND, EUROPE

tel. + 358 9 8193 8560 fax + 358 9 8193 8561www.panphonics.com

19.02.2007

Figure 4: Directivity of a 60 cm x 60 cm loud-speaker panel. Sound pressure is plotted in dBrelative to the on-axis level.

Frequency Response and SignalAccuracy

As the moving mass per area of the Pan-phonics audio transducer is very lowcompared to traditional cone speakers,the coupling from the diaphragm to theair is very good. Because of the low inertia,transient response is exceptionally accu-rate and minimal distortion levels areobtained. An impuse response is depictedin Fig. 5.

The size and shape of the transducerdrastically affect the frequency responseas predicted by the piston radiation theory.As an example, the sound pressure leveland the levels of two distortion productsare shown in Fig. 6.

Audio Receiver Usage

When used as a microphone, the transducermay be connected in several manners.Movement of the biased diaphragm causesmeasurable changes in the capacitancesmeasured between the diaphragm and thestators. Thus, the signal may be recordedfrom the stators, for example. Anotheroption is to apply opposite bias voltagesto the stators and to measure the voltageat the moving diaphragm. To protect thecircuit from electromagnetic disturbances,an extra counductive layer can be manu-factured on the element surfaces for shield-ing.

Figure 5: Panphonics transducer impulse re-sponse depicted as a spectrogram of the radi-ated sound pressure in dB.

T. Peltonen, Panphonics Audio Panel WhitePaper, Akukon Oy, Helsinki, Finland, 2003.

P. J. Walker, “New Developments in Elec-trostatic Loudspeakers,” presented at theAES 63rd Convention, Los Angeles, USA,May 15–18, 1979.

References

J. Borwick, Loudspeaker and HeadphoneHandbook, 3rd ed., Focal Press, Oxford,UK, 2001.

L. E. Kinsler et al., Fundamentals of Acous-tics, 4th ed., J. Wiley & sons, USA, 2000.

K. Kirjavainen, “Acoustic Element andMethod for Sound Processing,” PCT PatentWO 97/31506, 1997.

K. Kirjavainen, “Acoustic Elements andMethod for Sound Processing,” US PatentNo. 6483924, 2002.

H. Kuttruff, Room Acoustics, 4th ed., Sponpress, New York, USA, 2000.

P. M. Morse and K. U. Ingard, TheoreticalAcoustics, Princeton University Press,New Jersey, USA, 1968.

Figure 7: Frequency response of transducers ofthree sizes when used as audio receivers: S60 =60 cm x 60 cm, N20 = 20 cm x 60 cm, and oneslit = 2 cm x 60 cm.

Test results of three different sizes of receiverelements are shown in Fig. 7. Sound sourc-es located straight in front of the elementand 45 degrees to the side were used.Comparison with Figs. 4 and 6 shows thatsimilar directivity characteristics apply tothe transducer whether it is used as a sourceor as a receiver. In combination with itsflatness, this enables interesting usagepossibilities for audio interfaces in vehiclesand teleconferencing systems, for example.

Figure 6: Radiated sound pressure level at 1 mdistance (red) and two distortion productsmeasured at constant voltage of 10.95 Vrms.Measurement distance was 8 m, all levels arerecalculated to 1 m standard.