Piezoelectric speaker

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Piezoelectric speaker

  1. 1. PIEZOELECTRIC  MATERIALS  EMBEDDED  IN   THE  SURFACES  OF  A  CONCERT  HALL    Abstract The surface of concert halls is designed to reflect audio waves that arecoming from speakers or musical instruments. This ability to reflect the soundwaves depends on the “scattering coefficient” of the material, depth of thesurface and the design of the concert hall. This clearly constrains the design ofthe concert hall and increases cost of construction. Embedding piezoelectricspeakers in the surface of the concert halls would eliminate the need to have adesign that would optimise acoustic experience, as piezoelectric speakers wouldreplace this function by producing desired sound waves and destroying unwantedsound waves. In the future, the embedment of piezoelectric actuators in thesurfaces of concert hall would enhance the audience’s acoustic experience andat the same time reduce the cost of building a concert hall.Introduction Piezoelectric speaker is a technology well developed in the last century.This type of speaker is used widely in many types of audio systems. Althoughthis is the case, the usage of piezoelectric actuator in audio systems wouldusually mean that the audio system is of low quality.i Nevertheless, the cost ofproducing piezoelectric actuator has come down significantly with the discoveryof the method of manufacturing a piezoelectric actuator consisting ofpiezoelectric fibres in a polymer matrix on which electrodes are applied forcontrolling the fibres.ii With the falling cost of production of piezoelectric actuator,we now can embed it in the surfaces in concert halls. This is significant as beforethis, concert halls were designed to optimise the quality of audio transmission.With the embodiment of piezoelectric actuator in the surfaces of the concert hall,we now can have aesthetics value or lower construction cost as the mainpriorities of building concert halls. Existing concert halls are usually boxedshaped which reduces audience’s sightlines in order to have superior acoustics.   1  
  2. 2. Current Design For Concert Halls The physics behind building a concert hall is that the physical dimensionsof a space and the relationship of the surface locations, texture and material depthbetween the source of audio and audience will influence how one hears the soundreflections.iii This theory will determine the success or failure of a concert hall asthis theory will affect: • Loudness of the audio heard by the audience. Non- enthusiasts would understand this as the volume of the sound. If the concert hall does not have the ability to maintain the loudness of the sound or the design of the hall dampens the audio waves by destructive interference, audience experience would certainly diminish. • Clarity of the sound or the ability to make out fast moving melody within the overall reverberation in the hall. If a window exist between time the sound produced in the hall and the time that particular sound heard by the audience then it would ruin the acoustic experience. Audience should hear the sound instantaneously irrespective of the distance between them and the audio source. • Intimacy or the feeling of closeness to the sound produced and the source of the audio. This feeling can be improved by ensuring that the audience have sightlines to the source of audio. Currently, the design of concert hall has restrained, as audience sitting afar from stage would not be able to see the performer. • Envelopment or the feeling of the music surrounding the audience that would cause the audience to fell like being immersed in the sound.   Figure 1: Architecture illustration of Hakuju Concert Hall. Figure 1 shows the architectural design of a concert hall that is widely usedby concert halls across the world. The design of the concert hall is very common tothe extent that it would very hard to differentiate one concert hall to anotherbecause all of them are not unique and different to each other in terms of interiordesign.iv   2  
  3. 3. Embodiment Of Piezoelectric Materials In The Surface It is the intention of this paper to promote the usage of piezoelectric materialsto replace the need to have an acoustical design of the concert hall. Embedding thepiezoelectric materials itself in the surface does this. If the wall of the concert hall istaken as an example, materials are used to produce desired sound waves and atthe same time by destructive interference destroy unwanted sound waves. Thereare two factors that would need some considerations to embed the materials in thesurface of the wall: Location of the piezoelectric transducers and actuators and thematerials used in producing them.Location The choice of actuator location is an important issue in the design of theconcert hall, as this would increase the sound system to enhance acousticexperience. The actuators should be placed at the locations to excite the desiredmodes most effectively. Piezoelectric actuators, which locally strain the structure,should be placed in regions of high average strain and away from areas of zerostrain. This is so due to the fact that in the area of zero strain, piezoelectric effectcannot be achieved. As an example, piezoelectric effect can produce a sound waveon its own and sound waves that it produced have “loudness” element in it. Toachieve desired loudness of the sound, amplitude is calculated by this equation:[1]Where Qv is electrical charge, V is voltage , K is modal stifness and ζ is . Amplitudeis related to loudness as loudness, the quality of a sound, is primarily apsychological factor that correlates of physical strength (amplitude) of the soundwave. More formally, loudness is defined as "that attribute of auditory sensation interms of which sounds can be ordered on a scale extending from quiet to loud."vMaterial Having determined the appropriate location for placing the actuators,materials to produce the piezoelectric actuators should be determined. A widevariety of piezoelectric materials are currently available, including piezoelectricfilm, piezo-ceramics, and piezoelectric bimorph elements. In the selection of theone to be used in the manufacture of the piezoelectric actuators embedded in thesurfaces of a concert hall, certain criteria had to be considered. It is, of course,desirable to use a piezoelectric material that has a high piezoelectric-mechanicalcoupling effectiveness. The effectiveness of piezoelectric actuators is calculatedby using the following equation:[2]   3  
  4. 4. Where is maximum allowable piezoelectric field, is piezoelectric constant(strain/field) is x-coordinate of the centre of the piezoelectric, is = modulusratio of beam to piezoelectric and is non-dimensional bonding layer thickness. Ahigh is clearly a desirable feature, so that a large field can be applied to thepiezoelectric before de-poling occurs, which destroys the piezoelectric propertiesof the material. An actuator with a high effectiveness must also have a highpiezoelectric constant , since for a large , a large strain is produced for asmall voltage. If is small, then a large voltage will be required to produce strainin the piezoelectric device. The effectiveness of the piezoelectric material isimportant in the sense of it will affect the ability of the sound system to mimic orreplicate the original sound made. The general idea of embodiment of piezoelectric materials in the surfaces ofthe concert hall is to allow the piezoelectric materials to have piezoelectric effectsto produce or reduce sound waves.An Integrated System Of Piezoelectric Materials As ASound SystemSystem The sound that the audience hear in the concert hall is the result ofvibrations of particles of air. When it is transmitted through air and reflected at thesurfaces of the concert hall, sometimes the “quality” of the sound is greatlyreduced. Furthermore, the sound may be reflected more than once as the wavesmove in ripple, the idea that constructive and destructive interference may causethe “quality” of the sound to be diminished. To ensure maximum utility from thesound system, it is suggested that piezoelectric materials are used insynchronisation. What this means is that, a computer must be used to makecalculations on how much piezoelectric effect is needed in a particular actuator ata particular location. To understand this case better, an example should beexplained. Figure 2: An example illustration showing the locations of piezoelectric actuators (labelled as A, B, C, D, E, F, G, H, I and J)   4  
  5. 5. Let’s us assume that this simplified example is a concert hall. A, B, C, D, E,F, G, H, I and J are piezoelectric actuators. Since sound waves will be reflectedand dampened a few times before reaching the audience, actuators will play therole of producing constructive interference if the intended sound wave reach theaudience lesser than the level desired. For example, the sound wave will bedampened as distance increases. The amplitude of the sound wave at location ofactuator E will at a level that well below of the amplitude of the sound wave at thelocation of actuator A. To achieve the same level of amplitude at both positions, acomputer is needed to make calculations and send back instructions to thepiezoelectric actuators to make the same desired sound wave at both locations.So in the end, audience sitting near to A and E will hear the same “quality” ofsound.Active Noise Control This method is used to reduce unwanted sound. For example, a headphonethat has active noise control will be able to make the sound of aircrafts engineinaudible so that the person that the headphone on will hear only music. Popularmethods of suppressing unwanted sound using passive sound absorbersgenerally do not work well at low frequencies which means that certain sound maynot be prevented from being heard. Most common example of popular soundabsorber is Micro Perforated Plate used in recording studios and clubs and it iscommonly made up of porous material.vi If the budget for these studios were lowthen they would use mineral or glass wool although there is a higher risk of fire.The idea that conventional methods may not reduce noise is because at these lowfrequencies the sound wavelengths become large compared to the thickness of atypical sound absorber. Producing a “Quite Zone,” absorbing sound power, and minimising the totalacoustic power output of all audio sources, are each clearly distinct and differentobjectives in the active noise control. The “Quite zone” is achievable by makingsure that the sound wave reaching the audience is the same without any“unwanted” noise. The way in which any one of these acoustic objectives isachieved is distinct from each other and requires different method to achievethem. Previous researchers on this area such as Olson and May concentrated onusing no prior knowledge of the sound field, but feeding back the entire signal fromthe closely spaced microphone via an amplifier to the secondary loudspeaker.viiThis “feedback” policy is clearly different from that of Lueg’s duct control system,a system that the idea behind it is that the acoustic signal is obtained by using an“upstream” detection microphone. Lueg’s duct control system’s control strategycan be characterised as being “feed forward”. Piezoelectric materials play a part in this area and the usage ofpiezoelectric materials in active noise control is commonly called piezoelectricsmart structure.viii A piezoelectric actuator is capable of inducing more strain intothe host material if the extensional stiffness of the actuator is large. The problemof controlling vibrations in the concert hall in comparison to other piezoelectriccontrol applications is challenging due to both the high vibration levels present.This is so because the piezoceramic actuator chosen for this application, whichhad a high stiffness and a thick cross-section, was therefore suited for thisparticular problem.   5  
  6. 6. A few algorithms to calculate the behaviour of piezoelectric materialsneeded to be devised but it well beyond the scope of this paper to do so.Nevertheless this paper clearly state how piezoelectric materials can help inreducing noise.Conclusion Piezoelectric materials clearly have the benefit of producing “desired” soundwaves and at the same time cancel out “unwanted” sound waves. This functioncan be replaced by good functional design for a concert hall but then aestheticsvalue of the design and overall cost of construction may need to be sacrificed. Inthis sense, it is better to utilize piezoelectric materials in building a concert hall.Furthermore, a good acoustic design of the hall would cost more than instead ofusing piezoelectric materials as “speakers” and “sound absorber” to so it is moreviable for concert hall to adopt this option.AcknowledgmentThis paper was done with the help of Malaysian Philharmonic Orchestra as part ofits outreach programme. Correspondences with various academics such asProfessor Dr. Ahmad Kamal Yahya( UiTM, Malaysia) and Professor Dr. Md RahimSahar(UTM, Malaysia) contribute to formulating the idea for this paper.   6  
  7. 7.                                                                                                                Referencesi  JENSENIUS, A.R. , KOEHLY, R. & WANDERLEY, M.M, 2006, Building Low-CostMusic Controllers ,Lecture Notes in Computer Science (3902) 123-129.  ii  SEFFNER, L., SCHÖNECKER, A. & GEBHARDT, S. , Fraunhofer-Gesellschaftzur Forderung der Forschung e.V., 2000, Method for producing a piezoelectrictransducer ,US Pat. 10129748.iii BERANEK, L., 2011, Concert hall acoustics, Architectural Science Review, (50),5-14  iv  BARRON, M., 1993, Auditorium acoustics and architectural design, London,Taylor & Francis.  v H. FLETCHER & W. A. MUNSON, 1933, Loudness of a Complex Tone, ItsDefinition, Measurement & Calculation, Journal of Acoustic Society America (5)65-65vi  ELLIOTT, S.J. & NELSON, P.A., 1993, Low-frequency techniques forsuppressing acoustic noise leap forward with signal processing, SignalProcessing Magazine, (10) 4, 12-35vii  OLSON, H.F. & MAY, E.G, 1953, Electronic Sound Absorber, Journal ofAcoustic Society of America, (25), 829-829viii  KIM, J. & KO, B., 1998, Optimal design of a piezoelectric smart structure fornoise control, Smart Materials and Structures, (7) 6, 402-751     7  

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