General Paper - Outline

1. PIEZOELECTRIC
MATERIALS
EMBEDDED
IN
THE
SURFACES
OF
A
CONCERT
HALL
Abstract
The surface of concert halls is designed to reflect audio waves that are
coming from speakers or musical instruments. This ability to reflect the sound
waves depends on the “scattering coefficient” of the material, depth of the
surface and the design of the concert hall. This clearly constrains the design of
the concert hall and increases cost of construction. Embedding piezoelectric
speakers in the surface of the concert halls would eliminate the need to have a
design that would optimise acoustic experience, as piezoelectric speakers would
replace this function by producing desired sound waves and destroying unwanted
sound waves. In the future, the embedment of piezoelectric actuators in the
surfaces of concert hall would enhance the audience’s acoustic experience and
at 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. Although
this is the case, the usage of piezoelectric actuator in audio systems would
usually mean that the audio system is of low quality.i Nevertheless, the cost of
producing piezoelectric actuator has come down significantly with the discovery
of the method of manufacturing a piezoelectric actuator consisting of
piezoelectric fibres in a polymer matrix on which electrodes are applied for
controlling 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 before
this, 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 main
priorities of building concert halls. Existing concert halls are usually boxed
shaped which reduces audience’s sightlines in order to have superior acoustics.
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2. Current Design For Concert Halls
The physics behind building a concert hall is that the physical dimensions
of a space and the relationship of the surface locations, texture and material depth
between the source of audio and audience will influence how one hears the sound
reflections.iii This theory will determine the success or failure of a concert hall as
this 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 used
by concert halls across the world. The design of the concert hall is very common to
the extent that it would very hard to differentiate one concert hall to another
because all of them are not unique and different to each other in terms of interior
design.iv
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3. Embodiment Of Piezoelectric Materials In The Surface
It is the intention of this paper to promote the usage of piezoelectric materials
to replace the need to have an acoustical design of the concert hall. Embedding the
piezoelectric materials itself in the surface does this. If the wall of the concert hall is
taken as an example, materials are used to produce desired sound waves and at
the same time by destructive interference destroy unwanted sound waves. There
are two factors that would need some considerations to embed the materials in the
surface of the wall: Location of the piezoelectric transducers and actuators and the
materials used in producing them.
Location
The choice of actuator location is an important issue in the design of the
concert hall, as this would increase the sound system to enhance acoustic
experience. The actuators should be placed at the locations to excite the desired
modes most effectively. Piezoelectric actuators, which locally strain the structure,
should be placed in regions of high average strain and away from areas of zero
strain. This is so due to the fact that in the area of zero strain, piezoelectric effect
cannot be achieved. As an example, piezoelectric effect can produce a sound wave
on its own and sound waves that it produced have “loudness” element in it. To
achieve 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 . Amplitude
is related to loudness as loudness, the quality of a sound, is primarily a
psychological factor that correlates of physical strength (amplitude) of the sound
wave. More formally, loudness is defined as "that attribute of auditory sensation in
terms of which sounds can be ordered on a scale extending from quiet to loud."v
Material
Having determined the appropriate location for placing the actuators,
materials to produce the piezoelectric actuators should be determined. A wide
variety of piezoelectric materials are currently available, including piezoelectric
film, piezo-ceramics, and piezoelectric bimorph elements. In the selection of the
one to be used in the manufacture of the piezoelectric actuators embedded in the
surfaces 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-mechanical
coupling effectiveness. The effectiveness of piezoelectric actuators is calculated
by using the following equation:
[2]
3

4. Where is maximum allowable piezoelectric field, is piezoelectric constant
(strain/field) is x-coordinate of the centre of the piezoelectric, is = modulus
ratio of beam to piezoelectric and is non-dimensional bonding layer thickness. A
high is clearly a desirable feature, so that a large field can be applied to the
piezoelectric before de-poling occurs, which destroys the piezoelectric properties
of the material. An actuator with a high effectiveness must also have a high
piezoelectric constant , since for a large , a large strain is produced for a
small voltage. If is small, then a large voltage will be required to produce strain
in the piezoelectric device. The effectiveness of the piezoelectric material is
important in the sense of it will affect the ability of the sound system to mimic or
replicate the original sound made.
The general idea of embodiment of piezoelectric materials in the surfaces of
the concert hall is to allow the piezoelectric materials to have piezoelectric effects
to produce or reduce sound waves.
An Integrated System Of Piezoelectric Materials As A
Sound System
System
The sound that the audience hear in the concert hall is the result of
vibrations of particles of air. When it is transmitted through air and reflected at the
surfaces of the concert hall, sometimes the “quality” of the sound is greatly
reduced. Furthermore, the sound may be reflected more than once as the waves
move in ripple, the idea that constructive and destructive interference may cause
the “quality” of the sound to be diminished. To ensure maximum utility from the
sound system, it is suggested that piezoelectric materials are used in
synchronisation. What this means is that, a computer must be used to make
calculations on how much piezoelectric effect is needed in a particular actuator at
a particular location. To understand this case better, an example should be
explained.
Figure 2: An example illustration showing the locations of piezoelectric actuators
(labelled as A, B, C, D, E, F, G, H, I and J)
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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 reflected
and dampened a few times before reaching the audience, actuators will play the
role of producing constructive interference if the intended sound wave reach the
audience lesser than the level desired. For example, the sound wave will be
dampened as distance increases. The amplitude of the sound wave at location of
actuator E will at a level that well below of the amplitude of the sound wave at the
location of actuator A. To achieve the same level of amplitude at both positions, a
computer is needed to make calculations and send back instructions to the
piezoelectric 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” of
sound.
Active Noise Control
This method is used to reduce unwanted sound. For example, a headphone
that has active noise control will be able to make the sound of aircrafts engine
inaudible so that the person that the headphone on will hear only music. Popular
methods of suppressing unwanted sound using passive sound absorbers
generally do not work well at low frequencies which means that certain sound may
not be prevented from being heard. Most common example of popular sound
absorber is Micro Perforated Plate used in recording studios and clubs and it is
commonly made up of porous material.vi If the budget for these studios were low
then 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 low
frequencies the sound wavelengths become large compared to the thickness of a
typical sound absorber.
Producing a “Quite Zone,” absorbing sound power, and minimising the total
acoustic power output of all audio sources, are each clearly distinct and different
objectives in the active noise control. The “Quite zone” is achievable by making
sure that the sound wave reaching the audience is the same without any
“unwanted” noise. The way in which any one of these acoustic objectives is
achieved is distinct from each other and requires different method to achieve
them. Previous researchers on this area such as Olson and May concentrated on
using no prior knowledge of the sound field, but feeding back the entire signal from
the closely spaced microphone via an amplifier to the secondary loudspeaker.vii
This “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 strategy
can be characterised as being “feed forward”.
Piezoelectric materials play a part in this area and the usage of
piezoelectric materials in active noise control is commonly called piezoelectric
smart structure.viii A piezoelectric actuator is capable of inducing more strain into
the host material if the extensional stiffness of the actuator is large. The problem
of controlling vibrations in the concert hall in comparison to other piezoelectric
control applications is challenging due to both the high vibration levels present.
This is so because the piezoceramic actuator chosen for this application, which
had a high stiffness and a thick cross-section, was therefore suited for this
particular problem.
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6. A few algorithms to calculate the behaviour of piezoelectric materials
needed 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 in
reducing noise.
Conclusion
Piezoelectric materials clearly have the benefit of producing “desired” sound
waves and at the same time cancel out “unwanted” sound waves. This function
can be replaced by good functional design for a concert hall but then aesthetics
value of the design and overall cost of construction may need to be sacrificed. In
this 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 of
using piezoelectric materials as “speakers” and “sound absorber” to so it is more
viable for concert hall to adopt this option.
Acknowledgment
This paper was done with the help of Malaysian Philharmonic Orchestra as part of
its outreach programme. Correspondences with various academics such as
Professor Dr. Ahmad Kamal Yahya( UiTM, Malaysia) and Professor Dr. Md Rahim
Sahar(UTM, Malaysia) contribute to formulating the idea for this paper.
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7.
References
i
JENSENIUS, A.R. , KOEHLY, R. & WANDERLEY, M.M, 2006, Building Low-Cost
Music Controllers ,Lecture Notes in Computer Science (3902) 123-129.
ii
SEFFNER, L., SCHÖNECKER, A. & GEBHARDT, S. , Fraunhofer-Gesellschaft
zur Forderung der Forschung e.V., 2000, Method for producing a piezoelectric
transducer ,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,
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v H. FLETCHER & W. A. MUNSON, 1933, Loudness of a Complex Tone, Its
Definition, Measurement & Calculation, Journal of Acoustic Society America (5)
65-65
vi
ELLIOTT, S.J. & NELSON, P.A., 1993, Low-frequency techniques for
suppressing acoustic noise leap forward with signal processing, Signal
Processing Magazine, (10) 4, 12-35
vii
OLSON, H.F. & MAY, E.G, 1953, Electronic Sound Absorber, Journal of
Acoustic Society of America, (25), 829-829
viii
KIM, J. & KO, B., 1998, Optimal design of a piezoelectric smart structure for
noise control, Smart Materials and Structures, (7) 6, 402-751
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