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Basic theory of sound,
piezomaterials and vibrations
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Content of presentation
• Theory of sound
• Piezomaterials and the piezoelectric effect
• Basic principles of the Sonitron products
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Theory of sound
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Sound
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Noise
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Definition
• Sound = the vibration of molecules
around their balance position that can be
detected by the human ear
• The vibrations cause variations in air
pressure around the atmospheric
pressure → sound propagates as a
pressure wave
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Definition
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Theoretical aspects
• Pressure wave is characterized by:
1. Amplitude 2. Frequency
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Amplitude
• Human ear:
– Minimum pressure
variation: 20 µPa
– Maximum pressure
variation: 200 Pa
→ range = 107!!
• Atmospheric pressure
= 100 000Pa!!
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• Sound Pressure level (SPL):
SPL = 20 log (p/p0) (expressed in decibels = dB)
with p0 = 20 µPa (= threshold of hearing)
• Examples:– p = 20 µPa → SPL = 0 dB
– p = 1 Pa → SPL = 94 dB
– p = 200 Pa → SPL = 140 dB
Amplitude
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Amplitude (SPL)
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Frequency
• Human ear:
– Minimum frequency: 20 Hz
– Maximum frequency: 20 000 Hz
• Sensitivity of the human ear is not constant:
– Low sensitivity for the very low and high
frequencies
– Highest sensitivity between 2000Hz – 5000Hz
(speech!)
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Frequency
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Frequency
• Weighting curves are
used to evaluate the SPL
like it is perceived by the
human ear.
• 4 weighting curves exist
(A, B, C and D filter)
• Most commonly used: A-
filter. Measurements are
given in dB(A) to
indicate the weighting.
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Perception of sound
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• Addition of SPL’s
Logarithmic values can NOT be added like linearvalues. Two methods can be applied for the addition of SPL’s:
1. Converting the dB values to linear values (Pa):
SPLtotal = 10.log ( 10 SPL1/10 + 10 SPL2/10 + …)
Calculating with SPL
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Calculating with SPL
2. Using graphs
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Calculating with SPL
3. Examples:
SPL1
= 55 dB SPL2
= 51 dB
1. SPLtotal = 10.log(1055/10 + 1051/10)
= 56.4 dB
2. ∆SPL = 4 dB
SPL+ = 1.4 dB
SPLtotal = 55 dB + 1.4 dB = 56.4 dB
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• Conversions of specified SPL’s:
– Converting the specified distance:
∆SPL = 20.log (r1 / r2)
with: r1 = distance at which the SPL is given
r2 = distance at which you want to know
the SPL
Calculating with SPL
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– Converting the specified voltage:
∆SPL = 20.log (V2 / V1)
with: V1 = voltage at which the SPL is given
V2 = voltage at which you want to know
the SPL
Calculating with SPL
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Calculating with SPL
– Example: SPL = 90 dB @ 24V @ 1m
→ SPL @ 12V @ 0.3m ?
∆SPLdistance = 20.log(1 / 0.3) = + 10.45 dB
∆SPLvoltage = 20.log(12 / 24) = - 6 dB
→ SPL @ 12V @ 0.3m:
90 dB + 10.45 dB – 6 dB = 94.45 dB
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Piezomaterials and the
piezoelectric effect
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Definition
• Discovered in 1880 by Jacques and Pierre Curie
• Piezoelectricity is the phenomenon in which
materials develop an electric field when
subjected to pressure/force, or conversely,
exhibit a mechanical deformation when
subjected to an electric field.
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Piezomaterial
• Piezoceramic = mass of perovskite crystals
• Each crystal has a dipole moment (polarization)
randomly oriented so the ceramic element has no
overall polarization→ NO PIEZOELECTRIC EFFECT
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Piezomaterial
• Apply a strong DC field (> 2000V/mm) to
induce piezoelectric properties
• The crystals will align and roughly stay in
alignment→ remanent polarisation
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Piezomaterial
• Natural piezoelectric materials:– Quartz
– Tourmaline
– …
→ Small piezoelectric effect!
• Synthetic piezoelectric materials:– BaTiO3
– Lead Zirconate Titanate (PZT)
– …
(General formula ABO3 where A and B are metals and O stands for oxigen)
→ Large piezoelectric effect!
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Piezoelectric effect
• Direct effect: sensors (pressure/crash/accelaration), gas lighter,
power supply (shoe-mounted piezoelectrics), …
• Converse effect: acoustic components, micro engine, actuator
(inkjet printer/ diesel injection system), micropump, positioning system, …
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Manufacturing technology1. Pressing
2. Tape casting
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Problems during manufacturing
• Purity and particle size of the oxide powders
• Weight ratio of the oxide powders
• Type and quantity of the binder
• Firing method
• Cooling method after sintering
• Pitfalls
• Poling of the ceramic
• …
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Basic principles of the
Sonitron products
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Piezoelectric acoustic components
• Piezoceramic disc glued onto metal or composite
membrane:
• Alternating electric field: membrane vibrates =
variations in air pressure → sound
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Mounting methods and
mechanical vibrations• Nodal support:
– Vibration of a free strip is a standing wave in which the maximum amplitude (belts) occurs at both ends and in the middle.
– The position of these nodes is governed by the energy balance, the total energy inside the nodes is at each moment equal to the total energy outside the nodes.
– In this case, displacements are at maximum because the strip is in a free natural resonance, without extern boundary conditions that can limit this movement.
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Mounting methods and
mechanical vibrations• Edge support:
– Vibration of a free strip is a standing wave in which maximum amplitude (belts) occurs in the middle of the strip.
– Energy balance → energy lost in the points of support
– Suppresses the fundamental frequency by moving the node to the boundary of the membrane/plate (smaller displacements).
– The whole surface of the membrane vibrates in phase resulting in a lower resonance frequency.
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Construction of Sonitron products
• Nodal support:– Most efficient mounting method
– Higher resonance frequency
– Higher movement of the membrane
– Smaller surface to produce sound
– Very clear resonance peak.
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Construction of Sonitron products
• Information about mounting method:– Soft glue (silicones) → needs 2 - 3 days to dry
– Not waterproof
– Membrane has to be mounted precisely in the centre :
• Not easily to achieve: wires connected to the membrane and dry time 2 - 3 days
• If not mounted precisely in the centre, resonance frequency and sound pressure changes.
– Only for buzzers and transducers that generate a single working frequency
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Construction of Sonitron products
• Edge support:– More surface area to produce sound
– Less vibration of the membrane
– Wider bandwidth
– Lower resonance frequency
– No distinctive resonance peak
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Construction of Sonitron products
• Information about mounting method:– Hard (epoxy) or soft (silicones) glue
– Membrane is easily to place in the housing (always centred)
– SPL is very consistent
– Waterproof
– For speakers, multifunctional buzzers and alarms (products in which different frequencies are used)
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Comparison with electro-
dynamic speaker
• Electrodynamic speaker:Alternating current through voice coil
→ Alternating magnetic field
→ Interacts with magnetic field of permanent magnet
→ Movement of cone
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Comparison with electro-
dynamic speaker
• Piezoelectric speaker:Alternating voltage on piezoceramic
→ Alternating contraction and expansion of piezoceramic
→ Movement of membrane
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