Final Report

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Dublin Institute of Technology
Faculty of Science
Development of a
Thermoacoustic Refrigerator
_________________________
Richard Duffy
May 2014
Project report submitted in partial fulfilment of
examination requirements leading to the award of
Ordinary degree in Industrial and Environmental Physics
Supervisor: Francis Pedreschi

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Abstract
An inexpensive tabletop thermoacoustic refrigerator for demonstration purposes was built
from a boxed loudspeaker, Perspex tubing and sheet, carbon fibre rods, rubber plug and two
thermocouples. The purpose of a thermoacoustic refrigerator is to cause temperature
variations across the thermoacoustic stack using sound waves of a certain frequency. The
stack is placed in a resonance tube in a specified position to manipulate the sound waves
striking it into oscillating gas parcels inside the stack causing them to transfer heat up the
walls of the stack and give a cooling effect below the stack. Temperature differences of more
than 10 °C were achieved after running the apparatus for several minutes. The efficiency of
the device was increased by introducing an amplifier to the system for more speaker power,
by changing the speaker’s impedance and by placing the stack near the pressure maximum in
the tube. While the model could have been more efficient, and acts more like a heat pump
than a refrigerator, with more of an increase in temperature above the stack than a cooling
effect below the stack, this demonstration creates the temperature gradient needed for a
thermoacoustic refrigerator and the key principles for a thermoacoustic refrigeration system.

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ACKNOWLEDGMENTS
Firstly I would like to thank Dr Francis Pedreschi for his support and guidance throughout the
duration of the project. His enthusiasm and approachability over the 6 weeks made it a
pleasure to have him as my supervisor. To Dr Elizabeth Gregan who supported us all year
and give us great advice to achieve our goals. Thanks to the senior lab technician Joseph
Keogh who sourced the materials for the project and his knowledge of the lab equipment
were crucial in the success of the project. Also my family and friends who helped keep me
motivated with their invaluable support.

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TABLE OF CONTENTS
Abstract (ii)
Acknowledgements (iii)
CHAPTER 1 – General Introduction
1.1 BRIEF HISTORY ……………………………………………………………………..…2
1.2 THERMOACOUSTIC PHENOMENON………………...……………………………3
1.3 THERMOACOUSTIC STACK………………………………………………………....6
1.3.1 PIN STACK ARRAY ………………………………………………………….........7
1.3.2 HONEYCOMB STACK……………………………………………………………..8
1.4 RESONANCE FREQUENCY……………………………………………………………8
1.5 LENGTH OF TUBE…………………………………………………………………….9
1.6 CLOSED END PIPES…………………………………………………………………..10
1.7 THE SPEAKER…………………………………………………………………………11
CHAPTER 2 – Materials and Methods
2.1 MATERIALS………………………………………………………………………….14
2.2 BOXED LOUDSPEAKER……………………………………………………….......16
2.3 CARBON FIBRE STACK……………………………………………………….…….18
2.31 CATALYTIC CONVERTER STACK ………………………………………………..19
2.32 PHOTOGRAPHIC FILM STACK……………………………………………………20

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2.4 CALIBRATION OF THERMOCOUPLES……………………………………………21
2.5 AMPLIFING SOUND WAVE…………………………………………………………21
2.6 INCREASING THE EFFICIENCY……………………………………………………..23
2.7 EXPERIMENTAL SET UP……………………………………………………….…25
CHAPTER 3 - Results
3.1 INTRODUCTION……………………………………………………………………..27
3.2 TESTING OF ISOLATED TUBE ……………………………………………………...28
3.3 CARBON FIBRE STACK TEST ……………………………………………………….30
3.4 CATALYTIC CONVERTER STACK TEST …...............................................................32
3.5 EFFECT OF INCREASING THE AMPLIFIER GAIN……………………………….34
3.6 EFFECT OF STACK POSITION ………………………………………………..……..36
3.7 EFFECT OF SPEAKER IMPEDANCE AND SIZE ……………………………………38
3.8 TESTING THE 3rd AND 5th HARMONICS……………………………………………..39
CHAPTER 4 – Concluding Remarks
4.1 DISCUSSION……………………………………………………………....................43
4.2 FUTURE WORK ………………………………………………………….……….…..44
4.3 CONCLUSION …………………….………………………………………………..…45
BIBLIOGRAPHY………………………………………………………………..….….…47
RISK
ASSESSMENT………………………………………………………………………….…48

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List of figures
Figure 1.2 Thermoacoustic Refrigerator
Figure 1.21 P – V diagram showing the four stages in the thermoacoustic refrigerator cycle
Figure 1.31 Pin stack array inside resonance tube
Figure 1.32 Honeycomb stack design
Figure 1.5 Closed Cylinder
Figure 1.7 30W/ 8Ω Speaker
Figure 2.2 Thermoacoustic Refrigerator
Figure 2.3: Pin stack array made with carbon fibre tube
Figure 2.31: Catalytic converter stack
Figure 2.32: Photographic film stack
Figure 2.5: PA 100 Amplifier
Figure 2.51: Sound wave on Oscilloscope
Figure 2.6: Changing the speaker
Figure 2.61: New speaker set-up
Figure 2.7: Experimental set up
Figure 4.1: Russell and Weibull experimental data

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List of Graphs
Graph 3.2: Testing of isolated tube
Graph 3.21: Resonance tube without stack with 169Hz signal applied
Graph 3.3: Carbon fibre stack at 3cm from closed end
Graph 3.4: Catalytic converter stack in 50cm tube at 169Hz
Graph 3.5: Carbon fibre stack with amplifier gain of 3
Graph 3.6: Catalytic converter stack in optimum position
Graph 3.61: Catalytic converter stack not in optimum position
Graph 3.7: Higher impedance speaker at optimum conditions
Graph 3.8: 3rd harmonic f3 at optimum conditions
Graph 3.81: 5th harmonic f5 at optimum conditions

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CHAPTER 1
General Introduction

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1.1 BRIEF HISTORY
“Thermoacoustic refrigerators are systems which use sound waves to produce cooling power
(1)”. If the system has the ability to convert acoustics into energy it is hence, called a
thermoacoustic refrigerator. During the last two decades thermoacoustic refrigeration is
explored as a new cooling technology. The thermoacoustic device contains no adverse
chemicals or environmentally unsafe elements that are characteristics of the current
refrigeration systems. Thermoacoustics deals with the conversion of sound energy to heat
energy and vice versa. There are two types of thermoacoustic devices: thermoacoustic engine
and thermoacoustic refrigerator. In a thermoacoustic engine, heat is converted into sound
energy and the energy is available for the useful work. In this device, heat flows from a
source of higher temperature to a sink at lower temperature. In a thermoacoustic refrigerator,
the reverse of the above process occurs, i.e., it utilizes work (in the form of acoustic power)
to absorb heat from a low temperature medium and reject it to a high temperature medium.
For this project we will concentrate on the latter, thermoacoustic refrigeration. The efficiency
of the thermoacoustic devices is currently lower than that of their conventional counterparts,
which needs to be improved to make them competitive. Although thermoacoustic
refrigerators have many advantages which include:
 Mechanical simplicity
 No lubricants needed
 Use of cheap and readily available gases (air)
 Power saving by proportional control
 Lower life cycle cost

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Another major benefit includes the environmental aspect; the international restriction on the
use of CFC gives thermoacoustic devices a strong advantage over traditional refrigerators.
The gases used in these devices (air etc) are totally harmless to the ozone and have no
greenhouse effect.
1.2 THERMOACOUSTIC PHENOMENON
Acoustic waves are oscillations in a medium that cause it to experience pressure,
displacement and temperature variations. In order to produce thermoacoustic effect, these
oscillations in a gas should occur close to a solid surface. A stack is placed inside the
thermoacoustic device in order to produce such a solid surface. The thermoacoustic
phenomenon occurs by the interaction of the gas particles and the stack plate. The sound
wave (driven from a loudspeaker) is used in order to create temperature gradient across the
stack, which is used to transfer heat from low temperature medium to a high temperature
medium.
A thermoacoustic refrigerator consists of a tube filled with a gas, air for this system. This
tube is closed at one end and an oscillating device (loud speaker) is placed at the other end to
create an acoustic standing wave inside the tube.

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Figure 1.2: Thermoacoustic Refrigerator
To be able to create or move heat, work must be done, and the acoustic power provides this
work. When a stack is placed inside the resonator a pressure drop occurs. Interference
between the incoming and reflected wave is now imperfect since there is now a difference in
amplitude causing the standing wave to travel a little, giving it acoustic power. In the
acoustic wave, parcels of gas adiabatically expand and compress.

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Pressure and temperature change simultaneously; to understand the thermoacoustic cycle we
must consider the four processes in the Brayton cycle.
Figure 1.21: P – V diagram showing the four stages in the thermoacoustic refrigerator
cycle (2)
Solid circle shows the parcel state at the beginning of process and the dashed circle shows
the parcel at the end of the process.
1. Adiabatic compression of the gas. (temperature of gas increases). The temperature of
the gas parcel is now higher than that of the stack wall and heat flows from the parcel
to the wall.
2. Isobaric heat transfer. (constant pressure with decreasing temperature). The parcels
temperature is higher than that of the stack causing it to transfer heat to the stack.
3. Adiabatic expansion of the gas. (gas is cooled). The temperature of the gas is lower
than that of the stack.
4. Isobaric heat transfer. (constant pressure, temperature of gas increased back to its
original value) Heat is transferred from the stack back to the gas.

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1.3 THERMOACOUSTIC STACK
The stack is the most important and influential component in a thermoacoustic refrigerator.
This will determine the cooling effect at the set frequency of the fridge. The key to improving
the efficiency of the fridge is developing the stack. The primary constraint in designing the
stack is the fact that stack layers need to be a few thermal penetration depths apart, with four
penetration depths been the optimal separation. (2) The thermal penetration depth, dk , is
defined as the distance that heat can diffuse through a gas during the time t = 1/π f , where f is
the frequency of the standing wave.(2)
d k=√
𝑘
𝜋𝑓𝜌𝐶𝑝
(1)
 k = Thermal conductivity
 ρ = Density of the gas
 cp = Isobaric specific heat per unit mass
If stack layers are too far apart the gas cannot effectively transfer heat to and from the stack
walls. If the layers are too close together viscous effects hamper the motion of the gas
particles.

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1.3.1 PIN STACK ARRAY
The pin stack array was constructed using carbon fibre tubes. For optimum performance a
material with low thermal conductivity is required. The internal diameter of the tubes was
1mm and optimum separation four thermal penetration depths. This is the gas corridor the air
travels through.
𝟏 𝐱 𝟏𝟎−𝟑 𝐦
𝟒
= 2 x 𝟏𝟎−𝟒
m (dk) (2)
 dk = Thermal penetration depth
From this we can calculate the optimum frequency from the diameter of the tubes.
Then determine the length of the tube needed to create resonance at this frequency.
Figure 1.3.1: Pin stack array inside resonance tube(3)

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1.3.2 HONEYCOMB STACK
This stack is new to the market and is being introduced in thermo applications. We
constructed the design by using the catalytic converter from the exhaust of a car.
Figure 1.3.2: Honeycomb stack design (3)
1.4 RESONANCE FREQUENCY
Resonant frequency is the natural frequency of vibration determined by the physical
parameters of the vibrating object. (4) The resonant frequency of air columns depend upon the
speed of sound in air as well as the length and geometry of the air column. The speed of
sound in dry air is approx 334.1 m/s. For the purpose of this project this is accurate and we do
not need to consider room temperature variation effects.
The frequency of the system can be calculated using dk (equation 1)
dk = √
𝐾
𝛱 𝑓 𝑝 𝐶𝑝
(2)

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Rearrange for f gives
f =
𝐾
𝛱 𝑝 𝐶𝑝 𝑑𝑘2 (3)
Where K = thermal conductivity, p = density of gas, Cp = isobaric specific heat per unit mass
The density of air and isobaric specific heat per unit mass were calculated using an online
calculator at room temperature, which was measured with a mercury thermometer.
f =
0.0257𝑤/𝑚 𝑘
(3.14)(1.205
𝑘𝑔
𝑚3)(1.005
𝐾𝐽
𝐾𝑔.𝑘
)(2𝑥10−4 𝑚)2
f = 169 HZ
1.5 LENGTH OF TUBE
We can now calculate the length (L) of the tube needed
f =
𝑛 𝑉
4 𝐿
(4)
Rearrange for L gives L =
n V
f 4
L =
(1)(340)
(4)(169)
L = 0.5m
Where n = Harmonic number ( 1,3,5...) This tube produces only odd harmonics because it is
closed.
V = Speed of sound in air, f = resonance frequency , 4 = ¼ wavelength for closed end

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1.6 CLOSED END PIPES
The air at the closed end of the pipe must be a node (not moving) since the air is not free to
move there and must be able to be reflected back.
There must also be an antinode where the opening is , since that is where there is maximum
movement of the air.
Figure 1.5: Closed Cylinder. (5)
The red line represents sound pressure and the blue line represents the amplitude of the
motion of the air.
The pressure has a node at the open end, and an antinode at the closed end.
The amplitude has a node at the closed end and an antinode at the open end.
Therefore, optimum stack position in the tube should be close to the pressure maximum, but
away from the particle displacement minimum.
Even harmonics are absent as they would be out-of-phase , causing destructive interference
instead of constructive interference.

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1.7 The Speaker
The ohm (Ω) is the unit of measure for impedance, which is the property of a speaker that
restricts the flow of electrical current through it. (6) Study shows that the temperature
differences between the hot and cold sides of the stack increase with speaker power.
The amplifier will deliver maximum power to the speaker when the speaker impedance
matches the internal impedance of the amplifier. Too low impedance will result in weak
output and poor tone. If the speaker impedance is higher than that of the amplifier, its
output power will again be less than its capable of.(6)
For optimum speaker performance in our system the speaker impedance should equal the
amplifier impedance.
To calculate the impedance of an amplifier
Output impedance
The resistance was measured with a digital multimeter, with the speaker being the load on the
system. The load resistance is the resistance of the speaker.
Voltage measurement at the points at OUT:
V1 = Open-circuit voltage (Rload = ∞ Ω, that is without Rload, switch S is open)
Rload = Load resistance (Rtest is resistor to measure Ω value)
V2 = Loaded circuit voltage with resistor Rload = resistance Rtest
Zsource = The output impedance can be calculated

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8Ω x (
16.9 𝑚𝑣
7.7 𝑚𝑣
− 1 ) = 9.6 Ω (5)
Figure 1.7: 30W/ 8Ω Speaker

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CHAPTER 2
Materials and Methods

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2.1 MATERIALS
30W Speaker
60W Speaker
Carbon fibre tubes
Catalytic converter
Digital multimeter
Earplugs
Face mask
Lab coat
MDF wood
PA 100 Amplifier
Perspex tubing/ sheets
Power drill
Rubber O rings/cork
Safety goggles/gloves
Screws
Super glue
Silicon
Silver Varnish

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Styrofoam
Tektronix oscilloscope
Thermocouples x 2
Unilab signal generator
Vacuum grease

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2.2 BOXED LOUDPEAKER
The box for the loudspeaker was constructed using MDF wood; the sides were screwed
together using a power drill. The top of the box was drilled for the loudspeaker to fit snugly
into it. The speaker was fitted in and sealed with silicon. A Perspex sheet was fitted on top
of the speaker with a drilled hole big enough for the resonance tube. The Perspex was fitted
using silicon.
Figure2.2: Thermoacoustic Refrigerator

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Two circular Perspex rings were constructed with holes drilled in the centre to hold the
resonance tube. Using a lathe; notched groves in the Perspex were made to hold the rubber
O rings for an air tight seal.
The resonance tube was cut to length using a hacksaw.
A small hole was drilled in the side of the box for the thermocouple; the thermocouple went
up the tube and sat below the stack.
A rubber cork is placed in top of the tube with a hole drilled in it to fit the thermocouple
which sits above the stack. This hole was sealed with silicon.
The seals were also sealed with a vacuum grease to improve efficiency.
The system was placed on top of Styrofoam to dampen the sound level exposure.

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2.3 CARBON FIBRE STACK
Carbon fibre tubes were ordered from www.easycomposites.co.uk. They were cut using a
power tool with a fine grit edge. Safety goggles were worn. Insulation tape was used to
constrict movement of the tubes. The pin stack constructed was 50mm in length and a rubber
o ring was used for a seal.
Figure 2.3: Pin stack array made with carbon fibre tube

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2.31: CATALYTIC CONVERTER STACK
A catalytic converter was recovered from a car exhaust. It was cut to fit the resonance tube
using a handheld power tool with a sharp cutting edge. Safety goggles and a face mask were
worn as it contained harmful toxins.
cack
Figure 2.31: Catalytic converter stack

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2.32: PHOTOGRAPHIC FILM STACK
This stack was used in the original paper on tabletop thermoacoustic refrigerator by Daniel
A. Russell and Pontus Weibull.(2) The stack was designed using photographic film , fishing
line and a copper rod as the centre piece. Super glue was used to stick the fishing line to the
photographic film.
Figure 2.32: Photographic film stack
Testing of the stack proved problematic as the stack got damaged when changing the stack
position. Preliminary results were poor so this was not tested any further.

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2.4 CALIBRATION OF THERMOCOUPLES
For accurate results the two thermocouples were calibrated before the experiment was
conducted. A mercury thermometer was used as a control and the adjustment screw on the
thermocouples was changed to match the temperature on the thermometer.
2.5 AMPLIFING SOUND WAVE
The maximum temperature gradient achieved using the UNILAB signal generator was 2.9 °C
(see results). An amplifier was introduced to our system to improve the power output of the
speaker and increase the thermoacoustic effect. This increased our temperature gradient to 9
°C ( see results).
Figure 2.5: PA 100 Amplifier

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The signal was viewed on the oscilloscope to see what the maximum gain achievable is
before saturation occurs. Gain = output/input. The max gain of the amplifier before
saturation occurs, A = 7.
Figure 2.51: Sound wave on Oscilloscope

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2.6 INCREASING THE EFFICIENCY
To improve the efficiency of the system the speaker was changed. The speaker was very wide
for the small opening in the tube and some of the acoustic wave energy was being absorbed
by the Perspex walls.
Figure 2.6: Changing the speaker

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A piece of wood was placed between the Perspex top and the speaker to accommodate the
change in size of the speaker.
Figure 2.61: New speaker set-up
The new speaker had also higher impedance. The original speaker was 3Ω, whereas the new
speaker was 8Ω which is much closer to the desired 9.6Ω of the amplifier for maximum
performance. This increased our temperature difference a further 2.1°C giving us a change of
10.7°C (see results).

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2.7 EXPERIMENTAL SET-UP
This is the experimental set up used in the testing of the thermoacoustic refrigerator.
Figure 2.7: Experimental set up

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CHAPTER 3
Results

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3.1 INTRODUCTION
This section reports the results of the study. Following the testing off the system the carbon
fibre stack proved most efficient with the optimum stack position being 8cm from the closed
end. The efficiency of the system was increased by the addition of the amplifier and by
changing the speaker impedance. The study also viewed the difference in temperature
difference between the first, third, fifth harmonics. The system worked for the purpose
designed and demonstrated the thermoacoustic effect successfully with a maximum
temperature gradient of 10.7°C after 10mins being achieved.
NOTE: For the following sets of data Tc and Th will refer to the cold and hot sides of the
stack respectfully.
Data was recorded for time intervals at which significant changes happened, after this time
the temperature gradient between both ends of the stack all but stopped increasing.

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3.2 TESTING OF ISOLATED TUBE
Firstly the system was tested without the stack in place or the speaker connected to check for
any temperature variations. The two thermocouples were placed inside the tube in the
positions they would sit when the stack is in the tube.
Graph 3.2: Testing of isolated tube
As can be seen from the above graph the temperature does fluctuate inside the tube without
the stack or speaker connected. However, the variation is small with a maximum fluctuation
of 0.2 degrees Celsius for both Tc and Th . This could be due to ambient temperatures which
is the temperature in the room and around the thermoacoustic refrigerator. Room temperature
was monitored using a mercury thermometer and changes were very small and considered not
17 17 17
16.9
17
16.9
17 17 17
16.9 16.9 16.9
18.1 18.1
18.2 18.2
18.3
18.2 18.2
18.1 18.1
18
18.1 18.1
16.8
17
17.2
17.4
17.6
17.8
18
18.2
18.4
0 2 4 6 8 10 12 14
Temperature(C)
Time (s)
Temp vs Time
Temperatire Tc
Temperature Th

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important to the experiment. Data was recorded for 12 minutes as the fluctuations in this time
was steady and changes were not expected to happen after this time.
The speaker was then connected with the applied resonance frequency of 169 Hz.
Graph 3.21: Resonance tube without stack with 169Hz signal applied
The graph above if figure 3.21 shows a temperature fluctuation greater than that of figure 3.2.
This is due to the system being subject to the 169Hz signal applied. A rise in temperature is
evident with a maximum difference of 0.6 degrees in the tube after 10 minutes. This test was
done without the stack to see the effect of the applied frequency so the thermocouple Tc was
removed. Data was recorded for 10 minutes as temperatures did not rise after this time.
18.1 18.1
18.2 18.2
18.3 18.3
18.4
18.5
18.6
18.7
18.1 18.1
18.2 18.2
18.3 18.3
18.4
18.5
18.6
18.7
18
18.1
18.2
18.3
18.4
18.5
18.6
18.7
18.8
0 2 4 6 8 10 12
Temperature(C)
Time (s)
Temp vs Time
Temperatire Tc
Temperature Th

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3.3 CARBON FIBRE STACK TEST
The next test was the carbon fibre stack placed at different positions in the resonance tube to
search for the optimum stack position for maximum performance. The ideal condition is for
the stack to be close to the pressure maximum but away from the particle displacement
minimum. The UNILAB signal generator was used in this process.
The first test the stack was placed at 3cm from the closed end of the tube to the centre of the
stack.
After 10miutes of testing the temperature gradient ΔT = 1.4°C.
23.9
23.2
23
22.9 22.9 22.9
22.7
22.6
23.9
24.3
24.4 24.4 24.4 24.4
24.1
24
22.4
22.6
22.8
23
23.2
23.4
23.6
23.8
24
24.2
24.4
24.6
0 2 4 6 8 10 12
Temperatire (C)
Time (s)
Temp vs Time
Temperatire Tc
Temperature Th

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The second test the stack was placed at 5cm from the closed end to the centre of the stack.
The third test the stack was placed at 8cm from the closed end to the centre of the stack.
23.7
23.4
23.2 23.1 23 23 22.9 22.9
23.7
25.2 25.2 25.2 25.2 25.2
25.5
25.7
22.5
23
23.5
24
24.5
25
25.5
26
0 2 4 6 8 10 12
Temperature (C)
Time (s)
Temp vs Time
Temperatire Tc
Temperature Th
21.8
20.8 20.8 20.8 20.8 20.9 21 21
21.8
22.9
23.2
23.4 23.5 23.6 23.7
23.9
20.5
21
21.5
22
22.5
23
23.5
24
24.5
0 2 4 6 8 10 12
Temperature (C)
Time(s)
Temp vs Time
Temperatire Tc
Temperature Th

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These tests show that optimum position for the carbon fibre stack was 8cm from the closed
end. Further stack positions were tested but performance degraded significantly any further
distance from the closed end.
3.4 CATALYTIC CONVERTER STACK TEST
This test was to check the effect of changing the tube length and resonance frequency using
the catalytic converter stack. The catalytic converter stack was 25mm in length where the
carbon fibre stack was 50mm. The prime stack position was calculated to be 8cm for the
carbon fibre so the test was done at 4cm for catalytic converter as it’s only half the length.
The first test was using 50cm tube at 169 Hz
25.2
25.1
25
24.8
24.6
24.5
24.4
25.2
25.9 25.9 25.9
25.8 25.8 25.8
24.2
24.4
24.6
24.8
25
25.2
25.4
25.6
25.8
26
0 2 4 6 8 10
Temperature (C)
Time(s)
Temp vs Time
Temperatire Tc
Temperature Th

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After 9 minutes of testing the temperature gradient ΔT = 1.4°C.
The second test was using 25cm tube at 343 Hz. The resonance frequency was adjusted to the
tube length using formula f =
𝑛 𝑣
4 𝐿
.
These tests show that the catalytic converter was more efficient in the 25cm resonance tube
with 343Hz signal applied. This could be due to the stack length being half of that of the
carbon fibre. Further study of stack geometry would make interesting future work.
26.5
25.9
25.3
24.8
24.5
24.1
23.9
26.5
26.6 26.6
26.7 26.7 26.7 26.7
23.5
24
24.5
25
25.5
26
26.5
27
0 2 4 6 8 10
Temperature (C)
Time(s)
Temp vs Time
Temperatire Tc
Temperature Th

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3.5 EFFECT OF INCREASING THE AMPLIFIER GAIN
The amplifier was introduced to the system to increase the power of the signal from the input
to the output of the speaker.
The first test was using 50cm tube at 169 Hz and carbon fibre stack.
Gain = 3
The amplifier increased performance of the system hugely. The maximum temperature
gradient achieved using UNILAB signal generator was 2.9°C, this increased when using the
PA100 amplifier to 6.8°C.
The gradient achieved is due more to Th rising than Tc falling. This is the basis on which a
heat pump would operate and not a refrigerator. However, the principle behind the project is
24.3 23.2 23.1 23 22.9 22.9 22.9 22.8 22.8 22.8 22.9
24.3
26.6 27.3 27.8 28.2 28.6 28.9 29.1 29.3 29.5 29.7
0
5
10
15
20
25
30
35
0 2 4 6 8 10 12
Temperature(C)
Time (s)
Temp vs Time
Temperatire Tc
Temperature Th

42 | P a g e
to obtain a temperature difference across a thermoacoustic stack and this is achieved. All that
is needed is a pump to circulate the hot air which will give the refrigeration effect desired.
The same effect can be seen in the following results.
The second test was using 50cm tube at 169 Hz and carbon fibre stack.
Gain = 7 (max before saturation occurs)
The results show that by increasing the amplifier gain from 3 to 7 (max) our temperature
gradient increased from 6.8°C to 8.6°C while keeping the other parameters constant. This
23.9
22.5 22.4 22 21.9 21.9 21.9 21.8 21.8 21.8 21.8
23.9
26.6 27.2 27.5 28.2 28.8 29.3 29.5 29.9 30.2 30.4
0
5
10
15
20
25
30
35
0 2 4 6 8 10 12
Temperature(C)
Time (s)
Temp vs Time
Temperatire Tc
Temperature Th

43 | P a g e
shows that the gain has a direct effect on the performance of our speaker and therefore the
performance of our thermoacoustic refrigerator.
3.6 EFFECT OF STACK POSITION
This test looks at the effect of having the stack in position to out of position.
Catalytic converter stack in optimum position. (4cm)
Ideal performance conditions, f = 343 Hz , tube length 25cm , amplifier gain = 7.
Graph 3.6: Catalytic converter stack in optimum position
After 10 minutes of testing the temperature gradient ΔT = 6°C.
25 24.9 24.9 24.7 24.7 24.7 24.7 24.7 24.6 24.6 24.525
27 27.8 28.3 28.9 29.2 29.6 29.9 30.1 30.3 30.5
0
5
10
15
20
25
30
35
0 2 4 6 8 10 12
Temperature(C)
Time (s)
Temp vs Time
Temperatire Tc
Temperature Th

44 | P a g e
Catalytic converter stack NOT in optimum position. (8cm)
Ideal performance conditions, f = 343 Hz , tube length 25cm , amplifier gain = 7.
Graph 3.61: Catalytic converter stack NOT in optimum position
This data shows that the performance of the system decreased rapidly when the stack was
placed out of position. After 10 minutes of testing the performance decreased by 4.3°C.
Therefore, stack position is crucial in the set up of the system.
23.6 23.6
23.5
23.4 23.4 23.4 23.4
23.3 23.3
23.2 23.2
23.6
24.4
24.7
24.8 24.8
24.9 24.9 24.9 24.9 24.9 24.9
23
23.2
23.4
23.6
23.8
24
24.2
24.4
24.6
24.8
25
0 2 4 6 8 10 12
Temperature(C)
Time (s)
Temp vs Time
Temperatire Tc
Temperature Th

45 | P a g e
3.7 EFFECT OF SPEAKER IMPEDANCE AND SIZE
To increase the size of the temperature differential the speaker was changed. (see 2.6
increasing the efficiency)
The new speaker had higher impedance closer to that of the amplifier and a smaller diameter
to better suit the diameter of the resonance tube.
The test was done with the carbon fibre stack under the same conditions which achieved the
maximum temperature difference of 8.6°C. ( Gain of amp = 7, f = 169Hz, tube = 50cm, stack
position = 8cm)
Graph 3.7: Higher impedance speaker at optimum conditions
This increase in temperature shows us that changing the speaker made the system more
efficient. This is due to the new speaker having higher impedance closer to that off the
amplifier. (See 2.6 increasing the efficiency)
21.5 20.8 20.6 20.2 20.2 20.1 20 19.8 19.8 19.6 19.6
21.5
26.9 28.1 28.8 29.3 29.7 29.9 30 30.1 30.2 30.3
0
5
10
15
20
25
30
35
0 2 4 6 8 10 12
Temperature(C)
Time (s)
Temp vs Time
Temperatire Tc
Temperature Th

46 | P a g e
Another important factor is the diameter of the new speaker is smaller and more power will
therefore get up the resonance tube and not absorbed in the Perspex walls.
3.8 TESTING THE 3rd AND 5th HARMONICS
A harmonic of a wave is a component frequency of the signal that is an integer multiple of
the fundamental frequency. (6)
The wave displacement has only quarter of a cycle of a sine wave, so the longest sine wave
that fits into the closed pipe is four times as long as the pipe.
L =
λ
4
(6)
We can also fit in a wave if the length of the pipe is three quarters of the wavelength, i.e. if
wavelength is one third that of the fundamental and the frequency is three times that of the
fundamental. But we cannot fit in a wave with half or a quarter the fundamental wavelength
(twice or four times the frequency). Therefore this type of tube produces only odd harmonics.
f =
𝑛 𝑉
4 𝐿
f1 (1st harmonic) =
(1)(343)
(4)(0.5)
= 169 Hz
f3 (3rd harmonic) =
(3)(343)
(4)(0.5)
= 515 Hz
f5 (5th harmonic) =
(5)(343)
(4)(0.5)
= 858 Hz

47 | P a g e
3rd HARMONIC TEST
Test done under ideal conditions for performance, carbon fibre stack, stack position = 8cm,
tube 50cm, amp gain = 7, new speaker used.
f3 = 515Hz
Graph 3.8: 3rd harmonic f3 at optimum conditions
Performance of the system degraded from 10.7°C to 2.6°C from the first harmonic f1 to the
third harmonic f3.
This gives an efficiency drop of approx 75%.
20.7
20.5 20.4 20.3 20.3 20.2 20.2 20.2 20.2 20.1 20.1
20.7
21.5
21.7
21.9 22
22.2 22.3 22.4 22.5 22.6 22.7
19.5
20
20.5
21
21.5
22
22.5
23
0 2 4 6 8 10 12
Temperature(C)
Time (s)
Temp vs Time
Temperatire Tc
Temperature Th

48 | P a g e
5th HARMONIC TEST
Test done under ideal conditions for performance, carbon fibre stack, stack position = 8cm,
tube 50cm, amp gain = 7, new speaker used.
f3 = 858Hz
Graph 3.81: 5th harmonic f5 at optimum conditions
Performance of the system degraded from 2.3°C to 0.7°C from the third harmonic f3 to the
fifth harmonic f5.
This gives an efficiency drop of approx 75%.
In summary, the performance of the system decreases by approx 75% per overtone. This was
due to the standing wave pattern changing as the harmonics increased while the stack position
remained in the optimum position for the first harmonic and was not adjusted accordingly.
19.5
19.4 19.4 19.4
19.3 19.3 19.3
19.2 19.2 19.2
19.1
19.5
19.6 19.6 19.6
19.7 19.7 19.7
19.8 19.8 19.8 19.8
19
19.1
19.2
19.3
19.4
19.5
19.6
19.7
19.8
19.9
0 2 4 6 8 10 12
Temperature(C)
Time (s)
Temp vs Time
Temperatire Tc
Temperature Th

49 | P a g e
CHAPTER 4
Concluding Remarks

50 | P a g e
4.1 DISCUSSION
The project aims where achieved with the thermoacoustic refrigerator being built at low cost
and it provided a sufficient temperature gradient of 10.7°C to show the working principles of
the system. Carbon fibre proved a more efficient material for the stack than the catalytic
converter. The important factors in designing the heat stack include stack position, which is
crucial that the placement is near the pressure maximum in the resonance tube. For efficiency
purposes it is important to consider the power output of the speaker, a amplifier can give
more power to the speaker and thus a greater performance in the system. The speaker’s
impedance must also be close to that of the amplifier for desired performance. The amplifier
increased the temperature from 2.9°C to 8.6°C, and matching the impedance increased it a
further 2.1°C to our maximum gradient achieved of 10.7°C. Our temperature gradient
decreased on average by 75% per overtone. Therefore the fundamental tone n =1 is the most
efficient resonance frequency to work for the system.
In comparison to Russell and Weibull paper in the American Association of Physics
Teachers(2), this system worked more like a heat pump than a refrigerator with a large
increase in temperature above the stack and only a small cooling effect below the stack. We
can see from the following diagram this was not the case for Russell and Weibull who system
had a greater cooling effect below the stack like a typical refrigeration device.

51 | P a g e
Figure 4.1: Russell and Weibull experimental data (2)
However, the system designed is suitable for refrigeration, a simple heat pump could be used
to pump the hot air away and get the desired refrigeration effect.
The reason for the difference in performance is unknown and this is an interesting topic for
future work.
4.2 FUTURE WORK
Further development of the stack to increase performance, including stack length
optimization. Different resonator shapes to maximise power going into the tube could also be
investigated. General improvements on the seals could also improve the system. Investigate
the difference in performance between this system and Russell and Weibull system. (2)

52 | P a g e
4.3 CONCLUSION
The project was a success with reasonable and desired outcomes achieved; the temperature
gradient measured across the stack was 10.7°C. This temperature difference could be felt by
touching both ends of the stack which is a strong indication of the temperature gradient on
both ends of the stack. Both the carbon fibre and the catalytic converter were constructed
successfully and worked as a stack with the carbon fibre proving more efficient. The project
was built at low cost and was made more efficient than the original system designed by
introducing an amplifier for more speaker power and by changing the speaker impedance.

54 | P a g e
1. http://www.nevis.columbia.edu/~ju/Paper/Paper-
thermoacoustic/Construction%20therm%20refrigerator.pdf
2. http://www.acs.psu.edu/drussell/publications/thermodemo.pdf
3. http://www.nevis.columbia.edu/~ju/Paper/Paper-
thermoacoustic/Construction%20therm%20refrigerator.pdf
4. http://hyperphysics.phy-astr.gsu.edu/hbase/sound/reson.html
5. http://www.phys.unsw.edu.au/jw/pipes.html
6. http://www.prestonelectronics.com/audio/Impedance.htm
7. http://en.wikipedia.org/wiki/Harmonic

55 | P a g e
RISK ASSESSMENT
Researcher Details
Name (use block capitals): RICHARDDUFFY
Title: MR
Faculty/School/Department SCHOOLOFPHYSICS
Location ofWork
LAB KE 1-039 KEVIN STREET DUBLIN 8
Title and Descriptionof Work
Give brief detailsof task,materialsandequipment,frequencyandduration. Continue
on separate sheetif necessaryorattachmethodstatement,protocol etc.
DEVELOPMENT OF A THERMOACOUSTIC REFRIGERATOR,USING WOOD, PERSPEX,
LOUDSPEAKER,SIGNALGENERATOR, MULTIMETER, CARBON FIBRE. PROJECT
DURATION 6 WEEKS MONDAY TO FRIDAY10-5PM.

56 | P a g e
Hazards
For example:liftingandcarrying;repetitive movements;
heator cold; sharpedges;workingatheights;noise;
electrical. Give abrief descriptionof the injuriesthatcould
occur and how.
RISK
(High,medium, Low)
1. LOUD NOISE FROMSPEAKER
2. USING POWERTOOLS FOR CUTTING
3. USING GLUE ANDOTHER ADHESIVES
4. ELECTRICAL EQUIPMENT, AMPLIFIER
5. USE OF THE LADE
6. FUMES FROMCATALYTICCONVERTER
HIGH
HIGH
MEDIUM
MEDIUM
HIGH
MEDIUM

57 | P a g e
Who is at risk?
For example:staff carryingoutthe task;maintenance and
cleaningstaff;peoplenearby;visitors;contractors. Give a
brief descriptionof howandwhentheyare at risk.
RISK
(High,medium, low)
1. LAB TECHNICIAN WHEN SPEAKERIS ON
2. STUDENTS WHEN SPEAKERIS ON
3. MYSELF DURING USE OF POWER TOOLS ANDLADE
4. CLEANINGSTAFFWHEN GLUE WAS DRYING
5.
6.
HIGH
HIGH
HIGH
LOW
What physical or mental characteristics may alter the risk?
For example:pregnancy;illness(specify); disability(specify);height;leftorright
handedness

58 | P a g e
1. WORKINGFROMA BENCH INRESASESRISKOF FALLINGOBJECTS
2.
3.
4.
5.
6.
What measures are already provided to reduce risks to all those at risk?
(A) Safe working methods, materials, equipment

59 | P a g e
1. SAFETY GOGGLES
2. SAFETY GLOVES
3. LAB COAT
4. EAR PROTETION
5. PROTECTION COVERFOR LADE
6. LAB TECHNICIAN SUPERVISION
B) Location
1. Location:Identifyclearlythe exactlocation(s) of the risk
LAB 1-039, KEVIN STREET
2 Mobility:Will the hazardbe mobile?How often?
NO,ALL WORKIS DONE IN THE LAB
3 Access:What accessarrangementsare in place at the location,e.g.locks,
electronicsafetyinterlocksetc.

60 | P a g e
LAB IS LOCKED AT LUNCH TIME ANDAFTER 5PM ANDBEFORE10AM
4 Security:Issecurityrequiredatthe locationandwhatlevel:e.g.alarms,infra-red
detection,
securitycamera(s),panicswitchesetc
SECURITY CAMERAS IN OPERATION
5 Signsand warnings:Whatsignsandwarningsare neededatthe location?
NO EATINGOR DRINKINGIN LAB, DANGERHIGH VOLTAGE
C) Personal protective equipment or clothing
1. LAB COAT
2. SAFETY GOGGLES
3. EAR PROTECTION
4. FACEMASK
5. SAFETY GLOVES

61 | P a g e
D) Information, instruction or training
1. FIRE SAFETYDRILLS
2. EXITSCLEARLY MARKED
3. LAB SIGNS
4. FIRSTAID AVAILABLE
5.
6.

62 | P a g e
E) Emergency procedures
For example:firstaid;fire fightingandevacuation;communications.
1. FIRSTAID KIT
2. FIRE SAFETYDRILLS
3. WINDOWSANDDOORS OPEN FOR EXIT
4.
5.
6.
10.8 Are further measuresneededtoreduce risks?
For example:changesinworkingmethods;materials;
equipment;location;protective equipment;training.
ACTION
(DATE)

63 | P a g e
1. CLOSE DOORSTO REDUCE NOISELEVELS
2. GET BETTER EAR PROTECTION
3.
4.
5.
10.9 Sourcesof informationusedfor thisassessment
1. BOOKS
2. INTERNET
3. PUBLISHED PAPERS
4. PREVIOUSTHESIS
5.
6.

64 | P a g e
10.10 Person(s) completingthisassessment
Signature:
_________________________
Print Name: __RICHARD
DUFFY______________________
Title:
_______MR___________________
Date: _____21-5-
2014_____________________
Signature: _________________________
Print Name: ________________________
Title:__________________________
Date: __________________________
10.11 Approvedby Safety Officer(or Head of School)
Signature: ___________________________________ Title:
__________________________
Print Name: ________________________
Date: ________________________________
10.12 Approvedby Head of School

65 | P a g e
Signature: ___________________________________ Title:
__________________________
Print Name: ________________________
Date: ________________________________

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