This page contains online questions only. Jot down
your answers and check them against the worked out answers at the end of
the page
7. ULTRASOUND
- definition, examples described, uses and calculations
As already mentioned, you can use a signal
generators to produce electrical oscillations of any frequency.
The electrical oscillations are
converted to mechanical vibrations producing sound waves.
The sound waves can be produced to well
beyond the range of human hearing.
The high frequencies are above 20 kHz
(20000 Hz) and are known as ultrasound.
Ultrasound waves behave like any other
waves, they can be absorbed, reflected or refracted.
The reflection and refraction effects can
be used to measure distances and 3D scanning with sonar searching and
medical imaging applications.
In physics the term “ultrasound” applies to all acoustic
energy sound waves with a frequency above typical human hearing (20,000
hertz or 20 kilohertz). Typical diagnostic sonographic scanners used in
various applications operate in the frequency range of 2 to 18 megahertz
(2-18 x 106 Hz), which is hundreds of times greater than the
limit of human hearing.
Medical uses of ultrasound
Ultrasound of very high
frequency sound waves are used in scanning pregnant women to monitor the progress
of unborn baby.
The ultrasound waves enter the woman's
body and the echoes-reflections are picked up by a microphone and
converted into electronic signals from a which an internal picture of the
womb can be
constructed.
Ultrasound is considered a safe technique
for pre-natal scanning of a foetus, various soft-tissue organs and is much safer than using dangerous X-rays.
Tissues e.g. muscle, stomach, womb or fluids of different
density give different intensities of reflection and so differentiation of
the structure of the womb, foetus or baby can be seen - modern
ultrasound scanners can produce quite high resolution images.
The speed of
ultrasound is different in different tissues and the ultrasound scanner is
able to work out the distance between different boundaries and construct a
'3D' image of the developing foetus in the womb.
Ultrasound can also be used in the
'medical imaging' of soft tissue organs like the bladder, kidneys and
liver.
The scans can detect changes in the structure of these organs and
help diagnose medical conditions associated with them.
Although medical
imaging using ultrasound is quite safe, the images are not sharp enough to
replace the use of X-rays for investigating bone structure.
Note that a gel is placed on the
patient's body between the ultrasound probe and their skin.
The gel ensures that most of the
ultrasound would be refracted at the skin and a good image of the
internal structure of the body could not be obtained.
More
on
how does ultrasound scanning works
When sound waves (in this case ultrasound) are passing from one
medium to another (solid or liquid - body fluid, tissue, bone, organs etc.) some
are partially reflected, and some are transmitted and refracted at the boundary interface
(see the diagram on the right of wavefronts meeting a boundary between two media).
In foetal scanning, the two media would be
the fluid in the woman's womb and the skin and tissue of the growing foetus.
The time for reflections to take place at a particular angle is
measured i.e. the time it takes for the sound impulses to be emitted from the
transmitter and reflected back off a boundary and the return signals picked up
by the detector.
From the reflections, it
is the echo time intervals that allows a distance to be calculated by the
computer and their
distribution allows a 3D image to be built up.
The recorded data is processed by a computer to build up a 3D
video on the screen from which individual images can be viewed and even printed
out (for medical checks-investigation and hopefully for the delight of expectant
parents).
This form of medical imaging works because ultrasound waves
can pass through the body but if they meet a boundary e.g. between fluid and
tissue some of the waves are reflected.
It is the distribution of these echo
signals that enables the computer to build up the image.
Treatment of kidney stones
The patient lies on a water-filled cushion, and the surgeon
uses X-rays or ultrasound tests to precisely locate the kidney stone.
Ultrasound shock waves are then sent to the stone from a
machine to break it into smaller pieces that can be carried in your urine
and pass through the urinary tract and out of your body - job done!
Industrial uses of ultrasound
Ultrasound is used to detect flaws in manufactured products such
metal castings or pipes.
Ultrasound waves, after entering a
material are usually reflected back by the far side of a object.
If there is a flaw in the casting or the
welding of an object, when scanned with ultrasound the flaws show up as some
of the waves are reflected-deflected back where you might expect them to go right through the object
to the other side.
In other words if there is an internal flaw in the object
e.g. a weld joint, the ultrasound is reflected back sooner than expected.
The same technique is used on test
trains to check for flaws in railway tracks e.g. cracks in train rails.
An ultrasound beam can be used to measure
the thickness of a material by detecting the reflection from both surfaces.
Use of ultrasound for sonar - echo sounding
Ultrasound systems are used by small boats, ships and submarines
for echo sounding.
You need a transmitter and receiver and from the echo signals you can get the distance to the seabed.
With more sophisticated systems you can get an 'underwater
picture' of what's there e.g. shoal of fish, sunken wreck, dangerous underwater
rocks.
The signal time of the echo can be used to measure the depth of
water beneath a boat.
If d = depth of water (m), if t = time of
echo signal 'there and back' (s), v = speed of sound wave (m/s)
v = d / t, therefore depth d = v x
(t / 2) (m/s)
BUT, note that the time is halved because the
sound is 'going there and back' in the time interval t.
Bats and ultrasound!
Bats are amongst other animals, such as
cats and dogs, that can hear high frequency sounds.
Bats emit pulses of sound of 20 000 Hz to 100 00 Hz to
find their way around using echo location.
Their large ears pick up reflected sounds and their brain
builds up a 'picture' of the 3D world in front of them.
A tale of the mole! (but NOT
the chemical quantity the 'mole')
In September 2023 a European mole (Talpa europaea) decided to take up
residence in our garden and has had a great time excavating the lawn
and producing some very neat mole hills of excellent quality soil.
Internet research revealed that a humane method of inhibiting their
presence was to buy an ultrasound emitter and bury it into the
ground where mole activity was present and to be discouraged. They
don't like higher pitched sounds or loud sounds.
Theoretically ultrasonic mole repellers are supposed to emit
ultrasound waves that are inaudible to humans but extremely
irritating to moles. Moles can detect infrasound, that is sound
waves of less than 20 Hz.
I purchased a pair of these mole repellent ultrasound emitters,
fitted the batteries in and buried them at either end of the lawn.
These typically operate with frequencies of 400 to 1000 Hz and these
frequencies moles find irritating (technically this is not
ultrasound and better described as a sonic pulse).
However other ultrasonic pest repellers produce sound at frequencies
that are intolerable to pests like moles but inaudible to humans.
Electronic pest repellers' frequencies typically fall between 20 kHz
and 100 kHz, that is frequencies well normal human hearing.
The sonic devices I purchased did seem to work at first, but the mole just moved to other areas
in the garden, some I'm not really sure it was worth the financial
and physical effort involved in my sonic garden mole repeller
project. The general consensus is that they are not that effective.
However, it is another description of the use of sound and
might come in handy in a more open-ended exam question on the
applications of sound technology.
So, rather than resort to nasty chemicals I've decide to pay homage to
the evolutionary traits developed by the mole - however annoying
they are!
In evolutionary terms, the mole is an inspiring 'integrated science'
subject and well adapted for life underground. It lives off
worms, grubs and insects, but the problem is where they forage for
food, but some of the advantages evolutionary traits are:
(a) Greatly enlarged front feet that are strong and good for digging
and soil removal.
(b) Mole fur is short and smooth, almost like velvet and this
minimises friction when moving through narrow soil tunnels (physics).
(c) Moles have a thin layer of skin over their eyes to protect them
from soil falling on them, the eyes are only 1 mm in diameter.
(d) Moles do not have prominent ears, they have small holes in the
skull covered with a thin layer of skin to keep the soil out. They
can detect relative low frequencies of sound and do not like higher
frequency sounds (physics).
(e) Moles are tolerant to high concentrations of carbon dioxide in
the air, as carbon dioxide levels will build up at times in the
confined spaces of their tunnel network (biochemistry).
(f) They can re-breath in expelled air to extract more oxygen - this
links in with (e).
(f) The haemoglobin (hemoglobin) molecules in their blood are more
efficient in carrying oxygen than many other mammals including us
and this enables them to tolerate lower levels of oxygen in the
underground air (which of course will be coincident with higher
levels of carbon dioxide, biochemistry).
Calculations
involving ultrasound waves and echo sounding
Be able to use both the equations below,
which apply to
sound waves (and their rearrangements):
appropriate units used in ()
a) sound wave speed (metre/second, m/s)
= frequency (hertz, Hz) x wavelength
(metre, m)
in 'shorthand'
v = f x
λ
rearrangements:
f = v ÷ λ
and λ
= v ÷ f
b) sound wave speed (metre/second, m/s) =
distance (metre, m) / time (second, s)
in 'shorthand'
v = d ÷ t
rearrangements:
d = v x
t and
t = d ÷ v
This is the general formula for the
speed or velocity of anything moving.
Equation (a) 
equation
(b) 
Q1 A pulse of
ultrasound from a fishing boat takes 1.40 seconds to travel from the boat down
to the seabed and back to the 'microphone' detector.
If the speed of sound in seawater is 1530 m/s calculate the
depth of the water at that point.
Worked out ANSWERS to the 'echo' calculation questions
Q2
The speed of sound in seawater is 1530 m/s.
How long will it take an ultrasound
signal to be transmitted and received after reflecting back from a shoal of
fish swimming at a depth of 200 m? (give your answer to two significant
figures)
Worked out ANSWERS to the 'echo' calculation questions
Q3
In misty weather, a ship sounds a foghorn warning when near a coastline of
cliffs.
If the echo is heard 5.0 seconds later,
how far away are the reflecting cliffs if the seed of sound is 340 m/s?
Worked out ANSWERS to the 'echo' calculation questions
Q4
Suppose the time difference is 150
µs between two pulses from either side of the head of a fetus in a
woman's womb.
If
the speed of ultrasound is 1540 m/s, calculate the size of the baby's head
in cm.
Worked out ANSWERS to the 'echo' calculation questions
INDEX of physics notes
on SOUND
Keywords, phrases and learning objectives for
ultrasound waves
Know that ultrasound is very high frequency sound waves
that humans cannot hear but bats can, which they use for 'echo
location'.
Be able to describe examples of the uses of
ultrasound e.g. foetal scanning and depth sounding and scanning
using emitted ultrasound from a boat.
Be able to speed and echo sound wave calculations
Use your
mobile phone in 'landscape'?
SITEMAP
Website content © Dr
Phil Brown 2000+. All copyrights reserved on Doc Brown's physics revision notes, images,
quizzes, worksheets etc. Copying of website material is NOT
permitted. Exam revision summaries and references to GCSE science course specifications
are unofficial.
INDEX of physics notes
on SOUND
