Sound as a tool

Sound waves are reflected from obstacles. This is used to great effect in remote sensing, where the location, composition and motion of an object can be determined from large distances.

Detailed analysis of the intensities and frequencies of reflected sounds forms the basis for a whole range of imaging techniques. In most applications the sound is emitted as a series of pulses. The distance between the source and the reflecting surface is calculated from the time between the emission of a pulse of sound and the detection of the echo. The term echola-tion has been coined to describe the use of echoes to determine distances. Echolation is routinely used to measure the depth of water beneath a ship.

We can map the contours of reflecting surfaces by measuring the times for echoes to return from different parts of the surface. Echoes vary in intensity, loud for a highly reflecting surface and soft for a highly absorbing surface, enabling us to determine the nature of the surface. We can also measure the velocity of a moving surface from the difference in frequency between emitted and reflected sounds (using the Doppler effect).

Sound navigation and ranging (SONAR)

Sound travels large distances in water. Sound-based underwater communication systems were used as early as the 19th century when lightships equipped with underwater bells acted as navigation aids. The ringing sound was detected in passing ships using stethoscopes. These were the forerunners of today's passive sonar systems.

Passive sonar. A listening device which does not radiate any sound, it may used in studies of marine life and by submarines to minimise the risk of detection. Wartime use of passive sonar revealed many unexpected sources of underwater noise, such as the so-called 'snapping' of shrimps. There are numerous sources of noise: man-made (such as ships' propellers, engines and seismic drilling), biological (whales and other marine life) and natural (such as tides and seaquakes). Echoes from the seabed increase the underwater noise level. Many modern passive sonar systems have very advanced signal discrimination and extended receivers help to improve the direction and range finding.

Active sonar. Most sonar systems are active and gather information by echolation. The sound is emitted as a narrow pulsed beam of high intensity, often at ultrasonic frequencies.

The first active sonar system was an ultrasonic submarine detector developed in 1917 by Paul Langevin (1872-1946). The ultrasonic waves were emitted and detected by the same device, called a transducer. The reflected wave was detected using the inverse piezoelectric effect, whereby the incoming ultrasonic wave 'squeezes' a quartz crystal in the detector, producing an oscillating electrical signal. It was the first practical application of the inverse piezoelectric effect.

Current uses of active sonar are numerous. There is extensive military use, such as the detection of submarines and sea mines, navigation and acoustic missiles. Civilian applications include accurate underwater surveying, marine communication and the location of shoals of fish, sunken aircraft and shipwrecks. The

Boat Using Ultrasound Detect Fish

transducer may be fixed to a rotating platform mounted on the keel of a boat or towed along in a small 'towfish', just above the sea bed (side-scan sonar).

This 'ghostly' image of the sunken cruise liner Mikhail Lermonotov was recorded using side-scan sonar operating at a frequency of 675 kHz, purchased as part of a research project to map marine habitats.

Sonar Side Scan High Resolution
Sunken cruise liner. Courtesy of Ken Grange National Institute of Water and Atmospheric Research, Nelson, New Zealand.

Ultrasound is absorbed more rapidly than audible sound but has higher resolution and is ideal for searching for small objects such as mines. Submarine-hunting equipment operates at lower frequencies (mostly high frequency audible sound and very low frequency ultrasound).

Ultrasound in nature

Bats use ultrasonic waves to distinguish prey from obstacles when in flight. They can isolate echoes from general background sound. In a cave full of bats, a bat can differentiate between its own signals and those associated with other bats. The time resolution of bat sonar apparatus (about 2 millionths of a second) and the precision with which bats

Ultrasound Bats

locate and identify prey is the envy of manufacturers of sonar equipment.

Ultrasound in medicine

Ultrasonic energy is absorbed more rapidly in tissues such as bone, tendons and tissue boundaries which have a high concentration of a protein called collagen. This forms the basis for diagnostic and therapeutic procedures in medicine. Ultrasound does not produce the toxic side effects associated with ionising radiations such as X-rays.

Low intensity ultrasound as a diagnostic tool

The imaging of tissue and the velocity measurement of fluids within the body have become invaluable tools in medicine. The intensity of diagnostic ultrasound is kept as low as possible to minimise heat absorption. The times of arrival and relative intensities of the echoes are used to construct a detailed image of the tissue structure. The level of detail in ultrasonic images increases with frequency but the level of penetration decreases. As an example, body tissue absorbs about 50% of the energy of 1 MHz ultrasonic waves in about 4 cm but the same amount of energy at a frequency of 3 MHz will be absorbed in 2-2.5 cm. This limits the efficiency of the technique.

Sonography is a pulse-echo imaging technique which is widely used in medicine. It is almost 50 years since ultrasound was first used to produce images of a developing foetus and it is now standard medical practice. A transducer is placed on the patient's skin, close to the area being examined. A coupling gel applied to the skin ensures that about 99.9% of the ultrasonic energy enters the body. The

Foetal image. Courtesy of Wessex Fetal Medicine Unit, St Anne's Hospital, Southhampton.

ultrasound probe is moved over the abdomen of the patient to obtain different perspectives of the foetal image.

Doppler ultrasound

Pulsed ultrasonic waves are beamed into the body and are partially reflected by body tissue and fluids, as illustrated in Figure 7.3. The frequency of waves reflected from moving blood particles is different from the frequency of the waves emitted by the source. The difference between the two frequencies is directly proportional to the velocity of the blood (see Appendix 7.1 — Doppler effect). The velocities of blood particles in the aorta, the carotid artery, the umbilical chord and numerous other blood vessels can be calculated from measurements of this frequency shift.

The velocity of the blood particles depends on the diameter of the blood vessel and the uniformity of the walls at the point of measurement. It is possible to measure velocity profiles and detect abnormal blood flow patterns caused, for example, by the build-up of fatty deposits on the walls of arteries using this technique.

Heat treatment

As ultrasonic waves pass through the body, tissue expands and contracts thousands of times each second, producing heat. Highpower, well-focused ultrasound can create temperatures in excess of 100°C in targeted tissue without damaging surrounding

reflected waves

Figure 7.3 Doppler blood flow monitoring.

reflected waves

Figure 7.3 Doppler blood flow monitoring.

tissue, in the same way that a magnifying glass can be used to focus sunlight and burn holes in paper. Focused ultrasound can kill cells and is used to treat some forms of cancer.

Shock wave lithotripsy

Shock wave lithotripsy is the non-invasive destruction of kidney stones using ultrasound. Shock waves generated outside the body reduce the stones to a harmless powder. To avoid damaging surrounding body tissue, the shock waves are focused on the stone itself rather than being simply directed at the kidney. The ultrasound source S is situated at one focus of a silvered ellipsoidal reflector and the patient is positioned so that the kidney stone is at the other focus of the ellipsoid. Ultrasonic waves reflected from the silvered surface are focused on the stone and pulverise it. Waves not reflected from the silvered surface simply spread out and become weaker. The shaded areas in the diagram represent wavefronts converging on the kidney stone (only waves travelling to the stone are shown).

Acoustic cavitation

A liquid may be boiled by heating it to the appropriate temperature or by lowering the ambient pressure. In either case, the result is cavitation, the formation of bubbles of vapour within the liquid. Localised pressure changes, such as those caused by waves, can cause bubbles to form. Acoustic cavitation occurs when a liquid is exposed to intense ultrasound, causing bubbles to form, grow and collapse repeatedly. Energy is absorbed from the waves as the bubbles grow, and released when they collapse. The collapse or implosion of the bubbles causes localised

Lithotripters Internal Composition

Lithotripter. Courtesy of

Mark Quinlan, UCD Medical School, Dublin.

Lithotripter. Courtesy of

Mark Quinlan, UCD Medical School, Dublin.

heating. Research indicates that the internal temperature of almost completely collapsed bubbles may reach 10,000 K. When low frequency ultrasound bubbles expand and contract, flashes of blue light may be seen. This phenomenon is called sonolumi-nescense (from the Greek words for sound and light).

Surface erosion and cleaning

The erosion of ships' propellers is one of the earliest-known examples of surface erosion due to acoustic cavitation. The motion of a propeller creates sound waves and bubbles. Heat from repeated implosion of the bubbles erodes the surface of the propeller. Acoustic cavitation is an efficient and non-toxic method of cleaning. Equipment, immersed in a bath containing mild detergent, may be irradiated with high intensity ultrasound for 5-10 seconds to remove surface grime. It is often possible to see dirt being lifted from the immersed objects. In liposuction, another application of cavitation, sound from a probe inserted into the body attacks and erodes fat cells.

Infrasound in nature

Birds may travel thousands of miles in summer or winter migrations — a remarkable feat of navigation. One of the best avian navigators is the pigeon, once used to carry messages.

Pest control a well-equipped rat!

a well-equipped rat!

There are pest control devices which emit intense blasts of ultrasound at frequencies which totally disrupt the nervous systems of rodents, such as rats, but are harmless and often inaudible to humans and other mammals, such as cats and dogs.

The pigeon's extraordinary homing instinct is the basis for the popular sport of pigeon racing. Pigeons can fly home in cloudy and windy conditions and can be trained to navigate at night. It is well known that pigeons use the earth's magnetic field as a compass, but a sense of direction is not sufficient to return them to their home loft. They need to be able to relate their position to their destination — they need a map. It is thought that infrasound plays a part in this very precise homing process. Infrasonic waves, generated by natural sources such as seismic activity and ocean waves, propagate in the air. They are reflected off geographical features such as steep hillsides and may serve as a navigation aid for the pigeons. There have been instances where a large number of birds were inexplicably lost or delayed. In June 1997, 60,000 English pigeons were released in France and about one third of them failed to return to England. It has been suggested* that the birds were disoriented after crossing the path of low frequency shock waves generated by the Concorde supersonic aircraft.

* J.T. Hagstrum, The Journal of Experimental Biology, 203: 1103-1111 (2007); Proc. 63rd Annual Meeting of The Institute of Navigation, 2007, Cambridge, Massachusetts.

Elephants and infrasound

Elephants live in closely knit family groups sometimes dispersed over several kilometres. They frequently engage in coordinated activities, indicating that they have a well-developed communication system. Their sight is poor and remote communication takes place by means of sound. Conservation programmes use these calls to monitor elephant numbers, location and behaviour.

Most calls have fundamental frequencies in the range of 5-30 Hz, at or very close to the infrasonic frequency band, and may be transmitted over very large distances. A characteristic spectrogram of calls from forest elephants is shown in Figure 7.4, where frequency in Hertz is graphed against time in seconds. The loudness of the sound is represented by the darkness of the display.

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Figure 7.4 Elephant calls. Courtesy of The Elephant Listening Project — www.birds.cornell.edu.

Listening for danger

Networks of listening stations, originally set up to monitor explosions from underground testing of nuclear weapons, are now important sources of information on potentially disastrous events such as hurricanes, earthquakes and meteors.

In addition to furthering scientific understanding, such networks have the potential to become early warning systems in the event of impending natural catastrophies.

In 1999, infrared noise from the explosion of a meteor in the atmosphere was detected by scientists at the Royal Netherlands Meteorological Service.

Courtesy of Hans Haak,

Royal Netherlands Meteorological Service.

Courtesy of Hans Haak,

Royal Netherlands Meteorological Service.

7.3 Superposition of sound waves

The sounds we hear are usually the result of the superposition of a number of sound waves. Superposition of waves can result in amplification or in reduction of the intensity of the sound. Passive aids, such as ear-muffs, which reduce background sound are inefficient for low frequency noise. Noise cancellation, i.e. active suppression of noise, is widely used by airline pilots to suppress high levels of low frequency engine noise. Small microphones mounted on a pilot's headset convert the ambient noise in the cockpit to an electrical signal, which is inverted and re-emitted as sound by small speakers. The noise experienced by the pilot is the sum of the ambient and inverted noise wave forms and is substantially lower than the ambient noise in the cockpit. The technique is most effective against the steady low frequency sounds created by engines.

engine noise engine noise

sum of engine noise and inverted noise inverted \{ engine noise sum of engine noise and inverted noise inverted \{ engine noise

7.3.1 Standing waves Pipe open at both ends

A column of air in a pipe has natural resonant frequencies, in much the same way as does a string fixed at both ends. Standing waves can be set up in a pipe open at one or both ends. At an open end (or just outside), the air molecules are free to vibrate, constrained only by the elasticity of the air, so the open ends are anti-nodes. A closed end is an almost perfectly rigid boundary, so it must be a node. Figure 7.5 shows the first two harmonics for a pipe of length L, open at both ends.

The displacement is maximal at both ends of the pipe, so the basic symmetry is the same as a string fixed at both ends (the positions of the nodes on a string are the same as the positions of the anti-nodes in a pipe of the same length). Any whole number of half wavelengths can be fitted into the pipe.

first harmonic (fundamental):

first harmonic (fundamental):

second harmonic:

second harmonic:

Figure 7.5 Standing waves in a pipe open at both ends.

The natural frequencies of a pipe open at both ends are fn = nL where n = 1, 2, 3,----

Pipe closed at one end

In Figure 7.6 we see the first and second allowed modes of vibration in a pipe closed at one end. The situation is not the same as for the open-ended pipe — the symmetry has been broken because there is now a node at one end and an anti-node at the other end.

first harmonic (fundamental):

first harmonic (fundamental):

third harmonic:

third harmonic:

Figure 7.6 Standing waves in a pipe closed at one end.

Figure 7.7 Standing waves in a tube. Courtesy of Stefan Huzler, Trinity College, Dublin.

Only odd harmonics are found in the sound from a pipe closed at one end.

Breaking the symmetry removes half the natural frequencies.

The anti-nodes of sound waves in a perspex tube may be made visible by the presence of equally spaced soap films, as seen in Figure 7.7. The coloured soap films reveal the positions of the anti-nodes. The photograph is part of a novel experiment for the visualisation and measurement of standing waves in a tube.*

*F. Elias, S. Hutzler and M.S. Ferreria, Visualisation of sound waves using regularly spaced soap films. Eur. J. Phys. 28: 755-765 (2007).

t = 0 s wave 1: f1 = 60 Hz t = 0.1 s t = 0.2 s t = 0.3 s

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