In this lecture the following are introduced:
• Sound waves produced from a sudden local compression
• Infrasonic and Ultrasonic waves
• Ultrasonic transducers
• Reflections at interfaces
• Ultrasonic applications
• Thermoacoustics
• Opto-acoustics

The study of sound is called "acoustics" or "sonics". It is a very wide research field that includes the atmosphere, aviation, biology, ocean floor mapping, oil exploration, medicine, the military, music, welding etc. Sound is produced by a sudden local compression of a gas, liquid or solid. The compression may be a single pulse (bang!), a group of pulses (speech), or a periodic pulse (musical tone). It is then transmitted through the neighbouring media by pressure waves. The following picture shows compressions travelling in a spring.

Sound is only audible to the average human ear if the frequencies lie between 20Hz and 20kHz. The actual range varies from person to person.
Sound waves with frequencies less than 20Hz are called infrasonic or subsonic and those with frequencies above 20kHz are called ultrasonic.

Infrasonic waves

Elephant herds can communicate and synchronise their migrations even though they may be out of sight and separated by up to 20km. They do this by "speaking" and hearing with infrasonic waves which we cannot hear but can bend around obstacles rather than being absorbed like normal sound.
Infrasonic wave pulses are also produced by a number of geophysical processes such as avalanches, earthquakes and various explosions, geomagnetic variations, meteors, ocean waves, severe weather and volcanos. Early detection of infrasonic waves that signify dangerous situations may provide warnings to nearby populations.
Aircraft and other industrial machinery (e.g. jack hammers) also cause infrasonic waves that can be harmful to some human internal organs.
For further information see the National Ocean and Atmospheric Administration

Ultrasonic Transducers

Ultrasonic transducers are used to produce and detect ultrasonic waves ( >20kHz). An ultrasonic transducer is a device that changes its physical size when an electric potential is applied to it. As it shrinks and swells in size it sends out compression waves into the medium around it and they have the same frequency as the applied electric potential. To be effective, the transducer (often a piece of ceramic material) needs to vibrate strongly (i.e. resonate). To do this it has to be at least a half a wavelength thick.

Example
A ceramic material transmits sound through it at a speed 3.8 km.s^-1. Find the minimum thickness of the ceramic needed for a transducer that will resonate at a frequency of 1.5 MHz.

A transducer can also be used in reverse to detect sound. As sound waves compress and expand the transducer material, the transducer outputs an electrical signal that is an analogue likeness of the pressure wave that caused it.

Reflections at interfaces

When a sound or ultrasound wave comes to the boundary (or interface) between two different media, there will be, in general, some reflection backwards, some transmission forwards and some absorption by the interface. How much is reflected or transmitted depends on the acoustic "hardness" of the media involved. The bigger the difference the more reflection there is.

The acoustic impedance of a material measures the "hardness" and it is defined as the product of density (kg.m^-3) and speed (m.s^-1) and given a unit called the "Rayl".

Using transducers for transmitters and detectors together, pulses or continous waves can be sent out through materials and the reflections analysed to give information about the depth and types of layers in the material.

Biological applications of ultrasound

•Diagnosis

In diagnosis, ultrasonic scans (~1.5MHz) are used with low power ultrasonic transmitters and detectors to map and display human internal organs and structures. Scans can test for ovarian cancer and defects in foetuses. The speed and impedance for some biological tissues are shown in the table.

The wavelengths are about 1mm to 2mm. The different acoustic impedances of the various organs means there will be reflections of varying strengths at the internal boundaries between the organs. Also, the reflected pulses will return at different time intervals, or continuous waves will return with phase differences. Sorting out the various strengths and times/phases can give real-time pictures of the internal organs in two and three dimensions.

Some example foetal scans from Greggory R. DeVore

• Therapy

At higher powers, ultrasound can be used in therapy because it deposits heat in the deeper muscles, bones and joints. Kidney stones can be pulverised with ultrasonic waves by the intense vibrations that they can produce between layers in the stones.

Other Ultrasonic applications

Ultrasonic cleaners work because dust particles are small enough to be thrown around by ultrasonic waves.

Ultrasonic welders and cutters produce high temperatures in a small region and are used in the manufacturing industries when dealing with synthetic woven materials, rubber, fibre-glass, carbon-fibre and Kevlar.

Insects such as moths and ants have ultrasonic communications (~ 80 kHz) for mating and communal purposes.

Pulse-echo technique

Using a sound pulse and examining the echo that comes back is called the pulse-echo technique. If the speed of the sound through the medium is known then the distance to the interface can be measured. In the time between pulse and echo, the sound has travelled to the interface and then back again.

Since some sound will normally be absorbed while travelling through a medium, the echo will be less intense than the initial pulse. The relative size of the input wave and the echo will then show how much sound is being absorbed (or "attenuated") by the medium.

Fish Finders

Some fishing boats have a "fish finder" which sends pulses of sound into the water and picks up the reflections ("sonar"). They not only show the depth of the water under the boat but also any fish that are swimming underneath. (Submarines can also be found this way and used through earth some land mines can be detected).

Non-destructive testing

The pulse-echo technique can be used for non-destructive testing of materials. For example, cracks inside pillars will reflect waves back, so by sweeping a narrow beam of sound backwards and forwards, this will give the size and depth of any internal cracks as well as the wall thickness.

See the Magnetic Analysis Corporation

Seismic sections

In the oil exploration industry, "seismic sections" are produced. They are images that show the depth and thickness of underground rock structures.

Either a big truck will have a "thumper" attached to it, which will periodically bang the ground, or explosives are used to produce a line of pulses. Recording the various echos with their relative intensities and phases, enables the different types of rock, their thicknesses and depths, to be calculated and mapped.

Thermoacoustics

Thermoacoustics is about heat transport with sound. It applies to refrigerators, air-conditioners and some engines using the Stirling cycle.

Pictured is a thermoacoustic fridge for the space shuttle. Thermoacoustic fridges are 20-30% lower in efficiency than normal vapour compression fridges. However, this is in part due to intrinsic irreversibilities in the thermoacoustic heat transport mechanism. The irreversibilities are a good thing, because they give mechanical simplicity, i.e. few or no moving parts.

See Jeffrey Gold

The Stirling cycle has thermodynamic reversibility. See the Stirling Cryogenics and Refrigeration BV

Its main disadvantage has been the need for high-pressure, sliding piston seals. Scott N. Backhaus and Gregory W. Swift (Los Alamos) have constructed and tested a 1-kW Stirling engine with an efficiency that is greater than 35% of the Carnot efficiency. This is much higher than any previous thermoacoustic engine.

Optoacoustics

The interaction of light and sound can produce many effects and devices.
• Optoacoustic modulators can bend laser light.
• Light can be generated from sound (in sonoluminescence). See Dr. Felipe Gaitan at The Jamie Whitten National Center for Physical Acoustics, University of Mississippi.
• Laser beams can generate and detect acoustic waves. By controlling the position and size of a laser beam array, the points of ultrasound generation and detection are moved and an ultrasonic image is constructed.

This enables tissues to be sampled without removing them from the patient. See Dr. Matthew O'Donnell at the University of Michigan, Biomedical Ultrasonics Lab

Summarising:

Sound is produced from a sudden local compression of a gas, liquid or solid.
Infrasonic waves have frequencies < 20Hz: ultrasonic waves have frequencies > 20kHz.
An ultrasonic transducer is a device that changes its physical size when an electric potential is applied to it.
The acoustic impedance (density x speed) in Rayl, measures the "hardness" of a medium.
At an interface there is reflection, transmission and absorption.
The size of the reflected wave depends on the difference in acoustic impedance.
Ultrasound can be used in diagnosis and therapy.
The pulse-echo technique has many applications e.g. in fishing, oil exploration and non-destructive measurement.
Thermoacoustics: heat transfer by using sound.
Optoacoustics is the interaction between light and sound.