Acoustic impedance - view demo
Impedance is another word for resistance. In a way, it is a measure of how much the medium impedes the passage of sound waves. It has many similarities with the resistance (impedance) of components of an electrical circuit. Air has very low acoustic impedance, while water has very much higher acoustic impedance.

Amplitude - view illustration
Measure of a wave, for example a sound wave. In sound, amplitude is equivalent to pressure, provided the medium is constant. In preparation.

Auditory
Concerning the sense of hearing. The auditory system is the equipment an animal uses to hear, including the external and internal ears, the auditory nerves, and the entire auditory pathways into the brain. See also 'sonar receiver'.

Biosonar
= Biological sonar. An active sense possessed by some animals, notably bats and dolphins. The animal has a sonar transmittor (e.g. its larynx), with which it broadcasts pulses of sound, the sonar signals. The animal also possesses a sonar receiver (part of its auditory system). It uses this to recieve and analyse copies of the sonar signals reflected from objects in its surroundings (i.e. echoes). By doing so, the animal gathers useful information about these objects, such as their location, and can probably even form images of them.

Broadband
This term describes a signal that contains energy over a broad range of the frequency spectrum.

Correlation
Generally, a measure of similarity.

Cross-correlation
A method that of correlating an outgoing sonar signal with an echo of that signal. Bats might use such a method to extract precise delay information from an echo, by comparing the fine time information within an echo with the fine time information within the outgoing pulse. The two signals (outgoing pulse and echo) have a high correlation with each other only when they match exactly in time. The time at which they match exactly is taken as the delay of the echo, that is, how long it has taken to return, which indicates the distance to the target.

Bats seem to perform cross-correlation, or a process just as good as cross-correlation to determine the distances to objects in their surroundings.

Cycle - view illustration
One "cycle" or "period" is the period of time it takes for a wave to increase and decrease in pressure, and return to its starting point.

Decibels (dB), and the decibel scale
Decibels are part of a method of measuring the intensity, amplitude, or the Sound Pressure Level of sounds. In the case of sound pressure, the decibel scale compresses the vast range of audible sound pressures (from 20 micropascals to about 20 Pascals, a million-fold range) to a scale from 0 to about 120 dB (decibels), just so that talking about different sounds is more convenient and manageable. It works like this: a reference sound pressure is chosen. It is usually the smallest sound pressure audible to the human ear (20 micropascals). Then, the Sound Pressure Level of other sounds are expressed relative to the reference sound pressure, using an equation including a logarithm. The logarithm is the part that compresses the numbers. It has the effect that whenever the Sound Pressure Level increases by 6 dB, it also actually doubles in sound pressure. An increase from 70 to 80 dB, for instance, is actually more than a 3-fold increase. A 20 dB increase is actually a 20-fold increase in pressure.

The decibel scale is a logarithmic scale, not a linear scale.
See also: Sound Pressure Level

Doppler (effect)
The phenomenon of signal distortion that occurs when a sound source is moving. The frequency audibly changes, and the signal shortens or lengthens and distorts, depending on the velocity of the sound. Think of an ambulance siren moving past you. It starts with a high pitch (your impression of the sound's frequency) when the ambulance is moving towards ou (positive velocity), then changes to a lower pitch when the ambulance passes you and starts to move away (now having negative velocity).

Sound moves at about 330-345 metres per second (m/s). Bats fly at between 3 and 15 m/s. The velocity of bats is sigificant compared to the velocity of the sound signals they emit, so the Doppler Effect is important to bats. It can be a problem for the bat's sonar receiver if the received signal is distorted by the Doppler Effect. In biosonar, the Doppler Effect works in both directions (towards the target, and back, away from the target), so is twice as strong.

Echolocation
Another word for biological sonar, or 'biosonar'. Used moreoften in the world of bats than in the world of dolphins.

FFT
Fast Fourier Transform - a method of analysing the frequency content of a signal. When you perform Fourier analysis, you assume that the signal is composed of sine waves of different frequencies. By performing an FFT, you deduce the frequencies and amplitudes of sine waves you would need to reconstruct the time-domain signal. This distribution of frequencies and amplitudes is the power spectrum, and is a representation of the signal in the frequency domain.

Fourier
See FFT

Frequency - view illustration
The rate (or speed) at which something happens. In sound, it is the rate at which the wave 'cycles'. In other words, the frequency of a wave is the number of times it cycles in a second. Frequency is therefore the inverse of period (1 / period). Frequency is measure in Hertz (Hz) or kilohertz (kHz).

Hertz
A measure of frequency, or the number of times something occurs in a second. In terms of sound, 1 Hz (Hertz) = 1 cycle of the sound waveform per second. 1 kHz (kilohertz) = 1000 Hz.

Impedance
See 'acoustic impedance'.

Kilohertz
See 'Hertz'.

Loud, Loudness
The impression in the auditory system of the pressure or intensity of the sound. It is very different from sound intensity, though, because the auditory system, whether it be your own, or that of a bat, is more sensitive to some sounds than others. For instance, you are not very sensitive to sounds of frequencies above 20 kHz, so these sounds might be very intense, and have a very high amplitude and pressure, but would not be at all loud. Loudness, therefore, is a property of the sound and the auditory system together.

Medium
The substance through which a sound wave travels, for example 'air' or 'water'. Sound waves can travel through many media, including solids, gases and liquids. The sound compresses the medium as it passes (see sound demo ). Particles of the medium pass backwards and forwards in the direction of travel of the wave. The net movement of the medium is zero, because all the particles return to their original positions when the sound has passed, and when the pressure has returned to the ambient pressure.

Megachiroptera
A suborder of bats (order Chiroptera), these are the large, Old World fruit bats, including the flying foxes. They do not generally possess biosonar, but have very large, well-developed eyes. Only one small genus of three species, the Egyption fruit bats (Rousettus) uses biosonar, and it produces simple click signals with its tongue. This method of biosonar is separately evolved from that of the microchiropteran bats, and it is thought to be not so advanced, being suitable only for orientation in dark roost spaces like caves and tombs. Rousettus' cannot hunt by biosonar, their biosonar systems are not good enough.

Microchiroptera
A suborder of bats (order Chiroptera), these are the small insectivorous bats that use advanced biosonar extensively in their lives, not only for orientation, but often for hunting as well.

Narrowband
This means narrow in frequency range. A narrowband signal, once analysed for its frequency content, contains energy in only a small range of the frequency spectrum.

Oscillogram
A diagram of the changes in amplitude of a signal over time. A display of a signal in the 'time domain'. Can easily be displayed on an oscilloscope connected to a microphone or hydrophone.

Particles
It is convenient when talking about sound waves to picture the medium as 'parcels' which we call 'particles', but not in the usual sense of atoms or molecules. Particle in this sense is just a parcel of medium which we imagine behaving as one, for simplicity. When a sound wave passes, a particle of medium moves backwards and forwards in the direction the wave is travelling.
(see sound demo ).

Period - view illustration
One "period" or "cycle" is the period of time it takes for a wave to increase and decrease in pressure, and return to its starting point. Period is measured in seconds or milliseconds. Period is the inverse of frequency (1 / frequency).

Pitch
Pitch is the subjective sensation in the auditory system, brought about by detection of a sound's frequency. Frequency is a property of a sound. Pitch is a property of the stimulation of the auditory system by the sound. There is not a linear relationship between pitch and frequency, but rather an exponential one: see demo of pitch and ultrasound

Power Spectrum
A display of the frequency content of a signal. A display of a signal in the 'frequency domain'. The result of a Fourier analysis of a signal.

Sine - view illustration
A sine wave describes the changes in pressure in a medium due to a sound of pure tone (single frequency).

Sonar
SOund Navigation And Ranging. Used to describe the biological sonar of bats, dolphins etc., and artificial sonar developed by man and used by fishing trawlers, submarines, medical scanning etc.

Sonar receiver
A biological sonar receiver is the part of an animal's auditory system that does the computations necessary to 'see with sound'. Several different segments of the auditory system can contribute to the process; each has a job, from the outer ear to the auditory cortex. The sonar receiver is a theoretical concept, and its physical nature in animals' bodies has not been completely worked out. Man-made sonar receivers are antennae (microphones) and computer programs that analyse the incoming signal received by the microphone.

Sound - view demo
Sound waves are waves of pressure, or compression. They result from any physical disturbance of a medium, e.g. explosions, vibrations of loudspeakers, vibrations of vocal chords.

Sound Pressure Level (SPL)
Sound Pressure Level is part of a standard system of how we express the pressure, or amplitude, of a sound. Because it becomes harder and harder to measure very weak (low amplitude) sounds, we establish a standard weak sound to compare other sounds to. If we compare all sounds to the same, standard, weak sound, we have a standard way of measuring and reporting their amplitude. The most common standard weak sound is 20 micropascals (20 millionths of a Pascal, the unit of pressure), which is the weakest sound that is audible to humans at 1.0 kHz. We call 20 micropascals the reference sound pressure and make it 0 dB SPL (decibels of Sound Pressure Level). Then, most sounds that we record and measure have a Sound Pressure Level (in decibels) of more than zero, relative to a sound of 20 micropascals.

Spectrogram
A joint time-frequency display of a signal with time on the x-axis and frequency on the y-axis. You can usually see amplitude information on the spectrogram coded by colour or brightness of the display. A spectrogram shows the way in which a signal changes in frequency and amplitude over time. Because a click signal, like that of a dolphin, does not change in frequency over time, a spectrogram of a click is not usually useful. Dolphin experts usually use a single power spectrum, which shows frequency information over the entire duration of the signal.

Bat experts, on the other hand, find spectrograms very useful because most bat signals change in frequency (show frequency modulation, "FM") over time.

In effect, a spectrogram is built up from a multitude of power spectra of short, overlapping time segments of the signal.

SPL
See Sound Pressure Level

Target
Any object that is the focus of an animal's attention when it is using biosonar. It can be any object that reflects ultrasound, but is often an item of prey such as an insect or a fish.

Ultrasound - view demo
A word used to describe pressure waves (see 'sound') which cycle at frequencies greater than those to which the human ear is sensitive. Because we humans cannot hear ultrasound, it does not have the subjective quality of sound, i.e. it does not produce the sensation that we call sound. However, the 'ultra'-ness of ultrasound is really a feature of the human detection system. Fundamentally, it is just sound (pressure waves) of high frequency. Cats, dogs, bats and dolphins can hear ultrasound, and to them it must simply be high-pitched sound.