Box 2.2 Audiograms and spectral sensitivity curves
Light and sound have an important feature in common. Both can be described as waves. These disturbances are comparable to the peaks and troughs of a wave that we might watch travelling across the surface of a pond.
The distance between two peaks of a sound wave is referred to as the wavelength of that sound. Since the speed of travel of sound or light through a particular medium is constant (although sounds travel four times faster through water than through air, and even faster through solids such as metals and rocks) the wavelength bears a simple relationship to the frequency with which the waves pass by. It is this frequency that is usually used to describe one of the key properties of sounds. The unit of frequency used is hertz (Hz), the number of wave cycles per second, but sounds are commonly described in kilohertz, 1000 wave cycles per second.
The frequencies of different sounds vary, and it is these frequency variations that ears detect. We refer to sounds of high and low frequency as being of high and low pitch.
The relative heights of the peaks and troughs in a wave can also vary so that two sounds of the same frequency can contain different amounts of energy. These differences are detected as louder or quieter sounds.
The full spectrum of sounds in nature is remarkably broad, but any animal’s ears can detect only a portion of all possible sound frequencies. Across a range of frequencies an ear detects all sounds but sounds outside that frequency range are simply not detected. Knowledge of the upper and lower limits of the hearing range of a species is an important part of describing their biology. Different animals can hear sounds across markedly different frequency ranges, but no species can detect all possible sounds. Some species detect sounds in the highest frequency ranges, other in the lower ranges, and for others hearing is in a mid-range of frequencies.
Within the frequency range of an animal’s hearing, sensitivity to sounds is not the same across all frequencies. Some sound frequencies can be detected at lower intensities (the sound waves have low amplitude), while others can be heard only at higher intensities (waves of high amplitude). When hearing sensitivity has been determined across the range of frequencies that an animal can hear, an audiogram can be constructed for that particular species.
At a glance audiograms convey a lot of information about the average basic hearing abilities of a species. They define the upper and lower limits and also show which frequencies can be detected at high amplitude and which can be detected at low amplitude. Audiograms generally show a broadly ‘U-shaped’ function. This indicates that there is lower sensitivity (high-amplitude sounds are needed for their detection) to both higher- and lower-frequency sounds, and that the highest sensitivity (sounds of low amplitude can be detected) occurs in a middle range of frequencies. However, audiograms are typically not symmetrical, with rapid changes in sensitivity as a function of frequency in some parts of the hearing range, and less rapid changes in others.
The diagram here shows the average audiogram for Atlantic Canaries Serinus canaria (the common cage bird). It shows that maximum sensitivity to sound occurs at relatively high frequencies, the sounds of notes at the higher end of the piano keyboard, but above those frequencies sensitivity drops dramatically. At the low-frequency end, it looks as though a Canary will not hear the lowest notes of a piano, or at least they would have to be relatively loud to be detected.
If audiograms are available for a number of species, then differences in their hearing can be comprehended readily by comparing audiograms. This makes it possible to get a clear and rapid understanding of how the hearing of species differ one from another. Because of this, audiograms have become a valuable tool for characterising and comparing hearing across species throughout the animal kingdom.
Audiograms are also used as a clinical tool in humans. They are used to characterise different types of hearing loss by comparing an individual’s audiogram with a normal or average audiogram for humans. An individual’s audiogram is likely to change with age. Hearing loss at higher frequencies usually occurs with increasing age, and this readily shows up in an audiogram. This can be used to define the best characteristics of a hearing aid that can be recommended for a particular person.
Diagrams showing the differential sensitivity to light of different wavelengths are known as spectral sensitivity curves. Like sound, light can also be described as a wave phenomenon. However, rather than referring to the frequency of the wave it is usually more convenient to describe light by reference to its wavelength. The distance between the peaks and troughs in light waves is very short compared with sound waves.
The actual wavelength of light is too small to comprehend readily. However, by using the nanometre as the unit of measurement the numbers become manageable. A nanometre (nm) is one metre divided by 1,000,000,000, and using this unit it is possible to refer to visible light (for humans) as falling within the range of approximately 400–700 nm (Figure 2.5).
Within the visible spectrum of an animal, sensitivity to light is not the same across all wavelengths. Light at some wavelengths can be detected at lower intensities, while others can only be detected at relatively higher intensities. When visual sensitivity has been determined in a particular species at a range of wavelengths a spectral sensitivity curve can be constructed. The curve for the Rock Dove (Feral Pigeon) Columba livia at daytime light levels is shown here. It indicates that the eye is most sensitive to light in the yellow-orange wavelength range and that sensitivity falls rapidly in the reds and in the greens and blues, but there is a slight rise in sensitivity in the violet range.
At a glance spectral sensitivity functions convey a lot of information about the average basic vision of a species. These functions generally show a broad domed shape. This indicates that there is lower sensitivity (high-intensity lights are needed for their detection) to both longer and shorter wavelength lights, and that the highest sensitivity (lights of lower intensity can be detected) occurs in a mid-range. However, the position of the peaks in sensitivity and the shapes of spectral sensitivity functions are usually not symmetrical, and in some animal species more than one peak can occur.
If spectral sensitivity functions are available for a number of species, then differences in their vision can be comprehended readily by comparing the functions. Therefore, they are a valuable tool for characterising and comparing vision across species. They are also used as a clinical tool in humans to detect different types of vision loss.
Perhaps the most familiar of these are the differences that can occur in humans in their sensitivity to sounds, in our individual audiograms. If we have reason to have our hearing tested the results will be compared with a normal or average audiogram for humans. This shows how our own hearing may be more or less sensitive to particular frequencies than the hypothetical ‘normal’ individual. However, an inevitable process of ageing is that people start to lose their sensitivity, particularly to higher-frequency sounds. This is referred to as differential hearing loss, and the details of this loss describe how much our own hearing may have changed over time. However, it is worth bearing in mind that these changes can be the result of natural processes, although they can result from disease or physical damage caused by exposure to loud sounds.
When audiograms are compared across a wide range of animal species (mammals, birds, reptiles, and fish) very notable differences in sensitivity to sounds of different frequencies are found between these main animal taxa, as well as between individuals within a taxon. Some animals are able to detect sounds of very low frequencies, others are able to detect sounds with very high frequencies. The same applies to vision, where some species are able to detect light of short wavelengths and others light of relatively long wavelengths. Sensitivity to these shorter wavelengths of light is referred to as vision in the ultraviolet part of the spectrum. In hearing, sounds of low and high frequency are referred to as infrasounds and ultrasounds.
These infra- and ultra- labels are derived from comparisons with the range of light and sound that humans can normally detect. Although in an anthropocentric