The amplitude of a sound wave determines our perception of _______________________.

AMPLITUDE AND WAVELENGTH

Two physical characteristics of a wave are amplitude and wavelength (figure below). The amplitude of a wave is the height of a wave as measured from the highest point on the wave (peak or crest) to the lowest point on the wave (trough). Wavelength refers to the length of a wave from one peak to the next.

The amplitude or height of a wave is measured from the peak to the trough. The wavelength is measured from peak to peak.

   Wavelength is directly related to the frequency of a given wave form. Frequency refers to the number of waves that pass a given point in a given time period and is often expressed in terms of hertz (Hz), or cycles per second. Longer wavelengths will have lower frequencies, and shorter wavelengths will have higher frequencies (figure below).

This figure illustrates waves of differing wavelengths/frequencies. At the top of the figure, the red wave has a long wavelength/short frequency. Moving from top to bottom, the wavelengths decrease and frequencies increase.

LIGHT WAVES

   The visible spectrum is the portion of the larger electromagnetic spectrum that we can see. As the figure below shows, the electromagnetic spectrum encompasses all of the electromagnetic radiation that occurs in our environment and includes gamma rays, x-rays, ultraviolet light, visible light, infrared light, microwaves, and radio waves. These waves are everywhere around us at all times but for some waveforms we need to use sophisticated tools in order to translate this information into visible light waves we are able to see. The visible spectrum in humans is associated with wavelengths that range from 380 to 740 nm—a very small distance, since a nanometer (nm) is one billionth of a meter. Other species can detect other portions of the electromagnetic spectrum. For instance, honeybees can see light in the ultraviolet range (Wakakuwa, Stavenga, & Arikawa, 2007), and some snakes can detect infrared radiation in addition to more traditional visual light cues (Chen, Deng, Brauth, Ding, & Tang, 2012; Hartline, Kass, & Loop, 1978).

Light that is visible to humans makes up only a small portion of the electromagnetic spectrum.

In humans, light wavelength is associated with perception of color (figure above). Within the visible spectrum, our experience of red is associated with longer wavelengths, greens are intermediate, and blues and violets are shorter in wavelength. (An easy way to remember this is the mnemonic ROYGBIV: red, orange, yellow, green, blue, indigo, violet.) The amplitude of light waves is associated with our experience of brightness or intensity of color, with larger amplitudes appearing brighter. Animals that are able to see visible light have different ranges of color perception. Humans have three different types of color receptors (cones) resulting in a trichromatic organization of color, whereas most birds have four different types of cones resulting in a tetrachromatic experience including gray, blue, green and red. Dogs commonly thought to see in black and white actually do see in color, however their perception is limited to a more narrow arrangement of colors including black, yellow, gray and blue. Humans and animals perceive color by way of an opponent processing model of color vision where a small amount of primary color receptors mix their signals to create the perceptions of a variety of other colors (Herring, 1924). Behavioral methods have been designed which are used to better understand how many different colors animals are able to differentiate between (how many different colors are perceived) compared to how many different types of receptors they have (see Gregg, Jamison, Wilkie & Radinsky, 1924, for example of color differentiation between dogs, cats and raccoons). Where as human vision appears to operate on an opponent process model, some animals with more diverse varieties of color receptors have been show to operate on different methods of color perception. Ironically the mantis shrimp, the animal that could have the broadest, most detailed perception of color with 12 different color receptors, may not see in such the vivid arrangement that was previously thought. Recent research has demonstrated that although the mantis shrimp has 12 different types of color receptors (thus far the most known in the animal kingdom), the mantis shrimp’s visual system appears to be operating on a completely different, previously unknown color vision processing model which is based on temporal signaling combined with scanning eye movements, enabling a type of color recognition as opposed to color discrimination as in other animals and humans (Thoen, How, Chiou & Marshall, 2014).

Different wavelengths of light are associated with our perception of different colors. (credit: modification of work by Johannes Ahlmann)

FEATURES OF LIGHT

There are three main features of light waves which allows us to objectively define differences between what we experience as colors. The first factor, hue is what we are usually talking about when we refer to color (a red shirt has a red hue). The hue is basically the specific name for the specific wavelength that is reflected by the object. Violet has the shortest visible wavelength in the visible spectrum (~ 400 nm), and red has the longest (700 nm). Brightness refers to the intensity of the color and depends on the amplitude or the distance between the midpoint and the peak of the wave. The higher the amplitude of the waveform, the more intense and bright the color. Finally, saturation referred to color purity which is determined by uniformity of the wavelength. Higher saturations are recorded when many wavelengths have the same size and shape. Most colors we experience are not pure meaning there are many wavelengths entering the eye of which are different shape and sized waveforms. Due to differences between color hue, amplitude of the wave and saturation, the average human is able to perceive some 2.3 million different colors (Linhares, Pinto & Nascimento, 2008).

THE RETINA

After light passes through the cornea, pupil and lens, light waves travel through the jelly like vitreous fluid in the eye and land on the retina, a dense collection of neurons covering the back wall of the eye. The retina is where millions of specialized neurons called photoreceptors which absorb light waves and turn this information into chemical and electrical signals which are processed in the primary visual cortex of the occipital lobe, and the lateral geniculate nucleus of the thalamus. Rods and cones represent the two types of photo receptors that exist in the retina which get their names from their characteristic shape. Rods are are extremely sensitive to (fire in response to) single photons (quantum light units, the smallest packet of light, Rieke & Baylor, 1998). Rods create scotopic vision which encodes less intense light and are mainly responsible for humans ability for night vision. Rods are much more common in the human retina compared to cones with about 100 rod cells compared to about seven million cone cells (Williamson & Cummins, 1983). Cone receptors on the other hand allow us to experience the vivid diversity of different wavelength reflections from objects which create our perception of colors. It is important to note that color is not an innate property of object in the world and is created by they way our receptors respond to the way light is reflected off objects. Because one organism perceives an object as being blue and another experiences the same object as being gray does not mean one organisms perception is wrong or incorrect, it just means that they have receptors that are tuned to send different signals to color processing areas of their brains when experiencing the reflection of light off that object. Color is an interpretation that is created by mixing activation of the specific receptors we have and the signals those receptors send to higher processing areas of the brain. In addition to allowing us to see color, cones also process fine details and allow for visual acuity.

The human retina is a fascinating structure because light is actually processed seemingly in reverse, beginning with the pigment epithelium which is organized into receptive fields on the outside layer of the retina, and continuing toward the front of the eye through the rods and cones. The rods and cones transmit information to bipolar cells which transmit signals to to ganglion cells located at the from of the retina that bundle together and relay information to deeper structures of the brain by way of the optic nerve. The area where the ganglion cells bundle together to form the optic nerve exit the retina at the optic disc, which creates a natural blind spot in each eye. However the blind spot created by the exiting of the optic nerve is not perceived due to compensation of information from receptions surrounding the blindspot as well as information compensated from the other eye that is able to perceive information in the other eyes blind spot due to the light hitting the compensating eye in a different location on the retina. This will be additionally reviewed in the following section on vision.

SOUND WAVES

   Like light waves, the physical properties of sound waves are associated with various aspects of our perception of sound. Sounds waves are created by vibrations and can be thought of as ripples in the gasses that are constantly surrounding us. This is why sounds does not exist in space or complete vacuums. Without air or the presence of a gas to transmit the signal, sounds cannot exist. The frequency of a sound wave is associated with our perception of that sound’s pitch. High-frequency sound waves are perceived as high-pitched sounds, while low-frequency sound waves are perceived as low-pitched sounds. The audible range of sound frequencies is between 20 and 20000 Hz, with greatest sensitivity to those frequencies that fall in the middle of this range.

As was the case with the visible spectrum, other species show differences in their audible ranges. For instance, chickens have a very limited audible range, from 125 to 2000 Hz. Mice have an audible range from 1000 to 91000 Hz, and the beluga whale’s audible range is from 1000 to 123000 Hz. Our pet dogs and cats have audible ranges of about 70–45000 Hz and 45–64000 Hz, respectively (Strain, 2003).

The loudness of a given sound is closely associated with the amplitude of the sound wave. Higher amplitudes are associated with louder sounds. Loudness is measured in terms of decibels (dB), a logarithmic unit of sound intensity. A typical conversation would correlate with 60 dB; a rock concert might check in at 120 dB (figure below). A whisper 5 feet away or rustling leaves are at the low end of our hearing range; sounds like a window air conditioner, a normal conversation, and even heavy traffic or a vacuum cleaner are within a tolerable range. However, there is the potential for hearing damage from about 80 dB to 130 dB: These are sounds of a food processor, power lawnmower, heavy truck (25 feet away), subway train (20 feet away), live rock music, and a jackhammer. The threshold for pain is about 130 dB, a jet plane taking off or a revolver firing at close range (Dunkle, 1982).

This figure illustrates the loudness of common sounds. (credit “planes”: modification of work by Max Pfandl; credit “crowd”: modification of work by Christian Holmér; credit “blender”: modification of work by Jo Brodie; credit “car”: modification of work by NRMA New Cars/Flickr; credit “talking”: modification of work by Joi Ito; credit “leaves”: modification of work by Aurelijus Valeiša)

   Although wave amplitude is generally associated with loudness, there is some interaction between frequency and amplitude in our perception of loudness within the audible range. For example, a 10 Hz sound wave is inaudible no matter the amplitude of the wave. A 1000 Hz sound wave, on the other hand, would vary dramatically in terms of perceived loudness as the amplitude of the wave increased.

   Of course, different musical instruments can play the same musical note at the same level of loudness, yet they still sound quite different. This is known as the timbre of a sound. Timbre refers to a sound’s purity, and it is affected by the complex interplay of frequency, amplitude, and timing of sound waves.

SUMMARY

    Both light and sound can be described in terms of wave forms with physical characteristics like amplitude, wavelength, and timbre. Wavelength and frequency are inversely related so that longer waves have lower frequencies, and shorter waves have higher frequencies. In the visual system, a light wave’s wavelength is generally associated with color, and its amplitude is associated with brightness. In the auditory system, a sound’s frequency is associated with pitch, and its amplitude is associated with loudness.