The stereo image: how does the ear determine directionality of sound?
Stereophonic sound is the concept of similar, yet not identical, sounds from the same source and aimed at certain receptacles, in this case our ears. Using two ears rather than just one, we can locate the source of sounds without the benefit of seeing those sources, e.g. traffic noises when crossing the road, or when we can't see objects approaching us from behind. Science and technology has made great use of the fact that two ears are necessary for hearing by incorporating it into our daily lives. Stereophonic sound reproduction can be heard on the radio and in the movies, on television and on the records we buy or listen to every day.
Stereophonic sound reproduction originated in the 1930s as a result of the experiments in "two-eared hearing" conducted by Dr. Harvey Fletcher of Harvard University. He set up a normal shop window dummy but replaced its ears with microphones - one each side of its head. He then attached both microphones to a pair of earphones, the left mike to the left side of the headset and so forth. Dr. Fletcher then walked over to the dummy and began talking to it. As he did this, he could hear footsteps approaching and his own voice speaking to him less than six inches away from his ears. He then walked around the dummy whilst speaking and noticed that the sound he heard moved from one side of the headset to the other. Dr. Fletcher deduced from these results that one ear hears differently to the other so both must work as a pair for normal hearing. Before understanding binaural hearing or how the ears work together to understand the world around them, we must first understand how the ear works.
The action of hearing is the responsibility of four main parts of the body - the outer, middle and inner ears, and the brain. All the sounds that we hear are collected together as sound waves by the cupped shaped auricle of the outer ear (the only part of the ear which is actually visible to others. The vibrations are then transferred down the ear canal (or meatus) where they are amplified and strike the eardrum at ten times the air pressure outside the ear. The eardrum is a thin membrane, sensitive to any kind of vibrations, and in the case of high frequencies, movement as little as one-tenth the diameter of a hydrogen molecule. By expanding and contracting with these sound vibrations, the eardrum sets in motion three tiny bones which make up the inner ear, known collectively as the ossicles. These three bones are the smallest in the human body, and consist of the hammer, the anvil, and the stirrup (because of their shapes). The hammer vibrates with the eardrum, which then pushes the anvil, which in turn strikes the stirrup, and by the time these sound waves have travelled through the middle ear they will have been doubled or tripled in intensity. The vibrations travel from the stirrup into the oval window, and by now have changed from air pressure vibrations into mechanical energy. Certain microscopic hair fibres within the liquid contained in the curled cochlea are stimulated by certain frequencies, and like nerve-endings send messages to the brain determining the pitch at which we hear that particular sound.
We must remember that this is what goes on in one ear, and since both ears work simultaneously, the pitches heard in both ears may not be the same, as the experiments with the dummy prove. Many philosophers believed that only one ear was essential for correct hearing, but it has since been proved that two ears are essential for binaural hearing, and therefore, our understanding of the world around us.
The most basic of experiments into binaural hearing was conducted last century by Ernst Weber who found two watches, each with a distinctive ticking sound. A listener whose eyes were closed was asked to determine which watch was at which of his ears. Naturally he identified them correctly. In 1901 this experiment was extended by Arthur H. Pierce, who attached a telephone to a dome above a blindfolded subject's head. The telephone was rotated about the subject's head, and again was able to pinpoint the location of the sound despite its movement and the subject's lack of vision. Pierce assumed that the distance of the sound source from each ear decided where the brain pinpointed the sound, since the subject was confused when the telephone was directly behind or in front of him, i.e. exactly halfway between his two ears.
A link between stereophonic reproduction of sound and binaural hearing is time difference. In 1920, two German scientists, E.M. von Hornbostel and Max Wertheimer used an electronic device that played simultaneously two sounds of exactly the same pitch. A subject who sat in front of the machine identified the sound source as such. The scientists then separated the sounds, still at the same pitch, by thirty-millionths of a second, and the subject claimed that the source had moved slightly to the side. The larger the time gap, the further the subject claimed the source moved to the side. This idea of two separate signals from the same source is to be seen in modern stereophonic sound reproduction, and can be studied by looking at the grooves in a modern vinyl record.
Record grooves as viewed through a microscope
The sides of the groove on a stereophonic record were unequal in distance between them therefore the needle that ran along this groove sent out two separate messages, simultaneously- one to the left-hand speaker, the other to the right-hand speaker. The difference between the stereophonic groove and the monaural groove was that the sides of a monaural groove were parallel, sending the same message to both speakers.
This is how modern science aids the process of binaural hearing, but how do we understand the way in which it works? Perhaps looking at the natural world will help. Many animals, including dolphins and porpoise in water, and bats in the air, use a system called echolocation to determine directionality of sound. The eyesight of bats is very weak and so they must find their way about by other means, especially at night when hunting for food. The bat sends out a series of high-pitched, ultrasonic sounds and then listens for their echoes off objects in its path. The bat's ear is so sensitive that it can respond to echoes a hundred billion times weaker than the originally omitted sound. We humans learn from the animals, not only in the way science has based its complex radar system on the way dolphins use echolocation in water, but in the way blind people develop their sense of hearing to compensate for sight loss. Similarly to the bat's style, blind people can get about by listening for echoes off objects or furniture in a room. By doing this they create a mental 3-dimensional picture of the room in their heads, and in extreme cases can even correctly assess the dimensions of the room.
This all goes to show how the ear copes with locating not only objects, but sound sources and how this can be used to our advantage, particularly as an extra defensive, early-warning device when other senses are not enough. This is just as true today for the hectic, often dangerous lives we lead today as it was for the hunted man of the prehistoric ages.
- Introduction to Acoustics R.D. Ford (Elsevier, 1970)
- Sound and Hearing S.S.Stevens and Fred Warshofsky (Time Life Education, 1981)
- Into the Groove from the Austin Chronicle (Andy Langer)