Wyatt's research implied that we become increasingly sensitive to particular frequencies as we are exposed to them. Therefore, a musician's tone and pitch will improve with practice. Similarly, when learning a foreign language, speakers must have exposure to the new sounds of the language before they can develop a native accent. Graphs can be made of the frequencies produced by speech of different ethnic groups. There are obvious differences between ethnicities, which explain why some sounds may be undistinguishable for a non-native speaker. Despite the evidence of an ethnic ear, we also know that people can learn multiple languages and corresponding accents. This evidence shows that "hearing curves develop, without any apparent difficulty, as the child learns to speak or as the musician learns to play an instrument or sing" (Verlag, 49).
Generally, humans can hear sounds between 16 Hz and 21 kHz, but the most important auditory frequencies may be between 2 and 4 kHz (where most speech is heard). The frequencies between 400 and 700 Hz are also very important for the ability to distinguish a variety of constanents such as the sibilans (s, z, and sh) and the fricatives (p, b, k, and d) (Steinbach 33). We have the greatest range of hearing is our childhood, and hearing gradually deteriorates as a person ages for a variety of reasons. The upper limit of children has been observed to be near 21 kHz, but this decreases to about 12 kHz by the time a person is 50 years old. At our auditory peak, people can hear a 10-octave range and distinguish between 1500 different pitches (Steinbach, 33). Although this seems like a large auditory range, compared to other animals, it is considerably smaller and lower. For example, the upper limit of frequency perception for chickens is 38, and for dolphins, it is near 200 kHz (Steinbach, 34).
Typically, as we discuss hearing, we talk about the cochlea-more, specifically the basilar membrane and cillia-as the key physical component. However, sound therapy puts a larger focus on developing the muscles of the inner ear to indirectly repair damage to the inner ear.
In his book, When Listening Comes Alive, Paul Madaule relates audition to vision to explain how we develop sensitivity to various frequencies. We have two mechanisms operating simultaneously when we use our vision. The first permits visual perception, allowing us to see. The second allows us to focus in on particular objects. Audition works in a similar way. The inner ear consists of three small bones and two muscles. The role of these muscles is primarily protection. Exposure to loud noises causes an acoustic reflex where the contraction of these muscles reduces the intensity of the incoming sound vibrations and consequent stress on the eardrum (Handel, 467). More recently, however, the stirrup muscle has been associated with a second role: sound discrimination. Madaule describes the action of the stirrup muscle as the "auditory zoom" in the ear (Madaule, 40).
Repeated exposure to various frequencies, such as found in the hearing curve for a particular language, causes the stirrup muscle to strengthen and, in a way, specialize. Just as an athlete trains for a particular sport by strengthening specific muscles, the stirrup muscle is exercised and trained for music or a specific language. Difficulties responding to particular frequencies can be caused by a wide range of problems, from nerve damage to allergies (Fuller, 1994). Even if the sense of hearing can be regained, the auditory selectivity and discrimination skills will be disrupted. The stirrup muscles have to learn to contract quickly to new frequencies in order for hearing to be repaired.