Sensation and Perception Part 3 - Hearing Sound comes from pressure waves in a medium (e.g., solid, liquid, gas). Although we usually hear sounds in air, as long as the medium is there to transmit the pressure wave, there is the possibility for sound. Sound waves are relatively very slow compared to light. The more densely packed the medium, the faster the sound. Therefore, sound travels faster in water (1360 m per second or 3000 mph) then in air (340 m per second or 750 mph). Sound waves have three physical properties that affect our psychological perception: Amplitude corresponds to the psychological dimension of loudness or intensity Wavelength corresponds to pitch Timbre refers to sound quality, corresponding to saturation in color perception. Loudness is measured with the decibel scale, where zero point is the absolute threshold for audition. Pitch is usually measured in frequency in terms of cycle per second using a unit called a hertz (Hz). One hertz is equal to one cycle per second. Normal human hearing is in the range of 20 to 20,000 Hz. Some animals can hear sounds of much higher frequency. High- pitched sounds have short wavelength and high- frequency while low- pitched sounds have long wavelengths and lowfrequencies. 1
Timbre The timbre of the sound is evident when musical instruments, playing the same note (the fundamental frequency), sound different because each is producing different harmonics. Noise Noise, on the other hand, consists of a complex mix of unrelated sounds waves. The ear is composed of three parts: the outer ear, middle ear, and inner ear. In the outer ear, the pinna collects soundwaves and directs them down the auditory canal to the eardrum, or tympanum. The vibrations of the tympanum faithfully reproduce the frequency of the soundwaves in the air. The vibrations from the eardrum are transmitted to the bones of the middle ear: the malleus, incus, and stapes. The stapes usually rests on the oval window of the cochlea, a coiled and fluid- filled structure in the inner ear. Inside the cochlea is the basilar membrane, and it is covered with hair cells. The movement of the stapes against the oval window sets up a wave action within the cochlea The wave in the cochlea, again, matches the frequency and amplitude of the soundwave in the air. This wave energy causes the hair cells of the basilar membrane to vibrate; this vibration is transduced into auditory information. 2
From the Ear to the Brain The acoustic nerve or auditory nerve is formed from the axons of the sensory neurons coming from the inner ear. Information from the ear is first routed to the medulla oblongata, midbrain, and then the thalamus. From the Ear to the Brain From there sounds are mapped to the cortex in a fashion similar to the mapping in the visual cortex. Contralateral representations are the strongest, but ipsilateral (same side) connections are also made. How We Hear We are most sensitive to the middle range of the audible frequencies, which roughly corresponds to the range of the human voice. We are especially sensitive to changes of sounds, such as changes in pitch. Theories There are currently two theories proposed to explain the mechanism by which we hear: Place theory Frequency theory Place Theory Place theory, first proposed by Helmholtz, hypothesizes that the hair cells at certain locations along the basilar membrane are maximally sensitive to specific frequencies. Ear- damage data supports such a conclusion because overexposure to specific loud frequencies causes hearing loss for that frequency. Place Theory Von Bekesy s data supports the notion that the frontal regions of the basilar membranes are largely responsible for tones of high frequency. 3
Place Theory Unfortunately, Von Bekesy s data indicates that place theories cannot account for the full experience of audition by itself, in that medium and low frequencies were not found to be localized in one particular region of the basilar membrane; they were found along the length of the membrane. Frequency theory states that specific frequencies trigger neural activity at the same frequency. Frequency theory accounts well for frequencies in the lower range but fails to account for the higher frequencies because neurons cannot fire faster than 1000 times per second, 20 times less than the highest frequencies perceived. However, the volley principle attempts to deal with those higher frequencies by assigning those frequencies to groups of neurons rather than just one neuron. The groups, taken together, can match incoming auditory frequencies. Again, given that place theory and frequency theory cannot alone describe auditions, the theoretical picture is not yet clear. Duplicity theory attempts to reconcile the two positions by using the best parts of both theories, but the specific details of this dialectic remain unspecified at this point in time. 4
Locating Sounds How do we tell where sounds originate? Two methods have been proposed: the timing- difference method and the intensitydifference method. Time-Distance Method In the time- distance method, sounds are localized by an analysis of the time- lag between the arrival of a sound at our right ear and it s arrival at the left. The time- different method works best for low- frequency sounds. Intensity-Difference Method In the intensity- difference method, sounds are localized by an analysis of the difference in the intensity of sounds heard in the left and right ears. This method works best for highfrequency sounds. Sounds that hit both ears simultaneously often cause confusion. One way to overcome such confusion is to cock one s head so as to change the orientation of one s ears in relation to the source of the sound. In doing this, one improves his or her ability to localize the sound. 5