Can you hear any difference between a 100Hz and 101Hz sine tone? If you heard a 100Hz sine tone and a 100Hz trombone tone, would you perceive them as having the same pitch? How would you evaluate the relative pitch of a soft 100Hz tone, and a loud 100Hz tone? If a somewhat soft tone, and then 10 milliseconds later a somewhat louder tone were sounded, how many tones would you hear? Do right-handed people hear sounds the same as lefties? The answers to all these questions, and many more, comprise the realm of psychoacoustics, the study of human auditory perception.
The riddles of psychoacoustics have long intrigued researchers, but today the science is central to audio technology. Without an understanding of psychoacoustics, perceptual coders such as MP3, Dolby Digital and DTS could not operate, and the surround sound fields of home theaters would sound flat.
Psychoacoustics is the study of sound--in the space between the ears. It starts with the physiology of the ear, and follows the ear's electrical output signal into the brain, where it is subjectively interpreted. Whether you believe in creationism or evolution, the ear/brain system employs some marvelously complex engineering. Yet for all its sophistication, its acuity is quite variable; it may be expert at one acoustical task, and quite naïve at another.
From a purely physiological standpoint, the ear converts acoustical energy into mechanical energy, and then into electrical impulses. The outer ear (pinna) collects sound and its intricate folds help us determine directionality. The ear canal resonates at about 3KHz, boosting sensitivity in the critical speech frequency region. The first "bio-transducer" sound hits is our ear-drum, also called the tympanic membrane, which conveys the incoming sound to three bones in the middle ear, known as the hammer, anvil, and stirrup, which in turn efficiently convey sounds in air to the fluid-filled inner ear where the basilar membrane detects the amplitude and frequency of sound.
This detection occurs in the cochlea, a spiral-shaped, fluid-filled region of the inner ear that contains hair cells called cilia, which resonate at different frequencies depending on their location. Outer hair cells resonate at lower frequencies, and inner hair cells resonate at higher frequencies. These cilia convert sound vibrations into electrical impulses and send the information to the brain as neural data. While the ear's biological design is fairly straight forward, the operation of the ear/brain certainly is not. Consider one essential psychoacoustical phenomenon, localization, which is relied upon every time you fire up the surround soundtracks on a DVD-Video disc. We can perceive where sounds come from - above, below, behind, to the side--this is called spatial localization. In particular, our stereophonic ears can discern azimuth or horizontal (left-right) directionality, and zenith or vertical (up-down) directionality. We perceive directionality using localization cues such as Interaural Time Difference (ITD), Interaural Intensity Difference (IID), and pinna filtering.
The Interaural Time Difference (ITD) is the difference in arrival time of a sound at each of our ears. Because our ears are separated by about six inches, sounds coming from the left or right will arrive at the corresponding ear first. Although the delay time differences are slight, our brain halves can extract precise directional information from this data.
The head, as well as shoulders and upper torso, form a barrier to a sound's arrival at one ear or the other. This creates an acoustical shadow and an effect called the Interaural Intensity Difference (IID). For example, a sound coming from the extreme left has a lowered intensity in the right ear (in addition to an ITD time delay). The lowered intensity is due in part to the added distance (sound amplitude decreases over distance) to the right ear, but also from the acoustical shadow cast by the body itself. So each ear receives slightly different amplitudes from sounds that are not directly equidistant to the two ears.
The effect of the shadow is frequency dependent; high frequency sounds are more attenuated than low frequency sounds because low frequencies (those with wavelengths larger than the width of the head) can bend around obstructions and are not as easily blocked. For this reason, for example, the high frequency information in a complex waveform is more readily perceived at the incident ear--this relative difference in timbre is yet another cue used to determine directionality.
The outer ear provides still more information on a sound's directionality. Sound enters the ear canal through direct paths, and indirect paths that reflect from the complex folds of the pinna. When the reflections of the indirect sounds combine in the ear with the direct sounds, pinna filtering occurs, changing the received sound's frequency response. The ear/brain duo interprets this equalization, producing cues (assisting zenith localization, for example) from the filtering effect. To provide still more directional cues, small head movements allow the ear/brain to judge relative differences in the soundfield perspective. With our marvelous acuity, we can hear sounds coming from all around us, whether they are naturally created, or coming from the speakers of a stereo or surround sound system.
In some cases, where true directionality is not available, we can fool the ear. Cues from ITDs, IIDs, and pinna filtering enable us to perceive sound direction. These cues can be combined into one measurement called a Head Related Transfer Function (HRTF). An HRTF defines how a sound's frequency and amplitude responses are altered before entering the ear canal. Using an HRTF, we can take an original sound, process it with an HRTF, and create a sound that will contain the cues of a sound from a particular direction. The ear/brain will interpret the cues, and judge that the sound's direction is from another point in space. For example, in theory, stereo speakers could create a surround soundfield.
HRTFs are created from ITD, IID and pinna measurements associated with a certain azimuth and zenith (and possibly distance). The HRTF acts as a kind of filter that replicates that particular directionality. HRTF parameters can be measured using either real human heads or dummy heads. Tiny microphones are inserted into the ear canals, and a recording is made of a sound source from many different azimuths and elevations. Each particular measurement represents one sound source direction. HRTFs provide insight into localization cues. For example, inspection of HRTFs for sounds coming directly from the right at different elevations would show the effects of pinna filtering. The notches in the filtered frequency response would change in number and frequency position with different elevations as direct and indirect sounds combine in the ear. This particular data would help reveal how we detect elevation.
In addition, HRTF processing can be used to trick the ear. For example, a sound from a stationary speaker at an arbitrary elevation could be processed by this series of HRTFs. As its frequency response gradually changes, the ear/brain would perceive changing elevation from the stationary source. Similarly, complex surround sound fields can be synthesized from stereo speakers, but the ear isn't always entirely fooled. The realism of the created soundfield will suffer if the listener moves from the "sweet spot" between the speakers (headphones solve this problem) because the delicate balance of cues will be upset. Moreover, the folds of each listener's pinna are different; generic HRTFs cannot exactly match our own psychoacoustic expectations. In PC sound cards, using cross-talk cancellation techniques to improve perceived channel separation cleans up the 3D sound image in two channels, but headphones, with their superior real channel separation deliver a better two-channel experience. This type of sound field rendering in two channels is called binaural rendering.
Audio engineering was formerly a science of purely objective design. But increasingly we understand that audio is nothing until it is interpreted by the human auditory system. Thus, the final link in the chain, accounted for by psychoacoustics, must be considered. Whether satisfying its acute demands or exploiting its subtle weaknesses, modern audio systems must use the ear's own performance as the ultimate design criterion. After all, any sonic experience is purely subjective.
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This article first appeared in the June, 2001 issue of Extreme Tech Journal. Visit their website at www.extremetech.com
Music without words means leaving behind the mind. And leaving behind the mind is meditation.
Meditation returns you to the source. And the source of all is sound. — Kabir
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