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How we hear

To achieve a greater appreciation of the way the work is necessary directional sound basic understanding of how human hearing works and how sound processing allows the listener to locate the source of a sound with astonishing precision.

The human ear is essentially an extremely sensitive to several small changes in sound waves traveling through the air mechanical system. Sound waves are pressure changes in the air coming from the vibration of an object, such as a musical instrument, operating machinery or vocal cords people.

Our ears have three main parts: the outer ear, the middle ear and the inner ear. The outer ear is tapered and curved shapes that catch the sound and take him by the ear canal to the tympanic membrane, or eardrum. The eardrum is a thin, rigid layer of skin that separates the outer ear from the middle ear. When the sound reaches the eardrum vibrates, moving quickly to the higher-frequency sound waves while moves farther to the loudest sounds.

The middle ear is an air-filled cavity that houses three small bones, small bones, whose primary function is to amplify about 20 times the pressure received from the eardrum. The first of these bones, the hammer, is connected to the center of the tympanum and transfers vibrations from the eardrum to the other two bones. The last of the bones, the stirrup is connected to the cochlea, a channel in the fluid-filled inner ear. They vibrate the ossicles while the eardrum, allowing the bracket to act as a piston creating liquid waves in the cochlea that represent sound waves captured by the eardrum.

The cochlea, a structure shaped like a snail with three separate tubes filled with fluid membrane, converts or translates these physical vibrations into nerve impulses that the brain recognizes as sound. Liquid waves traveling along the basilar membrane of the cochlea stimulate the thousands of tiny hair cells in the organ of Corti, which lies on the surface of the basilar membrane and extends along the cochlea. When a liquid wave stimulates a particular resonant frequency, the membrane releases energy that moves the hair cells at that point. This, in turn, sends an electrical impulse through the cochlear nerve to the brain. Thus, the cochlea sends the raw data that the brain has to process, analyze and interpret. This is incredibly fast neural processing and explains our ability to detect the source of a sound.

Signaling the location of sound

Different basic factors explain our ability to locate a particular sound with surprising accuracy, especially if it is a wideband audio. 1, 2, 3, 4, 5

The ear anatomy and the fact that we have an ear to each side of the head allow us to capture subtle differences in the sound that we provide signals to locate a sound source. These signals are interaureales time differences (ITD), the interaureales intensity differences (IID) and the transfer function associated with the head (HRTF).

The concept of the ITD can be explained as sound waves traveling toward the ears. The crest of each sound wave will reach the nearest ear to ear before reaching the farthest side of the head (see Figure 1). So attentive to a sound subject tend to locate the source of the sound to the side of the first crest of the wave reaches the ear.

In the case of the IID, a difference in intensity occurs in both ears of a listener to pure tones still sound to one side, as one ear is overshadowed by the head (see Figure 2). As a result, both ears perceive a significant difference in the sound. To very low frequencies, the shadow of the head does not suffer impacts, and therefore there is no perceptible difference in sound. With higher than 5000 Hz frequencies, however, the difference in the perceived loudness between the two ears reaches 30 dB. With complex sounds such as speech, music and sound broadband exists not only a difference in loudness and intensity but also there is a change in the spectrum of the sound, since the high frequency components are lost in the ear the far side of the head.

Withington 1 and other sources 4, 5 establish that the differences in intensity and time can cause errors in the location for the listener when narrowband sounds or frequencies are used only. However, a confusion of this kind is almost never presented with fountains and sounds broad band with a sufficient duration to allow listeners move the cabeza.5

The outer ear is the key to the transfer function related to the head (HRTF). Ear shape attenuates some frequencies and amplifies other, filtering the sound field and is represented in Figure 3. The HRTF changes depending on the location of the sound source, providing additional location signal which is particularly important to determine if the source is in front or behind us. The HRTF operates over a range of frequencies, but it seems to be more effective from 5,000 Hz to 10,000 Hz. In combination with the movement of the listener's head, the HRTF provides a method of independent location that complements and enhances the capabilities of the ITD and the IID.

While these three signals provide complementary and redundant means to localize sound, fourth psychoacoustic phenomenon ensures that sound waves from too many highly reverberant spaces do not cause confusion. This is attributed to "the effect of priority." The ear can discern and look at the first sound receiving and not concentrate on subsequent sound or reflected sound. The signal arrives first suppresses the ability of the ear to hear other signals, including reverb, which receives up to about 40 milliseconds after the initial signal. A broadband directional pulse signal makes good use of the effect of priority and can compensate for listening conditions that were not optimal. Even in highly reverberant spaces, where every surface was reflective sound, subjects evaluated have not presented problems to determine the location of directional sounder.

Endnotes

1. Withington, Deborah J., "localisable Alarms," ​​extract from Human Factors in Auditory Warnings (eds.) Stanton, Neville A. and Judy Edworthy, Ashgate Publishing Ltd. 1999.

2. F. L. Wightman and D.J. Kistler, "Sound Localization" in W. A. Yost, A. N. Popper and R. R. Fay (eds), Human Psychophysics, New York: Springer-Verlag (1993).

3. Blauert, J., Spatial Hearing: The Psychophysics of Human Sound Localization, Cambridge, MA: MIT Press. (1997)

4. Stevens, SS, Davis, H., Acoustical Society of America, "Hearing, Its Psychology and Physiology," American Institute of Physics, New York, NY (1983), 169-172.

5. Small, Jr., AM, Wales, RS, "Chapter 17, Hearing Characteristics," in CM Harris (ed.), Handbook of Acoustical Measurements and Noise Control, 3rd Edition, Acoustical Society of America, Melville, NY, (1998), 17.2.

By Daniel J. O'Connor, PE, FSFPE

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