Emily Stover
February 2002
Stephen F. Austin State University
Auditory localization is the ability to recognize the location from which a sound is emanating (Goldstine, 2002). There are many practical reasons for studying auditory localization. For example, previous research states that visual cues are necessary in locating a particular sound (Culling, 2000). However, blind people do not have the luxury of sight to help them locate a sound. Therefore, the ability to locate sound based only on auditory ability is important. It is also important to study different auditory processes. For example, when studying a way for a blind person to maneuver through an environment, it is helpful to know that people can most accurately locate sounds that happen directly in front of them; sounds that are far off, to the side, or behind the head are the least likely to be properly located (Goldstein, 2002).
Three coordinate systems are utilized when attempting to locate a specific sound. The azimuth coordinate determines if a sound is located to the left or the right of a listener. The elevation coordinate differentiates between sounds that are up or down relative to the listener. Finally, the distance coordinate determines how far away a sound is from the receiver (Goldstine, 2002). Different aspects of the coordinate systems are also essential to sound localization. For example, when identifying the azimuth in a sound, three acoustic cues are used: spectral cues, interaural time differences (ITD), and interaural level differences (ILD) (Lorenzi, Gatehouse, & Lever, 1999). When dealing with sound localizaton, spectral cues are teh distribution of frequencies reaching teh ear. Brungart and Durlach (1999) (as seen in Shinn-Cunning, Santarelli, & Kopco, 1999) believed that as the frequency of the source on the azimuth increases, so does the individual's ability to detect accurately the source of the sound. Additionally, the auditory system utilizes localizing cues to determine from where a sound is originating (Goldstine, 2002). These consist of monaural cues from incoming sounds and binaural cues based on sound differences in both ears (Carlile, Hyams, & Delaney, 2001).
The cues mentioned above, as well as others, originate in the auditory cortex in the brain. When the auditory cortex becomes damaged, people may lose their ability to hear and/or they may lose the ability to locate sounds. In another line of research which utilizes animals, Rauschecker and Korte (1993) (as seen in Goldstien, 2002), found the specific area of the brain utilized for sound localization. This area is called the anterior ectosylvian sulcus (AES). After suturing kittens eyes shut, the kittens developed smaller visual areas in the AES. When a person is deprived of vision the sound area of the AES becomes larger. This is able to be applied to humans who are blind and rely heavily on auditory cues.
Applied research on the topic of auditory localization has been extensively explored throughout history. One of the first to exam this topic was Lord Rayleigh (1907) (as seen in Brungart,1999), who examined the ITD cues relationship to sound localization in hard-of-hearing individuals. through teh creation of the duplex theory of localization, he belicved that depending on the sources of the frequency spectrum, different localization cues will be dominant. Other research with hard of hearing individuals has been done by Viehweg and Campbell (1960) (as seen in Lorenzi, et al.). In this research Viehweg and Campbell concluded when in a situation with large amounts of background noise a hard-of-hearing individual will make maore errors when trying to localize a sound. Therefore, many hard-of-hearing individuals wear hearing aids to block out some background noise.
Research has been used to create hearing aids and environments that better suit the abilities of hearing impaired individuals. Because sound localization that takes place in front of a person relies primarily on ITD cues when occurring at low frequencies and ILD cues at high frequencies (Kistler and Wightman, 1992) (as seen in Lorenzi, et al.,1999), this information can be used to devise a way for hard-of-hearing individuals to have more ITD cues present since ILD are harder to hear and utilize (Lorenzi,et al., 1999). Also the ability of a blind person to locate a sound and better understand their immediate surroundings it is imperative to have multiple binaural clues present. For example, in a recent study, accuracy of sound location at a close proximity was greatly increased when binaural cues were present (Brungart, 1999).
The ability to accurately locate a sound that is heard from various distances has also been extensively studied. Sounds which occur over one meter from the head, are equal for any source falling on a cone centered on the internal axis (i.e., the "cone of confusion") (Shinn-Cunningham, Santarelli, & Kopco, 1999). The cone of confusion is an area in which an individual has a difficult time locating the exact location of a sound. For example, if a person were to hold a megaphone to their ear the cicumfrance at the end of the megaphone is the cone of confusion, and anything in that area is too close together to distinguish a specific sound (Shinn-Cunningham, et al., 1999). The primary way to eliminate this error of the cone of confusion is to evaluate the spectral content of the signals reaching the eardrum (Kulkarni, & Colburn, 1998) (as seen in Shinn-Cunningham, et al., 1998).
Auditory localization has previously been examined in terms of the relationship between visual perception and auditory cues. Carello, et al.(1998) found that when an individual hears leaves rustling or water dripping, the people not only can tell what they are hearing but they also create a visual picture of what they are hearing. This suggests that there is a link between the two perceptual senses. However, Carello, Anderson, and Kunkler-Peck (1998) also found that individuals were able to scale an object accurately with no previous knowledge of the object by simply hearing the object falling to the floor. In support of these findings McDonald, Teder-Salejarvi, and Hillyard (2000) found that a sound does have an effect on the processing of subsequent and concurrent visual stimuli.
A great deal of current research is aiding the development of better technology in sound location. As research gains more ground in determining how different factors influence our ability to locate sound, these factors are taken into account when designing new equipment in a variety of areas.One of these areas is emergency sirens on ambulances.
Emergency vehicle sirens have been extensively studied. The purpose for ambulance sirens is to warn drivers of their presence and give some direction to those drivers. However, ambulance sirens are perceived by the driver to be further away than they actually are when the ambulance moves toward the back of the driver (Caelli & Porter, 1980). According to this same study, siren localization errors are primarily due to reversals and also to the cycle of the siren. The traditional "hee-haw" siren of 30 cycles/min repetition rates resulted in an average localization error of 20 degrees (Caelli & Porter, 1980). This area of applied research demonstrates a need for the perceptual part of sound localization and the technological part of siren making to inform each other in order to better help drivers locate the approaching ambulance.
Currently, ambulance siren research is broadening to police sirens and other emergency alert signals that will allow a listener to determine where a sound is coming from and to not panic. With police sirens, drivers have been shown to delay emergency personnel by panicking because they can not determine from where the siren is originating. New sirens are being developed that have breaks in the siren cycle and are easier to locate.
An area where vision and sound localization are being combined is in the design of cockpits. Different frequencies and wavelengths are being utilized in the new designs. Also, colors and flashing lights are being associated with the different sounds in the hope that pilots will make fewer errors when flying therefore cutting down on the number of plane crashes. Overall, sound localization is often overlooked by individuals; however, it is an important area to aid in the development of ergonomics.
References
Brungart, Douglas. (1999). Auditory localization of nearby sources. III. Stimulus effects. Journal of the Acoustical Society of America, 106 (6), 3589-3602.
Brungart, D. & Durlach, N. (1999). Auditory localization of nearby sources II: Localization of a broadband source in the near field. Journal of the Acoustical Society of America , 106 (4), 1956-1968.
Caelli, T., & Porter, D. (1980). On difficulties in localizing ambulance sirens. Human Factors, 22 (6), 719-724.
Carello, C., Anderson, K., & Kunkler-Peck, A. (1998). Perception of object length by sound. Psychological Science, 9 (3), 211-214.
Culling, John. (2000). Auditory motion segregation: A limited analogy with vision. Journal of Experimental Psychology: Human Perception and Performance, 26 (6), 760-1769.
Goldstein, E. (2002). Sensation and perception (Rev. ed.). Pacific Grove, CA: Wadsworth-Thomsom Learning.
Lorenzi, C., Gatehouse, S., & Lever, C. (1999). Sound localization in noise in hearing impaired listeners. Journal of the Acoustical Society of America, 105 (6), 3454-3463.
Lorenzi, C., Gatehouse, S., & Lever, C. (1999). Sound localization in noise in normal hearing listeners. Journal of the Acoustical Society of America, 105 (3), 1810-1820.
McDonald, J., Teder-Salejarvi, W, & Hillyard, S. (2000). Involuntary orienting to sound improves visual perception. Nature, 407, 906-907.
Shinn-Cunningham, B., Santarelli, S., & Kopco, N. (1999). Tori of Confusion: Binaural localization cues for sources within reach of the listener. Journal of the Acoustical Society of America, 107 (3), 1627-1636.