Xref: utzoo comp.dsp:325 sci.psychology:2389 Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!tut.cis.ohio-state.edu!ucbvax!pasteur!helios.ee.lbl.gov!nosc!cod!martin From: martin@cod.NOSC.MIL (Douglas W. Martin) Newsgroups: comp.dsp,sci.psychology Subject: re: Psychoacoustics Keywords: binaural hearing, auditory spatial perception Message-ID: <1698@cod.NOSC.MIL> Date: 15 Nov 89 17:42:24 GMT Organization: Naval Ocean Systems Center, San Diego Lines: 92 In the past few weeks, there have been several articles about human perception of sound, and how such psychoacoustic cues are interpreted spatially. The two areas I wish to address are those of binaural recording, and the obstacle detection sense used by many blind people for navigation. I myself am totally blind, use this obstacle sense extensively, and have a MS in acoustics from Penn State. The ability to detect obstacles, find doorways, estimate the size of rooms, etc., was first discussed in the literature by D. Diderot in 1749. He thought that a blind person could judge the proximity of bodies by "the action of air on his face." The sensation of approaching an obstacle is somewhat like light pressure on the face. Thus, this sense has been misnamed "facial vision" in much of the early literature. The obstacle sense is more accurately referred to as echolocation; it is an auditory perception. Confirmation that this is an auditory sense and not some kind of "facial vision" was first obtained by researchers at Cornell University in the late 1940's. Obstacles can be detected either passively (using reflections of ambient sound in the room) or actively (using a self-generated noise such as a click or whistle). Learning to use this echolocation is sudden and insightful rather than gradual; a person merely needs to learn what to listen for. Of course, the use of this perception is not limited to blind people. Anyone can easily demonstrate this perception. Simply close your eyes, and walk with hard shoes on a hard floor toward a wall. You should sense the presence of the wall before actually contacting it. However, if walking barefoot across a carpeted floor, will usually result in impact with the wall, because there is much less reflected sound to work with. Many of the parameters of this echo detection capability were quantified by Charles Rice and his colleagues at Stanford in the mid and late 1960's. The ability to detect an object depends on its size, distance, and reflectivity. Rice found that blind people could detect obstacles spanning an angle of about four degrees. Area ratios between disks as small as 1.06 to one, could be discriminated. Some subjects could also reliably discriminate circles, squares, and triangles using their echolocation. Large obstacles can be detected at distances exceeding ten to fifteen metres. Distance cues appear to be related to both pitch and loudness, and directional cues result from the same auditory localization phenomena described by earlier articles in this group, mainly interaural time and amplitude differences. It was mentioned in an earlier article that binaural recordings can be made by separating two microphones by a distance equal to the diameter of the head. Actually, this is not sufficient to make a binaural recording; it will only make a stereo recording. In order to obtain the binaural effect of localization, an obstacle (like a head) must be present between the microphones. This is necessary to create the auditory shadow which is critical for high-frequency localization. When listening to a stereo recording through headphones, the sound image is "lateralized" as opposed to the image being "localized" using headphones with a true binaural recording. In lateralization, with stereo headphones, the sound image appears to be coming from somewhere inside the head, often closer to either left or right, but still within the head. When listening with headphones to a true binaural recording, the sound image is "out there in space" with a perceived distance and direction. Again, to make a binaural recording, it is necessary to have a head-sized obstacle between the microphones. The actual shape of the obstacle, the presence of hair or facial features, and other similar factors are not very critical. However, if sounds are to be localized in elevation as well as in azimuth, there must be a reflecting surface below the head, e.g. a torso. It has been mentioned that the ear has a different frequency response for sounds arriving from different angles. In fact, the structure of the pinna (outer ear) is such that an impinging sound wave undergoes multiple reflections in the pinna before reaching the eardrum. The amplitudes and relative time delays associated with these multiple reflections are, of course, angle dependent. An excellent paper on this topic was published by Wayne Batteau, 1965, in Proceedings of the Royal Society, London. I have hundreds of references in all these areas: blind echo location, binaural hearing and recording with dummy heads, and sound transformations in the outer ear. If there is interest, I will compile a bibliography as I have time, and will send it to anyone who wants it. Doug Martin martin@nosc.mil Naval Ocean Systems Center, San Diego, ca 92152. phone: (619) 553-3659.