Dear Jennifer Groh,
when localizing smoke detectors, localization of the horizontal direction (left/right) of the sound source based on ITD/ILD should work fine. However, localization in vertical or, more precisely, sagittal planes relies on spectral cues. So, if a smoke detector emits chirps with relatively long cycle duration, a listener perceives rather fluctuating source elevation, known as directional bands, than a stable elevation. Latter would require a broadband spectral profile at a certain time instance.
Moreover, localization performance in sagittal planes deteriorates with increasing sensation level, known as negative level effect. Thus, if the smoke detecter is very loud this does not help for localizing it.
Hope this helps you. If you need detailed references for what I've stated above, don't hesitate to contact me...
All the best,
Robert BaumgartnerPsychoacoustics and Experimental AudiologyA
coustics Research Institute
Austrian Academy of SciencesWohllebengasse 12-14, 1040, Wien
I am writing a book for a general audience on how the brain processes spatial information ("Making Space"). The chapter on hearing covers many topics in sound localization, but there is one that I'm currently still quite puzzled about: why it is so hard to localize a smoke detector when its battery starts to fail? Here is what I have considered so far:
- To my ear, the chirp sounds high frequency enough that ILD cues should be reasonably large.
- At the same time, it seems to have a broad enough bandwidth, and in any case it has onset-and-offset cues, that ITD cues should be usable.
- A possibility is that the chirp is too brief, and that limits dynamic feedback, i.e. changes in ITD and ILD as the head turns during a sound. However, in my laboratory we have obtained excellent sound localization performance in head-restrained monkeys and human subjects localizing sounds that are briefer than the reaction time to make an orienting movement.
- An additional possibility is that we have too little experience with such sounds to have assembled a mental template of the spectrum at the source, so that spectral cues are of less use than is normally the case.
I'm leaning towards a combination of the last two factors, which together would render the cone of confusion unresolved for these stimuli.
Jennifer M. Groh, Ph.D.
Department of Psychology and Neuroscience
Department of Neurobiology
Center for Cognitive Neuroscience
B203 LSRC, Box 90999
Durham, NC 27708