Greetings All, I was intrigued by the post titled 'catching flies' because distance-to information is an area of interest to me. As a few folks out there know, my interest in Ambisonics (aside from music) is its application to hearing research. It is important for safety reasons that a hearing aid (HA) or cochlear implant (CI) user be able to a determine source's distance.
Side note: It's interesting that a mic would be compared to the ear. No one should expect a microphone alone to do what the ear or auditory system does. A quality mic can accurately convert pressure variations to analogous voltage or current variations. That's about it. A laboratory grade mic and audio-analysis hardware or software can readily measure changes in relative phase, intensity, and frequency, and do this over a very wide dynamic range. But converting pressure variations (or particle velocity) to voltages is just the beginning of a chain of events that ultimately results in a listener’s perception of pitch, location, loudness, etc. If the goal is to reproduce a real-world sound field around the listener's head, then we need to add the following to the chain: Loudspeakers, signal processors, room acoustics, etc. Of course the mic is hugely important, and is at the heart of Ambisonics. Now back to distance approximation: I’m not sure how many readers are familiar with the book Ecological Psychoacoustics (edited by John Neuhoff). For those of you who are interested in loudness constancy, loudness of dynamically changing sounds, etc. this book addresses aspects of psychoacoustics that aren’t found in the best books on psychoacoustics (e.g. An Introduction to the Psychology of Hearing by Brian C. J. Moore). One of my mentors and an all-around great guy, William (Bill) Yost, wrote, 'The chapters in Ecological Psychoacoustics suggest many reasons why combining the rigor of psychoacoustics with the relevance of ecological perception could improve significantly the understanding of auditory perception in the world of real sound sources. Ecological Psychoacoustics provides many examples of how understanding and using information about the constraints of real-world sound sources may aid in discovering how the nervous system parses an auditory scene.' Although I don’t ascribe to a single 'school' of psychology, I do buy into James Gibson's idea that man (and animals) and their environments are inseparable (this is at the heart of Ecological Psychology). Here is where I find 'fault' or room for improvement with a lot of controlled laboratory experiments: The person (subject) is isolated from his/her environment, thus limiting the external validity of many experiments. As an example, there are ways of judging a sound source's distance that could be difficult to replicate using convention playback systems in the laboratory. It has been hypothesized that we are sensitive to the curvature (or flatness) of a wavefront, and that this shape provides cues as to distance. But when performing controlled tests of this hypothesis, free-field (anechoic) environments are limited in physical dimensions, so near-field / curved-wavefront conditions are difficult to avoid. Outside of the laboratory, reflections from surfaces are probable cues to distance. In a cafeteria (for example), the signal-to-reverb ratio grows as a talker approaches us, thus giving a viable cue as to the talker's distance. Naturally, intensity increases as well, but intensity alone isn't a great cue without a reference. A distant noise source could be equally loud but at the same time reverberant, thus compelling the listener to believe the noise source is at a distance. How well HA and CI recipients judge distance (and therefore safely avoiding disaster) is one of many questions I'm interested in. Again, I'm building a playback system designed to answer some of these questions. But if Ambisonics involves too much psychoacoustic 'trickery' (as some on the sursound list like to say), then it would not be the best recording/playback method for performing the aforementioned experiments. But to date, re-creating the sound field as it originally existed at the listener' s head via Ambisonics (while letting the ear and brain do the rest) seems to be one of the best research tools at my disposal. (Note: HRTF via headphones isn't a solution because headphones physically interfere with behind-the-ear HAs and CIs). So how good is Ambisonics in reproducing the original auditory 'scene'? If the reconstructed wavefield is close to the original, then what happens when you record the Ambisonics system itself? Will the playback of this recording yield the same spatial information as the first recording did through an appropriate first- or n-order system? Or will the recording of the playback capture the so-called 'trickery,' thus making the recording-of-a-recording useless. Anybody tried this? I think I’ll give it a go using a four speaker arrangement (horizontal only) while playing a live recording of persons talking at eight equally-spaced locations around a Soundfield mic. Upon playback, I’ll place the Soundfield mic in the four-speaker arrangement, record this, and then listen to the recording of the recording. How much localization info do you believe will be lost? Could be fun, plus I’m a firm believer in learning by doing. Thanks for reading, Eric -------------- next part -------------- An HTML attachment was scrubbed... URL: <https://mail.music.vt.edu/mailman/private/sursound/attachments/20120530/081156c8/attachment.html> _______________________________________________ Sursound mailing list Sursound@music.vt.edu https://mail.music.vt.edu/mailman/listinfo/sursound
