Fw: sursound: The pinna and Ambisonics/cross post (Peter Lennox )

Subject: Fw: sursound: The pinna and Ambisonics/cross post
From:    Peter Lennox  <peter(at)LENNOX01.FREESERVE.CO.UK>
Date:    Mon, 23 Oct 2000 14:04:48 +0100

Hello, My apologies for cross-posting; I would appreciate the wider perspective that cross-posting might bring to bear on this problem: Most of the world= 's audio engineers proceed on the assumptions mentioned below by Dave Malham= , namely that humans are able to more accurately locate a sound-source when that source contains significant energy at below 1000Hz. The research mentioned below is also used to support the argument that sound sources N= OT containing 'low frequencies' are correspondingly difficult to locate. Fro= m personal experience, whilst I might well agree in the special case of sou= nd generated by artificial devices (speakers), such as electronic tills, mob= ile phones, etc, such devices are ( from an evolutionary point of view) comparitive latecomers, and can be considered 'minority' special case audible objects. In the wider sense of the audible objects to be found in real environments, it does seem to me personally that there is no sense i= n which Lf facilitates location-identification better than Hf; - quite possibly the reverse. This is a problem I've puzzled over, on-and-off, fo= r some years. Is anyone aware of specific (and recent) research on this subject, or theories which incorporate perception-as-related to 'real environments' which speculate in this area? thank you for your time ppl Peter Lennox Hardwick House tel: (0114) 2661509 e-mail: peter(at)lennox01.freeserve.co.uk or:- ppl100(at)york.ac.uk ----- Original Message ----- From: "DG Malham" <dgm2(at)york.ac.uk> To: <sursound(at)darkwing.uoregon.edu> Sent: 23 October 2000 10:56 Subject: Re: sursound: The pinna and Ambisonics Hmm - seems several people bit at the bait I dangled over the side. I had noticed that several people were assumimng that the ear is better at discrimating high frequency based positions so I baited a hook and waited to see what would happen. My answer to those who think this is that they should look at the research. As early as 1958, Mills, the originator of the term "Minimum Audible Angle" (Mills, A.W. "On the Minimum Audible Angle" JASA, VOL 30, NO. 4, PP 237-246) had shown that, the ear could discriminate between two identical sound sources 1 degree apart for frequencies up to around 750 - 1000 Hz. However, he also showed that with significantly higher frequencies and at angles where the pinnae start to have a major contribution to position sensing, the MAA increase to 30 degrees or more. Later studies confirm this, for sounds without significant features at lower frequencies where other hearing mechanisms come in. Mills used tone bursts, which were largely, but of course not wholely, limited to the upper partials. Others who have used more continuous sound= s (sine waves, narrow band filtered noise) show similar or worse MAA's. Where high frequency results have been given which are significantly better than this (and sometimes better even than at low-mid frequencies u= p to 750-1500 Hz) it soon becomes obvious that the signals used (pulse trains, filtered music) contain significant cues at frequencies where other hearing mechanisms (ITD, IID) come into play. Deep intuition or not= , the facts simply don't bear out the argument. What I would agree with, and what the research base DOES bear out, is tha= t if the high frequency, pinnae based cues are right, they can play a very important role in enhancing the MAA resolution of the low frequency ITD/IID mechanisms. Conversely, if they are wrong (ie, in disagreement with the low frequency cue) they will degrade the low frequency ones. If they are simply confused, they will be rejected. This may well be related to survivsl mechanisms whereby high frequency cues are far more likely to be corrupted by nearby objects diffracting or reflecting them than low frequency ones (the average tree trunk has little effect on low frequency sound waves, which just flow round it, but can have pretty significant effects on high frequency sound waves due to diffraction - and to a far lesser extent, reflection - effects). When discussing any perceptual research, a great deal of care needs to be taken to ensure that what it i= s that is actually being tested is fully understood before making assumptions. This is made doubly diificult because it is usually impossible to reduce the perceptual "crosstalk" inherent in the geshtalt that is the average human being's view of the world. In the same vein, I like to point out that I feel there is as much of a problem at the bass end and that many of the MAA measurements have to be very suspect in this area since the common use of headphones, rather than speakers (Mills used speakers and an anechoic chamber) mean that whole body sound perception mechanisms (body cavity resonances, bon conduction etc.), which are vitally important in the bass region, are simply not brought into play. On Sun, 22 Oct 2000, Peter Lennox wrote: > Dave, > I'm sorry, I've got to chip in here; there's an implication in your mai= l > that, when push comes to shove, humans are terrible at locating sound > sources which do not generate ......"low to mid frequencies". Although = you > don't specify the frequency ranges, I've notion that (given your > familiarity with Bamford et al), you mean up to 1700hz (ok, 2kHz max). > I'm sorry, I simply don't buy the idea that resolution of spatial information > declines with increasing frequency; I don't just mean that it seems > counter-intuitive, I mean that accepted proofs as to the effiicacy of t= he > hearing system due to phase -locking ( / : ITDs) resulting in MAAs (minimum > audible angles) of the order of >1 degree wen compared to MAAs at hf, > utilising IIDs (intensity cues), is simply not comparing like for like. > The comparison relies on assumptions such as "static perceiver/ no head > movement". > My contention is that: given an ambulant organism with a temporally fin= ite > 'short term memory buffer', it may well be that 'hf'-borne information > actually yields 'better' spatial resolution with increasing hf. > I know I'm 'spitting into the wind' here, but I've a very deep intuitio= n > that, (up to a point), increased hf positively correlates with > increase-of-spatial-information-uitilisable-by-humans. > There's also a non-scientific argument here, that those creatures we kn= ow to utilise > sound to yield real-time detailed spatial information, use extreme hf, > rather than extreme Lf. - I mean Bats, Owls, and Dolphins. All of these > apparently achieve quite sophisticated environment-navigation > (/representation) using sound, seemingly equivalent to our own abilitie= s > using ambient light. > My own feeling is that there is a fundamental distinction arising here > between 'special case / static percipient' examples of so-called > 'perception', and 'real' perception which is evidenced *only* by > intelligent, mobile organisms such as ourselves. > I know I'm coming over all Gibsonian ("Ecological approach to [visual] > Perception) here, but I do feel that there is a genuine question as to = the > general applicability of laboratory-based conclusions to specific 'real > world' problems. > Actually, I'd like to cross-post to another audio list to glean some wi= der > perspectives; would you mind? > cheers, > peter > ----- Original Message ----- > From: "DG Malham" <dgm2(at)york.ac.uk> > To: "Rolv-karsten R=F8nningstad" <rolv-k-r(at)frisurf.no> > Cc: "Angelo Farina" <farina(at)pcfarina.eng.unipr.it>; > <sursound(at)darkwing.uoregon.edu> > Sent: 21 October 2000 20:01 > Subject: Re: sursound: The pinna and Ambisonics > > > On Sat, 21 Oct 2000, Rolv-karsten R=F8nningstad wrote: > > > Yes, I should possibly have been able to understand this myself. Anyw= ay, > > thank you very much for clarifying this with your answer. Can one the= n > > deduct that more real sources, i.e. loudspeakers, improve high freque= ncy > > localization? > > Yes, to some extent, since the errors add arithmetically (because they > change from speaker to speaker) and the correct cues add geometrically > (because they are consistent from speaker to speaker) so you can roughl= y > half the problems by quadrupling the number of speakers. As using this > many speakers also means that you can thing about using higher order > Ambisonics - and even going only to second order puts you much closer t= o > the ears limits - it is, after all, only capable of discriminating a 30 > degree minimum audible angle at high frequencies as opposed to between = 1 > and 3 at lower mid frequencies - you are beginning to get into the area > where things work in perceptual terms almost as well as they can withou= t > going to hyper dense transducer arrays or other advanced technologies. > > Dave > > > > > >

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