I can't help myself. I simply have to comment on this: > can we really detect the originating > direction of low frequency sound if you do not > have assistance of the over 80-120Hz overtones ?
from wikipedia: > As the frequency drops below 80 Hz > it becomes difficult or impossible to use either time difference or level > difference to determine a sound's lateral source, because the phase > difference between the ears becomes too small for a directional evaluation. Wikipedia is just wrong on this count. There is relatively little to go on in the psychoacoustic literature, because psychoacousticians don't seem to be interested in frequencies below about 250 Hz. This is no doubt due to the difficulty in finding a suitable anechoic environment and also the difficulty in producing undistorted high-level sounds at low frequencies. But Klumpp and Eady addressed the issue of localization at low frequencies in: Klumpp, R., Eady, H.; "Some Measurements of Interaural Time Difference Thresholds", JASA, Vol. 28 No. 5, (1956) In which they report that the threshold of detectability (what Blauert calls localization blur) is 11 uSec at 1000 Hz and 75 uS at 90 Hz. 75 uSec is still a very small number! Note that if the ear used phase as a measure of localization one would expect the threshold to have decreased by at least a factor of 1000/90, or about 11, whereas it only decreased by a factor of 7 or so. It seems highly likely to me that the determinant of the low-frequency limit of localization using time difference has to do with the threshold of hearing, which is about 0 dB SPL at 1 kHz and 76 dB SPL at 20 Hz. As the sound to be localized approaches the threshold of hearing it obviously will become difficult to locate the direction of the sound. There is no obvious reason to expect the Interaural Time Difference localization mechanism to fail abruptly at any particular low frequency. It would not surprise me to find out that it is possible to localize even very low frequency sounds, say 10 Hz, if the level were intense enough. As for localization at low-frequencies in rooms, the reason that we are able to localize at all in reverberant environments is that the auditory system is easily able to discriminate between the first arrival of sounds and the reflections or reverberation that follow. However, below the Schroeder frequency (about 150 Hz in small rooms) it becomes progressively more difficult to even define the difference between the first arrival and the reflections; they blend together. In experiments associated with my other publications I found that subjects experienced strong localization at very low frequencies. However, it was almost always in the wrong direction! Measurement of the acoustic ear signals shows why. At low frequencies the sound in a small room is constrained to flow in just a few modes. This means that a sound that originates from a particular location, let us say the direction of the right front loudspeaker, will quickly begin to flow in an axial mode, say the one associated with the length of the room. The listener will likely pick that direction (front) instead of the correct direction, which is right front. Furthermore, for an offset listening position the level in the room may change very rapidly with position, so much so that the level between the ears is quite different, even at a low frequency. Even though the ILD localization mechanism is not usually present for sounds in free space (say, out of doors) it is certainly active at low frequencies. That is why we can localize (lateralize) when listening to stereo recordings over headphones. The interchannel level differences which are converted to ITDs in normal stereo loudspeaker listening are now heard as ILDs and the lateralization is still correct, at least in so far as direction is concerned. Headphone experiments measuring lateralization show that direction can be discerned down to low frequencies. In limited experiments I find that subjects lateralize very well with sounds down to at least 40 Hz. The same requirement exists that the transducer shouldn't create significant distortion. Although most headphones generate audible distortion at high levels at low frequencies, the Stax Lambda Pro headphones do not, making them a preferred source for such experiments. I've probably said too much, so I'll simply sum up by saying that if you have the loudspeakers to do it, there is an advantage to reproducing the bass directionally. What I report above about the effect of the room on the directional perception is a problem in audio reproduction and it remains so. I believe, but have not demonstrated, that reproducing the bass in a wave front reproduction sense, say Ambisonically or with WFS, makes the directional reproduction better at low frequencies. Eric Benjamin ----- Original Message ---- From: jim moses <[email protected]> To: Surround Sound discussion group <[email protected]> Sent: Thu, June 9, 2011 8:03:56 AM Subject: Re: [Sursound] Fwd: Bass Problem in crosstalk cancellation my $.02 ... But the unanswered question is, can we really detect the originating > direction of low frequency sound if you do not > have assistance of the over 80-120Hz overtones ? > conventional thought says no. This matches my experience. This description from wikipedia http://en.wikipedia.org/wiki/Sound_localization#Evaluation_for_low_frequenciesseems pretty good to me. "For frequencies below 800 Hz, the dimensions of the head (ear distance 21.5 cm, corresponding to an interaural time delay of 625 µs), are smaller than the half wavelength of the sound waves. So the auditory system can determine phase delays between both ears without confusion. Interaural level differences are very low in this frequency range, especially below about 200 Hz, so a precise evaluation of the input direction is nearly impossible on the basis of level differences alone. As the frequency drops below 80 Hz it becomes difficult or impossible to use either time difference or level difference to determine a sound's lateral source, because the phase difference between the ears becomes too small for a directional evaluation. " > Or is the directional hint the the sensation of the impact on you body of > the sound preassure wave ( high volume needed)? There are research results indicating that it is not so important, that the > highfrequency pshychocoustic impressions overrides and creates the > directional impression. > > this relates to the impact of transients in the bass. and alignment of the subs with the mains can matter. not really a stereo/multichannel localization concern. > Do any one have the definitive answer to this? Or do we really have to > care, as far as "normal" use of mono channel for < 80Hz sounds. > > as far as localization i don't think one needs to worry about sounds below around 80Hz (and even below 120Hz perhaps). But frequency response is effected by the placement and number of subwoofers. jim > For the 5.1 specification satellite speakers are to be able to go down to > the cutover between 80 to 100 Hz, that some manufacturers of lowcost 5.1 > appliances have a much higher cutover frequency is the normal corner cutting > in low cost products. > > - Bo-Erik > > -----Original Message----- > From: [email protected] [mailto:[email protected]] > On Behalf Of Martin Leese > Sent: den 8 juni 2011 18:35 > To: [email protected] > Subject: Re: [Sursound] Fwd: Bass Problem in crosstalk cancellation > > Marc Lavall?e <[email protected]> wrote: > ... > > Ambisonics can supposedly reproduce > > bass from all directions; is it true? > > Yes, assuming: > > 1. The source contains bass from all > directions > 2. The standing waves in the room don't > screw up localisation of bass frequencies. > > But this is also true for 5.1 (assuming some idiot doesn't stuff all of the > bass into the LFE channel). > > Regards, > Martin > -- > Martin J Leese > E-mail: martin.leese stanfordalumni.org > Web: http://members.tripod.com/martin_leese/ > _______________________________________________ > Sursound mailing list > [email protected] > https://mail.music.vt.edu/mailman/listinfo/sursound > _______________________________________________ > Sursound mailing list > [email protected] > https://mail.music.vt.edu/mailman/listinfo/sursound > -- Jim Moses Technical Director/Lecturer Brown University Music Department and M.E.M.E. (Multimedia and Electronic Music Experiments) -------------- next part -------------- An HTML attachment was scrubbed... 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