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Dummy Head Recording

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Dummy Head Recording Dummy Head Recording Michael Gerzon FIRST, IT SHOULD BE emphasised that binaural recordings, i.e. Recently, there has been a lot of recordings made for reproduction interest in dummy-head or binaural via headphones, contain three main recording for reproduction via types of sound-localisation cue that headphones. This has been presented is absent from conventional stereo, by some as ‘the answer to and that there has been some quadraphony’, and some ill-informed conflict of opinion as to which cue comment has thoroughly confused is most important. The first cue is people as to the advantages and that of time delays between the disadvantages of binaural techniques. ears. It is clear that a sound from, Issues and available information are say, the left will arrive at the left summarised, together with an ear before the right ear. For a indication of areas of doubt. sound on the extreme left, this time delay at the right ear is about 0.62 ms, and for a sound arriving from (say) 30° from the left of front (or back, or up, or down), the time delay is about 0.24 ms. Clearly, the time delay cue cannot distinguish front from behind from above from below. All it indicates is the angle of arrival of the sound from the axis of symmetry of the ears. One technique of binaural recording makes use mainly or only of this cue, and that is the ORTF technique of using a pair of microphones spaced apart by about 17 cm. In order to improve compatibility with stereo loudspeaker reproduction, the microphones are directional ones pointing to the left and right respectively, angled about 110° one from the other (see fig. 1). For this application, cardioids are to be preferred, as the anti-phase lobes of hypercardioids Reproduced from Studio Sound, Vol. 17, pp 42-44 (May 1975), by permission of IPC Media Ltd, publishers of Hi-Fi News (www.hifinews.co.uk 1 Dummy Head Recording tend to give exaggerated width about 500 HZ). Many early requiring a smaller spacing. workers in binaural sound, such as de Boer and Blumlein, considered It is important that the spacing not that this head obstruction effect generally exceed 17 cm, as was important in sound otherwise the time delays are too localisation, and some large and everything is experimenters still do. However, concentrated at the extreme left or as will be explained, the evidence right. Certainly, ear-spaced seems to suggest that this is by far binaural recording gives much the least important cue. Indeed, sharper defined images via experiments in which a mono headphones than ordinary stereo, sound is fed to both ears but with even when recorded with a pair of differing gains show that a relative omni-directional microphones. An gain of as much as 15-23 dB is indication of the importance of required to create an illusion of a time delays between the ears is sound coming from 45° from the given if one makes one channel of a front, which is much more than the binaural recording 10 dB louder difference in the level at the two than the other. Surprisingly, the ears caused by a live sound from image shift thus caused amounts at this direction. Moreover, if the ears most to a few degrees, since the have just previously been exposed time delays are unaltered. Time to sounds with natural time delays delays around 0.5 ms are much less between the ears, such a panned important in speaker reproduction, mono sound can seem to come and this leads to the possibility of from only about 15° off front. In an amusing paradoxical recording other words, not only do the ears in which a sound appearing on the make poor use of differences in left via headphones appears on the level, but when they are provided right via speakers. Simply record a with other cues, they almost sound on the left of a pair of omni entirely disregard these level mics spaced 17 cm apart, and turn differences. up the gain of the right channel about 10 dB. The third cue is the effect of the pinnae, which is the name for the The second cue for headphone flaps on the ears. The various reproduction is the fact that the ridges on the pinnae reflect and head casts an acoustical shadow refract the sound waveform before across each ear for sounds from the it enters the ears, and the coloration opposite side. This effect is thus produced varies according to significant only in the treble (above the direction of arrival of the Reproduced from Studio Sound, Vol. 17, pp 42-44 (May 1975), by permission of IPC Media Ltd, publishers of Hi-Fi News (www.hifinews.co.uk 2 Dummy Head Recording sound. This coloration, which localisation. Roffler and Butler (ref. mainly affects frequencies above 5 2) describe experiments in which a kHZ, is now known to be of vital subject’s head was fixed, so that he importance to the ears in localising could derive no clues from head and positioning sounds, although movements, and in which a sound the way in which this coloration is source was moved in the plane of used by the ear to provide symmetry of the subject’s head, so information is still not understood. that it could be above, below, in front or behind. Since the sounds An intriguing experiment of reaching the two ears is then Batteau (described in ref. 1) identical, conventional theories of demonstrates this in no uncertain stereo hearing would suggest that fashion. In one room, he set up 16 height effect cannot be heard under loudspeakers (see fig. 2a) in a circle these conditions. However, Roffler around a pair of microphones and Butler found that a change of spaced apart by ear distance. The the sound source elevation as little outputs of these were fed to a as 5° could be clearly heard. On subject sitting in another room via the other hand, if the subject wore headphones. The various speakers a ‘pinna mask’ which covered up were then fed with sound and the the pinnae but allowed sound to subject was asked which of the 16 enter the ears, then no change of directions the sound appeared to elevation could be heard. be coming from. This test was performed both using ordinary So we see that the pinnae play an omni microphones, and with essential role in locating sounds, microphones fitted with accurate and that they should therefore be replicas of human pinnae, but with accurately modelled (preferably by no dummy head used in either taking moulds from human case. When no pinnae were used, pinnae) if used with a dummy the subjects found it difficult to head. It seems that most of the localise the sounds, assigning them recent commercial dummy head to more-or-less random positions. recordings have used However, with the pinnae fitted to inadequately-accurate pinnae for the microphones, localisation was optimal effect. We also see that the correct with no confusion between complication of an actual dummy front and rear. head between the microphones- with-pinnae may be omitted, Other experiments have also thereby improving the visual demonstrated that pinnae are of appearance of the microphones as vital importance for correct well as reducing some of the Reproduced from Studio Sound, Vol. 17, pp 42-44 (May 1975), by permission of IPC Media Ltd, publishers of Hi-Fi News (www.hifinews.co.uk 3 Dummy Head Recording coloration if the sound is played The trick is to reproduce the via speakers. Alternatively, a very dummy head recording via stereo idealised ‘dummy head’, such as a loudspeakers placed either to each simple baffle to separate the side of the listener (A in fig. 3), or microphones (as suggested by at least angled widely apart (B in Blumlein, ref 3), may be used. fig. 3). I have found that angles θ (see fig. 3) of more than 110° work Dr Edmund Rolls, of the well. The listener listens wearing a Department of Psychology, pinna mask. (For listening tests, it University of Oxford, has recently is adequate to use the hands to been conducting experiments in cover the back part of the pinnae.) dummy head recording, using Since the sound has been past small microphones placed in the pinnae once during the recording, ears of actual people (although one and since it is prevented from conjectures that they may resent going over them again by the pinna being termed “dummies”). This masks, the ears hear just the pinna microphone technique, so purist coloration inherent in the that advocates of Blumlein recording, and hence hear a correct technique must blush with shame, directional effect, including sounds is capable of giving very superior form behind or above. I have used binaural results, as would be this to demonstrate dummy head expected with such accurate recordings to an audience of about dummy heads. Recognising the a dozen via loudspeakers. importance of the pinnae described above, Dr Rolls has suggested a Correct localisation is not the only simple, ingenious and effective benefit produced by pinna method of reproducing dummy coloration. It is well-known that head recordings via loudspeakers. headphone reproduction always gives the effect of in the head localisation (ihl). This has been Reproduced from Studio Sound, Vol. 17, pp 42-44 (May 1975), by permission of IPC Media Ltd, publishers of Hi-Fi News (www.hifinews.co.uk 4 Dummy Head Recording thing, the pinna coloration still allows front and back to be distinguished to some extent.
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