This section covers binaural recording with an artificial (dummy) head.
The head contains a microphone flush-mounted in each ear. You record with these microphones and play back the recording over headphones.
This process can re-create the locations of the original performers and their acoustic environment with exciting realism.
You can substitute your own head for the artificial head by placing miniature condenser microphones in your ears, or on your temples, and recording with them. Some podcasts are made this way.
Thanks to the popularity of MP3 players with earphones, many people have the opportunity to hear binaural recordings.
Binaural Recording and the Artificial Head
Binaural (two-ear) recording starts with an artificial head or dummy head. This is a model of a human head with a flush-mounted microphone in each ear (FIG. 1). These microphones capture the sound arriving at each ear.
The microphones' signals are recorded. When this recording is played back over headphones, your ears hear the signals that originally appeared at the dummy head's ears (FIG. 2). That is, the original sound at each ear is reproduced (Geil, 1979; Genuit and Bray, 1989; Peus, 1989; Sunier, 1989a, 1989b, 1989c).
FIG. 1 A dummy head used for binaural recording (Neumann USA).
FIG. 2 Binaural recording and headphone playback.
Binaural recording works on the following premise. When we listen to a natural sound source in any direction, the input to our ears is just two one-dimensional signals: the sound pressures at the eardrums. If we can re-create the same pressures at the listener's eardrums as would have occurred "live," we can reproduce the original listening experience, including directional information and reverberation (Moller, 1989).
Binaural recording with headphone playback is the most spatially accurate method now known. The re-creation of sound-source locations and room ambience is startling. Often, sounds can be reproduced all around your head-in front, behind, above, below, and so on. You may be fooled into thinking that you're hearing a real instrument playing in your listening room. A catalog and demos of binaural recordings are available from The Binaural Source at binaural.com, headed by binaural expert John Sunier.
As for drawbacks: the artificial head is conspicuous, which limits its use for recording live concerts; it is not mono-compatible; and it is relatively expensive. Some sources for dummy heads are given in Section 12 under the heading "Dummy Heads and Headworn Binaural Mics." How It Works
An artificial head picks up sound as a human head does. The head is an obstacle to sound waves at middle to high frequencies. On the side of the head away from the sound source, the ear is in a sonic shadow: the head blocks high frequencies. In contrast, on the side of the head toward the source, there is a pressure buildup (a rise in the frequency response) at middle to high frequencies.
The folds in the pinna (outer ear) also affect the frequency response by reflecting sounds into the ear canal. These reflections combine with the direct sound, causing phase cancellations (dips in the response) at certain frequencies.
The human eardrum is inside the ear canal, which is a resonant tube.
The ear canal's resonance does not change with sound-source direction, so the ear canal supplies no localization cues. For this reason, it is omitted in most artificial heads. Typically, the microphone diaphragm is mounted nearly flush with the head, 4mm (0.16 in) inside the ear canal.
To summarize: The head and outer ear cause peaks and dips in the frequency response of the sound received. These peaks and dips vary with the angle of sound incidence; they vary with the sound-source location. The frequency response of an artificial head is different in different directions. In short, the head and outer ear act as a direction-dependent equalizer.
Each ear picks up a different spectrum of amplitude and phase because one ear is shadowed by the head and the ears are spaced apart. These inter aural differences vary with the source location around the head.
When the signals from the dummy-head microphones are reproduced over headphones, you hear the same interaural differences that the dummy head picked up. This creates the illusion of images located where the original sources were.
Physically, an artificial head is a near-coincident array using boundary microphones: the head is the boundary, and the microphones are flush mounted in this boundary. The head and outer ears create directional pat terns that vary with frequency. The head spaces the microphones about 6 1/2 inches apart. Some dummy heads include shoulders or a torso, which aids front/back localization in human listening but can degrade it in bin aural recording and playback (Griesinger, 1989).
The microphones in a near-coincident array are directional at all frequencies and use no baffle between them. In contrast, the mics in an artificial head are omni at low frequencies and unidirectional at high frequencies (due to the head baffle effect).
Ideally, the artificial head is as solid as a human head, to attenuate sound passing through it (Sunier, 1989c). For example, the Head Acoustics artificial head is made of molded, dense fiberglass (Genuit and Bray, 1989).
In contrast, the Sonic Studios GUY and LiteGUY artificial heads are made of absorbent Sorbothane.
As we said, you can substitute your own head for the artificial head by placing miniature condenser microphones in your ears and recording with them. The more that a dummy head and ears are shaped like your particular head and ears, the better the reproduced imaging. Thus, if you record binaurally with your own head, you might experience more precise imaging than you would if you recorded with a dummy head. This recording will have a nonflat response because of head diffraction (which I will explain later).
Core Sound (core-sound.com) is the world's largest manufacturer of binaural microphones. The company offers miniature omni con denser mics that can be clipped onto eyeglass earpieces. These mics make excellent binaural recordings. Sonic Studios (sonicstudios.com) has a similar product, DSM (Dimensional Stereo Microphones) that are worn on the temples rather than in the ear. Based on the head related transfer function (HRTF), DSM mics are said to provide better stereo over loud speakers than binaural mics can provide. An explanation of HRTF was given at the beginning of Section 12.
Another substitute for a dummy head is a head-size sphere with flush-mounted microphones where the ears would be. This system, called the Kugelflachenmikrofon, was developed by Gunther Theile for improved imaging over loudspeakers (Griesinger, 1989). See Section C under the heading "Sphere Microphones." CD track 17 demonstrates the stereo imaging of a sphere microphone. Listen to it over headphones as well as loudspeakers.
Some commercial products are listed in Section 12 under the headings "Dummy Heads and Headworn Binaural Mics" and "Stereo Microphones."
In-Head Localization
You might hear the binaural images inside your head, rather than outside. One reason has to do with head movements. When you listen to a sound source that is outside your head and move your head slightly, you hear small changes in the arrival-time differences at your ears. This is a cue to the brain that the source is outside your head. Small movements of your head help to externalize sound sources. But the dummy head lacks this cue because it is stationary.
Another reason for in-head localization is that the conch resonance of the pinna is disturbed by most headphones. The conch is the large cavity in the pinna just outside the ear canal. If you equalize the head phone signal to restore the conch resonance, you hear images outside the head (Cooper and Bauck, 1989).
Artificial-Head Equalization
An artificial head (or a human head) has a nonflat frequency response due to the head's diffraction, the disturbance of a sound field by an obstacle.
The diffraction of the head and pinnae creates a very rough frequency response, generally with a big peak around 3 kHz for frontal sounds.
Therefore, binaural recordings sound tonally colored unless custom equalization is used. Some artificial heads have built-in equalization that compensates for the effect of the head.
What is the best equalization for an artificial head to make it sound tonally like a conventional flat-response microphone? Several equalization schemes have been proposed:
• Diffuse-field equalization: This compensates for the head's average response to sounds arriving from all directions (such as reverberation in a concert hall).
• Frontal free-field equalization: This compensates for the head's response to a sound source directly in front, in anechoic conditions.
• 10° averaged, free-field equalization: This compensates for the head's response to a sound source in anechoic conditions, averaged over +/-10° off-center.
• Free field with source at +/-30° equalization: This compensates for the head's response to a sound source 30° off-center, in anechoic conditions. This is a typical stereo loudspeaker location.
The Neumann KU-100 and KEMAR artificial heads use diffuse-field equalization, which Theile also recommends. However, Griesinger (1989) found that the Neumann head needed additional equalization to sound like a Calrec Soundfield microphone: approximately +/-7dB at 3 kHz and +/-4dB at 15 kHz. He prefers either this equalization or a 10° averaged free-field response for artificial heads. The Head Acoustics head, developed by Gierlich and Genuit, is equalized flat for free-field sounds in front (Genuit and Bray, 1989), while Cooper and Bauck (1989) recommend that artificial heads be equalized flat for free-field sounds at +/-30°.
To provide a net flat response from microphone to listener, the artificial-head equalization should be the inverse of the headphone frequency response. If the head is equalized with a dip around 3 kHz to yield a net flat response, the headphones should have a mirror-image peak around 3 kHz (most do).
Artificial-Head Imaging with Loudspeakers
How does an artificial-head recording sound when reproduced over loud speakers? According to Griesinger (1989), it can sound just as good as an ordinary stereo recording, with superior reproduction of location, height, depth, and hall ambience. But it sounds even better over headphones.
Images in binaural recordings are mainly up front when you listen with speakers but are all around when you listen with headphones.
Genuit and Bray (1989) report that more reverberation is heard over speakers than over headphones, due to a phenomenon called binaural reverberance suppression. For this reason, it is important to monitor artificial head recordings with headphones and speakers.
Griesinger notes that a dummy head must be placed relatively close to the musical ensemble to yield an adequate ratio of direct-to-reverberant sound over loudspeakers. This placement yields exaggerated stereo separation with a hole in the middle. However, the center image can be made more solid by boosting in the presence range (see Griesinger's, 1989, recommended equalization previously).
Although a dummy-head binaural recording can provide excellent imaging over headphones, it produces inadequate spaciousness at low frequencies over loudspeakers (Huggonet and Jouhaneau, 1987) unless spatial equalization is used (Griesinger, 1989). Spatial equalization was discussed in Section B under the heading "Coincident Systems with Spatial Equalization (Shuffler Circuit)." A low-frequency boost in the L - R difference signal of about 15dB at 40Hz and +/-1dB at 400Hz can improve the low frequency separation over speakers.
References
Cooper, D. and Bauck, J. "Prospects for Transaural Recording." Journal of the Audio Engineering Society, Vol. 37, No. 1/2 (January-February 1989), pp. 3-19.
Geil, F. "Experiments with Binaural Recording." db (June 1979), pp. 30-35.
Genuit, K. and Bray, W. "The Aachen Head System: Binaural Recording for Head phones and Speakers." Audio (December 1989), pp. 58-66.
Griesinger, D. "Equalization and Spatial Equalization of Dummy Head Recordings for Loudspeaker Reproduction." Journal of the Audio Engineering Society, Vol. 37, No. 1/2 (January-February 1989), pp. 20-29.
Huggonet, C. and Jouhaneau, J. "Comparative Spatial Transfer Function of Six Different Stereophonic Systems." Preprint No. 2465 (H5), Paper Presented at the Audio Engineering Society 82nd Convention, March 10-13, 1987, London, p. 16, Fig. 13.
Moller, H. "Reproduction of Artificial-Head Recordings through Loudspeakers." Journal of the Audio Engineering Society, Vol. 37, No. 1/2 (January-February 1989), pp. 30-33.
Peus, S. "Development of a New Studio Artificial Head." dbMagazine (June 1989), pp. 34-36.
Sunier, J. "A History of Binaural Sound." Audio (March 1989a), pp. 312-346.
Sunier, J. "Binaural Overview: Ears Where the Mics Are, Part 1." Audio (November 1989b), pp. 75-84.
Sunier, J. "Binaural Overview: Ears Where the Mics Are, Part 2." Audio (December 1989c), pp. 48-57.
Several papers on binaural sound were presented at the 89th Convention of the Audio Engineering Society, September 21-25, 1990, Los Angeles. These papers are:
"Subjective Evaluation of Spatial Image Formation Processors," Elizabeth A.
Cohen and Charles M. Salter Associates, Inc., San Francisco, CA.
"A New Method for Spatial Enhancement in Stereo and Surround Recording," Dr. Wieslaw R. Woszczyk, McGill University, Montreal, Canada.
"Multi-Channel Sound in the Home: Further Developments of Stereo-phony," Gunther Theile, Institut fur Rundfunktechnik, GmbH.
"Development and Use of Binaural Recording Technology," W. Bray, K. Genuit, and H. W. Gierlich, Jaffe Acoustics, Norwalk, CT.
"Spaciousness Enhancement of Stereo Reproduction Using Spectral Stereo Techniques," D. J. Furlong and A. G. Garvey, Preprint 3007.
"An Intuitive View of Coincident Stereo Microphones," S. Julstrom, Preprint 2984.
More-recent Audio Engineering Society preprints:
"Investigations on a New Reproduction Procedure for Binaural Recordings," Ning Xiang, Klaus Genuit, and Hans W. Gierlich, Head Acoustics, Herzo genrath, Germany, #3732, October 1993.
"Temporal Localization Cues and Their Role in Auditory Perception," Martin D. Wilde, Wilde Acoustics, Chicago, IL, #3708, October 1993.
"Early Reflections and Reverberant Field Distribution in Dual Microphone Stereophonic Sound Recording Systems," Michael Williams, Paris, France, #3155 (R4), October 1991.
"Binaural Record/Reproduction Systems and Their Use in Psychoacoustic Investigations," Floyd E. Toole, National Research Council Canada, Ottawa, Ontario, #3179 (L6), October 1991.
"Development and Use of Binaural Recording Techniques," K. Genuit, H. W. Gierlich, and Wade Bray, HEAD Acoustics, Aachen, Germany, Norwalk, CT, #2950, September 1990.
"Further Developments of Loudspeaker Stereophony," Gunther Theile, Institut fur Rundfunktechnik GmbH, Munich, Germany, #2947, September 1990.
"Microphone Arrays Optimized for Music Recording," W. Woszczyk, McGill University, #3255, March 1992.
"Frequency Dependent Hybrid Microphone Arrays for Stereophonic Sound Recording," Michael Williams, Paris France, #3252, March 1992.
"Standard Stereo Recording Techniques in Non-Standard Situations," Albert G. Swanson, Location Recording, Seattle, #3313, March 1992.
"Improved Externalization and Frontal Perception of Headphone Signals," Soren Gert Weinrich, Oticon A/S Research Unit, Snekkersten, Denmark, #3291, March 1992.
"BAP Binaural Audio Processor," F. Richter, AKG Acoustics, #3323, March 1992.
"Transfer Characteristics of Headphones," Henrik Moller et al., Institute for Electronic Systems, #3290, March 1992.
"Improved Possibilities of Binaural Recording and Playback Techniques," K. Genuit et al., HEAD Acoustics, Herzogenrath, Germany, #3332, March 1992.
"Applications of Blumlein Shuffling to Stereo Microphone Techniques," Michael Gerzon, Oxford, UK, #3448 (S-1), October 1992.
Preprints can be ordered from the Audio Engineering Society, aes.org More articles in the Journal of the Audio Engineering Society:
"Measuring a Dummy Head in Search of Pinna Cues," H. L. Han, January- February 1994.
"Binaural Technique: Do We Need Individual Recordings?" Henrik Moller et al., Acoustics Laboratory, Aalborg University, Aalborg, Denmark, June 1996.
"Comments on 'Spaciousness and Localization in Listening Rooms and Their Effects on the Recording Technique'," Stanley Lipshitz, Audio Research Group, University of Waterloo, Waterloo, Ont., Canada, December 1987.
"The Effect of Head Shape on Spectral Stereo Theory," K. Rasmussen and P. Juhl, Acoustics Laboratory, Technical University of Denmark, Denmark, March 1993.
"On the Naturalness of Two-Channel Stereo Sound," Gunther Theile, Institut fur Rundfunktechnik GmbH, Munich, Germany, October 1991.
"A Computer Model of Binaural Localization for Stereo Imaging Measurement," E. Macpherson, Audio Research Group, University of Waterloo, Waterloo, Ont., Canada, September 1991.
"Room-Related Balancing Technique: A Method for Optimizing Recording Quality," M. Wohr, G. Theile, H. Goeres, and A. Persterer, September 1991.