BACCH® 3D SOUND Frequently Asked Questions (FAQ)
  1. What is BACCH® 3D Sound?

    BACCH® 3D Sound is a recent breakthrough audio technology (licensed by Princeton University) that yields unprecedented spatial realism in speaker-based audio playback allowing the listener to hear, through only two loudspeakers, a truly 3D reproduction of a recorded sound field with uncanny accuracy and detail, and with a level of high tonal and spatial fidelity that is simply unapproachable by even the most expensive and advanced existing high-end audio systems.

    BACCH® 3D Sound relies on canceling an undesired artifact, called crosstalk, that occurs whenever stereo sound is played through loudspeakers, thus allowing the 3D cues which the brain needs to hear in 3D, and which exist in abundance in practically all well-made stereo recordings, to naturally reach the brain of the listener.

  2. How Does BACCH® 3D Sound differ from surround sound?

    BACCH® 3D Sound has nothing to do with surround sound. Surround sound, which was originally conceived to make the sound of movies more spectacular, does not (and cannot) attempt to reproduce a true 3D sound field. What 5.1 or 7.1 surround sound aims to do is provide some degree of sound envelopment for the listener by surrounding the listener with five, seven, or more loudspeakers. For serious music listening of music recorded in real acoustic spaces, audio played through a surround sound system can at best give a sense of simulated hall ambiance but cannot offer an accurate 3D representation of the sound field.

    In contrast, BACCH® 3D Sound’s primary goal is accurate 3D sound field reproduction. It gives the listener the same 3D audio perspective as that of the ideal listener in the original recording venue.1 Soundstage "depth" and "width", concepts often used liberally in hi-end audio literature to describe an essentially flat image (relative to that in BACCH® 3D Sound), become literal terms for BACCH® 3D Sound. If, for instance, in the original sound field a fly cicrles the head of the ideal listener during the recording, a listener of that recording played back through the two loudspeakers of a BACCH® 3D Sound system will hear, simply and naturally, the same fly circling his or her own head. If, in contrast, the same recording is played through standard stereo or surround sound systems the fly will be perceived to be inside the loudspeakers or, through the artifice of the phantom image, in the limited vertical plane between the loudspeakers.

  3. Does BACCH® 3D Sound require special speakers and/or special room treatment?

    BACCH® 3D Sound will greatly enhance the spatial fidelity of sound reproduction through any loudspeakers. Loudspeakers that have high sound directivity2 will give the best and most accurate 3D imaging in a highly refelctive room with little or no sound treatment, as room reflections, which degrade imaging, are minimized by such loudspeakers.

    However, even loudspeakers with low directivity (i.e. omni-directional loudspeakers) will give a spectacularly spatial soundstage with BACCH® 3D Sound in a typical listening room. As the importance of room reflections is decreased (by increasing the ratio of directed to eflected sound through room treatment and/or higher-directivity speakers and/or nearfield listening) the image’s depth and 3D imaging approach the depth and spatial characteristics of the original sound field.

    An ongoing investigation of speaker directivity at Princeton University's 3D3A Lab, has shown that dipole speaker designs, electrostatic speakers, as well as speakers with horns and waveguides offer significant advantages in 3D imaging with BACCH® 3D Sound in highly reflective rooms, as they increase the ratio of direct to reflected sound. Abating room reflections with physical room treatment (i.e. using sound absorbers on sound-reflective surfaces) in a listening room is always beneficial to any audiophile-grade sound system. For BACCH® 3D Sound the effect of sound treatment is equivalent to using loudspeakers with high directivity, or listening in the nearfield. The more directive the loudspeakers are, the less sound treatment is needed for BACCH® 3D Sound to produce a full and accurate 3D sound image.

    Therefore, in a reflective untreated listening room, directive loudspeakers are more desirable. In a well treated listening room with sound-absorbing surfaces, any loudspeakers, even omnidirectional ones, will produce an excellent 3D image.2

    Although it may not always be practical for some listeners, a simple way to increase the ratio of direct to reflected sound, and thus further enhance the 3D imaging of a BACCH system, in even a very reflective room, is simply to listen in the nearfield of the speakers.

  4. Does BACCH® 3D Sound require a special placement of the speakers?

    While previous non-optimized crosstalk cancellation (XTC) methods required the speakers to be placed very close to each other (the so-called “dipole” configuration), this is not at all the case with BACCH® 3D Sound. To a zeroth-order, speakers placement (excluding the effects of room reflections) does not matter with BACCH 3D Sound. The 3D image you get through a BACCH filter created for a given speakers configuration will be essentially the same as the image obtained for a completely different (even asymmetric) speakers configuration as long as the BACCH filter corresponding for that configuration is used for listening. With BACCH 3D Sound the speakers are only the conduit of the sound waves to the ears of the listener, who then perceives the location of the original sound sources in the recording and not the location of the speakers.

    This is often a startling fact for audiophiles as they are used to a phantom image that is anchored in the speakers and whose dimensions (essentially width) is strongly dependent on the speakers span (the angle the speakers sustain from the location of the listener). This is why the recommended speakers span for regular stereo listening is the 60 degrees of the equilateral triangle (often called “stereo triangle”). With BACCH 3D Sound we recommend the same stereo triangle only because this is what audiophiles are used to and because it allows them to compare the sound through the BACCH filter to that without it (by hitting the bypass button) and hear the significant enhancement to the imaging compared to a case they are well familiar with.

    This unique independence of the BACCH-purified 3D image on speakers placement leaves the user with far more latitude in speakers placement (e.g. to satisfy esthetic or practical requirements) than is possible with regular stereo. The main thing to keep in mind while choosing a speaker placement for BACCH 3D Sound, is to minimize reflections of sound from nearby surfaces, as early reflections are the enemy of good imaging.

  5. How does BACCH® 3D Sound work?

    Imagine a musician who stands on the extreme right of the stage of a concert hall and plays a single note. A listener sitting in the audience in front of stage center perceives the sound source to be at the correct location because his brain can quickly process certain audio cues received by the ears. The sound is heard by the right ear first and after a short time delay (called ITD) is heard by the left ear. Furthermore there is a difference in sound level between the two ears (called ILD) due to the sound having travelled a little longer to reach the right ear, and the presence of the listener’s head in the way. The ILD and ITD are the two most important types of cues for locating sound in 3D and are to a good extent preserved by most stereophonic techniques.3

    When the stereo recording is played through the two loudspeakers of a standard stereo system, the ILD and ITD cues are largely corrupted because of an important and fundamental problem: the sound recorded on the left channel, which is intended only for the left ear, is heard by both ears. The same applies to the sound on the right channel. Consequently, an audiophile listening to that recording on standard stereo system will not correctly perceive the musician to be standing on the extreme right of the stage but rather at the location of the right speaker. Consequently the perceived soundstage is mostly confined to an essentially flat and relatively limited region between the two loudspeakers irrespective of the quality and cost of the hardware in the standard stereo system—the 3D image is greatly compromised.4

    In order to insure the correct transmission of the ILD and ITD cues to the brain of the audiophile, the sound from the left loudspeaker to the right ear, and that reaching the left ear from the right loudspeaker (called "crosstalk") should be cancelled.

    The technique of crosstalk cancellation (XTC) has been known for some time and can be applied by filtering the recorded sound through an XTC filter before feeding it to the speaker. This can easily be done digitally. However, until recently, XTC filters have had a detrimental effect on the sound as they inherently add a strong spectral coloration to the processed signal (i.e. they severely change the tonal character of the sound). This is why, until the advent of BACCHecently, XTC had not been widely adopted by stereo manufacturers and audiophiles.

    BACCH® 3D Sound is based on a breakthrough in XTC filter design, that allows producing optimized XTC filters, called BACCH filters, that add no coloration to the sound for a listener in the sweet spot (or even outside of the sweet spot). Not only do BACCH filters purify the sound from crosstalk, but they also purify it from aberrations by the playback hardware in both the frequency and time domains.

    The result is a 3D soundstage with a striking level of spatial and tonal fidelity never experienced before by audiophiles.

  6. What are BACCH Filters?

    There are two types of BACCH filters. The individualized BACCH filter (sometimes called i-BACCH) is custom-made using in-situ acoustic measurements of the audiophile’s entire listening chain, including his hi-fi hardware, loudspeakers, head, torso and ears. It is designed by sending special test tones through the hi-fi chain and recording the sound with miniature microphones placed at the entrance of the audiophile’s ear canals as he is sitting in a sweet spot of his choice. It takes about one minute to do this measurement. Theoretica’s BACCH-SP processor and BACCH-dSP application allow users to easily produce individualized BACCH filters for their systems.

    The universal BACCH filter (called u-BACCH) is the same as i-BACCH except a special dummy head, having microphones in its ears, is used to make the measurements instead of the audiophile’s own head and ears. A u-BACCH filter yields a bit less accurate 3D image then i-BACCH when used by the audiophile himself to listen to his hi-fi system, but is more compatible with other listeners (who do not have i-BACCH filters designed for them). Since the dummy head was designed to represent the sound diffraction characteristics of an "average" human head, the difference between the sound through the two types of filters is subtle but perceivable by a discerning audiophile.5

  7. How big is the sweet spot in which the listener can hear BACCH 3D imaging?

    Without Theoretica's advanced head tracking technology, BACCH® 3D Sound produces a sweet spot that is quite long (many meters long depending on the directivity of the speakers used) in the longitudinal (backward-forward) direction, but quite narrow (about 40 cm), in the lateral (side-to-side) direction. A listener in that sweet spot can move backward and forward by at least 2 meters without losing much of the 3D imaging, but, without the head tracking feature enabled, must stay within the 40 cm side-to-side limit of the sweet spot, which is not a big concern for a stationary listener.

    To completely remove the lateral sweet spot limitation, Theoretica has developed a powerful advanced head tracking technology, that uses either an infrared camera for head tracking even in pitch darkness (which works over a range of only about 2 meters), or a regular webcam for head tracking in regular and dim lighting (which works over a range up to more than 8 meters). All of Theoretica's products (i.e. all of the BACCH-SP models and the BACCH4Mac system) come equipped with this very easy to set up head tracking technology, which automatically and accurately (with millimeter accuracy) tracks the head of the main listener and adjusts the sweet spot dynamically in the lateral direction so that the listener is always in the optimal sweet spot as long as the listener's head is in the calibrated view field of the camera.

    Any listener outside the sweet spot, would hear regular stereo sound as if BACCH® 3D sound processing is bypassed (i.e. BACCH® 3D Sound causes no audible alteration to the sound outside the sweet spot, so listeners outside the sweet spot can listen without any detriments, albeit without BACCH's 3D imaging). On the other hand, due to the elongated dimension of the sweet spot in the longitudinal direction, additional listeners can sit immediately in front of or behind the main listener and remain in the BACCH® 3D Sound sweet spot, thus enjoying practically the same 3D image as the main listener (although this, admittedly, makes for an unorthodox multiple listener configuration, often seen at BACCH® 3D Sound demos in audio shows around the world.)

  8. Is BACCH® 3D Sound compatible with existing stereo recordings?

    Yes. Unlike other 3D audio techniques all of which require nonstereophonic recording techniques and coding, and many more than two loudspeakers for playback, BACCH® 3D Sound is fully compatible with all existing stereo recordings, and requires a single pair of loudspeakers. Therefore audiophiles can delight in re-listening to their existing collections of stereo recordings through BACCH® 3D Sound and discover the striking spatial and tonal fidelity that was missing or marred by standard stereo playback.

    The vast majority of stereo recordings (and practically all recordings made in real acoustical spaces) contain spatial cues (inter-aural level difference, ILD cues, and inter-aural time difference, ITD cues) that would allow the ear-brain system to perceive the location of the sound source in 3D space, completely independent of the location of the speakers, if transmitted correctly to the listener. The problem (well-known among spatial audio scientists and engineers, but not well advertised in the commercial audio industry) is that crosstalk inherent to speakers-based playback limits the range of these cues at the listener’s position and the listener perceives mostly an image that is artificially anchored at the speakers with no 3D extent except for some 1-D extent (the phantom image) between the speakers. All that the BACCH filter does is remove the artifice of crosstalk (without introducing any other artifacts) so these inherent spatial cues are perceived correctly by the listener. The brain of the listener does the rest of work by interpreting these (ILD and ITD) cues to locate the perceived sound source in 3D space.

    While binaural recordings contains very realistic ILD and ITD cues (since the recording is done with microphones inside the ears of a human-like head), and are obviously hurt by the crosstalk, any acoustic stereo recording is either based on ITD cues (if recorded with spaced omni mics) or ILD cues (if recorded with ORTF, XY, coincident etc. mics) which also get corrupted if played back through speakers without removing the crosstalk. Therefore one should expect the BACCH filter to improve the spatial realism of all such stereo recordings, not only binaural ones.

    Of course in the case of music not recorded in a real acoustic place (which is the case of most non-classical/non-jazz music) the ILD or ITD cues are artificial and are due to level and/or time-based panning done by the mixing engineer. This only means that the 3D image is artificial in the first place but it still contains ILD and ITD cues and one should still expect the BACCH filter, which purifies the audio playback from crosstalk, to project the image (albeit artificially) in 3D space (as to opposed to leave it spatially confined, also artificially, between the two speakers).

    The claim that playback of regular stereo recordings is compatible with, and is greatly enhanced by, BACCH® 3D Sound was verified independently by a well-known audio critic who reported on his experience after weeks of listening to various regular stereo recordings though the BACCH filter.

  9. Is the 3D realism of BACCH® 3D Sound the same with all types of stereo recordings?

    The stereophonic recording technique that is most accurate at spatially representing an acoustic sound field is, incontestably, the so-called "binaural" recording method6, which uses a dummy head with high-quality microphone in its ears.7 Until the recent advent of BACCH® 3D Sound, the only way for an audiophile to experience the spectacular 3D realism of binaural audio was through headphones. Many such recordings exist commercially, and more have recently been made thanks to the recent rise in the popularity of headphones.

    BACCH® 3D Sound shines at reproducing binaural recordings through two loudspeakers and gives an uncannily accurate 3D reproduction that is far more stable and realistic than that obtained by playing binaural recordings through headphones.8

    All other stereophonic recordings fall on a spectrum ranging from recordings that highly preserve natural ILD and ITD cues (these include most well-made recordings of "acoustic music" such as most classical and jazz music recordings) to recordings that contain artificially constructed sounds with extreme and unnatural ILD and ITD cues (such as the pan-potted sounds on recordings from the early days of stereo). For stereo recordings that are at or near the first end of this spectrum, BACCH® 3D Sound offers the same uncanny 3D realism as for binaural recordings9. At the other end of the spectrum, the sound image would be an artificial one and the presence of extreme ILD and ITD values would, not surprisingly, lead to often spectacular sound images perceived to be located in extreme right or left stage, very near the ears of the listener or even sometimes inside of his head (whereas with standard stereo the same extreme recording would yield a mostly flat image restricted to a portion of the vertical plane between the two loudspeakers).

    Monaural recordings contain very little, if any, spatial information10 and thus are not well suited for with BACCH® 3D Sound. Therefore, it is best to bypass BACCH processing for mono recordings.11

    Luckily, many of well-made popular music recordings over the past two decades have been recorded and mastered by engineers who understand natural sound localization and construct mostly natural-like stereo images, albeit artificially, using realistic ILD and ITD values. Such recordings would give a rich and highly enjoyable 3D soundstage when reproduced through BACCH® 3D Sound.

  10. Is BACCH® 3D Sound compatible with analog audio?

    Yes. The BACCH-SP Sound processor accommodates (balanced or unbalanced) stereo analog inputs and outputs. Since the BACCH filter is a digital one and must be applied in the digital domain, the input analog signal is converted to a high-resolution using audiophile-grade A/D converters inside the processor. The processed digital signal can then be sent out as a digital signal (e.g. for an outboard converter or a digital speaker) or converted to analog using an audiophile-grade D/A converter inside the processor.

  11. Why call it "BACCH® 3D Sound"?

    The word "stereo" was always associated with three-dimensional objects or effects until its later use, in the 1950s, in the word stereophony, which, ironically, is now a term that does not invoke true three-dimensional sound in the popular mind.12 In fact, the earliest use of "stereo", which comes from the word Greek στερες, (stereos) which means solid, goes back to the 16th century when the term stereometry was coined to denote the measurement of solid or three-dimensional objects. This was followed by stereographic (17th c.), stereotype (18th c.), stereoscope (19th c.) (a viewer for producing 3D images), and stereophonic (circa 1950). Stereophonic sound, alas, remained a poor approximation of 3D audio until the recent advent of BACCH® 3D Sound, which restores to the word stereo its original 16th century 3D connotation.

    The epithet "pure" in "BACCH Stereo Purifier" refers to the purifying action of the BACCH filters, which are at the heart of BACCH® 3D Sound. A BACCH filter "purifies" the sound from crosstalk for playback on loudspeakers, without adding coloration, and purifies it also from the detrimental effects of spatial comb filtering and non-idealities of the listening room, the loudspeakers and the playback chain.


  1. By the "ideal listener in the recoding venue" we mean the actual main stereo recording microphones, or the left and right channels of the stereo master recording, which represent the left and right ear of the ideal listener in the original sound field.
  2. Sound directivity is the extent to which loudspeakers beam the sound towards the listener instead of broadcasting it in all directions around the room.
  3. They are most accurately preserved if the recording is made with a dummy head (see Q&A 7).
  4. Aside from greatly compromising the 3D image, standard stereo (and even more, surround sound), inherently suffers from the problem of comb filtering, which significantly alters the tonal content of sound, and which is due to the interference of sound waves emanating from more than one speaker.
  5. BACCH stands for “Band-Assembled Crosstalk Cancellation Hierarchy”—a name that represents the mathematical filter design method and pays tribute to the great composer with a similar sounding name.
  6. The accuracy is due to the fact that binaural audio preserves not only the correct ILD and ITD cues discussed in Q&A4, but also contains so-called “spectral cues,” which are the effects the torso, head and ears have on the frequency response and which the brain uses, in addition to ILD and ITD cues, to locate sound, especially at higher frequencies.
  7. The spatial accuracy of dummy head recording is only surpassed by recordings made with microphones placed in the listener’s own ears—alas, a rare commodity that would have benefits upon playback for only that listener.
  8. This is because binaural playback through headphones or earphones is very prone to head internalization of sound (which means that the sound is perceived to be inside the head) and requires, in order to avoid this problem, an excellent match between the geometric features of the head of the listener and those of the dummy head with which the recording was made (This problem is surmounted by the BACCH-dHP module of the BACCH-dSP software ). BACCH® 3D Sound does not suffer from this problem as the sound is played back though loudspeakers far from the listener’s ears.
  9. The 3D realism is the same although the ability of reproducing a sound source at a location that accurately corresponds to the original location is relatively decreased due to the absence of spectral cues.
  10. Some stereo recordings have overwhelmingly strong mono content and may not yield much of an improvement in spatial realism when processed through BACCH.
  11. They contain no ILD, ITD or spectral cues but may contain some distance cues in the relative magnitude of the ratio of direct-to-reflected sound of the recorded sound sources.
  12. Despite the tendency of some audiophiles and audio reviewers for describing the sound from certain hi-fi components as “three-dimensional” or ”holographic”.