3D Microphony

By Pat Brown

Most effective multichannel microphone techniques were developed as a means of making pleasing recordings of live events. Since recording is as much art as science, many approaches have emerged that produce satisfying results. One of the main objectives of a good recording is the preservation of the ambiance and spaciousness of the listening space. This is increasingly becoming an objective for those measuring room acoustics, so many practitioners are borrowing multichannel microphone techniques from the recording world.

In auditoriums, sound can impact a listener position from any angle. The arrival times and relative levels are contained in the Room Impulse Response (RIR) and can be identified using the Energy Time Curve (ETC) in its various forms. When these are measured with an omnidirectional microphone the direction of wave propagation is lost. Two-channel measurements retain some horizontal directional information but no vertical information. These contain vastly more information that omnidirectional RIRs, and can produce very satisfactory results. A host of successful stereo miking techniques have been developed over the years. These include

• XY Cardioid
• Coincident Cardioids
• Split Cardioids
• Spaced Omnis
• MS Stereo

and a few others. Each method has its own set of pros and cons, which is why most practitioners have their favorites. Even with stereo miking techniques, vertical information is largely lost.

Other 2-channel microphone techniques emulate the human listener. These include binaural head and In- The-Ear™ ITE methods.

Multichannel acoustical measurements are especially of interest in system troubleshooting, where it may be necessary to know both the arrival time and direction of a loudspeaker response or room reflection. Many modern auditoriums have specular echoes that require spot treatment, so the ability to identify the surface producing a specific reflection is quite useful. The TEF Polar ETC  method described in Dale’s article is one such method.

What may prove to be the most versatile method of all for capturing spatial information was developed in the late 1960’s. It has been quite alive in the UK, France and Australia but never quite caught on in the US. That is changing and this method is finally getting the attention that it deserves.

Ambisonics and B-Format

Ambisonics was conceived in the late 1960s as a complete recording and reproduction system capable of recreating accurate three-dimensional sound stages from original recordings. The format was developed using complex mathematics and psycho-acoustics, all based on the original work on coincident stereo led by Alan Blumlein in the early 1930s.

Ambisonics failed to attract the significant attention of either manufacturers or the public in the ‘70s, principally because too many companies had their fingers burned by the quadraphonic fiasco. Fortunately, the numerous strengths of the system were recognized by enough professionals to keep it alive, if only on the periphery of the industry, and today it is beginning to receive the attention it truly deserves. Even though the technology is over 30 years old, it stands up to scrutiny, just like Blumlein’s coincident microphone techniques. A capable and mature way of capturing this direction-dependent information is through use of a B-Format microphone.1

Ambisonic technology is based on a meta-theory (a theory of theories) of sound localization developed by the late Michael A. Gerzon.

With Ambisonic technology, the directionality of the sound field is composed of spherical harmonic components. The zero-order component is termed W and is omnidirectional. The first-order components are figure-of- eight (lemniscate) responses which point forward, left and up. These are termed X, Y and Z, respectively. In practice, second-order and higher components are ignored. The W, X, Y and Z channels are collectively called B-Format.

The fact that the Z component can be recorded creates the opportunity for periphonic (full-sphere) reproduction. Periphony requires speakers to be placed and below the height of the listeners’ ears.

Readers familiar with microphone techniques will realize that the W and Y spherical harmonic components are equivalent to the M and S components of the MS stereo recording technique. Ambisonics is a natural extension of this recording technique to three dimensions. 2

The B-Format mic is actually an array of microphones. The typical implementation requires three figure-of-8 patterns and an omni arranged according to Figure 1.

• Mic 1- omni (W)
• Mic 2 – X-axis (front/back)
• Mic 3 – Y-axis (left/right)
• Mic 4 – Z-axis (up/down)

B-Format therefore consists of a WXYZ response. B-Format recordings theoretically retain all of the first order directional information of the sound. One benefit is that it allows the response of any first order microphone pattern (aimed in any direction) to be derived as a post-process. It is a popular method for recording live events that must be converted to 5.1 and the other surround formats. It has been used successfully for measurement work, but not on a widespread basis.

Where Do I Get One?

Commercial B-Format microphones are available, the most popular of which are the very high end Soundfield™ microphones www.soundfield.com. These are relatively expensive for casual use, but the theory is solid and lower cost alternatives are emerging in the marketplace. Soundfield has been unofficially adapted as the generic name for a B-Format microphone, with uppercase “S” indicating the brand and lowercase the B-Format mic in general.

It is also possible for motivated individuals to fabricate their own B-Format microphone. There is a wealth of information on the Internet describing various approaches in all price ranges.

A Manual Approach

B-Format recordings of live events require a soundfield microphone, whether commercial or home grown, since the WXYZ channels must be recorded simultaneously.

B-Format RIRs can be made with some readily available tools, including an omni microphone, a figure-of- 8 microphone and any single channel recorder. Figure 2 shows some WXYZ RIRs made in my office. The method described by Dale Shirk for PET could also be used, with the mic set for BiDi pattern.

Playback

One of the most attractive aspects of B-Format recordings is the large number of possible playback methods. The desired method can be determined as a post-process.

A B-Format decoder can process the WXYZ signals into mono, stereo, LCR, or surround. The surround options are endless, with a host multi-loudspeaker playback possibilities. Both hardware and software decoding is possible (Figure 3).

Conclusion

B-Format has never received the acceptance that it deserves. As is often true with technology, one can get there first and lose. But good theories don’t go away and B-Format may emerge as a primary method for collecting room impulse responses. pb

1 – Published in Sound-On-Sound October 2001

2 – http://members.tripod.com/martin_leese/Ambisonic/faq_latest.html