Tools of the Trade – By Pat Brown
Pat Brown gives an overview of the B-Format microphone and its capabilities.
An ongoing dilemma for those doing room acoustics measurements is the choice of microphone pattern. Omni’s are most accurate, but bear no resemblance to the response of the human listener. Stereo is more “human-like,” but there exist a large number of variables regarding polar pattern, relative angle and other parameters. The ideal measurement/recording microphone would be one that allowed these variables to be chosen after recording the event.
This has long been the promise of the B-Format microphone. A B-Format microphone allows recorded data to be post-processed to yield various microphone patterns, groups of patterns, and even playback formats (i.e. stereo vs. surround sound). A few years back I published an article (3D Microphony) concerning B-Format impulse response measurements. I used a Figure-of-8 microphone in 3 orientations plus an omni to simulate a B-Format response. If you are fuzzy on the details, I suggest a re-read of that article as a prerequisite for this one. At the time I said that this technique would eventually become more practical to implement, and would have much to offer sound system designers. That time has come.
First, let me jog your memory a bit concerning B-Format. Most room measurements are done with a single omni-directional microphone placed at a listener position. This is fine for tasks such as loudspeaker equalization and setting up crossovers, but the omni mic doesn’t gather any directional information. The observer can see the room reflections in the Room Impulse Response (RIR) but cannot know the direction they are coming from. Also, if the observer wishes to convolve the RIR with anechoic material, all spaciousness and envelopment are lost, and the resultant sound is far from reality. Stereo and binaural miking techniques provide a dramatic improvement, and most of my room measurement work involves 2-channel RIRs. The “holy grail” of room measurements and recording has always been to capture the three-dimensional characteristics of the sound, preserving accurate directional info for every event heard by the listener. This is what B-Format offers, and some additional benefits come along for the ride.
One of the “carrots” of B-Format is that once gathered, B-Format data can be decoded into any of the first-order microphone polar patterns, i.e. omni, cardioid, hypercardioid, supercardioid, and Figure-of-8. It also decodes into combinations of these responses, so that I can produce the response of coincident cardioids as a post process, and even rotate, tumble, tilt or zoom the coincident pair to any angle. In short, if I have B-Format data I can distill that into almost any microphone pattern, and even combinations of those patterns with the right software and a few mouse clicks.
B-Format can also be decoded for playback from various loudspeaker configurations, including stereo, 5.1 surround and even a 3-dimensional “cube” array. Imagine placing a single mic at a listener position, recording an event, and then playing back in full 5.1 surround sound with all imaging and localization preserved – cool to say the least.
There is no question that B-Format data is the way to go, but doing it “manually” by rotating a mike is cumbersome and time-consuming. We want B-Format data, but we want to have it at no additional expenditure of time or setup complexity.
While B-Format microphones have existed for some time, most are physically large, very expensive and require supporting electronics. What has been needed is a “poor man’s” B-Format microphone.
A-Format to the Rescue
One way to get B-Format data is via A-Format. An A-Format mic is a tetrahedral array of cardioids. The photos (above) show the TetraMic™ from Core Sound www.core-sound.com. It’s actually four mics in one. As one might expect, the placement of and performance of the elements are crucial, so the mics must be hand-built to exacting tolerances and utilize individual correction curves to match the the capsule responses. The TetraMic has four outputs, so a 4-channel recorder is required. Many audio interfaces support multiple channels, but due to the tight response tolerances, digital input level control is required to retain the match of the sensitivities of the individual elements. I am using an Edirol R-44 recorder for this purpose, as it has digitally-controlled input sensitivity and can be battery-powered. The whole rig fits into a small backpack, allowing the investigator to roam the venue cable-free to select the measurement position of interest. The mic is so small and lightweight that the Trekpod functions nicely as a stand.
The combination of the TetraMic and the R-44 provides a full-blown portable A-Format measurement/recording rig at an affordable price – about $2000 complete as of this writing.
Since the TetraMic outputs A-Format, decoding is required to get B-Format, and further decoding is required to convert B-Format to the desired output format. Figure 1 (see below) shows a flow chart of the decoding process. The TetraMic comes with Visual Virtual Microphone VVMic, an easy-touse software to perform these tasks (Figure 2 – see below). VST plug-ins are also available to allow the TetraMic decode in real-time – ideal for use in studios and listening rooms.
Recording vs. Measurement
B-Format has been most often used to record live events. I recently recorded an orchestra concert with the TetraMic. When played back on a 5.1 surround system the envelopment of the live experience is faithfully preserved. My real interest has always been to use B-Format for RIR measurements, as this would preserve directional information for each event on the Envelope-Time Curve (ETC). B-Format RIRs are easily gathered by recording log-sine sweeps played over the sound system. These can be de-convolved with the inverse sweep to yield the RIR. I have written much about this technique in recent years, and I demonstrate it at each Sound Reinforcement for Designers seminar. In future articles I will cover the use of B-Format RIRs as a powerful tool for gathering acoustical data in auditoriums. pb