Where should I place PZM-11LL mics for voice recognition?

I'll be using a voice-recognition system in a room 12 ft x 30 ft long and 12 feet high with bare walls, ceiling and floor. Where should I place two PZM-11LL mics in that room for best results?

Reply: A voice-recognition system works best when the mic picks up mostly your voice and not much of the room acoustics. To do that, the mic should be close to your mouth, and the room surfaces should be covered with absorbent materials if possible.

If you spend about half your time sitting and half standing near a wall, install the PZM-11LL on that wall midway between seating and standing mouth height. Maybe you can arrange your activities so that you are near a microphone most of the time.

Suppose you wander all around the room. If the two PZMs are in the middle of opposing walls, the miking distance will vary between about 2 and 16 feet -- so the amount of room acoustics in the signal will vary widely, confusing the voice-recognition system.

To make the miking distance more consistent no matter where you are in the room, try two PZMs on the ceiling, 14 feet apart, on the center line joining the two 12-foot walls (see figure below). That way, when you walk around the room, the miking distance will be fairly constant (about 7 to 10 feet from the mouth). However, 7-to-10 feet is very far from the mics. You might attach the mics to the ceiling temporarily with gaffer tape to see if they work with your voice-recognition software. If possible, carpet the floor or add some throw rugs to reduce room reverberation.

If the mics are on the walls, try to face the nearest mic when speaking. Suppose you "train" the voice-recognition system while speaking facing the mic. Later, if you speak at 30 degrees to the mic, the high frequencies ("s" sounds) will be slightly diminished -- so the recognition system might fail occasionally. However, I don't think this will be a problem.

A mic 3 feet behind your head is better than a mic 15 feet across the room. The sound-shadowing effect of the head reduces high frequencies ("s" sounds) behind the head, but the PZM-11LL has a high-frequency boost to compensate.

The mixer is not critical, just so it has enough amplification and low-enough noise for your voice-recognition system to work. PZM-11LL mics have a line-level output signal, so plug them into the mixer line inputs, not the mic inputs. Plug the mixer's line output into your sound card's line input, not the mic input.

If you can afford more mics and an automatic (gated) mixer, and you will be near the walls most of the time, install two PZMs on each 30-foot wall (14 feet apart) and one on each 12-foot wall. That way, no mic will be more than about 8 feet from you. Run all the mics into the automatic mixer. It turns on only one mic at a time -- the mic with the strongest signal. This greatly reduces background noise and room acoustics compared to having all the mics on at the same time. The Shure SCM810 is an 8-channel automatic mixer.

If possible, add carpeting, drapes, acoustic-tile ceiling, and stuffed furniture to the room. To absorb bass, get several rolls of fiberglass insulation (factory wrapped in plastic) and stack them from floor to ceiling in each corner. Cover them with muslin or burlap if looks are important.

The ideal voice-recognition system uses a headworn mic (wired or wireless). That way, the mic is very close to the mouth and picks up a uniform sound with almost no room acoustics. You do not need to treat the room acoustically if you wear a headworn mic. You could use a Crown CM-312A mic (wired) or CM-312AE mic into a wireless transmitter of your choice.