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PZM-11LL Generalities


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#1 will1234

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Posted 30 October 2006 - 10:26 AM

Hello all,

I would say that I am an audio novice but that would be an egregious overstatement.  What I am searching for, if any exist, are general rules of thumb regarding PZM microphone placement in an open room.  More specifically, I ordered 2 PZM-11LL mics for use in an open-air voice recognition system.  I pretty sure that this is a ‘that depends topic’ where everything influences everything else but I need a starting point of some kind.  I have bookmarked the technical resource documents on the Crown web site have not had time to read through them yet.  Also, I have been told that the room I am in is a worst case scenario with flat walls with nothing on them, hard wood floors, a 12 foot ceiling, and measuring in the neighborhood of 12’ x 30’.  I know this is probably going to be a lot of trial and error work but want to at least eliminate the ‘audio common sense’ variables.

Should the mics be installed at wall switch height, head height while sitting, while standing?  Is there any variance to where the mic is in relation to the voice?  For example, is it better to speak directly at the mic or would work as well with my head pointed 30 degrees to one side?  Would it be better to have one 3 feet behind my head or 15 feet across the room?

What about a mixer?  I currently have a cheap $40 mixer (you can probably guess where it was purchased) which helped while trying to use all of the wrong mics but have to believe that there is a different one at a less than heart stopping price somewhere.  I’ve heard of people using automatic mixers.  Is this the silver bullet that the name implies?

See, audio novice is an egregious overstatement!

Thanks in advance,
Rob

#2 brubart

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Posted 03 November 2006 - 11:11 AM

Hi Rob,

Thanks for asking. This subject is confusing to a lot of people, newbie or not!

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. 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.

Good luck,
brubart

#3 will1234

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Posted 03 November 2006 - 12:23 PM

Thank you so much for the reply.  From where I was this has propelled me light years ahead.....