Topic: PZM/boundary mics on ceiling? Taping in rooms with low ceilings in general

[poorlyconditioned]

I heard somewhere of a trick of putting a pair of spaced PZM or boundary mics taped to the ceiling.

Anyone try this?  I tried tonight with a pair of AT871 (boundary mics, omni, based on an AT853 capsule... what else?).  It was a rectangular room, about 20' wide and 30' long, with a low ceiling, maybe 8 or 9', and standard "acoustical" tile (what does that mean, anyway).  My mics were only about 5' apart, about 2/3 of the way back in the room.  Band was about 20' away.  I compared to a set of hypers (CK93 actives) in roughly ORTF hanging about 12" from the ceiling.

I noticed that the hypers had a lack of high end, while the boundary mics had more highs.  Can anyone explain this?  Is it the low ceiling, or are the mics too close to the ceiling (12")?  I've noticed this in other venues with low ceilings too.  I have to get my mics (hypers or cards) really close to avoid loss of highs.  I'm still digesting the ceiling sound.

So, two questions:
- does anyone like boundary mics on the ceiling?
- does anyone know why I lose high end quickly as I move my regular mics back?

Thanks,
  Richard

[poorlyconditioned]

bump.

Surely *someone* else has tried putting PZM/boundary mics on the ceiling!  Did you like it?

Certainly it is a good way to get up close without blocking the view.

  Richard

[Church-Audio]

The increased top end might be from the loading of the pzm capsule. Some pzm's have the capsule pointing down at the boundary yours is what is called a phase coherent cardioid the capsule is pointed across the boundary. There is a little lip where the pzm meets the surface of the ceiling this bump, caused a pump in the high end of the mic (more sibilance) that is where you’re getting the high end from. If you were to make the surface at the same exact height as the capsule and make it so it was counter sunk into the ceiling this high end loading would not exist.. It would make the high end much more flat but you still have the floor reflections to deal with.

If you want to analyze it further you can say that because the wave length is shorter the higher the frequency is, any disruption in the flow of sound as it meets the capsule will act as a acoustic filter adding or subtracting (phase cancellation) of mostly high frequency’s only because they are more directional.

The size of the boundary increases or decreases low end bigger = better for PZM so ceiling mount is very cool but there is one more effect that is the direct refection from the floor this is the "unknown" factor because it is subject to the absorption coefficient of the material the floor is made from IE: Marble floor= more high end then you can handle at the ceiling, wood floor a more warmer sound, concrete more of an echoey sound. This is one of the reasons I like PZM in the kick drum but I only like the one that is "phase coherent cardioid" like yours because it faces the drum head and captures more of the sound of the whole drum. PZM make great room mics.
 
Chris Church

bump.

Surely *someone* else has tried putting PZM/boundary mics on the ceiling!  Did you like it?

Certainly it is a good way to get up close without blocking the view.

  Richard

[rocksuitcase]

We used to use PZM's ( Crown, Radio Shack) on the stage footing to pick up dancerrs feet when I did Live PA work for Irish festivals.  Workes excellent for that purpose.  Later in 1988, -1991 we tried a combination of PZM's on the floor or walls of a small stage in Dallas at Club Da da.  They would let us use the snake to pass the signal to the board then mix with a 2 channel SBD feed and that was our matrix on the fly as it was recorded on 2 channels only.  Usually we would have 1 PZM at either corner of the stage, farthest away from the vocal monitors.

As Chris is saying, the boundary effects are the reason why teh high end seems exaggerated, the same reason PZM's are good at what they do is why they are bad at what we do unless applied perfectly.

Many tapers have tried the Crown PZM "head" or simply placed PZM's on a plexi glass sheet and thrown the whoel sheet up on a stand.  I remember seeing some dude do this with a 4 foot by 4 foot plexi behing EACH mic for the Phish shows in 1999 at Oswego NY.  Taoers section guys would ocassionally try this at Grateful Dead shows too.

Sound pressure drops about 3 db for every 10 feet away from the source you place the mics.  For Hypercards this effect may be more pronounced depending on the pattern of placement and other effects such as absorption and reflection characteristics of the room.

Does this same high end loss happen in other venues with approximately the same set up?  IF not, then it may be the room youi are referring to in your original post is highly absorptive and therefore reduces HF's at a greater rate than a less absorptive room.

Acoustics is tough to explain in person, or on a blackboard, this type of forum can be even harder.!!!!  I hope this helps.