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Old December 1st, 2013, 11:23 PM   #16
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Re: Boundary microphone advice

Quote:
... the output from a barrier microphone on a carpeted floor will have
a marked reduction of high-frequency sensitivity.
Aha, that's exactly what I suspected. So, unfortunately, this will require a little more thought if Mr. McDonald's mics are located on a carpeted chancel.

Quote:
Originally Posted by Colin McDonald
... a mic such as the Barlett recording mic comes mounted on a steel plate (13 cm long x 7.6 cm wide) so will that provide some offset for this HF loss?
For example, by my calculations, a 5KHz sound has a wavelength of .06864m so the steel plate might still be able to reinforce sounds of that kind of pitch or above.
I found an interesting paper here: https://pantherfile.uwm.edu/type/www...yMicsStudy.htm
which cites that "4 foot - 40 Hz rule." The formula given for the -6dB point, for a "too small" boundary area, is this: f(-6dB) = 188/D, where D is the side length in feet of a square boundary. Thus for a boundary plate 4' x 4' , D=4, so the -6dB point is 188/4 = 47Hz. (But they round this off to 40 Hz for mnemonic purposes.)

Can we roughly apply this for the dimensions of the Bartlett plate? 13cm = 5.1in = 0.43 ft. 7.6cm = 3in = .25ft. The area of the plate is 0.11 sq. ft. A square plate with that area would be 0.33ft on a side. So for the equation given, the -6dB point would be 188/.33 = 560 Hz.

The transition frequency, where LF rolloff begins, is given as 750/D. With the above dimensions, the mic would be flat down to 2,250 Hz. I would guess that a lot of carpet begins attenuating at a lower frequency than that. But if you make the plate somewhat bigger, say 1ft square, then the transition frequency moves down to 750 Hz, and the -6dB point moves down to 180 Hz. I would guess that's well below any significant attenuation from most carpet.

OK, there are a lot of assumptions and averaging there, but if all of that is true, could Mr. McDonald live with a boundary plate 1 ft square on each mic? It could, of course, be painted in a flat color that closely approximates the color of the carpeting. "Invisible"? No. "Unobtrusive"? I'd say yes. Out of everyone's sight lines? Most surely.

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When you stop to think about it, sound technology is lagging far behind picture technology. The picture guys have perfectly silent cameras... why don't we have invisible microphones?
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Old December 2nd, 2013, 12:23 AM   #17
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Re: Boundary microphone advice

This is a very unscientific comparison between 6 Shure SM-81s against 1 Bartlett TM-125 on a carpeted stage, in a high church of a children's choir concert I shot last night. Raw footage, no color correction or eq applied yet.

Microphone test - YouTube

Last edited by Warren Kawamoto; December 2nd, 2013 at 12:55 AM.
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Old December 2nd, 2013, 07:42 AM   #18
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Re: Boundary microphone advice

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Originally Posted by Paul R Johnson View Post
The turtles are great products, as I mentioned above, but they are not cheap!
But *very* much cheaper than a dedicated top end boundary mic. like the Neumann GFM 132 (which was about £2k +VAT when I last looked) and the Schoeps BLM3.

The reason I like the Turtle so much is that you can use any top-end SDC mic. that you already have when you need a boundary mic., and still use the mic. normally when you don't.
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Old December 3rd, 2013, 01:59 AM   #19
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Re: Boundary microphone advice

Quote:
Originally Posted by Warren Kawamoto View Post
This is a very unscientific comparison between 6 Shure SM-81s against 1 Bartlett TM-125 on a carpeted stage, in a high church of a children's choir concert I shot last night. Raw footage, no color correction or eq applied yet.

Microphone test - YouTube
Thank you Warren, that was most interesting.The comparison is very useful - I've watched and listened to it a good few times.
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Old January 18th, 2014, 08:57 AM   #20
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Re: Boundary microphone advice

If you are using floor mics on a carpeted stage, the high frequencies tend to be absorbed by the carpet, causing a dull or muffled sound. It helps to put the mic on a hard, foot-square panel such as 1/8” thick masonite, plywood, Plexiglas or Lucite. That keeps the sound clear.

Even though the floor mic has its own metal boundary built in, there is still some loss in high-frequency response when the mic is placed on a carpet.

The attached graph shows how the frequency response of a floor mic CHANGES when you move the mic from a hard panel to directly on top of ½” thick carpet. Specifically, there's a 5.5 dB loss at 5kHz and a 4 dB loss at 10 kHz. (The effect varies with the carpet thickness). The high frequencies are diminished, weakening the sibilance (“s” sounds) in the actor’s voices. So be sure to place a hard, thin panel under each mic when the stage is carpeted.

Quote:
Originally Posted by Colin McDonald View Post
After a bit of digging about, I found this PDF on Boundary mics from ElectroVoice
and it has this to say at the bottom of page 2 on the subject of carpets and boundary mic placement:


Fair enough, but what I'm wondering is that a mic such as the Barlett recording mic comes mounted on a steel plate (13 cm long x 7.6 cm wide) so will that provide some offset for this HF loss?
For example, by my calculations, a 5KHz sound has a wavelength of .06864m so the steel plate might still be able to reinforce sounds of that kind of pitch or above.
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Old January 18th, 2014, 09:22 AM   #21
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Re: Boundary microphone advice

Has anyone used this DPA boundary layer mount? I is meant to work with the DPA 4060/4061 tie microphone.
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Old January 19th, 2014, 11:52 AM   #22
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Re: Boundary microphone advice

I'm intrigued by the carpet 'issue' I have never encountered any drop in performance from boundary mics on carpet. Surely the entire point is that having the microphone on the boundary layer prevents refections from the boundary surface being captured by the mic. With the mic in the surface, these reflections cannot happen. So with a table, or a stage - the big reflective surface is effectively removed. Carpet doesn't change this? - it attenuates HF reflections of course, but these reflections don't hit the mic. Carpet also helps footfall noise.

Bruce is the expert, of course - but although the carpeted surface alters the sound in the room, I've never found much impact (if any) on the boundary mic sound.
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Old January 20th, 2014, 12:27 AM   #23
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Re: Boundary microphone advice

I have a pair of the DPA mics with the boundary mounts. One of them sort of lives under the lid of my wife's grand piano. It works OK, but I think being so close to the strings gives a more percussive sound which would be fine for some types of music but isn't my first choice for classical. I prefer the sound with an open lid and the mics at least a few feet away, but when that isn't an option for whatever reason I use the DPA.
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Old January 20th, 2014, 10:05 AM   #24
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Re: Boundary microphone advice

Quote:
Originally Posted by Paul R Johnson View Post
I'm intrigued by the carpet 'issue' I have never encountered any drop in performance from boundary mics on carpet. Surely the entire point is that having the microphone on the boundary layer prevents refections from the boundary surface being captured by the mic. With the mic in the surface, these reflections cannot happen. So with a table, or a stage - the big reflective surface is effectively removed. Carpet doesn't change this? - it attenuates HF reflections of course, but these reflections don't hit the mic. Carpet also helps footfall noise.

Bruce is the expert, of course - but although the carpeted surface alters the sound in the room, I've never found much impact (if any) on the boundary mic sound.
Note that many (most?) "boundary mics" come with their own reflective surface of some limited size. One could argue that at high frequencies, the mic's own boundary is in play, while at lower frequencies, the wavelength exceeds the mics hard surface diameter.
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Old January 20th, 2014, 11:09 AM   #25
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Re: Boundary microphone advice

Mr. Crowley,

That is exactly what's stated in the link that I posted above (post #16). So if the external surface is somewhat absorptive (carpet) rather than entirely reflective (e.g. hard floor) then it won't act as a proper boundary.
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Old January 20th, 2014, 01:19 PM   #26
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Re: Boundary microphone advice

Colin , I just noticed this thread and don't know if you are still looking for the boundary mic , noting you started the thread at the end of November ?

Anyway , I have an Audio Technica boundary mic ( can't remember the model but will check when I get home ) which I have used for recording interviews and it works quite well ; I also have a cheap and cheerful Realistic PZM . At work we have a Shure boundary mic , again I will need to check the model but it is very similar in appearance to the Audio Technica .

If you are still needing one of these you are very welcome to try one out .

PS , what about a miniature rifle mic such as the C747 or suchlike - almost invisible and might just work pointing up at an angle from the floor ?

Edit - just checked and the Audio Technica is an AT-851a , it comes with the AT 8531 power module which has XLR output .

I also have an AT-935 lectern mic on a short gooseneck ; this again is a miniature rifle mic and fairly inconspicuous , being about the size of a pencil and black , with the AT 8533 phantom powering module .

You are welcome to try either of these if you think they might work for you ?

Just send me a PM if you want to try them .

Last edited by Derek Heeps; January 20th, 2014 at 01:53 PM.
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