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Old July 2nd, 2005, 09:07 AM   #1
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How much should i attenuate?

Hi!

I have a Audio-Techninca AT835ST that feeds my cameras XLR-box, Panasonic AG-MYA30.

The problem is that the mic has a strong output and overloads the XLR-box when recording loud audio such as a rock concert. I am going to build attenuators that can be placed in the mic's XLR contacts but i dont know how many dB's i should attenuate.

The mic has three settings:
-30 dB (31.6 mV) - (Mid-Side)
-34 dB (19.9 mV) - (L/R Wide)
-36 dB (15.8 mV) - (L/R Narrow)

The XLR-box's specifications are:
-50 dBu/-60dBu (without or with the XLR-box's attenuator)

So, how many dB's should i attenuate to match the XLR-box's inputs when the mic is in Mid-Side-mode, -30dB (31.6 mV) and when the mic is in L/R-Wide-mode, -34dB (19.9 mV) when the XLR-box is set to -50dBu?

I am aware of AT's AT8202 Adjustable In-Line Attenuator but they are to long. Thats why i prefer to build the attenuator directly in the XLR-contacts.

I have tried to contact Panasonic to get more detailed specificatons on my XLR-box, but with no success. The specifications i have given here about it are taken from the manual that came with it.

Help appreciated! :)

/Roger

Last edited by Roger Averdahl; July 2nd, 2005 at 10:33 AM.
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Old July 2nd, 2005, 11:59 AM   #2
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Yeah, it's tough to get this kind of info from the manufacturer because the type of people who read your request are so many organizational layers away from theoretical design aspects that they don't even know who to go to.

Bottom line, -30 to -40 dB of attenuation should be right for close up recording of a painfully loud rock concert with your equipment, but I'd recommend building -20 dB and -10 dB attenuators for your kit as well.

Here are the details supporting that recommendation:

First of all, you can simply add and subtract dB levels in a common sense way to match components, but only when you know what is being specified.

Microphone specs
Microphone voltage output tracks with the amplitude of sound pressure. Both are expressed in dB, where +20 dB represents a tenfold increase in voltage or sound level. The mic outputs are specified for 1 Pa (Pascal), which is 94 dB of sound pressure. That's about the sound level of a lawnmower passing by at a few meters, or the level where its almost impossible to be heard by a person next to you even if you shout. For rock concert venues where you're close to the loudspeakers, the sound pressure is even higher, typically (according to most charts I've seen) about 110 dB. The mid side output of your mic would then be -30 dB plus the difference in sound pressure or -30 + (110-94) = -14 dB. So you're going to have to attenuate that -14 dB level down to whatever the receiving device expects.

Recording device specs
Unless otherwise specified, published input levels for devices in the recording chain are usually nominal, or the levels that will result in an average recording level of about -12 dB. A little curve your XLR box is throwing at you is that it is specifed in dBu. To convert dBu to dB, subtract 2.2. So when set for -50 dBu, your XLR box is expecting a -52 dB signal.

So theoretically you need -52 -(-14) = -38 dB of attenuation. Designing for -40 dB is close enough. This assumes that 110 dB is the average sound level. If it's actually the max, then -30 dB attenuation would be sufficient.

Many of your loud situations, including some rock concerts, will be less loud than we've discussed and even -30 will give you undesireably low recording levels during quieter passages. So you should be prepared to use less attenuation. Hence the recommendation to build a set of attenuators.
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Last edited by David Ennis; July 3rd, 2005 at 08:57 AM. Reason: To correct an arithmetic error
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Old July 4th, 2005, 09:12 AM   #3
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Hi Fred!

Thanks for the detailed answer, i really appreciate it! :)

I think the best solution for me is to buy a pair of AT's AT8202 Adjustable In-Line Attenuator. Itīs better than to have a fixed attenuation of the mic that sometimes is to much and sometimes to little.

The AT AT8202 Adjustable In-Line Attenuator can be set to either -10dB, -20dB or -30dB.

/Roger
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Old July 4th, 2005, 10:44 AM   #4
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Roger,
In practice, somehow there seems to be more lattitude than the calculations above indicate. Sometimes my initial guess is wrong, but once I set the right level of attenuation I don't find I have to change it during a given event. So your plan to build attentuated connectors could still work.

On the other hand, I have a pair of 8202s myself and like them very much. If you're concerned about their protrusion you might get a one or two foot XLR cable to plug into the camera and put the antentuator(s) between that and the feed. It would add a bit more stuff to secure if you're going handheld, but would be fine with a tripod. I normally gaf my cables to mic stands and tripod legs for stress relief anyway.
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Old July 11th, 2005, 04:39 PM   #5
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I have a pair of 8202's that I use with my 835ST also. I tend to use -20 for doing drum corps on the field and -10 up at press box level through a Beachtek DXA-8 with success.

Additional advantage of these is that they work with phantom voltage where some straight attenuators do not.
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Old July 12th, 2005, 06:26 AM   #6
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Quote:
Originally Posted by Roger Averdahl
Hi Fred!

Thanks for the detailed answer, i really appreciate it! :)

I think the best solution for me is to buy a pair of AT's AT8202 Adjustable In-Line Attenuator. Itīs better than to have a fixed attenuation of the mic that sometimes is to much and sometimes to little.

The AT AT8202 Adjustable In-Line Attenuator can be set to either -10dB, -20dB or -30dB.

/Roger

what XLR box do you refer to? I've yet to see a professional mixer overloaded by ANY mic.

Regards,

Ty Ford
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Old July 12th, 2005, 08:01 AM   #7
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He's talking about the Panasonic XLR box for his specific camera that has only -50db or -60db mic-level input. See the link he gave.
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Old July 12th, 2005, 12:31 PM   #8
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George:
I have ordered one pair of the 8202's. I recorded i rock band and realised that i really must have them to avoid distortion. I found out the hard way...

Ty:
It is Panasonics AG-MYA30 XLR box i am reffering to.

/Roger :)
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