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Help with Db SPL and Dbu

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Young Croc
Young Croc
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Joined: 26 June 2005
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    Posted: 20 March 2021 at 5:49pm
Haven't posted in quyite a while.....I'm an installer more used to pulling cable and building racks etc. I'm just starting to try and teach myself programming and system set up. I'm hoping someone can help with this problem.

I'm practising by writing a system file for a BSS soundweb Blu 50. The end client wants to utliise the Ambient Noise Compensator:

https://audioarchitect.harmanpro.com/aa_help/Soundweb_London/Signal_Processing_Objects/Dynamics/Ambient_Noise_Compensator_Non_Gap.htm

This processing object will automatically adjust program material (background music) to sit at a prescribed level above the ambient/background noise level. In this instance it's for a restaurant, through some research I can see the average level of conversation is about 65db (I assume this to be SPL or an A weighted measurement). The plan being that the program material will sit 3 - 6db above this. The ambient noise level will be measured using a condensor microphone and an ambient input channel on the compensator processing object.

The threshold of the processing is measured in Dbu which I understand is a reference of voltage. What I don't understand is how to set the threshold limit using the 65db SPL as the reference point for adjustment of the program material.

Am I right in thinking I need to convert DB SPL to DBu? If so how do I go about doing this?

Thanks to anyone who can heklp me with this problem!
Bassmentality: Ska, reggae, funk & Hip hop every month @ The Cellar, Oxford
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DMorison View Drop Down
Old Croc
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DMorison Quote  Post ReplyReply Direct Link To This Post Posted: 21 March 2021 at 2:03pm
The replies to your equivalent thread over at the LAB have pretty much covered this, namely you need to be using a processor that can be set to subtract a copy of the music signal from the mic input before using that for the compensation, otherwise you'll risk a runaway feedback loop. 
This process is also known as AEC (Automatic Echo Cancellation) and I believe one of the posters on the LAB pointed out that the BLU50 wasn't far enough up in the BSS range to include this feature.
If you do go with a processor with AEC, you'll be far better just calibrating in situ rather than trying to calculate as there are so many variables - the mic's sensitivity, how much gain (if any) the mic input of the processor adds, how representative the sound at the mic is WRT the average sound at the customers' locations etc which would all need factored into such a calculation.

HTH,
David.
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