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RTA measurements - the real deal

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Saul View Drop Down
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    Posted: 30 March 2010 at 6:34pm
ok this is a thread to clarify between everyone, the correct way to take RTA measurements of different cabinets. 

So for bass bins: i have been RTA'ing with 1m distance from edge of box, and the microphone as close to the ground as possible, but not touching. Any objections? 

mids + tops: 1m again, but with the mic centered in front of the box - 4pi style. 

Ive seen some whacky old measuring styles, hence me writing this, so we're all clear. 

another note: if the mic can't take the SPL, measure at 2m distance, and add 6dB to the result, iirc - must check this though - cant quite remember. 

i remember a thread where rog went through a fairly detailed process - any one know the one i'm talking about? - I think it was following pfly's measurements of 1850 horns.. 



obviously for ultra real measurements, you'd do it in a  anechoic chamber 


Edited by Saul - 30 March 2010 at 6:38pm
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Sinfinity View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Sinfinity Quote  Post ReplyReply Direct Link To This Post Posted: 30 March 2010 at 11:36pm
this one ?
http://forum.speakerplans.com/1850-horn-measured-plots_topic26669_page2.html

Matt at ukamp dot co dot uk
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Saul View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Saul Quote  Post ReplyReply Direct Link To This Post Posted: 30 March 2010 at 11:40pm
perfect
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Post Options Post Options   Thanks (0) Thanks(0)   Quote VPAS Quote  Post ReplyReply Direct Link To This Post Posted: 31 March 2010 at 1:35pm
nice, on this note...
Originally posted by Saul Saul wrote:


another note: if the mic can't take the SPL, measure at 2m distance, and add 6dB to the result, iirc - must check this though - cant quite remember. 


As i undestand it, levels dont follow the inverse^2 law in the near field. For subs it generally aplies from around a meter - 2 meters away.
and stacking multiples will extend the near field

In fact, measureing in the near field is prone to wildey unpredictable fluctuations. (even getting louder with marginal shift away from the subs)

So you could argue that you are better off measuring in the far feild and calculating the 1m response... but, if the near field is unpredictable can you calculate the levels correctly? is the 1m response even worth paying attention to?


hmmm, my head hertz

n^2 modulo -P = number wang
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Timebomb Quote  Post ReplyReply Direct Link To This Post Posted: 31 March 2010 at 3:04pm
Takeing absolute measurements is never going to be all that useful if you don't have very accurate kit and conditions to measure in, as they are meaningless until you compare them to something else, and that something else has to be measured the same way in the same conditions to give an meaningful insight.

It is kind of standard for bass cabs to measure at 10 meters at 100w, nearfeild measurements will be off for the reasons VPAS mentioned.

How you define 100W ia a question, id probably go for 90V RMS with a sine wave run into a dummy load, you cant just stick a meter on the output of an amp because the output will drop when you add a load.  I would look into how others do it though, not everyone does it the same way.... and if they dont do it the same way then comparing your result to theirs is a bit pointless

Another way to do it it is gat the 2 cabs you want to compare and measure one with a signal, then measure the other one with the same signal, that way you get a useful comparison. How you mesure / what voltage you use / what spl the meter says is less important, what can be useful is the relative difference.
James Secker          facebook.com/soundgearuk
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Post Options Post Options   Thanks (0) Thanks(0)   Quote torch1 Quote  Post ReplyReply Direct Link To This Post Posted: 31 March 2010 at 3:16pm
Are there places you can go to get absolute measurements and if so, does anyone have any idea of the costs involved?
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Saul View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Saul Quote  Post ReplyReply Direct Link To This Post Posted: 31 March 2010 at 3:21pm
universities usually have anechoic chamber facilities - but probably v expensive, if they'd let you at all
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