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Author Topic: SMAART help  (Read 29782 times)

Art Welter

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Re: SMAART help
« Reply #110 on: May 13, 2010, 01:23:19 pm »

Leo Melkote wrote on Thu, 13 May 2010 10:31

What are the criteria for ground plane measurements? If the stack is two dual 18 subs tall, can I still get accurate measurements in the ground plane?

What the microphone hears is a combination of the direct and reflected sounds, as well as on and off axis response if measuring an array. A  measurement can be “accurate”, yet not be indicative of the speaker’s actual response.

The most accurate response curve for a full range speaker would be made on axis at a large distance relative to the speaker size in a large anechoic chamber, or the speaker lifted far enough off the ground to be considered in “free space”.
However, a sub is usually used on the ground, so measurement on the ground will be  indicative of it’s actual behavior.

Measuring on the ground plane eliminates the reflected path length difference a microphone has at mid stack or head height, but when measuring close to two stacked dual 18 subs, the microphone will be hearing a large time (path length) difference between the top cone and the bottom. That stack at one or two meters would show a very different response than if the same  were measured at around 10 meters. Raising the microphone to the middle of the stack height would again show  different response curves at the two distances.

A 30 Hz wavelength is 37.5 feet long, a 100 Hz wavelength is 11.25 feet long. For the  100 Hz wave to be within 1/4 wavelength (where they combine constructively) requires a path length difference of 2.81 feet or less.
With an 8 foot high stack, to get the path length  difference down to 2.81 feet or less would require the ground plane microphone to be at around 10 feet or more from the stack. A further distance would be  required to avoid a bumpy graph above 100 Hz.

Art Welter
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Leo Melkote

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Re: SMAART help
« Reply #111 on: May 13, 2010, 02:32:59 pm »

Art Welter wrote on Thu, 13 May 2010 12:23


A 30 Hz wavelength is 37.5 feet long, a 100 Hz wavelength is 11.25 feet long. For the  100 Hz wave to be within 1/4 wavelength (where they combine constructively) requires a path length difference of 2.81 feet or less.
With an 8 foot high stack, to get the path length  difference down to 2.81 feet or less would require the ground plane microphone to be at around 10 feet or more from the stack. A further distance would be  required to avoid a bumpy graph above 100 Hz.

Art Welter


Thanks, that was very useful.
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Desmond Lee

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Re: SMAART help
« Reply #112 on: May 14, 2010, 04:26:26 pm »

Mac & Art,

both your post has gave me answers for my previous questions to Phillip. Thank you guys. Smile

Des
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Art Welter

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Re: SMAART help
« Reply #113 on: May 15, 2010, 12:12:26 am »

Desmond Lee wrote on Fri, 14 May 2010 14:26

Mac & Art,

both your post has gave me answers for my previous questions to Phillip. Thank you guys. Smile

Des

You're welcome.
Measurement is addictive.

Art Welter
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