| 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?
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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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Topic: SMAART help (Read 37190 times)