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Reliable frequency range for ground plane measurements
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There appears to be a discrpancy in the upper reliability reported in two sources.
I read in D'Appolito that the reliable upper frequency is:
for 1 dB error: 25.7*10^4/(l*m)
for 3 dB error: =42.9*10^4/(l*m)
where:
l = Distance from ground to the of center of the driver (cm)
m = Height of microphone (cm)
I also see on the Audioholics sight the formula:
IpI=2*A/r*sin((2*pi*b/l)*sin θ)/(2*sin(pi*b/l)*sin θ)
where:
|p| = magnitude of the rms sound pressureA = magnitude of the rms sound pressure at unit distance from the center of each sourcer = measurement distanceb = distance between the center of actual and virtual acoustic imagel = wavelength of frequency under considerationθ = angle to the perpendicular bisecting the actual and virtual acoustic images
I realize that the latter formula is calculating sound pressure from two non completely correlated sources but in calculating at different frequencies, I start to see a significant error by a few hundred hertz whereby D'Appolito's formula suggests that with a small enough baffle on the ground and enough reflection free clearance, you could get full range measurements. For example, for a 1 dB error with a 40 cm baffle and the mic center 0.5 cm above the ground, you get measurements to 25,700 Hz. I would love some insights and guidance from anyone.
Thanks,
__________________
Jay
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There appears to be a discrpancy in the upper reliability reported in two sources.
I read in D'Appolito that the reliable upper frequency is:
for 1 dB error: 25.7*10^4/(l*m)
for 3 dB error: =42.9*10^4/(l*m)
where:
l = Distance from ground to the of center of the driver (cm)
m = Height of microphone (cm)
I also see on the Audioholics sight the formula:
IpI=2*A/r*sin((2*pi*b/l)*sin θ)/(2*sin(pi*b/l)*sin θ)
where:
|p| = magnitude of the rms sound pressureA = magnitude of the rms sound pressure at unit distance from the center of each sourcer = measurement distanceb = distance between the center of actual and virtual acoustic imagel = wavelength of frequency under considerationθ = angle to the perpendicular bisecting the actual and virtual acoustic images
I realize that the latter formula is calculating sound pressure from two non completely correlated sources but in calculating at different frequencies, I start to see a significant error by a few hundred hertz whereby D'Appolito's formula suggests that with a small enough baffle on the ground and enough reflection free clearance, you could get full range measurements. For example, for a 1 dB error with a 40 cm baffle and the mic center 0.5 cm above the ground, you get measurements to 25,700 Hz. I would love some insights and guidance from anyone.
Thanks,
__________________
Jay