I found a formula on the Lansing Heritage site, derived by Frank Massa working for RCA at the time in the 1930's for calculating excursion of a compression driver.
I don't however know if the 'area' of the diaphragm is taken as a flat piston or considering the area of the dome. Anyone know which is correct? I am assuming that it is considered a flat disc when calculating the area.
"P = .0034 X f squared X d squared X Ap squared, all the foregoing divided by Ah"
Where:
Ap = diaphragm area in square inches
Ah = throat area in square inches
f = frequency in Hz.
d = peak diaphragm amplitude in inches
P = power output in acoustical watts
Assuming a 3" diaphragm compression driver diaphragm at 800 hz, given 0.060" to the phase plug ( I haven't measured that yet ) and a throat of 1/10th the 3" diaphragm ( 10:1 compression ) I get in the neighborhood of 78 acoustical watts at the point where the diaphragm hits the phase plug.
P = 0.0034 x 640,000 x 0.0036 x 49.96487 divided by 4.99648 = 78.33 Acoustic watts
The formula explicitly states that the area of the slots in the phase plug is considered the throat for the calculation of throat area.
If we say 75 peak acoustical watts, that works out to 138.75 dB sound power level at the phase plug exit. Coupled to a horn with a directivity Q of 10, and measured at a distance of 3 meters, this calculator: Sound power level SWL and sound pressure level SPL distance compare acoustic power sound source noise Conversion of sound pressure to sound intensity conversion sound level energy level strength directivity factor coefficient sound intensity SIL - se
The result of plugging in the above numbers says 128.21 dB pressure level at 3 meters distance.
Am I way off base, or does this seem correct? The driver in question has a 3" diaphragm and a 1.4" exit.
I don't however know if the 'area' of the diaphragm is taken as a flat piston or considering the area of the dome. Anyone know which is correct? I am assuming that it is considered a flat disc when calculating the area.
"P = .0034 X f squared X d squared X Ap squared, all the foregoing divided by Ah"
Where:
Ap = diaphragm area in square inches
Ah = throat area in square inches
f = frequency in Hz.
d = peak diaphragm amplitude in inches
P = power output in acoustical watts
Assuming a 3" diaphragm compression driver diaphragm at 800 hz, given 0.060" to the phase plug ( I haven't measured that yet ) and a throat of 1/10th the 3" diaphragm ( 10:1 compression ) I get in the neighborhood of 78 acoustical watts at the point where the diaphragm hits the phase plug.
P = 0.0034 x 640,000 x 0.0036 x 49.96487 divided by 4.99648 = 78.33 Acoustic watts
The formula explicitly states that the area of the slots in the phase plug is considered the throat for the calculation of throat area.
If we say 75 peak acoustical watts, that works out to 138.75 dB sound power level at the phase plug exit. Coupled to a horn with a directivity Q of 10, and measured at a distance of 3 meters, this calculator: Sound power level SWL and sound pressure level SPL distance compare acoustic power sound source noise Conversion of sound pressure to sound intensity conversion sound level energy level strength directivity factor coefficient sound intensity SIL - se
The result of plugging in the above numbers says 128.21 dB pressure level at 3 meters distance.
Am I way off base, or does this seem correct? The driver in question has a 3" diaphragm and a 1.4" exit.
"Given 0.060" to the phase plug" would be off by quite a bit (almost 6dB) as .06"=1.524mm, and HF compression drivers have less than 1mm (.8mm would be "kinda safe") between diaphragm and phase plug.
That said, using an 8 ohm B&C 3" diaphragm 1.4" exit driver, with a 1250 Hz 24dB per octave crossover on high DI horn (13 x 13 degree conical, about 117dB one watt one meter) measured the following using pink noise:
110 dBA 2 meters
115.2 dBA 1meter
119.7 dBA .5 meter, 19.5”
122 .5 dBA .25 meter, 9.75”
124.2 dBA .125 meter, 4.875”
126.3 dBA at horn mouth, 26” from throat screen
131.7 dBA 13” from screen
136.8 dBA 6.5” from screen
141.0 dBA 3.25” from screen
143.7 dBA at screen (144+ on impulse, flat) (about 8.2 V )
149.1 dBA at screen xover output turned up to +11dB (+6.5 dB over last test, reads 17.5V)
That level also happened to be just about the maximum clean SPL for the B&K 4004 test mic used.
JBL tech notes Volume 1, Number 8 also references Massa's 1942 work.
The measured distortion levels of the 1.75"(2425) and 4" (2445) drivers at relatively low drive levels is in agreement with Beranek's calculations.
Art
That said, using an 8 ohm B&C 3" diaphragm 1.4" exit driver, with a 1250 Hz 24dB per octave crossover on high DI horn (13 x 13 degree conical, about 117dB one watt one meter) measured the following using pink noise:
110 dBA 2 meters
115.2 dBA 1meter
119.7 dBA .5 meter, 19.5”
122 .5 dBA .25 meter, 9.75”
124.2 dBA .125 meter, 4.875”
126.3 dBA at horn mouth, 26” from throat screen
131.7 dBA 13” from screen
136.8 dBA 6.5” from screen
141.0 dBA 3.25” from screen
143.7 dBA at screen (144+ on impulse, flat) (about 8.2 V )
149.1 dBA at screen xover output turned up to +11dB (+6.5 dB over last test, reads 17.5V)
That level also happened to be just about the maximum clean SPL for the B&K 4004 test mic used.
JBL tech notes Volume 1, Number 8 also references Massa's 1942 work.
The measured distortion levels of the 1.75"(2425) and 4" (2445) drivers at relatively low drive levels is in agreement with Beranek's calculations.
Art
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The modern JBL drivers have a .020 mil .5 mm spacing between the phase plug and the diaphragm. Not accounting for the Dome will seriously underestimate area and don't forget about the area of the surround which can be tuned to a secondary resonance.
This is a datasheet for @ 4" large format driver.
2452h-sl
Rob
This is a datasheet for @ 4" large format driver.
2452h-sl
Rob
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Rob,
The 2452H datasheet you referenced indicates an Sd (the effective projected area of the cone or diaphragm) of 78.54 Sq cm, the same area of a flat disc of 4"/100mm.
Could you provide a link to the source you found JBL phase plug to diaphragm spacing distance?
Art
Memory fades back in... -though JBL does list the Xmax of the 476 Be at .5mm, the actual diaphragm to phase plug gap is probably very close to 1mm.
Some more discussion in this thread regarding Diaphragm to Phase Plug Space (DPPS):
https://www.diyaudio.com/forums/mul...m-spl-low-distortion-2-a-603.html#post6218632
18 Sound includes DPPS in their diaphragm replacement guide, the smaller diaphragms .8mm DPPS, 4” target DPPS at .9 to 1mm.
Art
Some more discussion in this thread regarding Diaphragm to Phase Plug Space (DPPS):
https://www.diyaudio.com/forums/mul...m-spl-low-distortion-2-a-603.html#post6218632
18 Sound includes DPPS in their diaphragm replacement guide, the smaller diaphragms .8mm DPPS, 4” target DPPS at .9 to 1mm.
Art
Hello Art
They call x max .5 mm and spacing is .020 from phase plug this is from the 2431/35 spec sheet. The 476Be also calls out a .5mm xmax. but doesn't specifically say what the spacing is. Could be different between the 3" vs. 4" drivers
Rob
They call x max .5 mm and spacing is .020 from phase plug this is from the 2431/35 spec sheet. The 476Be also calls out a .5mm xmax. but doesn't specifically say what the spacing is. Could be different between the 3" vs. 4" drivers
Rob
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