Trying to diagnose cause of unexpected resonances in my measurements for an enclosure with an 8 inch woofer (Dayton DC200-8). The enclosure will be used below a freestanding R-OSSE horn designed using ATH. Measurements were done with an analog microphone, external sound card/audio interface, amplifier, and REW software. Enclosure was measured at 12 feet high (3.65 meters), microphone 50 inches (1,270 mm) from baffle face. The tripod poles are 1 in x 10 foot threaded metal conduit pipe from Home Depot - I can feel them resonate during sweeps. I have ABEC simulations to use for comparison but they don't include materials or the woofer motor. Annnnnd...after writing this post I just realized I didn't try ground plane measurements which could have eliminated one of my primary suspects. I've never done ground plane measurements so that's why it wasn't in the front of my mind as far as a diagnostic tool.
First concern is the resonance at ~900Hz in the polar response. The resonance increases as the speaker is rotated off axis. However, the Nearfield measurement of the woofer in the enclosure has a dip ~800Hz. Green arrow shows an expected resonance according to the simulation. I don't know if the Nearfield woofer dip is related to the off-axis polar resonance or not. My first intuition is that maybe the Nearfield dip causes a resonance that transfers to the 10 foot long metal pole. I'm guessing that resonance is constant throughout 360 degrees of rotation, but lower in SPL than the on-axis measurements. As the speaker is rotated the frequency response from the woofer declines relative to the lower SPL resonance from the metal pole so it appears to grow as the speaker is rotated off axis. But I don't know why it's happening because I don't have enough experience with speaker measurements.
These are polars from the enclosure/woofer ABEC simulations.
Second concern is the resonance just about 300Hz. I don't see anything in the Nearfield or the simulation so I'm guessing it's from the ground reflection. Speaker measured 12 feet up (3.65 meters), 50 inches (1,270 mm) from microphone to baffle face. Does a ground reflection sound right or is it caused by something else? (green arrow shows expected resonance from simulation)
Simulation
A third concern is unrelated to the simulation, an Impulse Response error. Is that caused by a common mistake in a REW setting or maybe a setting on the external sound card/audio interface?
First concern is the resonance at ~900Hz in the polar response. The resonance increases as the speaker is rotated off axis. However, the Nearfield measurement of the woofer in the enclosure has a dip ~800Hz. Green arrow shows an expected resonance according to the simulation. I don't know if the Nearfield woofer dip is related to the off-axis polar resonance or not. My first intuition is that maybe the Nearfield dip causes a resonance that transfers to the 10 foot long metal pole. I'm guessing that resonance is constant throughout 360 degrees of rotation, but lower in SPL than the on-axis measurements. As the speaker is rotated the frequency response from the woofer declines relative to the lower SPL resonance from the metal pole so it appears to grow as the speaker is rotated off axis. But I don't know why it's happening because I don't have enough experience with speaker measurements.
These are polars from the enclosure/woofer ABEC simulations.
Second concern is the resonance just about 300Hz. I don't see anything in the Nearfield or the simulation so I'm guessing it's from the ground reflection. Speaker measured 12 feet up (3.65 meters), 50 inches (1,270 mm) from microphone to baffle face. Does a ground reflection sound right or is it caused by something else? (green arrow shows expected resonance from simulation)
Simulation
A third concern is unrelated to the simulation, an Impulse Response error. Is that caused by a common mistake in a REW setting or maybe a setting on the external sound card/audio interface?
You can window your measurement to eliminate the ground reflection's influence. Roughly 21 milliseconds for flight from speaker to ground and back. Anything around that time and beyond is suspect. See link below also. I'm ignoring a couple things because I'm lazy.
You could do one of your sweeps and put the microphone nearly touching the pole you're worried about to see what its up to.
Nearfield measurement accuracy is affected by the distance from the microphone to the source and source diameter. Your 800 Hz dip may or may not be real. Sometimes backing up and measuring at multiple distances can help try to get to the bottom of what's going on, even if what you're measuring isn't completely accurate. If there's a reflection interfering somewhere, this can be quite useful if you can move the speaker and the microphone independently as the frequency of the peak/dip will shift if it's a reflection issue.
https://audioxpress.com/article/measuring-loudspeaker-low-frequency-response
"Keele shows that a microphone distance less than 0.11 times the diaphragm effective radius results in measurement errors of less than 1 dB. As an example, a 6.5" driver will typically have an effective cone diameter of 5" or an effective radius of 2.5". For this driver, the microphone should be placed within 0.275" of the driver dust cap.
At higher frequencies, where cone break up begins, pressure waves from various areas of the diaphragm may arrive at the microphone out-of-phase, causing near-field response cancellations that are not observed at normal listening distances. For this reason, there is a practical upper limit to the nearfield technique given in terms of driver diaphragm diameter. For a driver mounted in an infinite baffle, the limit is:
fMAX = 4,311/D [2]
Here fMAX is in hertz and the driver diameter, D, is in inches."
You could do one of your sweeps and put the microphone nearly touching the pole you're worried about to see what its up to.
Nearfield measurement accuracy is affected by the distance from the microphone to the source and source diameter. Your 800 Hz dip may or may not be real. Sometimes backing up and measuring at multiple distances can help try to get to the bottom of what's going on, even if what you're measuring isn't completely accurate. If there's a reflection interfering somewhere, this can be quite useful if you can move the speaker and the microphone independently as the frequency of the peak/dip will shift if it's a reflection issue.
https://audioxpress.com/article/measuring-loudspeaker-low-frequency-response
"Keele shows that a microphone distance less than 0.11 times the diaphragm effective radius results in measurement errors of less than 1 dB. As an example, a 6.5" driver will typically have an effective cone diameter of 5" or an effective radius of 2.5". For this driver, the microphone should be placed within 0.275" of the driver dust cap.
At higher frequencies, where cone break up begins, pressure waves from various areas of the diaphragm may arrive at the microphone out-of-phase, causing near-field response cancellations that are not observed at normal listening distances. For this reason, there is a practical upper limit to the nearfield technique given in terms of driver diaphragm diameter. For a driver mounted in an infinite baffle, the limit is:
fMAX = 4,311/D [2]
Here fMAX is in hertz and the driver diameter, D, is in inches."
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I think my Impulse Response problem was because I had the wrong Preference settings in REW. I used the VituixCAD help file to adjust my REW preference settings and it seems to have repaired the response.
I also took ground plane measurements (inside, not outside yet) and it got rid of the 300Hz and 900Hz resonances. I'll take them outside during better weather to improve measurement detail.
I also took ground plane measurements (inside, not outside yet) and it got rid of the 300Hz and 900Hz resonances. I'll take them outside during better weather to improve measurement detail.