I'm hoping someone can help me understand what's going on with a new DIY speaker build. I am measuring a deep null, about 15dB, in the woofer response at 70Hz, and I don't know what is causing it or how to fix it. The null in SPL is accompanied by a peak in group delay (or sometimes it measures as a valley in group delay, probably just because the phase is changing so fast that the calculation gets messed up). This is a Satori MW19P-4 woofer in an 18L bass reflex enclosure with a 3"D x 10"L vent. The cabinet is 3/4 and 1" MDF, heavily braced. I'm using a MiniDSP 2x4HD crossover and ICEPOWER 125ASX2SE amp. I'm measuring with REW software and a MiniDSP UMIK1 microphone. I've tried changing following things and the null is very nearly the same in every case:
Measuring in two different rooms, and also outdoors lifted 6ft from the ground;
With and without the MiniDSP crossover;
With a different D/A converter and a different power amp;
With the speaker's port open and blocked;
With very little stuffing up to maximal stuffing in the box;
With two different samples of the woofer and the cabinet;
I also tried re-measuring my main system's speakers, a different design, using REW and UMIK1 with the same settings, and there is nothing remotely resembling this 70Hz null.
Measuring in different locations of course changes the response significantly, but that stubborn null stays at 70Hz.
The only thing I haven't changed is design of the woofer and the volume and shape of the cabinet. I'm out of theories of a mechanism of this response null. Any ideas?
The first picture shows the response of only the woofer, in my office (not a listening room), with tweeter muted, at several different gain levels in steps of 6dB. The second picture shows a typical group delay graph.
Measuring in two different rooms, and also outdoors lifted 6ft from the ground;
With and without the MiniDSP crossover;
With a different D/A converter and a different power amp;
With the speaker's port open and blocked;
With very little stuffing up to maximal stuffing in the box;
With two different samples of the woofer and the cabinet;
I also tried re-measuring my main system's speakers, a different design, using REW and UMIK1 with the same settings, and there is nothing remotely resembling this 70Hz null.
Measuring in different locations of course changes the response significantly, but that stubborn null stays at 70Hz.
The only thing I haven't changed is design of the woofer and the volume and shape of the cabinet. I'm out of theories of a mechanism of this response null. Any ideas?
The first picture shows the response of only the woofer, in my office (not a listening room), with tweeter muted, at several different gain levels in steps of 6dB. The second picture shows a typical group delay graph.
Did you save your outdoor measurements? My first thought was a room related thing, but you ruled that out yourself I guess.
The reflex output at about 50Hz (which should be about the tuning frequency) is there. So that would work I think. The dip is spot on the second impedance peak of your system (at least my sim shows that). It stays there when stuffing the vent or stuffing the enclosure (which makes sense). It's not an amplifier-driver related issue as you changed that. You could measure the voltage output of the amp (with a voltage divider you'd use for impedance measurements on a sound card input) just to be sure.
The vent and the location of it don't seem optimal to me though. It will produce quite a lot output at about 700Hz (1st order port resonance) and inside, the port entrance is too close to bottom and back to work properly. It might be some unlucky side effect of an ill-designed Helmholtz resonator.
The reflex output at about 50Hz (which should be about the tuning frequency) is there. So that would work I think. The dip is spot on the second impedance peak of your system (at least my sim shows that). It stays there when stuffing the vent or stuffing the enclosure (which makes sense). It's not an amplifier-driver related issue as you changed that. You could measure the voltage output of the amp (with a voltage divider you'd use for impedance measurements on a sound card input) just to be sure.
The vent and the location of it don't seem optimal to me though. It will produce quite a lot output at about 700Hz (1st order port resonance) and inside, the port entrance is too close to bottom and back to work properly. It might be some unlucky side effect of an ill-designed Helmholtz resonator.
I'd suggest going back outdoors and doing a ground plane measurement.
Put the speaker right on a driveway or other hard surface. Do the same with the mic. Measure at 2 meters or greater.
If this shows the 70Hz null, then I'd say the null is indeed real.....but not until then.......
Good luck !
Put the speaker right on a driveway or other hard surface. Do the same with the mic. Measure at 2 meters or greater.
If this shows the 70Hz null, then I'd say the null is indeed real.....but not until then.......
Good luck !
I had a similar problem a couple of years ago. I discovered my laptop, which generated the test signal, was imposing a bass EQ which included a notch at 68 Hz. The thread below explains the problem I had, and what I did to fix it.
https://www.diyaudio.com/community/threads/unexplained-low-frequency-measurement-mystery.364535/
https://www.diyaudio.com/community/threads/unexplained-low-frequency-measurement-mystery.364535/
Thanks for all your feedback, and sorry for my slow response. hifijim, you put me on the right track. It was a Windows 10 setting. I had been fooled when I compared to a test of my main system, thinking that the test system was the same, but I failed to notice that that test signal went out through an external d/a unit, not through the computer's d/a.
There is a setting in Windows 10. It's NOT visible at the dumbed-down "Sound Settings" that you find when right-clicking on the speaker icon in the system tray. But it IS visible at "Control Panel / Hardware and Sound / Sound Settings / (select playback device) / Enhancement". It looks like none of the "enhancements" are enabled, but even so I clicked "Disable all enhancements" and everything changed - including that the 70hz suckout disappeared. The picture here shows two frequency response graphs of the speaker in my room with no difference except the checking of that box. Thanks Microsoft for that stealth "enhancement".
There is a setting in Windows 10. It's NOT visible at the dumbed-down "Sound Settings" that you find when right-clicking on the speaker icon in the system tray. But it IS visible at "Control Panel / Hardware and Sound / Sound Settings / (select playback device) / Enhancement". It looks like none of the "enhancements" are enabled, but even so I clicked "Disable all enhancements" and everything changed - including that the 70hz suckout disappeared. The picture here shows two frequency response graphs of the speaker in my room with no difference except the checking of that box. Thanks Microsoft for that stealth "enhancement".
I am glad you got it sorted out. 
I struggled for many days with my issue. At the time I only had a USB mic, and no external sound card. So my ability to trouble shoot directly (like a loop test) was limited.
I believe that taking valid, accurate measurements is one of the most challenging areas of DIY speaker building.
I struggled for many days with my issue. At the time I only had a USB mic, and no external sound card. So my ability to trouble shoot directly (like a loop test) was limited.I believe that taking valid, accurate measurements is one of the most challenging areas of DIY speaker building.