I just picked up a DATs v3 to measure the port tuning of a large enclosure I built for my Lavoce 18" sub. Running an in-box impedance sweep though is showing a huge blip at 74Hz.
Driver: Lavoce SAF184-03
Volume: 17.3cf (76in L x 44.5in W x 10in H)
Tune: 14Hz
Port: 5.125in x 9.25in x 35in
One thing I discovered is that placing a pillow in the center of the box, along the width eliminates the peak. Though it also reduces the impedance a lot (by about half). Not sure what this means overall.
Here is my WinISD measurements before and after the pillow (you can see the 74Hz dip disappear)
Also I was going for a 14Hz tune, but DATs is showing about 12.5Hz. Should I think about shorting the slotted port I built into it?
Driver: Lavoce SAF184-03
Volume: 17.3cf (76in L x 44.5in W x 10in H)
Tune: 14Hz
Port: 5.125in x 9.25in x 35in
One thing I discovered is that placing a pillow in the center of the box, along the width eliminates the peak. Though it also reduces the impedance a lot (by about half). Not sure what this means overall.
Here is my WinISD measurements before and after the pillow (you can see the 74Hz dip disappear)
Also I was going for a 14Hz tune, but DATs is showing about 12.5Hz. Should I think about shorting the slotted port I built into it?
It means you have a large modal resonance, the size of the box says it should be closer to 90Hz mind you. Damping is required to try to calm it down, a decent thickness at the quarter points on that axis should be best.
Thanks. Yeah for fun I moved the pillows from the 1/2 point to the 1 of the 1/4 points, and same results as the 1/2. The peak disappears, but also the impedance goes down by about 50% too (for Fb from 97ohms to 50ohms). Not sure if this is bad or not.
While researching if this is "bad" I discovered that I can model this exact result in WinISD by changing the absorption coefficient (Qa) from 100 (no absorption) to 7 (above avg absorption). So those pillows are really doing a thing. So learned something new.
I guess in my mind its crazy that a couple of pillows can increase the apparent load on an amp by 2x. But I guess going from 100ohms to 50ohms are still pretty high values, so not really a factor.
FYI a picture of the box innards. This is a seating riser doubling as an enclosure. I stuffed the pillows in that 3rd section from the back.
While researching if this is "bad" I discovered that I can model this exact result in WinISD by changing the absorption coefficient (Qa) from 100 (no absorption) to 7 (above avg absorption). So those pillows are really doing a thing. So learned something new.
I guess in my mind its crazy that a couple of pillows can increase the apparent load on an amp by 2x. But I guess going from 100ohms to 50ohms are still pretty high values, so not really a factor.
FYI a picture of the box innards. This is a seating riser doubling as an enclosure. I stuffed the pillows in that 3rd section from the back.
Member
Joined 2003
Based on the very low tuning I would expect that the impedance peak is likely port resonant mode, not the cabinet walls. Big pillow will damp resonant modes at the expense of some efficiency. WinISD will tell you this port resonant information, look for the checkbox about "port transmission line". But, I would actually recommend looking in VituixCAD, look under Tools -> Auxiliary -> Box Volume. Here you can enter your cabinet and vent dimensions and see the frequencies where all the resonant modes will be located.
Based on your provided information, if you are aiming for 14-15Hz tuning it is probably a bit low, shared side walls with the port effectively makes the port longer which isn't taken into account in the computer model. Port resonance within the pass band is a common problem for low tuned speakers <25-30Hz.
For measuring the speaker to confirm the computer model, I'm not sure the measurement method above in REW but it doesn't look right. Measure nearfield of both the driver output and port exit, these measurements should look very similar to the computer simulation for driver and port output, and then can simply be added together for the total response. Port measurement amplitude will need to be adjusted, below tuning freq as frequency approaches zero the port output and driver output should converge, this can be done visually but can be difficult with such low tuning to begin with. Again, the computer model can be used as a guide.
Alternative is to calculate the adjustment mathematically, without getting out your calculator it can be done for you using VituixCAD merge tool. Load both nearfield driver measurement and port measurement to the "low frequency" portion, enter Sd for the driver, and cross-sectional area of the port. Select "no baffle loss" for near-field result to compare against the computer simulation.
You will get a benefit here with the nearfield port measurement of confirming the resonant mode as well, you can see in my screenshot above the port resonant mode occurring around 700Hz.
Based on your provided information, if you are aiming for 14-15Hz tuning it is probably a bit low, shared side walls with the port effectively makes the port longer which isn't taken into account in the computer model. Port resonance within the pass band is a common problem for low tuned speakers <25-30Hz.
For measuring the speaker to confirm the computer model, I'm not sure the measurement method above in REW but it doesn't look right. Measure nearfield of both the driver output and port exit, these measurements should look very similar to the computer simulation for driver and port output, and then can simply be added together for the total response. Port measurement amplitude will need to be adjusted, below tuning freq as frequency approaches zero the port output and driver output should converge, this can be done visually but can be difficult with such low tuning to begin with. Again, the computer model can be used as a guide.
Alternative is to calculate the adjustment mathematically, without getting out your calculator it can be done for you using VituixCAD merge tool. Load both nearfield driver measurement and port measurement to the "low frequency" portion, enter Sd for the driver, and cross-sectional area of the port. Select "no baffle loss" for near-field result to compare against the computer simulation.
You will get a benefit here with the nearfield port measurement of confirming the resonant mode as well, you can see in my screenshot above the port resonant mode occurring around 700Hz.
Another unrelated question though. I finally got everything put back together, but having what feels like an issue. I have to turn the gain/volume down considerably to get down to 75db SPL (for level matching). If I turn my AVR volume up to get my Left speaker to 75db (Audyssey off, level 0.0), the sub is then WAY too loud when playing the same pink noise.
My signal path is REW (-12 dBFS) > Denon AVR 6700 (volume -27dB, spkr gain 0.0 dB) > MiniDSP 2x4 HD (0.0dB in/out, no EQ) > Behringer NX6000D (gain 75%). The max I can attenuate the sub in the AVR is -12dB, and even that only gets me to a sub SPL of 85dB. I could instead attenuate on the output channel of the DSP, but that takes -24dB to get to where I need. Not sure where the best place to attack this is, or if I am missing something silly.
Any ideas here? I guess I feel like having set the gain so low (on some device) is going to give me signal-to-noise issues in that device.
My signal path is REW (-12 dBFS) > Denon AVR 6700 (volume -27dB, spkr gain 0.0 dB) > MiniDSP 2x4 HD (0.0dB in/out, no EQ) > Behringer NX6000D (gain 75%). The max I can attenuate the sub in the AVR is -12dB, and even that only gets me to a sub SPL of 85dB. I could instead attenuate on the output channel of the DSP, but that takes -24dB to get to where I need. Not sure where the best place to attack this is, or if I am missing something silly.
Any ideas here? I guess I feel like having set the gain so low (on some device) is going to give me signal-to-noise issues in that device.
Member
Joined 2003
High output at the source, lower the gain of the Behringer will generally provide better SNR vs providing low output from the source with high gain at the amp.
Make sure port area and length is entered correct in the VituixCAD box volume tool, hover over the traces will tell you what each colour is.
Make sure port area and length is entered correct in the VituixCAD box volume tool, hover over the traces will tell you what each colour is.
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