Hi,
I joined up here recently. I have designed several subwoofers and built a couple of 3 way kits but this is my first foray into real design. I am working on (slot) ported bookshelves based around the Dayton RS180P-8 mids and the HiVi RT2C-A planar tweeters. In retrospect I am not sure the HiVi’s were a good idea but committed now.
Working mainly in VituixCad - the boxes are built and for the last few days I have been running impedance and response measurements using REW and a Sonarworks calibrated microphone. Speaker is 1.5m off floor in a large cleared room (3m ceilings). I have merged Near and Far Field files with Diffraction files in VitruixCad and have started to play with crossover design but before going further I need to resolve a number of issues - lets start with this one:
I have a calculated Vb of 44Hz which as expected aligns exactly with the dip between peaks in the measured Impedance curve. As I understand it, the port maximum should also align with Vb however it is at 57Hz. I have measurements for two port lengths and for the ‘calculated’ port I have measurements for empty, lined, filled and lined + filled. The ~13db discrepancy between Vb and port maximum is consistent in all cases. This causes a significant bump in the bass response when combined with the cone measurement.
I then thought to check measurements with my UMIK-1 microphone. This was quick and dirty but behold the port frequency is now as expected. Moreover the shape of the curve is now very close to modelling while the measurements with the sonarworks microphone show exaggerated peaks and unexpected inflections.
Any input appreciated - thoughts on what I might be doing wrong or what to check next?
Cone Nearfield response. Blue is UMIK - offset for clarity. Other 3 are Sonarworks mic with various fill.
Port Nearfield - Green is Umik
I joined up here recently. I have designed several subwoofers and built a couple of 3 way kits but this is my first foray into real design. I am working on (slot) ported bookshelves based around the Dayton RS180P-8 mids and the HiVi RT2C-A planar tweeters. In retrospect I am not sure the HiVi’s were a good idea but committed now.
Working mainly in VituixCad - the boxes are built and for the last few days I have been running impedance and response measurements using REW and a Sonarworks calibrated microphone. Speaker is 1.5m off floor in a large cleared room (3m ceilings). I have merged Near and Far Field files with Diffraction files in VitruixCad and have started to play with crossover design but before going further I need to resolve a number of issues - lets start with this one:
I have a calculated Vb of 44Hz which as expected aligns exactly with the dip between peaks in the measured Impedance curve. As I understand it, the port maximum should also align with Vb however it is at 57Hz. I have measurements for two port lengths and for the ‘calculated’ port I have measurements for empty, lined, filled and lined + filled. The ~13db discrepancy between Vb and port maximum is consistent in all cases. This causes a significant bump in the bass response when combined with the cone measurement.
I then thought to check measurements with my UMIK-1 microphone. This was quick and dirty but behold the port frequency is now as expected. Moreover the shape of the curve is now very close to modelling while the measurements with the sonarworks microphone show exaggerated peaks and unexpected inflections.
Any input appreciated - thoughts on what I might be doing wrong or what to check next?
Cone Nearfield response. Blue is UMIK - offset for clarity. Other 3 are Sonarworks mic with various fill.
Port Nearfield - Green is Umik
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The low frequency response for such a small speaker looks pretty good to me. [ although not quite 'near-field' monitor standard ]
Because we don't have a crossover schematic diagram, it is possible that you may have a tweeter level + phase wiring problem.
PS.
It is my understanding that 'long tube' porting can work very well.
Because we don't have a crossover schematic diagram, it is possible that you may have a tweeter level + phase wiring problem.
PS.
It is my understanding that 'long tube' porting can work very well.
You have to account for surface differences between woofer and port.
Port has a smaller surface and higher (near field) SPL.
You can reduce the port response by the ratio of Sd and port surface as an approximation.
There is a strong port (and/or enclosure?) resonance at 400 Hz. Does the port have a similar length to one of the enclosure dimensions?
stv
You have to account for surface differences between woofer and port.
Yes accounted for in VCad "Merger"
There is a strong port (and/or enclosure?) resonance at 400 Hz. Does the port have a similar length to one of the enclosure dimensions?
I was going to tackle the 400hz resonance in a follow up question but you are on to something here. Yes the port is almost exactly the same length as the internal speaker height of 367mm. That said in simple harmonics 367mm resonance is 467hz not 400hz. Unless there is something else going on 400hz suggests a resonance length around 428mm.
Hmmm - the speaker has a removeable back panel. Just happens to be 420mm long.
You have to account for surface differences between woofer and port.
Yes accounted for in VCad "Merger"
There is a strong port (and/or enclosure?) resonance at 400 Hz. Does the port have a similar length to one of the enclosure dimensions?
I was going to tackle the 400hz resonance in a follow up question but you are on to something here. Yes the port is almost exactly the same length as the internal speaker height of 367mm. That said in simple harmonics 367mm resonance is 467hz not 400hz. Unless there is something else going on 400hz suggests a resonance length around 428mm.
Hmmm - the speaker has a removeable back panel. Just happens to be 420mm long.
