Edited to correct Vb to Fb ....
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 tuning frequency Fb 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 Fb 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 ~13Hz discrepancy between Fb 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
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 tuning frequency Fb 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 Fb 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 ~13Hz discrepancy between Fb 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.
Hi there Ramius2,
I was thinking about the picture of your speaker, and it occurred to me that because your port is so close to the woofer 'standard tuning practice'
may not work properly.
If you look at baby Genlecs for example, you'll find they place the ports up-high, basically as far away from the woofer as possible.
I was thinking about the picture of your speaker, and it occurred to me that because your port is so close to the woofer 'standard tuning practice'
may not work properly.
If you look at baby Genlecs for example, you'll find they place the ports up-high, basically as far away from the woofer as possible.
I don't have one yet - I am still working on taking box driver measurements for use in crossover design!
I'm not familiar with the Genlecs but most slotted bookshelf speakers have the port under the woofer. That's not to say it's ideal and if I have to build new boxes I will look into it further.
Cheers
Back to this..
As above - yes the port is almost exactly the same length as the internal speaker height of 367mm. My above calculation of resonance was simply based on Wavelength = V/F where a 367mm half wavelength gives 467hz. Interestingly First port Resonance calculated by WinISD aslo gave 467Hz.
However search for pipe resonance gave:
The first pipe resonance will occur at F1=c/(2*L+0.8*d)
Where
c=speed of sound (m/s)
L = vent length (m)
d = vend diameter (m)
Converting my rectangular port to circular based on equal area and plugging into this formula gave 410hz for the 267mm port - close enough to link the ~400hz resonance to the port. While 400hz doesn't appear to exactly align with calculated resonance for the internal 267mm height it seems likely there is some interaction going on.
I am still hoping for an answer to my measurement problem - summarised as: why does UMIK show Fb (IE port maximum) correctly as 40hz while the Sonarworks microphone consistently show it as ~55Hz?
This is strange indeed. I tried to think of anything that could lead to such a result. Did you try to do measurements with different mic distances (10 mm, 50 mm, 200 mm ...)?
For my research into port and enclosure resonances and how to deal with them you could have a look at my dedicated thread.
It might be possible to absorb some of the 400 Hz peak with an internal helmholtz resonator, located near the port interior end. Resonator air volume remains part of the bass reflex enclosure volume.
No but i might give that a try. I cant work out whether this is caused by a software setting, some physical interaction or bad hardware. Without resolution it makes it hard to trust the data or to know how to proceed.
I do have far field (1000mm) graphs but they dont provide any useful info on the low frequency respose.
What is your take on this 400hz peak. It it simply a standing wave resonance or is this related to the correlation between port length and the internal height. Also noting that nothing of significance shows in the far field charts do i need to worry about it?
Thanks - I glanced throught the linked thread and will read through in detail soon
Just to close the loop on this. After a lot of testing I traced the fault to the in-expensive CTEYUN Q-12 soundcard I was using. I have replaced it with a Scarlett 2i2 and the response curves now generally look as expected including that the port peak is now correctly at 40Hz.
The 400Hz peak in the port response remains but the data is now coherent so that I can progress the design.
The 400Hz peak in the port response remains but the data is now coherent so that I can progress the design.