Took out some of the stuffing around the port. It did even up the impedance peaks - not reducing them though.
Messed about trying to locate the damping medium just past the driver - thats the second trace - i can see how you can tune with just moving the damping wool around.
Did a frequency response measurement as well (close field) it hasn't really affected the hump in the response.
My throughts are i need to add more damping medium all over to reduce the impedance peaks. Before i took it out i had around 1.5 Ibs in the whole 6 ft3 enclosure. ( will have to get some more tommorrow)
Messed about trying to locate the damping medium just past the driver - thats the second trace - i can see how you can tune with just moving the damping wool around.Did a frequency response measurement as well (close field) it hasn't really affected the hump in the response.
My throughts are i need to add more damping medium all over to reduce the impedance peaks. Before i took it out i had around 1.5 Ibs in the whole 6 ft3 enclosure. ( will have to get some more tommorrow)
OK. The cabinet works 
First/lower peak is the port, second is the cabinet. You can reduce (as you've seen) lower peak with damping the port, but than the port output is reduced. You can't significantly reduce the second peak with damping - you can do it with RLC compensation in the crossover but I wouldn't do that.
I still think 35Hz freq resp peak is acoustic related.
First/lower peak is the port, second is the cabinet. You can reduce (as you've seen) lower peak with damping the port, but than the port output is reduced. You can't significantly reduce the second peak with damping - you can do it with RLC compensation in the crossover but I wouldn't do that. I still think 35Hz freq resp peak is acoustic related.
I am confused. My basic understanding of BR tuning was to divide and reduce the response into a couple of small impedance peaks, which i was assuming were the two peaks i was seeing. The MJK simulations do have a impedance peak ~ 2*fs, are we saying that this is the TL response? If this is the case, then from memory this would suggest that the TL needs to be longer to get this peak outside the usable frequency range.
After an evening playing with MJK's mltl workbook its clear that the test setup is following the predictions very well from an impedance perspective. Very different from TL stuff i did a decade or so ago. I modelled and measured the effects of increasing the port length, lowering the tuning frequency, which was evident in the FR measurements i made. Having now gained the confidence that i simulate whats happening with MLTL's, and likeing how the LF is presented hollistically, i am now ready to move to the next stage, once i have solved the source of the ~40hz hump.
I know its been stated that its a room coupling - i am not sure, as i had a pair of Decca/Volt speakers in my room a couple of weeks back, and they were suprisingly flat from 30hz upto 150hz, and would have throught a freq dependant coupling would have manifested itself.
Regarding the hump i have measured all the way around the room and it is there until you get to > 60 degs off axis, independant of distance from driver. The only source i have throught about is it how i have mounted the driver? ( attached on the inside of a 11" dia hole in 1" thick stuff)
I know its been stated that its a room coupling - i am not sure, as i had a pair of Decca/Volt speakers in my room a couple of weeks back, and they were suprisingly flat from 30hz upto 150hz, and would have throught a freq dependant coupling would have manifested itself.
Regarding the hump i have measured all the way around the room and it is there until you get to > 60 degs off axis, independant of distance from driver. The only source i have throught about is it how i have mounted the driver? ( attached on the inside of a 11" dia hole in 1" thick stuff)
Sorry, haven't noticed. Well that's strange, because it's almost impossible for a driver to have such isolated lowfreq peak and I wouldn't say it's cabinet related resonance because wavelenght is to big. And it doesn't seem to be a port output because peak's level and Q is to high (it's 6db peak!).
As it should.
Correct, though normally in a [ML]TL one concentrates the damping uniformly along the top half of the cab if using loose polyfil or similar or when using acoustic fiberglass duct insulation or similar, lining the top, one side and back its entire length and usually only adding more at the top if required since this is where it will affect the widest BW.
I’m confused, the sim we agreed was what you built is ~8.6 ft^3, so what am I missing? The sim uses 0.25 lbs/ft^3 = ~2.15 lbs of damping, though many folks that have built my designs say this is too much.
FWIW, I usually only line the top half initially and if it sounds ‘hollow’, only add as much further down as required until it goes away and only on rare occasions with heavily tapered cabs needing more at the top, so wondering why you want to damp down the impedance peaks anymore than necessary to achieve this.
If you’re using a high output impedance amp such as a SET and especially if a matching impedance one, then the goal is to tune it to Fs, i.e. the peaks should be at equal amplitude.
The 'hump' has me stumped ATM since it didn't shift much when you moved the cab, measurement distance.
GM
I confused you it is 8.6 ft^3. I have added only enough fibre starting from the top to get the "click to go to a "plop". The impedance traces match up with the simulation still.
I have noticed that the SPL trace is dropping off at the bottom end from 100hz, following the IB trace in MJK's worksheet suggesting the loading isn't working. I tried varying the lenght of the port without any success. Ideas?
Back to the hump - its still there. The attached trace shows what happens to it as the measurement distance goes to far field. Traces at 4", 24" and ~100" (listening position)
I have also attached a SPL trace of the 414z in an Onken cabinet in the same room- the hump isn't there (far field measurement). This suggests it cabinet related more than room to me.
I have noticed that the SPL trace is dropping off at the bottom end from 100hz, following the IB trace in MJK's worksheet suggesting the loading isn't working. I tried varying the lenght of the port without any success. Ideas?
Back to the hump - its still there. The attached trace shows what happens to it as the measurement distance goes to far field. Traces at 4", 24" and ~100" (listening position)
I have also attached a SPL trace of the 414z in an Onken cabinet in the same room- the hump isn't there (far field measurement). This suggests it cabinet related more than room to me.
Yeah, I figure I’m not seeing the ‘forest for the trees’ and it’s really bugging me. It has the double hump of a reflex and a peaking at Fb [~Fs] response similar to a much too large vented reflex, i.e. which it is when there’s no TL pipe action to ‘pump it up’; but how can a cab have what appears to be a major leak, yet still have the proper tuning???
What amp, output impedance are you using?
GM
What amp, output impedance are you using?
GM
I am using solid state amps - either a 20W t amp (test amp) or my 600W bass amp - both give the same answer w.r.t. impedance and frequency response.
This now is becoming an academic challenge to me now, it don't like thinks i cannot understand. I have been considering using mathematic modelling resource available to me to do an explicit analysis, however as with everything it will only be as good as the assumptions and inputs.
This now is becoming an academic challenge to me now, it don't like thinks i cannot understand. I have been considering using mathematic modelling resource available to me to do an explicit analysis, however as with everything it will only be as good as the assumptions and inputs.
I went back to basic- to try and understand where i have gone wrong not understood correctly. Firstly i have build a cabinet with a folded line looking similar to this (dimensions different as per MJK workbook.
I blocked off the vent today to see how it would perform as an IB. Impeddance trace looks as you would expect.
FR response - very similar to the vented one - the "hump" is still there - becomes pronounced in the far field. The difference in the ground plane (floor) may be having an effect? Due to the size of it i haven't tried lifting it off the floor yet.
Hopefully this may give some clues to someone cleverer than me where i am going wrong.
The
An externally hosted image should be here but it was not working when we last tested it.
I blocked off the vent today to see how it would perform as an IB. Impeddance trace looks as you would expect.
An externally hosted image should be here but it was not working when we last tested it.
FR response - very similar to the vented one - the "hump" is still there - becomes pronounced in the far field. The difference in the ground plane (floor) may be having an effect? Due to the size of it i haven't tried lifting it off the floor yet.
An externally hosted image should be here but it was not working when we last tested it.
Hopefully this may give some clues to someone cleverer than me where i am going wrong.
The
Last post was rushed - and did not come across coherent ( family time pressures)
I hoped the logic from my previous post was to show, the response from a know situation, i.e infitie baffle. This showed what i was expecting from an impedance response - highlighting i think that the cabinet isn't leaking ( expect via the port)
Obviously with the "hump" still being there for the IB it isn't some port interaction - baffle effects - to lower freq for this? Therefore must be room coupling i suppose.
At least i can now seperate the issues - the lack of LF response above that you get from a IB is the one i hope to solve with the aid of those on the forum.
I hoped the logic from my previous post was to show, the response from a know situation, i.e infitie baffle. This showed what i was expecting from an impedance response - highlighting i think that the cabinet isn't leaking ( expect via the port)
Obviously with the "hump" still being there for the IB it isn't some port interaction - baffle effects - to lower freq for this? Therefore must be room coupling i suppose.
At least i can now seperate the issues - the lack of LF response above that you get from a IB is the one i hope to solve with the aid of those on the forum.
Would tuning it slightly higher deal with the dip? or at least make it a smoother transition lower. could it be possible that the wave from the rear port is reflecting off wall(room coupling) and canceling part of the front wave, sorta like you would expect from a dipole or OB. Could it be back wave coming back through cone?
BTW, thanks for doing this. I am learning a lot.
BTW, thanks for doing this. I am learning a lot.
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