Right, plus any vent 'organ' pipe 1/2 WL harmonics comb filtering with the driver's output around/at Fb and why for HIFI apps prefer/recommend 'critically' damping the vent, reducing excessive group delay [ringing] that Bret is apparently hearing.
We use to call this 'sticking a sock in it', literally a cotton sock till we found out it can be accurately done with a 'click' test, though nowadays using software to measure it with an impulse response is the most accurate: Click Test | GM210 | Flickr
GM
If I had my way, this type of test would be mandatory before folks are allowed to post these types of threads since there's no fundamental baseline of reference of the poster's hearing and their room's impact on speaker response.
GM
Interesting Bret, especially as this will be built to JL audio’s own specifications and presumably for car use. The tuning frequency is about 30Hz +-1Hz and modelling it on WinISD shows that with a 20Hz 12db/octave subsonic filter will take the excursion below tuning pretty well.
I’ll try it in the car and living room but I think I will build a second, sealed, box for both rather than one of each.
I could of course buy a couple more of these drivers and simply own both...
In the Flickir image link - What exactly is driver z
'Res = driver z/damping factor.'
I looked around to try to find what it was but did not manage to find out exactly.
Wikipedia has
Znom - The nominal impedance of the loudspeaker, typically 4, 8 or 16 ohms
Zmax - The impedance of the driver at Fs, used when measuring Qes and Qms.
Z m a x = R e ( 1 + Q m s /Q e s )
You're welcome!
Just depends on what you're using. I've historically used [layers] of double knit fabric or pet screen tightly stretched in place covered with a traditional speaker grill while others have used a variety of damping materials in a screen/prosound driver cover/whatever 'sandwich', with coffee filters being popular for small drivers/vents with all manner of cosmetics designed to 'taste'.
GM
Educational certainly, but perhaps just a touch Draconian?
Obviously I don't at this point; been doing this on a regular basis on a variety of forums since early '97 and the number of man/months, if not man/years me and others have spent on the same fundamentals of audio that could have been better spent........
GM
I was shooting for 30hz on my box and it ended up 29hz. It had a few triangles involved so I didn't get it perfect. When I tested it, I didn't have a proper mic to use, but I think my phone was good enough, there is still a peak at 40hz due to the cabin in my car. Once I get my dsp installed I'm going to see if I can bring that down a bit without affecting the 30hz output too much. I have no experience with an eq but I understand the "q" value so Ill see if it solves the problem. You will learn something tying this in two different spaces, I reckon.
So...
After building the box and experimenting with various cardboard templates I have decided that my idea of a central port is needlessly complicated.
I'm building a sort of wedge shaped rhomboid and adjusting the port length after it is built will be difficult without holding too much of the thing together with screws and makes it impossible to build decent bracing in from the start.
I've decided to go for a slot port at the bottom of the box which is easier to model and can be adjusted for length much more easily.
Thanks to all for the help, I'll experiment with this port design but on a smaller scale and for not quite such an critical project.
After building the box and experimenting with various cardboard templates I have decided that my idea of a central port is needlessly complicated.
I'm building a sort of wedge shaped rhomboid and adjusting the port length after it is built will be difficult without holding too much of the thing together with screws and makes it impossible to build decent bracing in from the start.
I've decided to go for a slot port at the bottom of the box which is easier to model and can be adjusted for length much more easily.
Thanks to all for the help, I'll experiment with this port design but on a smaller scale and for not quite such an critical project.
Quick update:
The temporary port extension is in place and gives a total length of 39cm for a tuning of 33hz ( calculated ) in a 62 litre box.
given that this is now a front firing port, over the woofers at the front of the box, with the extension heading down the back wall, how much lower can I expect the actual tuning frequency to be?
I would be happy with a 30Hz tuning, could this be close?
The temporary port extension is in place and gives a total length of 39cm for a tuning of 33hz ( calculated ) in a 62 litre box.
given that this is now a front firing port, over the woofers at the front of the box, with the extension heading down the back wall, how much lower can I expect the actual tuning frequency to be?
I would be happy with a 30Hz tuning, could this be close?
It turns out that tuning frequency is around 31.5hz, judging from the rough and ready frequency sweep/cone motion method.
This is 1.5hz off the predicted tuning frequency so not bad. My options are to shorten and lengthen it to see whether a higher or lower tuning frequency is aurally preferable or whether to keep it as is.
It’s early days and the box needs bracing, that can’t be done until the port is finalised, but I’m not sure I like the “ported” sound... sounds softer and more blurred than a sealed 12w4 but again, it’s early days.
This is 1.5hz off the predicted tuning frequency so not bad. My options are to shorten and lengthen it to see whether a higher or lower tuning frequency is aurally preferable or whether to keep it as is.
It’s early days and the box needs bracing, that can’t be done until the port is finalised, but I’m not sure I like the “ported” sound... sounds softer and more blurred than a sealed 12w4 but again, it’s early days.
Not surprised, all but one simming program I've used tune lower than predicted, but should have been somewhere between what the base pipe tuned it and a pipe the size of the flare mouth, which will be longer for a given tuning, so for the same length as the smaller pipe it would be tuned higher.
Yeah, the bigger the port, the more it affects performance and why not the best for HIFI, though some folks [me included] are fine with it once the port has been critically damped, but the trade-off is a roll-off of some/a bunch of its gain. Not many 'free lunches' in audio system design: Click Test | GM210 | Flickr
GM
Yeah, the bigger the port, the more it affects performance and why not the best for HIFI, though some folks [me included] are fine with it once the port has been critically damped, but the trade-off is a roll-off of some/a bunch of its gain. Not many 'free lunches' in audio system design: Click Test | GM210 | Flickr
GM
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For car audio use, I've tried many types of designs, and ended up going with sealed alignments. Box size is smaller, and power is cheap these days.
If I was going to choose a vented alignment though (e.g. if I only had one driver on hand and I wanted to reduce distortion by minimizing excursion), I'd probably opt for a MLTL arrangement, to at least minimize the impact of the first pipe resonance peak on the subwoofer's usable passband. This will allow the use of a large vent and extend the subwoofer's raw response above 100 Hz.
Critical vent damping isn't really required for a subwoofer, BTW, and putting any stuff in the vent may likely increase distortion at higher SPL levels. A lot of the "noise" that you hear out of a subwoofer's vent is actually out of band noise, and best dealt with by other means (e.g. lining the walls of the subwoofer, careful choice of the positions of the vents and drivers, etc.).
If I was going to choose a vented alignment though (e.g. if I only had one driver on hand and I wanted to reduce distortion by minimizing excursion), I'd probably opt for a MLTL arrangement, to at least minimize the impact of the first pipe resonance peak on the subwoofer's usable passband. This will allow the use of a large vent and extend the subwoofer's raw response above 100 Hz.
Critical vent damping isn't really required for a subwoofer, BTW, and putting any stuff in the vent may likely increase distortion at higher SPL levels. A lot of the "noise" that you hear out of a subwoofer's vent is actually out of band noise, and best dealt with by other means (e.g. lining the walls of the subwoofer, careful choice of the positions of the vents and drivers, etc.).
Mobile audio?! I thought this was for in home. For mobile, neither fine tuning nor damping the vent is normally necessary due to cabin gain over-damping the speaker system.
We'll have to agree to disagree WRT HIFI/HT vented woofers not ever needing critical damping and the 'stuffing' bit was just what some folks have done, but only recommend what I outlined, which historically hasn't reduced output much and as someone who promotes maximizing acoustic efficiency, never recommend the amount of cab stuffing density required to quell large vent pipe harmonics.
GM
We'll have to agree to disagree WRT HIFI/HT vented woofers not ever needing critical damping and the 'stuffing' bit was just what some folks have done, but only recommend what I outlined, which historically hasn't reduced output much and as someone who promotes maximizing acoustic efficiency, never recommend the amount of cab stuffing density required to quell large vent pipe harmonics.
GM
thanks chaps, I shall research port stuffing and experiement.
This isn't actually for a car although it does use car subwoofer.
I have somehow started collecting old school JL Audio subs and am building boxes for them for fun and knowledge.
I refoamed these drivers before making the box and the sound is getting better as they break in.
They are starting to gain "chest thumping impact" but still don't produce as clean bass as the single sealed 12.
I'm not sure how much of this is the ported alignment and how much of this is two woofers sharing a box?
This isn't actually for a car although it does use car subwoofer.
I have somehow started collecting old school JL Audio subs and am building boxes for them for fun and knowledge.
I refoamed these drivers before making the box and the sound is getting better as they break in.
They are starting to gain "chest thumping impact" but still don't produce as clean bass as the single sealed 12.
I'm not sure how much of this is the ported alignment and how much of this is two woofers sharing a box?
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