16cu/ft 24hz T-line for two Eclipse 8122

[r0llinlacs 10+ year member]

Just a rough draft with google sketchup. Measurements not exact. I used this mainly to get the line length figured out but also some basic rough dimensions.

Ends up being 16.6cu/ft. Line length 11.75ft, tuned at 24hz.

Plan on starting some time soon.

Click to read more...

 

[keep_hope_alive Premium Member]

your line looks too small - in free area. does it match the combined Sd of both woofers? i see about 136 sq. in. of TL area and the subs should be around 150-190 sq. in. of cone area (i don't have the exact specs of that sub).

it just increases your box depth by a foot or so.

i'd also be concerned with how well both subs "couple" to the TL given different pathlengths.

this is a chamber-coupled TL for a single 10"

 

[r0llinlacs 10+ year member]

your line looks too small - in free area. does it match the combined Sd of both woofers? i see about 136 sq. in. of TL area and the subs should be around 150-190 sq. in. of cone area (i don't have the exact specs of that sub).

The line length ends up being exactly 11.75ft which is 1/4 the wavelength of 24hz. I count the first part of the line though, right? The wall right behind the subs still counts? Or does the line actually start on the second wall? If I count the first wall right behind the subs, it's 11.75ft.

I can't find the Sd of the woofers, people just throw out ballpark figures for average 12's and say 550sq/cm but I needed to know exactly what the Sd was, so I measured myself and came up with 408sq/cm per woofer which would be 816sq/cm for both which is 126sq/in which is the "port" area 7"x18"=126sq/in. So if my Sd measurements and calculations were right, the TL measurements should be right too, but how does 408sq/cm sound compared to other 12's? The only reason I doubt my measurements is because the cone is concave and I figured for the area of a flat circle, but I cut paper towel to the form of the cone, stretched it out flat and still got the same measurements. The cone is 9" which pi*4.5squared = 63.62sq/in = 410sq/cm, so I just re-did my math and came out a tiny bit different (410 vs 408) but does it sound about right for 9" of cone area? I also doubt myself because other 12's seem to have a lot higher Sd, but honestly the cones on these Eclipses really aren't all that big.

i'd also be concerned with how well both subs "couple" to the TL given different pathlengths.

I'm also concerned about that, which is why I offset the subs to try and keep them both behind the first part of the line, but the first sub still sticks over farther than I'd like so this design is more than likely not final and I may raise the box and re-work the TL area (something like 6"x21" vs the 7"x18" on the blueprint) so I can offset the subs even more to keep them closer to eachother and 100% behind the first wall. I don't think I'll 100% like the look of this but I'd have to sit and brainstorm ways to work it out but for now all I can think of is offsetting the subs even closer. I'm limited to a 36" box width but I'm pretty free as far as depth and height.

The subs are Eclipse SW8122.4 if anybody knows where to find T/S parameters for them, all I can find are posts from other forums with slightly varying parameters and no Sd parameter.

Anyways thanks for the reply, can't wait to hear what else you have to say.

 

[CSCStang]

To measure the SD all you have to do is measure the center of the surround from one side to the other. Most 12's will be roughly 10.5ish, but certainly more than 9.

 

[r0llinlacs 10+ year member]

To measure the SD all you have to do is measure the center of the surround from one side to the other. Most 12's will be roughly 10.5ish, but certainly more than 9.

Okay, I measured from the center of the cone to the beginning of the surround. So I actually count half of the surround as cone area?

 

[garychoffmann 10+ year member]

That is correct ^

 

[CSCStang]

Okay, I measured from the center of the cone to the beginning of the surround. So I actually count half of the surround as cone area?

Yeah. Center to center is your SD or piston diameter.

 

[r0llinlacs 10+ year member]

Wow. Huge difference. 5.5" from center of cone to center of surround. Came out to 613sq/cm per woofer. Thanks a lot guys. Looks like I have more work to do.

 

[CSCStang]

Wow. Huge difference. Came out to 613sq/cm. Thanks a lot guys. Looks like I have more work to do.

Gonna be a monster lol. Gotta see pics!!

 

[r0llinlacs 10+ year member]

Yeah, should dig pretty deep with the 24hz Fs which is what I want. My SUV just doesn't hit the lows like my car did even with 28hz tuned boxes so now that I have the space, I figure why the hell not. Gonna take lots of wood! I need a new jigsaw too so I still have a few things I need to do before I can start.

Also, I've showed the design to my friend who still works at the car audio shop I used to work at, he's the electrical genius but not the best at boxes and he showed it to some of the other guys there and they all say it's "tuned too low", I'm like uhhh, it's a t-line lol you're supposed to tune to the Fs of the subs... I don't take it too hard because I've seen the boxes they build (everybody here would lol at them) so I know they really have no idea what they're doing, but does anybody here have any experience with really low tuned boxes? I know it's a t-line, so it's different, and technically the "perfect box" for the subs, but I'm still curious how it will turn out. I know the t-line will be more even than a 24hz tuned box but I'm wondering if it will have the same output around 24hz as a 24hz tuned box, which would obviously be peaky around that frequency. Input on that, anybody?

The lowest tuned boxes I've ever experienced are the ones I built at 28hz and they sounded more even than my 32hz single 12 box, so I'm not all that afraid to dig deeper than 28hz on the tuning frequency of a regular ported box and now I'm pondering whether or not to even build the t-line, if I can get more output from a 24hz port tuned box, because I have a feeling the acoustics of the cabin will even out the 24hz peak I would get with a port tuned box.

keep_hope_alive, what would you say about my situation? What should I do for sub-30hz output if the cabin acoustics drop off at sub-30hz? I have no way to measure it, but I have a very decent ear and I can tell it drops off around 30hz and it doesn't matter if the windows are up or down, it just gets quieter with the windows down. 30hz and below still moves plenty of air and shakes everything but the sound isn't there, and I know for an absolute fact I can hear down to the 17hz range because my t-line in the house hits unbelievably low and you can hear the actual sound and not just the air moving, all the way down to 17hz, so I'm not 100% sure but I'm pretty sure it's the cabin acoustics that I'm not happy with so whatever I build, I want to tune it around the cabin to get rid of the sub-30hz drop off.

Also, keep_hope_alive, you said some other time that a box in the trunk of a car creates a 3rd order bandpass and with the windows down a 6th order bandpass or something like that (sorry, I'm not familiar with bandpass boxes), so would a certain bandpass box be ideal for re-creating the sound of a trunk car in an SUV, so to speak? Or should I just go with a t-line?

 

[keep_hope_alive Premium Member]

I'll get back to you when I'm on a PC next.

 

[r0llinlacs 10+ year member]

I'll get back to you when I'm on a PC next.

Sounds good.

If mods want to move this to enclosure design/help I won't complain, didn't see that category when I posted this.

 

[keep_hope_alive Premium Member]

Yeah, should dig pretty deep with the 24hz Fs which is what I want. My SUV just doesn't hit the lows like my car did even with 28hz tuned boxes so now that I have the space, I figure why the hell not. Gonna take lots of wood! I need a new jigsaw too so I still have a few things I need to do before I can start.
Also, I've showed the design to my friend who still works at the car audio shop I used to work at, he's the electrical genius but not the best at boxes and he showed it to some of the other guys there and they all say it's "tuned too low", I'm like uhhh, it's a t-line lol you're supposed to tune to the Fs of the subs... I don't take it too hard because I've seen the boxes they build (everybody here would lol at them) so I know they really have no idea what they're doing, but does anybody here have any experience with really low tuned boxes? I know it's a t-line, so it's different, and technically the "perfect box" for the subs, but I'm still curious how it will turn out. I know the t-line will be more even than a 24hz tuned box but I'm wondering if it will have the same output around 24hz as a 24hz tuned box, which would obviously be peaky around that frequency. Input on that, anybody?

The lowest tuned boxes I've ever experienced are the ones I built at 28hz and they sounded more even than my 32hz single 12 box, so I'm not all that afraid to dig deeper than 28hz on the tuning frequency of a regular ported box and now I'm pondering whether or not to even build the t-line, if I can get more output from a 24hz port tuned box, because I have a feeling the acoustics of the cabin will even out the 24hz peak I would get with a port tuned box.

keep_hope_alive, what would you say about my situation? What should I do for sub-30hz output if the cabin acoustics drop off at sub-30hz? I have no way to measure it, but I have a very decent ear and I can tell it drops off around 30hz and it doesn't matter if the windows are up or down, it just gets quieter with the windows down. 30hz and below still moves plenty of air and shakes everything but the sound isn't there, and I know for an absolute fact I can hear down to the 17hz range because my t-line in the house hits unbelievably low and you can hear the actual sound and not just the air moving, all the way down to 17hz, so I'm not 100% sure but I'm pretty sure it's the cabin acoustics that I'm not happy with so whatever I build, I want to tune it around the cabin to get rid of the sub-30hz drop off.

Also, keep_hope_alive, you said some other time that a box in the trunk of a car creates a 3rd order bandpass and with the windows down a 6th order bandpass or something like that (sorry, I'm not familiar with bandpass boxes), so would a certain bandpass box be ideal for re-creating the sound of a trunk car in an SUV, so to speak? Or should I just go with a t-line?

Tuning to the driver fs is the point of a T-Line. It's not "too low" for that sub. The allure is a flatter frequency response without a large peak at the tuning frequency (which any vented box can do depending on the Q).

It's questionable what you're "hearing" around 20Hz. You are likely hearing harmonics of the fundamental and not the fundamental itself. in a home, it's easier to hear harmonics simply due to the orientation and placement of sub and listening position.

I measured this in my old IB setup (2001 Accord with a pair of Oz Audio 300H on an xtant 3300c) - 12Hz sine wave (i generated the signal), measured and recorded on a 24-bit system, analyzed on custom software with 1Hz resolution. The OZ-300H has a fs of 40Hz.

As you can see, plenty of harmonics there in the audible range.

I think the loudest trunks act like a 6th order bandpass - sub in a ported box firing into a ported chamber. the trick is to build the ported box for the trunk airspace and vent. then maybe vent the trunk through the rear deck or rear armrest to actually tune the 6th order you create.

i'm in desperate need of a new sub box. the combination of my box in my trunk is pitiful. i'm tuned to high and the box isn't the right size for the trunk. very large trunks make it difficult to make small boxes work well in anything but a rear corner. best location in my car is rear corner but i don't like that physical arrangement for storage and access. I need to go IB. I also have a pair of RF HE2 10's sitting around, one of which is destined for a TL. If my amp wasn't under my rear deck, i'd cut through my rear deck and load off the rear glass.

 

[keep_hope_alive Premium Member]

LDC 7th Ed

"TLs are a class of the device that perform as a phase inverter for the low-frequencies, allowing the energy of the rear of the woofer to be combined with the energy of the front of the gone (which is the goal of a vented or PR design), and simultaneously provide an approximately first-order low-pass transfer function to attenuate high frequencies... TL speakers are short compared to the wavelength being produced and have horrendous resonant modes that require significant damping (the inclusion of a seriously resistive element)... Transmission line speakers can roughly be characterized by:

1. Low cabinet resonance

2. Low frequency response

3. Highly damped impedance peak

4. Deceased cone motion around fs with increased infrasonic cone movement (need for an infrasonic filter)

5. Low degree of midbass coloration

6. Low efficiency

7. High -pass roll-off and group delay similar to a well damped sealed box (Qtc 0.6-0.7)

"

while most of the LDC information is in regards to full-range speaker cabinets, the above still hold true for subwoofers.

 

[keep_hope_alive Premium Member]

The other factor to concern is wave development. Long wavelengths never fully develop in the car but can in a room.