Quarter Wave / T-Line tutorial-UPDATED

[Casparado]

OK I could use a little help. I read 34 pages into this thread and ran out of time/patience/brain power (sorry) so I am hoping I can just get an answer. 

I am designing a new box for my B2 Ref12 and after reading this thread I really want to go T-line. However my overall box volume is limited to 17"x21"x42". 

The speaker parameters are:

Fs: 20.8hz.  Qts: 0.26 (Qms 3.4) 

I feel like it would work well, but the resonant frequency is so damn low!  In order to hit that resonant frequency I would need a port length of roughly 13.5ft, a port volume of 95in^2 (on the lower side) and using 3/4"MDF, my box would have to be 28" tall, which would block my rear window and pretty much create a wall. If I lose one of the folds in the port, thereby reducing the overall length to around 10 feet, it would fit my space perfectly... however it would raise the tuning to around 28hz, which is well above the Fs of the sub. 

Is this going to eliminate the benefit of going T-line if I go 28hz tuning on a 20.8hz Fs sub? Can I use polyfill to mitigate that? I can't lose my rear window as my truck is a daily driver for work... 

Thank you VERY much for any help! 

 

(edit) after a lot of pen on paper, I have two choices that will kind-of fit my area, but both are compromises. 

1) 80in^2 port area, 13.5 feet long, = tuned to 21hz (perfect for Fs of sub) but is a few inches too tall and the port might be constricted just a tiny bit... I can fit it but it starts to obstruct my rear window. Also the baffles would have to be 1/2" MDF to make it fit, but they're only baffles and not outer walls, so I don't think that will hurt anything. 

2) 96in^2 port area, 10ft long(ish) = tuned to 28hz. Port area is perfect, but length is a little short for the Fs of the sub. Fits my area perfectly... no obstruction. 

 

Sorry for long winded post.... my last question is how many octaves above tuning will these boxes play? 

[Casparado]

ok I have officially read this whole thread. It really got interesting around page 140++, and a lot of my questions got answered there. I am going with this design... slightly modified to fit the back of my ranger: (The 1/3 mount) 

However I still am wondering how it is going to play one and two octaves above tuning. From what I am gathering T-line boxes play well within one octave of their tuning, in either direction, but once you go beyond that they start to have issues. Am I correct in assuming so? So if I tune to 28Hz, it should play well at 55-60hz but will start to cancel itself out at around 110-115? Also, how will it respond to the all important 40Hz and 80Hz, which are respectively almost exactly half way in between the octaves? I know I am oversimplifying things but I am just trying to get my head wrapped around it. FWIW I downloaded Hornresp but ... yeah... that's WAY beyond me. 

[Triticum Agricolam]

2 hours ago, Casparado said:

ok I have officially read this whole thread. It really got interesting around page 140++, and a lot of my questions got answered there. I am going with this design... slightly modified to fit the back of my ranger: (The 1/3 mount) 

 

However I still am wondering how it is going to play one and two octaves above tuning. From what I am gathering T-line boxes play well within one octave of their tuning, in either direction, but once you go beyond that they start to have issues. Am I correct in assuming so? So if I tune to 28Hz, it should play well at 55-60hz but will start to cancel itself out at around 110-115? Also, how will it respond to the all important 40Hz and 80Hz, which are respectively almost exactly half way in between the octaves? I know I am oversimplifying things but I am just trying to get my head wrapped around it. FWIW I downloaded Hornresp but ... yeah... that's WAY beyond me. 

Ok. I'm going to try to answer some of your questions.  

First off, you don't HAVE to tune the t-line to the Fs of your sub.  Sometime it makes sense to do that, but the Fs of your sub is so low its not practical.  Instead tune to a few Hz above the lowest frequency you want to play well.  

Secondly, you don't HAVE to make the t-line area equal to cone area.  Making the t-line area bigger increases output around tuning, but lower mechanical power handling.  The trick is finding the right balance.  

T-line do boost output around tuning, the output falls off below that though. You are not going to get good output one octave below tuning, and you will mostly likely damage your sub if you try to play that low.  You can get good output much more than one octave above tuning, especially if you place the driver 1/3 down the line so you eliminate the first resonance dip.  

My suggestion for an enclosure would be about 62 sq in of line area tuned to around 30 Hz (~108" total line length) with the sub placed 1/3 from the closed end.  Here is the raw frequency response: 

Making the line 62 sq in instead of something larger reduces the peak in output around tuning.  There is still a significant peak, but you can dial that down by putting some polyfill in the line and adjusting it until you get the sound you want.  Its better to error a bit on the high side and not eliminate the peak completely by reducing line area because you can always add polyfill to take the peak down, but you can't do anything if you made the line area too small.  

By placing the sub 1/3 down the line you eliminate the big peak that would be at 90 Hz and subsequent dip that would be right around 120 Hz.  As such you should get good output to well over 100 Hz if you so desire.  

 

Just in case you want to play around with HornResp, here is the inputs I used:

[Casparado]

Thanks, man! That looks mighty peaky... Based on that graph, do you think it will sound good? 

[Triticum Agricolam]

1 hour ago, Casparado said:

Thanks, man! That looks mighty peaky... Based on that graph, do you think it will sound good? 

It does look peaky, but my experience is those peaks aren't as pronounced as HornResp makes them look.  A little polyfil will tone that peak down significantly.  Here is how the frequency response looks with just 4 oz of polyfil added, and you could certainly add more:

[Casparado]

Holy cow I finally got a little time to watch some tutorial videos on HornResp, and WOW is it powerful! I just modeled my sealed box to compare the output chart to the T-line, and holy cow I can't wait to build one now! 

Thank you SO MUCH for all your help! 

 

Another quick question. According to the driver displacement chart, it's going to exceed the Xmax around 25Hz, and at 20 Hz and below it's exceeding it drastically. I have some music with extremely low frequency sweeps in it... How worried do I need to be  about bottoming out the woofer and damaging it? Also the excursion drops to nearly 0 at the tuning frequency. Is this normal? 

[Triticum Agricolam]

4 hours ago, Casparado said:

Holy cow I finally got a little time to watch some tutorial videos on HornResp, and WOW is it powerful! I just modeled my sealed box to compare the output chart to the T-line, and holy cow I can't wait to build one now! 

Thank you SO MUCH for all your help! 

 

Another quick question. According to the driver displacement chart, it's going to exceed the Xmax around 25Hz, and at 20 Hz and below it's exceeding it drastically. I have some music with extremely low frequency sweeps in it... How worried do I need to be  about bottoming out the woofer and damaging it? Also the excursion drops to nearly 0 at the tuning frequency. Is this normal? 

Lol, yeah there is a LOT you can do with HornResp. 

Nice job looking at the driver displacement.  So T-lines are very similar in function to a ported box, they pretty much accomplish the same thing, just in a different way.  Like a ported box T-lines unload much below the tuning frequency and you do need to give that some consideration.  Hopefully your amp has a subsonic (highpass) filter.  You should set it to a half octave below the T-line's tuning frequency.  This will protect your sub.  If you want to be able to play at lower frequencies you will need to tune the t-line lower, which is as simple as just making the line longer.  You can simulate the effect the subsonic filter will have in HornResp.  Most subsonic filters are 24 dB/octave (4th order).  

It is perfectly normal for the excursion to drop to nearly zero at the tuning frequency.  Pretty much all resonant enclosures (ported, t-lines, bandpass) do this.  This is where the cone control come from.  At the tuning frequency all the output comes from the t-line/port and pretty much nothing comes from the cone.  

[9264RANDY]

This is an amazing thread.

I have (2) 08S8L72 Kicker Solo Barics and had a KX600.1 powering them.  These are the DVC 2 ohm version of the square subs.

Is it possible to create an isobaric transmission line enclosure with them and using the KX600.1?

Well, I appreciate any help anyone can give me.  I used to be in CarAudio back in the 80's-90's but now I am trying to build my 17 year old son a surprise system for his 18th Birthday.

Take car,

Randy

[Triticum Agricolam]

5 hours ago, 9264RANDY said:

This is an amazing thread.

I have (2) 08S8L72 Kicker Solo Barics and had a KX600.1 powering them.  These are the DVC 2 ohm version of the square subs.

Is it possible to create an isobaric transmission line enclosure with them and using the KX600.1?

Well, I appreciate any help anyone can give me.  I used to be in CarAudio back in the 80's-90's but now I am trying to build my 17 year old son a surprise system for his 18th Birthday.

Take car,

Randy

Sure, you could make a transmission line for them, but why isobaric?

[9264RANDY]

Just do to space issues.  I wanted to get really clean bass.  I was thinking isobaric because it would minimize the enclosure size.