opinions of T/L box V ported OR sealed - diyAudio
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Old 27th November 2003, 06:21 AM   #1
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Default opinions of T/L box V ported OR sealed

I am thinking of making a set of T/L speakers.

Could someone explain what is the main advantge or dis advantge of them over ported or sealed boxes
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Old 27th November 2003, 01:39 PM   #2
Wizard of Kelts
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A) Ported boxes have lower F3's for the same size box than sealed does. If you take a speaker with an Fs of 30 Hz, a Qts of .4 and a Vas of 2 Ft³, (56 liters), and put it in a box equal to it's Vas, (2 Ft³), which is tuned to 30 Hz, you will have an F3 of 30 Hz.

Put it in a sealed box of the same volume, and you will have asn F3 over 50 Hz, albeit one with a more gradual cutoff slope that splices into "room gain" better.

Lower F3 can be obtained with sealed boxes with a higher Q. when put into a sealed box, the driver Qts raises up.

Below is a chart where a driver with a Fs=30 Hz and Vas=2Ft³,(56 liters), is put into a 2 Ft³ box. The Qts of the driver is varied to yiels the Qtc, (Q in Closed Box), of 0.5, 0.7, and 1.0, respectively. They are compared to a ported box with Qts=0.38, 2 Ft³ tuned to 30 Hz, put into the same box.

Blue is a ported box, tuned to 30 Hz.
Orange has a Qtc of 1.0
Purple has a Qtc of 0.7
Red has a Qtc of 0.5


Note that in all cases, the box is 2 Ft³, the Fs=30 Hz. The drivers all have the same sensitivity-88 dB. Only the Qtc varies among the sealed boxes, the blue is a ported box with the same box volume.

There is quite a difference in bass output there, with the ported box holding an edge in F3.
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File Type: gif sealed and ported comparison.gif (18.3 KB, 276 views)
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Old 27th November 2003, 01:51 PM   #3
Wizard of Kelts
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As you can see, the edge in F3 goes to the ported box, with a sealed box of Qtc=1 not far behind.

Further comparison points-once a ported box goes below it's tuning frequency, the woofer excursion goes out of control. However, sealed boxes can go below their resonance frequency without going out of control.

Big point in favor of the ported box-cone excursion. The deeper the bass, the more air must be moved. The driver in a sealed box must move all the air to produce a note itwelf-and as those bass notes get lower and louder, it can run out of ability to move the required air.

A ported box, at it's tuning frequency, is required to move only one quarter as much as a sealed box to produce the same bass output. How? the ported box reacts with the air in the box so the bass output explodes out the port-it is very efficient in the lowest octave above the tuned frequency. Below the tuned frequency, the ported box is pretty useless. On the other hand, at 30 Hz, you can see that the sealed boxes of 0.7 and 0.5 really aren't putting out much at all. the Qtc=1.0 is putting out pretty good, though.
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Old 27th November 2003, 02:05 PM   #4
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Transient response, "ringing", and overshoot varies for the various Qtc of sealed box. Although the Qtc=1 has a small "hump" in the response, it's transient response is still much better than the ported box. I don't have the graph handy, but the ported box has a transient response approx equal to Qtc=1.5.

So sealed boxes of Qtc = ).5, ).7, and 1.0 all have transient response superior to the ported box.

It should be noted that the lower the bass, the less such things are hearable. What is noticeable up where the guitar and violins are playing, and where the tenor singer is singing, is not noticeable down where the electric bass is playing. Still, the response is snappier in the sealed box, if you can notice it.

The graph below is by Richard Small, of the Thiele-Small parameters, from his famous article on loudspeakers in closed boxes. The values in purple were added by me, just for clarity's sake.
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File Type: gif step response-closed box.gif (5.2 KB, 282 views)
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Old 27th November 2003, 02:17 PM   #5
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I cannot speak to much of Transmission Lines, having never built one. But I can make a few comments.

Once mysterious structures that only occasionally came out correctly, a set of equations by Martin J. King has now made the Transmission Line predictable. Apparently, the Transmission Line requires a little more volume than the Ported Box to achieve the same F3, but also augments the bass response of the driver. Throughout the heavy bass range, one woofer in an open-ended Transmisison Line is about equivalent to two sealed woofers as far as bass output goes. Transmisision Lines have had a reputation for many decades of giving smooth bass, even when nobody knew that much about how to build a good one. Now they do.

Graphing capability for Transmission Lines are available at Martin J King's site,
www.quarter-wave.com.

If you have Norton AntiVirus on your computer, let me know before you download these-there are special instructions you must follow or you will have big trouble.

The moderator of this forum, Planet 10, also has a very instructive Transmission Line site, located at:
http://www.t-linespeakers.org/
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Old 27th November 2003, 04:35 PM   #6
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Thanks
That's a whole lot of knowledge you dropped on us here.
Very educative.great staff.

Bartek
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Old 27th November 2003, 09:28 PM   #7
GM is offline GM  United States
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>I don't have the graph handy, but the ported box has a transient response approx equal to Qtc=1.5.
====
Oh really?! If you compare the impulse responses of a max flat BR (Qtb = 0.7, what you simmed) to a max flat sealed (Qtc = 0.7) you'll be hard pressed to see any difference.

GM
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Old 27th November 2003, 10:40 PM   #8
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Quote:
Originally posted by kelticwizard
Transmission Lines
There is something to be said about the low pressure nature of a TL, and the flattening of the impedance curve....

dave
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Old 27th November 2003, 10:50 PM   #9
ronc is offline ronc  United States
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Without pulling out a bunch of graphs and getting into the math, lets talk about sound.
I am not a real big fan of BR as to me it is a more or less single note bass. A sealed box well constructed gives little away to a BR, sure the BR may hit a lower frequency with a higher SPL at that frequency but the sound is almost singular while a sealed will give a better LF BW response.
Next up is TLs, they can go around 1/2 octave below tuning frequency.A properly tuned TL is far superior in LF sound quality than either of the 2 above. You may have some comb effect on some designs but with proper tuning it can be reduced to an acceptable level.
IMHO the best LF resonse (outside of horns) is a TQWT.Yes you have to fiddle with the stuffing and they are larger than some TLs and for sure larger than BR or sealed, but the results are worth it.
My last TQWT was based on the RS1354a with a silk dome tweet rolled in at 10Khz and had an aperiodic port(easy to tune) and the results were outstanding.
ron
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Old 27th November 2003, 10:56 PM   #10
ronc is offline ronc  United States
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Default Like This!

Yes larger than a BR or sealed, but......


http://community.webtv.net/roncla/RS1354ainaFolded


ron
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