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Old 16th September 2014, 06:52 PM   #1
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Default Portable Boombox in a T-Line

Hello all, I'm wanting to create a portable boombox and want to use this oppertunity to expand my scope of builds into my first Transmission Line. I've been doing some cinsiderable reading on the subject and I *think* I'm getting the hang of most of it.

Before I continue further I thought I'd ask your advice... I don't have access to MathCad /sketch up at school, but I wanted to post a mockup of the t-line and dimensions so that you guys could yay or nay my current design...

I'm building the enclosure around the Tang band 5 1/4" speaker (fs 55hz).
Tang Band W5-704D 5-1/4" Woofer
crossed with a 3" fountek driver:

After reading MJK's work and tables I've concluded that the line needs to be aprox. 49" in length with a cross section of 14.57^2" (94cm^2... the reduction from a 61.1" to 49" line is due to the .8 fudge factor- if I'm understanding that right?)

My concern is how close the woofer(s) will be to the mouth of the line and if this is feasible?

The enclosure is already nearly 25" wide, 14" high and 10" deep... I feel if it get's any larger it will no longer be considered in the realm of a "portable boombox."

Any suggestions?
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File Type: jpg bbox-front-view.jpg (101.6 KB, 319 views)
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Old 16th September 2014, 11:54 PM   #2
xrk971 is offline xrk971  United States
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You might try the Accidental Mass Loaded Transmission Line (AMLTL) technique to size your TL and port based on a bass reflex assumption. Search for thread with that name. If the driver is moderate (0.45) to higher Qts it can work well. Does not work as well for low Qts drivers.
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Old 17th September 2014, 02:16 AM   #3
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No need to operate in stereo. Speakers are simply too close together to have much effect. Better to sum mono. Can look at it like this, it's now half the size or has better bass extension when maintaining same internal overal volume as is.
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Old 18th September 2014, 08:08 PM   #4
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I've been considering mono, and it seems more and more likely if i want to do a portable T-Line that gets into the 50s Hz range, however I'd really like to go stereo (I'm getting back into my Zepplin albums and would miss the cross-fades).

I'm planning on making the xo around 150hz for the Tang Band's and the full ranges handling everything above.

Is this a lost cause (without going bass reflex/ported)?
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Old 18th September 2014, 08:42 PM   #5
zobsky is offline zobsky  India
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Perhaps this amp would allow you to cross active and sum bass too
Lepai LP-168HA 2.1 2x40W Mini Amplifier + 1x68W Sub Output (amazon has it too)?

I've been considering something like this for a deep but low profile sound bar to work with our TV table, The wireless soundbar we use has horribly flabby bass and too high a crossover
"Not everything that can be counted counts, and not everything that counts can be counted." - Albert Einstein
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Old 18th September 2014, 08:46 PM   #6
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You won't hear those cross fades unless you are centered and close. So unless it's sitting on a shelf above a work bench, stereo goal is pointless.

I like my Zep too. When designing speakers I always test in mono. Stereo just makes it sound fancier. I wonder how many people listen to that new pair of speakes with only one hooked up and run mono would feel about their purchase
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Old 18th September 2014, 10:36 PM   #7
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I looked into the Lepai amp... but I have a friend aboard now on the project that wants to use the opportunity to build the amp/ switchboard/ fm tuner/LCD display etc. himself while I do the woodwork and cabinet design.

I plan on having the sides/front/back made of wood (maybe pecan since we have a lot of dead trees here), but the top made of plexiglass so the 'guts' are visible - considering all the effort and thought being put into this I'd like to showcase it's innards.

You make good points. I guess I'm just reticent of losing the 'fancy' sound/look of stereo. The trade off of Summing to mono should increase the db/spl levels, yes?

my initial inspiration was Paul Carmody's Sprite:
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Old 18th September 2014, 11:06 PM   #8
sreten is offline sreten  United Kingdom
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TL is not the way to go for portable full stop, bloated and oversized.

Go sealed, outdoors bass is a joke without room gain, or bass vented.

i'd go one bass, L+R mid treble, strereo amplifier with one
channel inverted, one L or R wired out of phase and the
bass unit across L+R positive for bridging mono output.
Simple series L for bass and C for L and R.

rgds, sreten.
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Old 19th September 2014, 12:20 AM   #9
dangus is offline dangus  Canada
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I'd be inclined to keep it simple. Two fullrange speakers, and a highpass active filter so you don't waste headroom on bass they can't handle. Try prototyping using junk particleboard if you want to try anything fancy.
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Old 19th September 2014, 12:50 AM   #10
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I didn't realize a transmission line was that dependent on room gain... I've made several ported and sealed enclosures and was looking to push myself - while extending the bass, and have a visual "cool" factor when viewed from top down (plexiglass clear top).
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