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Author Topic: Sealed, ported or TL....??  (Read 4771 times)

OTR-jkl

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Sealed, ported or TL....??
« on: September 09, 2004, 12:30:23 PM »

- Which type of monitor cab design do you prefer?
- What do you like/dislike about each?
- Does one work better in certain room sizes? (My room is small...)
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Level

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Re: Sealed, ported or TL....??
« Reply #1 on: September 09, 2004, 01:14:05 PM »

Loudspeaker design is an art of optimization. Using thiel/small parameters, certain woofers based on these parameters work better in certain enclosures.

A ported box has a 12dB/Octave slope below the fe3 (lowest frequency at 3db down in a free field environment) but usually have a higher sensitivity within its range.

A sealed enclosure will have a 6dB/Octave slope below its fe3

Sealed can ultimately go lower but sealed systems are generally lower in sensitivity. (db/1w/1M) The internal "air spring" keeps the woofer in a controlled environment.

TL enclosures can be described as variable VAS in the acoustic sense and variable QTS and QMS in another. (acoustical/electric) The added advantage is the ability to reach the lowest octaves but the enclosures are quite large. I do not like to use a TL in a monitor environment unless the crossover is so very low that it only gets signal below 35hz. D Collins mentioned how "slow" a TL design sounds.

One of the most promising ideas is that of the ported transmission line. In a TL, the end of the line is not a port but a pressure release. In the Ported TL, an actual tuned port is used part way down the line. Wierd, but they can go very low with a reasonable sized enclosure.

Of course their are folded horn enclosures as well. Huge and efficient.

In retrospect, their is no golden rule of one being better than another. In my useage, I used ported JBL's in the 70's, Sealed satellites with TL subs in the 80's, Sealed in the 90's and I still use a sealed loudspeaker system today.

All loudspeakers are a controlled set of trade offs and compromises.

I honestly can't tell you what is better. They all have their ups and downs. I like a sealed enclosure due to the ability to hit the bottom octaves in a reasonable sized system but they require a bit of power to work well.
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chrisj

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Re: Sealed, ported or TL....??
« Reply #2 on: September 09, 2004, 02:17:45 PM »

I'm using an evil combination of 'all of the above' Wink
If you combine sealed and ported, you get a ported where the port doesn't seriously boost bass- it just humps up the deep-bass roll-off a bit, which sounds good. Then if you don't have the port in front to be listened to, the effect is like unloading the pressure of the enclosure a bit.
If you combine internally open and transmission line, you get an arrangement where the back wave in the enclosure is somewhat obstructed- not as confined as in a TL, but not as open as a truly spacious box. This restricts the bass somewhat, but it also controls it, where the open box is at the mercy of its resonances (and stuffing it will give it TL-like qualities)
I'd say just experiment. There is no substitute for experimentation and willingness to listen.

genericperson

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Re: Sealed, ported or TL....??
« Reply #3 on: September 09, 2004, 03:52:36 PM »

good question, great answers.

one little tidbit of info: ported designs have a resonance spot where the port is tuned to.  this can cause an amplitude rise at that frequency point.  ports can also have a somewhat deliterious effect on pitch around the port center frequency.  the messing-with-the pitch thing is something that makes me steer away from ported designs.

i'm a noobie to the m.e. world, so discount what i say by 90%. but i do have a keen ear for pitch, and would personally avoid spending a lot of money on a loudspeaker system that skews the pitch in any way.

i just wanted to put that out there, and i'm hoping the experienced pros will offer elucidation on this aspect of ported enclosures.
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chrisj

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Re: Sealed, ported or TL....??
« Reply #4 on: September 09, 2004, 09:08:28 PM »

No, a port is not necessarily tuned to any given resonance point. That is what you do if you want a car stereo thunder box, the way to get the highest SPLs out of the thing. You can tune it lower, and if you do that it broadens that resonance point considerably- take your pick how broad and well-behaved you want the bass boost, anywhere from the resonance point down to effectively sealed box.
Just because online calculators can be found that tell you how to tune the port for maximum output doesn't mean that's what you would ever want to do.

genericperson

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Re: Sealed, ported or TL....??
« Reply #5 on: September 10, 2004, 12:53:58 AM »

Isn't there an unavoidable minimum amount of resonance in a port?  Many musical instruments will resonate louder at certain pitches that have a strong harmonic symmetry based on the instrument's dimensions and other characteristics.  I would imagine that intelligent engineering can adjust and/or reduce this effect in a ported loudspeaker enclosure.  But when a note is in the wavelength-sweetspot of the port, I can't help but assume that the fundamental of that note would receive extra loudness due to sympathetic resonance.  The very intention of a port is to increase output efficiency.  Unfortunately the static dimension of the port will favor certain frequencies.  (Unless I'm completely missing something here.)

Maybe they will come out with a variable dimension port with digital look-ahead features and high-speed precision adjustment mechanics.  But that's probably an impossibility in this century.  Shocked  Smile
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Immanuel Kuhrt

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Re: Sealed, ported or TL....??
« Reply #6 on: September 10, 2004, 08:16:02 AM »

Think of port design as using a parametric equalizer. You choose a certain resonant frequency, but it is not just one frequency getting a boost. The boost is spread around the resonant frequency - just like with a parametric, where the Q value decides the spread. I have not heard of ways to control the Q of a port though (there could be a way I do not know - in that case, I would like a link to information on the subject.
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chrisj

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Re: Sealed, ported or TL....??
« Reply #7 on: September 10, 2004, 12:54:39 PM »

No no- it's not controlling the Q of the port exactly (though surely you could do that the same way you control the Q of a Helmholz resonator?). What I'm saying is that tuning all the resonances of the system together causes a BIG peak in the bass. Spreading them out (tuning the port lower) means you don't get the peak, you don't get as much bass power, but you can take the bass extension down farther, and it'll be more well behaved. It's strictly about taking the same amount of energy and distributing it differently. The port doesn't actually have a different Q, but the overall system sure does.

Phillip Graham

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Re: Sealed, ported or TL....??
« Reply #8 on: September 10, 2004, 02:36:32 PM »

Level wrote on Thu, 09 September 2004 13:14

A ported box has a 12dB/Octave slope below the fe3 (lowest frequency at 3db down in a free field environment) but usually have a higher sensitivity within its range.

A sealed enclosure will have a 6dB/Octave slope below its fe3


Very close.  A sealed enclosure will roll off at 12dB/octave, and a vented box at 18-36dB/octave.  A driver in air free air will also roll off at 12dB/octave below its FS.  This is a consequence of the behavior of radiation impedance.

Quote:


Sealed can ultimately go lower but sealed systems are generally lower in sensitivity. (db/1w/1M) The internal "air spring" keeps the woofer in a controlled environment.


It should be mentioned that even though the sealed box generally has more extension, the vented box has more output, because of the excursion protection afforded the driver by the resonant air mass in the port.

Quote:


TL enclosures can be described as variable VAS in the acoustic sense and variable QTS and QMS in another. (acoustical/electric) The added advantage is the ability to reach the lowest octaves but the enclosures are quite large. I do not like to use a TL in a monitor environment unless the crossover is so very low that it only gets signal below 35hz. D Collins mentioned how "slow" a TL design sounds.


TL designs are fractional wave pipes which use (typically) the 1/4 wave eigen mode to support the driver.  The systems are then stuffed w/ lossy material to quell the q of the resonance, and are often overdamped in the process.  This, and the lack of harmonic content being reradiated through the cone out of the cabinet is likely where the "slow" reputation comes from.  In a sense the TL may be thought of as very bad horn, which only had a  good impedance match the air at one or two eigenmodes, whereas a real horn has a pretty good impedance match over a reasonably extended bandpass.

Quote:


One of the most promising ideas is that of the ported transmission line. In a TL, the end of the line is not a port but a pressure release. In the Ported TL, an actual tuned port is used part way down the line. Wierd, but they can go very low with a reasonable sized enclosure.


I am not familiar with this design but it sounds like they are trying to moderate the above issues w/ the TL.  You make it shorter and increase it's modal density, and then use a helmholtz resonator to give you some extension.

Quote:


All loudspeakers are a controlled set of trade offs and compromises.


Agreed heartily.  The sealed box and horn, however, emerge as the designs with the best overall phase response.  Also, the horn works as a series lowpass to the driver, attenuating the extensive out of passband harmonics all moving coil speaker drivers produce.  Almost all the deigns have their place, though, and the inherent flaws in certain designs can actually be euphonious for certain types of music.

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Phillip Graham

Phillip Graham

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Re: Sealed, ported or TL....??
« Reply #9 on: September 10, 2004, 02:46:13 PM »

chrisj wrote on Thu, 09 September 2004 21:08

No, a port is not necessarily tuned to any given resonance point. That is what you do if you want a car stereo thunder box, the way to get the highest SPLs out of the thing. You can tune it lower, and if you do that it broadens that resonance point considerably- take your pick how broad and well-behaved you want the bass boost, anywhere from the resonance point down to effectively sealed box.
Just because online calculators can be found that tell you how to tune the port for maximum output doesn't mean that's what you would ever want to do.


A vented box system does indeed have a definable resonant frequency, which appears at the minima in cabinet impedance b/t the two peaks, and as the observed minima in the driver excursion point.

Richard Small extended Theile's analysis to include 16 different ported box equivalent circuit alignments, all with defined Q, passband ripple, phase response, and rolloff points.

As a general rule, the lower you tune a vented design, the lower in frequency its phase response, and therefore group delay, begins to suffer.  Your ear is also less sensitive to these effects at the lowest frequencies.  The tradeoff here is that if you tune a ported box too low, it will not provide excursion protection over the drivers entire low end bandwidth.

This becomes an issue in the design of ported boxes for professional audio applications, where tuning the box too low results in over excursion in 50-70hz range, where most of the important low end thump of live music lies.
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Phillip Graham

Phillip Graham

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Re: Sealed, ported or TL....??
« Reply #10 on: September 10, 2004, 04:41:50 PM »

chrisj wrote on Fri, 10 September 2004 12:54

No no- it's not controlling the Q of the port exactly (though surely you could do that the same way you control the Q of a Helmholz resonator?). What I'm saying is that tuning all the resonances of the system together causes a BIG peak in the bass.



The system's resonance (driver box/combo) is a function of both elements, the driver's prior resonant frequency doesn't have much meaning.

In fact, for drivers with qts of approx. 0.3, and picking the 4th order Butterworth alignment exposited by Small, box Fb is basically identical to Fs of the driver.

Quote:


The port doesn't actually have a different Q, but the overall system sure does.


Both Leo Beranek and R. A. Benson deal with changes in the Q of the resonance of a vented box.  Multiple factors come into play.  One is the resistance of the wire between the amplifier and the driver (influencing Qes).  Another is the loss due to port turbulence.  Another is the loss due to errors in construction and/or loss through the cone.  A final consideration for high power applications is the reduction in Qes from the increase in resistance of the voice coil wire as it heats up.

Pro sound manufacturers often use T/S parameters measured after extensive high power driver preconditioning, and presume approx. 50ft of 10ga. cable as the equivalent series resistance with the driver.
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Phillip Graham

Roland Storch

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Re: Sealed, ported or TL....??
« Reply #11 on: September 10, 2004, 06:01:14 PM »

Phil Graham wrote:

TL designs are fractional wave pipes which use (typically) the 1/4 wave eigen mode to support the driver. The systems are then stuffed w/ lossy material to quell the q of the resonance, and are often overdamped in the process. This, and the lack of HARMONIC CONTENT being reradiated through the cone out of the cabinet is likely where the "slow" reputation comes from.


Do you mean harmanic distortion with "the lack of harmonic content"?
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Phillip Graham

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Re: Sealed, ported or TL....??
« Reply #12 on: September 11, 2004, 02:36:35 PM »

Roland Storch wrote on Fri, 10 September 2004 18:01

Phil Graham wrote:

TL designs are fractional wave pipes which use (typically) the 1/4 wave eigen mode to support the driver. The systems are then stuffed w/ lossy material to quell the q of the resonance, and are often overdamped in the process. This, and the lack of HARMONIC CONTENT being reradiated through the cone out of the cabinet is likely where the "slow" reputation comes from.


Do you mean harmanic distortion with "the lack of harmonic content"?


Well, loudspeakers can be pretty nonlinear devices, especially the little minimonitor types when pushed to high monitoring levels.  All nonlinear devices will produce both harmonic and anharmonic signals that we call distortion.  Also, any excessive resonances of the cabinet structure will alse be reradiated out through the speaker cone.

Does that answer your question?
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Phillip Graham

Roland Storch

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Re: Sealed, ported or TL....??
« Reply #13 on: September 11, 2004, 03:57:03 PM »

Yes Phil, that answers my question and I completely agree the point with the slow reputation (for some here in this thread) of TL.

As I stated before TLs are not at all slow for me, I find them faster than ported systems - and cleaner (meaning more transparent) too.

And your explanation fits exactly in my hearing experience, thanks.
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Phillip Graham

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Re: Sealed, ported or TL....??
« Reply #14 on: September 12, 2004, 02:01:24 AM »

Roland Storch wrote on Sat, 11 September 2004 15:57

Yes Phil, that answers my question and I completely agree the point with the slow reputation (for some here in this thread) of TL.

As I stated before TLs are not at all slow for me, I find them faster than ported systems - and cleaner (meaning more transparent) too.

And your explanation fits exactly in my hearing experience, thanks.


Hey Roland,

I personally have almost no experience with TL's, but I'm glad to hear the science aligns with what you hear.  The speakers I have designed are mostly for pro sound apps, and the TL has no real place there.

As an aside, the little Bose "wave" radio, and their latest generation of "acoustimass" bass products are both essentially transmission line designs.
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Phillip Graham
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