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Old 3rd March 2007, 04:33 AM   #11
Wizard of Kelts
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Quote:
Originally posted by audiobomber



I've heard that argument about less excursion in ported designs, but it only applies if the signal is above the port's resonant frequency. If you apply a signal below F0, the driver goes into extreme excursion.
Correct. However, the port frequency can be tuned really low.

Take a 12 inch driver in a sealed enclosure.

Take another 12 inch driver in a ported enclosure tuned to 30 Hz.

Drive both so that they produce 110 dB at 30 Hz.

The driver in the ported enclosure will be moving only one quarter as far to produce that 110 dB as the driver in the sealed enclosure.

At some point, the driver in the sealed enclosure will run out of excursion-but the driver inthe ported enclosure will be able to go 12 dB higher SPL.

Yes, below 30 Hz the ported driver will begin to excurse quite a bit more than than the sealed driver. But 30 Hz is pretty low, and down to 30 Hz the ported driver has it all over the sealed driver, SPL, (volume), wise.


Of course, you can tune a ported driver lower than 30 Hz if you wish and stil get the same benefits. I just selected it because it was a common tuning frequency.

Bass is about producing SPL's with a limited excursion ability, (ability of the driver to move back and forth). A ported enclosure, down to it's tuning frequency, simply produces a much higher SPL, (volume level) in the bass for the same excursion ability of the driver.
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Old 3rd March 2007, 09:32 AM   #12
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Wow! How many replies!...

Now I'm a bit confused...

Anyway, I see all around almost exclusively bass reflex enclosures, but I'm agree with some of you about the fact that their bass are unnatural, likely "coloured", while acoustic souspension seems to be more accurate.

From my part I ask why,then, acoustic suspension speaker systems are really unfindable?

They seems to be cheaper and easier to build than Bass reflex ones...

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To me the highest quality, or most realistic sounding is actually a Infinite baffle with a low frequency boost.
Well,

How to get low frequency boost?
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Old 3rd March 2007, 11:53 AM   #13
bjorno is offline bjorno  Sweden
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Quote:
From my part I ask why,then, acoustic suspension speaker systems are really unfindable?
No, To get this right an acoustic suspension speaker system is a subset of the more general sealed enclosures that can be defined if the rear leaks (losses) have a very long time constant in comparison with the system cut-off frequency and the rear side features is not contributing to the system output at all.

Instead the output is solely relying on the control from the contained volume, the acoustical compliance that can take any value but for the case of acoustic suspension this volume is much more dominating than the driver compliance.

The system compliance ratio (alpha=Vas/Vb) for an acoustic suspension is typically >=4 but for a general closed box system alpha can be of any number thus a low value of box volume is selected for an acoustic suspension speaker when compared to the driver Vas volume.

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Old 3rd March 2007, 01:52 PM   #14
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Quote:
Originally posted by kelticwizard
Correct. However, the port frequency can be tuned really low.
With some drivers the ports can be tuned really low, with many others it can't.
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Old 3rd March 2007, 03:40 PM   #15
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I was not really aware that ported systems had pushed sealed systems out so completely. Time was when you could not get a ported system. Perhaps it is just the dealers in your area.

As statistics go, the bass is measured at the -3 dB down point. In other words, we consider a mediocre bass output as being -3 dB @ 50 Hz, a deep bass output as -3 dB @ 30 Hz.

A -3 dB down @50 Hz is not bad for say, a 6.5 inch woofer. For a 12 inch woofer, that would be considered pretty lackluster.

Sealed systems-whether "acoustic suspension" or not-roll off at a rate of 12 dB per octave. Ported systems roll off at the rate of 24 dB per octave-usually. So if you have a ported system and a sealed system, both -3 dB @ 50 Hz, the ported system will seem to have a little more bass, since below that 50 Hz line it will still have some hearable output. The ported system will not have hearable output below 50 Hz.

However, for advertising puposes, the ported and the sealed will seem to be the same if they have the same -3 dB down point. And the ported enclosure will be considerably smaller if both enclosures have the same -3 db down point.

So when the consumer goes shopping, he sees a sealed unit of 28 liters, and a ported unit of 14 or 15 liters, and they both have a -3 dB down point of 50 Hz. The consumer will seem to be instantly drawn to the smaller unit which seems to have "equal" bass to the larger unit.

That is one reason.
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Old 5th March 2007, 09:54 PM   #16
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Quote:
How to get low frequency boost?
Sorry I tuned out and missed the question.

My personal experience of this was in a cinema that I designed and custom built the speakers for. The boost was from a 1/3 octave equalizer within the dolby processor , so simply a 6 db boost at 32 hz.

I would assume that a cheap kit from an electronics shop would do the same thing bass extender
Boosts are also found in some plate sub amplifiers.

Its simply a compromise of quality verses efficiecy. If you like music which features the Double Bass, the difference is noticable. to me its sounds more natural.

Easiest way would be to design a large bass reflex box, listen to the results, temporarily seal the port, boost the sub bass electronically, and then decide for yourself.
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Old 5th March 2007, 10:21 PM   #17
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Quote:
Originally posted by pjpoes
There are too many factors involved to really say which is better. Actually I recently saw an article in The Absolute Sound talking about Subwoofer design inwhich they claimed the superiority of Acoustic Suspension, suggesting that the only reason to use bass reflex is to get lower boomy bass. I disagree, but unfortunatly this is the sort of myths perpetuated in the media, and its also a result of many poorly designed speakers.
Omigosh! I agree with TAS! In about 95% of cases with BR boxes they're correct. In most cases, the boxes are mis-aligned and you don't really have a good connection between the Helmholtz resonator and the driver (ie. if the driver doesn't 'almost' stop movement at some point, then there's not a really good coupling). This is when you start to get boominess. It's louder, but it doesn't sound like an instrument, just noise. I'm not sure that EQ'd sealed enclosures sound much better (apologies to Linkwitz). Basically, there's not substitute for cubic inches...IMHO

Quote:
Originally posted by kelticwizard
As statistics go, the bass is measured at the -3 dB down point. In other words, we consider a mediocre bass output as being -3 dB @ 50 Hz, a deep bass output as -3 dB @ 30 Hz.

A -3 dB down @50 Hz is not bad for say, a 6.5 inch woofer. For a 12 inch woofer, that would be considered pretty lackluster.
Oh no!, my 12 inch boxes are -3dB at 50Hz, -12dB at 20Hz, -21dB at 10Hz. _
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Old 5th March 2007, 10:40 PM   #18
Geoff H is offline Geoff H  Australia
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Cloth ears, don't worry, your twelves are OK. Where are these 6 1/2 inchers that do -3db at 50Hz? It can be done, but at what SPL/watt?
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Old 5th March 2007, 11:15 PM   #19
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oh come now, plenty of 6.5" woofers can do a -3db at 50hz, and with plenty of output. In a ported enclosure anyway, hehe. In a sealed box, I tried modeling some and had trouble, I think the Extemis was the only one that did, and in a very large box.

Only going with subwoofers for the moment, a ported box is more efficient than a sealed box in the low bass. Taking my own Dayton case in point, the Dayton has an F3 of around 47hz, and thus rolls off at 12db's per octave, the ported system will be 12db's per octave more efficient below the point the sealed system starts rolling off. Also, that isn't completely accurate in that the ported system isn't even rolling off at all until 20hz, which is what, about 1 octace down, a little more.

TAS is not right I'm sorry, and I'm currently putting together a test rig to prove my own point. However, there are many far more knowledgable than I that already could not only argue, but show with graphs how what they say isn't so. I currently have in my hands two 12" dayton Refrence subs, one in a 2.5 cubic foot sealed enclosure, with an F3 of 37hz, and the same woofer in a 4.5 Cubic foot enclosure tuned to 21hz, and an F3 of 19hz. I have some preliminary measurements, but I'm still calibrating everything. Even using my ol' RS digital meter modified for a flatter response, and using the associated correction factor, I still have at 1 watt a 2db increase in efficiency from the ported system. Also, Running it outside and doing my measurements as close to free air as I can, same as Stereophile does, my Impulse response from 19hz to 45hz is better on the ported system than on the sealed system. That overhang they show on the ported system has more in common with what I'm seeing from my sealed box. So far distortion measurements are showing it to have as much as 20% less distortion in the very low end, and I calculate quickly an average of 5% better. The only place the sealed system is showing better impulse response, better distortion, and otherwise identical measurements is from 45hz on up. Also, my focal based main speakers have a better measured response, including impulse response, from its low end, an F3 of 42hz on up. Though I actually cross the sub over at closer to 60hz as I prefer the transition, I still feel that designed right, the ported box is the better choice here.

I also think its unfair to characterize a design by the misuse of the design by many manufacturers. Even if you want to use the ol' if it was better everyone would be doing it, look at the majority of high end speakers. In the bass area, most seem to still be going with ported enclosures, rather than sealed enclosures. I know, not 100%, and there are ways to make the sealed boxes perform great, but I still see them preferring ported as a whole.
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Old 6th March 2007, 12:33 AM   #20
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Quote:
Originally posted by pjpoes
TAS is not right I'm sorry, and I'm currently putting together a test rig to prove my own point. However, there are many far more knowledgable than I that already could not only argue, but show with graphs how what they say isn't so.
pj,

I'm not trying to start a fight , I just prefer the sound of sealed to ported. Them plucked or stroked double-bass notes just don't sound like the instrument through a hole. But that's my opinion (and I'm sticking to it !).

And when it comes to efficiency, well, the Lambdas in my bass bins can handle 300w all day, and the amp that drives tham can also. I'm just looking around for a 800W-er to drive my planned subby (15" Lambdas). Something under $400Aus, because I'm cheap...he! he!

I think if you looked at my outfit with graphs, you might say it was down on bass extension. But it sure doesn't sound like it. And when we move somewhere bigger, then there will aways be the bigger sub to handle the extra bass required.

Ported does make more bass noise from a smaller box with less power. But, if you've got the power and the space then you might prefer the sound of the sealed.

Or O/B. Or T/L. Or horn. Which are all outside the scope of this thread.
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