Sealed enclosure Q vs Linkwitz Transformed Q? - diyAudio
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Old 31st December 2006, 01:14 PM   #1
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Default Sealed enclosure Q vs Linkwitz Transformed Q?

Simple question (hopefully) for those who have experience of this...

If you were to take two identical woofers, put one in a sealed enclosure with Qtc = 0.707 and put the other in an enclosure where Qtc = 1.1 or something but use a Linkwitz transform to bring the Qtc and f3 back into line with the first speaker.
Would the second speaker sound the same as the first, or would it have the same characteristic sound (transient performance ect) of a higher Q alignment but with the FR of the first speaker? Would it emulate the first speaker in every way?

Hopefully I worded that so it makes sense
I know what I mean
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Old 31st December 2006, 01:25 PM   #2
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Default Re: Sealed enclosure Q vs Linkwitz Transformed Q?

Quote:
Originally posted by FullThrottleRic
Simple question (hopefully) for those who have experience of this...

If you were to take two identical woofers, put one in a sealed enclosure with Qtc = 0.707 and put the other in an enclosure where Qtc = 1.1 or something but use a Linkwitz transform to bring the Qtc and f3 back into line with the first speaker.
Would the second speaker sound the same as the first, or would it have the same characteristic sound (transient performance ect) of a higher Q alignment but with the FR of the first speaker? Would it emulate the first speaker in every way?

Hopefully I worded that so it makes sense
I know what I mean

Hi Ric

The short answer is no unfortunately. The LT version just bashes the FR into the same sort of shape as the unassisted Q=0.7 sealed. So the tonality would be somewhat similar.

However you can expect all the negatives that go with a smaller enclosure - more power needed like for like, less SPL etc. To top it off you'll find the CSD will be different and possibly worse giving yet further distance to the two.

So it should be clear that the sealed vs. the LT isn't comparable in performance but rather the LT is a compromise where you need a compact sub and a willing to sacrifice SPL and performance in general. The two subs however will sound similar given that they share a similar FR and driver.
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Old 31st December 2006, 01:48 PM   #3
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Argh boobies, bigger box it is then

Thanks mate
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Old 1st January 2007, 04:30 PM   #4
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Hi FullThrottleRic,

I will slightly change your initial conditions, simplifying them, to approach your question.

Let’s consider two drivers with identical parameters, fs = 25 Hz and Qts = 0.5.

The first one is mounted in an infinite baffle of which a good approximation is on a wall between two rooms. The parameters fs and Qts won’t change.
The frequency response, room effects being neglected, is:
-6 dB at 25 Hz, -3 dB about 38 Hz, -2 dB at 50 Hz, -1 at 70 Hz, -0.5 dB at 100 Hz, -0.28 dB at 140 Hz and –0.13 dB at 200 Hz

The second driver is mounted is a closed box with a Vab = Vas/3. The parameters are now fb = 2 x fs = 50 Hz and Qtb = 2 x Qts = 1.
The frequency response, room effects being neglected, is:
-11 dB at 25 Hz, -3 dB about 38 Hz, 0 dB at 50 Hz, +1.25 dB at 70 Hz, +0.9 dB at 100 Hz, +0.5 dB at 140 Hz and +0.25 dB at 200 Hz.

Which gives a difference of local efficiency (in dB) for the second unit:
-5 dB at 25 Hz, 0 dB at about 38 Hz, +2 dB at 50 Hz, +2.25 dB at 70 Hz, +1.4 dB at 100 Hz, +0.78 dB at 140 Hz and +0.33 dB at 200 Hz.

The local efficiency at all frequencies between and 38 Hz and 200 Hz is higher
for the driver mounted in the closed box. To get the same level at all frequencies as the first driver, a transform will lower the voltage level applied between 38 Hz and 200 Hz, which means less power.

Due to the spectral distribution, the average level at 25 Hz is usually –12 dB compared to the average level at 50 Hz (some hard rock music) or at 100 Hz (classic music). For the same acoustic power delivered at 50 Hz by both drivers, the one in the closed box will require 2/3 for the electrical power applied to the other one.

The stiffness of the air in the closed box will dominate that of the driver. As it is usually much more linear than the stiffness of the driver (this is the principle of the acoustic suspension), the overall linearity is increased over the driver on the infinite baffle : the distorsion is less in the closed box.

So mounting a driver in a closed box on which a Linkwitz transform is applied can show some virtues : the enhancement of the efficiency and the lowering of the distorsion.
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Old 1st January 2007, 04:45 PM   #5
sreten is online now sreten  United Kingdom
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Hmmm......

The short answer is they will sound similar, the LT will need more power.
The transient performance will be essentially identical, not different.

If you have power to spare (amp will cause overexcursion of sealed
box), then using a smaller box and a LT is completely reasonable.

/sreten.
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Old 1st January 2007, 06:43 PM   #6
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Sreten,
I've shown quite the contrary. In my example, the Linkwitz transforn needs more power for frequencies where there is not much signal (25 Hz) and less power where there is more (above 40 Hz).
It all depends on the resonance frequency of the closed box.
As stated by Dudley Harwood from the BBC thirty years ago, too low Q fundamental resonances make speakers less efficient than when of higher Q, this is very often forgotten.
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Old 1st January 2007, 07:35 PM   #7
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Quote:
Originally posted by forr
Sreten,
I've shown quite the contrary. In my example, the Linkwitz transforn needs more power for frequencies where there is not much signal (25 Hz) and less power where there is more (above 40 Hz).
It all depends on the resonance frequency of the closed box.
As stated by Dudley Harwood from the BBC thirty years ago, too low Q fundamental resonances make speakers less efficient than when of higher Q, this is very often forgotten.
Correct me if I'm wrong but we are talking about a subwoofer here aren't we?

I assumed that with my initial reply anyway.

For example, as you can imagine 3dB more at 25hz is a big deal. If this is got via LT because its lagging behind then you need twice the power and roughly twice the displacement to hit that.

I'm baffled as to why you'd consider the stuff generally over 50hz a big deal in this case. The LT is simply flattening the peaky nature of a high Q enclosure. Sure you get lower distortion but at this point its completely overshadowed by the additional strain needed to get down low at the same SPL.

If you've got the option to not use an LT and get where you need to go through using volume then its the better solution for obvious reasons of higher SPL at greater depth and less power needed to do so and depending on the specifics quite probably improvements in other area's. The one area that I can see LT being useful is simply for a smaller design or when using robust drivers tailored to high power input and large excursion to reach extreme lows. You can also improve group delay and phase characteristics as a side effect of the minimum phase filtering of an LT.

The way I see it and LT shouldn't be used at all if possible in this context, it really should be used to extend the sub for greater output down below ie. flat to 20hz instead of 30hz etc. But at least get the box resonance as low as possible then add to get the extra output down below. An LT which electrically bashes the response into the shape resembling an enclosure with a physical Q of 0.7 isn't equal to one that's naturally 0.7. What is useful is a sub with a Q of 0.7 that has an LT applied to get the response down to 20hz or whatever, at least in the application of a sub.
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Old 1st January 2007, 08:04 PM   #8
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When I origonally asked the question I was looking into using a SS 18w8531G00 in a smaller than ideal enclosure as a lower mid/upper bass driver.
I'm pretty set now on building Troel's Ekta design but I wanted to add a larger driver to cover the bottom couple of octaves. The Scan woofer seems to get a lot of praise for it's bass performance but a single 7" woofer per side seems a bit girly to me
I want to keep the whole thing around 50L total volume and don't want the mid enclosure taking up half the entire speakers volume as the two woofers I'm looking at (Scan 23w4557T02 or TC Sounds 10" LMS-4000) need around 35-40L to do what I want.
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Old 1st January 2007, 11:31 PM   #9
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ShinOBIWAN

---Correct me if I'm wrong but we are talking about a subwoofer here aren't we?---
As the initial post did not mention any crossover frequency, I do not think the discussion is restricted to subwoofers.

It has been said that if 100% satisfaction is obtained for a system with a flat room response in the range 20 Hz -20 kHz, a 99% satisfaction is obtained for a flat room response in the range 40-15 kHz. As there is much more energy above 40 Hz than below in the vast majority of records, it is preferable to concentrate on the best rendition around 50 Hz than around 25 Hz.

The 5 dB lift required at 25 Hz, less than 4 times more power, in my example is even less than that because of the room gain which is about 6 dB at this frequency.


---The LT is simply flattening the peaky nature of a high Q enclosure.---
The definition of this transform (there are other transforms making the same job) is to change the second hi-passed tranfer function of a speaker to another one where the apparent resonance frequency and its damping can be either decreased or increased. Increasing the apparent resonance frequency is rarely used but is undoubtedly useful in the design of uncompromised active crossovers.

I used to be very skeptical towards transforms till the day I tried one : I heard bass giving a feeling of authority which made me think of motion feedback. Already in the camp of the closed box addicts, it changed my point of view on what can be done with a few electronic components.
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Old 2nd January 2007, 08:48 AM   #10
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Quote:
Originally posted by forr
Sreten,
I've shown quite the contrary. In my example, the Linkwitz transforn needs more power for frequencies where there is not much signal (25 Hz) and less power where there is more (above 40 Hz).
It all depends on the resonance frequency of the closed box.
As stated by Dudley Harwood from the BBC thirty years ago, too low Q fundamental resonances make speakers less efficient than when of higher Q, this is very often forgotten.
Hmmm.......

If you pick the example you can show almost anything you want.

I stuck to answering the question.

Too low Q affects bass efficiency but conversely the lower the Q
the higher the midband sensitivity for most cases, as ever its
a compromise best understood rather than "stated".

/sreten.
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