Critically Damped (Q=0.5) Subwoofer - Page 3 - diyAudio
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Old 19th November 2003, 07:13 AM   #21
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Im very familiar with those Qtc graphs,but only for sealed boxes.

Quote:
If you survey the step response in the thread I gave you before, you should know that that the cleanest ported box's ressponse is worse than a sealed box with a Qtc of 1.3.

The "classic" bass reflex, where Vas=Vb, and Fb=Fs=F3, has step response worse than a closed box with a Qtc of 1.6.
did he thoroughly test it?


it cant be true.....

Why does my EBS style overdamped ported box jbl 15" Sound so bloody good then/?

i would NEVER build a qtc=1+ sealed box system....

*reels in shock at this new revelation*

i was thinkn that a highly damped (overdamped )ported box would sound FINEE and is nothing like Qts=1.3

how does one come to that conclusion-Maths or simply measuring it?

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Old 19th November 2003, 01:32 PM   #22
Wizard of Kelts
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Quote:
Originally posted by mikee12345
Im very familiar with those Qtc graphs,but only for sealed boxes.
These are for sealed boxes.



Quote:
it cant be true.....


It is from Richard Small's, (of Thiele-Small fame), paper. If it isn't true, most of the loudspeaker enclosures of the past 30 years have been built under false pretenses.


Quote:
Why does my EBS style overdamped ported box jbl 15" Sound so bloody good then/?


As stated perviously in this paper, the lower you go, the less distortion is hearable. As also stated earlier, I also prefer ported boxes, because they give greater output with less cone excursion.

You can talk all you want about distortion, but isn't aren't you distorting the music if a bass note is there and your system doesn't play it?

I find it much preferable to having a low bass note actually be played, slightly distorted, than not being played at all. As Woody Allen once said, "Eighty percent of anything is just showing up".


Quote:
i would NEVER build a qtc=1+ sealed box system....


Suit yourself. But the distortion is much less than even the cleanest ported systems.

With all this accent on low Qtc sealed boxes, a few facts are being lost. When sealed boxes became figured out back in the fifties, designers aimed for a Qtc between 0.7 and 1.0. A Qtc of 1.0 was the aim of many box designers, because at resonance, the output is still at a level with the midrange. If you have sufficient excursion, a Qtc=1 gives output not far behind a ported box. Therefore, a little above Qtc=1 is not going to sound bad.



Quote:
how does one come to that conclusion-Maths or simply measuring it?
[/b]

I believe Small simulated it. But it apparently works out close enough in the real world.


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Come on now. Things are not as gruesome as all that.
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Old 19th November 2003, 01:44 PM   #23
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Mikee:

By the way, I have not mastered the program, but I understand you can simulate any setup using Unibox.

Also, I believe that Martin King has a MathCAD worksheet for ported boxes that will also simulate the step response of any ported box.

I am not sure if the step response will be affected by the equalization necessary with an EBS box. But at least the simulations will give you an idea.

I have been meaning to post Small's simulations for ported boxes for a long time. Maybe later today or tomorrow. But you can go ahead and simulate yourself if you wish.

By the way, what is the JBL model number, what is the size box you put it in, and what is the Fb of the box?

Thiele-Small numbers would be appreciated, but I can probably look them up.
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Old 19th November 2003, 07:55 PM   #24
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jbl2226h
Fs=40hz
Qts=0.31 i think
Vas=175L
xmax=7.6
Pe=600

175L box (internal)33hz tune
www.geocities.com/sc00byd0159
Im very happy with it ! sounds very good,the response should be shown on my site.i dont know what my room gain is but its a small low ceiling room ,about 4mx2.2m with a hollow bassment underneath! :-S


Quote:
If it isn't true, most of the loudspeaker enclosures of the past 30 years have been built under false pretenses
*is very sad*

but i thought ported boxes had a sort of equivalent Q to sealed,by the response peak

Ive just never heard anything about ported box Q beign that bad.

eg ported box flat response= Similar to Qtc=0.7 but slight resonance due to port stored energy etc..

I dont EQ my jbl,its sort of rolled off ,when i get more power(eg more than 10watts rms ) i may EQ it flat

Quote:
I believe that Martin King has a MathCAD worksheet for ported boxes that will also simulate the step response of any ported box.
OH yeh i remember doing that for Tlines it was interesting.How does one calculate that stuff!!not easy to predict? combination of port Q and driver Q and whatever else/?

Yep the only worry here for me ,was the Ringing/qts POrted vs sealed issue the rest i know,eg the visable ringing on the Qtc values i know those.

ive just never heard anyone say that my dear to my heart ported boxes with flat response exhibit Qtc=1.3 sort of Q responses

il get the mathcad sheets and model up my JBL sometime soon(exam time at moment )
Thanks guys
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Old 19th November 2003, 07:57 PM   #25
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oh crap GEOCITIES has mucked up my MSWORD html page,the jbl one haha,the response pic is now a blown up image of a box!

so il go fix that very soon.

the actual efficiency of the jbl in this box is like 93db

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Old 19th November 2003, 10:44 PM   #26
Bose(o) is offline Bose(o)  Canada
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"aren't you distorting the music if a bass note is there and your system doesn't play it?"

Aren't you distorting the sound of the signal by adding to it via resonances caused by venting techniques?
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Old 20th November 2003, 12:50 AM   #27
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Quote:
Originally posted by Bose(o)

Aren't you distorting the sound of the signal by adding to it via resonances caused by venting techniques?
Yes, you are. But considering that is generally understood by all that the lower the bass frequency, the less that distortion is hearable, I think playing a slightly disotrted bass note is preferable to no bass note at all-or very reduced bass note.

I would also point out that these step responses are at resonance. As you move up the scale away from resonance, the overshoot becomes less.

So if you have a sealed sub that cuts off at 42, and a ported sub that cuts off at 30, (half an octave below), the ported sub will have overshoot much reduced at 42 Hz. In the 30 Hz range, where the sealed sub puts out little, the ported sub will have big output, albeit somewhat distorted.

So the distorted output is sort of a "bonus" on the ported sub. the ported sub cleans it's act up the higher up the scale it goes.

LOL, I feel like I am putting myself between a rock and a hard place. Defending the sealed sub to the ported fan, and the ported sub to the sealed fan.
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Old 20th November 2003, 02:41 AM   #28
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Hi guys

> I should point out that a sealed sub, even with a Qtc of 1.3 or
> higher, has less distortion than a ported sub. It is usually
> easier to get higher output at lower frequencies with a ported
> or Drone Cone, (Passive Radiator) sub. To which can be added
> the fact that distortion is less hearable the deeper the bass
> frequencies.

It's known that a bass reflex box can generate less THD (Total Harmonic Distortion) down low because of the reduction in cone excursion. In the vented box we have an acoustical impedance rise which damps the critical resonance region. Starting from the Helmholtz resonance, and moving towards the upper resonance event Fh, the two resonating systems are stiffening each other and cone excursion is greatly reduced from Fb to Fh. It's typical that as cone excursion is reduced, almost all driver non-linearities are reduced also. It's demonstrated more in systems highly loaded acoustically, i.e. horns, where the acoustical impedance is very high at the throat which reduces excursion and also partly describes thier excellent transient response. The sealed box on the other hand does nothing to reduce cone motion and because of that, distortion is higher. This is the case when the ported box has been tuned to reduce excursion in the mechanical resonance region. Bare in mind that the majority of low frequency distortion is caused by the motor itself, and not the box. From about a half octave above resonance and down to resonance and below, distortion on all motors dramatically increases. Every single reputable manufacutre I know of has shown in published data that the majority of low frequency distortion is caused by the magnetic structure itself. And a reduction in excursion by the vented box means that distortion is minimized more than what a sealed box possibly can.

On the issue of transient response.... it's very realistic to tune a bass reflex box to achieve the same 'tightness' as a sealed enclosure. Because we have an additional resonator, we can further dampen the system.

There's two things to consider - that of phase, and that of resonance overring. Static phase changes are undectable, and ~one cycle of shift in the bass region is inaudible.

Resonance overring is a phenomonen in both sealed and vented systems. It's when theres residual but attenuated cycles "still coming" after the initial cycle has been removed.

Essentially the extra resonator - the Helmholtz can cause conditions between the two resonators to be in phase and others that cause them to be out of phase. It introduces additional components for damping, by introducing another resonator. You can think of it like another spring which acts as a counterweight to accelerate damping at resonance.

At Fo, is where the sealed system resonate's so thats where the potential is for overring and control is the least. The box modifies the mechanical properties of the speaker by pneumatic means to damp the cone, and Fs can be said to no longer exist and Fo is the new resonance point. It acts as though it changes the speaker's mechanical suspension by modifying the spring's stiffness and resistance. That's good because the system is more damped at Fo when installed in the enclosure than when its free air and ringing at Fs.

But a properly tuned bass reflex cabinet cabinet on the other hand can be made to damp the cone further. The enclosed woofer's resonant frequency Fo is usually nearly coincident with the upper resonant event Fh, and they are near enough that the port damps the woofers resonant frequency by a great deal. So for the octaves between Fh and Fo, the system is more rigid as the port is tightly coupled with the woofer. Distortion is reduced because excursion is reduced, down to Fb.

Frequencies much higher than resonance have damping very similar to the sealed box, above Fh the port does little because pressure changes are too rapid. It is as if the port is not there at all. But around resonance the interaction of the two systems are significant. One thing we can all agree on is that the bass reflex box provides an acoustical impedance rise around resonance, so we're capable of providing more damping. It can more effectivly damp the system in its weakest area, resonance. It's not really a single frequency phenomonen, but the band around resonance which is described by its Q figure, just like in any other electrical resonant system.

System Q is a measure of damping, describing the response curve. Underdamped conditions give a peak, and represents augmentation from uncontrolled movement of the cone. Essentially systems which are more highly damped have better control over their moving mass. That peak represents the region where it will 'ring', much like when you blow across the top of a bottle. Resonant ringing is the process that creates sounds from and object when it is excited in some way, such as by striking a drum or bell, pushing a pendulum or swing, or blowing across a bottle neck or flute.

Motor damping is also another part of the system, and is more effective where impedance is low. Thats because motor strength is a function of both magnet strength, the amount of coupling between the magnetic fields of the voice coil and of the drive magnet, and voice coil current. When impedance is low, more current can pass through and the motor is able to damp the system providing that voice coil current and magnetic flux is strong enough. It also requires an amplifier that has good current sourcing and sinking capabilities.

The motor is accelerated and decelerated when back-EMF is applied, and that also means there is a requirement for it to "deccelerate". Motors which are strong can accelerate a large mass and also overcome large mass and/or resistance. It sets a large part to the overall system Q. There's a large amount of properties in the system which all add to damping; the mechanical properties of the driver itself, the pneaumatic properties of the box and the interaction between the driver, and the electrical properties of the voice coil and amplifier - the speaker motor can damp the system most when current capacity is high, which means electrical impedance is low, both in the loudspeaker and the amplifier.

There's a handful of things to consider in any system. Basically systems which are more damped have better control of their moving mass, so they won't ring as much. The trade off is that they sacrifice some of the low end. But again, the larger the woofer is typically the lower it can go even in an overdamped condition. Bigger motors, bigger diaphragms with more mass, it just takes more power really.

The vented box places this emphasis on the resonance area and damps it better here. Higher damping can also be achieved in a smaller volume compared to the sealed enclosure. Getting back to the issue of phase in ported and sealed systems, remember that its linked to frequency response. I'll quote Brian Davies on this one from his 1981 paper:

One of the criticisms which is sometimes levelled at the vented system is that it is not "sufficiently fast." In this regard, it should be remembered that for frequencies below the crossover point, the acoustic output begins its life at the rear of the woofer, and must therefore be a half cycle delayed. The problem, however, is not unique to a vented system. It is one of the fundamental laws of linear systems that if their output depends only on previous input - that is, if they cannot see into the future - then the phase response is completely determined by the amplitude response. Applied to the woofer, this means that if a sealed box is chosen and its input is electronically equalized so as to extend bass response, then the gain in amplitude bandwidth will be at the expense of extra phase shifts according to the same inexorable laws of nature. The only perfect solution is to design a system with a resonant frequency of about 20-25Hz with a total Q of about unity - and this depends on buying woofers which are not available, or on mounting a woofer in a very large box , so as not to bring its resonance up too high, and using sufficient series resistance to bring up the total Q. For most people, connecting 10-20 ohms in series with their woofers is not a practical idea.

And note that if the frequency response curve for the vented system is the same of the sealed system, then in its passband, their phase response is almost the same and phase shifts are very low. Also note that one cycle of shift in the bass region is inaudible. The other benefit to the ported box is that, we can achieve deeper bass extension without the use of electronic EQ needed on a sealed box. As the passage above notes, the EQ introduces extra phase shifts as an expense for the deeper bass extension covered. But the vented box can be made to do this without EQ, and also at the expense of extra phase shifts. Davies is an important person in loudspeaker history and we use his implementation of Thiele/Small that we use today, and so his work includes sealed (Thiele) as well as vented (Small) systems. His title is Reader in Theoretical Physics, Australian National University and that above excerpt is from one of his published 1981 works.

Iím just about approaching the maximum word limit for this post, so Iíve continued it directly below in the next post.
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Old 20th November 2003, 02:42 AM   #29
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I myself am a vented box man, I tend to find them better academically, and I find they sound better too - if tuned correctly. Certainly a poor vented system, tuned for maximum bass extension is going to have worse transient response than a sealed enclosure. Chosen correctly, then the right woofer in a vented box is advantageous over a sealed box. The reduction in distortion is nearly reason enough to choose the vented box in most cases. And the vented box is also 'fast' when tuned correctly too, the acoustical impedance of the enclosure becomes very high at the Helmholtz resonant frequency, which serves to limit cone movement. This is why I expect that the properly tuned bass reflex cabinet provides better transient response - it more effectively dampens the woofer in it's weakest area - its resonant frequency.

Adrian
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