Damping material not only has the thermodynamic effect, it also has an acoustical resistance property. This resistance is more effective where the velocity is high and it is very high near the port in a vented system. Think of high velocity as high voltage so a lot of current and thus power is dissipated in the damping resistance. As I stated previously the boundary of the enclosure is a zero in terms of volume velocity but a high pressure (pressure being analogous to current) point. Lining the walls is only a convenient place to put damping material, it is better to put it toward the middle of the box where the velocity in standing waves is higher.
I agree with your observations about the tall slim box, it is difficult to eliminate those standing waves because the damping materials are not as effective at low frequencies.
Perhaps you are not familiar in utilization of these devices to cancel standing waves. Wiki pages don't tell you7 everything.
Earl has pointed out what I intended everyone to think about. Whenever we look at TLs or any other kind of vented enclosure, we cannot just consider one aspect of the design and conclude one's point of view is more correct than others.
So give me a reference to what you are talking about, you give none just half baked ideas.
I only use Wiki because you seem to need some help, I don't like it at all - I did suggest Beranek to people here.
I would not say that Earl pointed anything out of value, he did disagree with you however.
I thought perhaps we were getting this thread back on track, it seems not ....
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This is the point I wanted others to think about. Estimations of TL is based on reflection, Estimations Helmholtz resonators are volume ratio based. In a TL, both characteristics are interelated, and uless we model a TL considering both perpectives, there is surely some significant issues that we have missed. When we measure devices like these in a room to prove a math model, we should also know what kind of acoustics properties of the room the measurment was conducted in.
I would have guessed 64, 58 and 52. Close enough.
There's definitely a strong downward shift, more than I would have expected.
Something else grabbed my attention though. In the later plots it becomes increasingly obvious that impedance minima and maxima do not correspond exactly with zero phase. What's with that?
😕
It doesn't "look right" to me, but then I'm more used to looking at electronic circuits and simple LCR networks. Is this an effect that only crops up when dealing with delayed reflections?
Cheers - Godfrey
It just means that the system is not minimum phase. That's certainly possible when there are delays. It also could mean that the measurements are bad.
Thanks, Earl! 🙂
Playing in a simulator, I've now gotten similar results with a couple of different circuits (of the sort I wouldn't normally have anything to do with).
Cheers - Godfrey
Playing in a simulator, I've now gotten similar results with a couple of different circuits (of the sort I wouldn't normally have anything to do with).
Cheers - Godfrey
It is very close to the zero degree mark which indicates that it is moving toward a purely resistive input impedance. I would say that it is swamped by the high damping. The line is simply a leak at very low frequencies and then the phase does go positive indicating that the suspension compliance is the dominant reactive component.
It is a coupled resonant system, the driver being a parallel circuit and the TL around l/4 being analogous to a series resonant circuit.
I remember discussing this with Professor Wadsworth (got the spelling right this time) and he pointed out that this could happen in lossy resonant circuits. I believe that the resistance (loss) has to be in one of the reactive legs of the parallel resonant circuit, or in the leg of the coupled series resonant circuit. I'm a bit rusty on this and a bit tired tonight to look at (or think about) the theory, perhaps one of our circuits experts will explain it better.
Are you familiar with where the upper and lower peaks come from in a vented design?
Looking at the curve again it seems like the phase is shifted down toward negative phase (massive); if you shift it back up then they would line up again.
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The peaks indicate high cone velocity, whereas the dip between indicates minimum cone velocity.
In the pic below, with everything referenced to the electrical side:
The impedance dip corresponds with the series resonance of the enclosure components.
The peaks correspond with parallel resonances between the total cone impedance and the total enclosure impedance, where one is predominantly inductive and the other capacitive.
Looking at the acoustical circuit (i.e. the other side of the transformation), everything is reversed e.g. parallel becomes series etc.
I'm too tired to try draw that one as well now. Hope I managed to get the rest right, though...
Attachments
Yes that's about right, the flip on the other side depends if you use an impedance or mobility circuit analogy for the mechanical circuit.
The easiest way to understand it is to consider a B4 alignment, or any alignment where Fs = Fb, although this is not required. The series resonant circuit is massive below Fb and compliant above Fb. The lower peak is then the parallel resonance of the driver being shifted down in frequency due to the additional mass load, and the upper peak being it shifted up due to the compliance from the series circuit. It is clear then that these resonances are dependent on the driver.
There are SPICE models here, post #18 is particularly good:
http://www.diyaudio.com/forums/subwoofers/66373-driver-electrical-model-2.html#post2117669
More:
http://users.ece.gatech.edu/mleach/papers/spice_electro.pdf
http://users.ece.gatech.edu/mleach/papers/Filling.pdf
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I might as well jump into the fire to.
Hi Pete I've been reading your posts.
Since I'm in one of them I'll write this little blurb.
Disclaimer.
I am not a Bsc Eng
I am a designer of loudspeaker drivers and enclosures, crossovers and wave guides. I am a very good engineer. I am a very good cabinet maker and machinist. And I can read and understand what is required to design and build something. I do this every day in a great many fields of endeavor. Heck I even review classical music!
But I am not an expert. Experts are people who do not know how to do, so they tell people what to do.
In the long section about lumped modeling of boxes that don't fit into any particular molds I can throw the UNHORN into the pot. I posted an account of the trials and pitfalls of trying something a little out of the ordinary. And to my surprise it worked. There are a couple of gents who built the design and are thoroughly enjoying it.
I'm puzzled as to why you cannot figure this out. It is a bandpass enclosure. Plain and simple. It is not new at all. I think that after really checking the prior art the Karlson coupler was one of the first examples of this type of open concept box.
Method of operation is brutally simple. The rear chamber acts as a resonator. The small coupling section a port to the larger part of the flare. Careful design of the cross sectional area will either create two large peaks about 2 and 1/2 octaves apart or a rather smooth response. Size is everything in the coupling section. As is the case when correctly constructing and tuning a double ported bandpass. Very much the characteristics of a bandpass enclosure.
Bjorno who I consider quite an intelligent fellow beat me over the head with reasoning until it sunk in. Thanks Bjorno! There is no horn loading of any kind. So the name UNHORN was chosen. Even though it kind of looks like a horn.
The only thing that sets this box apart from a band pass enclosure is it's ability to couple into a room corner without the attendant peak in the mid-bass frequency response. This is due to the secondary flare of the ported section that exits out of the box.
A simple 56 liter box using two TRIO8's has been roughed out and will be compared just to see what the heck is going on. So ever onward and upward.
Mark
Hi Pete I've been reading your posts.
Since I'm in one of them I'll write this little blurb.
Disclaimer.
I am not a Bsc Eng
I am a designer of loudspeaker drivers and enclosures, crossovers and wave guides. I am a very good engineer. I am a very good cabinet maker and machinist. And I can read and understand what is required to design and build something. I do this every day in a great many fields of endeavor. Heck I even review classical music!
But I am not an expert. Experts are people who do not know how to do, so they tell people what to do.
In the long section about lumped modeling of boxes that don't fit into any particular molds I can throw the UNHORN into the pot. I posted an account of the trials and pitfalls of trying something a little out of the ordinary. And to my surprise it worked. There are a couple of gents who built the design and are thoroughly enjoying it.
I'm puzzled as to why you cannot figure this out. It is a bandpass enclosure. Plain and simple. It is not new at all. I think that after really checking the prior art the Karlson coupler was one of the first examples of this type of open concept box.
Method of operation is brutally simple. The rear chamber acts as a resonator. The small coupling section a port to the larger part of the flare. Careful design of the cross sectional area will either create two large peaks about 2 and 1/2 octaves apart or a rather smooth response. Size is everything in the coupling section. As is the case when correctly constructing and tuning a double ported bandpass. Very much the characteristics of a bandpass enclosure.
Bjorno who I consider quite an intelligent fellow beat me over the head with reasoning until it sunk in. Thanks Bjorno! There is no horn loading of any kind. So the name UNHORN was chosen. Even though it kind of looks like a horn.
The only thing that sets this box apart from a band pass enclosure is it's ability to couple into a room corner without the attendant peak in the mid-bass frequency response. This is due to the secondary flare of the ported section that exits out of the box.
A simple 56 liter box using two TRIO8's has been roughed out and will be compared just to see what the heck is going on. So ever onward and upward.
Mark
What I always heard as the defintion of an expert was "somebody that travels more than 50 (or 100) miles to make a presentation to your company".
But we said that entirely in jest, of course, and stating that experts can't do what they are expert at is ridiculous.
Paul
But we said that entirely in jest, of course, and stating that experts can't do what they are expert at is ridiculous.
Paul
Indeed. What the poster doesn't realize is that when he makes such a statement, immediately everything else he writes loses any credibility. Talking about shooting yourself in the foot 😉
jd
One of the few ways, in the US, that an engineer can make some lawyer-level money.
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