Interesting, but here is something to consider, we are talking about a vented box!
There has to be a different explanation with a vented box because your explanation does not seem to fit the facts. Please let me explain:
When vented, the tuning frequency is independent of the driver. That is the bottom of the saddle of the Z plot defines that frequency. Change the driver and the Z plot may well look different, but the bottom of the saddle will be the same frequency.
Now make the box more 'resistive' by carefully adding fill. Keep away fill from the vent. Now look at the Z plot and the bottom of the saddle frequency is no longer the same, but different. It is lower in frequency.
Based on a simple program like WinISD (or any other simulation program), we can see what the saddle frequency should be like and it works out. But if it is 10 Hertz lower, increase the box volume in WinISD to match what you are getting with fill, now you can see that effective box volume has been increased considerably.
The vent is now seeing a larger volume, that is what is lowering the tuning frequency. But the physical box volume has not changed. Then the only other conclusion open is that the velocity of air has slowed down sufficiently so that the vent is seeing a different volume.
Can any other conclusion be drawn? I am open to suggestions.
Cheers, Joe
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It comes down to models and language and how we use them.
There is a difference between 'it acts like' and 'it works like'.
When on land, we usually use a flat-earth model to give directions(even though there is a cliff next to our north pasture where you will go up 960 ft in less than 100), because it usually works well.
The same goes for air damping modeling. When we physically filter out higher frequencies, the pressure front acts like it's in a different medium than air.
It's not the same behavior as any homogenous medium, but we can get a feel for how it will act re: lower frequencies by pretending that we have an in-between medium giving us a different speed of sound.
It is going to take a lot to model mixed-media transmission.
Stiffness, elasticity, density, morphology of the materials, just to start.
Like the old saying goes "it's a small world, but I don't wanna paint it”
I don't mind the speed-change model for box modeling, I'm not married to it, it's not a belief system.(but it's useful)
There is a difference between 'it acts like' and 'it works like'.
When on land, we usually use a flat-earth model to give directions(even though there is a cliff next to our north pasture where you will go up 960 ft in less than 100), because it usually works well.
The same goes for air damping modeling. When we physically filter out higher frequencies, the pressure front acts like it's in a different medium than air.
It's not the same behavior as any homogenous medium, but we can get a feel for how it will act re: lower frequencies by pretending that we have an in-between medium giving us a different speed of sound.
It is going to take a lot to model mixed-media transmission.
Stiffness, elasticity, density, morphology of the materials, just to start.
Like the old saying goes "it's a small world, but I don't wanna paint it”
I don't mind the speed-change model for box modeling, I'm not married to it, it's not a belief system.(but it's useful)
The damping material does not change the speed of sound much(*). What happens is that the material adds tortuosity, so the sound travels for a longer distance and therefore takes longer to travel between two points in the box.
(*)There may be a small change related to the increased heat capacity provided by the damping material. This will affect the the adiabatic index and hence the thermodynamics of the pressure propagation a bit, resulting in a small change in the speed of sound. See here: Speed of sound - Wikipedia
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Yes, sure, but that does not matter. Just remember that with a vented box, the impedance curve shows what's happening to the woofer, not the port. At the port tuning frequency, the woofer does not move, as all energy is going into the port resonance. If you add damping to the box, the same thing happens as with a sealed box: the damping is more efficient at higher frequencies than at lower frequencies, which results in a shift of the resonance to a lower frequency.
I notice that I have been misquoted in post #3465, it should have been attributed to "boswald" as I would not have said that.
Also, I believe my point about vented boxes may have been misunderstood. The box frequency, the frequency of the saddle, changes when you increase the volume and the port stays the same. It goes lower. The same thing happens when you make the box more resistive, it now acts as if the volume has been increased. Since the saddle frequency is a function of the box alone, what explanation is there? The Helmholtz frequency is lower because it is acting as if the volume was increased.
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Also, I believe my point about vented boxes may have been misunderstood. The box frequency, the frequency of the saddle, changes when you increase the volume and the port stays the same. It goes lower. The same thing happens when you make the box more resistive, it now acts as if the volume has been increased. Since the saddle frequency is a function of the box alone, what explanation is there? The Helmholtz frequency is lower because it is acting as if the volume was increased.
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This is not entirely correct. The bottom of the saddle, the Helmholtz frequency, is independent of the driver.
I have heard this said many times and I think this has become folklore. Feed it a sine wave and match the Helmholz frequency and stick a microphone nearfield to the driver and you will get output. There are enormous pressure to stop it moving, but it still moves, just not very much.
No, this is a Helmholz resonance. The mechanism you describe cannot, as I see it, change that. I just can't see how that works. Unless there is more to that explanation you haven't told me. To me it is a reaction between the port and the volume of the box, not damping. The damping may change it indeed and show up in the Z plot, but not the frequency.
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It's been damped. More seems to fit into a smaller box because you've made some of it disappear and is no longer there to increase the bass output. This level of resonance resembles a larger box.
Yes, I've fixed that for you.
I am quite well aware that Wikipedia doesn't get everything right, but clearly what it says below is incorrect to some here:
"A significant increase in the effective volume of a sealed-box loudspeaker can be achieved by a filling of fibrous material, typically fiberglass, bonded acetate fiber (BAF) or long-fiber wool. The effective volume increase can be as much as 40% and is due primarily to a reduction in the speed of sound propagation through the filler material as compared to air."
Quote from: Loudspeaker enclosure - Wikipedia
So the minimum output at the Helmholtz frequency is a phantom resonance?
I just can't see that. The peak in the output of the vent, is likewise phantom?
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"A significant increase in the effective volume of a sealed-box loudspeaker can be achieved by a filling of fibrous material, typically fiberglass, bonded acetate fiber (BAF) or long-fiber wool. The effective volume increase can be as much as 40% and is due primarily to a reduction in the speed of sound propagation through the filler material as compared to air."
Quote from: Loudspeaker enclosure - Wikipedia
So the minimum output at the Helmholtz frequency is a phantom resonance?
I just can't see that. The peak in the output of the vent, is likewise phantom?
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Reflections on fibres inducing longer travel time would be OK when considering small particules and is then a reduction of air velocity (the displacement of air particules) and not the propagation of the wave itself (speeed of sound) These are two different things too often swapped when thinking to physics (into Loudspeakers).
To me, Joe is correct when speaking of a air-velocity reduction when adding stuffing.
Boswald is not wrong either in considering that internal space of a stuffed box is another medium than air...
To me, Joe is correct when speaking of a air-velocity reduction when adding stuffing.
Boswald is not wrong either in considering that internal space of a stuffed box is another medium than air...
As long as the box looks larger when presented to the vent, that's all I want.
The Helmholtz frequency will change, to a lower frequency. It does not get damped down to a lower frequency as some say here. That might (?) seem more logical with a sealed box system, but the vented box action says something different. Unless I am missing something that hasn't yet been pointed out to me. I do keep an open mind, something I don't always see from others.
Hope somebody doesn't correct me, but I don't think I used the speed word. If I did, I think it was always meant to be more correctly velocity. They are not exactly the same even if sometimes used interchangeably, but sometimes context makes them the same, except they are not.
Acoustic impedance (resistance if you want to simplify it
Acoustic impedance, I go along with that.
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It's simple physics, not folklore. If you set up a microphone per your suggestions, the microphone will pick up the sound output from the port. In other words, the microphone measurement will just show the crosstalk from the port. A better approach would be to measure the cone excursion as described here: Open Source Monkey Box
That said, I agree that the cone movement may not be exactly zero since we're dealing with real-world, lossy systems.
Joe, please carefully read my explanations again. The damping materials are added to the insides of the box, not to the driver itself. The damping therefore does have an immediate effect on the Helmholtz resonance, which is the result of the resonant system formed by the air mass in the port and the spring action of the air enclosed in the box.
I am not sure what you mean by this. Of course it's silly to say that the frequency gets damped, simply because it does not make sense from a logical/language point of view. The damping is applied to the resonant system, not to a frequency. However, the tuning frequency of the bass reflex system (i.e., the Helmholtz resonance frequency) does change as a result of added damping, as I explained above and in my previous posts.
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