The reaction to the accelerating mass of cone assembly plus air is a body force within the magnet. The size of the force is mass times acceleration. The acceleration follows from a simple lumped model of the driver. The mass of the air is given by the reactive part of the air impedance which is likely to be fairly constant and modelled fairly well by that of a piston in an infinite baffle though a more sophisticated model should be easy enough. If your software doesn't support body forces within the elements of the magnet then apply an equivalent pressure to the back of the magnet in the way you are doing for the air pressure on the inside of the cabinet. The magnet is going to move effectively as a solid body so it will have the same effect.
Here's a goldmine: https://www.avsforum.com/threads/a-monster-construction-methods-shootout-thread.3154560/page-4
Andy: I will try a coupled acoustic-structural model instead. Thank you for the input!
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Coupling the structural motion and the acoustics is complex and unecessary. The load of the air on the cabinet is weak and pretty well modelled with an impedance. Simulate the cabinet structure with relevant boundary conditions applied for the external forces and then simulate the external acoustic field with an acoustic BEM or FEM software using the calculated motion of the cabinet as velocity boundary conditions. If keen one could recalculate the motion of the structure using the calculated surface air pressure rather than the originally assumed ones but it almost certainly won't make a big enough change to be worthwhile. Might be worth doing once as a check though.
Should be obvious.
Air movement — half the energy goes into the box and the other half is radiated out into the air. Furthermore air is not a great conductor of mechanical energy. Not to mention any fibrous damping material.
Direct connection to the box via the baffle: ALL the energy goes into the "box + baket".
dave
I believe i posted this from a study (publihed in the AES Journal) earlier:
Wasnposted here: https://www.diyaudio.com/community/threads/bracing-vs-damping.428225/post-8024367
dave
It's a little more complicated than that because it varies with frequency. At LFs the box is very rigid and is just another mass to the drivers frame, not so at all at frequencies where the box has mechanical resonances. AT LFs the air in the box pressurizes the box with a uniform pressure throughout creating a breathing mode (monopole) which is an extremely efficient radiator. So it's clearly different with frequency.
The fundamental question of this thread was about isolating the driver from the box. That is absolutely the wrong thing to do. An isolated driver has to be on springs. The drivers mass and this spring will resonate. When this happens all the energy will go into the moving the drivers mass and there will be a big hole in the response.
If you are calling the demonstration that you showed as "data" it is seriously flawed in so many ways. i.e. if the displacement of the glass is normal to the water surface then the surface will show nothing. And this is precisely the direction of motion that we are most interested in. Then there is the signal and the "meter" (our eyes!?) In other words, your "data" is not worth considering. There is lots of data that cabinet walls vibrate. Is it significant? I do not and have never considered radiation of the box as a major concern. It is on my list, just pretty far down. Only in a superb loudspeaker would this ever become an audible issue (assuming of course that the speaker is not blatantly poorly designed.)
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Isn't that a dual opposed sub? If so, I think that should be mentioned.
If not, how much does that dang sub weigh not to vibrate ? 🙂
It is. But everything varies with frequency.
Ballooning cn be a probelm with a subWoofer. But SubWoofers can be treated differently. Build light and stiff and take advantage of active push-push reaction force cancellation and once sufficiely braced ballooning becomes quite unlikely.
dave
No, the article I posted originally with actual room measurements showing no difference between isolation and no isolation. Here it is again:
http://ethanwiner.com/speaker_isolation.htm
Ethan, i see nothing there that tells us anything about the DDR. Isolation/decoupling would show up first in the small details.
dave
dave
What is DDR? And why do you think my very carefully controlled tests are not proof that decouping makes no difference? Please be very specific.
DDR = Downward Dynamic Range. A term coined by Allen Wright. A giant in the field. A term to describe all the small stuff still there even in the face of a big signal.
This analogy may help: The FR measurements we take are a measure of the surface of a body of water (mirror smooth is the target). DDR measures the depth of the water (deeper the better). This is one of the significant reasons a loudspeaker with a rough “surface” can “outperform” is that it is has "greater depth” — better DDR.
A system with good DDR will have all of the small details that give voices body and make them distinct, differences in instruments, and a good solid 3D soundstage/image. The difference between a good system and a great onne.It still eldes being quantified. Why Floyd Toole said:
Of course this information has to be on the software being played, and that the signal chain, loudspeakers and how they interact with a room need to lose as little of this information as possible. That is the whole job of a hifi system.
Collums recently wrote a bit:
So despite the fallibility and plasticity of the ear/brain hearing system it is still how we have to do “final” evaluation of the system. And given the large number of degrees of freedom that means alot ofpeople happy with a lotofdifferent hifis.
dave
This analogy may help: The FR measurements we take are a measure of the surface of a body of water (mirror smooth is the target). DDR measures the depth of the water (deeper the better). This is one of the significant reasons a loudspeaker with a rough “surface” can “outperform” is that it is has "greater depth” — better DDR.
A system with good DDR will have all of the small details that give voices body and make them distinct, differences in instruments, and a good solid 3D soundstage/image. The difference between a good system and a great onne.It still eldes being quantified. Why Floyd Toole said:
Of course this information has to be on the software being played, and that the signal chain, loudspeakers and how they interact with a room need to lose as little of this information as possible. That is the whole job of a hifi system.
Collums recently wrote a bit:
So despite the fallibility and plasticity of the ear/brain hearing system it is still how we have to do “final” evaluation of the system. And given the large number of degrees of freedom that means alot ofpeople happy with a lotofdifferent hifis.
dave
There's a lot to unpack here, but I'll try my best. Much of what you wrote is not scientific, and seems based more on feelings and subjective impression than logic and evidence. For example, equating low levels of noise and distortion (the "DDR") with water depth makes no sense. A system that is clear and well defined is a system that has little distortion and noise too soft to harm clarity. This is the domain of measuring, not subjective listening. Further, imaging is mainly a function of room acoustics - having absorbers at the side-wall, ceiling, and floor reflection points. The untamed early reflections present in most hi-fi rooms does more damage than many people realize. When Floyd Toole talks about ears being "better" than one microphone, that's what he's talking about. He is not saying that ears always best measurements.
Asking, "Are your feet tapping unconsciously in time to the performance?" is right out of Stereophile magazine. The difference between a good system and a great system does not elude being quantified. Other than damaging reflections that are outside of the equipment, there are only four parameters that define all of audio fidelity. This article from Skeptic magazine explains in great detail:
http://ethanwiner.com/audiophoolery.html
Asking, "Are your feet tapping unconsciously in time to the performance?" is right out of Stereophile magazine. The difference between a good system and a great system does not elude being quantified. Other than damaging reflections that are outside of the equipment, there are only four parameters that define all of audio fidelity. This article from Skeptic magazine explains in great detail:
http://ethanwiner.com/audiophoolery.html
Subjectivity plays a role, that is where we start wehn we are trying to figure out how to measure something we cannot yet measure. A whole lot of evidence, logic, and experience in what is said. Because we can’t measure it, coen’t mean it doesn’t exist. Jitter is a good example of nfinding a way to measure something previously unmeasurable.
That statement is so far off the mark as to say “are you out to lunch?”.
What is the loudspeaker’s diffraction signature? Includes any “decoupling” that keeps it from exciting direct mechanical room resonances.
What is the loudspeaker’s (4π Steradians) radiation pattern? Speakers like omnis activate a lot of the room, “Geddes-style" speakers aim to elimnate the room asmuch as possible. As does near-field oudspeaker placement.
What is the loudspeaker’s response from lowest levels to highest levels? From the driver and from the box? (a subset of the first point)
A whole lot depemnds on the shape of the room and the plastisities aof the boundaries.
You are, unfortunately, missing a lot of the picture. Pontificating as if an expert with only part of the information is not helpful. I am sure you are very experienced with damping poor rooms (and maybe overdoing some). Do you do diffusors?
If you pay some attention to the broad swath ofknowledge here i am sure you can learn a lot.
dave
I see some members talking past each other in this thread, perhaps to be expected as some specialise in speaker design, some in room acoustics and some see the two as inseparable.
I think your qualifier about "competent" speakers was wise. The thing is that the issue is best dealt with at the cabinet level. I know you didn't start the original question about decoupling the box from the stand, but the speaker crowd here is likely making this distinction.
The accelerometer test was a worthy suggestion. There have been tests done in the past to show cabinet vibrations, and what affects them.
Take a speaker playing music and place, lift, place,... on a desktop. Repeat on a concrete floor. It's the same speaker but the result is likely to be different. The cabinet radiation may well be inaudible and remain inaudible in both cases. The issue is not primarily to do with the speaker (though a pair of force cancelling woofers will help) but the mechanical impedance between the speaker and what it stands on plus the very large increase in radiating area for unwanted sound.
Recalling from school physics classes, the work done by the speaker on the desktop or floor is the product of force and displacement. So solutions can involve reducing the size of the (fluctuating) force and/or the (fluctuating) displacement. For example, a slab under a speaker spreads the load on the floor reducing the (fluctuating) displacement whereas a speaker on a soft spring with a resonance of a few Hz will increasingly reduce the (fluctuating) force with frequency starting at roughly 1.5 times the resonant frequency. A spring that is too stiff will have a resonance in the audible frequency band and will increase the displacement around the resonance rather than reducing it making things worse around this frequency and possibly a lot worse if there is little damping (and if there is a lot of damping it won't isolate effectively at higher frequencies).
Some expensive audiophile isolators have soft enough springs and will reduce the level of unwanted sound if there is an issue with what the speaker is standing on. Others are too stiff. It's unlikely the marketing material will distinguish given the stories will have been created to appeal to audiophiles. It's a rare example of expensive audiophile accessories doing something positive in the physical domain albeit not in a cost effective way.
I'd draw your attention to two different behaviours. The plain back and forth of the cone mass against the box as a single unit, and cabinet panel vibrations including separate panel vibrations.
It seems you've described the former. Gedlee also addresses this, noting the driver should be rigid in space to radiate as intended. In this context it could be argued that this extends to the cabinet itself as well.
Ethan Winer has chosen to address the latter using the example of the glass on the top panel. I'm not sure if there was supposed to be a further connection than that..
It seems you've described the former. Gedlee also addresses this, noting the driver should be rigid in space to radiate as intended. In this context it could be argued that this extends to the cabinet itself as well.
Ethan Winer has chosen to address the latter using the example of the glass on the top panel. I'm not sure if there was supposed to be a further connection than that..
Not sure I understand your distinction and my last post didn't talk about the details of cabinet vibration though some earlier ones did.
The vibration of a cabinet is driven by two sets of forces: the air inside the cabinet exerting a force on the inside of the cabinet walls and the drivers bolted to the cabinet exerting a force directly. When considering unwanted sound radiating from the cabinet the magnitude of the forces from the air is only of significance at low frequencies whereas it is some of the resonances at higher frequencies that can be loud enough to be audible. Therefore the former is often ignored and only the forcing from the drivers directly on the cabinet considered.
Now in the case of a speaker cabinet coupled to a desktop or a sprung floor creating unwanted sound things are different. For example, if you listen to a speaker driving a sprung floor in the room below it is mostly low frequency sound and not the louder cabinet resonances that are heard. This means the forcing from the air pressure inside the cabinet cannot be neglected unlike when considering just the audibility of the cabinet radiation. The large surface area of the floor or desktop is more effective at radiating low frequency sound than the small cabinet surface area.