there is no difference between an aperiodic vent and a box plugged with foam. think of as subdividing one large chamber into many small ones that are interconnected with aperiodic vents.
velocity at the wall is irrelevant as you can also find spots in an aperiodic system where velocity is zero. the point is velocity is nonzero in some spots as well - in both systems.
now naturally velocity in a sealed box will be an order or two of magnitude lower than in a aperiodic vent. all this means is that it will take an order or two of magnitude more foam to provide the same damping.
now obtaining purely resistive ( or approximating ) behavior may indeed require some ingenuity. but when there is a will there is a way 🙂
by the way i think aperiodic venting is more commonly used not in cars but in headphones. i think electrostatic headphones are the only ones that don't do it.
i remember when i was like 15 i bought Aiwa headphones with 50mm drivers and decided to improve them.
First thing i did was put some foam damping in their enclosures which did improve sound. But then i also removed the filter paper from the back of the driver because i wanted to get more bass and i felt it was restricting airflow. i didn't understand then that i would be effecting the Q and thus the frequency response 🙂
i remember when i was like 15 i bought Aiwa headphones with 50mm drivers and decided to improve them.
First thing i did was put some foam damping in their enclosures which did improve sound. But then i also removed the filter paper from the back of the driver because i wanted to get more bass and i felt it was restricting airflow. i didn't understand then that i would be effecting the Q and thus the frequency response 🙂
In this discussion I have heard no mention of the presure to shear transition effect.
If a pressure wave in a gas or liquid impinges upon an elastic medium then part of it can be propagated as a shear wave in that medium.
rcw.
If a pressure wave in a gas or liquid impinges upon an elastic medium then part of it can be propagated as a shear wave in that medium.
rcw.
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in fairness...
i think it would be fair to say that MJK and other have agreed that the 'fibres-slowing-the-speed-of-sound-thus-lowering-Fb' theory to be rubbish...maybe adiabatically of isothermally but slowing speed of sound? NO WAY!
besides...doesnt the speed of sound INCREASE as the density increases? kinda like youd expect it would transistioning from air to a more dense matter...like water, foam, etc? or did i make that bit up?
either way(ie if if got that all wrong) purely resistive loading of a driver may achieve some benefits, and may be the perfect solution in theory, but i believe this is where some of the aging theory falls down.
After all there is no alignment tablature for aperiodic loading, not even an agreed alignment, as many examples vary wildly..
IMO opinion a box designed the correct size for a sealed design, PLUS as much extra room in depth (and other dims if you want) to accomodate foam is only a goood thing, and the increased thickness will absorb lower freqs,m but im sure there is a practical limit, which is defined by how much space you really want to give to it.
i think it would be fair to say that MJK and other have agreed that the 'fibres-slowing-the-speed-of-sound-thus-lowering-Fb' theory to be rubbish...maybe adiabatically of isothermally but slowing speed of sound? NO WAY!
besides...doesnt the speed of sound INCREASE as the density increases? kinda like youd expect it would transistioning from air to a more dense matter...like water, foam, etc? or did i make that bit up?
either way(ie if if got that all wrong) purely resistive loading of a driver may achieve some benefits, and may be the perfect solution in theory, but i believe this is where some of the aging theory falls down.
After all there is no alignment tablature for aperiodic loading, not even an agreed alignment, as many examples vary wildly..
IMO opinion a box designed the correct size for a sealed design, PLUS as much extra room in depth (and other dims if you want) to accomodate foam is only a goood thing, and the increased thickness will absorb lower freqs,m but im sure there is a practical limit, which is defined by how much space you really want to give to it.
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Review these two links and decide for yourself:
http://auralization.blogspot.com/?zx=2c158bfb00a190f8
and fig. 5.5 at this link: http://www.quarter-wave.com/TLs/Damping_Coefficient.pdf
Bradbury also theorized back in 1976 the speed of sound thru entangled fibrous structures was diminished. However, King's work with both theory and measurements showed the degree of the reduction in speed was much less than Bradbury originally postulated.
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i have an idea
iil mount my speakers in the wall seperating listening area and box room, and then fill the box room last 6ft with solid poly foam...that should maybe absorb up to 100hz if im lucky
iil mount my speakers in the wall seperating listening area and box room, and then fill the box room last 6ft with solid poly foam...that should maybe absorb up to 100hz if im lucky
Absolutely. Too many here confuse the speed at which air moves with the speed at which sound travels.
right well i can see the graph you mean and i agree it does show a reduction in sound velocity....im at a loss what to say as i was sure i read the opposite in another of his papers...(or maybe mis- read)...either way the effect is not all that significant, like you say, and not as significant as bradbury thought.
rather than the speed of sound being reduced may i postulate that the fibres absorb and re-radiate the sound, with an inherent delay, and also by this action absorb some of the energy in exciting the fibres, which is lost as heat.
THerefore the speed of sound is not reduced at all, merely the wavefront is delayed partially, from different 'fibres' at different times leading to the perceived effect of reducing sound velocity, NOT an actual reduction in velocity. I mean that the constant velocity of sound is decelerated and re-accelerated thru the absorption and re-radiation of/from the fibres. HENCE, speed of sound remains constant but the time to cross a given stuffed distance increases, as we are no longer dealing with a volume of fluid with uniform density. This is much like what id expect to happen with a region of cold air contained magically, and isolated magically, from the main air mass
i would liken this action to the action of the 'buckets' of capacitance in a bucket brigade delay line IC, where charge/sound energy, is passed along a line from one bucket/fibre to the next, the fundamental velocity of an electron is not altered, merely stored and hence delayed
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mondo there is no contradiction.
everything i said is perfectly consistent with slowing of sound with stuffing.
your confusion arises because just like damping can refer to many things so can density.
density may refer to increase in mass or increase in stiffness. added mass slows sound while added stiffness speeds it up.
sound in dense media tends to be faster but this is due to stiffness, not mass.
stuffing imparts no extra stiffness, only mass therefore i would be very surprised if the effect was anything but slowing of sound.
of course you also have to remember that straight line is the shortest path and stuffing makes any line less straight.
everything i said is perfectly consistent with slowing of sound with stuffing.
your confusion arises because just like damping can refer to many things so can density.
density may refer to increase in mass or increase in stiffness. added mass slows sound while added stiffness speeds it up.
sound in dense media tends to be faster but this is due to stiffness, not mass.
stuffing imparts no extra stiffness, only mass therefore i would be very surprised if the effect was anything but slowing of sound.
of course you also have to remember that straight line is the shortest path and stuffing makes any line less straight.
Another thing that happens is that if you look at an acoustic absorber as a bunch of small diameter parallel tubes, there comes a point at which the viscous forces exceed the inertial ones and the flow increasingly becomes a Poiseuille flow, that has a pure acoustic resistance, this ratio is known as the shear wave number.
rcw.
rcw.
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deflection turning a straight path into a curved or otherwise, YES that is valid, as is reflection, diffraction, absorption and re-radiation of sound by the absorbent media. in over stuffed high density stuffing, mass loading is apparent. However, NONE of these things CHANGE the velocity of sound, merely divert, delay, disperse the wave front resistively, the velocity remains the same only the path length changes.
i sincerely wish i did misunderstand, as ignorance appears to be blissful....but the velocity of sound is constant within a constant density media, at a constant temperature, that IS widely accepted FACT.
Like i said, a change in density WILL change the velocity, as conversely, sound energy storage and release will slow the 'apparent' velocity, like a delay line effect.
Stiffness only imparts a small effect due to the amount of absorption a 'softer' media will have in comparison to a 'stiffer' media., and this impacts pretty much solely on absorption and reflection; again a delaying effect on the wavefront.
Beyond average stuffing densities, the media no longer acts as individual fibres and damps by mass alone or in vast majority, and the fibre delaying effect is overcome by the mass damping mode.
so again as v=st
v is constant in air at constant temp.
s is the distance and is not constant due to fibres deflecting or otherwise changine the path of the wavefront
t is the time taken,
As you cannot effectively measure the sound path (s)length thru the absorbent media, or the time (t) as it is deflected randomly by many fibres as it progresses thru the media; it is approximated, or worse still, given as the straight line distance. THIS is not accurate and is an empirical ASSUMPTION. this leads to the BELIEF that the velocity of sound is slowed when in fact the time (T) and the path length(S) are actually increased(as you have stated), and a false calculation of reduced velocity is created.
This is semantics. Does the speed of light actually change within glass? If not then a lens would not be possible because refraction could not exist. On a microscopic (quantum) scale the speed is indeed invarient and the path length changes, but as far as any measurement that YOU could actually perform would result in the conclusion that the speed of the light changed. The situation with sound is exactly the same thing. Microscopicly there may be an added path length or there may be a thermal effect or a density or stiffness change, but that is academic and semantic. Any real world measurement that you make will show that the wave speed has changed.
i agree it is semantics, but just like the light refracted by two layered media of different density, so is the sound refracted when encountering a layer of stuffing. yes it would be measured that sound velocity had changed, but myself personally(in RE to TL design at least) find that the 'effective length' of the line with differing stuffing density to be a better measure than change in sound velocity.
also in this case(as in light refraction) the energy is refracted back towards the lower density media, depending on the difference in densities, as im sure any 16yr old physics student will have learned. therefore a higher density stuffing only serves to increase the refraction angle, giving the audible subjective effect of sound reflection. However this is not the full story obviously as some absorption occurs, as does some mass damping.
So semantics aside, which i agree are pretty annoying(i only stated them for the benefit of others that may be confused), the current theories that ANYONE has modelled are ALL simply too simplistic to fully account foe the behavior of stuffing to a satisfactory degree.
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Well thats where I don't agree. My model was very good and very few assumptions were used in its derivation. A real situation like this COULD be made and tested, but as I said, its not something that interests me so I'm not about to do it. It predicted all of the features that we see in the real world - I'm satisfied. So unless you can point to a specific error, it is presumptuous of you to draw the conclusions that you have.
The dispersion relationship, i.e. the ratio of frequency to wave number is linear in free air but non linear in a duct. Since the only variable in this is the free air phase velocity this must be less in a duct than in free air.
rcw.
rcw.
How can this be? Surely you don't mean this to be true in general. It can be true in some situations but not all. Take for example a rigid wall duct with plane wave propagation. The wave propagates just as if it were in free space - same speed, same wavenumber, same everything. Basically it doesn't even know the walls are there. The higher order modes are dispersive of course, which is the whole point of my HOM audibility claims, the plane wave or lowest mode is not dispersive however.
presumptious maybe, i accept that, but so are some of the models, which presume quite alot relatively. They ARE satisfactory however, I totally agree with that; except that with judicial guesswork and a bit of good ol fashioned commonsense, one can 'guesstimate' and obtain results almost as accurate.
This unfortunately makes any model other than a very comprehensive one, rather less than necessary, IMO of course. Modelling behavior is great, but not perfect. Experimentation is great and even more hit and miss, so i guess what im saying is that a really really SIMPLE model would be just as good as a perfect one. In as much as it would point the boundaries for experimental tinkerings. BUt personally, i think ill still just tinker and wait til i hear something that sounds good instead as its much more fun!
Bit like my father did when tinkering with the goodmans protos back in the mid 70's...not perfect results but damn listenable.
what needs to be done is taking impulse response of stuffed versus unstuffed pipe.
the impulse will be delayed but the shape and magnitude will change as well. this should bring effects such as attenuation and bandpass filtering to light.
then you would have to play sinewave tones with and without stuffing to watch for generation of harmonics or other distortions.
perhaps do an intermodulation test ...
only question is where do u find a source capable of generating a perfect signal ? is there any way around this ?
you would probably need to record the error in unstuffed pipe, invert it and add to the original to make it generate a perfect impulse or sinewave. then you can move to stuffed pipe for real measurements.
anyway we got sidetracked again.
my claim was that bass CAN be attenuated by foam for EVERY mode of the enclosure down to the fundamental.
my other claim was that aperiodic is not different from stuffing.
but now i actually think i need a disclaimer. aperiodic is only not different from stuffing IF ( and that's a big if ) the stuffing is immobilized to the same degree as resistive material in an aperiodic vent.
because the foam can be said to have 3 separate effects:
1 - resisitive
2 - inertial
3 - elastic
and these will work differently on different frequencies giving foam nonlinear frequency response. immobilizing the foam not only at the edges of the cabinet but throughout the entire box volume may be the key to obtaining ideal bass performance.
my claim was that bass CAN be attenuated by foam for EVERY mode of the enclosure down to the fundamental.
my other claim was that aperiodic is not different from stuffing.
but now i actually think i need a disclaimer. aperiodic is only not different from stuffing IF ( and that's a big if ) the stuffing is immobilized to the same degree as resistive material in an aperiodic vent.
because the foam can be said to have 3 separate effects:
1 - resisitive
2 - inertial
3 - elastic
and these will work differently on different frequencies giving foam nonlinear frequency response. immobilizing the foam not only at the edges of the cabinet but throughout the entire box volume may be the key to obtaining ideal bass performance.
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