For wavelengths equal to and below the largest dimension of the cabinet, air inside the cabinet acts as a spring, i.e., it gets compressed as a whole. I assume there are no waves and nothing to reflect or bounce around inside the cabinet to come back out.
I'm not able to express the full physics behind it. I guess I need to think through it fully.
I'm not able to express the full physics behind it. I guess I need to think through it fully.
I realize there are several other things going on, but does this measurement not indicate at least some of the effect? It is a 1cm nearfield of a woofer (measured out on the cone, not in the center) in a large cabinet, with absorptive material being added.
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Unpowered driver in an infinite baffle (eg. closed door between rooms). Various noise sources one side (from known transducer).
Microphone otherside of baffle
Fun part comes when you start to electrically damp the speaker - from open circuit to short circuit. Oh, add some reactive (cross over) components too.
Ps - remember to repair the door before your other half gets home!
Its a rough indication, but there are several things going on. And the issue isn't "is there?" of course there is, the issue is "how much?" and is it a significant audible effect.
Ra7 has it right. For the most part the internal reflection is just a stiffening pressure inside the cabinet. Of course it could only do this if in fact the cone is highly opaque to sound (not transparent as was initially claimed.) By the time the wavelength become sufficiently short that it could actually travel through the cone as say a bending wave or something like that, some interior absorption does the job very well.
I hear this claim all the time as a reason not to have a box and I just don't buy it.
I would say that the effects of not having a box are much greater than anything the box itself could create. Allowing the rear radiation to do what it wants is a magnitude or more greater than any reflective internal wave could ever do compared to what the interaction of front and back waves are going to do. I'm with Earl on this front.
I'm a little surprised that asserting that speaker cones are fairly acoustically transparent would be controversial. I've been mounting small to large speakers on IEC-or-larger baffles for twenty years now, and it's pretty obvious that sound goes through a passive (undriven) speaker cone. Just put your head next to the disconnected speaker and listen.
Can you hear ambient sounds coming through the hole where the speaker is mounted? I do. Compared to the open hole with no speaker mounted, yes, there's some attenuation, but it's not like 3/4" solid birch ply, which is pretty opaque acoustically.
As a side note, I grew up in Japan, where the paper shoji dividers (between rooms in a traditional village inn) are notoriously acoustically transparent ... the original "paper-thin walls" that apartment dwellers in big cities like to complain about. No, I didn't live in a village inn, but I've been a guest more than once. Rooms with shoji dividers aren't very warm, either, which is why each room will have a small heater in the center to keep guests warm during the winter.
Taking the argument to a logical extreme, if paper cones are acoustically opaque, why go to all the trouble of making enclosures out of heavy, expensive, hard-to-machine MDF or birch ply? Why not just use paper conveniently formed into cones? It would weigh a lot less and you could make an acoustically opaque enclosure with nothing more than scissors and tape.
Can you hear ambient sounds coming through the hole where the speaker is mounted? I do. Compared to the open hole with no speaker mounted, yes, there's some attenuation, but it's not like 3/4" solid birch ply, which is pretty opaque acoustically.
As a side note, I grew up in Japan, where the paper shoji dividers (between rooms in a traditional village inn) are notoriously acoustically transparent ... the original "paper-thin walls" that apartment dwellers in big cities like to complain about. No, I didn't live in a village inn, but I've been a guest more than once. Rooms with shoji dividers aren't very warm, either, which is why each room will have a small heater in the center to keep guests warm during the winter.
Taking the argument to a logical extreme, if paper cones are acoustically opaque, why go to all the trouble of making enclosures out of heavy, expensive, hard-to-machine MDF or birch ply? Why not just use paper conveniently formed into cones? It would weigh a lot less and you could make an acoustically opaque enclosure with nothing more than scissors and tape.
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For this test to be a valid test, the cone would have to be glued so that it cannot move as designed.
Just putting a piece of wire between the two terminals to simulate an infinite damping factor, or plugin it to an actual amplifier, would be more representative of what is really going on inside the speaker: the cone is not glued in place, and its movement can possibly influenced by the "back wave" (or whatever is should be called)
....Just putting a piece of wire between the two terminals...
Off Topic , but it's a good practice while shipping big fragile speaker drivers , especially vintage .
On Topic
Lynn you mentioned an option with two woofers and were considering one 416 and one 515 . Would it be 2.5 kind a system ? I have that option and I'm thinking how it could be done.
Rgrds, L
Off Topic , but it's a good practice while shipping big fragile speaker drivers , especially vintage .
On Topic
Lynn you mentioned an option with two woofers and were considering one 416 and one 515 . Would it be 2.5 kind a system ? I have that option and I'm thinking how it could be done.
Rgrds, L
Lynn, it is not controversial that paper cones might be acoustically transparent. But this is never a problem is speaker design. Above the wavelength corresponding to twice the largest dimension of the cabinet, there should be absorption inside the cabinet to kill the rear wave. Below that wavelength, the air moves like a mass/spring system. This is the same principle that allows us to design sealed and vented boxes.
Now, if you have a leaky dustcap, that might a source of unwanted sound coming through the speaker. But as long the cone itself is sealed, it shouldn't let any sound out at very low frequencies.
Now, if you have a leaky dustcap, that might a source of unwanted sound coming through the speaker. But as long the cone itself is sealed, it shouldn't let any sound out at very low frequencies.
Lynn stated "1 dB" transmission loss and that made it sound like a scientific experiment. I objected to that level of specificity if it is unsupported.
Cones and steel plates are both "transparent" but to different degrees. I am saying that in all of my experimentation I could find no substantial evidence to support a belief that a cabinet degrades performance because of internal pressure that "leaks" through the cone and becomes audible. If it were truly "1 dB" then the cabinet would not even pose a spring stiffness on the cone.
Shorting the terminals is not quite enough because the electrical damping is not 100% efficient.
Lynn,
Since your proposition is acoustical transparency of paper cones (obviously dependent on the thickness of said) the out-of-phase reflected radiation from one driver will go through it's own cone to the same degree as it would a second, other than the inverse distance law should attenuate it's transmission through the second to some degree.
The "additional time delay thanks to the transit time between the two drivers" also would be a similar effect as the time delay from longer reflections from the bottom end of a typical aspect ratio cabinet compared to the shorter reflections from the back through a single driver.
Art
50/50 =100%, e.g. I see the driver cone as both transparent and opaque. Visualize a single driver on a theoretical infinite baffle would not create issue because it is a point source (despite the cones imperfect behaviour). But when we introduce a second driver onto the scene not only do these pressure waves form upon the opposing drivers (flexibly thin) cones as bending waves along the surface of our infinite baffle but the distance between the two comes into play as a time delay and would expect distortion to rise. Enclosing this space would only worsen this condition. Dampening helps, somewhat in this regard. Moving a single driver to it's own box would minimize this interaction. Yes I said minimize not eliminate. It still exists in the opposite direction but then we're not listening to the dampened backwave inside the box... OR ARE WE? 
Lynn,
I would think that the requirement for this seperate enclosure per driver in a TL design would only need to be partially seperated along it's length to vastly reduce driver interaction in the low mid bands up.
Take a simple straight MLTL with two drivers loading the line. If the length were seperated by a divider (damped) half it's overal length wouldn't that go a long way toward reducing these surface pressure waves? I feel the random backscattered waves formed in cabinets are of little concern as long as they are diffuse enough.
Lynn,
I would think that the requirement for this seperate enclosure per driver in a TL design would only need to be partially seperated along it's length to vastly reduce driver interaction in the low mid bands up.
Take a simple straight MLTL with two drivers loading the line. If the length were seperated by a divider (damped) half it's overal length wouldn't that go a long way toward reducing these surface pressure waves? I feel the random backscattered waves formed in cabinets are of little concern as long as they are diffuse enough.
(Emphasis mine)
Back when I was building my Ariels ('95 MkIIs) I spent some time in correspondence with a "Jackius" (sp?) who was trying to write a paper on TLs (IIRC).
Long story short: keep them separate. Vastly goes to zero, at least until you hit the room itself
(Anyone tried the door experiment yet? Or do we just theorise here?
)
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