SaSi said:
Hello SaSi,
i'd like to add, that at low frequencies, where wavelength of sound in air is much smaller than the inner dimensions of e.g. a small subwoofer, there is no "radiation" of sound into the enclosure.
Of course there is some oscillating "force per area" (aka "pressure") acting on the cabinet walls, but there is a different "main mechanism" exciting cabinet vibrations:
That is the oscillating force acting directly on the baffle where the driver is mounted.
The force per area is much higher here than anywhere else in a woofer enclosure, thus direct mechanical excitation via the driver's motor and basket is a dominant excitation mechanism for cabinet vibrations, especially when the enclosure is stuffed and resonances of the "enclosed body of air" are highly damped.
This is why the kind of mounting of a woofer and the design of the baffle is important in designing a subwoofer or woofer cabinet.
The pictures on my websites below show a "bell like" mode occuring at about 220Hz in two small subwoofer cabinets, that i made and tested years ago. This "bell mode" is the lowest vibrational mode of that particular cabinets.
As the cabinets are used as subwoofers up to 80Hz having sufficiently sloped low pass filters of 2nd to 3rd order at least, the cabinet resonance is "well above" the used bandwith and this cabinet may be seen as a "valid design" in this regard.
You can find pictures of that mode at the bottom of the page:
Cabinet vibrations made visible (the old school way ...)
The woofers (invisible in the pictures) have been driven well above their continouus power to make these pictures, so without control of the power fed into the drivers please
"Don't try this at home !".
The signal was switched off periodically, allowing for the voice coils to cool down ...
Kind Regards
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This is why the kind of mounting of a woofer and the design of the baffle is important in designing a subwoofer or woofer cabinet.
This is so true; yes, it is the ultimate of importance, yet rarely addressed.
That's why I use a reaction-forced cancelling mounted technique where 2 identical woofers are bolted together, magnet-to-magnet. I'll post a picture
as soon as my latest project is assembled.
Hello Scott, hello all,
my statement should instead read:
"i'd like to add, that at low frequencies, where wavelength of sound in air is much larger than the inner dimensions of e.g. a small subwoofer, there is no "radiation" of sound into the enclosure."
... but i guess, i was understood anyhow. Sorry for confusing.
LineArray said:
my statement should instead read:
"i'd like to add, that at low frequencies, where wavelength of sound in air is much larger than the inner dimensions of e.g. a small subwoofer, there is no "radiation" of sound into the enclosure."
... but i guess, i was understood anyhow. Sorry for confusing.
@Linearray: I am with you on your comments. Perhaps stating the same in different ways or from different points of view.
Good point about pressure, that property should be considered when optimizing a design.
Mechanical vibrations obviously are a big and important issue and not only to sealed designs.
BTW, I feel I should add that the "backwash" from the driver radiates not only through the cone but through the panels as well. I've measured the radiation from a 20mm enclosure of 250lt containing a totally enclosed 12" driver in a sealed compartment and the 20mm of plywood are pretty much semi transparent to the sound levels you would expect the driver to produce "under normal listening conditions"...
Adding several layers of bitumen sheeting all around the inner panels reduces this emission by 10-15dB depending on frequency.
Since we are not talking about an "acoustically transparent" material here and the enclosure joints were pretty airtight, it's all about resonances triggered by changes in pressure in the enclosure. At low frequencies (below the panel resonance points) things are rather controlled. Once the lowest resonance is reached things get pretty wild. Adding mass to the inside of the panel or making a sandwitch solution goes a long way to reduce unwanted emissions.
Good point about pressure, that property should be considered when optimizing a design.
Mechanical vibrations obviously are a big and important issue and not only to sealed designs.
BTW, I feel I should add that the "backwash" from the driver radiates not only through the cone but through the panels as well. I've measured the radiation from a 20mm enclosure of 250lt containing a totally enclosed 12" driver in a sealed compartment and the 20mm of plywood are pretty much semi transparent to the sound levels you would expect the driver to produce "under normal listening conditions"...
Adding several layers of bitumen sheeting all around the inner panels reduces this emission by 10-15dB depending on frequency.
Since we are not talking about an "acoustically transparent" material here and the enclosure joints were pretty airtight, it's all about resonances triggered by changes in pressure in the enclosure. At low frequencies (below the panel resonance points) things are rather controlled. Once the lowest resonance is reached things get pretty wild. Adding mass to the inside of the panel or making a sandwitch solution goes a long way to reduce unwanted emissions.
Hi Allen,
those "internal baffles" or "dividers" may not be intended as "braces" to make the outer enclosure walls more stiff.
But those "internal walls" in Post #1 e.g. will - when executed as hard walls in a usual way - act like braces nevertheless.
Kind Regards
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