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RE: "New" Quads and Stands

Posted by josh358 (P) on February 28, 2017 at 06:52:59

In Reply to: RE: "New" Quads and Stands posted by cawson@onetel.com on February 28, 2017 at 04:47:27:

Consider the way you feel the bass through a wood floor. The floor is vibrating so much that you can feel it. What's worse, it's acting exactly like a sounding board.

Think of the woofer's vibrating coil as a vibrating string of a guitar, the cabinet or baffle the bridge, and the floor itself as the guitar body. Or of how you touch a tuning fork to a table top, it gets a lot louder. That's the physics of it -- the floor and sheetrock are acting as sounding boards or loudspeaker cones, which are acoustic transformers.

Intuitively, it's the difference between waving your hand back and forth rapidly and waving it back and forth while holding a Japanese fan. In the former case, you'll feel little air resistance, in the latter case much more, because you're pushing on many more air molecules.

But it gets worse than that -- floors and sheetrock are typically resonant at audio frequencies. You can hear the resonances by tapping on them and you'll hear a lot in the audio range. So now they're acting like organ pipes as well and that will tend to give you one-note bass.

So if the floor and sheetrock vibrate, more of the acoustic energy from the vibrating enclosure will be coupled to the air, and more of it will reach your ears rather than being dissipated by the internal damping of the speaker enclosure (turned into heat, or randomly-vibrating rather than systematically-vibrating molecules -- it is the *randomness* here that insures that we can't hear it), and it will tend to resonate at certain frequencies as well.

On the other hand, if that vibrating enclosure could be solidly coupled to a massive, low profile building structure like the studs and joists, you'd potentially have a better situation. The studs and joists are also acoustic transformers, but wired the other way around. They have a high mass density, they're rigid, and their resonances are pretty random. So in effect, they make the mass and stiffness of the enclosure higher if they're well coupled to it. For the most part, the benefit isn't that these solid structures are *absorbing* the vibrations but rather that they're reflecting them because of the extreme impedance mismatch. This, coupled through the enclosure, keeps more of the acoustic energy in the moving mass of the driver where it is dissipated through electrical and mechanical damping.

It's really a complex system though since so many factors can vary -- you have flimsy enclosures that vibrate like crazy, and at the opposite extreme you have speakers like Magicos that take heroic measures to keep the enclosure stiff, massive, and damped. And different kinds of floor and building structures as well -- old resonant wood ones, concrete, what have you. Maggies for example benefit from being coupled to building structures with rigid, massive speaker bases like the Mye.

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