Originally Posted by
Lancer
Just for fun - here's the 1500AL mounted in a 4.00 cubic foot sealed test cube. One should note the standing wave at ~ 350 Hz.
The standing wave can be seen in the impedance curve. It is the blip at ~ 350 Hz. With the oscillator set at that frequency the most horrid sound emanates from the system as would be expected. The internal dimensions of the enclosure are 484 mm cubed.
Note that 484 mm corresponds to a wavelength of ~ 712 Hz hence the standing wave will have a frequency of ~ 356 Hz.
The addition of 2" thick OC fiberglass on all panels is sufficient to reduce the standing wave ~ 350 Hz. Impedance drops to "normal" and the objectionable sound is greatly attenuated.
This doesn't necessarily have anything to do with the project. I just thought it might be interesting to some that the standing waves generated in a cube are definitely objectionable and are viewable in an impedance run.
Some closed box data:
driver mounted normal
4.0 cu ft gross
no fill
Qtc = 0.56
Fc = 49.8
driver mounted normal
4.0 cu ft gross
2" OC fiberglass on all panels except baffle
Qtc = 0.54
Fc = 47.8
Like pointing to a distant Moon gazing at you on a still night there are also the more ponderious issues a lower frequencies:
That of Pressure waves as detailed by the quote below are best attenuated by a thick loosy layer of open cell foam with a laminated thin film plastic layer such open cell carpet under felt to attenuate such pressure resonances.
"Unless an enclosure is spherical or ellipsoidal, all or some of its sides
will consist of plane surfaces clamped at their edges. Such a clamped
panel will have its own acoustic output when forming part of an
energised loudspeaker, derived from sound energy within the enclosure.
The output consists of standing-wave modes at higher frequencies and
pressure modes at those frequencies where the wavelengths exceed the
internal enclosure dimensions. Adjacent panels may be similarly excited
by vibrational energy from the drive unit chassis.
Theoretically a clamped panel has a well defined vibrational series in
both longitudinal (volume stiffness) and bending modes. A further mode
is due to the panel mass resonating with its own and the enclosure's air
volume stiffness. Stevens found that in a typical reflex cabinet this
latter resonance appearedat almost twice the fundamental enclosure
resonance, a condition verified over a range of tuned system frequencies.