Wavelength & accoustic "short circuit"?

[Jennice]

Hi all,

I need some help in brushing up on my theory.

I was feeding a "random noise" (white noise) to a woofer, which is about 210mm diameter. It is placed with magnet to the floor, without enclosure. What the microphone picked up seemd to relate well to the audio source signal, exccept that it dropped off below about 780 Hz.

I then recalled something about accoustic short circuit, where the air travels directly from the front to the back of the driver, without creating an audible sound pressure.
With a frequency of about 780 Hz, the wavelength should be (in my math) = 334 / 780 = 0,428 m This would give a very nice fit with the driver diameter being 1/2 * wavelength. Too close to be coincidence, I think.

Is that what I experienced?

Is it the same math for baffle step on enclosures?
I have read sources saying that baffle step happens at 1/2 * wavelength, and others say it's at 1/4 wavelength.
Can anyone enlighten me, to put theory and practice together the correct way?

Jennice

[planet10]

Originally posted by Jennice
Is that what I experienced?

yes

Is it the same math for baffle step on enclosures?

Baffle step isn't a cancelation it is a transition from 2 pi to 4 pi radiation... an empirical formula based on the work of Olson if f3 = 4560/w where w is the baffle width in inches... now this would be for a circular baffle and the real world is more complex... there is software called The Edge that does more accurate modelling.

For more BS info:

http://www.t-linespeakers.org/tech/b...intro-bds.html

dave

[Hans L]

You're right about the acoustic cancellation. See Linkwitz' site for all the theory behind dipole speakers.

Bafflestep is centered (theoretical -3dB point) around 1/4 wavelength.

[Jennice]

Thanks guys!

It's the first time I get to compare theory and reality by this kind of measurement, so it's great to have experienced guys around!

Jennice