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Lowering Fs.
Lowering Fs.
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Old 10th February 2013, 10:59 PM   #31
tinitus is offline tinitus  Europe
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Lowering Fs.
cool looking new Mivoc woofer

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Old 10th February 2013, 11:54 PM   #32
OscarS is offline OscarS  United States
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Join Date: Jan 2011
Originally Posted by OMNIFEX View Post
The fs of the loudspeaker in the cabinet usually shifts downwards if the cabinet is optimum sized or larger than the loudspeaker's requirements.

The link below offers the TS Parameters of one of my older Beyma 18-inch drivers.

Beyma Speakers - Beyma 18LX60 speaker - Beyma 18LX60 1200 watt 18" woofer for all bass applications. Beyma 18LX60 and other Beyma 18" speakers here.

This is the measured impedance curve of the loudspeaker in the box.

Click the image to open in full size.

As you can see, the loudspeaker offers an fs of 27 Hz in free air and, 19.04 Hz in the box. A 3 cubic foot box for a 15-inch pro driver is not going to give you low bass for the limiting factor is the enclosure size.
The issue with this is that when you run a "parameter test" once a speaker is installed in a box, the software doesn't know that the 1st impedance peak is not a raw-driver's Fs (remember, Fs= free-air(!) resonance)----and since it doesn't know, it still goes ahead uses that 1st peak to calculate what it thinks is the driver's free-air resonance, and spits out the results without alerting you that the driver is not really suspended in free-air.

But it is flawed because there is no Fs once a driver is enclosed. There is nothing "free air" about a driver "enclosed"---a kind of oxymoron. The reason for this is that once a driver is enclosed it now forms one driven resonant system with the enclosure and vent, that are now inseparable from each other. This goes back to University Physics 101 with the studies of a driven damped double-mass double-spring harmonic oscillator.

This is why a vented enclosure's primary system is called Fb, not Fs (similarly for a sealed box, the system resonance is Fc). Rest assured, the primary system resonance (Fb, or "port tuning" as it is known) for that enclosure you posted is just slightly below 40Hz, right where the impedance dip is.
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