Need info on operating below Fc

[mashaffer]

I have seen a trick mentioned in sub design where the sealed box is deliberately made smaller than normal (Qc >> 0.7) and resulting sub driven entirely in the band below Fc with a 2nd order compensating filter. Unfortunately I can't find the information. Does anyone have a pointer to a discussion of this approach?

[bobodioulasso]

May be a search on "linkwitz transform"?

[chris661]

Quote from the web

An interesting variation on the sealed box is the Extended Low Frequency (ELF) design. (Note that "ELF" is a trademark of Bag End Loudspeakers, one of several companies supplying commercial subwoofers based on this design principle.) An ELF system is simply a sealed box which is operated only below the low frequency -3 dB point (F3). This has several advantages as well as several obvious disadvantages. The advantage include:

* Very small boxes are not only possible, but actually desirable since F3 becomes the upper limit rather than the lower limit of the passband.

* The impedance is quite constant and stable below F3. thus presenting an easier load for an amplifier.

Nothing's free, however, and ELF designs suffer corresponding disadvantages:

* Since the amplifier must equalized to put out an extra 12 dB/octave below F3, efficiency is extremely low and the required amplifier power is exceptionally high. For example, Bob Carver's Sunfire "True Subwoofer" is an ELF design which includes a built-in, dedicated 2,700 watt amplifier!

* The driver must be capable of handling all this additional power.

* Generally speaking, the more power a driver must dissipate, the more distortion it can generate due to the effects of voice coil heating.

* The driver must be designed with a high enough excursion to be able to achieve its equalized low frequency potential.

For more information on ELF designs, see the Bag End (for theory) and Elliot Sound Products (for information on the required electronic crossovers) web sites.


Question - why are you considering such a thing?

[Moondog55]

Sub-Woofer Controller

P48 Sub-Woofer Controller (Rev-A)

Use multiple drivers to allow use of cheaper subwoofers, instead of 30mm X-Max use 4 * 8mm

[mashaffer]

Thanks. I have an application (portable "mini-console") where I am looking at options for getting the best low bass output in a small package. This seemed like one reasonable approach.

mike

[mashaffer]

I think this is worth playing with. I think the LT is unnecessarily complex for my situation.

I have an 8" driver from a 1 ft^3 vented sub to play with and I assume that I can determine the resonance in box by simply finding the Zmax frequency so tweaking a box for the desired top end should not be too tough. I have a 200W amp that I can use to power it and would just need to knock up an appropriate LP filter. Should be interesting.

My thinking is that the final driver that I purchase should probably be high Xmax, lowish Qts, lowish Vas, and high electrical power handling. Sound about right?

[Moondog55]

Re-reading the page on ESP I don't think you need any filters before Rods board.

He says you need the full bass signal < which makes sense, the ELF works below where your current bass drops off

[GM]

Originally Posted by mashaffer

My thinking is that the final driver that I purchase should probably be high Xmax, lowish Qts, lowish Vas, and high electrical power handling. Sound about right?

Works for me! FYI, Qts = 0.312 gives the maximum bass extension for the minimum box size.


GM

[Moondog55]

Originally Posted by GM

Works for me! FYI, Qts = 0.312 gives the maximum bass extension for the minimum box size.


GM

Interesting observation GM, never thought along those lines before, although looking quickly at my favourite woofers( Vifa M-22 and Cerwin-Vega Vega154) they have Qts of 0.33; very similar??

My woofers have much higher Vas and Qts so I was thinking of putting them in an undersized but Aperiodic box.

[GM]

Yeah, unfortunately I wasn't sharp enough to notice it though, it was pointed out to me way back when by an acoustical engineer employed by the local Altec distributor that helped me to get through all the math in Altec's T/S tech bulletins. Seems to me that djk? has posted it too.

Sounds like a plan as long as they aren't too undersized since less is always less in acoustics.........

GM