Need A 50Hz Crossover Network

[tbryanh]

I need a 24 dB per octave high pass filter set to 50 Hz for an Eminence DeltaPro12 driver (Speaker Detail | Eminence Speaker).

Any ideas?

[Chuck H]

Won't a simple inline capacitor give about 20dB/octave? Do you need to Xover below 50hZ to a dummy load? another speaker? That'a a big inductor, but that's how it's done.

[Jazz P Bass]

Why?
Help with high pass filter in Bass Guitar Cab - Techtalk Speaker Building, Audio, Video, and Electronics Customer Discussion Forum From Parts-Express.com

[tbryanh]

The crossover is recommended in this Eminence document, http://www.eminence.com/pdf/Delta_Pro_12A_cab.pdf. It is to protect the driver from overexcursion.

Actually, depending on the cabinet, 4 different cutoff points are recommended, 50, 70, 100, and 150 Hz.

A simple capacitor might work, but what about the roll off rate?

[Jazz P Bass]

That is pure & simple cover your ass BS.
The excursion at 50Hz is 16mm.
What are you planning on playing through this thing?

[R.G.]

A simple capacitor might work, but what about the roll off rate?

Depends on whether the speaker is acting resistive, inductive, or capacitive at the frequency. Speakers in enclosures are fourth-order all by themselves. I can't remember for horns; maybe only second order.

If you assume resistive, it's 6db/octave, 20db/decade. 24db/octave is a four-pole highpass. The problem is they tell you that resonance is 51Hz, so it's inductive below that, capacitive above it and resistive only right on the top of the resonance, so the 50Hz one will be ugly. You may only move the resonance around with a single cap, or in fact any non-conjunctive passive crossover.

Unless you're already committed to doing it with a passive filter, go for an active crossover and don't feed it anything below 50/70/100/150 depending on the switch setting.

[tbryanh]

..

[tbryanh]

Depends on .... You may only move the resonance around....go for an active crossover...

Looking closer at the Eminence document, it appears the low pass filter is used to prevent the driver from being operated at its resonant frequency. It appears the different cabinets cause the driver to resonate at different frequencies, hence the need for different cutoff frequencies for the low pass filter.

I think operating the driver at its resonant frequency could cause overexcursion.

Using the "low pass filter" to lower the resonant frequency of of the driver could work. If the driver needs to reproduce low notes on the bass guitar, keyboards, or kick drum, this might be better than active filtering.

[Chuck H]

I think cabinet design (ie: ported, folded horn,etc) is a better goal for efficiency. Other than that juse base the volume of the enclosure on the speaker parameters. Using a rough SPICE simulation (a bastardized duncan TSC format) with an 8 ohm source impedance, an 8 ohm load impedance and a DC resistance of 6 ohms (approximating) you'll have about 18db-20dB octave rolloff with a simple capacitive high pass filter of 140uf with a -3dB knee at 50Hz. Since I don't know all the formulas this is part of how I do things. Corrections accepted. It's noted that R.G. is right in that resonance and DC resistance/AC impedance specifics will play an important roll in accuracy.

[tbryanh]

If the driver needs to reproduce low notes on the bass guitar, keyboards, or kick drum, it appears that a different driver should be used.

[Chuck H]

That's up to you. Plenty of effective PA cabs that aren't very efficient below 50Hz that sound just fine though. The filter issue remains WRT your OP.

[R.G.]

I think operating the driver at its resonant frequency could cause overexcursion.

OK. I understand the goal now. Yes, loud notes at an undamped resonance can kill the driver.

It's odd for a speaker company to put things that way. Controlling excursion at resonance is one of the fundamental reasons for existence of the bass reflex enclosure, and they should know this. Maybe they got fancy with the speaker design for some tonal reason and stuck this into the literature to give themselves an out for guitarists who insisted on using it in ways that caused it to fail. They for sure know that musicians won't in general do an enclosure design to work properly with it.

Using the "low pass filter" to lower the resonant frequency of of the driver could work. If the driver needs to reproduce low notes on the bass guitar, keyboards, or kick drum, this might be better than active filtering.

It doesn't work that way. The low frequency resonance of the driver is a mechanical one. Feeding it through a capacitor doesn't help with the cone resonance it just phase shifts and changes the magnitude of the driving signal. The cone resonance is affected by enclosures by moving UP in frequency. That's why they are talking about higher frequencies depending on the enclosure.

As I remember, a transmission line enclosure can lower the cone resonance. I don't think other types can, although I haven't played speaker enclosure for a long time now. A bass reflex splits the main resonance into two on either side of the original and damps them both if done right. Horn leaves the bass resonance at the same frequency but damps it by increasing loading, but gets you into resonance problems with the driver chamber and resonances in any folds or imperfection in the horn itself. Infinite baffle gets you to the free-air resonance, undamped, and acoustic suspension is a special case requiring special speakers optimized the other direction from guitar speakers.

[tbryanh]

So active filtering might be best after all for this driver. Then a subwoofer might be needed. This leads to a bi-amped system.