Turning a woofer into a subwoofer

[DavidWardlaw]

Hello,

I want to build a passive crossover for my woofer that I bought. It is a 4Ohm woofer rated at 400wattsRMS in a 1.5x1.5x2 ft. box. I would like to filter out all frequencies over 80 Hz (Basically I want it to become a subwoofer). I read somewhere that a 12mH inductor placed in series with the woofer would filter out all frequencies over 75 Hz. Is this true? If so, will it work? If not, how should I go about making a passive crossover for my woofer? Are there any other factors to consider when making a crossover for 1 speaker?

[nunayafb]

From xover simulator, a 12mH inductor in series w/ 4ohms will yield an Fc of 53hz. But remember, one inductor is a 6dB/octave crossover(@100hz it will only be down about 6dB). About 8mH will yield the Fc of 80hz.

Question, do you have a subwoofer xover built into your receiver?

[lndm]

Crossing over this low has its challenges but it can be done. Finding an inductor as big as nunayafb suggested that has a low DCR and an air core, means a little more than a few dollars.

When you do this, you might find that the rolloff around the crossover frequency is a little sudden. This will be due to the impedance peak, you may need to play with the values to get it right.

If the impedance peak is too close to 80Hz, you may find you can't cross there without compensating for the peak or going active.

You will probably want to add an RC network across the woofer to balance out the impedance rise in the midrange so you don't get a peak there.

[DavidWardlaw]

Originally posted by nunayafb
Question, do you have a subwoofer xover built into your receiver?

No, I do not. I simply have a 2 channel, 4 speaker outs (2 speakers per channel).

Originally posted by lndm
You will probably want to add an RC network across the woofer to balance out the impedance rise in the midrange so you don't get a peak there.

I don't have a midrange. It's only 1 speaker in a box that is connected to a reciever. What is an RC network?

[peace brainerd]

As a releated question..
What is the result of mechanically choking a woofer?
Many years back I threw together a very quick and dirty
passive "subwoofer" and choked it by putting a ring
of dense foam around the speaker face. Placing it
face down and adjusting the height from the floor (very
close) to just about the point where I could no longer
localize the sound, I was suprised at how well it seemed
to integrate with my speakers. I had no way to measure
what was taking place.

What is, after all, the effective difference between
electronically choking high frequencies and physically
doing so (let's say momentarily, with a pillow)? ...assuming,
for arguments' sake, that the "pillow" is able to be
situated so that it doesn't damp cone movement.

[Bazukaz]

Hi,
It is better to use active filters for subwoofers. Subs need at least 12db/oct (better 18db / oct or even 24 db/oct) filters. With passive filters , you would need to equalise woofer's impedance , as already mentioned. You need additional RLC circuit. The huge quality coils and capacitors are needed.This is usually quite expensive.That's why almost all subs use active filters.
A single coil (6db/oct) filter is not good for a sub.

Regards,
Lukas.

[lndm]

Originally posted by DavidWardlaw
I don't have a midrange. It's only 1 speaker in a box that is connected to a reciever. What is an RC network?

The inductor would be working on the woofer's impedance. Impedance is kinda like resistance, only it's a little different with an AC signal. The woofers impedance will get higher into the midrange due to the voice coil, which happens to be like an inductor itself. Try reading this: http://www.trueaudio.com/st_zobel.htm

When you read that, notice the impedance peak below the region the zobel deals with. This is due to the woofer's natural resonance, the one most spoken about. Your crossover will not work on top of that peak. You have three choices:

1. Cross over a bit higher.

2.

Originally posted by Bazukaz You need additional RLC circuit

3.

Originally posted by Bazukaz It is better to use active filters for subwoofers.

The third option works because if you have an electronic filter (crossover) somewhere before your amp, the crossover does not see the speaker's impedance and can't have a problem with it.

Originally posted by bluebeard
What is the result of mechanically choking a woofer?

This is a good point. Doing it with foam can be difficult at such a low frequency. The common method for mechanically crossing over a sub is the bandpass enclosure, which is quite popular in autosound applications.

[DavidWardlaw]

Originally posted by lndm

The third option works because if you have an electronic filter (crossover) somewhere before your amp, the crossover does not see the speaker's impedance and can't have a problem with it.

So are you saying if i were to put an 8 mH inductor in series with my reciever, the speaker will act as if it had a 80 Hz crossover in it? (This would mean that the signal will start at the source (cd player, laptop etc.) go through the crossover, through the reciever and then to the speaker)

[lndm]

Yes, you can filter the signal so that only the bass is amplified and fed to the speaker. You won't, however, do it with an 8mH inductor. The 8mH applies only to the 8 ohms which is only found at the speaker.

There are many options, some as simple as using a well selected and placed capacitor in your signal path. If this is your first use of active filtering, you might find it useful to purchase an active filtering kit consisting of an op-amp with filtering components and instructions for selecting your desired frequencies.

Others may have a better idea.

[peace brainerd]

Is that basically the function of these FMOD crossovers?

http://www.partsexpress.com/webpage....nufacturer=265

I've often wondered about the 25.00 price of these.
Are they simply a "well selected and placed capacitor"?