Hey everyone, so I want to make a low pass filter for my portable speaker, and only the powered wires are available. It has two speakers and I want to make one of them a subwoofer.
I’m new to electrics, but I know I need a capacitor and a resistor, but don’t know what rating they have to be.
I have linked to the capacitor I will use, just need help on the resistor part
Any help is appreciated!

 

You need an inductor, not a capacitor, for a low-pass filter to the speaker.
A resistor and capacitor LPF will seriously reduce the power.
Parts-Express has a good selection of components for that.

 

A portable speaker is usually too small to produce low frequencies.
A $25.00 Parts Express 4" woofer in a fairly large vented enclosure goes down to 73Hz which is far from lower sub-woofer frequencies.
Some 10" woofers in a huge vented enclosure are good sub-woofers.

 

Making a passive lowpass filter from a capacitor and a resistor is just going to waste power and produce disappointing results. In any case, you can compute the corner frequency of the lowpass filter from the following:
\( f_c\;=\;(2\pi RC)^{-1} \)
Suppose we wish to have a corner frequency of 80 Hz. So anything less than 80 Hz will be passed with minimum attenuation, and anything above 80 Hz will be attenuated. So using a bit of algebra we se that:
\( RC\;=\; (2\pi f_c)^{-1}\;\approx\;2\times10^{-3} \)
The solutions are not unique, in fact there are an infinite number of them. The usual approach is to pick and available capacitor value and comput the corresponding resistor value. As an example you could pick:
\[ C=100 \mu F;\;R\;=\;20\Omega \]
To see if we have done this correctly we can simulate the circuit to see the AC response

You are down almost 11 dB in the passband and our corner frequency is a bit higher than expected due to the rounding. We can make the corner frequency lower but there is not much we can do, with an RC filter, to the relationship between R1, which is part of the filter and R2 which is modeling the speaker impedance.

 

To follow PB's simulation, below is a simulation with an inductor:
Note that ideally, there is little signal loss below the filter corner frequency of 85Hz.

 

Your RC can be used to feed the input of a high input resistance amplifier but is slope of higher frequencies reduced is very gradual. A single series inductor also produces a very gradual reduction in higher frequencies.

With the 20 ohm series resistor then -3dB is 1/2 power, -6dB is 1/4 power, -9dB is 1/8th power and -12dB is 1/16th power which is a large reduction in loudness.

 

Your RC can be used to feed the input of a high input resistance amplifier but is slope of higher frequencies reduced is very gradual. A single series inductor also produces a very gradual reduction in higher frequencies.

With the 20 ohm series resistor then -3dB is 1/2 power, -6dB is 1/4 power, -9dB is 1/8th power and -12dB is 1/16th power which is a large reduction in loudness.

It is not a very attractive tradeoff to be sure.

 

Sorry, meant inductor, not resistor. Would something around a 1000uH capacitor be fine, or should I add an inductor as well?

 

You need an inductor, not a capacitor, for a low-pass filter to the speaker.
A resistor and capacitor LPF will seriously reduce the power.
Parts-Express has a good selection of components for that.

An inductor isn't the only component of a low pass filter. Depends on your configuration.
http://www.bcae1.com/xoorder.htm

 

Sorry, meant inductor, not resistor. Would something around a 1000uH capacitor be fine, or should I add an inductor as well?

The simulation in post #5 shows an inductor of
\( 15\;\text{mH}=15\times10^{-3}\;\text{Henries} \)
There is no need for a capacitor of 1000μF or any other value for that matter. You can compute the impedance of that inductor at the geometric mean of 1Hz and 85 Hz as follows:
\( X_L\;=\;2\pi fL\;=\;(6.28)(9.22)(15\times 10^{-3})\;=\;0.87\Omega \)
which is pretty low compared to a typical speaker impedance.
This rises to 8Ω at 85 Hz. which is to be expected since it makes the filter impedance equal to the supposed speaker impedance. If your speaker impedance is different you'll have recalculate.

 

An inductor isn't the only component of a low pass filter.

True.
The speaker impedance is the other component.

 

Would something around a 1000uH capacitor be fine, or should I add an inductor as well?

That would be µF not µH for a capacitor, but you don't need (or want) a capacitor, only an inductor.
And the inductor must have a low series resistance, with an air core type preferable for low distortion.

 

You are making a bongo drum, not a sub-woofer. A little portable speaker probably resonates at the frequency you want it to cutoff. The resonating speaker needs the extremely low output impedance of the amplifier to damp its resonance. A series inductor creates an impedance too high.

What is the diameter of the speaker, what is its resonant frequency, what is the size of its enclosure, vented or sealed enclosure and what frequencies does it need to produce??

Most portable little 2-way speakers use separate amplifiers and active filters for its woofer and tweeter. No big, heavy and high impedance inductors.

 

You will NOT get "more-Bass" than You already have,
without getting some different, larger speakers, that can move a substantial amount of Air.
Small Speakers = No Bass.
.
.
.

 

At Costco they had a portable little Name-Brand fairly expensive speaker on demo producing plenty of good-sounding bass.
It made my skin have goose-bumps and I think my hair stood up. It was unbelievable.
My son also heard it then he bought a Chinese copy for Fathers Day. It was junk and it produced no bass.

 

True.
The speaker impedance is the other component.

And a capacitor, and more inductor and more capacitor depending on the order.

 

You will NOT get "more-Bass" than You already have,

Relatively you will.
Since only one speaker will now be generating the higher frequencies, the low frequency power will double (of course that's only 3dB).

 

One stereo channel for the booms and the other channel for the squeaks?

 

At Costco they had a portable little Name-Brand fairly expensive speaker on demo producing plenty of good-sounding bass.
It made my skin have goose-bumps and I think my hair stood up. It was unbelievable.
My son also heard it then he bought a Chinese copy for Fathers Day. It was junk and it produced no bass.

Was there a subwoofer at the demo, hidden out of sight? Sold as an "optional extra"?

 

Was there a subwoofer at the demo, hidden out of sight? Sold as an "optional extra"?

No hidden sub-woofer. That is the second Bose demo that surprised me with plenty of bass from little speakers.
The first demo had two huge speakers producing very good sound with plenty of bass. Then two pretty young ladies came and removed big fake covers over the real little speakers that were producing the sounds. Again, no sub-woofer.

My local shopping mall and a few large stores installed large two-way Bose ceiling speakers that sound good.
My nearby large stadium installed many large Bose speakers that sound awful.
My nearby Opera House installed an unknown brand of a speaker system that also sounds awful.
A guy often sits at a nearby walkway playing loud RAP on a tiny little speaker that sounds terrible.