# Audio frequency filtering

I would like to make a Audio frequency filtering that would take in audio signals of music and only export <80 hertz. How can I do this?

"Export" in what way?

Why not just build a simple low-pass filter?

You could make an RC (resistor-capacitor) low pass filter with nothing but a 200 Ohm resistor and a 10 uF capacitor.

The equation for centre frequency is: fc = 1 / ( 2 pi R C)

If you choose a 100 Ohm resistor and need to pick the right capacitor, this becomes: C = 1 / ( 2 pi R fc) C = 1 / (2 * pi * 100 * 80) C = 0.00001989 F, or 19.89 microFarads

Or if you have a capacitor lying around, say 10 uF, it's probably easier to buy a specific resistor, or use a potentiometer to get the right resistance: R = 1 / ( 2 pi C fc) R = 1 / (2 * pi * 0.00001 * 80) R = 198.94 Ohms

Please correct me if I've done this wrongly - I haven't done any elec eng since college!

arduinoPi:
I would like to make a Audio frequency filtering that would take in audio signals of music and only export <80 hertz. How can I do this?

Are you making a subwoofer crossover circuit?

If so, you will want better than a first order filter. Let me know and I can show you how to do it.

BTW, I assume you plan to do this with discrete parts and not the Arduinoâ€¦ right?

Since you can sample at upto 10kHz or so with the Arduino I would strongly suggest doing so and then using a digital filter to get your 80Hz low-pass characteristic. Something like this calculator can make it easy and give upto 5th order chebshev response... http://www.schwietering.com/jayduino/filtuino/index.php?characteristic=ch&passmode=lp&order=5&chebrip=-0.5&usesr=usesr&sr=10000&frequencyLow=80¬eLow=¬eHigh=&pw=pw&calctype=float&run=Send

Actually fixed-point representation will calculate a lot faster.