Hi , jules,

```
void IIRFilter::makeLowPass (const double sampleRate,
const double frequency) throw()
{
jassert (sampleRate > 0);
const double n = 1.0 / tan (double_Pi * frequency / sampleRate);
const double nSquared = n * n;
const double c1 = 1.0 / (1.0 + sqrt (2.0) * n + nSquared);
setCoefficients (c1,
c1 * 2.0f,
c1,
1.0,
c1 * 2.0 * (1.0 - nSquared),
c1 * (1.0 - sqrt (2.0) * n + nSquared));
}
```

I did some calculation with my pen and according to the code above, which is using a 2nd order butterworth, I think the param n has some problem. Param n here is 1/w, and w is nomalized angular frequency.

My question is: shouldn’t the first line of the code be replaced by this:

`const double n = 1.0 / tan (0.5 * double_Pi * frequency / sampleRate);`

Because the frequency conversion equation(from normalized freq to analog freq) is :

Wa = tan (Wn / 2) * ( 2 / Ts);

where Wa means analog filter frequency, Wn is normalized freq( 2 * pi * fc / fs), and 2/Ts is usually ignored.

Correct me if wrong.

thx