1

Here's a little something that puzzles me; do I add up the roll off point (-3db) to the attenuation per octave (eg. 18dB/va) or not?

Am I correct that the roll off point is not to be considered important when calculating the attenuation?

My test: Protools and Waves Q1, Rchannel,EQ3 filters

```
signal generator at 300Hz at 0dBFS
highpassfilter is set at 300Hz
result in level: -3dBFS
signal generator at 150Hz at 0dBFS
highpassfilter is set at 300Hz
result in level: -18dBFS
signal generator at 75Hz at 0dBFS
highpassfilter is set at 300Hz
result in level: -36dBFS
```

Conclusion: slope at 18dB/va So far so good! (with Q1 the slope is 12dB/va with corresponding results btw)

Now this: Used: Protools and Scheps 73 and VEQ3

```
signal generator at 300Hz at 0dBFS
highpassfilter is set at 300Hz
result in level: -3dBFS
signal generator at 150Hz at 0dBFS
highpassfilter is set at 300Hz
result in level: -21dBFS
signal generator at 75Hz at 0dBFS
highpassfilter is set at 300Hz
result in level: -39dBFS
```

Both the 1 oct down and 2 oct down do differ with the first test, they have 3 dB extra? Has this to do with the rolloff point (an extra 3dB) or is it something else?

I've gotta give an upvote for an answer that sounds just so right, yet I didn't understand a single, solitary word of ;) – Tetsujin – 2017-12-20T12:44:40.497

That's what I thought indeed, do not add the roll off point. Thanks for confirming. – user3810130 – 2017-12-20T14:09:10.043

@Tetsujin Look at this graph: https://en.wikipedia.org/wiki/Low-pass_filter#/media/File:Butterworth_response.svg The 3 dB that you don't add are shown in the gap between the actual response curve in red and the idealized straight lines in blue. The red line approaches the sloped blue line asymptotically, which is what this answer means by "the attenuation per octave is only asymptotic".

– Todd Wilcox – 2017-12-26T19:43:03.173