[Edgar77]

I just (re)watched Danny's video about Open Baffle Basics.

https://youtu.be/7ipMq7KlWwM?t=947

Danny shows different frequency response curves from bad to, what he calls, flat. But they are not really flat. There are still a few dB up and down.
I guess we can't expect +/- 0dB and probably also not +/- 1dB.
When is a frequency response curve considered to be flat? What is "good enough"?
And is it more of a problem when there is i.e. a short 2dB peak? Or is it worse to have 3db difference with smoother up an down?

When is a frequency response curve considered to be flat?

[S Clark]

+- 3dB is considered a very good curve.  It varies how sensitive we are to slight loudness variation. A 1 or 2 dB rise or fall is not likely to be noticed. 

[Danny Richie]

Anything in the +/- 2db range is pretty flat. However, a response can be in that range and still have narrow band peaks. And sometimes those can be pretty rough. The real indicator of problems is to look at the spectral decay. Look for a curve with no sudden movements.