Crossover frequencies

Okay..   yes, i've done my searches like a good little n00b, but my questions weren't directly answered.

How does one decided what frequency to cut off at when designing passive circuits?

Obviously, it's going to depend on two things, 1) what order pass you set up, and 2) what the frequency response of the component is.

So, for argument's sake, let's set up the hypothetical. a 4 ohm 6.5" 2-way speaker with a frequency response listed at 35Hz to 25KHz.

If we set the cross frequency at 35 Hz, there's going to be some frequencies below this that get through, albeit at reduced power, depending on the order of the filter.

How much is too much power outside of the stated operating range?

Should we be setting the cross at some arbitrary number above the low operating range?

Are attenuation curves of pass filters really straight lines, or are they exponential curves?

And last, but not least, for a speaker with this range, can I safely assume that i don't need to worry about low passing at 25KHz?

The following are highly generalized comments to your general questions.  There are exceptions to any of these comments:

6.5" speakers may list response to 35Hz, but in practical application they do not respond well below about 100Hz.  The lowest high-pass for a 6.5" for best results is about 80Hz.

Do not exceed the stated RMS power limit of the speaker system.

The cross setting should be the same as that used for the next speaker in the system.  For example, if the sub is low-passed at 80 Hz, the mid-bass should be high-passed at 80 hz.

The attenuation curves are neither straight lines nor expotential curves, but for estimates use straight lines.

You should never low-pass a tweeter unless you are trying to eliminate certain noises or harmonics.

A good moral to this story is that speaker frequency response ratings mean NOTHING.  Listen to the speaker and see if you like how it sounds. 

Just to add-on to what DYohn posted:  The closer to the controlled limits of a driver the steeper the slope should be.  For example, a crossover point of 120hz on a 6.5" speaker doesn't require as steep a crossover slope as the same speaker crossed over at 80hz.  I totally agree with DYohn, the lowest I'd go on a 6.5" is 80hz. 

120hz might be fine at 12db/octave while 80hz might need to be bumped to something steeper like 18 or 24 db/octave.  You have to play around and see. 

I run my 6.5" quart at 80hz 18db/octave.

Very cool information. Thanks for the newbie help.

Lesson learned: size matters, not ratings. In speakers, that is..

kfr01, thanks for the input. I did find a set of 6.5s that I really liked, and wanted to know how to protect them from harmful frequencies. Thanks.