filtering and tweeter fs

[crazyhub]

Hi guys,

At the begining of a new filter design process, besides all other considerations as distortions, directivity, x-max and on, do you have a general goal about the resulting level at tweeter resonance? For example: -10 or -15dbs at fs.
Thanks.

[bjorno]

Hi,
As you rule out X-max thus what’s level dependent on the excursion only power consideration remains.

I try to keep the tweeter not to heat up to more than sin RMS half level i.e. - 3 dB of max RMS power at peaks, that is about 0.707 Vmax RMS.

From that level (power) the average music power level is often lower than 10 dB down keeping the tweeter most of the time cool.

The voltage level mentioned is also the maximum RMS sin voltage that I would apply at fs where the impedance could be of any value depending on tweeter type (Ferro fluid cooled or not), or i.e. when filter, notch filter is used or not (that would further decrease the level at fs).

I don’t have a rule for a general ‘dB down’ at fs as the applications could vary considerably with the tweeter type/brand and what I find is a suitable filter to use.

Through out the years I’ve used tweeter fs levels ranging between 0 to –24 dB down and the levels always set depending on the resulting achieved overall sound quality, thus sometimes restricting maximum available SPL.

But generally as long as you keep the voltage level below 0 dB = V max RMS at fs and at an octave below >=12 dB down, valid for most tweeters, the excursion would be constant from fs and below or slightly falling and x-max should occur at or above fs for most tweeters where advertised max tweeter power should be valid.

Watch out for the specifications, max power can be expressed as a function of an advised lowest frequency and recommended minimum filter slope to use.

b

[crazyhub]

Thanks bjorno for your reply.

I projected to use my Focal Utopia be (fs 1500Hz) 3rd order cutoff frequency = 2500Hz. Result: fs is -17dbs / 87db maxi at 8500Hz (95db at 8500Hz without filter) and -40db at 750Hz. Thus largely enough protection!