Bass Reflex - Port Freq. Response

rjbond3rd

Member
2007-01-24 2:28 pm
Hi folks, I have an extreme newbie question. I've gone through quite a few books on speaker design. I can't seem to find an answer to this question.

What is the general theoretical frequency response (the curve) of a properly designed QB3 bass reflex port? Specifically, is it extremely narrow around the tuning frequency or does it slope gradually at both ends?

Stated another way, does the port generally add less than a half-octave, or is it supposed to add an entire octave (or more)?

(I think I'm dealing with a QB3 alignment in the small 35-liter Fostex recommended box for the FE 207e.)

The reason I'm asking is I'm creating some test tones to give the port a workout in the hope that I can get an ultra-smooth transition between the speaker and the port (by varying port width). But I can't tell what the frequency response of the port is supposed to me (so I can't really compare it to anything).

Thanks in advance for any help or guidance!
 

Bob Brines

Member
2003-01-31 10:11 pm
First off, IMO QB3 is a very bad alignment for a home speaker. Unless you are using your speakers out of doors or in a gymnasium, you are going to get significant room lift which leads to boomy, one note bass. My experience is that the in-room F3 is somewhere around anechoic F8-F10. If your QB3 F3 is 40Hz, add 10dB for room lift. Is that the sound you want to hear?

A much better approach is to use some form of EBS (Extended Bass Shelf) alignment. I don't do an actual shelf, but try for a curve that starts to droop at 80-100Hz and slopes gradually down to the expected room lift.

Yes, the Fostex recommended BR @ 35l is a QB3 alignment. Try 45l. Much better sound and extended bass.

If you want to understand how the driver and port interact, you need to use a model that shows driver, port and combined outputs. MJK's MathCAD does this, as does Unibox.

Bob
 

rjbond3rd

Member
2007-01-24 2:28 pm
Hi Bob, thank you! I will definitely simulate, as I can see now the intuitive / trial-and-error approach is going to take more time than I have. I appreciate the help!

Godzilla, I will humbly attempt to build whatever FE207E box you design. My only limitation is that I'm using pine shelving for everything (which I've seen that you have done a few times as well). Here in Santa Fe, New Mexico, pine is the traditional material of the great Spanish craftsmen and it's used everywhere, and for everything.