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In Reply to: RE: Bass reflex with port on the front or on the back ... any difference ? posted by Kal Rubinson on December 07, 2014 at 08:31:59
Hi and thanks a lot but i am not sure to understand rightly what is the reason of the port on the back
Does it improve the low freq response ?
Kind regards,
bg
Follow Ups:
beppe61,
Employing a port on the rear allows the manufacturer to build a smaller speaker than an identical front ported example.
Al
Hi and thanks and this is what i wanted to hear
That at the end of the game is mainly for practical reasons but the effect is identical.
Good to know. Issue closed.
Thanks again.
Kind regards,
bg
Air moving out of front port creates cancelation with next driver above. Don't believe me? Try using an appropriate sized post em note over port and the difference is not the bass but the midrange.
I disagree with your assertion. Its all about the bass and not the midrange as easily shown by the frequency response curves. The black trace is a sum of the blue trace which is the driver and the red trace which is the port. Look how far down the amplitude is at 1Khz ..
Hi and thanks. Interesting.
If i understand rightly the bass port adds to the midrange ?
this i did not know. I was thinking that the main effect was in the bass actually.
Good to know.
Kind regards,
bg
No it is almost always a BASS port. The air coming out of a port is designed to resonate at a particular bass frequency. At the upper end of the frequency response the air in the port moves diametrically opposite to the low end response of the driver above (Midrange in a three way, tweeter in a two way). Temporarily blocking such a front port will reveal significant detail in the range of the crossover.
Using a post em note allows the paper to flap when large bass dynamics are present, but blocks lower velocity air movement which causes cancelation.
Above tuning frequency, port in phase with bass, below tuning frequency out of phase (that is the port resonance, not the midrange leakage from the back of driver to wall to port exit, which might be what you are trying to explain)
Very rare that port will interact with tweeter as they are at least one but usually well over three octaves away from tweeter xover point.
In a two way system?
About the worst/highest of significance port measurements I've seen or done show the midrange leakage through port in the 700 to 800 hz. So a xover of 2k to 3.5k (fairly typical of speakers), that is the one to two octaves.
And with good number having 400hz to 500hz that then approaches 3 to 4 octaves. So even with fairly shallow 12db slopes your looking at 36 to 48 db down for tweeter output at the interaction point of the port leakage to the tweeter
Now interaction with midrange, that can clearly have an affect. Another thing is port exhibiting resonance (not leakage), that resonance will ADD to the output (see the graph I posted in original response of the Opera Callas).
So with all that going on, for most speakers setups in rooms, the port on the back is the chosen design.
Hi and thanks for the valuable explanation
i think i have understood better now
Kind regards,
bg
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