height placement considerations for port for a floor stander

tajanes

Any thoughts / considerations (or links to previous discussions) on the height placement of a port for a floor stander?  I’ve seen discussions re front / rear placement options, and would expect some advantage of near-floor placement for floor coupling, but curious as to advantages relative to attenuating disruptive floor/room interactions by moving port towards mid-level and above (i.e. for a rear port design)???   Thanks

rjj45

any ports close to the woofer can cause baffle-diffraction related artifacts in the FR.
I'd advise close to the floor, front or back.

PWRRYD

This is a great question.  I don't have the answer, but I suspect having the port down close to the floor and the woofer up much higher would result in a peaking of bass centered around the port tuning frequency.  I think that is why I didn't like the one MLTL I built where the port was centered 5" from the floor and the woofer was 30" from the floor.  I have no measurements to back that up though.

4thtry

My dual exhaust speakers have a front port near the floor with the woofer up high about 30 inches or so.  When we played them at Dakota last year, the bass was very anemic compared to Curt C's floor standing speakers.  I'm still trying to figure out what I did wrong.  The Leonard Audio TL program allows the user to move the drivers, stuffing, and port location around, but this program only gives you the anechoic response that you would get with the speaker without floor or wall room reflections.  It doesn't tell you what happens when you put the speaker in a real room. 

PWRRYD

Bill, that is what I suspect may be the problem.  I have great respect for Paul K. and his use of Martin Kings MathCAD sims.  But when I built his designed MLTL exactly to spec there was a very pronounced peak around 40 Hz.  That was the tuned frequency of the port which was only 5" from the floor.  The port got boundry gain whereas the direct radiated driver output did not.  I was not the only person who heard it.  Several guys at Iowa said the same thing.

jhollander

Beyond boundary reinforcement imo there's not much issue with port location.  Playing with horn response moving the port further from the woofer changes the summation a bit in the higher frequencies. 
 
I think Bill's problem maybe the nearfield splicing of the woofer response.  I saw the same issue with Bill's Coax project.

4thtry

Craig, makes sense to me.  When we were playing my dual exhaust speakers in Dakota, we moved them to about 6 inches from the rear wall, then moved them out to about 2 feet from the rear wall.  Near the rear wall, the bass below 300Hz or so increased dramatically.  Even with the speaker several feet away from the rear wall, I'd imagine that the port picks up a 2dB or so floor bounce reinforcement in the same manner.   But at 40Hz, wouldn't the wavelength be so long so as to cancel this gain?  Not sure on this.

John,  you are correct.  I messed up big time trying to apply the NF/FF blending process on both my dual exhaust and coax speakers.  But I think I have the blender figured out now. Time will tell.   

rjj45

I never knew that ports close to a woofer could/would cause serious FR irregularities.  Recently been working on a 4 inch desktop with a slot port. building the slot port right next to the woofer caused +2 dB swings from 700-2.5K.  Plugging the port smoothed out the FR to a great extent. 
As always, experience is the best teacher....  Here's the baffle with the port plugged.

Woofer FR, port open in blue, port blocked in red.

jhollander

With the synergy horn there was the same problem with the driver outlet holes causing dips.  Ironically the port holes, even tough inside the horn, did not create any dips.

The dip FR location seemed to be influenced by the size of the hole and how close it was to the driver under test.

rjj45

Thanks John.
Considering the possible physics, it's probably a worst case scenario - the slot is about 4 1/2 inches wide, and it's very close to a 4 inch woofer, so there's a concentration on 3 wavelengths. As I said, never again.  The woofer tested very flat in a flat baffle box with the port on the back.

6thplanet

Rjj45, what do you have inside controlling the rear wave of the woofer? The FR wiggles your seeing with the port open could be rear waves coming out of the port and interacting with the front wave. We had this issue on a buddies speaker when he used poly fill inside. We took that out and used FG batting and the SQ was like night and day. 

rjj45

Thanks for the feedback
In this case, I had Dow 703 on the side walls and top and a good handful of poly stuffing so I'm pretty sure it's not the backwave.

Guess I could cover the woofer and try to measure port output separately.
Always fun with this hobby

4thtry

I was thinking about tajanes original question about the effect of port location and was wondering if the attached comparison would be valid.  I used Jeff B's "Diffraction & Boundary Simulator" to compare a port located 6 inches from the floor verses a port located 3 feet from the floor.  Would this be a valid comparison?    If so, these curves should be used as correction files to normalize the port's near field response curve.  They should also be used to normalize the woofer's near field response curve, based on the position of the woofer.  Overall, this will have a significant impact on how much baffle step compensation (BSC) one decides to apply, depending on the crossover frequency and the height of both the port and woofer from the floor.

On my current coaxial speaker project, I had to reduce the overall BSC by about 2-3dB to compensate for the fact that the woofer is 7.5" (on-center) above the floor, the port is 2" (on-center) above the floor, and the crossover is 410 Hz.  The near floor positioning of the woofer and port significantly boosts the response below 500Hz or so. 

Bill