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building tannoys...beginners help please

I have a question on port tuning:

- If the modelling shows a flat response to Fb, and Fb happens to coincide with an axial room mode, will this cause a greater in-room response peak than if the axial mode were at a higher frequency than Fb?
- Does port location influence this, i.e. how close the port is to the room boundaries compared to the driver? Is this why rear-ported speakers cause issues when placed too near the front wall?
 
ToTo Man said:
- If the modelling shows a flat response to Fb, and Fb happens to coincide with an axial room mode, will this cause a greater in-room response peak than if the axial mode were at a higher frequency than Fb?
- Does port location influence this, i.e. how close the port is to the room boundaries compared to the driver? Is this why rear-ported speakers cause issues when placed too near the front wall?
Click to expand...

I don’t understand much at all about the math of speaker cab design, but I’ve certainly found from experience that if you must port/vent a cab then you want to do it well below any room node and at as low a ‘q’ as you can possibly get away with. I’ve had 15” Golds in three cabs in my room; a very heavy sealed 75L corner cab, DIY Yorks which had a tuned slot port, and the current Lockwoods which have an odd aperiodic design with a very large non-tuned flat vent. All worked well in my room unlike many ported stand-mounts which almost always set a 45Hz node off. The Yorks worked the least well sounding rather ‘ported’ (i.e. loose and chuffy) on some notes, the sealed cabs had superb bass, deep enough for real impact, and just so fast, clean and tuneful, and the Lockwoods are a nice compromise between ‘ported’ and ‘sealed’ to my ears. Thunderous weight and scale, but still quite fast, agile and tuneful. I’d certainly argue that in typically small UK living rooms the last thing one wants to do with a large Tannoy (or any other large speaker) is to aggressively port to get that last ounce of extension. These drivers are not short of bass, I’d just aim for the cleanest, fastest, freest sound possible. I bet they could do a bottom E fine in an open baffle in most rooms!
 
ToTo Man said:
I have a question on port tuning:

- If the modelling shows a flat response to Fb, and Fb happens to coincide with an axial room mode, will this cause a greater in-room response peak than if the axial mode were at a higher frequency than Fb?
Click to expand...

I can't see why it would, but it is very easy to get the port tuning wrong and end up with a peak at Fb (Especially with smaller speakers where a slight increase in size can make quite a difference).


ToTo Man said:
- Does port location influence this, i.e. how close the port is to the room boundaries compared to the driver? Is this why rear-ported speakers cause issues when placed too near the front wall?
Click to expand...

I've seen this talked about on diy speaker building forums that I use, and according to the threads I've read it shouldn't make any difference as long as the port is not too close to the wall as to affect the tuning frequency, BUT I've seen a lot of people on pinkfish complain about boomy bass with rear ported speakers. Without making an identical set of speakers, but with the port on the front, It's impossible to know. Maybe the people that complain of boomy bass are just placing them too close to the rear wall? Typical port tuning frequencies are omnidirectional so in theory it shouldn't matter.
 
Fatmarley said:
I can't see why it would, but it is very easy to get the port tuning wrong and end up with a peak at Fb (Especially with smaller speakers where a slight increase in size can make quite a difference).
Click to expand...
This is my thinking too.

Fatmarley said:
I've seen this talked about on diy speaker building forums that I use, and according to the threads I've read it shouldn't make any difference as long as the port is not too close to the wall as to affect the tuning frequency, BUT I've seen a lot of people on pinkfish complain about boomy bass with rear ported speakers. Without making an identical set of speakers, but with the port on the front, It's impossible to know. Maybe the people that complain of boomy bass are just placing them too close to the rear wall? Typical port tuning frequencies are omnidirectional so in theory it shouldn't matter.
Click to expand...
I'm (perhaps wrongly) thinking of the port as a narrow band speaker driver, so my logic is that the further the port is located away from the driver, the more difference in the extent to which it excites room modes. E.g. in a sealed enclosure, the closer the speaker driver is to the floor the stronger the room height axial mode will be excited, the closer the speaker driver is to the front wall the stronger the room length axial mode will be, etc.

Presumably the magnitude of the dip in the 'ultra nearfield' frequency response measurement of the driver at Fb remains the same regardless of where the port is located, as long as it isn't obstructed?
 
ToTo Man said:
This is my thinking too.


I'm (perhaps wrongly) thinking of the port as a narrow band speaker driver, so my logic is that the further the port is located away from the driver, the more difference in the extent to which it excites room modes. E.g. in a sealed enclosure, the closer the speaker driver is to the floor the stronger the room height axial mode will be excited, the closer the speaker driver is to the front wall the stronger the room length axial mode will be, etc.

Presumably the magnitude of the dip in the 'ultra nearfield' frequency response measurement of the driver at Fb remains the same regardless of where the port is located, as long as it isn't obstructed?
Click to expand...

It's not something I've looked in to because I always place my ports very close to the driver.
 
Rockmeister said:
Would I better swap my port to be 1/3 down from the driver rather than I/3rd up from the bottom of the cab?
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Assuming port location does indeed affect excitation of room modes, which has not yet been confirmed, then unless your ceiling height is 6 metres (thus producing an axial height mode that coincides with your 28Hz port tuning), I don't think it will make any difference in your case.

I suspect it's more likely to affect me, if I decide to put my hard-edge MG12 (Fs=49Hz) into Lancaster cabs tuned to 50Hz, because my ceiling is 3.25m high and therefore produces an axial height mode at 52Hz. My three axial modes are grouped fairly closely together (my axial width mode is 41Hz and my axial length mode is 45Hz), so if port location does matter then I expect the worst place for it in my circumstances would be at the rear, bottom, outer corner of the cab as this would likely excite all 3 modes!

PS - My above speculation purely relates to the effect on room modes. There may be other advantages/disadvantages of locating the port close to the driver which I am not aware of.
 
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My ceiling probably IS around 6m...It's an open stairwell room with double height up to the sloping roof, so!?
I think I'm just going to run away now, build the damned things and say, 'they are perfect' regardless...all this mode stuff and 18.724 cm ports needing tuning to a possible 17.41 after downloading room resonance tones for measuring.
HELP!
:)
 
Rockmeister said:
My ceiling probably IS around 6m...It's an open stairwell room with double height up to the sloping roof, so!?
I think I'm just going to run away now, build the damned things and say, 'they are perfect' regardless...all this mode stuff and 18.724 cm ports needing tuning to a possible 17.41 after downloading room resonance tones for measuring.
HELP!
:)
Click to expand...

I wouldn't worry too much, because even if you make the odd bad decision they will still sound good. The crossover is by far the most important ingredient. Talking of crossovers, make sure any electrolytic capacitors are measuring as they should, if they aren't replace with like for like (Don't replace electros with polyprop thinking it's a guaranteed upgrade).
 
Rockmeister said:
My ceiling probably IS around 6m...It's an open stairwell room with double height up to the sloping roof, so!?
I think I'm just going to run away now, build the damned things and say, 'they are perfect' regardless...all this mode stuff and 18.724 cm ports needing tuning to a possible 17.41 after downloading room resonance tones for measuring.
HELP!
:)
Click to expand...
Using the REW Simulator to model the effect position has on room mode excitation, and again assuming that port location affects room modes in the same way as driver location, then moving the port a few inches closer to the driver makes naff all difference to the size of the peak at 28Hz (we're talking tenths of a dB which is basically inaudible). It does appear to affect the 28Hz multiples more significantly (56Hz, 84Hz, 112Hz, 140Hz, 168Hz, etc). However, a lot of this will also depend on the other dimensions of your room and how these multiples coincide with the 28Hz multiples, and also the position of your speakers and listening seat relative to the room's other boundaries. Too complex to explain in writing, you'd be best to download REW and play about with the simulator yourself to see what I'm talking about. We are getting into OCD territory here though so I'd probably be inclined to stick with your original plan otherwise you'll never get started! :D
 
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Ill Post you a picture of the room later and you can see why the resonances are all over the place. A more irregular shape you could not imagine! Getting on with it is I think, the plan :)
 
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