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Open Baffle Mark Audio project

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KJFAudio View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote KJFAudio Quote  Post ReplyReply Direct Link To This Post Posted: 02 Jan 2017 at 9:51am
Starting to come together this, I'm really looking forward to seeing it evolve. You might want to consider curving the array as it's so tall or even giving the drivers some way of tilting, although I guess you'll be able to beam steer a bit with delay.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote morris_minor Quote  Post ReplyReply Direct Link To This Post Posted: 02 Jan 2017 at 11:39am
Originally posted by KJFAudio KJFAudio wrote:

You might want to consider curving the array as it's so tall or even giving the drivers some way of tilting, . . .

You mean like this: LOLLOLShocked


Bob

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Drewan77 Quote  Post ReplyReply Direct Link To This Post Posted: 02 Jan 2017 at 12:33pm
I thought about that Stefan but I'm able to manually adjust time delay per driver set by down to 0.001ms (the processor algorithms will do timing/phase automatically once I get a semi-anechoic measurement so this should prove unnecessary). 

'Anechoic' readings are best taken outdoor in the winter when the birds are quieter but I suspect it will be several months before I am ready, which is a pity. I once did this in the summer and every tree around lit up in unison like a twitchers discotheque! To average-out random sounds like that this means at least 24x thirty second bursts of loud rising frequency which makes the valley behind our garden sound like an alien attack is incoming! In the winter, it's easier with fewer, shorter bursts.

Basically, the further the speakers are from any other surfaces, including soft ground, the better the reading & the greater the frequency range that can be corrected. I have been able to achieve a clean 36ms reading before any reflection (a tiny wiggle from the mic stand only) which means accurate data right down to 28hz and even the subs sound significantly tighter once calibrated.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Drewan77 Quote  Post ReplyReply Direct Link To This Post Posted: 04 Jan 2017 at 10:26am
Prompted by Grahams' other thread, I have been doing some basic baffle step simulations for these speakers. Fortunately, using DSP I can mitigate many of the issues that he is wrestling with but it's nevertheless an interesting exercise.

Starting with the largest drivers, OB rear out-of-phase cancellation can be clearly seen in the first graph below: -6dB at 40hz (I have also shown the same parameters for a box speaker version for comparison. This only calculates the step effect and doesn't take into account any other aspects of the drivers themselves).

Beyma 15":

OB:

Box:


Edge step cancellation can be reduced on OBs to varying degrees by applying relatively short open side 'wings' and this is what I plan to do. I don't have the means (or the brain!) to simulate this but on my existing OBs these have a very beneficial impact so I will prototype a few variations, the goal being to take these particular drivers down to around 40-50hz and align my existing subs below that.

I still consider myself a relative novice with this subject but have enough understanding, experience and most of all the processing hardware to enable this to work well. In general terms it's better to a apply larger processor dB reduction rather than >3dB additions & using separate amplification, then equalise output of the various driver sets.

Simulations for the Alpair 12P & 7P baffles:

12P:




7P:




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Post Options Post Options   Thanks (0) Thanks(0)   Quote Drewan77 Quote  Post ReplyReply Direct Link To This Post Posted: 02 Feb 2017 at 4:57pm
1. I now have everything necessary to build these speakers:

- Drivers (partly burnt in)
- Baffles (manufactured by Stefan at KJF Audio, in the photo the panels aren't yet aligned by grain)
- Proprius Amps (partly burnt in running the tweeters on existing OBs)
- Materials for the rear frames, including timber, aluminium bracing, bolts/nuts, mounting feet etc.
- Spatia cabling & speaker binding posts

Unfortunately, domestic chores have intervened & this weekend will be spent wallpapering so 'normal service' will resume with these next week.



2. Seeing Grahams' ongoing tribulations with his room/cabinets, I decided to take some measurements of a single Alpair 7P & then correct the frequency response using an HDP-5 processor (to see just how far this little driver can be 'pushed' in its naked state, although I will only be using it for high frequencies).

Taken from a near-field in room measurement with the driver surrounded by a mattress, outdoor seat covers, cushions, middle room & ear listening height. A calibrated Earthworks M23 mic was used.



The three frequency plots show:

- The raw driver measurement.
- Flat correction created by the processor.
- Final in-room measurement with a correction filter applied. Note how room effects overlay the correction filter but nevertheless, frequency changes from the native driver are significant. The processor algorithms also automatically correct timing & phase errors across all frequencies so everything is as it should be.

Although this was a simple setup without any form of baffle, comparing a piece of music playback via the raw driver & then the corrected driver, the difference was significant (as I would expect). Midrange/mid bass was now apparent, together with a smoother overall sound. It was actually very pleasant and listenable. Smile That last plot shows how the driver was now basically 'flat' down to just above 100hz which would of course create less processing strain once assisted by a baffle.


Showing group delay plots, firstly from the correction filter & then the final measurement, there is a slight room impact on both at ~800hz & ~6khz. Group delay on this driver prior to correction was several magnitudes higher. In all other respects: impulse response, step response and phase, digital processing has also 'corrected' the Alpair 7 to some degree.


I have shown all this as an example of just how far digital speaker correction has come and what is possible. When I build these speakers, I will run them actively in-room for a few weeks with just basic crossover & manual time alignment until I'm satisfied that the drivers and Proprius are fully run-in. After that I will disassemble, re-build and measure outdoor on a dry day. Then I will run some more sophisticated calibrations back in the listening room, firstly to achieve a flat frequency response and then slightly tweak to my listening preference.

To be continued...





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Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 05 Feb 2017 at 6:33pm
Thanks Chris. Helpful to see DEQX earning its way and I like the wood panels.
Jon

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 05 Feb 2017 at 6:36pm

If anyone's interested REW can do a similar correction if you ask it to identify the filters needed to correct an uneven response, probably more useful for bass management rather than integrating open baffle drivers as DEQX clearly does well.

Jon

Open mind and ears whilst owning GSP Genera, Accession M, Accession MC, Elevator EXP, Solo ULDE, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links, and a Majestic DAC.
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