'LGT' Construction Diary - Page 129 - diyAudio
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Multi-Way Conventional loudspeakers with crossovers

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Old 7th September 2007, 05:27 PM   #1281
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Quote:
Originally posted by m0tion
Shin:

You take your measurements indoors right? Do you do anything special with the placement of your speaker and microphone before measuring or do you just rely on the gating to clean everything up? About how long is your gate?
Not normally. Instead for the initial driver linearisation I go outside, usually the only time I do in room measurements is to correct for the room. The LGT's are far far too heavy, even when broke up, for one person to safely carry for any distance and especially when negotiating stairs. So for now I'm doing inroom measurements which is full of problems.

Shuffling the speaker iteself well into the room and away from walls, I tend to measure the drivers with the mic on the axis of each and with a distance of 30-40cm to avoid cancellations yet still minimise the room. I try to avoid any gating or minimal gating if possible and this usually is OK down to around 500hz.

Once each driver has been linearised I set the mic back 1.2m on axis with the tweeter. I play a logsweep with all the corrected drivers working and then begin to level match and check overall performance. It doesn't have to be perfect at this point. The above two steps are best done outside if possible or in a decent sized room with the loudspeaker at or close to the center.

From there the mic is moved to listening position and I check the time alignment of the drivers using logsweeps for each driver in turn but with a specific and deliberate delay between each. These sweeps are overlayed and you can see the delays needed by identifying the impulse peaks for each driver and looking at the time differential between each and comparing this to you deliberate delay you engineered into the logsweeps. That's the driver time aligned at the listening position. Then do another single logsweep with all drivers working and check the results. Step response should be markedly improved and the impulse should be cleaner too.

Still with the mic at the listening position you perform another log sweep with the whole speaker working and with the finalised XO. This forms the basis for the room correction.
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Old 11th September 2007, 11:02 PM   #1282
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Thought this might interest folks.


Here's the filter transfer functions for the LGT, crossover types are Transient perfect Neville/Thiele 1st order. Also in the overlay is an actual un-smoothed mid driver measurement after correction. The black line shows the perfect summing of the filters.

The design is 3.5way:

W = 8" upto 200hz
M = 5" 200hz - 800hz
T = Ribbon 2500hz+
M = 5" 200hz - 2500hz
W = 8" upto 200hz

Click the image to open in full size.

Plots showing the impressive improvement after driver correction:

Frequency Response
Red = Filter Response
Green = Corrected Driver response(unsmoothed)
Brown = Uncorrected Driver Response(unsmoothed)

Click the image to open in full size.

Step Response

Red = Filter Step Response
Green = Corrected Driver Step Response(unsmoothed)
Brown = Uncorrected Driver Step Response(unsmoothed)

Click the image to open in full size.

Phase

Green = Filter Phase
Blue = Corrected Driver Phase(unsmoothed)
Black = Uncorrected Driver Phase(unsmoothed)

Click the image to open in full size.
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Old 11th September 2007, 11:15 PM   #1283
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To provide a bit more background to the why's for the above, here's a post that I made on another forum whilst discussing EQ:

The filter transfer functions are as follows and these, when summed together, create a flat response(the black line). The filters in this example are Linkwitz Riley 4th order:

Click the image to open in full size.

This next one is showing time domain behaviour of the above. You can clearly see the individual filters are ringing, which is typical of any crossover and increasingly severe with steeper filtering, but when summed together the pulse response in black can be seen to show perfect behaviour due to the invidividual filters ringing cancelling each other out when summed as one.

Click the image to open in full size.

Note: These are not driver responses but the visualisations of the mathematical filters and anything can be made to look perfect with maths! So don't pay much heed here. Just know that if the drivers were perfect then so would the speaker. The good news is we know the crossovers themselves are perfect in the time and amplitude domain, at least until a driver is stuck on the end of them and everything is ruined

So what happens when you put your driver in?

Well take a look at this horrible mess showing a typical midrange and its intended passband:

Click the image to open in full size.

There's three elements here:

Red is the filter tranfer function like the ones we demonstrated above, the ideal and what were aiming for in order for the crossover to work accurately and as close to correct as possible.

Brown is the actual driver response with the filter in place and is gain shifted for a better view and not to clash with other the other elements in the graph. Ideally this should perfectly follow the red line but the truth is it couldn't be further from it!

Black(bold) is the minimum phase correction filter needed to bring the brown(driver response) in line with the red(filter transfer function)

Why the need to correct?

Well if you saw that the crossover filters alone are perfect so we must try to make the drivers follow suit and match them to the filter. Its a balancing act here because over-correction ie. correcting every single defect however small or large will lead to a very strange sound that really only works in one very very small sweetspot. An excellent solution is to smooth the correction filter so as to provide a gentler effect that works on the more severe errors but largely ignores the smaller ones that if corrected wholesale could do more harm than good.

Below is a comparison of without correction in green and with correction in blue. Note that the measurements are unsmoothed so what you see is the unblemished truth at the measurement position. You can clearly see the big improvement in overall shape and you can also see that excess correction hasn't taken place because we aren't seeing over correction of the response which brings its own problems. In the end what we have is a driver response that closely follows the filter and should you apply 1/3 octave smoothing to remove the aliasing present in the measurement you'd see that it almost perfectly follows the filter. Do this for every driver, combine the responses and it brings us one step closer to accurate.

Click the image to open in full size.

Further affirmation in the form of a before(red) and after(blue) step response confirming driver time correction was very effective:

Click the image to open in full size.
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Old 12th September 2007, 06:34 AM   #1284
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WOW! Very informative!! Thanks Shin
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Old 12th September 2007, 04:49 PM   #1285
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Quote:
originally posted by ShinOBIWAN:
To provide a bit more background to the why's for the above, here's a post that I made on another forum whilst discussing EQ:
Hi ShinOBIWAN,

I'm happy to see you have understood the basic idea and I truly believe you like it

You also should share your experiences at another user group, you certainly know which I mean. Therefore you should also feel free to advertise looking at this completely interesting thread.
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Old 12th September 2007, 05:12 PM   #1286
schauki is offline schauki  Austria
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Hello!

This is a very very impressive speaker. I can only say, respect an superb work.

Looks really fantastic.

Many grats to you!!

cheers
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Old 14th September 2007, 05:35 PM   #1287
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Quote:
Originally posted by uli.brueggemann


Hi ShinOBIWAN,

I'm happy to see you have understood the basic idea and I truly believe you like it

You also should share your experiences at another user group, you certainly know which I mean. Therefore you should also feel free to advertise looking at this completely interesting thread.
Ah the man himself!

Uli your a genius. The superb software you have created is... well... superb! IMO it has furthered the art immeasurably. It's now possible to correct a loudspeaker to the point where it becomes not the weakest link in the audio chain and instead is overtaken by the all important room and to a lesser extent the recording.

Anyone half interested in creating an excellent sounding speaker, and in particular an accurate reference monitor, would do well to seek out Uli's Acourate program.

I am preparing a PDF document that covers the latest advances in PC crossover technologies including sections dedicated to Acourate and Thunau's Frequency Allocator program. Both of these are the leaders in their respective categories IMO. Hopefully it will help other users and people who are intrigued by this approach and would like to try it out. There's a lot of information out there but it seems spread all over the place with a lot of searching required to find the right answers to any question one might have. This is really targeted at those people who tread this path for the first time where failure is highly likely because of the perceived complexity, sadly that also means they miss out on a potential first class audio experience as well.

Once finished I'll put it up on the forums.
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Old 14th September 2007, 05:49 PM   #1288
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Quote:
Originally posted by schauki
Many grats to you!!
Thank you Schauki
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Old 14th September 2007, 08:57 PM   #1289
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Would anyone local be willing to come and give me a hand getting these sounding right? Now that the initially wow factor has worn off I can hear problems. In terms of getting the sound right, this design is the toughest I've done so far. Initially I thought "yeah, the problems will be worked out with time and experimentation" but its been a good while now and I'm starting to think it might never come together or at least not with my skill set alone. It has crossed my mind that the design itself is compromised rather than pointing the blame at the drivers. So what I really need is another pair of ears and preferably someone with experience who has a good ear. I find myself listening so much that I don't have the greatest perspective any more.

Maybe I should go out and listen to some high end kit so as to get some of that perspective back.

Main problems are a thin upper midrange and a treble that can exacerbate this problem. There are other problems but this is the one that immediately sticks out and is particularly noticeable with female vocals which sound thin and horrible at times.

I can post the full range of measurements if that will help folks but they could be considered very good already so I don't think making the speaker more technically correct is going to help much but please let me know if you think me wrong here, at this point I'm willing to try anything.

Any questions that you have that will lead to results, I will do my best to accomodate.

Thanks
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Old 14th September 2007, 09:09 PM   #1290
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Ant, i'm just up the road in sheffield and wiling to lend a hand, i do a lot of compartive listening to other peoples rigs and have recently been shopping for a new set of speakers to replace my Kef ref 203's, so have been listening to some 'daddies' in the 3-5k range.

i could even whack the boys in the boot and bring them over to compare, they aren't the most dynamic speaker but they resolve with a beautiful balance and tone and dig out all the detail you could want.

Also i work from home so can bin off a day pretty much whenever i want.
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