"The Flatback Banned" OmniMet

Dedicated to those large boxes at one end of the room
Cressy Snr
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#106 Re: "The Flatback Banned" OmniMet

Post by Cressy Snr »

The directivity has been slightly altered, bringing the direct to reflected sound ratio, slightly more in favour of the direct, and makes better use of the ceiling reflection, lending a sense of air and height to the soundfield wihin the room.

With the improvements made to the low bass, by tuning the port lower and the increased image focus, weight and solidity, brought about by the tilt mod, I think we have a speaker that can easily hold its own in the company of any comparably sized transducer.

This sloped top baffle will be applied to the larger one, when I build the top boxes.

What I'm looking for with the larger one, is all the positives of the flatty, with that last degree of expension and bass power, provided by the sealed bass cabinet. A tall order? We'll see.
Last edited by Cressy Snr on Wed Jul 26, 2017 5:57 pm, edited 1 time in total.
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#107 Re: "The Flatback Banned" OmniMet

Post by IslandPink »

Sound like good steps to take Steve. I was concerned about a lack of output in the top end of the mids and lower treble, so tilting the driver a bit more towards the listener would allow some more direct energy in that band.
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Cressy Snr
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#108 Re: "The Flatback Banned" OmniMet

Post by Cressy Snr »

IslandPink wrote: Wed Jul 26, 2017 5:56 pm Sound like good steps to take Steve. I was concerned about a lack of output in the top end of the mids and lower treble, so tilting the driver a bit more towards the listener would allow some more direct energy in that band.
Yes, that is the general idea. It is a delicate balancing act, setting up the direct/reflected sound ratio. It is something I'm enjoying; probably in the same way as you are with your horn experiments. I'll be at it for a while yet.
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#109 Re: "The Flatback Banned" OmniMet

Post by Cressy Snr »

Anyway, I have been concerned about the looks of these things since day one to be quite honest.
So today I simply turned them sideways and mirrored them.

Image

The transformation in the looks is most pleasing. Now we're getting somewhere.
I simply fitted my other pair of SB ring tweeters and disconnected the existing.
It's not ideal as the speaker connections are now on the sides too, but the looks are worth the temporary loss of a tweeter until I figure out a decent looking way to blank off the hole.
The speaker terminals are another matter. The answer to that is a cabinet rebuild but that can wait a while.
Last edited by Cressy Snr on Thu Sep 14, 2017 4:45 pm, edited 1 time in total.
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#110 Re: "The Flatback Banned" OmniMet

Post by Cressy Snr »

The bass/mid unit is now as close to the wall as it can possibly get and despite it only being around 1.5" closer, the increased wall coupling is very evident in the resultant sound, which is now very solidly grounded. The lower bass has come up a tad due to the closer proximity of the line exit port to the back wall.
It is all tight, focussed and solid. Not a night and day difference, but I can't imagine going back to the other arrangement.
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#111 Re: "The Flatback Banned" OmniMet

Post by Cressy Snr »

Long post.

OK,

So off I toddled to David''s speaker bakeoff with the semi-omnis and as soon as I saw the room, superb that it is, I thought I'd better get my coat. My farty little semi-omnis were dwarfed by everything else in the room. And I left them in the little vestibule, just outside the room so I could sneak them back into the car before anyone noticed.
However after the Doc and Savvy had demmed a pair of cubes I felt a bit better about filling the room with sound.

The OmniMets performed better than I expected, given the size of the room and produced a sound devoid of boom and with plenty of rhythm, tunes and drive. The bass end worked very well and the near wall placement incorporated into the design of the things was spot on. Those were the positives.

HOWEVER all was far from well at the upper mid and lower treble boundary. Out of the bass and lower mids area they sounded as flat as a fart, swathes of detail and harmonics missing in action, boring as hell and at least 70% of the people in the room began to ignore them and start having conversations.

That had not happened with any of the other speakers in the room and frankly I was fucking pissed off, quite angry in fact and it was written all over my face how I was feeling. Poker faced I am not unfortunately, which is not good in these sorts of "public" circumstances because it inevitably comes over as "sore loser" syndrome.

After leaving the room, taking a walk to Tescos and calming down I pondered what had happened and put on my objective head. Most of the audience had lost interest for a good reason, they didn't do it on purpose just to piss me off, and a couple of the guys there struck up a conversation about harmonic details of guitar strings, cymbals being shut off because there was a hole in the treble response, air and space just not happening.

PRaT was excellent, bass even and punchy, not a lot else but they played alright. Jesus! I began to realise that I had actually gone and designed a bloody Flat Earth speaker and after all I had said about never going there again, there I was once more, and to make matters worse, I had done it without any help from Linn/Naim!

This is not something I had noticed at home and neither had anyone else who had heard them in my own room, in fact they had been used in several rooms and the problem had never reared its head, but it bloody well had now! Indeed they had measured relatively flat through the third octave spectrum analyser, the measurements appearing to confirm their goodness.

BUT this had by now happened twice in large rooms; once at Owston and now in MCRU's dem room, so something was going pear shaped when they were taken out of their domestic context, which I'm afraid is just not good enough in my book. Just goes to show that measurements need to be treated with a healthy degree of skepticism and caution.

OK so between us, myself Colin and Chris, after a long conversation out of the dem room had agreed that it had to be the crossover to the tweeter that was causing the problem with the missing bits of the music and I resolved to do something about it as soon as I got home.

Looking at my calculations for the crossover capacitor value, it all looked right and double checking it all, it was still correct, so no clue as to what was going on. I examined the 45 degree, off-axis response curve of the bass mid driver, with its white tack induced roll off and the curve that the first-order tweeter crossover capacitor gave with my calculation, and in theory, the curves married up to give a flat response. In other words, everything lined up perfectly. There was much head scratching and beard ruffling (well there would have been, if I'd had a beard) I mean, what the hell was going on?

This was getting a bit silly now. I've been building speakers for nigh on forty years. OK, yes, granted; this is a semi-omni, but come on, I do know what I'm doing. I'm missing something obvious, but I'm damned if I can work out what it is, these things should work and they do.....yet they don't if you get my drift.

As a last resort I went right back to the SB Acoustics catalogue and got out the spec for the tweeter:
SB.Acoustics SB29RDNC-C000-4 Tweeter

Efficiency, 94dB
Impedance - 8 ohms......err....no..4 Ohms.. :shock: ...OH FOR FUCK'S SAKE....YOU BLEEDIN' NUMPTY!

The crossover cap was half the value it should have been, meaning the crossover frequency was twice as high as it should have been, hence the hole in the treble. Oh poo!

So the cap value was doubled with another one the same value in parallel with it and it all fell into place soundwise, musicwise and everything elsewise. The other three omnis I have built all had the same error in the crossover, due to me somehow misreading the HF driver data. Yet they had all sounded very good. In small to medium rooms, the error had been there, but not glaring enough soundwise, to have rung any alarm bells.

Here endeth the first lesson. Check, check and check again. Make sure you have the bloody right data before you design your crossover! :error:
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#112 Re: "The Flatback Banned" OmniMet

Post by simon »

Glad you posted that Steve, I was wondering what the improvement was. I can certainly see why a few kHz hole would do the damage to the sound you say.

What I can't quite get my head round was that's not what I heard when I trialed your originals at home - if anything it was the other way and I felt the drivers were doubled up rather than creating a hole. Funny old game this audio lark.
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#113 Re: "The Flatback Banned" OmniMet

Post by Cressy Snr »

Yes, it is a strange one, but I think I have an explanation.
I remember playing around with the crossover cap on my old 5ft Mets trying to get rid of shout in female vocals. This was before I discovered the benefits of black tak rings at the dustcap/cone boundary.
I found that the higher I pushed the tweeter crossover freq. the more exposed the tweeter became; therefore, the narrower the band, the tweeter was operating within, the easier it was to hear its contribution; not the seamless distribution that was actually required.

Once the black tak ring had tamed the shout, the tweeter could be given a wider ranging job to do and the discontinuity vanished. I know that one of the other things, apart from the even, uncoloured bass and easy wall placement, that attracted Dave to the big Mets, was the seamless integration of the treble into the sound picture.

I think we were hearing the tweeters at your place not because the crossover was too low, but because it (due to the previously discussed error) was too high and they were exposed because of a hole in the response and appearing to be fighting with the main driver; except that they weren't; they were just out on a limb.

Mentally, I had not made this link until you posted above, simply because I had forgotten the process I'd gone through with the big Mets. It's probably age, and I ought to start making notes, so I can go back and refer to previous findings. I can no longer keep stuff like that in my head these days.

In short, setting up the crossover cap in a first order configuration, to the give the correct range of tweeter operation, relative to the main driver INCLUDING the right amount of overlap between the two, is the key to creating a great speaker as opposed to a so-so one. I would also argue that it is bloody difficult to get right, though not impossible, and that particular difficulty, is one reason why high-order, steep slope networks get used more often than not. They are a quick and dirty way of integrating drivers.
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#114 Re: "The Flatback Banned" OmniMet

Post by simon »

Could be Steve, really difficult to say I agree. That's the benefit off measurement, though you did that too. :?:
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#115 Re: "The Flatback Banned" OmniMet

Post by Cressy Snr »

Yes I did measure. Maybe I'll try measuring the revised crossover version.
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#116 Re: "The Flatback Banned" OmniMet

Post by Cressy Snr »

image.png
Niiiice :D
Click for full size.
Almost flat from 800Hz, all the way out to 10 KHz, only a slight drop to 16KHz.
Well pleased with that.
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#117 Re: "The Flatback Banned" OmniMet

Post by Cressy Snr »

Crossover point is 3.5KHz with 11.2uF of Jantzen Standard blue cap to tweeter (2x5.6uF ||)
Bass/mid is running wide open.

My view is that you have to be pretty ruthless in your analysis of the sound and need to be prepared to say so up front, without trying to make excuses, when something isn't working as you think it should be working. Granted, speakers are very much a personal thing, but nevertheless, there is a minimum standard of performance that should be taken as read, and in my opinion, on that day, in that environment, the damned things were not meeting that minimum and, as I said in the previous post I made, it simply wasn't good enough.

These semi-omnis are a mash up of ideas from Roy Allison, Richard and Martin J King. It is a bit "outside the box" to be using such disparate ideas as semi-omni operation and quarter wave bass loading, so there is not much available in terms of help if problems are encountered; you are a bit out on a limb with stuff like this, but I enjoy being outside the mainstream, though it can cause a hell of a lot of grief and you need to be highly self critical of every step, whilst at the same time, believing absolutely in what you are doing; a difficult, often potentially explosive mixture and oftentimes I wish I could just conform and be done with it. It would make life so much easier.

It was great however on the day to hear the speakers in that dem room. Listening past the obvious top end problem, it was good to hear the against-the-wall, quarter-wave loaded, down-firing bass policy, demonstrating a quick, clean, even, performance at the bottom end, and the uncoloured vocal performance was gratifying. Now that the correct tweeter cap value is installed, the performance further up the range matches the rest of the frequency spectrum. I certainly have no qualms anymore about their performance; not now anyway.
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#118 Re: "The Flatback Banned" OmniMet

Post by Cressy Snr »

Why did I develop this thing?
Maybe the HMV Stereomaster, "Projected Sound" Radiogram my parents had from 1970 to around 1985 was a subconscious influence and part of the answer.
image.jpeg
image.jpeg (51.42 KiB) Viewed 8622 times
Side firing bass/mid, forward firing tweeters in sealed boxes at either end of the cabinet.
It was placed across a room corner with a standard lamp behind it in the triangular gap. After that, my sister's budgie in its cage occupied the space behind it.
The radiogram produced a wash of sound that filled the room, invariably accompanied by the budgie singing along to everything played on it.

My dad always bought HMV: telly, transistor radio and radiogram.
My mate Swanny's dad had an HMV music centre with "4D" sound in about 1973. It had four speakers and the turntable was a BSR Mc Donald MP60.
Memories. :flower:
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#119 Re: "The Flatback Banned" OmniMet

Post by Cressy Snr »

A bit more progress with the omnis today:

Image

Though the crossover cap is now sitting within the correct range of values for the tweeter impedance and the upper mids and lower treble tone is greatly improved, there was a discontinuity in the polar response on the vertical axis.
This was easily heard by slowly standing up from the sitting position in front of the speakers.
As I moved further off the tweeter's vertical axis, onto the bass/mid horizontal axis, there was a clearly audible step down in the treble response happening at around 4ft 6in from the floor.
Doing the same thing again but starting from around 8ft away, in a standing position and walking towards the speaker, it was again possible to hear the step down occur, once I got to within 5ft of the cabinets.
It was a simple matter then to cross-reference the step down, to a point 4ft 6in off the floor at a distance of 5ft from the cabinets.
Conclusion then had to be that the tweeter was rolling off more rapidly moving off the vertical axis than the top driver was rolling in on its horizontal axis to compensate. IOW the polar response of the driver array was unacceptably bumpy; just what you absolutely DON'T want with an omni.

What you DO want, as far as is possible, is an even polar response in all directions. I've found out empirically for myself, that there's nothing better than a pair of ears and a tape measure for doing polar response investigations.

The solution to the step in the polar response was to reset the range of the up-firing driver to its factory spec, by removing the white tack, which allowed the driver to operate with a wider frequency range and fire its HF towards the ceiling, the off axis natural roll down, preventing it from interfering with the front firing treble unit output.

The handover from one to the other on the vertical axis is now seamless and the music now has air, height, width, and depth. Reverb tails are much longer and there is a far greater impression of the size of the studio or the live venue. Cymbal sizzle and decay is far better and guitar and piano have the requisite upper harmonics present.

It's good.
What I need now is another pair of nearby ears to help evaluate the changes.
I like te results, but then I liked them before until Huddersfield.
Trusting your ears in one thing, whether or not your ears are made of cloth is another matter entirely.

I was talking to Colin at Huddersfield and it went something like this:

"You know what Colin? I have come to the conclusion lately, after nearly forty years of involvement in hi-fi, that my idea of what is a good sound is completely different to what everyone else's is"

So at the eleventh hour I have decided that I'd be best off accepting the fact that maybe its not them that are wrong but (horrors) it is me actually. Believe me that's quite a step for somebody like the person behind this keyboard.
I still do left field stuff in the speaker dep't (can't help it) but with a better sense of the value of other people's views.
The Metronome speaker concept did have lots of other people's input, and worked out superbly, so I'm unsure why it is that I didn't keep going with that attitude.
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#120 Re: "The Flatback Banned" OmniMet

Post by IslandPink »

I think you're doing exactly what you should - learning what you want and how to engineer it - at the end of the day it's you that uses that system. Also with a lot of the music you have from the 60's it'd be too harsh on many flat-response or up-tilted speakers that others like.
One thing from your comments above though - how could an Omni ( I mean if it has an up-firing mid-woofer ) have a stable polar response vertically ? If the woofer's treble is being lost by pointing it upwards, doesn't that mean you hear more of that treble if you stand up, and less if you sit down ? Seems like it would be an inevitable result of the design.
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