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The Goals for an "Ideal Loudspeaker"


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#161 Zilch

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Posted 08 May 2010 - 10:45 PM

I'm typically doing calculations of best height behind the screen and right CD horn to use, to balance out the level across a cinema audience. That isn't an issue for domestic systems.

Wouldn't they be similar, directivity-wise, for a domestic home theater, particularly a multi-row one?

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#162 tysontom

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Posted 09 May 2010 - 01:45 AM

I think the goals of accuracy would be similar for speakers of any intended use, but obviously sound reinforcement or studio use places extra demands and some compromises may be called for. Needs for higher SPL will always force some compromise. I put a lot of effort into getting KEF KM1's to play loud enough for studio use without degrading fidelity.

David


Hi David,

What role did you play in the design of the KEF KM1? Was Laurie Fincham still there when it was designed?

For those not familiar with that design, it was (and is) a fine, no-compromise studio loudspeaker. It seemed capable of very high acoustic outputs (something like 120 dB SPL in the 60-20 kHz range at one meter). I don't have the impression that it was a commercial success as a studio monitor, but I always felt that it was "as good as it got" for such a speaker! Extremely heavy and extremely expensive!

--Tom Tyson

#163 speaker dave

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Posted 09 May 2010 - 11:47 AM

Wouldn't they be similar, directivity-wise, for a domestic home theater, particularly a multi-row one?

The issue with cinemas is the large distances and greater depth involved. Typically the back row is 2 to 3 times the distance of the front row so you might have 6 to 9 dB drop of the direct sound. A well behaved horn at the right height can have the back row on axis, the front row sufficiently off axis to largely compensate for the level difference. I haven't seen a home theater with that depth. Usually they are set up for the "money seat"(he who pays the bills).

There have been some horns optimized for oblique coverage of an audience by JBL, Altec and EV.

#164 speaker dave

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Posted 09 May 2010 - 12:00 PM

Hi David,

What role did you play in the design of the KEF KM1? Was Laurie Fincham still there when it was designed?

--Tom Tyson


I was hired by Laurie from JBL when the early prototypes of the KM1 were running. I was to turn it into a viable pro monitor. Ric Cicconi and I did the design work and then I was in charge of building and selling it to studios.

It wasn't a commerical success but was a good new-idea test bed. The biggest challenge was hitting the SPLs required without using horns and compression drivers. Besides the usual thermal issues we would have bextrene cones crack and fail from acceleration.

Initially we had power protection that injected a little DC through each voice coil and shut off the amps when the DCR rise indicated excessive temperature. That was replaced by an analog simulator that used two time constants for each driver. A fast time constant represented the voice coil temperature rise. A slow time constant represented the heating of the frame and magnet structure.

Over the top engineering in the usual KEF way.

David

#165 tysontom

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Posted 09 May 2010 - 01:58 PM

I was hired by Laurie from JBL when the early prototypes of the KM1 were running. I was to turn it into a viable pro monitor. Ric Cicconi and I did the design work and then I was in charge of building and selling it to studios.

It wasn't a commerical success but was a good new-idea test bed. The biggest challenge was hitting the SPLs required without using horns and compression drivers. Besides the usual thermal issues we would have bextrene cones crack and fail from acceleration.

Initially we had power protection that injected a little DC through each voice coil and shut off the amps when the DCR rise indicated excessive temperature. That was replaced by an analog simulator that used two time constants for each driver. A fast time constant represented the voice coil temperature rise. A slow time constant represented the heating of the frame and magnet structure.

Over the top engineering in the usual KEF way.

David


What became of the KM1? How many were produced, and how many years was it in production? I supect that replacement parts would be very difficult to source today. This speaker had a sort of AR-LST appearance, and the design goal was about the same: greater SPL while maintaining linearity and accuracy. Obviously, it was a very sophisticated design.

The KEF KM1 reminds me of the B&W 800, the large speaker designed for studios, but never particularly successful. I think that B&W took the design criteria and put it into the hugely successful 801 and the family of speakers that followed -- arguably the most successful of the "high-accuracy" monitors of that era.

--Tom Tyson

#166 speaker dave

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Posted 09 May 2010 - 03:25 PM

What became of the KM1? How many were produced, and how many years was it in production? I supect that replacement parts would be very difficult to source today. This speaker had a sort of AR-LST appearance, and the design goal was about the same: greater SPL while maintaining linearity and accuracy. Obviously, it was a very sophisticated design.

We didn't make that many, either 50 units or 50 pair, I can't remember exactly. Some went to studios around the UK, such as the BBC Maida Vale facility. Others were bought by the rich including in Saudi Arabia and Hong Kong. I know a year or two after I left they tossed the central frames and a few cabinets that were still unbuilt. The woofer was stock B300, so it would have been available. The tweeter was stock T52 except for a better adhered rear damping ring and special braids. The mid was full custom, initially bextrene SP1052 derived, then became a custom polypro cone. Electronics were Quad 405 mkII output amp cards and proprietary input and EQ. Peter Baxandall did consulting on a special transformer balanced input stage (He was a brilliant engineer. I still have the long and lucid engineering reports he wrote.)

If you want the story behind them, the BBC was on strike one summer. The non-union people were still at work with nothing to do so they invited Laurie to come in and give a talk on speakers and sound reproduction. During that talk somebody challenged him to make a "really good sounding" studio monitor. The goal was to take the KEF 105 and "add 12dB". Four woofers would achieve that at LF but we needed two mids and one tweet to make a sensible array, in dispersion terms. We used 8 Quad amp cards running with 4 ohm loads: 150 watts each or a total of 1200 watts. There were only 7 drivers so the tweeter got 2! It was our first use of ferrofuid and we worried about the fluid boiling (turns to sludge) hence the special thermal protection. The cabinet looks a bit like 4 105 cabinets turned sideways and stacked. It was also good for dispersion and response. Ric told me he took the central midrange cabinet and put some hinged wings on it. The response was best when the wings were folded back to the angle in your photo.

The KEF KM1 reminds me of the B&W 800, the large speaker designed for studios, but never particularly successful. I think that B&W took the design criteria and put it into the hugely successful 801 and the family of speakers that followed -- arguably the most successful of the "high-accuracy" monitors of that era.

--Tom Tyson

I remember an 808 also, or are we thinking of the same model?. I believe the 801 came first and then they wanted to make something with a more useful form factor for studio use. I remember seeing them at Abbey Road (name dropper!)

Regards,
David

#167 Zilch

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Posted 09 May 2010 - 05:43 PM

I think the goals of accuracy would be similar for speakers of any intended use, but obviously sound reinforcement or studio use places extra demands and some compromises may be called for. Needs for higher SPL will always force some compromise.

The home theater DIYers demand that the mains meet THX spec, which they calc as implying 115dB max continuous SPL for mains. Sounds like studios want similar capabilities, which would also be compatible with many SR applications.

One formula for getting there is high-efficiency (read "Pro audio") woofers, compression driver horns/waveguides, and multiple subs. That little 8" AC18 specs at 116dB, and the dual-woofer variant AC28 at 120dB:

http://www.jblpro.co...f...&docid=1083

If KM1 had similar SPL capability, then as a goal, multi-use may not be unrealistic.... :)

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#168 tysontom

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Posted 09 May 2010 - 07:02 PM

We didn't make that many, either 50 units or 50 pair, I can't remember exactly. Some went to studios around the UK, such as the BBC Maida Vale facility. Others were bought by the rich including in Saudi Arabia and Hong Kong. I know a year or two after I left they tossed the central frames and a few cabinets that were still unbuilt. The woofer was stock B300, so it would have been available. The tweeter was stock T52 except for a better adhered rear damping ring and special braids. The mid was full custom, initially bextrene SP1052 derived, then became a custom polypro cone. Electronics were Quad 405 mkII output amp cards and proprietary input and EQ. Peter Baxandall did consulting on a special transformer balanced input stage (He was a brilliant engineer. I still have the long and lucid engineering reports he wrote.)

If you want the story behind them, the BBC was on strike one summer. The non-union people were still at work with nothing to do so they invited Laurie to come in and give a talk on speakers and sound reproduction. During that talk somebody challenged him to make a "really good sounding" studio monitor. The goal was to take the KEF 105 and "add 12dB". Four woofers would achieve that at LF but we needed two mids and one tweet to make a sensible array, in dispersion terms. We used 8 Quad amp cards running with 4 ohm loads: 150 watts each or a total of 1200 watts. There were only 7 drivers so the tweeter got 2! It was our first use of ferrofuid and we worried about the fluid boiling (turns to sludge) hence the special thermal protection. The cabinet looks a bit like 4 105 cabinets turned sideways and stacked. It was also good for dispersion and response. Ric told me he took the central midrange cabinet and put some hinged wings on it. The response was best when the wings were folded back to the angle in your photo.

I remember an 808 also, or are we thinking of the same model?. I believe the 801 came first and then they wanted to make something with a more useful form factor for studio use. I remember seeing them at Abbey Road (name dropper!)

Regards,
David


David,

Thanks for that update! Yes, you are right, the B&W version (introduced at the 1984 CES by John Bowers, I believe) was indeed the 808. It also was an excellent, high-power monitor speaker. I recall that it was fairly successful in recording studios, but the 801-series was considered sufficient and sold in large numbers, and only a wealthy few people bought the 808 for home use. Nevertheless, both the KM1 and the 808 demonstrated the technological accomplishments from KEF and B&W!

--Tom Tyson

Image 1: B&W 808 in Rosewood
Image 2: B&W 808 Cutaway Drawing

#169 Carlspeak

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Posted 09 May 2010 - 09:48 PM

I presume the 808's were sold as mirror image pairs?
IT'S ALL ABOUT THE MUSIC!

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#170 Pete B

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Posted 09 May 2010 - 10:14 PM

The B&W 800 Matrix was an interesting design also:
http://www.stereophi...akers/691bw800/

#171 tysontom

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Posted 10 May 2010 - 03:02 PM

I presume the 808's were sold as mirror image pairs?


Carl,

Yes, I believe that the B&W 808s were available only as "mirror-image" pairs, but I think most all of the KEF and B&W high-end speakers were made as left-right pairs, and only sold in this fashion. For example, I don't believe you could buy just one B&W 801 unless it was a special-order item, and the onwer's guide contained performance data (such as fr and distortion curves) for the specific pair of speakers. Both KEF and B&W also kept records on each specific speaker sold, and the companies (at one point in history) maintained replacement drivers that would closely match the batch that were installed in each speaker.

By the way, the front of the 808 was massive, but the midrange-tweeter array (MTM) was at least placed close to the left or right side of the front baffle, and the side then was trancated and slanted back away from the front, so the diffraction was probably minimized.

--Tom Tyson

#172 Steve F

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Posted 10 May 2010 - 03:55 PM

Both KEF and B&W also kept records on each specific speaker sold, and the companies (at one point in history) maintained replacement drivers that would closely match the batch that were installed in each speaker.

--Tom Tyson



I remember once asking Andy Kotsatos during a factory tour about BA's QC (which was pretty tight, since they made all their own drivers and had very good process control) and how it compared to Snell's, B&W's, et al., companies that hand-tweaked specific x-overs to specific drivers.

He derisively waved his hand and muttered that such a practice was a tribute to poor driver QC, and was not necessary if the drivers themselves were within a tight response window to begin with (BA's were routinely cookie-cutter identical), especially if the x-o components were +/- 5% or less.

Ironically, a few years later, BA bought Snell, and each company continued their respective practices.

Ya never know.

Steve F.

#173 tysontom

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Posted 10 May 2010 - 04:25 PM

I remember once asking Andy Kotsatos during a factory tour about BA's QC (which was pretty tight, since they made all their own drivers and had very good process control) and how it compared to Snell's, B&W's, et al., companies that hand-tweaked specific x-overs to specific drivers.

He derisively waved his hand and muttered that such a practice was a tribute to poor driver QC, and was not necessary if the drivers themselves were within a tight response window to begin with (BA's were routinely cookie-cutter identical), especially if the x-o components were +/- 5% or less.

Ironically, a few years later, BA bought Snell, and each company continued their respective practices.

Ya never know.

Steve F.


Hi Steve,

Well, that's a damned-good answer from an outspoken man who was fast on his feet! What else was he going to say? I would think, however, that those companies had excellent quality control, and I believe both companies built many of their drivers in-house. That is interesting about Snell, however. Thanks, Steve!

--Tom Tyson

#174 speaker dave

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Posted 10 May 2010 - 05:26 PM

I remember once asking Andy Kotsatos during a factory tour about BA's QC (which was pretty tight, since they made all their own drivers and had very good process control) and how it compared to Snell's, B&W's, et al., companies that hand-tweaked specific x-overs to specific drivers.

He derisively waved his hand and muttered that such a practice was a tribute to poor driver QC, and was not necessary if the drivers themselves were within a tight response window to begin with (BA's were routinely cookie-cutter identical), especially if the x-o components were +/- 5% or less.

Ironically, a few years later, BA bought Snell, and each company continued their respective practices.

Ya never know.

Steve F.

I got pressure from Andy to discontinue the practice but I managed to evade it while I was there. It was part of the Snell DNA, hence a marketing tool as well as a quality objective, I argued.

BA consistency was pretty good but I think there was a little bit of fooling themselves as well. For example, they would argue that they wanted +- 2 (on sensitivity and broad errors) and the measuring system had and inherent +-1dB uncertainty, so they would allow +-3 for a pass. I argued that was backwards: if you want +- 2, and the measurements give you +- 1 as a potential error, you would have to reject outside of +-1 to be sure of achieving your desired tolerance.

I didn't expect to win that arguement but the underlying notion is correct.

The Snell approach was brute force and I think a sorting approach could have done about as well.

At KEF, Reference series was built in batches of 96 and the computer would sort them into best pairs. Woofer pairs and tweeter pairs of the same sensitivity were then linked together. With crossovers, capacitors would be sorted into tolerance bands and inductors would be wound and linked to each cap group in a compensatory way. It was all very clever but a bit too cumbersome to really be practical. I wonder how the Chinese factory is handling it today??

David

#175 Carlspeak

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Posted 11 May 2010 - 04:02 PM

Dave Wilson of Wilson Audio has a few ideas about the 'ideal' loudspeaker. He is a good salesman.
Give a listen to his 6 brief talks here:

http://www.wilsonaud...ions_part1.html
IT'S ALL ABOUT THE MUSIC!

Carl
Carl's Custom Loudspeakers




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