There was no instruction from the BBC to change impedance, it just happened along the way. What they wanted was an exact match at all frequencies to selected earlier units. Their thinking was that if any unit blew in service they should be ale to grab a second unit of any serial number and slot it in.
To my knowledge all the early units should have the autoformer for setting tweeter level. I was instructed to ditch it (expense and uncertain supply) and we came up with a nice multistep L Pad approach that gave steps fine enough to compensate for tweeter sensitivity variations.
As to crossovers outside the box, I'm not sure it will make much difference, as crossover volume is minimal. Still, the cabinet is a little undersized so it won't hurt.
David
Would it be fair then to say then that there is no particular advantage in the 15 Ohm version over the 11 Ohm version? Similarly does the transformer version offer no benefit? I ask this in the context of the various crossovers offered by Falcon Acoustics where the 15 Ohm version with transformer costs an additional £50!
How undersized? If one wanted to make a near but not exact clone using a T27 & B110, how much larger would you make the cabinet and would you still use relatively thin plywood? i.e. the same construction principles but a bit bigger.
And finally, back on topic, if making a near clone, would you front-mount or rear-mount the B110?
Another aspect of the Ls3/5a design (and an lot of speakers of that era) but which is far less often seen now is the recessed baffle. Was this purely for mounting the standard grill frame flush with the front edge of the box?
The main reason why manufacturers stopped mounting the driver from the correct side, ie. the inside of the baffle, was that it made speakers much cheaper to produce. It is the same reason why manufacturers switched from using plywood to MDF.
Almost any mid/woofer sounds and measures better when mounted from the inside. Primarily due to the shadowed diffraction effect of the suspension but secondly due to compressions/reflections occuring between the baffle and the chassis frame.
It is strange that people universally agree that recessing drivers into the baffle when front mounted improves measurement and sound but totally ignore that most of the diffraction distortion comes from the suspension itself especially when the driver is in heavy use as the diffraction will then vary with cone (and suspension) excursion.
Mounting from the inside and using a calculated round over or chamfer almost completely removes these problems.
Almost any mid/woofer sounds and measures better when mounted from the inside. Primarily due to the shadowed diffraction effect of the suspension but secondly due to compressions/reflections occuring between the baffle and the chassis frame.
It is strange that people universally agree that recessing drivers into the baffle when front mounted improves measurement and sound but totally ignore that most of the diffraction distortion comes from the suspension itself especially when the driver is in heavy use as the diffraction will then vary with cone (and suspension) excursion.
Mounting from the inside and using a calculated round over or chamfer almost completely removes these problems.
Not sure where your "facts" are coming from but I've never seen woofers measure better when inside mounted. The effect can be minor if the panel thickness is small, but for any significant thickness the response degrades and the network will need to deal with it otherwise the sound will suffer.
Not sure what "diffraction distortion" is.
This isn't so hard to understand. Rear mounting deep in a panel is to create a short horn or pipe. What do we see when we mount woofers at the end of a pipe???
David
The reason for the crossover re-design was variability and drift in the driver response. If the drivers were still good then the BBC would have happily soldiered on with the original design. If you pay extra for the original design with the autoformer then you have to face the issue of finding drivers that match the early drivers that it was designed for.
The network redesign is primarily to allow the switch from SP1003 to the more stable SP1057.
You will have to do your own modeling on this.
If I was cloning I would rear mount. If I wanted to start from scratch and design something better, I would front mount.
David
If you haven't seen woofers measure better when mounted from the inside, I assume it's because you have never actually done any such measurements. With a few exceptions almost all drivers I have measured does measure better when mounted from the inside especially when measured with larger scale signals in the 20V+ range instead of the traditional small scale signals at 2.83V. And even the few that didn't measure better sounded better mounted from the inside to me but as I am obviously biased I don't regard that as having much significance.
In fact, this would be common knowledge to anyone who have a bachelor or higher in acoustic engineering taken within the last 20 years or so. You will also find that many of the more serious sound monitor manufacturers either use inside mounting of the woofers directly, or have a mounting ring that simulates the effect.
Mounting speakers from the inside eliminates the pipe effect of the driver being mounted at the end of the pipe section that is the baffle cut-out but I already wrote that above so I just assume you're glossing over my response. You also wrong in that there would not be a horn/pipe effect at all. If anything it would be a wave guide effect, as I already wrote by saying that you have to calculate the round over or chamfer to fit the suspension shadowing effect.
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So you are saying you have a large signal effect that is corrected by a small signal response aberration (i.e. rear mounting). This would be worthy of a technical paper, but what is the solution when you are listening at less than 20 volts??
Sadly my schooling predates that and so I must live with my ignorance. I've attached a page from Olson that shows and describes the problems of rear mounting, but sadly his education predates your 20 year limit as well (I think he has been dead longer than that).
Clearly the physics of loudspeakers has changed recently and I was not informed.
Here we are just not speaking the same language. How does mounting from the inside eliminate the pipe effect? Clearly it is the cause of the pipe effect. It adds the dimension of the pipe.
To call this a waveguide rather than horn or pipe is to fall into semantics and ignore physics. A driver at the end of a tunnel (waveguide, horn or pipe) is likely to see resonant effects at the usual 1/4. 3/4, 5/4 harmonic series. That is, unless that too has changed in the last 20 years.
I'd be interested on your view of Olson's data.
David
Hi Dave,
What would you say is the effect of mounting like this (note the absence of the front cavity as mentioned in your posted paper):
http://www.stereomojo.com/CES 2010 Show Report/images/sonist.jpg
Or, http://www.soundsrealaudio.com/images/sonist/Concerto2MediumCherry.jpg
Of course, there are thos that haven't read the paper as well: http://www.musictech.net/wp-content/uploads/2013/07/CMS-502.jpg
What would you say is the effect of mounting like this (note the absence of the front cavity as mentioned in your posted paper):
http://www.stereomojo.com/CES 2010 Show Report/images/sonist.jpg
Or, http://www.soundsrealaudio.com/images/sonist/Concerto2MediumCherry.jpg
Of course, there are thos that haven't read the paper as well: http://www.musictech.net/wp-content/uploads/2013/07/CMS-502.jpg
First of all I can note that Olson had the speakers mounted from the inside (as they are supposed to be) in both his examples. One is just flush mounted, ie. recessed into the baffle enough from the inside.
Well. Yes. And you do know about this.
What is different to when Olson did his limited studies on the subject and the reason it is commonly seen as erroneous in modern design, so much so that it is in fact the complete opposite, is the use of semi-circular rubber suspension compared to the old fashioned cloth suspension. This suspension by itself creates diffraction but it also creates diffraction distortion at larger signals as we see a variable strength vortex created behind it which creates non-liner distortion especially at larger excursions.
This is demonstrated today in classes with slow motion cameras and smoke. The effect on measurements is very similar to those found by Olson just the complete opposite result. What you do is to calculate the round-over of the baffle cut-out so that you break up this effect so that the baffle acoustically is seen as unbroken and the suspension invisible to the wave front at the desired frequencies. In other words a simple wave guide.
Apparently. Mounting from the inside removes the pipe effect because there then is no pipe. It is only when you mount from the outside that there is a pipe effect because you are then introducing the pipe in the first place. I fail to see how this can be viewed any differently.
Well. Yes. And you do know about this.
What is different to when Olson did his limited studies on the subject and the reason it is commonly seen as erroneous in modern design, so much so that it is in fact the complete opposite, is the use of semi-circular rubber suspension compared to the old fashioned cloth suspension. This suspension by itself creates diffraction but it also creates diffraction distortion at larger signals as we see a variable strength vortex created behind it which creates non-liner distortion especially at larger excursions.
This is demonstrated today in classes with slow motion cameras and smoke. The effect on measurements is very similar to those found by Olson just the complete opposite result. What you do is to calculate the round-over of the baffle cut-out so that you break up this effect so that the baffle acoustically is seen as unbroken and the suspension invisible to the wave front at the desired frequencies. In other words a simple wave guide.
Apparently. Mounting from the inside removes the pipe effect because there then is no pipe. It is only when you mount from the outside that there is a pipe effect because you are then introducing the pipe in the first place. I fail to see how this can be viewed any differently.
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That is a very typical modern monitor design which incorporates the lessons learned from rejecting Olson's limited studies (on this specific matter) as I have mentioned earlier. Almost all current manufacturers of modern professional monitors such as Genelec, M-audio, JBL etc etc have the same or similar approach, and almost exclusively use rear driver mounting (as they indeed originally were intended to be).
Nope.
The Genelecs do it the right way, by shaping the 'pipe' section as a waveguide, slightly horn-loading the woofer. Notice the shape of the cavity, the Focal does not bother to chamfer the edge or round it over (I assumed that is what you were talking about in your posts).
My assumption is that if you were to shape the area in front of the speaker properly, the reflections are in fact minimised and it actually helps with diffraction effects.
http://www.genelec.fi/documents/images/previousmodels/1022b.jpg
Compare the area just next to the woofer surround, completely different to the Focal.
All rear mounting is not the same, IMHO. There may be a good way to do it.
The Genelecs do it the right way, by shaping the 'pipe' section as a waveguide, slightly horn-loading the woofer. Notice the shape of the cavity, the Focal does not bother to chamfer the edge or round it over (I assumed that is what you were talking about in your posts).
My assumption is that if you were to shape the area in front of the speaker properly, the reflections are in fact minimised and it actually helps with diffraction effects.
http://www.genelec.fi/documents/images/previousmodels/1022b.jpg
Compare the area just next to the woofer surround, completely different to the Focal.
All rear mounting is not the same, IMHO. There may be a good way to do it.
If you see from the suspension and out then Focal use a curved front which ends in a lip. Then a small gap. This has exactly the same effect as what Genelec does on their monitors. It's the same principle, it's just 2 different ways to do the same.
It's better to compare the Focal monitor with another similar compact near-field monitors like Genelecs M-series or JBLs LSR series
http://www.hifisentralen.no/forumet/attachments/vennetra-der-hifimerker/215465d1378586832-venner-av-genelec-genelec_eb40b14573.jpg
http://www.jblpro.com/images/default-source/product-slider-images/r-b/3-series/lsr305_front_z.jpg
Quite obviously, rear mounting midwoofers with cloth suspension can and does create the exact problems Olson demonstrates. I don't know of many modern midwoofers that don't have rubber suspension though so rear mounting is quite obviously the most correct way from an engineering point of view.
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I have no problem with those designs. Although rear mounted they have taken steps to deal with the problems by flaring, or in one case having a driver that recesses in from behind so that little effective depth is left. I have done so myself with some designs that made it to market and had rear mounting and a flare. It gave a clean look (no visible mounting screws) and no particular response issues.
This aside, there will always be response problems if the baffle thickness is too great and the recess resembles a short pipe. The job of the crossover is to deal with this and if the designer is competent he will do so. The discussion began with the BBC designs and I stand by the statement that rear mounting gave them response issues that they had to deal with via the crossover design. specifically with the LS3/5a, the midrange mound was considerable and the crossover didn't fully deal with it. Every published curve that I have seen, along with my own measurements, showed an elevated response in the 500 to 1k region. I created at least one prototype with a network modified to deal with the issue.
David
Ah, so it is the suspension material that determines whether the recess creates the diffraction, reflection, and cavity resonance issues that Olson shows.
I am learning much today.
David
No. But I have already written why Olsons result are seen as incorrect with modern drivers, you just not reading it, so why should I care to repeat it..
Didnt look like a curve of any kind to me, also there is a pretty visible gap between the edge of the first lip and the deco ring. Actually the cavity between the two is quite visible.
Anyway the reason I was asking is because I need to add 1" baffle thickness in front of a driver and wanted to flare it, which would have a very similar effect to rear mounting. Obviously the flare is required and was planned for, just needed to be sure that it would indeed ameliorate the effects of adding such.
Guess I'm in the clear 🙂
Well. There is a quite pronounced curve on the Focal monitor. You can clearly see it when look at them in real life.
Yes, there's a gap. It is intentional just like I wrote in what you quoted. It has the same effect as what Genelec and JBL are doing on their monitors. It's just different approaches.
Would this apply equally with a thick baffle where the driver is mounted from the front? Should baffles be made of thin(nish) material and/or be chamfered on the inside?
I ask because I would like to build some TL cabinets but was thinking of using 25 mm MDF rather than 18 mm - in hopes of reducing resonance.
Yes. Exactly. A driver front mounted on a thick baffle creates the pipe effect on the inside. Although the effect is smaller it is easy to eliminate simply by rear mounting a driver that have a semi-circular suspension (as opposed to a the almost flat suspension of a old fashioned cloth surround).
It is often wiser to use calculated bracing instead of using thicker material to reduce resonances. Especially if you can brace the driver itself it will significantly reduce the need for baffle thickness.
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