Something stirs in the Undergrowth

[ed]

fwiw I see l/d characteristics as having been important in both technologies from the very beginning....

http://www.vitalstates.org/diy/software ... m#airfoils

the l/d coefficients are available for most of the foils that the above can print......

[IslandPink]

Article about slot-loaded ( Heil AMT ) OB bass now available from JC. Interesting comments from Thomas Dunker about fabric over the slot to control Qm .... :
http://www.labjc.com/?p=4230

[Max N]

fwiw I see l/d characteristics as having been important in both technologies from the very beginning....

http://www.vitalstates.org/diy/software ... m#airfoils

the l/d coefficients are available for most of the foils that the above can print......

Sorry just seen this, but actually I don't have much to add....you guys have it covered......oh all right then....
As a vehicle dynamicist (as opposed to an aerodynamicist) I would of course say that all the effort and expense that goes on in the wind tunnel and in the CFD super computer clusters is only there to push down on our tyres......don't know why they bother really.....
And of course, as we all know, the last year that F1 was any good was 1967, before the wings appeared......
Which gives me a good excuse to post this link, Jim Clarke's Lotus crashing at the Monaco historic...weep...was a few years ago actually...
https://jalopnik.com/nooooo-jim-clarks- ... 1575223896

Seriously though, Ed is almost right. F1 should have been about lift/drag ratios from the very beginning, but actually up until '68 people just minimised drag. You only have to look at the '67 Lotus 49 (the last one without wings) to see that its just a cigar shape. As soon as the first wings appeared, the genie was out of the bottle. But even then, the really powerful idea, ground effect, was discovered almost accidentally. Patrick Head tells a brilliant story about a floor tray they stuck on the bottom of one of the cars, and they couldn't figure out why it kept coming adrift. They kept beefing up the fixings, and it kept coming off. Then finally they calculated just how much force it would take to pull all those fixings out.....ground effect was born!
Then before you know it Patrick was creating the 6-wheeler with full-length venturi tunnels on each side of the car...L/D about 8:1....current f1 cars are about half that.....they needed something brilliant to try and beat the turbos with a normally-aspirated Cosworth....but it was banned. Then Keke won the title anyway
Here's the 6-wheeler taking the record at Goodwood. The record was finally broken by a '98 McLaren I think, so about 16 years of development to get back to this level of performance. The only reason for using front wheels on the back was to allow the full-length venturi tunnels. But of course the car would be unbalanced with the small tyres on the back, so they needed four of them.
https://www.youtube.com/watch?v=NtERkxx_VbU



Sorry Mark for dragging your thread off topic again....

[chris661]

Article about slot-loaded ( Heil AMT ) OB bass now available from JC. Interesting comments from Thomas Dunker about fabric over the slot to control Qm .... :
http://www.labjc.com/?p=4230

Sorry Mark, but some of the stuff this guy goes on about just isn't true:

"the mass and inertia of the pistons also creates delay… more mass and more excursion is more delay. this is time domain distortion, that makes integrating a multi driver array difficult. we might experience this as hearing the sound coming from several sources (draws attention to the bass as separate) or as “drag” or “slow”. don’t misunderstand me. low frequencies are slow! i am not talking about that. i am talking about integration. having the entire system sound like one complete whole. this is also made worse when the frequency dividing network is added to the system. even if some electronic compensation is possible in the analog domain, the complexity of the intervention can and often does offset the fix. there is no simple solution for this. digital can do some interesting things here, but that is even more complex…

reducing excursion, all by itself, limits time domain issues and lowers harmonic distortion."

Cone excursion has nothing to do with time delays. Larger cone excursion means more current through the voicecoil, but the magnetic system can easily move fast enough to move a cone back and forth very slowly at, say, 50 cycles per second.
If what he's saying is true, you'd see group delay varying with drive level, and that has never ever been observed.

I could go on, but a lot of what's written there is subjective and no comparisons to equivalent-sized dipole systems have been made. Pass's SLOB showed 9dB of gain at the front from slot loading, but only when the measurement mic was at the slot. Moving away, the high-pressure zone dissipates as it expands into the room. A free 9dB gain would violate various physical laws, particularly the conservation of energy.

Chris

[IslandPink]

Typing limited here on laptop, so I'll be concise.
JC is prone to hyperbole at times, so I also don't agree with everything he says.
First bit, his explanation is a bit 'conventional' and I tend to agree with you. However remember that with a mass loaded driver the output ( pressure ) is 90 deg out of phase with the driving signal as Tom Danley has very nicely explained, so this means there is some group delay across its flat ( amplitude ) band. However I don't see the SLOB be worse than anything else in this respect.
OK re, efficiency, I disagree. While even Nelson didn't claim 9dB at the listening position, if what you say is true about violating the laws of physics then horns wouldn't be >100dB efficient. A horn can give 10 to 15dB of gain at the LF end - and you can't invoke narrow directivity at the LF end, so the gain MUST be due to better impedance matching.

[Nick]

I have no dog in this race but

A free 9dB gain would violate various physical laws, particularly the conservation of energy.

If you want to look at it this way then it would seem the fact that a 90dB/W and 100dB/W loudspeaker both exist would break the first law. In practice a 90dB/W speaker is about 1% efficient, so there is a lot of scope for extra gain.

[IslandPink]

What you're arguing against, Chris, is the very principle that makes compression drivers so efficient. It does matter both the volume of air you move and its velocity. Nick - correct, the potential is there as you say , some compression drivers including the JBL cone midrange comp drivers are acoustically 30% efficient.

[chris661]

Nick, you seem to have misinterpretted me. Firing a driver through a slot will not magically grant 9dB more output, unless you put the mic to the slot, where the pressure is very high and the reading will increase accordingly. When measured at a sensible distance, there will be minimal difference in output.

I've no problem with high efficiency drivers. Where did I say they can't be >100dB@1w?

Mark, compression drivers use exceptionally high compression ratios. The total area of the slots in the phase plug would be of the order of a couple of square centimetres, while a 3" dome is more like 50cm^2. They get to do that because they're using exceptionally strong diaphragms that undergo minimal excursions. A large paper cone wouldn't survive treatment like that (indeed, some tapped horns have that problem and will shred cones if driven hard), so, IMO, for all intents and purposes, the idea isn't applicable at low-frequencies.

FWIW, I also ran a couple of simulations to find out the compression ratio needed to drop the resonance of the system down by an octave. With the example driver I chose (a good 15" subwoofer), it needed a 21:1 compression ratio. A 4:1 CR changed Fc by a few Hz, which would be within production tolerances of the original driver.

A further FWIW, John K posted this about such LF techniques: http://musicanddesign.com/Nelson_Pass_S ... ed_OB.html
Seems to me like it's worth just bolting the drivers to the flat baffle, unless you're willing to trade cavity resonances for force cancellation.

Chris

[Nick]

Nick, you seem to have misinterpreted me. Firing a driver through a slot will not magically grant 9dB more output, unless you put the mic to the slot, where the pressure is very high and the reading will increase accordingly. When measured at a sensible distance, there will be minimal difference in output.

Just suggesting that invoking the first law in this case is at best an appeal to authority fallacy.

[IslandPink]

Let's stop talking about 9dB gain please, Nelson claims around 2 to 2.5dB from the slot loading at sensible listening distances !

Mark, compression drivers use exceptionally high compression ratios. The total area of the slots in the phase plug would be of the order of a couple of square centimetres, while a 3" dome is more like 50cm^2. They get to do that because they're using exceptionally strong diaphragms that undergo minimal excursions. A large paper cone wouldn't survive treatment like that (indeed, some tapped horns have that problem and will shred cones if driven hard), so, IMO, for all intents and purposes, the idea isn't applicable at low-frequencies.

FWIW, I also ran a couple of simulations to find out the compression ratio needed to drop the resonance of the system down by an octave. With the example driver I chose (a good 15" subwoofer), it needed a 21:1 compression ratio. A 4:1 CR changed Fc by a few Hz, which would be within production tolerances of the original driver.

A further FWIW, John K posted this about such LF techniques: http://musicanddesign.com/Nelson_Pass_S ... ed_OB.html
Seems to me like it's worth just bolting the drivers to the flat baffle, unless you're willing to trade cavity resonances for force cancellation.
Chris

1. Compression is typically much higher in real compression drivers as you say. But in terms of diaphragm robustness, have you actually looked (felt) the diaphragm on an Altec 288 ? - it's like a piece of kitchen foil . I guess the concave shape and the way the phase plugs loads must keep the loading OK for it to survive. I think re. compression ratio for bass the limitations are from 'chuffing' out of the slot and JC has commented privately that by 5:1 it gets to be a problem.

2. Re. resonance, you simulations must be inaccurate because if you look back a few pages, I measured the Fs of the Supravox for the two cases and the Fs dropped from 54.5Hz to 36 Hz with the 4:1 compression.

3. I will look at John K's page, thanks for the link.
My intention soon ( after final mods to 8FE200 ) is to do a decent 2-3 hour comparison of the Supravox as conventional OB bass and slot-loaded bass - I have them side by side.

[Dave the bass]

This maybe waaaaay off topic and of no interest but Trace Elliot did some cabinets that were slot-loaded.

[IslandPink]

No, Dave, very relevant. I think this kind of slot loading was around before the SLOB idea. I don't know enough about the subject yet to understand what the pros/cons of the different methods are. There's also the 'Karlson' which attempts to blend the loading over a wider frequency range.

[jc morrison]

not to crash any party but i HATE to be misrepresented.

there is NO controversy at all in the statement: efficiency (good loading) and excursion are the two primary methods contemporary engineering tackles the first octave. none. a 12" driver is pathetically efficient at 20Hz, and a 21" is slightly better. this lack of efficiency can be made up for with excursion. i have just described 95% of contemporary loudspeakers.

pro audio deals with it very differently. many drivers or horns. better loading saves money in the arena setting.

however, and this is not either controversial from the engineering or physics pov: excursion creates problems of its own. one is distortion. over hung woofers, are completely obvious in this regard. but, especially with sealed box baffles and bass reflex baffles, notwithstanding the problems with the drivers themselves... sealed boxes have increased 2nd harmonic inherent at large excursions (the box is a wimpy diode, has a higher impedance in one direction), and reflex boxes have a complex impedance across the first octave with "all" bass drivers... (you could do a bag end thing with fs much higher than the first octave... and then it's identical with the sealed box), which now makes the amplifier a part of the transfer characteristic.

but now the part that pisses me off. no where did i say 9 dB of gain! wtf are you talking about?

i have four similar drivers in parallel.... and the free air efficiency at 20 Hz is 75 dB/watt at one meter. the low efficiency is not only the back wave canceling but the resonance impedance climbs to about 120 ohms in each driver. i lowered Fs with the compression ratio by roughly 10Hz... that put Fs at about 18Hz... now the efficiency per driver is 80 something through the first octave. that is because the impedance has been lowered. 4 drivers (one pair are 24 ohms and the other pair are 16 ohms) in parallel give 9dB more efficiency... what's the problem? the baffle might give 2dB in front... maybe not even that?

and the final thing. mass creates delay. it's called inertia. excursion multiplies it. you want to argue about that? it's taught in physics 101. this is the reason why "group delay" is even an issue... why low frequencies are delayed compared with higher frequencies in the same driver. you really want to argue about that?

i couldn't give a rat's ass what you think of me, but criticize me right. don't make shit up.

[jc morrison]

or, make shit up and i will come and remind you that you made it up.

[jc morrison]

and if, just if, you were referring to the mirrorphonic system (M2), it has 4 each 18" woofers (24 ohms) with electromagnets with permendur pole pieces (half cobalt - half pure iron). the 50Hz efficiency is roughly 106 dB/watt and the drivers are horn loaded. add 9 dB of efficiency for the parallel operation... where's the controversy?