It is not. It is a mass loaded quarter-wave resonator. The "vent" is a restricted terminus.
If you take the volume of the Met, make it a more regularily proportioned box and used the same "vent", you would find that the resulting BR is significantly mis-tuned.
Martin King, in his landmark article on his ML-TQWT has some ANSYS sims that clearly show significantly different behavior vrs a "similar" BR.
dave
No magic, just physics. If you do the math for enclosures long in one direction compared to the other two dimensions, the standing waves ignored by the Thiele / Small lumped parameter models start to become important and play a major roll in the bass output. The shape of the enclosure in the long direction is important and influences the tuning frequency. But you have to do the math to understand.
Martin
As Dave & Martin say, the length & geometry of a cabinet have a profound effect on its behaviour. No magic / witchcraft, just physics. If you stretch one dimension of any vented cabinet sufficiently far, it will move from pure Helmholtz resonance toward one where the standingwaves / eigenmodes / whatever you prefer to call them become the dominant behaviour. In practice, the transition from one type of box to the other might be said to occur when the one dimension is elongated sufficiently relative to the others that the vent will need to be retuned to maintain the intended Fb.
As an aside, personally I class the Metronome as a tapped, mass loaded horn, on the basis that anything that expands toward the terminus is technically a horn, it has a resistive (mass loading) vent, and the driver is tapped into the horn at x distance from the throat. Not that it matters -we could spend the next week discussing nomenclature to little purpose.
As an aside, personally I class the Metronome as a tapped, mass loaded horn, on the basis that anything that expands toward the terminus is technically a horn, it has a resistive (mass loading) vent, and the driver is tapped into the horn at x distance from the throat. Not that it matters -we could spend the next week discussing nomenclature to little purpose.
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Scott,
I have several problems with your paragraph above. It starts with the your definition of a horn as anything expanding as you go towards the terminus. Consulting Wikipedia for a definition of an acoustic horn I find the following statements.
"A horn is a tapered sound guide designed to provide an acoustic impedance match between a sound source and free air. This has the effect of maximizing the efficiency with which sound waves from the particular source are transferred to the air."
The definition has the statement about a tapered geometry which is easy to visualize, but the subtle and more important part of the definition is the "acoustic impedance match between a sound source and free air" which requires a deeper understanding. A horn couples efficiently to the air in the room when the mouth's acoustic impedance is primarily resistive, this in turn dictates a certain size as a function of frequency. For bass frequencies, even including room boundary conditions that limit the free space being radiated into, the mouth of a horn still needs to be fairly large. In fact the size requirement for the mouth of what we commonly label as back loaded horns BLH is much greater than the openings in almost all of the BLH designs found on the Internet. What most of us are really designing is transmission lines that evolve into back loaded horns at frequencies in the 100's of Hz. So any horn math used to justify the design of many BLH is not applicable at 50 to 100 Hz and a better job of optimization could be done if you toss the horn labels and sizing restrictions and work the low frequency design from a TL perspective.
You did pick up on the resistive nature of the horn mouth, but missed the fact that a mass load from a port is not resistive. A resistive boundary condition dissipates energy from the horn enclosure into the room over a range of frequencies, this can be very efficient. A mass boundary condition reflects sound waves back into the enclosure leading to discrete resonances and narrow spikes in efficiency with nulls between the spikes. So a Metronome enclosure is a form of quarter wavelength resonator that is mass loaded by a port at the larger end of the geometry. It operates in the same way as what we have often refered to as ML TL and ML TQWT designs, they all work the same but are tuned a little differently due to the cross-sectional area definition along the length.
Martin
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Hi Martin,
Apologies for the screw up re the resistive reference WRT the vent. I wasn't really concentrating when I wrote the above. I think we're largely on the same page as it happens; it's just ye olde definition trap rearing its head again. I don't regard impedance matching down to cut off as a defining characteristic of a horn; merely a potential design option. I just view a horn as being a pipe that expanding toward the terminus, and therefore possessing at least some 1/2 wave characteristics, however minimal. I believe Greg has a similar take; IIRC, he once told me that Altec et al traditionally classed a straight pipe as a pipe or column, an expanding pipe as a horn, and a reverse taper pipe / (or reverse taper horn if you prefer) as a TQWT. Works for me, although I suppose it's idiosyncratic these days.
Apologies for the screw up re the resistive reference WRT the vent. I wasn't really concentrating when I wrote the above. I think we're largely on the same page as it happens; it's just ye olde definition trap rearing its head again. I don't regard impedance matching down to cut off as a defining characteristic of a horn; merely a potential design option. I just view a horn as being a pipe that expanding toward the terminus, and therefore possessing at least some 1/2 wave characteristics, however minimal. I believe Greg has a similar take; IIRC, he once told me that Altec et al traditionally classed a straight pipe as a pipe or column, an expanding pipe as a horn, and a reverse taper pipe / (or reverse taper horn if you prefer) as a TQWT. Works for me, although I suppose it's idiosyncratic these days.
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Scott,
In my opinion, you cannot assign a name/label based only on geometry. An expanding line does not to me automatically mean a horn. You have to look at how it functions, the physics of the air motion in the geometry. A name/label should tell you how the device functions and not how it looks. This is not always obvious and a more difficult task. The right answer is not always the "obvious" answer.
Martin
In my opinion, you cannot assign a name/label based only on geometry. An expanding line does not to me automatically mean a horn. You have to look at how it functions, the physics of the air motion in the geometry. A name/label should tell you how the device functions and not how it looks. This is not always obvious and a more difficult task. The right answer is not always the "obvious" answer.
Martin
Martin,
Oh I agree, we obviously just have different criteria for what we describe as being a horn. You define it quite precisely by whether a pipe is impedance matched down to Fc. I define it more loosely by whether a pipe posesses some 1/2 wavelength characteristics as a result of its geometry. I think both of these definitions have a regard for the physics involved, it's just that one is far broader than the other.
Scott
Oh I agree, we obviously just have different criteria for what we describe as being a horn. You define it quite precisely by whether a pipe is impedance matched down to Fc. I define it more loosely by whether a pipe posesses some 1/2 wavelength characteristics as a result of its geometry. I think both of these definitions have a regard for the physics involved, it's just that one is far broader than the other.
Scott
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If I may opine on the last few posts, the description that Scott made, invokes an enhanced picture in my mind that Martin's doesn't. I believe that to the unsophisticated mind (like mine), Scott makes it a bit easier to comprehend, while realizing that Martin's is the more technically precise answer, it certainly becomes much easier to understand Martin's concepts once the "general idea" is formed.
I have several friends that insist that they need to have the mathematical formulas in front of them to understand things, while it seems that I need to see it in a more mechanistic, or physical manner.
I think that between the two of you, I have finally gotten a much better idea of the concepts involved.
Best Regards,
TerryO
I have several friends that insist that they need to have the mathematical formulas in front of them to understand things, while it seems that I need to see it in a more mechanistic, or physical manner.
I think that between the two of you, I have finally gotten a much better idea of the concepts involved.
Best Regards,
TerryO
Update on my F120a Met:
It was the summer before last that I built Fostex F120a Mets wanting to see what all the fuss was about regarding Alnico magnets. You can look back at my earlier posts, but the bottom line was that these drivers seem to be extremely sensitive to the amp driving them. I (and MJK) had bad luck with the F120a with SS amps. But they really come alive with tubes!
Now I’m getting ready to build another pair of Mets using Fostex FE167e. And I was also reading another thread on AudioKarma where the posters were rabidly wild about their Sansui SS amps & receivers--many speak of them as having a ‘tubey’ sound. Well, I really like my Sansui 4000 that I bought back in 1972, as well as the little 350A (early 70’s vintage) I picked up a couple of years ago. It occurred to me that I had never paired the F120a’s with the Sansuis. Note that the Sansuis use very different topology from the Yamaha CR-1000 receiver and amps I had tried earlier, including MJK’s Luxman amps. The Sansuis are capacitively coupled outputs, while the Yamaha and others are direct coupled.
Guess what? The F120a’s sound pretty darned good on the Sansui 4000. Sound stage and imaging aren’t as good as the single ended tube amp I have been using with them, but still, very engaging. As before, no BSC needed.
I remain puzzled by the serious lose of highs when the F120a’s are paired with the Yamaha--I used a treble boost of 8-10 dB (at 10 KHz) to make them sound something like reasonable. (No BSC used, as bass was perfect.)
Bottom line: it all comes down to synergy. I had almost given up on the F120a’s until MJK brought his tube amp over and ‘showed me the light’ (so to speak). And now I have another good combo.
PS: I now think that the ’magic’ is in the driver as a whole, not the material used to make the magnet.
Crazy, man. Crazy!
Cheers, Jim
It was the summer before last that I built Fostex F120a Mets wanting to see what all the fuss was about regarding Alnico magnets. You can look back at my earlier posts, but the bottom line was that these drivers seem to be extremely sensitive to the amp driving them. I (and MJK) had bad luck with the F120a with SS amps. But they really come alive with tubes!
Now I’m getting ready to build another pair of Mets using Fostex FE167e. And I was also reading another thread on AudioKarma where the posters were rabidly wild about their Sansui SS amps & receivers--many speak of them as having a ‘tubey’ sound. Well, I really like my Sansui 4000 that I bought back in 1972, as well as the little 350A (early 70’s vintage) I picked up a couple of years ago. It occurred to me that I had never paired the F120a’s with the Sansuis. Note that the Sansuis use very different topology from the Yamaha CR-1000 receiver and amps I had tried earlier, including MJK’s Luxman amps. The Sansuis are capacitively coupled outputs, while the Yamaha and others are direct coupled.
Guess what? The F120a’s sound pretty darned good on the Sansui 4000. Sound stage and imaging aren’t as good as the single ended tube amp I have been using with them, but still, very engaging. As before, no BSC needed.
I remain puzzled by the serious lose of highs when the F120a’s are paired with the Yamaha--I used a treble boost of 8-10 dB (at 10 KHz) to make them sound something like reasonable. (No BSC used, as bass was perfect.)
Bottom line: it all comes down to synergy. I had almost given up on the F120a’s until MJK brought his tube amp over and ‘showed me the light’ (so to speak). And now I have another good combo.
PS: I now think that the ’magic’ is in the driver as a whole, not the material used to make the magnet.
Crazy, man. Crazy!
Cheers, Jim
Jim,
It seems you've found a good combination. I've bragged about how I've found my Sansui AU-7900 to have a very nice sound, often to the groans of others.
I paid $14.95 for it at St. Vinnie's 25% off day and after using contact cleaner on the pots and switches it became one of my favorites.
A good SET will certainly, well almost always, sound better, but some of those can Sansui's work very well in a decent system.
I look forward to reading your further investigations into the world of single driver and Open Baffle speakers and appreciate that you've taken the time to share with the rest of us.
Best Regards,
TerryO
more like there's a smokey Argentinian tango in the interplay between amp/speaker system (dare I include cabling to make that a 3-some?)
when the partners move together well, it's poetry to watch and thrilling to participate, but when not, well it's not much more fun that dancing with yourself
in other words, I doubt there's a single factor that contributes - that Sansui might sound lackluster on a big-a$$ ELS or magneto-planar drive system
Shortcomings and all, the F120A is quite a beguiling performer
Thanks, Carmen Miranda, but I will stick with my opinion -- a somewhat high output impedance amplifier is gonna sound better with the mets!
What is really interesting about our experiences with my tube amp and various speakers was that is really sucked on everything else we tried. It did not like the two way passive dipole systems at all, bass just disappeared. But on the one speaker, Jim's F120A Metronome, it was magic. Until that time we had both been underwhelmed with the F120A driver, in his Metronome and also my OB/H frame system, driven by SS amps.
Martin
Martin
Does that count as back-leading? (I dance tango and suck, but my wife is awesome and occasionally gets bored with my lead and decides she's going to take control...)
So, I told DaveD that I was building, and he said "Pictures!". Unfortunately, as is reported by many of those with sub-standard wood skills, it is taking me a long time. Also, I only have shop access on weekday afternoons/evenings, so I only get a few hours each week. It has been fun so far, however, and one of the two measures within 1/64" of the design (the other is about 1/16" off, too small in every direction.) These are for 127eNs and I'm using 3/4" apple-ply maple. Finished ones soon, I hope!
