john k... said:
Yes, in a certain respect the foam is a LPF, but it is very small for the main wave and much larger for the HOMs, which travel a longer path through the foam, as do the reflections. So the foam does LPF the main wave, but it LPFs the diffraction and reflections to a much greater extent, resulting in, once re_EQ to flat, a net benefit.
Earl Geddes' posts describing the flattening of room modes in the bass, by employing multiple subs strategically sited throughout the listening room (w/ quasi-random phase relationships) transformed my room from one in which bass was virtually unlistenable into some of the best bass I've heard. The frosting on the cake is that it is the most efficient known method to achieve the sound of large room bass in a domestic listening room.
At age 54, having done this for many decades, as best I can surmise Dr. Geddes provided more audible benefits than everything else I've learned prior combined. The benefit can not possibly be overstated.
The benefits infinitely outweighed $8k equivalent value of ASC acoustic soffit & any prior benefits from EQ.
I am well familiar w/ the sound of live acoustic instruments, pretty good player of a choice Martin HD28LSV (adirondack top). Just ordered a Ryan Thorell custom (IMO one of the top luthier's in the world at the incredible age of 29). Also regularly hear great pianists play my wonderful Chang grand.
Earl also personally is responsible for about $5500 USD saved in components. Kudos to Earl. I owe Earl a nice dinner if I ever cross his path.
At age 54, having done this for many decades, as best I can surmise Dr. Geddes provided more audible benefits than everything else I've learned prior combined. The benefit can not possibly be overstated.
The benefits infinitely outweighed $8k equivalent value of ASC acoustic soffit & any prior benefits from EQ.
I am well familiar w/ the sound of live acoustic instruments, pretty good player of a choice Martin HD28LSV (adirondack top). Just ordered a Ryan Thorell custom (IMO one of the top luthier's in the world at the incredible age of 29). Also regularly hear great pianists play my wonderful Chang grand.
Earl also personally is responsible for about $5500 USD saved in components. Kudos to Earl. I owe Earl a nice dinner if I ever cross his path.
Quoting Lynn, again
So the unavoidable question is - which complex symphonic or choral material do you find highlights this?
I know this runs the risk of the "audiophile approved recording" meme however it's always interesting to see what causes our illusion engines putter out.
As an Ariel builder with a relative who wants to build something similar I've been watching this thread with some interest.
Lynn Olson on page 194 said:
So the unavoidable question is - which complex symphonic or choral material do you find highlights this?
- Allegri's Miserere? (eg. '64 Kings College recording)
- Khachaturian: Spartacus? (eg. '91 Armenian Phil)
- Schuetz Magnificat?
- Mahler's 5th?
- Or something more Wagnarian?
I know this runs the risk of the "audiophile approved recording" meme however it's always interesting to see what causes our illusion engines putter out.
As an Ariel builder with a relative who wants to build something similar I've been watching this thread with some interest.
Hello,
Feedforward amplifier is a very interesting concept, and well known technique and living strong at RF amplifiers still today. I used to deal with those in the lab at my work.
I've been also thinking how to implement feedforward in tube amps. One think I haven't solved yet is how to achieve directional summer with transformer that is needed to perform the feed forwarding. Any ideas?
- Elias
Lynn Olson said:
Feedforward amplifier is a very interesting concept, and well known technique and living strong at RF amplifiers still today. I used to deal with those in the lab at my work.
I've been also thinking how to implement feedforward in tube amps. One think I haven't solved yet is how to achieve directional summer with transformer that is needed to perform the feed forwarding. Any ideas?
- Elias
thoglette said:
Nothing as audiophile as all that. The simplicity vs density requirement in a playback system comes down to little more than the type of music you enjoy. If your favorite genre of music is baroque chamber music, blues, jazz, or anything with a small number of musicians, this will generate a less dense spectrum than an ensemble of 60 to 120 musicians (or a large choir).
The denser the spectrum presented to the playback system, the more sum-and-difference IM distortion terms it will create; these IM terms increase at much faster rate than simple harmonic distortion terms. As a result, a system that sounds just fine on small ensembles can fall apart rather dramatically on large ensembles. But if you don't listen to large ensembles, what does it matter?
Heavily produced rock music falls between the two - thanks to the generous amounts of distortion in the guitar amp & speaker, combined with intentional postprocessing and FX in the mixdown, you get a lot denser spectrum than an acoustic guitar with minimalist miking and recording techniques. The difference between rock and large acoustic ensembles is expectation: many of us are familiar with the sound of a large acoustic ensemble, and expect the recording to sound have some kind of approximate relation to the acoustic event; with rock, though, only the musician (and maybe the producer) really "know" what it should sound like.
The same applies to movie soundtracks, which are wholly synthetic confections with hundreds of tracks and unlimited FX applied in postproduction. Only the director really knows what it "should" sound like, since there is no analog in the real world.
So with rock and movie soundtracks, anything goes - the word "realism" doesn't have any context, unless you want to replicate the sound in the control room. Music like this is spectrally dense and relies on power to achieve the esthetic effect, so speakers with modest SPL limits are going to get in trouble. If you've ever heard a good guitar amp at a distance of a few feet, you know they can play a lot louder than most audiophile systems - loud enough to feel as well as hear.
An extra 5 to 10 dB of headroom can make all the difference here, and the most direct way to get that headroom is raise the electroacoustic conversion efficiency. "Normal" audiophile speakers have tweeters and woofers that are 0.3% to 1.0% efficient, so nearly all the power is doing nothing more than heating a tiny voice-coil.
With acoustic music, there is a vague, approximate reference, but there still remains an immense gulf between a live performance and the reproduction. With small-ensemble music, you get the luxury of relaxing the headroom and IM distortion requirements - single-driver speakers come to mind here. Large-ensemble music has the most difficult requirements in terms of headroom, since the spectrum is dense (inviting more IM distortion), instantaneous peaks are loud, and there are listener expectations of the acoustic event.
Lynn Olson said:
I'm really looking forward to the sonic outcome of your horn experiment and how your findings will be in the light of the above.
You remember we once (way, way back) discussed the issue of "missing spectral lines" in the audio band.
My position then was that there are *no* such missing parts in the reproduced audio spectrum ( like there can be in colors reproduced by monitors or beamers due to the uneven spectra of the light source for example).
In the meantime my own investigation about back diaphragm mirror distortion has proven me wrong. There are – under certain circumstances – mechanisms that can suppress discrete frequencies within the audio band – way more effectively than usual comb filter effects due to wall reflections or diffraction.
Could you keep an "eye" on that when auditioning your beautiful horn and report your impression?
Any chance you can operate your compression driver without back wall for comparison?
Michael
Yes, that will be part of a step-by-step investigation, although making valid IM measurements on loudspeakers are not trivial. From a measurement perspective, the most obvious difference is going to be in the time domain, since the back chamber only has a thin felt pad to "absorb" the backwave from the aluminum diaphragm.
My own feeling on the back chamber is that it originally served as a highpass excursion-control device back when crossovers were nothing more than 1st or 2nd-order electrical filters - the bad old days of "M-derived" matrix theory. With crossover theory in its infancy, the only means available for effective excursion control were mechanical, or more precisely, decreasing the effective compliance of the diaphragm through acoustical means. (Stiffening the diaphragm surround has undesirable effects at the top of the band, so acoustical means are preferable.)
Thanks to the development of color television along with military communication systems during the Fifties, we received the benefit of modern filter theory (Chebyshev, Butterworth, Bessel, Gaussian, et al), and are able to apply it to crossovers and bass-cabinet design. A more sophisticated crossover can now perform the highpassing previously done by the compliance shift of the back chamber, without the downside of reflections off the felt pad.
But - in practice it may not matter that much, with performance of the overall system so powerfully dominated by the horn, phase plug, and internal flare inside the compression driver. Most of the horns on the market, particularly commercial constant-directivity horns used in modern movie theaters and SR systems, have gross problems with internal reflections and diffraction, which are then brute-force "corrected" by digital equalization.
From my vantage point, just getting a horn to approach (never mind surpass) the time-domain performance of a good-quality direct-radiator is a tremendous accomplishment, and that will be my main focus. I think you have to get the basics right before focussing on the more esoteric stuff. As mentioned earlier, I focus most on the integrity and quality of the first-arrival wave. If that is degraded by excessive energy storage or too much diaphragm excursion, the other parameters don't matter that much.
My own feeling on the back chamber is that it originally served as a highpass excursion-control device back when crossovers were nothing more than 1st or 2nd-order electrical filters - the bad old days of "M-derived" matrix theory. With crossover theory in its infancy, the only means available for effective excursion control were mechanical, or more precisely, decreasing the effective compliance of the diaphragm through acoustical means. (Stiffening the diaphragm surround has undesirable effects at the top of the band, so acoustical means are preferable.)
Thanks to the development of color television along with military communication systems during the Fifties, we received the benefit of modern filter theory (Chebyshev, Butterworth, Bessel, Gaussian, et al), and are able to apply it to crossovers and bass-cabinet design. A more sophisticated crossover can now perform the highpassing previously done by the compliance shift of the back chamber, without the downside of reflections off the felt pad.
But - in practice it may not matter that much, with performance of the overall system so powerfully dominated by the horn, phase plug, and internal flare inside the compression driver. Most of the horns on the market, particularly commercial constant-directivity horns used in modern movie theaters and SR systems, have gross problems with internal reflections and diffraction, which are then brute-force "corrected" by digital equalization.
From my vantage point, just getting a horn to approach (never mind surpass) the time-domain performance of a good-quality direct-radiator is a tremendous accomplishment, and that will be my main focus. I think you have to get the basics right before focussing on the more esoteric stuff. As mentioned earlier, I focus most on the integrity and quality of the first-arrival wave. If that is degraded by excessive energy storage or too much diaphragm excursion, the other parameters don't matter that much.
OT but about GPA
I am currently using the GPA 604 Series III in a 4 cubic foot sealed box with a custom crossover by Selah Audio. I'm not trying to toot my own horn or threadjack...all I wanted to say was that it makes for an extremely simple, nearly fullrange solution that sounds amazing. Never in my audio dreams would I have thought that an archaic technology like a 15" duplex with a short biradial horn could produce such dynamic, room filling, musical sound. It is of course not quite as transparent as the open baffle it was in prior to this, but I feel the compromise may be worth it with respect to dynamics and a size that is easy to live with.
Best,
Chris
I am currently using the GPA 604 Series III in a 4 cubic foot sealed box with a custom crossover by Selah Audio. I'm not trying to toot my own horn or threadjack...all I wanted to say was that it makes for an extremely simple, nearly fullrange solution that sounds amazing. Never in my audio dreams would I have thought that an archaic technology like a 15" duplex with a short biradial horn could produce such dynamic, room filling, musical sound. It is of course not quite as transparent as the open baffle it was in prior to this, but I feel the compromise may be worth it with respect to dynamics and a size that is easy to live with.
Best,
Chris
