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In Reply to: RE: physics posted by Swamis Cat on March 11, 2017 at 07:09:43
I heard a pair of DWMs with 20.7s in a very large room. They were set up by the dealer and it was not a success ... although the owner of the 20.7s was using a huge German amplifier to drive them, when the DWMs were also driven by the amp ... it no longer had the grip to drive them.But the dealer explained that the main use of DWMs was to get bass pressurising the room from other positions in the room (than the 20.7 bass panels) - to smooth out the bass - not to deliver low bass. So his idea was the same as yours.
Andy
Edits: 03/11/17Follow Ups:
The owner of these 20.7 is complaining about lack of power below 50 Hz. A while ago he got some 40 year old Tympani IIIA basses to work with.
It's a beautiful room, but a terrible one for line sources (and probably anything else, but mostly line sources). Line sources want a ceiling, they depend on the reflections from ceiling and floor to approximate an infinite line.
So why do peoble use sound deadening ceiling panels? It is not common here in Europe and most acoustic Consultants do not recommend them except for placement just behind/above conventional radiating speakers.
With boxes, you'd normally put absorption on the ceiling at the first reflection point, which is to say a bit in front of the speaker. I'm not sure why anyone would want to do more than that, unless they're trying to lower the Rt of the room and they can't use the side or front and rear walls or perhaps if they want to suppress the reflection to a lower frequency, which wouldn't be bad except that there's a risk with a large area of making the room too dead.
Full-length dipoles are entirely different and you really don't need absorption on the ceiling or floor, or want it at frequencies at which wavelength becomes large compared to driver height.
Interesting room; great looking setup. I have what many may consider a modest setup powered by modest components. Dual XPA-1 mono amps on the bottom of my MG20r cranks out 'enormous' bass; so much I have to turn down the low gains on my Marchand XM44 to #2 on the dial. Neither muddled nor boomy, but nice and controled bass. I hesitate to use the word 'tight' since my room isn't acoustically setup for correct bass response (yet).Now what I'm about to say will raise a few eyebrows but here it goes: I'm convinced my 20s have hit 20hz-30hz at times (yes you read that right!) I'm only speculating since I don't have equipment to measure such. But I've never heard bass reach as low and controlled even with past subwoofers I've owned. Keep in mind this doesn't happen frequently at that level, in fact only once or twice and I also feel it's source specific- meaning if your album, cd doesn't send it, Maggies won't receive it. My jaw dropper source is John Barry's "Dances with wolves" soundtrack, "Main Title - Looks Like a Suicide". Out of nowhere a kettle drum kicks in and lays a blanket of bass so deep it literally -if only momentarily, reaches into the bottom of your soul.
Room and engine(s) dictate such; and high current is the key. I had a single Krell KSA 200 that couldn't do it; whereas these Emotivas handle it with ease.
Edits: 03/18/17
Stereophile measured this.
The 20.7's definitely can go down that low.
Also, if your XPA-1's are like my XPA-2 was, they have a wooly, prominent midbass compared to my A-21. I have no idea why that should be the case since they both have high damping factors and they're feeding a mostly non-reactive load. There shouldn't be a difference, but there is.
Some sonic differences are easy to explain -- crossover notch distortion, harmonic distortion spectrum, impedance interaction, instability, RFI -- but I continue to be mystified by some of the differences that I hear.
I think the key difference is in the Marchand XO vs. the maggie speaker level XO box. The XPA might have a better time driving the panel direct rather than dealing with the XO box's reactance.
My own experience was that the XO box is woolly sounding. All the amps I tried on the bass panels sounded much more extended and tight without the XO box. I also tried the bass panels as full range speakers out of curiosity. Even the Crown 5002 sounded woollier with the speaker level XO box. But it definitely sounded worlds ahead of the Bryston 4BNRB Classe DR9 whether in stereo or bridged mono and Nuforce 8b .
No problem believing that you get audible 20-30 hz content, particularly with some wall loading. Wall loading the T IV bass panels and bracing them brings out organ foot pedal fundamentals that are inaudible without wall loading and weak without bracing.
The Stereophile bass measurements are done at close proximity so don't show what it actually sounds like in the far field - where room modes also come in. My own 1m measurements show a resonance at 30'ish hz. But the particular frequency changes with the angle between the panels, appearing lowest when the panels are flat and face forwards. all this from memory as this was done over a decade ago and the PC used for it is long dead without a full backup.
Without looking at the schematic (too late, I'm about to head for bed) I'm guessing that the issue with the passive XO is that the amp isn't seeing the driver above the crossover point and is losing electrical damping as a result. But that doesn't explain why the amp would make a difference. So -- it might have something to do with the amp's ability to deal with the back EMF. But I'm not sure why that would be an issue, since these amps generally have plenty of reserve unless you're really cranking them, and the feedback loop of a modern amp should have enough bandwidth and slew rate. And I don't think the harmonic spectrum would have that much of an effect.
Really, there is something going on here that I don't understand from an engineering perspective -- not just the wooly bass but the different ways amplifers handle the ADSR envelope/20 dB transients on piano etc. This stuff isn't happening at a high frequency, but at a very low one.
These Stereophile measurements say they were taken at 100", so it isn't near field. But they say anechoic. Don't have the original review and I was wondering what the setup was. Were they done outside? Did they try to take a quasi-anechoic gated measurement at 100", which you *can't* do with a line source unless you measure it in a room the size of a gymnasium? In any case, as you say, the only meaningful measurement of the bass response of a dipole is in a room, because you have cancellation from the rear wave reflection. And that of course is going to vary widely depending on room and setup -- as bass always does.
Reminds me that I have to measure my IVA's in their new position . . .
The caption says the bass was measured separately below 300 hz in "quasi anechoic" conditions which is long form for near field.
The speaker level XO should reduce the damping available from the amp. At least vs. not having it in the way.
The transient dynamics are a matter of power supply and the IV curves of the output devices. Pass points out somewhere in his white papers that besides power supply sag the output devices are just not linear outside the narrow range at their center where the IV curve is straight. so he chooses where to operate the output devices to get the least compression and then to make that compression come off as naturally as the drop off of SPL in air. That way he is thinking that the effect would be similar to sitting a bit further away from the sound source rather than being perceived as drooping output.
Pass also discusses the effects of transformer saturation. The effect is compression of dynamics and he gauged its audible effect by using two giant spools of magnet wire on pallets .stacked on top of each other as an air core transformer for the same amp design. There he noted how it differed in there being a lack of compression, among other things. He then lamented the impracticality of air core transformers since they would be as big as speakers beyond a few watts and be entirely immobile..
I'd say, just off hand, that maggies don't store enough energy for amp damping to be a Huge Factor.
Too much is never enough
Remember this room?
http://i331.photobucket.com/albums/l454/cmbfoto/_P0F5690.jpg
It is now slightly different, Tympani IIIA Bass Speaker added.
http://tinyurl.com/lwxdcwh
Any idea how those are crossed over?
I see he is using the new concentric U profile diffusers. They really took off recently.
Originally his room housed some conventional box speakers. In order to tame them the room was filled with a lot of Svanå Diffusors. After switching to Magnepan 3.7 and later on to 20.7, the room has been changed a couple of times. The side walls are now without most of the diffusors, the wall behind the speakers have them partially coverd with absorbers from Sounds of Silence (http://www.sofsci.com/). The transparent diffusors are from Svanå (http://www.diffusor.com/). The Tympani Bass Speakers are driven by a LAB poweramp with built-in crossover (maybe even DSP).
Yikes! I'm having a somewhat difficult time believing a 20.7 is that deficient in delivering "lower bass" that it needs to be supplemented by four Tympani Bass panels. Perhaps their individual total bass output has been adjusted (by use of a X-O of some type?) to meet the task. Nevertheless, just too much of a good thing?
The panels couple together. So having them as an add on increases lower freq output and extension beyond that of either set.. Besides which the T III bass panels are likely crossed over in the vicinity of 50hz while the 20.7 likely play full range.
Thanks Satie, points taken.
They spec'd the measurement at 100", though. That isn't near field. And if they tried to gate it at that distance to make it quasi-anechoic, they screwed up *unless* they were outside or in a huge space in which the reflections were outside the window.
Some other wrinkles -- if the gating is too short, you won't get a contribution from the full height of the line source or the floor and ceiling reflections that you do want. And if you do it outside, you won't get a contribution from the ceiling reflections.
Just very hard for these reasons to do meaningful quasi-anechoic or even anechoic of a planar. What counts is the in-room response -- and then you have to accept that bass levels vary widely between rooms and speaker/listening position.
Sounds like an interesting white paper. I remember Pass saying that he tries to mimic the harmonic distortion of the air but I'm afraid I'm missing the effect of device linearity -- that's what feedback is for. Of course you'll hear effects of feedback such as the adverse effect on the harmonic spectrum, but I'd expect the feedback to take care of obvious effects of the magnitude I was hearing. That I think is too high for the harmonic distortion spectrum of a modern amp, usually what I hear of harmonics is subtle except for crossover notch distortion and this didn't sound like that to me.
They say it is nearfield right on the caption. That is how they get "quasi anechoic" in the bass. Atkinson details these measurements somewhere on the Stereophile website.
Part of Pass' philosophy is to avoid using too much feedback, so he can't rely on it to entirely flatten out non linearities in the output devices. He is mainly avoiding high order odd harmonics so far as I understand it and that relates to the HD spectrum in air - which does not contain those. And his DIY projects going into the First Watt concept also try to avoid time domain issues due to overly aggressive feedback, which is something he also worked into his "regular" AB and Class A amps.
He's wrong, though. For the typical planar, 100" isn't near field, it's medium field -- on the border of the near and far field, which is to say it's near field at some frequencies, far field at others. This is very apparent when you look at a plot of dispersion as a function of both distance and frequency.
"Quasi anechoic" just means the measurement has been gated to suppress room reflections and at that distance in the typical room you *can't* gate out the reflections. Unless he measured outdoors, but then it would likely be described as an outdoor measurement and would include the reflections from the ground -- which are actually crucial to the LF response of a dipole since they effectively double the baffle size.
I seem to remember having read Pass on the harmonic distortion of the air some years back. The man is an artist. On the other hand, high order harmonics sound more dissonant and have to be weighted in a harmonic distortion measurement because they're a good deal more audible. One of the main arguments against high levels of negative feedback is that the distortion spectrum changes to emphasize higher-order harmonics. They really should dispense with THD and use the formulas for audibility to create a meaningful weighted measurement. It would take into account both the relative audibility of the harmonics and the Fletcher-Munson curves.
I have seen in-room measurements of the MG 20, 20.1 and 20.7. There use to be a drop in the lowest octave. They cannot simply move that much air. My Tympani IIIA basses are similar, the lowest resonant frequency is 33-34 Hz and from there it drops by about 18 dB/octave. The 3.6 I am using today are also driven by an active crossover and four channels of power amplification. They are not doing much below 40 Hz. Maybe I have to continue my modification of the Tympani IVa...
PS. Kettle drums = timpany? Timpany is playing at higher frequences, from about 55 Hz and upwards. What you hear in that track is the orchestral bass drum at about 35 Hz or so. Here an analysis of that track.
http://www.forumbilder.com/image/E85
The room reinforcement from early front wall reflections would reinforce the bass by 3 dB at half a wavelength path length difference, so if the speakers are 5' from the front wall you'd get cancellation at 100 Hz (10' wavelength and path length) and reinforcement at about 50 Hz (20' wavelength and 10' path length) rather than the bottom octave. At 20 Hz you're talking a 50' wavelength so you'll get a fair degree of cancellation from a 108 degree net phase shift. So it seems to me that a very effective bass trap could potentially increase bottom octave response, though of course it will ultimately be limited by the woofer's resonance.
I didn't see much bottom octave bass when I measured my IVA's but there are two important caveats -- I've moved them and there's delam on the lowest frequency resonant sections. So the measurements probably aren't meaningful and since I'm in the middle of some long-overdue Ebay selling it's going to be a few days before I have a chance to take some new measurements (as a matter of curiosity, I want to compare the response before and after the delamination is repaired).
I think it is a complex mix of reinforcement and cancellation from reflections. Not only from the wall behind and beside the speakers but also from the area around the listening posistion.
Definitely. A room mode calculator will predict what happens pretty well for a rectangular room if you remember that with dipoles, a node is an antinode and vice-versa, and that dipoles excite fewer modes than omnis because of the dipole null, which I think is one of the main pluses for planar bass.
Josh, I'll gladly display my ignorance in exchange for you explaining your statement "with dipoles, a node is an antinode
and visa versa". If you share your knowledge with me (and others), assume I know nothing, which ain't far from the truth! I understand that bass frequencies collect in a room at locations (nodes/modes. Are they one and the same? Also called standing waves, right? More things I don't know!) and frequencies determined by the rooms dimensions. What is an antimode, and why the reversal with dipoles? Thanks---Eric.
Edits: 03/20/17
I can't believe he is having trouble with bass in that room given the hardwood and all of the diffusion.... I can't see his electronics... do you know what they are? Unless he is looking for Wilson Alexandria xlf bass.... Can't get under 50Hz??? That just doesn't sound right.....
So where would he put his T III bass panels? behind him?
The response certainly goes below 50 Hz but he feels there could be more bass. I think he had 20.1 earlier. Power amps are probably LAMM. Not sure how he would use the T-IIIA basses. Will have to visit him soon... Lot of acoustic stuff from Svanå.
Here is a thread from a Swedish forum.
http://www.euphonia-audioforum.se/forums/index.php?/forums/topic/11142-v%C3%A4stkustskt-v%C3%A4lljud-fr%C3%A5n-maggiesar/
Most of the bass traps/ diffusion are no longer there. He is using Lamm M2.2 monos.
Hasse,
Have you been there at Claes' place? Yes, his room changed over time.http://i331.photobucket.com/albums/l454/cmbfoto/_P0F5690.jpg
http://i331.photobucket.com/albums/l454/cmbfoto/_49U3326.jpgMaybe this is what Claes is refering to? The lack of bass.
http://i331.photobucket.com/albums/l454/cmbfoto/image1.jpg
Edits: 03/15/17
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