Proof pudding
Join Our Community Subscribe to Paul's PostsNo matter how much we wish to believe in something the proof’s often in the pudding. It tastes good or it doesn’t.
For years I have been a disciple of servo-controlled woofers. And, for good reason. Proper servo control has a number of advantages: lower distortion, reduction of overhang, flat response irrespective of the enclosure and driver parameters. That’s a lot to like.
Every Genesis Technologies woofer system I helped design was servo-controlled. It just worked and sounded great.
Not until our senior analog engineer, Darren Myers, and speaker designer, Chris Brunhaver, joined the PS engineering team did I begin to question my long-held beliefs. If memory serves me it was Darren that first questioned the actual sound from the servo system. It wouldn’t take long for Chris to join him. Their beef? It didn’t sound right.
They said the pudding would taste better without the servo.
The idea of letting go my love of servos was at first abhorrent. Hard to change that which you have truly believed in for as many decades as I. Yet, it didn’t take but a few hours of demonstration to flip my switch. What they argued wasn’t all that complicated. Servos did indeed produce cleaner bass but, they argued, at the loss of audible slam and impact.
Over the course of a few weeks, multiple experiments were conducted on every kind of music we could come up with. The results were always the same. With the servo in place some of music’s excitement was lost—something one doesn’t notice until a better example is at the ready for comparison.
It’s always a good reminder that no matter how great the recipe, the proof of the pudding is in the eating.
Paul, even if your strong mission for and conviction of your initial opinions (which others already questioned before) challenges one’s belief in orientation by your current opinions…it’s absolutely admirable that and how you take management measures as soon as you question them yourself.
Thanks, Jazznut. Not too bad for an old guy like me. 🙂
Not everyone might agree with you on everything in audio matters, but your (old) management skills and (proactive) strategies for the company are quite unquestionable I’d say 😉
Congratulations, Paul, on being open-minded and introspective and intellectually-honest enough to re-examine a long-held and deeply-held belief.
WHAT? Two posts in one day?
Paul, is this your idea of an early Christmas present, or have you been pushing the wrong buttons again? 😉
I’m sure that this post ties in with the one from a couple of days ago about keeping an open mind.
A couple of weeks ago someone said that “the proof was in the pudding, but that the devil is in the detail” when it comes to certain aspects of audio.
I believe that the devil is in the pudding…especially at Christmas time 🙂
Oops. I shouldn’t schedule posts when drunk. Thanks for that catch. That post was supposed to be out the day after Christmas. Fortunately, only a few people saw it and it didn’t mail out. Appreciate the help, Martin.
This closely ties in with the ‘wisdom’ post of 2 days ago. Maybe Paul is from Missouri…. “the show me state”
I guess the real question one should ask themselves is…. how could I be wrong for so long?
Finally, something I can fully agree with…
… now all we need is for Paul to agree some good designed speaker systems don’t need separate subs, unless you want to recreate an earthquake 😉
And no it’s not because I’m in the U.K. – my room maybe a bit smaller than large USA apartments, but at 3.3m by 4.4m which leads through a 1.4m archway into a smaller room has it’s own merits if some issues. The floors are solid concrete, stone tiled with thin’ish carpets while the walls are vinyl paper and the ceilings textured. I have some corner bass management, but my speakers are designed to be closer to the back wall – most domesticated folk can’t lose a 1/3 of the back wall space.
So in my opinion for the music I listen to, just as Paul has changed is opinion with servos, my twin 43 inch high but slim towers each with two 8 inch bass/low bass to 30Hz (OK -6db) drivers sound better than when I tried a dedicated Sub. Maybe I can’t create earthquakes or 20Hz massive organ pipes – I can live without that… just.
It will be real interesting how the PS-Audio Speaker puddings finally turn out…
… at least I know Paul and his good folk always listens more than measures kit.
Same here Alan.
I love my sub for a good movie night – but for music not so much. I’m still stuck with stand mount speakers due to downsizing so I do “need” the sub. But I’ve tweaked the level down so many times its probably sitting on 2 right now, lol! I’d rather lose some low end than hear “muddy” bass.
Eventually I’ll upgrade to some tall towers again. Until then, its the hobby of research mixed with the process of elimination.
Lower the xover point djB.
Bass “muddiness” is usually a combination of bass modes from bad room acoustics; excess resonance from high order enclosure design; high moving mass and low suspension damping to get more extension from a smaller cabinet; bass boost; and muddiness in the content that is masked by typical weak bass systems, or sound design and engineering for boom over PRaT.
It also arises from Doppler Inter-Modulation Distortion from having too small woofer and subwoofer. 12″ mid- bass is good to 100Hz or so, real bass starts at 15″.
I have a 140 liter sealed cabinet for a 15″ woofer that has voice coil inductance of a mere 300 microHenrys. The Bessel response with ~35Hz corner and 2 liter volume displacement blows away commercial subwoofers both consumer and pro. I can reproduce a kick drum at full volume and drummers think it is real.
I have a pair of JBL 4343’s with 15″ woofers and a pair of Velodyne DD-15’s that do a pretty good job.
Four fifteens usually gets the job done!
Did you bi-amp the JBLs? I had a pair of 4333’s and the passive crossover left a hole in the middle because they did not properly compensate for the high woofer voice coil inductance, which also means the woofer is too slow. I used a JBL electronic crossover, a Yamaha B2 100WPC FET amp for the the horns and a BGW 350WPC for the bottom. These were set up as nearfield monitors, my first 120dB system.
You could also add a Zobel in parallel with the woofer, that is easier to get to as I recall.
The 4343’s are 4 ways with a 10″midwoofer. They are in storage at the moment because we are moving. But I am replacing all of the drivers with updated versions except for the supertweeters. And the crossovers will be replaced with outboard charge coupled models. The xovers have been designed by Ian Mackenzie from Australia and a member of the JBL Heritage site. The 4343’s are amongst the best speakers I have heard but they do have some harshness in the upper midrange. I hope the mods will remove it.
http://www.audioheritage.org/vbulletin/showthread.php?37379-JBL-4343-restoration-progress-and-quite-possibly-a-lot-of-question-along-the-way&highlight=4343
These are not my speakers, just an example.
Wow I have a lot to learn still! Thank you – will research and experiment more.
If there’s an earthquake on the recording, I want to hear it.
HAHA! Earthquakes go below 1Hz. I wanted to make an 8Hz speaker for 64′ organ stops but found it impractical. AFAIK, 15Hz is the limit of commercially available subwoofers. Units with usable output on the Fletcher Munson curve (130dB) are built for stage like the big Danleys (the size of a coat closet) with the SAF of a tarantula nest, and they are five octaves short of “sounding like an earthquake”. For that you need an industrial shaker table like they use to test military hardware.
acuvox. I hope you can see the satirical nature of my post. I don’t really believe an earthquake will be on a recording.
I did have an earthquake-like audio experience. I bought a CAD-Equitek E300 microphone on eBay and plugged into a mixer to make sure it worked using my old trusty Koss Pro-4A’s. I heard something that sounded like a VLF sonar pulse, which repeated a few seconds later. The mic is specified to 10Hz in omni pattern and quite high sensitivity (LDC). I ripped the headphones off to see what physical phenomenon could generate sub-bass sounds. I found they where blasting the granite ridge the house was sitting on to expand the sewer system, so yes, an small earthquake waveform was being reproduced.
Wait! Someone fully agrees with me? I must be losing my touch. 🙂
Also in the UK, swapped out Harbeth (8″ mid/bass driver + BK subwoofer) for Wilson (8″ dedicated bass) and ditched the subwoofer. I have a similar sized room. Plenty of low end for me.
The servo system was electro-mechanical. I’ve been using the Devialet SAM system for the last 5 years. It corrects the bass using software, based on the measured performance of the speaker, which you download as part of the system set-up. You can turn it on and off from the remote or dial it in from 0 to 100%. This is separate from Dynamic EQ software that allows you to adjust performance for the specific room.
There are now systems that use remote sensors to monitor the bass in the room and feeds it back to the amplifier and adjusts the sound.
I don’t think there is anything wrong in principle with what the servo woofer was trying to do, it’s just a rather old-fashioned way of doing it, and there have been software solutions for some time. The software solutions are preferable because the servo system can only take account of the speaker, when the main issue is often the room. Of course the software can be applied to almost any speaker.
As far as I understand the goal behind Devialet’s SAM correction mode it is about getting a near perfect step response meaning “phase-aligned” drivers. This of course also yields in better bass performance. However there will be still some efforts for getting the speakers set up for optimized in-room response including room treatment.
There is separate room correction parametric EQ facility called Sweet Room. You have to measure the room with REW, put your chosen settings into a file and copy it onto your system SD card. It’s similar to the parametric EQ in Roon, but you can have two sets of settings, there is more flexibility and you can switch between them and turn them off from the remote. You can switch on/off in Roon, with a break of a few seconds as it resets.
This has been around for ages, and there are more aggressive systems like Dirac, but it’s nice to have in one box and for no extra charge.
I have some significant problems with Paul’s description. I can’t understand the difference between “cleaner” and “slam and impact”. I am not sure if they are measurable. Cleaner probably deals with distortion, but I don’t understand if slam and impact are the opposite of distortion or what? Again, Paul uses ambiguous terminology that makes it difficult to understand what he means. Every person will use their own interpretation to those words.
It is absolutely irrelevant if the woofers have servos or not. What matters is frequency response, distortion and impulse control. And performance in YOUR room.
My husband is going crazy trying to “tame” the room’s bass nulls and peaks. He uses REW to try this. Now, as Steven says, speakers are designed to “see” the room and provide the proper adjustments. It is “measurable”. DSP into active speakers is the best way to deal with bass issues in rooms. If Paul “tames’ his speakers based on his room, how do we know if they are going to work in ours?
Madam, them’s fightin’ words!
“It is absolutely irrelevant if the woofers have servos or not. What matters is frequency response, distortion and impulse control. And performance in YOUR room.”
🙂
Seriously, you’re correct on the basic needs of a woofer system, but there’s a very big difference in woofers having or not having servos and THAT is easy to measure. For example, distortion is typically reduced by a factor of 10 and step response is quite noticeably better. One can get close to a 20Hz square wave response from a woofer with a proper servo where without it, it’s more a sloppy sine wave.
Now, I will agree that you can get close to the same performance by means of EQ, but not entirely. And it all sounds quite different.
Lastly, one of the problems is implementation. In days past at Genesis when I did the designs for the servo systems (and BHK did the designs for Infinity’s servo systems), our fearless leader, Arnie, would use cheap woofers. Why? Because the servo system “corrected all measurable ills”.
So part of the problem here is comparing apples to apples. In the new FR30 series we went the opposite route, designing a state of the art low distortion linear woofer system with crazy motor systems etc. to accomplish our goals. This is very hard to then compare that system to a servo system based on a cheap woofer.
In any case, it’s late and a tire.
But then you made my point. And thanks for that. You reached the same “sound” by improving the speaker’s design. As I said, the way you get there may not be important. You said that servo was used because it wasn’t a good speaker so you designed a better one. I’m sure the new one has lower distortion than the old one and this is why you didn’t need the servo. I also seem to recall that the perception of distortion in the low frequencies is much worse, or we can tolerate higher distortion without noticing it. Correct?
I still don’t know what is slam and impact.
What troubles me of your posts is the ambiguity of the terms you use. For example, you now say “sounds quite different”. What does that mean?
Sorry Paul, I should have clarified that one deals with the inherent quality of the sound produced by the speaker (design, servo or non servo) and the other deals with the interaction of the sound with the room (DSP and EQ). I think that with “adequate” speakers, the room has a much larger impact. Of course, it is better to start with a good speakers. My apologies for my “ambiguity” this time.
I would agree the room, by far, has the biggest impact. No bout a doubt it.
I know you’re unhappy with audiophile terms and their ambiguity. I understand. If I am trying to describe the way two kinds of chocolate taste I have at my disposal ambiguous words or measurements (or maybe both). Same with HiFi.
Here’s the thing. I write for the main group and they understand what slam, pacing, impact, and all those words mean. Were I to change my style and instead write that the step response was improved from 1v/uS to a whopping 3v/us, the vast majority of my readers would scratch their collective heads and say, “gee, whatever the f’ that means I guess it must be good.” The few percentage of engineering nerds who actually understand that and can then iterpolate that into more slam and quicker transient speed are happy while the rest are left in the dark.
As a writer I cannot please everyone and I have only limited time. I understand the perfect scenario would be for me to write how the step response improved from X to Y and then that means greater slam and impact, but that would then require me to spend more time in the lab measuring all this shit that honestly I don’t care about.
I can hear it, I have 50 years of design experience to understand how to affect it in the system and circuits, so that’s what we do. Our design team is attuned to this and it works well.
So, let’s just suggest I stick with the 80/20 rule. If I can effectively build for and communicate with 80% of my audience then I am quite happy. That ain’t nuthin’. 🙂
OK, you claim to have 50 years of experience but you can’t explain to me what “slam” means. Even the ambiguous wine industry has developed a vocabulary to explain flavors and smells.
Let me give you three examples of very low frequency and maybe you can explain this to me.
Listen to this sort of loud (it is not my type of music):
https://www.youtube.com/watch?v=inHT_huiPL4
Then you have this one:
https://www.youtube.com/watch?v=EPt8xVyV6K0
These two pieces have bass that truly fills the room and you feel it in your chest. But they have limited impact. They appear as “pure” tones. Artificial instruments and only some pipe organs can reach this low.
You then have the bass drum at the end of Firebird (no except here). This one may not go as low as these modern pieces but the “impact” is provided by the higher frequency harmonics of the drum. You feel more because of the higher frequencies.
All three pieces generate enormous pressures at home when played loudly. And you need really good woofers to “hear” them.
Is this what you mean by slam?
Diavalet seems to set extreme effort in SW solutions.
When researching about the mastering of their Fondamenta digital and vinyl releases (I own a few of them), I experienced strange behavior 😉
Their mindset is to even digitally modify when a snare drum resonates from another instrument playing in that same recording. Independent of what one thinks about extensive digital processing like removing noise etc. generally, IMO this is a kind of “optimization”, which goes too far into the recorded art itself. But it opened a view into their thinking about “improved sound”.
SAM does what a servo tries to do.
Sweet Room is very similar EQ to what you find in Roon and some other devices.
I have some Fondamenta recordings and they have not been cleaned up as you suggest, quite the opposite, in particular the Gilels set, but then he plays a piano not a snare drum.
Their phono stage has a digital noise reduction system called RAM.
So they are big on timing and noise issues, but not shaping the sound. For that you need a valve amplifier.
My first audition demo track is Sonny Rollins “Old Cowhand” and I had no issues with the percussion, Devialet into Wilson. I use a lot of percussion test tracks, the same ones when I bought Devialet and Wilson. I really don’t find it a problem.
I don’t think Devialet is any different from the vast number of other software-driven products, from dCS downwards, or the Bluesound system I’m listening to now.
I didn’t hear if or where they did such modifications,, I just read Fondamenta’s description of how their “Phoenix Mastering” SW works.
I have two vinyl sets, the Dizzy and the Bill Evans as well as several as digital hires versions of their albums. I like the Dizzy and compared the Evans to the same release on Resonance Records. The Diavalet sounded thinner, which was a main difference and in this case didn’t help the anyway less good source recording.
What I just want to say is that Diavalet/Fondamenta certainly seem to be strong supporters of extensive digital processing (within music production as well as hardware concepts), which I’m not a big fan of I admit.
I mainly bought them because I wanted to get the otherwise unavailable recordings. I believe they are mostly Dutch radio tapes. I heard Gilels live shortly before he died and wanted some more.
My apologies for listening to the music rather than the sound! The Dutch were (are?) heavy smokers, and you can hear it on some of these recordings.
I bought them for the same reason…that they all (on either label) sound not as good as many officially released albums of the era is inevitable. But my topic here was the lot of digital processing of Fondamenta and the audio quality of their releases compared to the exact same one of other labels.
Parametric room correction fails for more than one seat, and for problem rooms. Individual bass frequencies can vary +12dB and -20dB in extreme cases, and they also vary this much from seat to seat. Where there is a bass peak, it takes several cycles for the buildup to occur so the steady state response is different from the transient response so you need several seconds of latency to include the full time response. Further, getting that 20dB hole up to full output takes a hundred times the amplifier power, likely resulting in driver distress. (I’ve seen fire and I’ve seen smoke, and drivers that never once more spoke…)
If you are serious about accurate bass, you have to tune the room acoustically, which usually involving architectural level mods instead of mail order products.
Oh no, I hope he never reads this. I tolerate his coffin sized speakers and some room treatment, but after all, it is not only about his “stereo”. It is supposed to be a multiple purpose room.
Don’t worry about it. I don’t use EQ even though it is available. As for bass, as long as drums don’t sound like you’re hitting the side of your head with a box of Cheerio’s, there’s not too much to worry about.
I’ve got a perfect solution as our new room is I hope going to be rebuilt by a builder who has just done a recording studio, plus she was until recently a he, so she can see both sides of the equation.
The furniture may well have legs with wheels, so I can shuffle around for the sweet spot.
Well, he thinks I am a Tomboy intruding in this old boys club.
We actually negotiate sound quality and decoration. I “usually” win but I do let him test a lot and then discuss options. We do reach a good balance, even if he wants to add more stuff. I “see” the issues but I can’t say I can “hear” them to the same degree. Occasionally, you can tell that some bass notes are too loud while others appear less than normal.
Cheerios? He eats porridge, result of his mother’s English family side.
Only if his mother was Scottish.
When my wife chose the speakers last week, at 12:47 or thereabouts she said “If we’ve decided to buy them, why are we still listening to them?”. She had a point. Men like the audio journey, women prefer lunch.
Lunch and a nice glass of bubbly.
Family is from near Chester.
That’s where all the Diva’s come from. He might have to explain that to you.
🙂
Steven, nice word play, but when did it reach its Apogee.
In engineering you have to be willing to let go of preconceived notions because what seemed true before may not be true now. Iy’s not an easy thing to do, it’s one of those lessons I had to teach myself over and over.
” …Servos did indeed produce cleaner bass but, they argued, at the loss of audible slam and impact…”
There is no free lunch !
Perfect does not exist.
So the descriptions in this post make me wonder if the servo design possibly sounds better playing acoustic stand-up bass guitar – and the non-servo excels at reproducing the kick drum? If so, my novice mind asks – can you achieve the best of both worlds by having both? Each cabinet having two different bass drivers – each with its own unique skill set. Or does that introduce overlap issue that do more harm than good?
And bravo! Your discovery illustrates the beauty of teamwork – and having engineers on your team that are also musicians provides great perspective!
NO!
The universal problem with spring suspension bass drivers is the inherent resonance, which adds an off-key note when the bass starts and/or stops abruptly. They are fine for Organ pipes and movie sound effects (which are mostly synthesized and have no real world equivalent), but for bass RHYTHM instruments they all suck. Bass speakers with secondary resonators (Passive radiator, vented, bandpass) suck more, and the WORST of all is the Bose WAVE(TM) which has three resonance frequencies. Servos can’t fix this time distortion/spectral contamination. See below.
Hi Paul,
Please send my regards to Darren and Chris. What I have found is, thus far nothing else has the slam and weight in a bass system as does the Newtonian forced-vibration canceling technique. Looking forward to the day I can finally come out there to visit with you all. Perhaps Summer, 2021. Best regards, Scott Freimann.
When I first added a REL sub to my floor standers I was very pleased with the results, not just in the bass but to the overall sound just as REL had described. Much as I like bass I didn’t want it to over power the sound so tuned the sub to have a subtle influence, again as REL recommends. Later I added a second sub, not twice as good but a further improvement.
I now have larger floor standers which definitely go deeper and I have started to wonder about the influence and necessity of the subs but it’s hard to let go of the very positive impression they first created.
So talk about shaking long held beliefs, the easy thing to try is turn them off and listen without but even that is proving a difficult step to take. Hopefully, mentioning it here will act as a kind of therapy and I’ll give it a go…..tomorrow.
… really keen to hear how you get on Richtea.
Paul unwittingly put a lot of pressure on me to retry with sub(s), I’m still glad I resisted this…
Alan, I’ll try to remember to mention it then. The ‘tomorrow’ was slightly tongue in cheek as in tomorrow never comes. I’m actually listening now, did think about, but then stuck with what I know.
Yes, after a reread, I wondered if ‘tomorrow…’ was mañana
… sounds like subs are for you, which is fine, there’s a lot of them about and lots of folk can’t be without them.
… “long held beliefs” are just that, long held 🙂
Alan, later that same evening….I wouldn’t want to be without my subs.
I decided to give it a go otherwise it was likely to get lost in time. Firstly I listened to a relatively unfamiliar track without the subs and was initially surprised that not so much was missing as I expected. I think sometimes the brain automatically inserts what you heard before, subliminal psycho acoustics or something, but that’s another topic. Next I tried much more familiar tracks with no excessive bass, the subs still off. They sounded good but a little thin. I couldn’t get over the feeling that something was missing. Was that because I knew the subs were off. I don’t think it’s always that easy to pin point exactly what’s missing, most of us can listen to lower quality sound and find it acceptable, it’s still the music that we enjoy. Turning the subs back it was obvious what had been lacking. To try and put it into words it had lost weight and texture and remember this is music without a lot of obvious bass content, but now the low notes had returned where previously they’d gone missing. I found, like fans of subs will often say, they underpin the music, subtle low level and low frequency resonances, things like that. For information my main speakers go down to 27Hz and I have the subs set to roll off at 23. No pressure Alan, I think you ought to give the subs another go. On the other hand, if you’re satisfied with what you’ve got why change? Hope that helps, but I may have just caused an itch which is going to need to be scratched.
PS. BTW I hadn’t read your 5.16pm comment when I posted this.
Some people here have a pretty assertive tone… 😐
NO ! I don’t like that. I decide for myself what SOUNDS best. Period.
I find it ironic that while Darren was an immediate opponent of servo woofers, he used massive negative feedback on his first try for the Stellar Phono Preamp.
What I love about observing this industry is how committed people are to ideas. They hold tenaciously to certain truths like priests telling the gospel and then they see the light and switch religions, sometimes with a 180 degree U turn.
https://www.youtube.com/watch?v=vtLYJ4STuXY
Servo good. Servo bad.
Preamplifiers bad, preamplifiers good
Line arrays good, line arrays not necessary
Tubes bad, tubes good … sometimes in some places.
IRS killer good. IRS killer replaced by FR30
Class D good, class D bad, class D good
Were you lying then or are you lying now?
When did you stop beating your wife?
Where were you on the night of the 24th? Got any witnesses? Oh so you were home alone watching TV. What program?
What were you doing with that other woman I saw you with?
Lipstick on your collar.
Get away from my iphone, those are private messages.
Now I think the sofa looked better where it was. Put it back honey…. please. Yeah, and the breakfront too.
Congratulations! You have not understood my postings on this topic, you had to hear it for yourself. I told you how to set up the tests many years ago. My first experiment with a low resonance woofer (back wave damper) was obvious to non-audiophile family and friends.
This problem was what started my speaker design efforts. I used to play bass guitar, but I was having trouble copying a lick of bassist/producer Liminha on Jorge Ben’s ‘Pega Ela de Montao”, until I made an attempt after midnight when I had to turn the amp off because of thin apartment walls. Suddenly it sounded right coming from my un-amplified Fender Jazz Bass, and I realized that all bass speakers suck. I took my bass to Anatel where I was a design consultant and plugged it into a DSO to verify that when I played it right, the note was an integral number of half cycles, and all half cycles were full voltage from first to last.
Therefore, a proper musical test signal is a rectangular synchronously gated sine wave (tone burst). My amateur fingers created this waveform so you can’t argue it does not exist in music (as did Linkwitz), and I found it in a a very popular track played by a solid gold producer. In fact, all rhythmic bass instruments where you load energy into a store and release it to start a note have a rectangular gated front end – percussion like bass drums and plucked instruments where you stretch the string and release it. Skilled wind players can also produce percussive attacks.
All dynamic drivers have a resonance at the low frequency corner. The air coupling decreases proportional to ratio of diameter to wavelength, so the amplitude doubles for every octave drop in the inertia limited range to maintain flat frequency response. This rolls off when in the compliance limited band, and at the transition frequency stored energy builds up in the resonance, shifting from kinetic energy of the mass to potential energy of the spring and back.
A rectangular gated sine wave is like an AM modulated rectangular wave, and it has a response to the rectangle wave step of a damped sine wave at the box resonance frequency or frequencies at both the leading and trailing edge of the rhythmic note. This is anharmonic (off-key) and anarhythmic to the music, producing “muddiness” and reducing PRaT so your toe won’t tap.
To compensate for this ill effect in studio monitors (vented boxes as a rule, with two resonances), producers and engineers make frequent use of the low cut filters on the mics and preamps, use cardioid and supercardioid mics with attenuated bass, limit and compress the bass, etc. so that only organ recordings have flat response below 80Hz! (Greiner & Eggars, JAES 1989) (OK, this pre-dated hip hop which can have exagerated bass, but rarely if ever below 60Hz). Studio bassists also learn playing techniques that compensate for the time distortion. So as a rule, tone bursts are edited out of recordings.
The classic Fender Jazz Bass Guitar has the neck pickup at about 22% of open string length, which attenuates the fundamental over 10dB wrt to the harmonics! Further, the classic Bassman twin twelve and Dual Showman twin fifteen cabinets have a sealed box response that rolls at 12dB/octave below 80Hz, as do the Ampeg 8×10 SVT cabinets.
Even going back to orchestral instruments, the string Contrabass, Concert Grand Piano and bass drum have radiating surfaces less than a wavelength in circumference so the bass fundamentals are attenuated acoustically.
But back to the servo question. The resonance has a phase shift of over 360 degrees, as the damped sine wave response is more than one cycle. Trying to reproduce the modulation waveform step transient accurately also requires extraordinary voltage to overcome the voice coil inductance and moving mass inertia. Remember, rhythmic instruments go from zero to full pressure slope instantly, turning the corner hard. The phase delay from stimulus to response further makes feedback loops go chaotic.
This grossly audible time distortion can’t be fixed through conventional feedback because you are trying to create “negative time”, counter-acting events that haven’t occurred and are too sensitively non-linear to predict.
So what does fix the PRaT?
1. Lower resonance bass system: Back Wave Damper/Absorber (Nautilus), Aperiodic, Dipole, Infinite baffle, Bessel sealed box, Butterworth sealed box, Passive Radiator (counter-intuitive that 5th Order system has wider flat phase bandwidth than vented box)
2. Low Qms – suspension damping
3. Lower Le
4. Higher BL to Mms ratio (higher efficiency)
5. To a small extant, lower amplifier damping factor and thicker/shorter speaker cables.
I also have an idea for a patentable circuit that works in simulation, but the implementation is beyond my current capabilities.
Paul, doesn’t your IRS System include servo woofers, at least originally? If so, have you now overridden that for bass you like better?
m3 lover,
FYI…when Paul noticed that some of the 12″ drivers in his IRSV’s were starting to disintegrate he replaced them all with 12″ drivers from ‘Dayton’ & at the same time Chris Brunhaver completely removed the servo systems & reworked/redesigned the crossovers between the sub towers & the fronts. This happened about ten months ago, from memory.
Thanks Fat Rat. I do recall his mention of replacing the woofers but didn’t remember discussion of the servo system or crossovers. That makes sense given his comments today.
You’ve got a good memory FR. That’s why I have to be careful what I say 😉
Rich,
As long as you’re not still jumping to conclusions for your cardio work-outs my friend 😉
(Seriously though, I am amazed at my memory recall considering all the high-grade marijuana that I smoked over thirty seven years…it’s astounding!)
FR, It’s not been forgotten but I’ve reduced my exercise regime, now it’s just running through my mind.
Oh, touche! 🙂
“Servos did indeed produce cleaner bass but, they argued, at the loss of audible slam and impact.”
Could some of that some of this “slam” and “impact” is actually group delay (phase vs. frequency) variation in the area of the bass resonance, or ringing in the response. Both of which are related and could be much less in a well-designed servo system, which extends the bass cutoff to well below the audio range. And by doing so, resulting in a flatter group delay and less ringing in the bottom two octaves of the audio range?
This recalls a subwoofer design I did a while back which was not a servo design but included a line-level network to extend the bass cutoff two octaves lower. It is a sealed design with several drivers and a (non-eq) cutoff at 45 Hz. With with EQ network, the -3 dB point was 10 Hz. With rock and jazz program material, having transient bass energy in the 40 to 60 Hz range, there was actually a perception of *more* bass without the equalizer!
Yes, group delay and ringing are alternate descriptions of the universal bass reproduction problems. In terms of time distortion there are a few engineers that consider phase distortion, but then they ignore phase wraps. You have to look long and hard, even with the assistance of Herr Google, to find cepstral measurements and rectangular sine bursts are more arcane.
Back the 1960s tone burst measurements where fairly well known, General Radio and Rhode & Schwartz made tone burst generators and Harry F. Olson called tone bursts “the single best measurement of loudspeaker quality”, with a graphic depiction of tone burst testing on the dust jacket of “Modern Sound Reproduction”.
Resonant speaker behavior tends to be too high Q with the damped transient response extending several cycles, and often conflated with box resonances of vents, transmission lines, etc. The voice coil resistance prevents loading and unloading the energy stored in the resonant system fast enough for wide bandwidth, waveform-accurate servo control.
It was nearly 25 years between the time I had the analytical tools of Newton’s laws of motion and the time I applied them to understand how the mechanics of woofer/enclosure systems actually work. This is the classic mass, spring, dashpot problem applied to forced oscillation. There are three factors that matter, moving mass, springiness, and damping. Of all the designs the only one that has springiness and damping independent of frequency and amplitude is the acoustic suspension design invented by Edgar Villchur in the mid 1950s. It was an intuitive invention which even he didn’t fully understand. The design has other advantages too. Its one disadvantage is the tradeoff of low efficiency for better low frequency performance. There are other designs now that are even less efficient and don’t work nearly as well.
As far as I can tell there are no woofers on the market today optimized for this design. The best of them are compromises for use in other designs as well. A true acoustic suspension woofer has a long throw, very high compliance, a high mass cone giving a very low free air resonance. If the speaker isn’t in a suitable air tight enclosure it can easily be damaged by bottoming out due to lack of air pressure as the primary restoring force. The invention also introduced the roll surround as a necessity. Up to that point all woofers used an accordion pleated surround. So the current designs like Dayton RSS315-HF are not optimal for this design. Its recommended sealed design has an F3 of 53hz which is much too high. However, due to the 12 db per octave linear falloff below resonance it can be equalized to play at much lower frequencies without distortion using more power within the mechanical excursion and thermal limits of the driver.
The design has inherently low harmonic distortion due to the uniformity of the restoring force on the cone resulting in no twisting or shearing forces causing the cone to break up into harmonic modes like mechanical suspensions do. Servo control is not necessary due to the driver’s inherent low distortion and linear and equalizable frequency response. Servo control like negative feedback is a useful tool for those who know how to use it correctly in applications where it is of value. Used incorrectly it can cause far more problems than it was intended to solve including uncontrolled oscillation and even destruction. The idea that more is better there is wrong. It is an error correction system by introducing the same error at the same magnitude but 180 degrees out of phase. Add too much and you are adding distortion back in. Don’t expect most designers in this industry to get it. The course is so difficult it is designed for senior students and masters degree candidates in electrical engineering. Lots and lots of calculus and much more math.
I went from Genesis V’s with the servo to Green Mountain Audio “Imagos” without it.
I love the “Imagos” transmission line bass port.
They are time aligned and phase coherent 4-ways using only first-order cross-overs.
I completely re-did the speakers including re-wiring twice, finally with Dueland’s four nines silver ribbon wire with a natural silk
dielectric embedded with oil at high pressure.
The external cross-over boxes, the fad of the era, were moved to the bottom of the speaker, upgraded and hardwired in.
They contain a King’s ransom of Dueland (Denmark) and Audio Consulting (Switzerland) caps & resistors.
I wasn’t aware that external cross overs had been a fad but thought you might be interested that it is still offered as an option by ‘Living Voice’ on the top model in their ‘Auditorium’ range, the OBX, O for outboard.
Looking at the pictures it’s easy to see why it didn’t gain wider acceptance. Another two boxes to accommodate, and that’s before you bring on the subs.
It turned out the external boxes had very poor connectors and wire, which were really hurting the signal, the difference was pronounced!
It can be a hard thing for us old guys when one of our long held audio beliefs comes into question. I have very recently found myself at a similar crossroads, and it’s all your fault Paul!
I have long been a big fan of tube pre-amps, and particularly phono stages, especially when combined with a good step-up transformer for MC’s. I always seemed to prefer them for their smooth, rich sound and extra large presentation. This ease has meant that I can listening for hours to my rather large and still growing LP collection. My systems are certainly not state of the art, as I live on the east coast of Canada and the nearest real Hi Fi store is about a thousand miles away, but they are none the less very good and quite satisfying. I tend to buy used gear online to audition when the upgrade bug hits. This way, if I am not totally happy with my latest purchase, I can usually move it on at minimal or no loss. I also have reasonable DIY skills, so I often do some mods to gear like capacitor upgrades, etc. so that can generally get me close enough to the audio nirvana I seek.
My latest foray has been to take a firm step into the 21st century and start archiving some of my LP collection to 24/96 FLAC files. I first used a couple of pro audio A to D’s, which yielded reasonable results, but were often glitchy or just plain inconvenient to use. After a little internet research, it seemed that the PS Nuwave Phono Converter might be just the ticket. It took me several months to find a used one online, (people don’t seem to want to part with them, which was a good sign) but one showed up on US Audio Mart, and I immediately pulled the trigger.
Despite the high praise the phono stage of the NPC received in reviews, I just assumed that my beloved tubed phono stages that I had spent years tweaking and optimizing would be better, and I would just use them into the line input of the NPC. (For reference, my usual set-up is a re-tipped Shelter 502 or some sort of DL-103 derivative on a Roksan Xerxes/Artemiz into a Lundahl SUT into either an ARC SP-16 or SP-8, both of which have been endlessly tube rolled and tweaked)
Well, my brand new used NPC arrived last week and I eagerly set it up in the system to give it a go. Before I got into doing some vinyl rips, I thought a little A/B between the NPC and my SP-16 would be fun. It was fun, but not in the way I thought.
Simply put, the NPC phono stage was better, and not just by a little. It was cleaner, quieter, more detailed, and threw a bigger soundstage than the ARC/SUT combo. The bass in particular was much better with more weight and articulation. I was blown away, to use a tired adage, and it’s been a very long time since I’ve felt that way about any component. I totally forgot about the vinyl rips, and just started playing record after record, every one of which seemed to reveal some little nuance that I hadn’t noticed before. I seem to have a new reference phono stage! I would have gladly paid what I did for the NPC just for the phono improvements. The fact that I now also have a near state of the art A/D to play with is just the cherry on top.
It can be challenging when some of our very long held beliefs come in to question, but in the famous words of Red Green, “I’m a man, but I can change, if I have to, I guess.” 🙂 In the end, it’s whatever brings us closer to the music that is important, and the type of technology that gets us there is only of secondary concern.
Thank you for producing such a wonderful product in the NPC. It will likely have pride of place in my system for a very long time to come!
I wish you and all the folks at PS Audio the very best of the season, and let’s hope that the new year brings in happiness and joy to overcome the madness we’ve had in 2020.
Cheers!
Phil Griffin
Prince Edward Island
Canada
I don’t know if you heard, but Tim de Paravicini died, and he was one of the most brilliant audio designers who ever lived, Very Sad!
The way I see it the servo mechanism works all the time to control the movement of the cone. This reduces the cone’s excursion which results in comparative lack of slam and impact. But it also loses something called the “thrumm” factor which results from very fine movements of the cone which the servo inhibits. Thus the bass is tight a little on the dry side , a little unnatural and of course lacking in full force perceived as slam as mentioned before. Remove the servo and the cone movement is restored to full excursion, the slam returns and with it the “thrmm” which makes the bass sound natural. Servos were used to reduce cone hangover but with current speakers with very strong magnets and excellent cone restoring mechanisms servos are not necessary. Speaker hangover is little and can be controlled by careful cabinet design. Well made infinite baffles are the best. Bass reflexes are quite prone to hangover no wonder their wooly bass comparatively speaking. Something similar happens in analogue front ends. Using record weights and clamps over dampens the system and sparkle is lost. One can get used to it but music is a little dead sounding. But there is a place for servos and record weights and clamps. This is in speakers of poorer quality and comparatively inferior frontends with resonance problems irrespective of price. clamps and weights also help with poorly recorded vinyl where too much reverberation is added to the recording. The bottom line is do not over dampen a system. Regards.
WOW!!! This has to be one of the best bass discussions I have seen in a long time. I am sorry I missed this thread yesterday, but things were a little crazy here. I will keep this short. After much listening mostly between Wilson Audio Alexia’s (2) and Magico S7’s I choose the S7’s because drums sounded more life like on them. The S7’s have three 10″ woofers per speaker where as the Alexia’s have an 8″ and 10″ woofer per speaker giving the S7’s 55% more woofer cone area. To compensate the Alexia’s are ported and the S7’s are sealed enclosure. Being ported allows the Alexia to produce a bigger bass sound that one would expect from the small woofer cone area. However, the porting is what, in my opinion, hurts the Alexia’s bass performance. It makes the speaker’s bass performance much more room and room placement dependent and the bass is not as crisp as the bass from the S7’s. My room is 30′ x 27′ x 11′ and I use no room correction of any type. As always YMMV.
Great attitude and one I keep close in this lovely hobby of ours. “Don’t knock it till you’ve tried it.”