Dustcover Blues

You say this whole problem came to light while you were listening to Herbie Hancock’s "Secrets". How does that square with your inference that my own preference for jazz ("old" and new) limits the demands I place on my systems? I also listen to classical orchestral and chamber music, big band jazz, latin or afro-cuban jazz which is usually big band with lots of drums of all types, and I love R&B, which I do like to play "loud", also to certain pop artists where the sonics can be very demanding at low frequencies if you want to feel satisfied. I would also say that dynamics of any kind of music, not just heavy duty bass notes, are what places stress on our audio systems and is a real determinant of how close the system can come to sounding real. But I admit, I will never ever want to listen to "Soundgarden’s Badmotorfinger" at any SPL, let alone 100db.

You also wrote, "Now for the juicey part. I put felt strips under the dust cover to seal it when closed and..... the feedback got worse. I hate when that happens:-( " So much for the Helmholtz Resonator theory, which I couldn’t buy in the first place.

My speakers are 4 inches wider according to you; I don’t know the dimensions of your SLs, so I cannot comment. But if the difference in size between your panels and mine is only 4 inches in width, that would equate to a nearly 400 sq in per channel difference in surface radiating area, since my panels are 8 feet tall (96 inches). If your panels are also that tall, I was not aware of that. If they are less than 8 feet tall, then the difference in surface area between mine and yours would be greater than 400 sq in by that height difference factor. I also use tube traps and wall-mounted absorbent panels tuned to low frequencies behind each speaker, in order to absorb as much as possible of the rear radiation, so as to minimize bass cancellation. Furthermore, I have 100 lbs of lead weight sitting on each SL backplate, so as to minimize the tendency of the panel to sway when asked to reproduce low bass. The positive effect of those weights was immediately noticeable in terms of bass definition. (This is why I completely disagree with anyone who wants to put soft spongy feet or springs under any speaker.) Anyway, you don’t need huge panels because of your predilection for electromagnetic woofers (not "subwoofers" if used up to 120Hz). I am not for one second claiming that my SL system can compete with yours for bass SPLs that can blow your head off below say 40-50Hz, but I do claim that my SL speakers get all the music at low frequencies.

@lewm , my SLs are also 8 feet tall. They are a custon Job because the full width 845's were just too wide for my 16 foot wall with the theater screen. According to Roger West bass performance is the same but I am not so sure about that. If you want to "stabilize" the speakers you have to add weight to the top of the speaker. The bottom is fixed by the floor and the interface. Sound Labs speakers will make bass. I never said they did not. The problem is at higher volumes vigourous bass will distort everything else. It is a phenomenon I have noticed will every single ESL I have owned. Taking the bass out of them cleans things up and I am sure you would notice the improvement if you would try it. Unfortunately, analog crossovers do not work near as well as digital ones. 18 dB/oct is too slow. You will be getting subwoofer up into your midrange where it certainly does not belong. 

I had forgotten when I switched to the Sound Labs I changed the crossover to 100 Hz 48dB/octave. That really does not matter. The drivers being used are specifically subwoofer drivers. There is a marked difference in parameters between subwoofers and woofers given the much higher x max of subwoofer drivers.

I hate to be a stick in the mud Lew but, you have no idea what your system is doing unless you measure it. Throwing tube traps and other room mods in an empirical fashion depending on what you think you are hearing is a sure fire way of screwing things up. I know, it is your system and you are entitle to screw things up any way you want.

The helmholtz resonator theory is in no way shape or form dead. What would you venture is boucing the sub chassis at 24 Hz? That is not it's natural resonance frequency nor that of the tonearm. Something else is resonating at 24 Hz and it certainly is not the granite the turntable is sitting on. Look at the design of the RH Labs subwoofer. The enclosure is described as a helmholtz resonator. At any rate I think I know how to stop it. My brother said to think of sound waves as waves in the ocean. What would happen if the turntable was rhythmically immersed in water? What he was getting at is you have to keep the water (air pressure) from getting into the turntable. 

The easy thing to do here is just keep the cover open during play. Never take the easy way out. It is not sporting and you never learn anything. I do not think there can be any question that a closed dustcover keeps dust off the record. I have also demonstrated that a closed dust cover does attenuate sound by as much as 10 dB (not 15 dB) at certain frequencies. 

@secretguy , that is common mythology. It was perpetrated by manufacturers that can't or won't add a dust covers to their tables. It is very much like the tube vs solid state and  Analog vs Digital arguements. Most people with dust covers will tell you that they can not hear any difference dust cover up or down. I do not know for a fact whether or not I can. Once I get this straightened out I'll see if I can set up a blinded AB comparison. My own mentality is to protect my records first and work on the sound second. 

@tomic601 , very interesting point tomic. Yes, it has the magnetic bearing. But, it is the whole subchassis moving. There is no give that  can feel in the thrust bearing. If it has a resonance it would be way up high. 

@rauliruegas , whenever you get new equipment you can run into teething problems like burning up the high frequency balance control in your brand new $45,000 speakers and this one with the turntable. This is where the hobbiest approach comes into play. What would life be like without the occasional challenge? 

Thinking about time can get very depressing. Keep yourself busy having fun. This is just one way to have fun and a good one at that.

Golly! I am so glad to be instructed by you after only 50 years of messing about with audio.  First, I wouldn't argue for a moment that my SL experience would be improved if I were to add SUBwoofers to the Sound Lab system.  (In other words, woofers that come into play at below 50-60Hz and never above those frequencies.) You are absolutely right about that. I already explained to you privately that I have made a choice not to do it, partly because the system is in our living room, which is already too dominated by audio equipment.  But if I did do it, I would never add a woofer that comes in at 120Hz; there is too much real music at that frequency and down below to 40-50Hz. Your 12-inch electromagnetic woofers, impressive and loud though they may be, can never match the speed and articulation of an ESL at 100Hz.  But that's just my opinion based on some prior experience.  The only woofer I would even think of pairing with an ESL at those frequencies would be one based on a transmission line enclosure using a much faster woofer than average.  With the Beveridge system (in my basement), I am not constrained by decor, and the 2SWs do require woofer supplementation; that's how they were originally marketed.  There I do use KEF B139 woofers in a large and ungainly TL cabinet that I built myself decades ago.  But even there, the crossover, as determined by Beveridge and not me, is at 80Hz.  Likewise, the slope of the hi-pass filter is Beveridge, 18db/octave.  Consequently, I use an outboard electronic crossover for the woofers that has a complementary 18db/octave slope. If I had my druthers, I would use the Linkwitz-Riley slope of 24db, because that maintains phase between the high an low frequency reproducers. The woofer x-over has controls for level and frequency, but I keep it around 80Hz.  Finally, there is no free lunch in audio. Every electronic x-over I have ever heard has a "sound".  I've never heard a digital x-over, but I am sure it has a sound too, not to mention the extensive amount of digital room correction that you have chosen to incorporate.  I admit to being almost too pure a purist, but I like to avoid x-overs, both passive and electronic, if it makes sense. This is all fine. The goal is to satisfy onesself.  I would never be so bold as to sit here and tell you what you are hearing over there. Perhaps you should adopt the same approach.  Sorry for the very long digression, but you raised my hackles.  I didn't know I had hackles.

Helmholtz Resonator.  Every definition I can find, including the one on Wikipedia, states in one way or another that you need a closed container with a large hole at one end and a small outlet at the end of a narrow neck at the other end.  Apparently Helmholtz built several of different sizes to demonstrate how size of the enclosed volume of air, neck length, and aperture area all determine the frequency heard at the small outlet. Based on what you wrote, it seemed to me that your problem persists after you seal the dust cover to the plinth surface.  Thus there is no pathway for air to go in or out. So, you cannot have a "Helmholtz Resonator".  For sure, you do have a resonance problem, just not the one envisioned by Dr Helmholtz.  Where have I gone wrong in this reasoning?  I think you may have the forme fruste of "dust cover blues", as you so aptly put it in the first place.  In other words, you have an extreme and unusual problem that nevertheless falls under the category of why some of us eschew the use of a dust cover when playing LPs.

@lewm , thank you Lewm. It is a pleasure instructing you:-)

As I mentioned before I dropped the crossover frquency from 120 to 100 when I got the Sound Labs as they have so much more surface area than my old Acoustats. Given you have even more you could try a little lower maybe 80 Hz but I would never go lower than that. If you have a digital crossover you can cross up higher and still keep the sub out of your midrange with steeper slopes. There is no comparison between digital and analog bass management. They are totally different worlds. If you replaced the crossover in the Beverages with a digital one you could cross up a little higher using a much steeper slope without phase issues. Would be fun to try. 

Lew, the speed of a driver will dictate it's upper frequency limit. Most subwoofer drivers, even the 18" ones are capable of reaching 1000 Hz before cone breakup.The electromagnetic forces generated by ESLs are several orders of magnetude weaker than a modern subwoofer motor. It is not a speed or transient response issue. The problem is matching the subwoofer up to a dipole ESL. The very muddy midrange that a sub produces clashes with the incredibly detailed midrange of the ESL. If you turn off the main speakers and just listen to most subwoofers you will clearly hear some midrange leaking through. This is made even worse by timing errors. This is why early on we were keeping crossover points so low, to keep the sub out of the midrange. The trade off was decreasing distortion in the ESL. The higher you cross, the less distortion you get out of the ESL. It was digital bass management that finally solved this problem.

I still have some issues with the subs I have now. I think there is some distortion added by the enclosures and although they will go very loud I think they could do it with less distortion from the drivers themselves. I am working on new enclosures and will use 8 instead of 4 drivers which will halve their excursions. 

There are different forms of helmholtz resonators. With the dust cover closed you have a sealed system with a large opening at the bottom (under the plinth) leading to a constriction at the opening for the tonearm board into the blind cavity under the dustcover. This is complicated by the springloaded platform hanging underneath. Regardless, as you suggest,  I have a resonating system and I need to figure out how to make it a non resonant system. My next approach will be to make a tight fitting skirt that closes off the bottom of the plinth so that air cannot enter from below. I'll use cheap poplar just to see if it works. If it works I will make a new plinth cover using the old one as a jig. 

You definitely have hackles. 

Mike