5 posts

Ultra-fast dual capacitor high pressure sensor

I’ve been thinking about creating my own high pressure sensor, simply because the weight of commercial high pressure sensors is too large to be practical for the bike shock “chamber” I’m creating. I was going to build a capacitive sensor based on the idea that at high pressures the permittivity of air varies not insignificantly. My main concern there is current fluctuation over tiny time intervals. This might kill the idea, since I need capacitance readings +/- 20% over 20-50 millisecond intervals. This may be too short. So I’ve come up with something different I’d like to pass by you. It would be a circuit measuring differential current between two capacitors. It would essentially be measuring the magnitude of fluctuation of current. I think this could work well to at least measure pressure gradients accurately.


T-man! Welcome to my Packets post! I go by the name “mohini” here – hopefully that won’t confuse you.

I won’t tell you why it’s named “packets” – maybe you can figure it out later. OK, I thought it might be fun to first throw out a couple questions. You know the first one but you didn’t answer correctly, so let’s figure this one out:

Question 1.
If hot air rises why are mountain tops almost always colder than lower elevations? For you to answer, you need to understand a little bit about what heat and temperature are. I didn’t know those two things until I was 15, so no reason that you will. Let’s dive into this subject though, because it’s a good place to start a discussion about Physics.

Question 2.
What if you could build a perfectly rigid tower as high as you wanted? And what if you could take a fast elevator to the very top in very little time? And what if you were wearing a space-suit that allowed you to breathe and move your arms and legs around freely in outer space?

Imagine that.

Now bring a baseball with you when you go up that elevator. Don’t forget your space-suit.

If you get high up enough you can throw the ball into an orbit around the Earth and it will stay in orbit for a very very long time.  What I think is most interesting about that is this: you don’t have to throw it straight out in a direction that is level with the tower platform. You can throw it out at a bunch of angles including straight ahead which is the zero angle. You can throw it up at some angle or down at some angle and if you are high enough and throw it fast enough it will stay in orbit.

Why is that?


Displacement Current within Good Conductors: Gone MIA but why?

bob. try these two links and let’s discuss

(the following is from an email to Constant314 who is editor of the Wikipedia entry for “Speed of Electricity”)

I believe we’ll end up having to jump at least three hurdles before all is said and done …

1.) We really need to study these books in detail and that means more time is needed for me (at least) because I have other things to do. I know you do as well.

2.) We need to come to terms with the reasons for the different approaches, at least in the cases Hayt vs. Jackson

  • I’ve glanced at Hayt and displacement current is ignored entirely 1/3 of the way down p359. This does not make it zero. He means tiny in relation to conduction. I don’t see discussion on this point afterwards. I think this omission leads Hayt into making incorrect statements at top of p360. I believe I know why he did and and how it’s gone for so long without correction. I explain this in the second to last paragraph at bottom of this email.
  • Jackson has a similar treatment and statements about *most* of the energy propagation normal to surface. He then includes a footnote about the ignored displacement term. I’ve attached a relevant page with footnote at bottom
  • Hayt makes an important point at top of p358 about the “loss tangent”. He moves on here to invoke equations and constants that I need to guess at. I need the relevant pages to know that I am making good assumptions
  • Both Jackson and Hayt are being (using your word) “terse” with respect to clarity in relation to the importance that skin depth plays in their arguments. For instance, at this point on Monday morning I ran out of time and quite can’t figure out if Jackson’s treatment later in the book involves *only* thin skin depth relative to cross sectional dimensions of the wire. I need to walk away and come back to it. If he is treating only a case of small skin depth relative to wire radius then I really need to get my hands dirty to apply his equations to the opposite case. It is that opposite case which is relevant for electricity flowing in most wires of interest to most people reading this wiki entry. Yes that’s my opinion and I hope you agree. Case in point, even if we are talking high frequencies, thin wire is often chosen (but not always!) such that its radius is small in relation to skin depth. It think it will be necessary to make this distinction in the wiki section eventually.

3.) We need to agree on what I think will end up being *the* crucial point: The topic is and always has been the “Speed of electromagnetic waves in a good conductor”. That should never have been interpreted as meaning “Speed of electromagnetic waves in a good conductor only for the component of largest magnitude of energy transmission”. It is an enormous distinction and I believe this distinction is what resulted in (my opinion) careless statements at top of Hayt p360.

Maybe #3 should be #1 on the list, because I think we need to start here first. The section is entitled “Speed of electromagnetic waves in a good conductor”. What follows should be an explanation starting with which E&M waves can and do propagate within a conductor. For my purposes as a physics guy, I want to know what the speed is of E&M field propagation in the axial direction. I want to know that first and foremost, because I want to know primarily how an electric field *within the interior* can fluctuate and propagate *within the interior* so quickly from one end of a wire to another. As it is E&M propagation it does not need to carry much energy to be able to sustain itself without attenuating to zero at the other end of the wire. Dipole motions everywhere they exist are sustaining it here (NOT at the exact center however for unpolarized waves. there shouldn’t be any problem posed by that point). Its Poynting vector does not need to be significant in relation to the normal flow. Rather, it needs to carry phase information and yes, some amount of field energy, from one location along the wire to another quickly. Is that interior E&M wave in phase with axially propagating waves elsewhere? I want to know this primarily. That would mean that for a 60 hz signal, fields in the interior must involve waves of very long wavelength moving at a speed close to that of light.

Can a conducting medium sustain such a wave?

If I’m reading Hayt correctly he is saying “No”. That is incorrect. Please read on …

Early on in our discussion this past weekend I mentioned my surprise about the magnitude of the magnetic field. Jackson makes it clear that almost all the energy within the interior of the conducting medium is magnetic. It should be very clear now to both of us that this large H is responsible for the most significant component of *energy flow*. Solve for the Poynting vector inside a conductor and you see it is *almost* exactly normal to the surface. That is a statement about just how large the magnitude of energy flow is relative to the axial direction. This is not proof that axial energy flow can be discarded! Rather, what it implies is mathematical proof that by far *most* of the energy flow is normal to the surface. By far most, but not all. Fine. I think even Hayt is fine with this point.

Please look at my Jackson page I’ve attached. At bottom is a footnote clarifying that inside the conductor there’s a small electric field *normal* to the surface. Note the imaginary symbol i here. This is a fluctuating electric field in the direction normal to the surface. It is small as expected. But how fast is its phase information propagating within the wire? We know it’s frequency, but what is its wavelength?

Except for the following two paragraphs I need to stop here for three important reasons. First, I/we really need to know if Jackson is considering a case of small skin depth relative to wire radius. Second, although terse, Jackson doesn’t hesitate to remind that we are making approximations about the direction of the fields right at the surface. We really need to know to what degree other components of the fields are being ignored for this treatment. Third, when Jackson treats energy propagation in a good conductor for lower frequencies he also ignores terms related to displacement current. However he does not say these terms are zero. I want to find out why he didn’t choose to come back to this point. Maybe he did and I missed it. Maybe that’s the entire point of the footnote attached.

Finally two points to be leave with for now …

I believe Hayt and EEs simply just DO NOT CARE about the tiny energy flow in the axial direction. You made a significant point when you wrote “There is a well developed theory that connects the field values in the dielectric with the field values in the conductor and it accounts for the transmission lines frequency dependence, time domain behavior, delay, dispersion, conductor loss, power drawn, power delivered, and skin effect.” I’ve been thinking about this, and I think it all makes sense now. These are the things EEs and perhaps 99% of others are going to care about most. Hayt’s treatment *does* get energy flow right. It *does* correctly assume the axial flow of energy is negligible relative to the normal flow. His incorrect statements at the top of p360 simply do not matter at all with regard to how well all these things mentioned are calculated (I mean the things within the quotes above). It is absolutely true that energy transmission due to axial displacement current can be *almost* entirely ignored when figuring those things out. So who cares?! Well I do, because I don’t want to know about relative energy transfer. I want to know about the various speeds and directions of E&M waves within a conductor. With regard to the wiki section in question, I do not care about the relative magnitudes of the respective E&M energy flows.

The last point to be made here is that Jackson mentions something that is a big hint and should guide me when I get time to jump back into all this. He mentions that H and E are out of phase inside a good conductor with a phase angle and wave number magnitude that do *not* depend on penetration distance into the conductor. This screams to me that somehow the fields related to conduction current inside the conductor are varying in the axial direction “in step” with those varying outside. I am back to questioning how the *total* current inside can maintain a constant phase relationship with the voltage. How can this be accounted for if a 60 hz signal is penetrating normal to the surface at 3.2 m/s? How can there be no lag outer to inner along the axial direction without the existence of an axial component propagating *inside* the conductor at near light speed? What else can maintain such an E&M profile that keeps “in step” with the outside? The questions are mainly for myself, because it’s quite possible Jackson makes assumptions that I missed here. Maybe he means the profile to be just within a very tiny distance into the conductor from the outer surface. So more time is needed for me to better grasp this.

More on this later this week, but not sure exactly when. I really do need to get to other things. In the meantime, more than likely I will passing this email along to others who know Jackson. I’m considering hitting up a physics prof or two or three to get their “feeling” on all this.

Do speaker cables matter? Is an A/B/X test worth your time?

Almost every speaker cable ABX test has the same result: inconclusive or no difference between cables. Does that mean that no one is able to discern differences between cables? A lot of people swear they can. Are they lying or maybe just fooled by something they’re not considering, or is something about their experiment that’s different from the others?

I feel I have a good perspective and set of experiences leading me to conclude that there *is* something different about their experiment. I don’t know how to explain without a lot of detail and history however. Since this is a long explanation it might be best to just start at the end …

The End:
I believe I have a statistical argument demonstrating that only a small % of people in high-end audio have heard a properly setup system. In my opinion a properly setup system is one that reveals as much information of the recording as is possible and does it with the lowest amount of distortion possible while reproducing a frequency response that is as close to the original recording as possible. If you haven’t heard a properly setup system then you likely won’t be able to hear cabling differences.

I’m including everyone here: dealers, manufacturers, reviewers and consumers. And “tweakers” (neurotic sound engineers for this discussion) who are specifically not dealers and are rarely consumers/reviewers. Some manufacturers might qualify as tweakers, but usually only speaker manufacturers and not even half of those qualify. The tweakers go to great lengths to tweak everything to death (room, source material, equipment). And they never stop tweaking. Going into greater detail here takes paragraphs so I’ll cut it short by stating that I have driven all across the country stopping in at well known manufacturers, many dealers, many consumers homes and many rooms at audio shows. I’ve had direct and lengthy communications with consumers everywhere, and some manufacturers to some degree. My communications with dealers are almost always very brief. They are almost always completely full of $#it and almost always rely on manufacturers to decide how to setup equipment. And it is almost always poorly done. I can go on in detail, but the takeaway is this: Excluding the category of tweakers and manufacturers, I feel I have a sound statistical argument which strongly implies that only a small % of everyone else has ever heard an excellent setup.

You should be very skeptical about that last paragraph. I would have been extremely skeptical if I had read this mid way through my journey, and that was after obsessing on this topic for quite some time. It took me about 3 years to reach this opinion. Here was my path which began about 8 years ago:

First, after reading a large amount on the topic of high-end audio, I proceeded to have several uncomfortable online correspondences with “audiophiles”. Those correspondences proved I had no idea what goes in this industry. It’s a mess with regard to accepted definitions and standard practices. Primarily, it begins with the goals of the high-end listener. What goals are they trying to achieve? I thought it was obvious 8 years ago, and now I realize it’s mostly about personal preference. And even if the goals are strongly tied to “faithful representation of an audio recording”, that gets one almost nowhere. At first I thought there might be just a handful of definitions for what faithful means. Now I see there is no set of accepted definitions. There is only preference. Surprisingly, faithful isn’t even an important goal for some audiophiles.

What a surprising discovery to find that a good % of high-end consumers are not spending their money on a quest for faithful playback!

Second, I started buying and selling equipment and playing around with everything. I had a fairly nice listening room, and I got to a point after about 1 year where I was hearing excellent (my opinion) high fidelity from my own system. It was here that I had my first interconnect experience. I doubted ICs made much of a difference so I tested some out and the difference was dramatic. Eye opening. I thought it was a very important discovery, but it wasn’t really. I simply discovered that some ICs are bad, and it makes no sense to buy cheap interconnects. But what is cheap? I don’t know exactly, but after this discovery I realized that I couldn’t hear any difference at all between good ICs. I expected a gradation here where $300 ICs would sound better than $100 ICs, and then as one climbs above $300 I should expect diminishing returns but improvements nevertheless. I never heard that however. What’s significant about this part of my path is that I inched forward and realized a couple things: improvements in sound were likely to come incrementally at this point, and whatever happened next would have to be completely up to me.

As the wisest in the industry will often say, “get out there and listen to everything that you can”.

Third, as I went around to shows and dealers and audiophiles homes I realized that my cheapo system always sounded better to me. My $3000-$6000 system always sounded better than $50K+ systems. Begs the question of what “better” means of course, and that’s where I went next – trying to figure out what quality all these other guys were looking for. So bit by bit as I traveled around I came away with new discoveries with regard to tone that I wasn’t hearing at home (usually the tone of strings/pianos/horns) and the soundstage which on some occasions was just better than what I had at home. You likely know what soundstage means, but I’ll define it loosely here anyhow as “distinct placement of instruments/vocals/sounds across the space in front of the listener accompanied by some amount of depth”.

Maybe it’s not necessary to spend a lot of $$ to create the best playback possible?

Fourth, 2.5 years into this my system was sounding better as I started moving things around the room and playing with angles and heights and listening positions. I was beginning to think of my soundstage as realistic. Not just convincing. Then two things happened that took me in a completely new direction:

1. I bought a VAC 70/70 300b tube amp. A disgustingly dirty amp that when played though my Joseph Audio Pulsars did something I had never heard anywhere. Prior to this I had no idea it could be done: vocals were now placed about 4-6 feet behind the speakers with speakers about 11 feet apart and 8 feet in front of the listening position. The first few days of this were just eerie – very strange to hear these voices coming from a very well placed spot well behind the speakers. Prior to that I had heard maybe 1-2 feet behind. Never anything close to this though. I couldn’t explain it.

2. I bought Magnepan panels. I set them up as instructed by experts. Horrible sounding. I researched a bit more and setup according to other experts. Much better but disappointing. I gave up on the experts and just started playing around with speaker setup and discovered that the experts have no idea what their speakers are supposed to sound like – every person that I’ve ever known or read about that owns Magnepans sets them up way too close to rear walls. They need to – their rooms aren’t big enough to do otherwise. Once I decided on 12 feet away from the walls (as opposed to the suggested 3-6 foot “ideal” position) the stage opened up and now I heard what panels are supposed to sound like. By far the most realistic sound stage I had ever heard. Great depth especially when I drove them with the VAC 70/70. But how much of this new experience was due to speaker and listening positions?

I thought I was getting closer, but now I was back to having little knowledge as to the common goals and standard practices of high-end audio.

Fifth, I moved to a house with a listening room sloping from 8 feet to 23 feet. Mostly carpeted, but I covered most of the hard floors with rugs. I made and installed very large curtains to split the room in half, made my own baffles, covered windows and hung huge and heavy rugs on walls and coming down from the stairwell. Played around with an enormous number of speakers and placements. Soon new things began to happen. Images were incredibly well defined and placed on the stage with good depth. Performances now took on a presence I hadn’t heard before, because now I was hearing excellent low distortion, high powered, low frequency energy inserted into a larger waveguide. For the first time ever I heard a well recessed and powerful tympany erupting in my home replete with reverb straight off the recording. My 20’x40′ room became a much larger concert hall. Did NOT know that could happen. Not that kind of spatial presence. Never heard that before. At this point I believed I was approaching as good as could be expected, because it was as good as anything I had heard plus/minus the differences that come with speaker choice: more/less/lower bass, more dynamics, and differences in frequency response and dispersion.

Ah ha! Only by putting a lot of effort into this am I achieving something that very few people are hearing with their own systems!

Sixth, after all the traveling and approx 100 rooms visited at 3 audio shows (Denver + Newport) and many discussions turned into friendly relationships with audiophiles and manufacturers and one dealer (the other 15 or so weren’t worth a minute of my time) I was settling down and deciding to take the advice of others and start putting money into cabling and a DAC and a much better transport. Mostly I had decided it was time to actually fork over bigger bucks for my own speakers rather than continue to buy and sell. I slowed all the traveling down and stayed local (Phoenix, AZ). One day I ended up at a Phoenix audiophile’s home to listen to an amp to purchase for a friend. His was another $50k – $90k system. I walked into his listening space with few expectations. Twenty minutes later I was hearing something I had never heard anywhere before. The entire performance came together with the best detail and realistic depth I’ve heard. But that wasn’t the big revelation. The main thing that was new was the “air” surrounding the sound-stage. Hard to explain, and maybe better to say I was hearing all the information off the recording and that included all the information from the soundstage itself. Even if that meant the confines of the recording studio I was now hearing it. I thought I knew these favorite recordings, but it was clear that I did not. The low level acoustic guitar of Adele’s “Lovesong” for instance was convincingly placed at a precise distance and depth from Adele, and I could now hear much more detail of fingers on the fret and the air in between and around vocals and guitar. I could hear the extra information from the recording environment, regardless of studio or live. Regardless of how it was engineered there was clearly a lot of information I had never heard. It was extremely low level but it was there and was a missing piece convincing me of a presence of instruments and musicians in the room.

I had heard something that was much better than my own system. Soon after I visited a new dealer location in downtown Phoenix. I went and listened to the expensive system in a large room of its own. Almost the same experience, but not as much detail. Still, I heard much more air than I did at home and I was blown away by another favorite recording that I was sure I knew: Dire Straits “Your Latest Trick”. I would simply describe it as a “better” sax. A better recording overall. Mostly just more detailed. It was clear to me now my speakers weren’t even close to what I needed, which to match what I heard at these two locations, needed to be at least in the $12K+ USED range ($20K+ new). And I had no idea if my amp was good enough to match up with everything else.

These new discoveries weren’t good things though. I put a lot of time into tweaking my room, and now it appeared that the big boy audiophiles were right after all: I needed extremely expensive source material, cabling, and speakers.

… but two months later I discovered every bit of that thinking was wrong.

The problem was distortion, distortion, distortion and it was all coming from my speaker-room interactions. I purchased $2500 Quad ESL-2805s, a $5500 VAC Phi 200 tube amp along with $7000 Ayre MX-R monoblocks for comparison. My own speaker cable. A lot more tweaking of baffles especially right behind the quads. My own ABX box and I started testing. Cutting to the chase over the course of a week I had moved the speakers around 25-30 times about 1″ at a time playing around with angles and the positioning of baffles behind and in between the speakers. By the time I was done the amps made little difference in terms of the sense of realism. I started using my cheapo $2500 Primluna tube amp just to prove this. Up until the very last minute a week later, and before I needed to pack up and sell the Quads, the sound got better and better and there seemed to be no end to where it could go. At this point the soundstage and air was better than what I heard anywhere – for the first time I had a feeling of everything being in my house. Not just the instruments and soundstage but whatever was in the air around the soundstage. I know that may not make sense – it’s hard to explain. There was a *visceral* conviction that everything involved with the recording was all right there and the soundstage was extremely wide and extremely deep. There was information that I wasn’t able to hear before. Prior to all these new changes my “high end sound” was mixed up by phase interference of the room and speaker placement and distortion coming straight from the speakers. And yet I had no sense that my previous setup was anything but excellent.

I hadn’t ever heard excellent. I thought I had many times, but it was now clear that most of those earlier listening sessions involved improper setups.

When I write that I have a statistical argument demonstrating only a very small % of listeners have ever heard an excellent setup I mean that I’m not even sure how close I came. I know I could have kept on going, but I sold that system (2805s + MX-Rs). I am convinced that only the most neurotic tweakers would do what I did to get to where I ended up. There are plenty of us no doubt, but % wise of all involved in high-end audio? That’s an incredibly small number. I was using a middle of the road DAC (Auralic Vega), a barely qualifying transport (OPPO BDP-105), excellent recordings off of standard digital sources, good ICs (Purist) and my own speaker cables. That entire system cost about $8500 and was mostly pre-owned equipment. The Phoenix dealer (a self-proclaimed guru) was clear that his system was about the top of the line. All told that was about a $120K system. I have no reservations when I claim that my system sounded better. Not in terms of dynamics and deep bass, but overall just “better”. Quads produce better strings and piano – his speakers did not compare in this regard (his were $50K Rockports if I recall). Horns and vocals are excellent on both systems. Tone overall is excellent on both systems, but the detail, imaging and soundstaging simply beat what he had. Why? Distortion. It’s very difficult for dynamic (cone) drivers to approach the crazy-low distortion and excellent frequency response of electrostatic panels. Difficult but not impossible. To do it correctly one needs to tweak the room like crazy, and that means way more absorption than the “industry” recommends (a different topic for a different time). Speaker placement and the areas in between and behind speakers must be tweaked and tweaked some more. And then even more.

So what does all this have to do with cabling? If you haven’t heard as much information as is possible off your favorite recordings then you likely won’t be able to hear cabling differences. You *might* be able to hear differences going from CD to SACD, DSD, DXD etc, but being certain that one difference is “better” than another? That’s the hard part unless your system is setup extremely well.