A System Vibration Sensitivity Test Method

[Karma]

HI All,
I posted this test methodology as part of another thread. After writing it, I thought it might carry more gereral interest among the more technicaly minded members. I decided to expand it a bit and post here. It might prove useful to anyone who has a system capable of powerful, fully extended, low bass.

Many audiophiles have claimed that the sound vibrations in the air could couple into the equipment and act as a vibration source. I agree. After an aggresive vibration reduction program, I designed the following test to see if I could determine the effectiveness of the program.

My main system (not my solid state HT system) is based on tube equipment. My preamp (Audio Research SP-11 Mk2), the main speaker amp (Audio Research D-250 Servo Mk 2) and DAC (Anodyne) all are tube based. Tubes, as you may know, are infamous for microphonic problems excited by external vibrations. Besides this, I was VERY interested in my turntable system. Turntables are very sensitive to vibration and can produce acoustic feedback that is not obvious but detrimental to all aspects of playback.

I'm not going to go into detail about the various vibration reduction techniques I used. Suffice it to say that they are extensive.

The subwoofer amp is solid state Levinson No. 23. My subs are dual Kinnergetics SW-800's which are rated at 4 ohms and go to 15Hz with power. The Levinson can output an honest, continous, 400W per channel into that load. This performance is important because the subs function as an important part of the test equipment used in this test.

I disconnected the Levinson's input from the crossover. I ran a Tektronix pulse/function generator, the kind where you can program the number of sine wave cycles and frequency in a burst and control the burst duration and repetition rate, into my sub amp. I monitored the output of the main tube amp with a very sensitive Tektronix storage oscilloscope. I could measure micro volts. The preamp was switched to the Moving Coil photo input, the most vibration sensitive mode because of its very high gain. I placed the stylus on the turntable platter. The turntable was not turning. Preamp gain was adjusted to about 3dB higher than my highest normal listening setting. With these settings the entire amplification chain was engaged in the signal path.

I then triggered the generator to produce a series of very low frequency, high power sine wave bursts through the subs. Believe me, every loose thing in the room was rattling. All, that is, but the tubes and turntable. If the amps or the turntable were vibration sensitive I would be able to see it indicated by a varying voltage on the scope. I saw NOTHING. I tried different frequencies of sine wave bursts to identify resonances but found NOTHING. I even tried a continuous frequency sweep. NOTHING! I was pretty happy with my vibration reduction program.

Anyone with a subwoofer and access to the required test equipment can duplicate my test. There are many variations of test equipment that can be used. Prior to my tests my gut feeling informed me that I had no significant vibration problems. Now I have confirmation. I only wish that I had run a test series before I started the vibration reduction program so I could compare the before and after performance.

I do know the system now sounds better. More solid, better focus, a precise and stable soundstage and bass that is a clean as a whistle with no overhang no matter which program source is in use. It's almost as though a subtle fog has been lifted from the sound. The fog was not something I identified as a problem. I only noticed it after it was gone. My system sounded good before. Now, ........ well, it's hard to be humble. :T

Sparky

[SteveCallas]

It sounds like you went to a great deal of work and I'm glad you are satisfied with the results. What I instantly ask myself is even if there are some miniscule vibrations the equipment is seeing, will it actually result in audible differences? With your tube equipment and turntable, perhaps it will - with solid state, I highly doubt it. I think a lot of it has to do with the distance between the equipment and subwoofer as well.

[Karma]

HI Steve,
Well, that's the important question, isn't it? Is vibration actually a problem? I have argued both sides of the issue in slightly different contexts. There is a current thread asking if it is alright to stack equipment. The replies are very interesting. In general, the majority of opinions are dead set against stacking. Everybody was correctly concerned with heat build up.

But what if the equipment did not produce heat, I asked? Now the opinions shifted to prevention of vibration. I asked for real evidence of a vibration problem. No one could produce any but that did not change strongly held opinions. These opinions run counter to yours. BTW, how do you feel about stacked equipment? Why? The point is this: these are just opinions. It's an open market for snake oil salesmen.

My test, as far as I know, is first one I have seen that was specifically designed to uncover vibration sensitivity of lack thereof. I can make my case without having to resort to opinion. In my system, there were at least two mechanisms that have a solid technical reputation for problems. In solid state systems without turntables the issue is not as clear.

Very expensive CD/DVD decks go to great extremes (and expense) to prevent vibration sensitivity. Does it work? Does it solve a real problem? I don't know but I do know that a number of people buy those decks.

I must point out that your statement is one based on opinion. Boy, it would nice if you had real data so we would have to guess. Run my test.

BTW, distance is important in real world sound. But if the volume is high enough distance is not an issue. In my tests the volume level at the equipment rack was scary loud. That's why I ran bursts rather than continous sine waves. It gave the voice coils a chance to cool. Otherwise, my subs would have smoked. The SPL's were that loud. I had no way to measure SPL. I'll guess about 110dB but it's really hard to estimate bass that low in frequency. My RS SPL meter just does not go that low.

Sparky

[SteveCallas]

I must point out that your statement is one based on opinion. Boy, it would nice if you had real data so we would have to guess. Run my test

Unfortunately, due to the setup of your experiment, by using my sub to play low frequency sine waves to induce vibration in my equipment, I wouldn't be able to conduct any serious blind listening, as the sine waves would be distracting. Without being able to do any serious blind listening, how could I tell whether or not the vibrations are causing a degredation in sound quality?

I can say that a friend of mine who built a replica of my sub had his dvd player skip when it was placed low to the ground and near the sub while low bass was playing, but that is an extreme case. After moving it further away, the problem disappeared. As for stacking equipment, as long as there are no heat buildup issues, I'd have to imagine that stacking would help reduce vibrations, as you are mass loading the chassis.

[Karma]

HI Steve,
Yes, I agree that they would be distracting. I guess you understand that my test did not depend on listening. While bursting the system, there was no other program being played. No CD's , no records, no nothing. The system was just idling while being bombarded by sound pulses. The scope told the story. Any vibration sensitivity would cause a voltage to be generated somewhere in the signal path, be amplified and then observed with the scope.

I specifically designed the test to eliminate all distractions.

FYI, here is a post concerning stacking on this very forum:

http://www.htguide.com/forum/showthread.php4?t=19823

Sigh, no data anywhere. It's a desert out there. Just opinions.

Sparky

[SteveCallas]

Getting back to my first point though, even if you eliminate all traces of vibrations, how can you know if it truely made an audible difference or not? Blind listening tests would be needed. I think vibration, just like almost every other "hi end" issue, has its root in science, but gets far, far overexaggerated, and most of of the issues aren't actual issues at all.

[Joe L.]

I specifically designed the test to eliminate all distractions.
Sparky

Having no signal on your scope trace is a really great start, and it indicates you have done a great job of isolating the subwoofer signal from getting to the turntable "at the frequencies you tested" and "in the plane in which you initroduced the vibrations", but I'm not sure it is the only test needed to prove actual signals cannot be affected.

I can envision a well designed amplifier having "common mode" rejection of signals such as those introduced by your vibration tests, but still slightly non-linear such that they would "mix" the low level vibration induced signals with ones from your audio source and produce "intermodulation-distortion" as a result when signals are present.

I have a hard time believing that the turntable is isolated from all sounds in the room. The air in the room must couple some energy to the platter the stylus is resting on. There might not be much coupled from the equipment rack because you have isolated it, but if you tap on the turntable platter and see anything on the scope then sound waves must be doing the same but on a much lower level.

[Dennis H]

Getting back to my first point though, even if you eliminate all traces of vibrations, how can you know if it truely made an audible difference or not? Blind listening tests would be needed.

Not really. Measurements are much less subjective than listening and much more accurate. I think his method of playing the sub and measuring the output from the turntable (and all the other gear) is quite good. As an old vinyl guy in the past (we didn't have anything else), I know it was a problem to even be able to walk across a wood floor without generating subsonics that made the woofers go crazy. You could see them move even if you couldn't hear it.

[SteveCallas]

But you can supply all the measurements, numbers, charts, and graphs you want - in the end, if you can't hear an audible difference, what's the point?

[Karma]

HI Joe and Steve,
Joe
You can imagine any number of alternative tests. I wish you would put your imagination to use and design and run some alternative tests. That way you can add to our knowledge base. I have no problem with a critical attitude toward my tests. I posted both to educate and to learn. Right now you appear to be just a simple naysayer. Like a music critic who has never composed or performed a piece of music in their lives.

Think about your comment about different planes of sensitivity. That could well be true. However, the test I ran duplicated actual operating conditions with the tools I had available. I was not worried about theoretical results. I was only concerned about how my system performed in the real world.

BTW, balanced operation (my system is not balanced) would make no difference to the results.

BTW, have you ever owned state-of-art turntable systems? I can't tell from your profile. I have, for over 40 years. This is important because most folks have not, especially younger ones. Why? Well, they have always been extremely expensive. If you have one to look at every day, you are struck by the very creative design that reduces vibration sensitivity. They are built like tanks but they are also subtle. Some are better than others but all are concerned with the problem. What can't be controlled is the way they are installed into the users system. My vibration work concentrated on providing the most viable vibration environment. What I did seems to have worked. My results also speaks well for the design of the turntable (and the air bearing tone arm) itself. I'm impressed.

I'm not claiming total isolation. I can only say that within the confines of the test ran, I saw no vibration issues. I'm sure the isolation is not total but occurs at SPL's higher than I generated. Remember, I was only concerned with my real world situation.

Steve
I believe in listening as the final arbiter in audio matters. However, there are certain performance areas that lend themselves better to technical testing. This is one of those cases because of the question I was asking.

I do not believe in blind listening tests. Blind tests have been performed numerous times by serious researchers and have never been shown to be effective. So your concerns about blind tests are not mine. I really do trust my ears.

As for your point wondering if vibration has audible consequences I think it is a matter of degree. With the system under test, there are no questions that acoustic feedback and microphonics are most definitely audible. As I stated, my system improved after vibration treatment. Are you questioning that conclusion? You should not.



In solid state equipment, vibration mechanisms are no different though microphonics would occur at a much lower level because there can be no loose tube elements which are the most egregious offenders. On a theoretical level, any time a conductor moves relative to a magnetic field, a voltage is produced. This basic idea holds for any type of component from wires to resistors, capacitors, tubes or anything else that conducts electrons. Is it audible? It certainly could be. Is it actually audible? The manufacturers think so. Check out the mechanical design of a top end amp or preamp. You will see extensive anti-vibration design everywhere from shock mounting to thicker circuit boards. Personally, I don’t know but I suspect so. You must judge for yourself or run some tests.

Sparky

[SteveCallas]

I do not believe in blind listening tests. Blind tests have been performed numerous times by serious researchers and have never been shown to be effective

That's a very bold claim. I'd ask for some basis, but I think I have a good feel for where you are coming from and I realize it would be pointless to continue. I should have kept my mouth shut after reading the last paragraph in your first post. I'm glad you found a solution that eliminates vibration in your components when playing low frequencies.

[Dennis H]

But you can supply all the measurements, numbers, charts, and graphs you want - in the end, if you can't hear an audible difference, what's the point?

Ah, you skeptical kids..... Believe me, anyone can hear the woofers bottoming when someone walks across a wood floor and vibrates the turntable. It's not a pretty sound.

[Karma]

HI Dennis,
Yes, you are right. Not pretty. In truth I had those types of gross problems fixed a long time ago. I was more concerned with the subtle effects of acoustic feedback. These types of problems would not be immediately noticible but still prevent the turntable sound from attaining its best. These would show up as time smearing, transient loss, detail leveling, loss of precision of the focus and soundstage and other such audiophile concerns.

I suspect that many turntable based systems are affected by the subtle type problems and the owners are not aware of the issue. If so, I hope they read this thread and maybe be moved to improve their vibration isolation.

Sparky

[Joe L.]

Joe
You can imagine any number of alternative tests. I wish you would put your imagination to use and design and run some alternative tests. That way you can add to our knowledge base. I have no problem with a critical attitude toward my tests. I posted both to educate and to learn. Right now you appear to be just a simple naysayer. Like a music critic who has never composed or performed a piece of music in their lives.

Think about your comment about different planes of sensitivity. That could well be true. However, the test I ran duplicated actual operating conditions with the tools I had available. I was not worried about theoretical results. I was only concerned about how my system performed in the real world.

BTW, balanced operation (my system is not balanced) would make no difference to the results.

BTW, have you ever owned state-of-art turntable systems? I can't tell from your profile. I have, for over 40 years. This is important because most folks have not, especially younger ones. Why? Well, they have always been extremely expensive. If you have one to look at every day, you are struck by the very creative design that reduces vibration sensitivity. They are built like tanks but they are also subtle. Some are better than others but all are concerned with the problem. What can't be controlled is the way they are installed into the users system. My vibration work concentrated on providing the most viable vibration environment. What I did seems to have worked. My results also speaks well for the design of the turntable (and the air bearing tone arm) itself. I'm impressed.

I'm not claiming total isolation. I can only say that within the confines of the test ran, I saw no vibration issues. I'm sure the isolation is not total but occurs at SPL's higher than I generated. Remember, I was only concerned with my real world situation.

Karma,
I appologize if I sounded like a naysayer. That was not my intent.

No, I've never been able to afford a "state of the art" turntable. I do still own a decent quality DUAL 1229 turntable from the mid 1970's. It had a fairly heavy platter for its day (I think weighing over 10 lbs) Compared to what you own, it is a toy, but decent for its day. Perhaps not "audiophile" grade, but way better than average.

At the time I owned a pair of Ohm Acoustics Model "F" speakers. It was very easy to get a feedback loop going from the turntable with vibration induced signals at low bass frequencies. (The amplifier I was using was DC coupled and it was easy to see the cones on the drivers move with even slightly warped records...) I was not using a rumble filter.

I ended up having to make my own vibration isolation base to allow me to play records at any reasonable volume.

Now, my isolation base consisted of a precast "patio-square" (basically a 3 inch thick slab of concrete 18" on a side) sitting on 4 foam rubber ball halves. With a coat of flat black paint, it looked pretty decent and it had a prominant place in my living room.

Again, my isolation base sounds like a crude ancestor to anything you have in your system, but the basics are the same... Mass and Isolation.

With it I could play at any level I wanted without feedback. Without it, the turntable was unusable.

So... I do have a little bit of experience to draw upon from over 30 years ago.

I can appreciate what you accomplished. All I was saying was that IM distortion will only occur when signals are present. Your tests showed no vibration induced signal. I don't know what effect the vibrations would have on amplified signals. Odds are high the effects, if any, are not audible.

Joe L.

[JonW]

I'm not going to go into detail about the various vibration reduction techniques I used. Suffice it to say that they are extensive.

Actually, I'd be very interested to hear what type of dampening techniques you have used. It's something that's been on my mind a bit.

I can say that a friend of mine who built a replica of my sub had his dvd player skip when it was placed low to the ground and near the sub while low bass was playing, but that is an extreme case. After moving it further away, the problem disappeared.

That's me. The DVD player skipped when it was sitting on a few pieces of wood on top of the sub amp, on top of a couple pieces of wood, on the floor. About 2-3 feet from the sub. It skipped, of course, with heavy bass output (e.g., explosion in a movie, etc.) If I put a folded bath towel under the DVD player, it didn't skip anymore. Now I have the DVD player on a couple pieces of wood, on top of the prepro, on top of an amp, sitting on a small 4 leg table. Maybe 4-5 feet from the sub. No more skipping. Although, come to think of it, I don't know if I've really pushed things hard since having it located there.

[Dennis H]

In truth I had those types of gross problems fixed a long time ago. I was more concerned with the subtle effects of acoustic feedback.

Yup, understood. I was just trying to smack Steve upside the head with a 2x4 example to get his attention. He seems to think you're some kind of kook and the problems aren't real. People who haven't lived with turntables and tubes have no idea. Tap a power amp tube gently with the soft part of your finger and you'll hear it through the speakers bigtime so it's only logical a powerful sub could do the same thing.

I like your system of measurement very much. Heck, this whole site is devoted to measuring things to take the mysticism out of them. We measure driver distortion, speaker frequency response, room response, etc, etc. Measuring feedback is very useful to see if you have a problem or not. If you don't have a problem, then there's no need to fix it.

[SteveCallas]

will it actually result in audible differences? With your tube equipment and turntable, perhaps it will - with solid state, I highly doubt it

you can supply all the measurements, numbers, charts, and graphs you want - in the end, if you can't hear an audible difference, what's the point?

:T Let's not turn this into AVS.

[Dennis H]

Peace, Steve! :T

Just so you know where I'm coming from, I'm trying to avoid the AVS nonsense. I've read your amp comparisons and agree. Some people claim "this amp doesn't measure any better but it sounds better." So it's perfectly reasonable to do an A/B to see if it really does sound better even though it "can't be measured." But this is different. The measurements really are different. No listening required to know there is a difference. One is better than the other. Don't fall into the subjectivest trap of "measurements are crap, all that matters is what I hear." You're joining the dark side when you do that.

[SteveCallas]

Don't fall into the subjectivest trap of "measurements are crap, all that matters is what I hear." You're joining the dark side when you do that.

Lol, no worries, I would never use that golden ear logic. I don't pretend my hearing is better than precision measurement equipment like they do, instead, I'll openly admit it's not as sensitive, even though my hearing measured above average according to physical evaluations. What I'm getting at is that in some circumstances, you can measure large differences and think it's the end of the world, when in reality, the differences may very well not be audible.

P.S. I'm not saying that is the case in this instance.

[Dennis H]

you can measure large differences and think it's the end of the world, when in reality, the differences may very well not be audible.

True enough. In this case it's easy to translate the measurements into % distortion. Set your prepro to reference level and play some pink noise. Measure the output of the mains with a DVM. Then play pink noise through the sub only and measure the output of your main channel amps with the DVM. The ratio is your % distortion due to feedback. Anything over 0.1% is something to be concerned about.