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Effects of damping factor on planar magnetics/orthodynamics ?

Discussion in 'Sound Science' started by frenchbat, Jul 10, 2012.
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  1. MaciekN

    Dynamic drivers are only indirectly controled my magnet (if amp's Z out is low enough), the resistance you speak of comes from driver suspension, and that is why some drivers are less sensitive to high impedance outputs- they have stiffer suspension that does most of the job of controlling the membrane. I'm not an expert but it's logical that ortho drivers do not simply sit loosely between magnets but are suspended just as regular dynamics, something holds them in place.
    Btw there is no magnet attached to the membrane (or ribbon, whatever you call it) in dynamics, just the voice coil.
    Even if we speak of an open- back ortho like Audeze there still is mechanical damping from the felt or similar stuff that sits behind (and, I guess, in front) of the driver. For example there is a mod for LCD- 3 which uses toilet paper (purrin's idea if I'm right): it simply restricts the airflow allowing less driver movement after it was excited by the signal.
    As far as my basic knowledge of accoustics goes I can say that orthos are indifferent to output Z variations (and therefore high Z out) at least in FR terms. If they are a purely resistive load you could aim for current bridging. If Z out is equal to the load, then current transfer is most efficient.
    Low Z out allows for more efficient voltage transfer, meaning more dB at your ears at a given gain, so if you used like 10kOhm Z out to drive 50 Ohm ortho headphone you would need a lot of a gain for satisfactory loudness, and most of the voltage would be wasted into heat.
  2. frenchbat
    We are talking about the same thing [​IMG] No problem here. Obviously the driver needs to be suspended in both cases, but orthos are supposed to be much less stiff than the dynamic drivers. Hence the discussions I've found on the magnepans' boards. However I have no idea if and how it can apply to our headphones. There are most likely significant differences between magnepans and our ortho drivers, if not in construction, at least in operational conditions. And Steve's experience shows that at least the Audeze, can deal with a very high Zout.
    True, that was a poor phrasing on my part.
    Here I guess there's a difference between HifiMan and Audeze. Besides the earpads and the outside grill, there's nothing besides the driver in the HE500.
    That's what the theory predicts, and most likely it's the case until a certain point. As I said earlier, I wish I could measure the distortion with different Zout, between say 50 Ohms and 500 Ohms, to see if there's a difference. I'll keep searching for information when I have the time.
  3. daveDerek Contributor
    hopefully that heat is dissipated via the amp and not the cans...
  4. Steve Eddy
    For a given loudness, the headphone will dissipate the same amount of heat whether it's driven by a 1 ohm or 1,000 ohm source.
  5. MaciekN

    Yes, output resistor will change excessive voltage into heat, not the cans.
    @frenchbat, I'm afraid you would be measuring the wrong thing, there are no mechanisms that would increase distortion in ortho drivers when driven by high Z out amp, only efficiency can be affected. You will find all the information if you google some books about acoustics and how loudspeakers and headphones are built, there is plenty of them.
  6. frenchbat
    I see what you mean, a Zout set too high will reduce the available voltage, and by consequence the available power. I should have thought about it from Steve's first post, since that's exactly what his amp is doing, converting the speaker taps' power in headphones output with a high Z.
    On a different note, the flapping thing seems to have been limited to the earlier designs of the Magnepans, where magnets were aligned on one side only. Later on the double sided drivers, similar as the design of our orthos, are said to have solved that problem. So I guess there's no real worry on that side either. 
  7. impermanente
    Here my 2 cents about the question, unfortunately I don't know any article specific for orthos:
    In principle a very low impedance headphones amp output (1 ohm or less) allows the generated current from the headphones (generated by the inertia of the diaphragm of the orhto headphones after a stimulus) to flow "easily".
    This means that the inertial oscillations of the diaphragm (due to the inertia of the diaphragm=mass + stifness of the joint to the frame) of orthodynamic heaphones are also damped by the low output resistance of the amp, just like the diaphragm of normal dynamic headphones or speakers. Please notice that orthodynamic and dynamic diaphragms use exactly the same principle of physics (electrons moving in a magnetic field).
    However this is not the end of the story and I see that there is a lot of misconception about damping a diaphragm out there. If I understand well what you wrote about the low damping factor, I think you as well have a bit of misconception about the damping phenomenon. This misconception is clarified eminently in my opinion, in the article of your link. Here what I mean:
    I have read many articles here and there (even from some good amp companies), which stress on the point that a low impedance amp should always be used to properly damp the diaphragm of speakers and headphones. The whole purpose of electrically damping a diaphragm is getting a better control of the amp over it...in other words the diaphragm must do what the amp says and not going all over the place when the signal from the amp is gone. Lack of control results in lack of clarity and variation of the frequency response of the speaker. As clearly explained in the article from your link, this lack of control tipycally caused by high output impedance amps is called underdamping.
    But the question that many people seem to miss all the time, is that exists also the overdamping (again clearly explained in the article) which is caused tipycally by a very low impedance amp output. The overdamping causes exactly the same problems that the underdamping causes, that is, lack of clarity, distortion and change in frequency response, but in opposite direction, the sound is dull, flat, without dynamic.
    What people should really take in account is the Critical Damping Resitance (CDR) which is a parameter that has been around since the invention of the speakers. This parameter is the total resistance seen from the speaker, so CDR=amp output impedance+speaker impedance (ortho or dynamic) that is able to damp the diaphragm just right. (This can depend on the mass of the diaphragm, strength of the mag field and other factors).
    So to proper electrically damp a diaphragm (ortho or dynamic, headphones or loudspeakers) one needs to match this single value, the CDR (within a range obviously). This means that the output impedance of an amp must not be low, super low or high, but just the right one=CDR-speaker impedance. Only in this way an optimal damping can be obtained (please amp makers think about this and make a variable output impedance amp with a nice knob!).
    In the article of your link there is even a nice and simple electrical circuit that describes how to obtain the CDR of a speaker in a simple way (given a good quality speaker). Obviously the story can be broadened and refined but the principle is very sound.
    Also, given the concept described in the article, I strongly believe that it is not possible to damp many many headphones, because the CDR is probably lower than the impedance of the transducer itself, so the condition CDR=amp output impedance+speaker impedance cannot be reached. This could be the cause of the need to mechanically damp the transducers in many headphones, or the cause of many bad sounding headphones.
    As well this could be a valid explanation on the matching headphone/amplifier.
    So according to what I understand and in my opinion, here the summary:
    - Regarding the electrical damping of diaphragms, the orthodynamic headphones behave in the same way as dynamic headphones or speakers.
    - The right amp output impedance to obtain proper electrical damping of the transducer is amp output impedance=CDR-transducer impedance and not necessarily as low as possible, otherwise the result is an overdamped headphone.
  8. Steve Eddy
    But damping has everything to do with resonance. And I don't see any evidence of resonance in orthos in the audio band. Their impedance curves are ruler flat.
  9. impermanente
    "But damping has everything to do with resonance. And I don't see any evidence of resonance in orthos in the audio band. Their impedance curves are ruler flat".
    I am not sure if I understand well what you are saying, are you saying that if a speaker (dynamic or orthos) does not have any resonance there is no need of damping?
    A diaphragm attached to a frame behaves basically like a spring (ortho, dynamic or whatever), that's why it is a good thing using electrical damping (using a proper amp output impedance) to attenuate the "anomalous" oscillations caused by the inertia of the diaphragm (which has a mass, it is attached in a certain way...etc). This works in theory at any frequency but mostly at low frequencies when the amplitude of the oscillations is large (the electrical damping is proportional to the amplitude and speed of the oscillations).
    Now for some diaphragms resonances will occur (most dynamic ones), this is caused by the mechanical resonance of the diaphragm at a certain frequency or frequencies (for a speaker in a cabinet the volume and shape of the cabinet also contribute), consequently this will give rise also to electrical impedance. When a diaphragm resonates it is more "free" to move, this is obviously bad for good music reproduction because the amp has not good control at the resonance frequency. Usually this kind of resonance it is attenuated by an RLC band pass filter in parallel to the speaker, which decreases or flatten the electrical impedance peak, damping also the oscillations. In our case, damping with a proper amp output impedance works at the resonance frequency of the diaphragm (the critical damping resistance only depends from the resistance of the coil and amp + cables), and for the rest of the spectrum especially at low frequencies where the oscillation of the diaphragms are larger.
    So in a few words coupling the system speaker/amp using the Critical Damping Resitance principle is an effective way to damp the diaphragm oscillations through the entire spectrum. This electrical damping system works more when large oscillations of the diaphragm occur and this happens at low frequencies and at frequency resonances, which is exactly when we need it more. So basically, once one properly calculates the amp output impedance to reach the CDR condition this will be the same throughout the spectrum and will work at all frequencies with larger beneficial effect at low frequencies and at resonance frequencies.
    It all makes sense to me but I could be wrong so this is just my humble evaluation of the problem.
  10. xnor
    Why input impedance? The input impedance of amps is usually several kOhms to not load down the DAC.
    Just call it output impedance, Zout or Zs to avoid more confusion.
    Problem with most (dynamic) headphones is that they seem to be underdamped even with Zout=0.
  11. impermanente

    My mistake you are right, output impedance!
    ...and yes I agree with you, probably for most of the headphones out there it is not possible to reach the critical damping, as rule of thumb I would say that this would be more possible for headphones with very low impedance and very strong magnets. I am thinking about the Tesla drivers and low impedance beyerdynamic drivers.
  12. impermanente
    changed input to output...sorry :)
  13. Steve Eddy
    What I'm saying is that "damping" is a meaningless term outside the context of resonance. If that resonance exists outside the audio range, then there's nothing that will excite it and damping becomes moot.
    Yes, they do behave as spring/mass systems and like any other spring/mass system will resonate at a certain frequency. Not sure what you mean by "anomalous" oscillations.
    No, it doesn't work at "any frequency." It only works at the resonant frequency.
    Again, damping is only germane to resonances. Here is the impedance plot of an LCD-3. Where do you see any signs of resonance?
    By contrast, here's the impedance plot of an HD-650:
    You can see its resonance centered between 60-70 Hz.
    namhkim likes this.
  14. impermanente
    "No, it doesn't work at "any frequency." It only works at the resonant frequency."
    Please I don't understand this, could you please explain why in your opinion should the damping work only at the resonance frequency? It seems obvious for you but I don't see it.
    The electrical damping works all the times the diaphragm moves, the movement in itself generates a current which generates a force opposite to the movement (damping force) this always happens just like for the same reason the diaphragm moves when the current flows from the amp into the coil (or a flat circuit printed on the diaphragm as in the case of the orthos). A speaker (orthos or dynamic) is just like an electric motor and just like a motor it works also as current generator. But probably I don't see exactly what you mean.
  15. Steve Eddy
    Because as I said, damping has everything to do with resonance. It has no meaning beyond that. Damping is the reduction of the amplitude of a resonance. Unless there is a resonance, or at least one within the audio range, then the whole idea of damping is moot.
    That in itself has nothing to do with damping.
    That's a whole other issue.
    Damping is all about controlling the resonance of a spring/mass system or its electrical equivalent. A spring/mass system will resonate at a particular frequency, it's "natural frequency." If that resonance is too great, you want to damp it. Damping is loss. Friction in a mechanical system or resistance in an electrical system. As you add resistance, the resonance becomes more and more damped.
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