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Introducing the New RST Tweeters From Dayton Audio

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  • Introducing the New RST Tweeters From Dayton Audio

    http://www.parts-express.com/dayton-...4-ohm--275-141

    http://www.parts-express.com/dayton-...4-ohm--275-131


  • dlr
    replied
    My earlier description was a little off. The ideal (infinitely rigid) dome tweeter has a gradually increasing phase delta on-axis between tip and the rest of the driver. Actually, this is a distributed phenomenon, requiring an integration over the entire dome surface at every frequency since all points on the dome radiate with their own delay, so the actual dome shape will have a significant impact on the phase response from every point on the surface of the dome. The result is that there is a gradually increasing integrated phase delta with increasing frequency. It reaches a maximum, but not at the frequency where the wavelength is equal to twice the depth of the dome. The maximum destructive interference is always where the phase delta = 1/2 wavelength. But this is not related to the total dome depth, meaning the distance from tip to surround attachment. It's a distributed phenomenon, so is probably somewhere part way between this distance, at a" ring around the dome. It's not half way because the shape of the dome determines this. Only an integration of the dome shape can determine this ring.

    The demonstration of the response, both ideal and for two simulated non-ideal materials is in this paper by B&W. I always thought this one to be a fascinating study.

    The key results from their modelling are in the set of graphs below. They modeled an ideal (infinitely rigid) dome, an aluminum one and a diamond one. They don't specify the dome shape. I suspect that is probably because there is no ideal shape, even for a theoretically infinitely rigid material. Change the shape and the interference pattern changes on-axis with likely a change to the minimum point, but also the off-axis changes as well, so the shape, as for real drivers, needs to take the off-axis into account as well as the on-axis. The shape and the material affect what they call "phase rolloff". Note that the driver breakup actually alters this significantly. They point out later that flat response is only attainable by utilizing breakup.

    Click image for larger version  Name:	800_Development_Paper_Comparison_Graphs.png Views:	1 Size:	455.5 KB ID:	1375531

    Next are some interesting quotes from the paper.

    The rigid response shows a characteristic roll off and deep null at approximately 70kHz caused by the interference effects owing to path length differences, commonly referred to as phase loss.
    The response of the diamond tweeter is much closer to that of the perfect rigid tweeter below 20kHz than the aluminium tweeter. More specifically, at 10kHz, the response of the aluminium tweeter is approximately 0.8dB higher than the rigid tweeter whereas the diamond tweeter's response differs by only 0.1dB. At 20kHz, the difference is increased to approximately 4.6dB for the aluminium tweeter but is less than 0.5dB for the diamond tweeter.
    Regarding figures 6 & 7 (Structural Acceleration):
    Note: this flatness of acceleration response is really what should be quoted in specifications; it indicates a purity in real performance which is not obscured by acoustic effects such as phase roll-off. A flat acoustic frequency response can only be achieved by utilizing break-up.
    In the end, the response due to speed of sound in the material used is not necessarily what allows for shallower dome shapes and improved response, it's but one factor of several, including structural rigidity. It was enlightening to learn that flat and extended response is based largely on utilizing breakup. A perfectly rigid tweeter would, in fact, exhibit a somewhat early roll off. This would probably require a phase shield, but for exactly the opposite reason they are used now. It would be used to raise the output at the top end rather than attenuate it as needed now for hard dome breakup peaks.

    dlr

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  • dlr
    replied
    Certainly they shape metal cones differently and partly for the reasons you state, but they still make domes (or cones), so it's all about maximizing response with tradeoffs. You can find many examples of drivers made from similar materials, yet they often are quite different in physical dimensions, even within a single manufacturer. Each will have its own on-axis vs off-axis response, so there aren't any set rules, so-to-speak, they are generalizations. Probably more variability is in soft domes since manufacturers use various methods to optimize the response of a diaphragm.

    I personally prefer the sound of soft domes to hard domes and that could be due to several issues, though I'm currently using the DXT in my main system for a property not in other tweeters and not having to do with the material.

    dlr

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  • TMM
    replied

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  • dlr
    replied
    Originally posted by TMM View Post
    Also since a ring radiator is still a soft diaphragm it still takes some time for the movement of the coil to propagate towards the center.
    This is true of all materials, it's just faster in hard domes.

    Theoretically if the dome is just the right stiffness, the propagation time of a soundwave inside the dome material from the coil to the tip of the dome will equal the propagation speed in air over a distance which is the height of the dome, so the sound emitted from where the coil is attached will be perfectly in phase with the sound emitted from the tip, at any listening position directly on axis with the tweeter.
    Theoretically this is only true at a single frequency, but we see the effect "phase in" so-to-speak, with gradually increasing phase delta below that single center frequency (or possibly even decreasing due to the various influences), then gradually increasing phase delta above it as well. The reason it's not an issue below that frequency is that the phase delta is small due to the wavelengths involved. The off-axis as similar, but a more complicated situation occurs due to shape and partial occlusion of parts of the radiating diaphragm. I think that this is the reason for slightly worse off-axis response of ring radiators. Without the dome shape, less of the diaphragm is occluded off-axis. This allows for more destructive interference patterns at various frequencies.

    Metal domes have a problem because the propagation speed inside the metal dome is much faster than the speed of sound in air, so sound from the tip arrives at the listening position slightly earlier than the sound emitted from where the coil is attached (= out of phase).
    I think that it's also true of soft domes as well. They simply have much better internal damping.

    Often this is why the soft dome version of a tweeter will have a taller more spherical profile while the metal version has a flatter profile.
    Actually this is primarily due to dome structural integrity. One tweeter manufacturer still makes a catenary (somewhat egg) shaped dome profile. This is why cones are most often used for larger drivers as well, coupled with techniques to damp transmission at higher frequencies where phase deltas are an issue and to damp internal diaphragm resonances and simultaneously providing improved off-axis performance, true of cone and dome.

    The last consideration is that the tip area of a dome is much smaller in radiating surface, so it's impact is less when there is a phase delta between it and the rest of the radiating area, dome or ring. But on-axis even a soft dome might benefit in a small way with a phase shield. Usually the off-axis is worse, however, with a phase shield. It's all a balance.

    dlr

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  • TMM
    replied
    Also since a ring radiator is still a soft diaphragm it still takes some time for the movement of the coil to propagate towards the center. The SB29RDC clearly still has breakup or minor phasing issues at the top like most dome tweeters of a similar size. In my experience the SB29 behaves better in waveguides than most soft domes, which is probably due to it's shallower diaphragm shape and slightly shorter distance between the coil and most protruding part of the ring. On the XT25 because of the diameter of the phase plug it is equivalent to an even smaller diameter dome. In addition to this they seem to have engineered the soft parts better than the SB29 so all the breakup is ultrasonic.

    Back on topic: The Usher 9950-20 (same as RS28F?) have almost dead flat response. I don't see the need for a phase shield on that. Theoretically if the dome is just the right stiffness, the propagation time of a soundwave inside the dome material from the coil to the tip of the dome will equal the propagation speed in air over a distance which is the height of the dome, so the sound emitted from where the coil is attached will be perfectly in phase with the sound emitted from the tip, at any listening position directly on axis with the tweeter.

    Metal domes have a problem because the propagation speed inside the metal dome is much faster than the speed of sound in air, so sound from the tip arrives at the listening position slightly earlier than the sound emitted from where the coil is attached (= out of phase). Often this is why the soft dome version of a tweeter will have a taller more spherical profile while the metal version has a flatter profile. One solution to the metal dome problem is to add a phase shield so sound from a portion of the dome is blocked.

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  • fdieck
    replied

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  • dlr
    replied
    There are issues with this as well. The maximum output is affected since the volume displacement is reduced vs. the same diameter dome. The other issue is slightly worse off-axis performance.

    dlr

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  • Pete Schumacher
    replied
    Hence the reason for "ring radiators.* With a soft dome, the center usually starts "flopping" and gets out of phase with the edges, so why not just pin that center down and get it over with?

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  • fdieck
    replied
    Are you telling me to go outside and play? I took my dog for a walk this morning went out in the back with her a couple of times. It may be warm enough to go swimming pretty soon. She has been several times this spring. She gets hot after her walk or her sun bathing and jumps in the pool to cool off.

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  • jhollander
    replied
    fred, we're just killing time until someone starts posting results, how's the weather in texas?

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  • fdieck
    replied
    There is only one thng worse that being talked about, not being talked about. Oscar Wilde.
    i would settle for this tweeter not being talked about anymore unless it is about how it sounds and measures acoustically. Anybody?

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  • DDF
    replied
    Originally posted by Dirk View Post
    It's a little unusual here because normally fabric domes have enough internal damping or are coated in such a way that sound propagates more uniformly. It's usually more of a "metal" problem.
    Soft domes are into break up at those frequencies so don't vibrate like pistons and don't have the path length cancellation issues.* The damping helps keep the response in control while break up is having its good ol time

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  • Face
    replied
    Originally posted by hongrn View Post

    That's what I'm going to do next, but as the opening of the tweeter is much larger than that of the wg, you're not going to have good results. Shaving the back of the wg to create a smooth transition between the tweeter and the wg is the only solution, but that will require some special tools which I don't have.
    You only need a router table, a jig, and the ability to take good measurements(not acoustic :D).

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  • KEtheredge87
    replied
    Originally posted by Dirk View Post
    Phase shields are used because tweeters are domes, and not flat. Sound propagating from the center of the dome would arrive sooner than sound arriving from the edges of the dome. Not a big deal with lower frequencies, but here the wavelengths are sufficiently short that you'd have phase cancellations and ragged response. So you cover part of the dome with the phase shield.

    It's a little unusual here because normally fabric domes have enough internal damping or are coated in such a way that sound propagates more uniformly. It's usually more of a "metal" problem.
    Thanks bud! That makes sense. I learn something new every day

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