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[Helge A Bentsen]

What is the definition behind the term "piston range" regarding loudspeaker drivers?

I tried google, but it came up with contradicting answers.
I've read about it in a few discussions regarding loudspeakers but so far I've not seen a good definition of it.

[Frank Koenig]

What is the definition behind the term "piston range" regarding loudspeaker drivers?

I tried google, but it came up with contradicting answers.
I've read about it in a few discussions regarding loudspeakers but so far I've not seen a good definition of it.

I'll give it a shot. The piston range of a loudspeaker is that range of frequencies, from zero up to some maximum, where the diaphragm may accurately be modeled (for a given purpose) as a rigid piston. In conventional cone drivers, where the cone is mechanically driven from its center, sound propagates radially outward in the cone material as a transverse wave (unlike in air where sound is a longitudinal compression wave). As long as the radius of the cone is small compared with the wavelength of this wave then the cone may be treated as a rigid piston. This is assuming the cone does not "break up" into some crazy higher modes. As an aside, one purpose of the outer suspension of the speaker is to terminate this transmission line to minimize the wave reflecting back.

Now the real loudspeaker designers can answer the question.

--Frank

[Helge A Bentsen]

I'll give it a shot. The piston range of a loudspeaker is that range of frequencies, from zero up to some maximum, where the diaphragm may accurately be modeled (for a given purpose) as a rigid piston. In conventional cone drivers, where the cone is mechanically driven from its center, sound propagates radially outward in the cone material as a transverse wave (unlike in air where sound is a longitudinal compression wave). As long as the radius of the cone is small compared with the wavelength of this wave then the cone may be treated as a rigid piston. This is assuming the cone does not "break up" into some crazy higher modes. As an aside, one purpose of the outer suspension of the speaker is to terminate this transmission line to minimize the wave reflecting back.

Now the real loudspeaker designers can answer the question.

--Frank

Thanks.

Any idea how to calculate upper frequency for a given cone diameter?
Or is one of those "it depends" questions?

[Chris Hindle]

Thanks.

Any idea how to calculate upper frequency for a given cone diameter?
Or is one of those "it depends" questions?

Definitely "It Depends".
Cone material and or stiffness, suspension compliance.
BUT, I am no speaker wizard. There's probably some rough guide, but then "season to taste" depending on a particular driver.
Chris. 

[brian maddox]

...

In conventional cone drivers, where the cone is mechanically driven from its center, sound propagates radially outward in the cone material as a transverse wave (unlike in air where sound is a longitudinal compression wave)....

--Frank

I've been doing this sound thing for decades and the simple truth of this statement has never occurred to me.  But it's obviously correct.  Simple inertia and physics dictate that it must be.

I've always visualized the cone moving in lock step with the coil as though they were all rigidly one unit.  But while i'm sure the driver designer would love for it to behave that way, it's just not physically possible for that to happen.

I've no idea what this newfound insight means from a practical standpoint.  But i'm pleased to have learned something new today.

[Chris Grimshaw]

Thanks.

Any idea how to calculate upper frequency for a given cone diameter?
Or is one of those "it depends" questions?

It does rather depend.
For instance, metal coned drivers will usually stay as a rigid piston until a fairly high frequency, where they'll become a huge mess of bell-mode resonances and ring like crazy. An 8" metal cone might be in cone breakup (where the cone is resonating and no longer acting as a piston) around 5kHz.
Paper isn't as mechanically stiff as metal, but it does self-damp better. Many 15" subs have a breakup peak around 1kHz. An 8" might be doing that around 2kHz.
Plastic cones are another step again in that direction - even more bendy, but usually better damping. So the 8" cone might be breaking up around 1kHz, but the response will usually stay pretty smooth until the high-frequency rolloff.

We usually use paper cones in the PA world, but once the cone is treated with coatings etc, the cone resonances will decrease in frequency and Q. How much, of course, depends on the cone's material and the damping applied to it.

As an order-of-magnitude rule-of-thumb, the cone break-up resonances will usually start to occur when a half-wavelength can fit between the surround and the dustcap. They're not always in that particular bending mode, but whatever mode they are in will be of roughly similar size.

Chris

[Frank Koenig]

It does rather depend.
For instance, metal coned drivers will usually stay as a rigid piston until a fairly high frequency, where they'll become a huge mess of bell-mode resonances and ring like crazy. An 8" metal cone might be in cone breakup (where the cone is resonating and no longer acting as a piston) around 5kHz.
Paper isn't as mechanically stiff as metal, but it does self-damp better. Many 15" subs have a breakup peak around 1kHz. An 8" might be doing that around 2kHz.
Plastic cones are another step again in that direction - even more bendy, but usually better damping. So the 8" cone might be breaking up around 1kHz, but the response will usually stay pretty smooth until the high-frequency rolloff.

We usually use paper cones in the PA world, but once the cone is treated with coatings etc, the cone resonances will decrease in frequency and Q. How much, of course, depends on the cone's material and the damping applied to it.

As an order-of-magnitude rule-of-thumb, the cone break-up resonances will usually start to occur when a half-wavelength can fit between the surround and the dustcap. They're not always in that particular bending mode, but whatever mode they are in will be of roughly similar size.

Chris

Thank you Chris for that guidance. Not for design but just for general visualization, it's probably safe to say that 15 inch paper cone drivers are good to about 500 Hz, and for other sizes frequency scales inversely with diameter.

Unfortunately, this is a spec that manufacturers are not in the habit of publishing. I suppose for a spec to be meaningful we'd need a standard for what constitutes the edge of the piston range, and that gets messy.

A good paper cone woofer is pretty flat in the region between resonance and the top of the piston range so I'm thinking that an experienced eye can make a good guess looking at a not overly smoothed frequency response. And while stroboscopes and laser interferometers come to mind as the tools to really measure this, I wonder if it's possible to glean anything from a careful examination of the electrical impedance curve? More questions than answers.

--Frank

[Chris Hindle]

More questions than answers.
--Frank

A sobering thought. The more we learn, the less we realize we actually know.
Chris.

[Chris Grimshaw]

More questions than answers.

--Frank

Hi Frank,

Yep, looking at the impedance curve is a good way to spot potential problems.
For instance, there's a small peak in the impedance curve on my Beyma 15P1200Nd-based subs at 1.2kHz, which is right where there's a peak in output, followed by a sharp dip before things get messy.
There's also a small peak in the impedance curve at 160Hz, which was predicted by the simulation software I use. That's a pipe resonance from the port - the length of the port is one half-wavelength of 160Hz, so there's a secondary resonance there after the designed Helmholtz resonance at 38Hz.
The Faital Pro 15HP1060 drivers are pretty much a drop-in for the Beymas, but they have a smaller impedance peak up at 1.9kHz. If we consult the published graphs, things start getting messy around there, too.

Generally, the speaker is starting to show narrowing directivity before cone breakup occurs.
As a rule, when the cone is equal in size to one half-wavelength, it's going to be starting to narrow. ie, a 15" cone will be starting to beam around 500Hz*. Going up a little higher would mean you can get the directivity of the cone to match a horn, with a, say, 60-degree horn needing a higher crossover point than a 90.

I found 800Hz to be good with the 10" drivers I use, matching to a 90-degree (horizontal) horn. As a compromise, though, for the big gigs I take the crossover up to 1.2kHz to give the HF driver an easier time. The result is the 10"s are now covering a range where they're beaming a little, so people stood off to the sides have a dip from 800Hz up to 1.2kHz. Not a big deal, really - better that than lose the HF when the going gets tough.

* Which is why I detest cheap 2x15" cabs with a 1" HF unit and a 2kHz crossover point. Sure, they might've got the response flat on-axis. Off axis, however, will be a mess. It takes a good HF unit to do a 2x15" properly.

Chris

[Art Welter]

Generally, the speaker is starting to show narrowing directivity before cone breakup occurs.
As a rule, when the cone is equal in size to one half-wavelength, it's going to be starting to narrow. ie, a 15" cone will be starting to beam around 500Hz*. Going up a little higher would mean you can get the directivity of the cone to match a horn, with a, say, 60-degree horn needing a higher crossover point than a 90.

I found 800Hz to be good with the 10" drivers I use, matching to a 90-degree (horizontal) horn. As a compromise, though, for the big gigs I take the crossover up to 1.2kHz to give the HF driver an easier time. The result is the 10"s are now covering a range where they're beaming a little, so people stood off to the sides have a dip from 800Hz up to 1.2kHz. Not a big deal, really - better that than lose the HF when the going gets tough.

* Which is why I detest cheap 2x15" cabs with a 1" HF unit and a 2kHz crossover point. Sure, they might've got the response flat on-axis. Off axis, however, will be a mess. It takes a good HF unit to do a 2x15" properly.

Chris

Chris,

In the pistonic range, narrowing does occur as you stated. Out of the pistonic range, the various ranges of decoupling make for very odd dispersion, as the virtual piston diameter changes with each different frequency, so some upper frequencies may actually have wider dispersion than lower frequencies. This behavior makes * all the more detestable, get it right on axis, and various off axis "gack" shows up off axis, spikes at various frequencies in the upper band of the woofer's range.

To be able to use the 10"woofer in  cabinets with a crossover around 1500 Hz, I experimented with various width slots, arriving at a 6" width to match the  -6 dB at 90 degree dispersion of the HF horn. As a "bonus", the slot also acts as an acoustical bandpass, making a passive crossover easier to get right without lots of extra parts.

4" and 5" slot  resulted in a bit too much upper dispersion, the 6" matched the HF dispersion nicely through the crossover region.

Art

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