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Influence of driver shape (dome, cone) on sound

q3cpma

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#1
After seeing multiple well-known pro gear manufacturers arrive to the almost same design for midrange - a 3" textile dome - I started searching about hard data about the way driver shape (cone, dome, phase plugs, etc...) influences its sound but found nothing. Anyone with such information and/or books or articles about it?

Intuitively, I'd say that directivity is the main thing being influenced but intuition is not of value to me, here.
 

oivavoi

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#3
I don't think there's any grand mysteries here. It's usually the case that the optimal size of drivers follows from the wave lengths they are asked to reproduce. Tweeters are usually small - have you seen any 15-inch drivers asked to reproduce frequencies above 10 khz? Woofers are usally big - or they have very long excursion (or both). Midrange drivers are usually somewhere in between.

And yeah, directivity is important. The ideal is that all frequencies have the same dispersion, but this is almost never the case. But smaller midrange drivers will often have a dispersion characteristic at the crossover frequency which is more similar to the dispersion of the tweeter.
 

617

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#4
After seeing multiple well-known pro gear manufacturers arrive to the almost same design for midrange - a 3" textile dome - I started searching about hard data about the way driver shape (cone, dome, phase plugs, etc...) influences its sound but found nothing. Anyone with such information and/or books or articles about it?

Intuitively, I'd say that directivity is the main thing being influenced but intuition is not of value to me, here.
I think the domes generally have wider directivity than a midrange cone of similar size, and generally have low mass which allows for extended treble. 3" is about as big as you can make a dome - the ATC ones are very good but expensive and have huge motor systems. The advent of more robust tweeters has lessened the need for really extended midrange drivers - Spendor and a/d/s used to make a lot of systems with mid domes, but back then tweeters couldn't go down to 1.8khz like they can nowadays.

You don't see midrange domes much smaller because the low end response of the driver suffers and at that point you just get a tweeter. SEAS makes a tweeter which is firmly between the dome midrange and dome tweeter categories - 35mm:
http://www.troelsgravesen.dk/large_domes.htm

As far as driver shape, pistonic drivers directivity is primarily influenced by size. The profile of the cone - conical, shallow, deep, curved - is a structural issue which impacts breakup more than anything, and shouldn't influence sound too much if it's in the stopband as it should be. The sb satori woofers have probably the widest, smoothest linear response of any current hifi driver and they use a relatively deep cone, but to get a 6" driver to play to 5k or whatever requires a lot of optimizations across the whole driver. Everything about those satori drivers is a little special - the wire leads, the frame, the surround, the cone material.

The one driver type which breaks all the rules are BMR drivers such as those produced by Tectonic elements. These are little drivers - normally smaller than 3", with flat diaphragms, which have better HF dispersion than most tweeters, and can produce upper mids well. There are some diyers using these, and if I remember correctly at least one person prefers them to the morel 55mm dome mids.
 

Cosmik

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#6
KEF say that the ideal shape is a hemisphere that expands and contracts, and that their tangerine waveguide converts a conventional moving hemisphere into something that approximates the expanding one. I think.
 
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#8
After seeing multiple well-known pro gear manufacturers arrive to the almost same design for midrange - a 3" textile dome - I started searching about hard data about the way driver shape (cone, dome, phase plugs, etc...) influences its sound but found nothing. Anyone with such information and/or books or articles about it?
ATC worked on a dome midrange. It had fairly high sensitivity and low distortion, so naturally, some companies found it interesting and used it in their designs. Then, ATC withdrew from the public market, so other companies were left dry. Volt has since made its own version to sell to certain companies.

Lower mass does not imply extended upper response but a higher efficiency in the mass controlled region, above Fs. Lightweight and stiff materials, if sufficiently light or stiff (high sound velocity), may push higher modes well beyond the band of interest. The driver would behave a reasonable approximation to a rigid membrane in the band of interest.

The low-velocity materials would have the portion of the membrane in-phase with the driving coil steadily collapse upon it as frequency increases. This would result in a mild decrease in the driving surface at higher frequencies, an annulus for edge-mounted coils or a smaller effective membrane size for a cone. This can result in the cone's directivity remaining low over a slightly wider bandwidth.

Regarding KEF, a pulsating sphere would be an optimal source to present to a conical horn, the midrange driver. But as they note, a spherical source is not readily made by the axial motion of the tweeter. The radial phase plug should be there to ease the wave transition into the throat.
 
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617

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#9
ATC worked on a dome midrange. It had fairly high sensitivity and low distortion, so naturally, some companies found it interesting and used it in their designs. Then, ATC with from the public market, so other companies were left dry. Volt has since made its own version to sell to certain companies.

Lower mass does not imply extended upper response but a higher efficiency in the mass controlled region, above Fs. Lightweight and stiff materials, if sufficiently light or stiff (high sound velocity), may push higher modes well beyond the band of interest. The driver would behave a reasonable approximation to a rigid membrane in the band of interest.

The low-velocity materials would have the portion of the membrane in-phase with the driving coil steadily collapse upon it as frequency increases. This would result in a mild decrease in the driving surface at higher frequencies, an annulus for edge-mounted coils or a smaller effective membrane size for a cone. This can result in both the cone's directivity remaining low over a slightly wider bandwidth.

Regarding KEF, a pulsating sphere would be an optimal source to present to a conical horn, the midrange driver. But as they note, a spherical source is not readily made by the axial motion of the tweeter. The radial phase plug should be there to ease the wave transition into the throat.
Thanks for sharing your expertise and welcome to ASR.
 

oivavoi

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#10
Regarding KEF, a pulsating sphere would be an optimal source to present to a conical horn, the midrange driver. But as they note, a spherical source is not readily made by the axial motion of the tweeter. The radial phase plug should be there to ease the wave transition into the throat.
This is part of the reason why I'm getting increasingly unsure about the whole waveguide/horn idea. How can it be good to pressure and massage the soundwaves into a different form right after they're emitted from the driver? One might think, for various reasons, that a waveguide or horn is nevertheless a trade-off that is worth it (directivity, efficiency etc). But it seems intuitively reasonable to me that it would be better to achieve even directivity without the use of waveguides.
 

617

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#11
This is part of the reason why I'm getting increasingly unsure about the whole waveguide/horn idea. How can it be good to pressure and massage the soundwaves into a different form right after they're emitted from the driver? One might think, for various reasons, that a waveguide or horn is nevertheless a trade-off that is worth it (directivity, efficiency etc). But it seems intuitively reasonable to me that it would be better to achieve even directivity without the use of waveguides.
All drivers are loaded into some kind of acoustic environment which bends them. A flat baffle is just a 180 degree waveguide with dramatic discontinuities at the edges. Same phenomena. Your intuition about discontinuities near the diaphragm itself is on the money, which is why tweeters have such carefully shaped faceplates and why phase plugs in compression drivers are so critical.

An interesting thought experiment. Imagine a 1" dome loaded into a 6' 1" tube. It's going to be louder at the exit of that tube than if it radiated into free space. Now imagine it mounted on a wall. Quieter than the tube. Now imagine mounted like on a b&w speaker with no baffle around it. Quieter still, probably. A waveguide is simple between the wall and the tube in that continuum. A wide baffle speaker loads a given driver differently than a narrow baffle.

You can achieve directivity using arrays to a certain extent but it's not ideal at HF. Alternative methods of pattern control are a really interesting idea. Unfortunately sound waves really like to spread out in space.
 
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oivavoi

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#12
All drivers are loaded into some kind of acoustic environment which bends them. A flat baffle is just a 180 degree waveguide with dramatic discontinuities at the edges. Same phenomena. Your intuition about discontinuities near the diaphragm itself is on the money, which is why tweeters have such carefully shaped faceplates and why phase plugs in compression drivers are so critical.

An interesting thought experiment. Imagine a 1" dome loaded into a 6' 1" tube. It's going to be louder at the exit of that tube than if it radiated into free space. Now imagine it mounted on a wall. Quieter than the tube. Now imagine mounted like on a b&w speaker with no baffle around it. Quieter still, probably. A waveguide is simple between the wall and the tube in that continuum. A wide baffle speaker loads a given driver differently than a narrow baffle.

You can achieve directivity using arrays to a certain extent but it's not ideal at HF. Alternative methods of pattern control are a really interesting idea. Unfortunately sound waves really like to spread out in space.
Excellent reply! You have a gift for explaining complex physical sound phenomena in an easy and comprehensible way.
 

RayDunzl

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#13
Is there really nothing about it?
Cones are driven near the center.

Domes (I believe) are driven at the edges.

That, I suspect, is the main consideration.
 

RayDunzl

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#14
Regarding KEF, a pulsating sphere would be an optimal source

I've wondered if pulsating pressure applied to the inside of a "balloon" would create a viable omni speaker...

Maybe there are modern materials that would contract and relax with applied voltage, eliminating the problematical mechanical aspect of my hypothetical design.

Hmm...

https://en.wikipedia.org/wiki/Electroactive_polymers
 
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617

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#15
15727099069586767804999611432967.jpg


I've actually been trying to make an Omni mid treble unit but 3d printing it has been a pain the ass. I found the perfect driver for it. 12 drivers.
 
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#16
I did see some attempts at building a dielectric polymer speaker, one even with its Class D electrostatic amplifier. These are not near commercialization, but the class D-electrostatic angle is interesting enough considering efficiency alone.

This is part of the reason why I'm getting increasingly unsure about the whole waveguide/horn idea. How can it be good to pressure and massage the soundwaves into a different form right after they're emitted from the driver? One might think, for various reasons, that a waveguide or horn is nevertheless a trade-off that is worth it (directivity, efficiency etc). But it seems intuitively reasonable to me that it would be better to achieve even directivity without the use of waveguides.
Most cones, being less than rigid, should have some midrange effect that is recognizable as horn loading. Horn effects aside, the directivity limitation of conventional designs, ones that are based on rigid pistons, are well known as to be textbook. While material choice can mitigate the presence of higher-order modes from the diaphragm, this limits you to a well-defined pattern.

The BMR can be likened to an extreme case of a carefully damped flexible cone. It yields an extended high-frequency response on-axis from the flat profile. The modes beyond the zero mode, the piston, fill in the response off-axis. The obvious problem with the BMR is that the added masses, as well as the thick membrane, result in low efficiency above Fs, even by the standards of compact drivers. The natural applications are limited to close use and lower SPL, such as a compact speaker.

Scaling up, there is also the distributed mode loudspeaker. AFAIK only sold by Tectonic Labs, as with the BMR. Presents a diffuse sound source and can offer a very wide off-axis response. But being diffuse, it should sound quite different from conventional loudspeakers both in the direct sound and reflections. Most of the DMLs also include ribbon tweeters, which makes me wonder how they fare in the upper octaves.
 
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jhaider

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#18
My hunch is that dome midranges are often used in pro monitors not because of any unique property of the diaphragm shape, but rather the voicecoil. A dome is edge driven. A 3” dome will have roughly a 75mm diameter voicecoil. A 3”-4” cone will have a 20-25mm voice coil. The advantage of the larger coil is theoretically lower power compression (more conductor, more cooling area) and therefore theoretically higher reliability at sustained high SPL use.

However, newer companies (such as Barefoot) often use cone midranges. Modern drivers have much better voicecoil heat management than older drivers, with lots of ventilation above and below the spider and forced air movement through the coil.


View attachment 37641

I've actually been trying to make an Omni mid treble unit but 3d printing it has been a pain the ass. I found the perfect driver for it. 12 drivers.
Which driver? SB 2.5”?
 

Juhazi

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#19
Cones and domes have fundamentally different off-axis radiation pattern. Shortly, domes have smoother off-axis behaviour. On the other hand cones have more effective on-axis radiation area (with same diameter) which helps in low end of passband, higher spl/less distortion. Difference in "sound" is not so big, if any, unless either one is pushed out of it's comfort zone.

Legendary Zaphaudio measurements, first 3" dome , then 3" shallow cone





More basics from Klippel
https://www.klippel.de/fileadmin/_migrated/content_uploads/KLIPPEL_Sound_Radiation_Poster_01.pdf

We have some modern large domes, eg. from Devialet and Accuton.

 
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#20
The TB's dome area is clearly much larger than the Hi-Vi's cone area. The "3 inch" domes have nominal Sd closer to "4-4.5 inch" cones, such as the Scan-Speak 12MU. Most domes are described by VC diameter and cones by the frame.
 
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