Ask an Acoustic Engineer (me)

[dgage]

Speaking of unique surrounds, take a look at the unique surround from the fairly new company Purifi Audio.

https://purifi-audio.com/transducers/

[Jscoyne2]

Speaking of unique surrounds, take a look at the unique surround from the fairly new company Purifi Audio.

https://purifi-audio.com/transducers/

https://www.madisoundspeakerstore.co...-hr-15-woofer/

Similar

[Holmz]

...
Taking picture, upload from computer, selecting it... clicked upload file...? Just disappears, no image here...)
....

I know I have to hit the "upload" under where I select it (i am on an iPad)... and then that takes some patience.

[Justin Zazzi]

Morning Justin,
I have read this thread twice from start to finish, and it's like taking a Master's level course in acoustics. The level of knowledge displayed by you and other members of CAJ is astounding!

I'm really happy you are enjoying this!

With a lot of discussion around reflection inherent in the interior of vehicles and the necessary considerations you must account for in tuning, I am wondering what are some twists that must be considered when designing a system in a vehicle that will lack many of those reflections, such as a Jeep with no top or doors or another convertible type? One of the many features that DSP holds for me is the preset option's; I imagine a scenario where I save one tune for top on/doors on, but create another for summer months. I am reminded of how lighting systems in outdoor arenas (football fields) have gone from a flood effect which used to light up multiple city blocks, to now being so directional that you can stand across the street from a bank of lights at a stadium and it's dark as normal. Any thoughts? Thanks in advance!

A modern convertible vehicle like a Jeep Wrangler sometimes has a sensor that detects when the roof is on or off and it can adjust the stereo's tune based on this. Roof on means more reflections and the frequencies that are reflected on those broad hard surfaces (windows) are higher frequencies. So with the roof on the higher frequencies bounce around a lot more and they tend to seem louder than they really are.

Roof off means those high frequency reflections (and midrange too) are less reflecty and so the midrange and especially the treble seems to drop off. An open-air car like a Wrangler with no roof or a dune buggy or a motorcycle will need more treble compared to an enclosed vehicle at rest, and even more in motion because of wind noise.

Roof on vs off can also have some cabin gain effects on the bass performance which might need adjusting as well, especially in something like a Wrangler if the subwoofer is in the rear cargo area and is now exposed to open-air.

Speaking of presets, I always thought it would be neat to use the sensor in the passenger seat and then switch to a 2-seat tune when a passenger is present. Then switch back to a one-seat tune when you're driving solo.

Also, remember to change out your Winter air for Summer air in your sub enclosures. I usually do mine when I get an oil change

[Justin Zazzi]

Is there a general list or study or test result, of cone materials [even understanding there's going to be some variance possible from one vendor to the other - all aluminum cones aren't the same, all paper cones aren't the same, etc] or even a forum thread somewhere... just to at least give a general clue, showing what frequencies that even those first bending modes occur at?

For example, if aluminum *generally* has a higher first bending mode frequency than paper, and I'm looking to push my tweeter crossover point up higher for some installation design target - it might help to know "you might want to look into aluminum cone mids (along with whatever other materials might have 'higher than paper' bending mode frequencies). Does such a list exist?

You can look at the speed of sound in a material as a rough way to compare materials. A higher speed of sound means the first bending mode should appear at a higher frequency, which is better.

Attachment 11593

table borrowed from https://www.engineeringtoolbox.com/s...ids-d_713.html

The table above shows speed of sound for aluminum is about 6km/s whereas something fancy like Diamond or Beryllium is twice that at 12km/s. A low performing material might be something like cork with a speed of only 520m/s.

The speed of sound in a material is related to the elastic (Young's) modulus divided by the density. A higher modulus (stronger) and a lower density (lighter) is going to have a higher frequency for the first bending mode.

All that said, some materials have a really nasty breakup behavior and some materials have a really gentle pleasant breakup behavior. This is also driven some by the damping of the material so a larger damping is better which is another material property.

I don't think we can learn a whole lot from looking at tables and whatnot. I prefer really practical things like handling a piece of paper with my fingers and then handling a piece of thin aluminum and steel and titanium and .... so on. The sound you can hear just from handling the material with your fingers is really powerful.

[Blu]

Speaking of presets, I always thought it would be neat to use the sensor in the passenger seat and then switch to a 2-seat tune when a passenger is present. Then switch back to a one-seat tune when you're driving solo.

THAT would be cool!

[mauian]

Ok, I have a few questions that I'm curious to get some input on....


Speaker Surrounds
1a. I've noticed that some higher-end speakers like to use an "inverted roll" speaker surround. What is the advantage of an inverted speaker surround, if any?
1b. Is it true that speakers with a higher Xmax, typically have a "bigger"/"beefier" rubber surround? I'm assuming that the higher Xmax requires the surround to allow for more cone movement? I ask this because I noticed (for example), the 2.5" GS25 wideband speaker has an Xmax of 4mm and the GS690 6x9 speaker has a 5mm Xmax - yet their rubber surrounds are not even close to being the same "size". I honestly can't imagine that the GS25 really has an Xmax difference of only 1mm compared to the GS690. Just doesn't make sense to me.


Frequency sounds via different speaker materials
2. Does the same sound frequency sound different depending on speaker material? For example, does 18khz sound different on a metallic tweeter than it does on a paper cone wideband speaker? Or if a speaker produces the same frequency at the same level, will it sound identical regardless of the speaker material used? Another example - will a 200hz signal sound the same on a 6x9 woofer as it will on a 2.5" wideband speaker (again, assuming they are playing the frequency at the same level)? I *have* noticed that 200hz on a 6x9 will *feel* different than 200hz on a 2.5" wideband! I can *feel* the vibrations from the 200hz signal on the 6x9, where I can't on the 200hz signal on a 2.5" wideband speaker.

Thank you!

Hi jtrosky, this is an interesting question! I have these speakers and my guess on the discrepancy of why the Xmax for the GS25 and gs690 are so close is probably due to multiple factors and my understanding is as follows:
- even if those speakers are moving the same distance (4mm) and playing the same frequency, the smaller one will sound quieter as it’s not moving as much air
- To sound just as loud as the GS690, at say 300 Hz, it’s going to have to extend much farther to displace the same amount of air as the larger 6x9.
- the smaller speaker can probably get away with a smaller surround to move almost as far as the 6x9 because it is much lighter and the forces needed to move that lighter/smaller cone (and the stresses on the surround) are less than those of the 6x9. This depends on the motor (see below) and other factors I’m sure (cone material, etc)
- the motors of the two speakers also likely play a role. I can imagine two speakers that are otherwise equal but one has a better motor design that controls the cone better; this speaker may need a less beefy surround compared to one where the cone movement is less well controlled and flying all over the place.

Also interested to hear what Andy would say about this.


Sent from my iPhone using Tapatalk

[geolemon]

Hi jtrosky, this is an interesting question! I have these speakers and my guess on the discrepancy of why the Xmax for the GS25 and gs690 are so close is probably due to multiple factors and my understanding is as follows:
- even if those speakers are moving the same distance (4mm) and playing the same frequency, the smaller one will sound quieter as it’s not moving as much air
- To sound just as loud as the GS690, at say 300 Hz, it’s going to have to extend much farther to displace the same amount of air as the larger 6x9.
- the smaller speaker can probably get away with a smaller surround to move almost as far as the 6x9 because it is much lighter and the forces needed to move that lighter/smaller cone (and the stresses on the surround) are less than those of the 6x9. This depends on the motor (see below) and other factors I’m sure (cone material, etc)
- the motors of the two speakers also likely play a role. I can imagine two speakers that are otherwise equal but one has a better motor design that controls the cone better; this speaker may need a less beefy surround compared to one where the cone movement is less well controlled and flying all over the place.

Also interested to hear what Andy would say about this.


Sent from my iPhone using Tapatalk

I think you make some good points - some of them mirror what I noted, so that helps validate as well.

You and I both are basically essentially saying - we really don't think he (or anyone) would expect a 2.5" to play as loud as a 6x9, and we don't think he (or anyone) would expect a 2.5" to play as low as a 6x9.
The former is simply the basics of the difference in cone size, and it's definitely something to think of how much more excursion you'd need to have a 2.5" match the displacement of even a low-Xmax 6x9.

The latter is really the reason I think the surround difference truly exists, though.
It's simply unrealistic to think a 2.5" would be expected to play as low as a 6x9... especially considering a 6x9 is pretty comparable to an 8" round. Even for a wideband, it's just not realistic.
So, given the loudness differential, it makes more sense to use more cone, less surround.
...which likely happens to also add a touch more moving mass, lowering Fs a touch, also towards the design goals of a wideband - even though it's not expected to kick out the thunder on AC/DC drum tracks.

[jtrosky]

Just to clarify - my question was just based on the fact that I just didn't understand how the GS25 speaker cone could possibly move up to 4mm with it's surround, especially since the GS690 speaker cone only moves a max of 5mm with it's MUCH "bigger" surround.

Maybe I don't understand the Xmax "spec". I am assuming that an xmax of 4mm means that the speaker cone is capable of moving out by as much as 4mm (and that the "size" of the surround is relative to the amount of movement that is "allowed" by the xmax spec). I just didn't quite understand how the GS25 speaker cone could possibly move out 4mm with it's puny surround while the GS690 speaker cone is only able to move out by 5mm with it's pretty significant surround.

Hopefully I'm explaining myself properly. :-) The surround on the GS25 just doesn't seem "big" enough to allow 4mm of cone movement being that the surround on the GS690 only allows for 5mm of cone movement.

The surround on the GS25 is also significantly "smaller" than the surround on any of the other 3"/3.5" speakers I've seen (HAT S3SE, multiple 3" or 3.5" coaxials, etc) - all of which have xmax values less than 4mm.

Just something that I noticed and thought was strange, that's all. In reality, I can't imagine that the GS25 speaker cone would ever really need to move anywhere near 4mm in real-world usage - it barely seems to move at all, unlike bigger speakers that play lower freqs.

[geolemon]

Just to clarify - my question was just based on the fact that I just didn't understand how the GS25 speaker cone could possibly move up to 4mm with it's surround, especially since the GS690 speaker cone only moves a max of 5mm with it's MUCH "bigger" surround.

Maybe I don't understand the Xmax "spec". I am assuming that an xmax of 4mm means that the speaker cone is capable of moving out by as much as 4mm (and that the "size" of the surround is relative to the amount of movement that is "allowed" by the xmax spec). I just didn't quite understand how the GS25 speaker cone could possibly move out 4mm with it's puny surround while the GS690 speaker cone is only able to move out by 5mm with it's pretty significant surround.

That's why I differentiated Xmax from Xmech in my response earlier.
In general (exceptions in just a second)-

• Xmax is limited by the motor. It's simply the distance the motor can move before the coil starts exiting the magnetic field.
• Xmech is limited by the suspension. That's the distance the cone unit can move before the suspension bottoms out, or something strikes hard (cone on surround landing, leads pulling, VC former on backplate).

I say "in general" because there are trade-offs in motor design.

You can put all your windings right in the magnetic field, so that when they are energized you get ALL your motor strength... but essentially then you have 0mm Xmax, since as soon as the voice coil moves in either direction, you start pushing windings out of the magnetic field. This isn't far off from how lots of SPL subs are designed though, where BL/Re (motor strength) is "the" goal.

You can spread out your windings, over a long distance, covering the whole voice coil former below the spider, if you like. Now, you have tons and tons of Xmax, but only a tiny portion of the energized windings are actually IN the magnetic field. So most of that power is wasted, and your BL/Re is very low. Lots of old-school long-throw subs use this technique.

And independently, your suspension and basket (and to some degree the length of the VC former) comprise Xmech, which define how far the whole assembly can actually physically move.

Usually, your Xmax is reached before your Xmech, which usually keeps people out of trouble - no hard parts hitting.
But there have been exceptions. I think it was the original RE XXX that had such a long-throw XBL^2 motor, but used an off-the-shelf basket and spider, such that the Xmax and Xmech were either nearly the same, or maybe even the Xmech was less than the Xmax. At any rate - if I'm thinking of the wrong sub, it's happened... so it's still possible, but unlikely to be something you run into.

...and my main point is "the alignment will be determined by the intended usage", which brings me back to my theory:

The 6x9 is much more similar to a bass driver than a midrange. So it needs excursion capability. A user will expect it to play right down to their subwoofer - or even to BE a subwoofer. So it makes sense to ensure the Xmech is significantly longer than the Xmax... whatever the Xmax target might be.

A 2.5" speaker is going to be expected primarily to be used as a midrange - maybe even as a tweeter. The whole "wideband" application is a bonus, and realistically can still only play so low. And your available hard parts don't have much excursion room... plus you might be designing a speaker to fit into a small sail panel, so can't be too deep, etc...

So if that 2.5" has an Xmech that's nearly the same as the Xmax, that's fine... it's not expected to play that low. The Xmax in this case might simply be here for linearity - because the BL curve for any given speaker will be flatter when played at the center of it's motor excursion, and that reduces intermodulation distortion (intermodulation literally means "fluctuations when moving", and the distortions could be caused either by motor strength changing across the excursion range, or the suspension compliance changing across the excursion range), which also makes sense for a midrange since it's the most audible type of distortion, and we most easily perceive audible distortion in the midrange realm.

So it's not the Xmax that's different between these two drivers, it's Xmech - and I truly believe that's by design because of how they will/can be used:
Having a similar Xmax makes sense for two different reasons (raw excursion for bass vs flat BL for midrange), and having different Xmech makes sense for the same reasons.