Preliminary IMD measurements for subwoofers (2 Viewers)

[Ilkka R]

OK, so you're coming up with a dual driver sub soon, right?

But I would still like to know how your new '3HO compares to your previous subs, for example VTF-3 MK2 regarding IMD? They both have 12" drivers, so cone area will be the same. It was said that '3HO's driver has more Xmax, so does this translate to higher IMD at max output level?

 

[Peter Marcks]

That is a great question. IMD is made up of various non-harmonically related distortions, of which driver excursion-related distortion is just one component. Higher excursion clearly increases the driver excursion-related distortion component of IMD, but there are other elements of the driver design that affect IMD. The 3 HO driver should be a significant improvement in terms of overall distortion versus MK2 because it is using newer and more advanced motor technology.

Sincerely,

 

[Ilkka R]

I think there is only one way to find out? But it looks like '3HO won't be in stock when my next measurement session is due to happen. Maybe after summer then?

 

[Peter Marcks]

Another great question Ilkka, to which I really don't know the answer to, HA!

 

[LanceJ]

Anybody else getting the feeling this is more of an academic debate i.e. all these subs sound great/can crack windows/etc but some people are insulted that their sub is said to only create a four inch crack compared to the other guy's 4.5 inch crack?

Sheesh.

Anyway...........

Heres' something to cogitate on: from what I know of, a woofer that has X excursion capability needs a voice coil of a specific length so the correct amount of windings at any one time is always in the magnetic gap. If that excursion capability is increased, the vc needs to be lengthened also. But that will reduce the efficiency of the woofer because there will be a smaller percentage of windings in the magnetic gap at one time (since the windings are now spread out over a longer area), which means more power will be needed to move the cone in the desired manner. FYI: This issue always popped up in discussions of ported vs. sealed loudspeakers back in the olden days of the "East coast sound" and the "West coast sound".

Does this issue have anything to do with the distortion being discussed here?

 

[ScottCarr]

Lance,
I am on the same page as you BUT I am learning a lot from these discussions.

It is kinda like listening to the discussion of how lens distortion can affect the speed at which the transistor gates can be switched.

Sure the average consumer couldn't care less on how it happens jsut that it happens.

The wonderful thing about these threads is that some of the factors and theories that make these subs so good are shared in an open forum.

Sure it may seem it is my sub is better than yours but to be honest, before following these threads I didn't know what IMD was. (well it could have been an inter metal layer defect)

The other great thing is when I am get tired of the thread I simply skip over it. But I am not going to kid anyone, I was eagerly waiting for this thread to heat up. Sure there may be some mud slinging but in that exchange there is always information hidden in there.

As far as being discussed in the Subwoofer/Speaker section I couldn't think of a better place, and you can't beat the price of the beer or the hours in which this place is open.
Scott

 

[Robert SW]

Lancej,

Read this following thread carefully and report back in the morning.


http://forum.carstereos.org/f-offici...nologies-56398

Dan Wiggins on speaker motor tech -- from Adire's forum

Split gap (XBL^2) gets flat BL without a huge increase in coil windings. Used in the 'HO and also in my DIY's Tumult.

Bob

Sorry not enough posts here to post a real URL. Maybe someone else can post it cleanly?
Edit: Thanks Ilkka (his following post does it)

 

[Ilkka R]

http://forum.carstereos.org/f-offici...nologies-56398

 

[SteveCallas]

But what was your original statement that started this whole thing? Something to the effect of 'Drivers with high excursion will produce more intermodulation distortion than drivers with less excursion' - that's not a quote, just a general restatement. Then, when we found out that was an incorrect statement, it was changed to something like 'Drivers with high excursion will produce more doppler distortion' with doppler distortion being only one aspect of IMD. Then, we found that when a driver is mounted in a down-firing position, doppler distortion is seemingly cancelled out. Next, your company comes out with a sub that uses a driver that has 2" peak to peak excursion, and from then on you define "high excursion" as > 4" peak to peak excursion.....do drivers with a 50.8mm xmax even exist?

The whole argument is a fraud, as the only time in which the higher excursion could result in more IMD, "all else being equal", is at a point in which one driver is outperforming another in extension or spl. It's like you're saying that I'll get more sore from lifting weights than you will, yet I end up lifting 500lbs and you only lifted 300lbs. Sure, I may be more sore, but I lifted 300lbs much easier than you did, and then I went on to lift 200 more lbs. From Ilkka's test, the higher excursion driver is not only performing better within both of their limits, but it still has something left in the tank to extend deeper and louder than the other driver. So the driver with less excursion, in this instance, is a two time loser.

Until you can provide some proof or evidence to back up your claims, like Ilkka has, you are simply spinning your wheels holding on to a belief and looking quite foolish in the process - no offense intended.

 

[Peter Marcks]

Steve, frankly it is a bit insulting and disappointing to see you act and post with such hostility. It's no big deal though, it's always nice to be called a "fraud" and "foolish".

I don't know why it is so difficult for you to understand my argument. Ultra high excursion is ultra high excursion, did you think I meant 1" or 2" peak to peak? I've already explained many times before what we would rather do than go for ultra high excursion. Higher excursion, all else equal, exacerbates IMD. Of course, that is just one component of IMD, and there are other components involved too. The ironic thing here is that Ilkka's test results on IMD strongly support the philosophy that I discussed above about preferring to forgo ultra-high excursion for lower excursion and more cone area!

I am not going to respond anymore to this thread. Everyone else in this thread has been very respectful and thoughtful, which I am grateful for.

Sincerely,

 

[LanceJ]

Robert (and Ilkka): thanks for the link - just skimming over it for now already reveals a lot of good info to think about.

 

[SteveCallas]

Because it is flawed. The higher excursion may have more IMD, but only at a point in which the lower excursion driver can't "hang" anymore. It's like you wish to penalize the higher excursion driver for offering more capability, it makes no sense.

I apologize if I have offended you - not my intent. But at this point I'm not very interested in being politically correct - why can't we just tell it like it is?

 

[Peter Marcks]

OK, one last response

Steve, you are misunderstanding what I am saying. I am not saying that a driver with higher excursion will have higher overall IMD (including all the non-harmonically related components that make up IMD, including driver excursion-related distortion and other distortions) than a driver with lower excursion. I am saying that instead of moving to an ultra high excursion driver, I would rather use more cone area and lower excursion to maintain a given SPL, for various reasons including lower IMD. Read this again for why Ilkka's data supports this philosophy: http://www.hometheaterforum.com/htfo...75#post2979075

That is all, I am finished now, promise. Have a nice night.

 

[FeisalK]

no, really?

 

[dave alan]

Mark,

I fail to see the vague point. I think it's painfully obvious that Peter means this: Given the same basket, motor and cone, a design with more Xmax will have less Sd due to the surround (generally speaking), and therefore more excursion for the same output as a design with less Xmax...all else equal.

The other part of the point is more extremely the same point, which is multiple shorter throw drivers vs a single extremely long throw driver.

I doubt anyone here would argue against that 'philosophy'?

Bosso

 

[dave alan]

Ilkka,

Questions:

"Used 30 Hz and 70 Hz fundamentals are therefore good, because neither one of them matches with the tuning frequency of these subwoofers. That means the place of the vent doesn't cause a large difference/error here."

This is not confirmed by the test. For a measurements kinda guy, this statement seems out of place, IMO.

"I tested both subwoofers for IMD and THD (calculated for both 30Hz and 70 Hz fundamentals) using three sound pressure levels: 110 dB, 115 dB and 120 dB. Both subwoofers could have gone even higher, but unfortunately my microphone started to clip at higher levels, so I couldn't measure them."

Here you say clearly that both subs could have gone higher, then later you say...

"Axiom suffered from around 0.5-1 dB of compression at 120 dB test level, so adding input level by same amount was needed."

Hmmm, I find it hard to believe that the Axiom was compressing while delivering 120dB at 4". In your shootout, you have the Axiom delivering 105dB @ 2M GP @ 30Hz before compression. This should translate to around 130dB at 4", outside.

Also, in your shootout, you show the Axiom outputting 103dB @ 32Hz with 10.5% THD. This would translate to 128 or so dB @ 4", outside. In this test, you show 120dB @ 7.9% THD @ 30Hz which leads me to believe the numbers I'm imagining are correct, or at least more correct than the Axiom being compressed a dB at 120dB @ 4".

Is it the fact that there are 2 fundamentals simultaneously vs a sweep?

Bosso

 

[LeeLee]

So this is where everybody is!!!!

Wheeee! First post.

 

[Jack Gilvey]

I made that point any number of times in an AVS thread in which this came up. Dividing a given Vd over multiple woofers is desirable. Granted. My question is this: If a design is limited to a single 12" driver, is it "better" to have a lower-excursion driver? Or is it "better" to have more headroom available with the possibility of raised IMD at output levels the lesser driver would perhaps never reach?

In what ways will the lower-excursion single-driver subwoofer outperform the higher-excursion model?

Hsu's position is that they'd rather increase cone area than excursion, but they appear to do neither (unless we're saying that the smaller surround of a lower-excursion driver results in the "increased cone area" being referred to. What might the percentage increase be?)

I'd also like a definition of "dynamic distortion". It's the kind of term that gets fed to and wielded by fans, but without definition. I'm assuming it could refer to a distortion of dynamics, or "compression", but it may have other meanings. Is it measureable?

 

[LeeLee]

In fact, HSU is going the opposite direction, by using a longer excursion driver in the new VTF3-HO, which necessitated a fatter surround, and thus less cone area.

But of course they explained that their view of the downfalls of high excursion doesn't come into effect until the peak to peak excursion is something over 2.5" (thereabouts).

And I still don't understand why they got off so easily on the issue of IMD when Peter claimed a while back that their issue with IMD was primarily focused on the doppler effect, which as many have pointed out is nil when the woofer is at 90-degree orientation with respect to the lister, as with *all* down-firing subwoofers including all current HSU subwoofers.

HSU makes excellent subwoofers, and the new VTF3-HO is a beast and a great buy for the asking price. I even like the concept of the turbo, though I think the execution could have been more elegant. But I just don't like all the posturing that they are doing, with this holier-than-thou engineering philosophy that they try to use as a point of differentiation. If big box high excursion is so contrary to a "balanced design", why is HSU's latest/best subwoofer is such a fine example of a big box high excursion design.

 

[Ilkka R]

Well naturally it affects. If we think of two extreme examples, single sine wave and wide band pink noise. It is pretty obvious that the output is higher with a sine wave before compression.