Dumax Xmax....unfair????

With such an intelligent forum community, I thought I would start a "discussion" thread that would solicit opinions on an issue that I am sure has had it's share of attention. People rate Xmax so highly and I am beginning to wonder why so much emphasis goes into designing that spec into the driver.

All other parameters being equal you have 2 drivers:

driver #1. Xmax=20, Bl=27, (Xsus=30)
driver #2. Xmax=23, Bl=18, (Xsus=30)

All other parameters are equal, (just accept it). If it helps, assume Bl^2/Re of drivers are equal.

Wouldn't driver #1 likely be less distorting at high excursions? What really matters is the cone control, as measured by Bl/Mms at that excursion. At Xmag, the Bl of Driver #1 is 19 (.71 x Bl), which is higher than driver 2 at neutral. What is the point of measuring Xmag at 71 percent of the peak strength?
Distortion is the issue for discussion.

If motor Horsepower is important, I pick the stronger motor, driver #1.
 
The BL product alone has nothing to do with distortion. It's perfectly possible to create a driver with a BL product of 2 that has near-zero distortion (look at some high-end tweeters, for example)

What *is* important to look at however, is the flatness of the BL vs. X curve, along with the flatness of the Cms versus X curve.

Ideally, you'd want these to be a horizontal line for as wide of an excursion range as possible. If these aren't "flat", then that means that the driver responds to each part of the signal differently, depending on where the cone is positioned - this creates distortion. Also, you want both the BL vs. X and Cms(or Kms) vs. X curves to be as symmetrical as possible - asymmetry of either of these will also contribute greatly to distortion.

Have a look at the XBL^2 motor tech paper on Adire Audio's website for some more information on the subject.

(www.adireaudio.com)

Also, keep in mind that many people, including myself (not so important) and Nick McKinney (more important - he designs driver motors for a living) believe that BL/Mms and BL^2/Mms is a worthless derived number, and should be ignored.

BL^2/Re is a little better, but even that doesn't tell the true story.

I tend to agree with Dan Wiggins (from Adire Audio) in that Qes is probably one of the best power -> weight indicators - lower Qes, better power -> weight ratio, and vice versa.

For further reading on the subject, take a look at Nick's article here:

http://www.lambdacoustics.com/library/whitepapers/bl_mms.htm

At any rate, the point I'm trying to get across is that motor horsepower *is* helpful, but if it's not relatively constant throughout the driver's usable excursion limits, then it's useless.

I'd much rather have the "BL=27" driver reduced to a BL of 15, if it could keep that BL product absolutely constant over a +/-25mm excursion range.

- Rick
 
Also, it should be noted here that there is no need for a "fast" subwoofer.

The measure of driver "speed" is simply bandwidth - since 99% of our subs are bandlimited to below 80hz by the crossover anyway, subwoofer "speed" is a non-issue.

People talk on and on for lengths about "fast" and "slow" bass, but neither of those have any basis in the actual speed of the driver per se - they're just subjective opinions about the bass quality. Much of this perceived "speed" can be attributed to alignment differences, which will produce differences in response peaking, ringing and overshoot in the system's transient response, and the like.

- Rick
 

CHRIS8

Disabled Account
2001-12-12 8:47 am
VA, USA
I agree ThingNess. I am also tired of hearing about 'fast' speakers. At one time, I too thought that their was such thing as a 'fast' speaker. I felt silly when I realized my error.

I've gotten into arguments over this too many times with people. However, IME I have noticed that a woofer that is called 'slow' is in actuality a result of:

1. Inappropriate bass alignment.
2. A poorly designed driver with a relatively flexibible cone
3. A poorly degigned motor that quickly become non-linear into even moderate excursion levels
4. Crossing two drivers such as a midbass and subwoofer, with the drivers being approx. 15 degrees(subjective value) or more out of phase with each other at the listening position.
5a. Has a lower effective frequency response(some people misinterpret lack of some low bass as 'fast')
5b. Has a lower effective frequency response, exciting room modes that were previously not a concern with the last loudspeaker that they used.
6. Placement in room of their 'new' speaker.

In the event all other things are equal, a driver with a lower motor strength to weight ratio results in one thing: lower sensitivity

BTW, here is a good paper on driver linearity:
http://www.klippel.de/pubs/aes2000/klippel paper 109.htm

-Chris
 
CHRIS8,

Those were some very good points, and I strongly agree with 5a, 5b, and 6 in particular - I had overlooked them in my original posts, so thanks for bringing them to light.

It's funny, becuase I was just sitting here thinking about the BL product and how its only real effect on the subwoofer system was efficiency and the change in Qes/Qts, and was just about to post a response to that effect, when I saw yours. :)

Good work!
 
A high BL driver is better because all else being equal, the high BL driver will be more efficient (as CHRIS8 said above.)

Changing the BL product will also change the Qes, and thus the Qts, and thus the final alignment of the system as well. I happen to prefer low Qes/Qts drivers due to their high efficiency - a high BL product just happens to contribute to this.

As for the Brahma, it is a very linear driver, but one that doesn't have a particularly "strong" motor - this is by design, though. Adire designed it specifically for car high SPL applications - in the car, enclosure size is very limited, and many (perhaps most) enclosures are small sealed boxes.

For a small sealed box, taking into account the car's natural transfer curve, a Qts of approximately 0.4-0.6 can be considered "ideal" - I think the Adire driver was designed with this in mind, and the BL product was manipulated to achieve the intended Qes, and thus the intended Qts. In this case, to achieve the proper Qts, the BL product was made relatively low, and the rest fell into place.
 

CHRIS8

Disabled Account
2001-12-12 8:47 am
VA, USA
"With respect to your discussions about "fast" bass. Can you cite the advantages then of multiple smaller drivers in place of fewer large drivers?"

Is this an interogation? LOL!

Advantage of smaller drivers? Their are no theoretical advantages IF in both instances thier is an adequate amount of displacement, linear distortion, IM distortion and Harmonic distortion being equal, and the bandwidth that you require is possible.

However, their are other REAL issues such as cost effectiveness, typical behaviour, etc. Here are some brief points:

- If you require a final SPL that requires many small driver, as compared to few large driver you will find that it is cheaper to simply buy two 12" or 15" drivers as compared to 6-10 8" woofers, 4 10" drivers, etc. depending on displacement capabilities.

- Less labor is required to build an enclosure for a couple of 15" drivers rather than 10 8" woofers, as using many small speakers will require very extensive front baffle renforcement and alot more hole cutting.

- In many large diameter drivers, the same model speaker with the smaller cone unit will typically have margninally lower linear distortion because it will have a more rigid cone that will remain more pistonic at moderate to high excursion levels RELATIVE to the excursion of the larger unit. This is not true for all driver makes, but is a factor in some instances.

- Even if the same SPL level output and lower frequency extension are met with relatively equal levels of distortion in both cases, many people state that a larger driver sounds more natural for lower frequency applications. I have yet to see measurements explain this phenomena, and I am undecided since performing a test to actually evaluate this is difficult. However, because of reality; the afformentioned linearity issues and lower extension abilitites and how they are percieved, the smaller driver may be found to be 'faster' by people. If you are building a mono subwoofer using 2 drivers it would be a good idea to use the woofers in the same volume in a push-pull configuration in order to further reduce linear distortion.

- The last statement usually does not apply, since typically larger drivers have a more extended lower frequency response than smaller drivers. It is rather difficult to match LF extension, transient response behaviour, and distortion levels thus an accurate comparison is very difficult to perform in order to conclude this issue IMO.

Well, the above points should provide a better basis for judgement. But, in the end if high SPL and/or very low frequency extension is required, you should just stick with large drivers.

-Chris
 

AndrewJ

Member
2001-09-05 6:37 pm
I largely agree with Chris8 about "fast bass". The loudspeaker company KEF was involved with experiments on the audibility of phase response at low frequencies, and built speakers that through equalization were adjustable in low frequency response and alignement, even including responses flat to 5hz (though with limited output capability at that frequency!). Additionally through DSP processing they could alter the phase response and amplitude response independantly. It was found that the impression of bass "quality/speed/timing/etc" is tied to the low frequency alignement, independant of the upper bandwidth of the driver, since as noted, the bass unit is filtered at around 80hz anyway. Subsequently, during the development of the KEF 107/2 speaker, with its built in low frequency equaliser, I did many additional experiments on the audibility of the low frequency alignement. Generally I found that an extended low frequency response, even if this involved a gentle downtilted (ie very low Q) response, gave a more dynamic up-tempo sound compared to a a maximally flat fast roll-off response.
After talking to a bass player (Red Mitchell) we realised that the explanation may be that when an instrument such as a string bass is played, the lowest notes have to be plucked slightly ahead of the beat compared to higher notes, so that ,as he described it, the note "explodes" on the beat. This is because the fundamental of the lowest tuned strings take longer to build up to full level than the harmonics (and the lower the note the longer the build up time), so the apparent timing is altered unless compensated for. Therefore, if the relative level of the fundamental compared to the harmonics is changed in reproduction , then the apparent timing of the "explosion" is changed and hence the impression of playing on , before or behind the beat. This changes the whole impression of the tempo of the music.
As for is high Bl better, hence the more the better, this is not true either. Although higher Bl will increase efficiency, this is only in the high frequency range. At the lower frequencies the output will decrease as Bl is increased (lower Q). For a given moving mass/cone area/box size there is an optimum Bl that will give the desired response alignement. If you want to pay for more magnet, you should spend this by spreading that Bl over a longer more linear gap and so increase output capability and lower distortion
 

CHRIS8

Disabled Account
2001-12-12 8:47 am
VA, USA
"Although higher Bl will increase efficiency, this is only in the high frequency range. At the lower frequencies the output will decrease as Bl is increased (lower Q)."

Excellent observation AndrewJ.

I almost overlooked this inverse effect on low frequencies, where Q itself is the primary factor to efficiency, since in this circumstance resonant magnification decides output level efficiency(higher Q results in less non-acoustical energy dissapation around resonance in a given alignment).

-Chris
 

CHRIS8

Disabled Account
2001-12-12 8:47 am
VA, USA
masterp2, while I do enjoy helping(as do many many others), it personally believe it would be better FOR YOU to do more research and find the in-depth answers that you seek. Please don't misunderstand, I enjoy answering your questions but this is supposed to be for your benefit. In that perspective, I believe that you will learn/discover alot more if you find some answers for yourself. I know that I learn 10x more by searching for the answer myself, and I imagine it's the same for anyone.

I mean this in the nicest possible way, it's just a suggestion.

-Chris