[fredk]

I was told on another forum that this is a bad idea as the driver looses linearity. By the designs and tunings posted here, that does not seem to be a concern though.

Why is that and what might the effects of tuning lower than the f3 be?

 

[fredk]

DOH! Should be fs not f3...

 

[Mike P.]

Tuning below the Fs results in more low end extension at the expense of overall SPL. Non linearity can occur when a sub is asked to reproduce frequencies below the tuning point, a situation that is rectified by using the proper Hi-Pass filter.

 

[fredk]

So, other guy writes:

"xmax has no importance on how low a driver should be tuned. It has everything to do with Fs and Qt. Generally a speaker is best tuned a little above F3. If you try tuning below Fs, then all you get is high distortion and poor output. The models are often deceptive if you do this. You have to model knowing what works and is practical. That takes a little experience."

You write:

non-linearity may happen with tuning below fs.

These may two very different statements. Why might he feel that tuning below fs will result in distortion (non linearity?) and you feel that is not always the case?

My appologies if I am mixing terminology between distortion and non-linearity. I'm just trying to get a better understanding of the design side of a DIY sub.

 

[Mike P.]

non-linearity may happen with tuning below fs.

Below the tuning frequency a sub will "unload" causing distortion. A Hi-Pass filter controls this.

I can't speak for anyone but me. The link in my signature will take you to the Subwoofer Database. May a sub is tuned well below the Fs with no complaints from the owners.

If you try tuning below Fs, then all you get is high distortion and poor output

It's apparent from the above statement the author has never heard a large low tuned ported sub.

 

[fredk]

Actually he has, and doesn't like them, but thats neither here nor there. Could simply be bias.

He has also had quite a bit of experience with designing speakers (as an amateur) and recording live classical performances, so he is not clueless.

My first thought after reading his post was the Mal-x 18. It seems to do just fine tuned well below its fs

I am wondering if this guy is just used to older tech drivers that do not exhibit the kind of linearity that the newer high excursion drivers show.

I would like to understand the relationship between excursion and fs.

 

[Ryan Anderson]

IMHO it sounds like you're talking 2 different things. I've been watching the thread since it was started... I too am curious.

I think Mike is speaking of what happens below tuning in a vented/ported enclosure's F3 (Fv or Fb? maybe that's the confusion...)

So what happens if you tune below the driver's Fs?

How about in a sealed, 4th order, 6th, 8th, TL, TPQW, horn..... what happenes below driver Fs?

Edit:


`Fc'
System resonance (usually for sealed box systems), in Hz

`Fb'
Enclosure resonance (usually for reflex systems), in Hz

`F3'
-3 dB cutoff frequency, in Hz

 

[fredk]

You may be right Ryan since I originally used F3 vs Fs. The original context was for a ported enclosure.

I just want to get a better understanding of the non-theoretical impact of tuning below Fs. Models asume that the driver is perfect, but we know otherwise.

 

[fredk]

OK, I finally had a little time to play with WinISD so I'm going to take a stab at answering my own question.

I'm going to use the CSS SDX15 as an example using 3 different models: the default 7cuft ported tuned to 19.31Hz (Yellow) which is Fs, 9 cuft tuned to 17Hz (grey) and 11cuft tuned to 15Hz (blue).

Max SPL:

My understanding is that this is where I would look at the tradeoff of headroom vs extension. Looking at these three graphs, I have little or no headroom loss in upper bass (above 50Hz??). I loose ~1db between 25Hz and 35Hz with the 17Hz tuning, and loose 1-2db between 21Hz and 46Hz.

In return, I get an extension of flat room response from say 17Hz down to about 13Hz (assuming 5db room gain). 2db of headroom for 4db of extension seems to be a good tradeoff.

Now lets look at excursion, and by extension distortion (maybe??). I am going to make an asumption here that I can get linear excursion to about 80% of Xmax. I don't know if that is realistic or not.

So, 80% of 30mm = 24mm and look at that, we get clean excursion all the way down to 16.6Hz with the tuning at Fs. With a tuning of 17Hz, we exceed maximum linear extension at 28Hz down to 21Hz. Thats definately in audible territory. Similarly, we exceed linear excursion between 30 and 17Hz with the 15Hz tuning. The amount of excursion for a given frequency (and thus distortion) also increases.

So, how did I do? Is it E for effort of B+ for [ahem] brilliant deduction?

If I got it more or less right, the question is, how much distortion does it really add and is it audible?

 

[Mike P.]

Most subs are linear to their rated Xmax, only when Xmax is exceeded does distortion become a factor.

As your modeling shows, the lower the tuning the more low end extension you get at the expense of overall SPL.

 

[fredk]

Thanks for the reply Mike, but your answer is too simple. Me, I'm all about complicated so I did some searches and some reading.

It seems that we do get distortion below Xmax, but it may well not be a factor until after Xmax is reached. How much distortion seems to vary from driver to driver and I gather some sort of representation of a BL curve needs to be generated using something like a Dumax or Klippel thingy to determine distortion.

This has me curious to understand if there is a relationship between fs and the BL curve. It also leaves me wondering if you can extrapolate a driver's distortion from the t/s parameters (I suspect not).

More reading...

 

[SteveCallas]

There is no problem tuning below driver Fs - this does not create excess distortion. It's a myth just like so many other porting myths. Check Ilkka's measurements of the SVS Pb Ultra 13 tuned to 15hz to confirm.

 

[fredk]

Funny you should mention the PB13. I was thinking of those measurements earlier and wondering what the specs for this driver looked like.

I take it that you cannot generalize where distortion starts and how bad it gets. It depends on the design of the driver.

 

[Ryan Anderson]

There is no problem tuning below driver Fs - this does not create excess distortion. It's a myth just like so many other porting myths. Check Ilkka's measurements of the SVS Pb Ultra 13 tuned to 15hz to confirm.

well I got what I wanted out of your thread, Fred. I hope you did as well. Research... you got it! You are now infected!

 

[fredk]

I actually found the short answer in the right context on the AE Speakers Europe site. Once upon a time not so long ago, there were no Dumax or Klippel systems and people had fixed assumptions about Xmax and driver behavior. That all got thrown out the window when you could do real world measurements of real drivers.

Apparently the poster on the other forum is old school.

So, the 70% mark on the BL curve represents 10% THD and thats how Xmax is supposed to be set. This does not include things like suspension distortion, so you are really over 10% ad Xmax, but then, at such a low frequency, you probably can't hear distortion until its significantly higher.

All in all, unless I have either of the above mentioned pieces of equipment, I have to trust that the manufacturer I choose is being relatively honest about how Xmax is determined and that they did a reasonable job limiting other distortion. Having worked in Hi Tech marketing for a bunch of years, thats a tall order, but I'll get over it eventually...

Well, that was fun. It gave me a chance to revisit some reading that made little sense in the past. Now it makes a little sense. A few more spins on the hamster wheel and I should have it figured out.

Thanks for your help and patience.

 

[Ryan Anderson]

Xmax (70% Bl) indicates 10% THD?

 

[Ricci]

In theory...yes. It's not quite so cut and dry. Lot's of things can affect the amount of distortion put out by the driver. The enclosure alignment, the type of signal sent to it, the power levels, heating of the vc/motor, manufacturing tolerances, etc.

I'm sort of with Fred. I figure that the driver is going to provide clean useable output up to 75 or 80% of xmax at least. Most drivers are exhibiting some signs by the time you get all of the way to xmax.

 

[fredk]

Interesting comment Ricci. I lack the experience, so I'm taking stabs in the dark. In your experience, are the signs of stress quite subtle, or more obvious? My guess is subtle because of the nature of hearing and our lack of sensitivity down low.

The only real world example I can think of is comparisons I have seen between two commercial subs, the pb13 ultra and Axioms ep600. One shows less than 10% distortion through most of its range, the other jumps up to 20-25% as you go below 20Hz. Yet, when people who were unaware of the relative levels of distortion made comparisons, they felt the differences between the two subs were very subtle.

I know CSS suggests the 19Hz alignment for the SDX15 even though, from my perspective, the 15Hz tuning looks very good on paper.

I have been toying with the idea of building a cylinder sub with adjustable volume to test the different tunings. I just haven't figured out a good way to do this yet.

 

[Mike P.]

A 19Hz alignment for the SDX15 is just one option, tuning it lower to get more low end extension is another. Both need to be kept within Xmax with the appropriate hi-pass filter for the best results.

 

[ec114]

In your experience, are the signs of stress quite subtle, or more obvious? My guess is subtle because of the nature of hearing and our lack of sensitivity down low.

Thing to remember is that the nature of harmonic distortion (which, by definition, just about any nonlinear distortion will create) is that higher frequencies are created where there should be none. And those products, even while absolutely smaller in scale, can often be perceived to be even louder than the fundamental. Of course there are then real signals that offer other sounds to mask and such, but still...

Bottom line? Once you hear a really low-distortion, high-output system, you just may recognize it as such...