On Xmax and Spl

[martyh45]

First, Thanks bfish for the link in the 604E thread and thanks GM for your input at Hi-Eff.

On the subject of Xmax;
bfish, I followed that last link in the 604E thread and found a paper at Klipple (Link below)?interesting stuff. I wonder what the Xmax of the classic Altec underhung motors would be if you used 10% distortion as the limit, I think it would probably be much higher than what is published. IIRC, the classic Altecs have a relatively uniform flux density in the gap. Excluding excursions of the coil out of the gap, Bl should be much more linear than the overhung motor examined in the paper. Again IIRC, this is part of the reason that distortion is so low with Altecs as compared to modern big Xmax woofers.

On the subject of max SPL;
I have been playing around with a few options for a woofer to match up with the aforementioned 604 below 100Hz in a bi-amp system. The 604 looks to do about 114dB max at 1 meter in the cabinet I have chosen without exceeding geometric Xmax. Near as I can tell, this should be plenty for my purposes. The 416-8As I am gravitating towards as the woofers look to do about 109dB 1M/W while staying within published Xmax (which I assume is geometric based on the time they were published) This as near as I can tell is right on the edge of what will work for me. After GMs reply in another forum I?ve done some more measuring of my current system. I listened to rock classical and Jazz Lps at a volume I consider about as loud as I can stand. I held a calibrated checkmate meter set to C weighted/Fast/peak hold and came up with peaks at 101dB on the classical. Doing the math for reduction in Spl due to distance gets me to 108 dB at 1M/1W, right about at the limits of what the sims say the 416-8A will do within Xmax. Now for the questions and thanks for reading along?..

So what do you think, is measuring in room peaks a valid way to determine what kind of headroom I need?

Considering that I am working with two speakers below 100 Hz, is it safe to assume + 3dB based on both channels operating?

And last of all, has anyone compared predicted Spls to actual levels in place? Are they close or is all this fussing a waste of time?

Thanks!
http://www.klippel.de/pubs/Klippel%2...nt_XMAX_02.pdf

[bfish]

You're welcome Marty.

Just based on visual judgement of displacement, and given the large difference between xmax and maximum excursion before damage, I agree that it seems like acceptable linear excursion would exceed rated xmax for many Altecs. Assuming that underhung designs are inherently more linear than overhung might be a mistake though. Ignoring compliance effects, overhung designs can allow much greater excursion while remaining linear, but at the expense of efficiency, since much of the energized coil area is out of the primary field at any given instant. Many modern designs also employ shorting rings and other methods of extending/controlling a symmetrical field on both sides of the pole plate that offer increased linearity as well. If you study the flux patterns of designs like your 604, it's easy to see why when they do go non-linear, they do it in a hurry, as the fringe is greater on the backside than the front.

There's a common perception that underhung Altec drivers are wimpy in the excursion and power handling departments, but one has to consider the applications they were intended for. The heavy acoustic loads that reduce excursion and the acoustic gain of a horn cab are ideal for them.

An often overlooked aspect of getting the most from your underhung Altecs is DC offset. Considering the excursion range is already small and the efficiency is high, even a few millivolts of DC offset gobbles up usable excursion in a hurry, subsequently causing non-linear distortion at much lower levels. Another aspect of this that I've encountered but seldom see discussed is the potential for digital media to have the entire signal signifigantly offset in relation to a zero voltage reference. This can be a real buggar to detect, as you can have the entire chain powered with 0VDC on the speakers, but when the corrupt media is played, offset is introduced.

I like to consider headroom in a back-to-front manner. Convert the average SPL needed to 1m values, and tack on the 10-20 dB of dynamic range for transient peaks, and there you go. This has to be considered from a complete system POV though, as having speakers that can hit 120-130dB peaks is of little use if the electronics can't support it in a clean linear manner.

Another aspect of headroom to consider is how it relates to frequency and source material. Given the size/shape/nature of things, it's way easier to run out of steam on the low end before the high.

If you don't have your LF components yet, you might consider 411s or 3154s. Personally, I think either would work better for you than a 416, assuming separate/partitioned boxes. Since you're cutting the 604 at ~100Hz, are you treating it as a sealed midrange? I think that would help with your concerns of reduced xmax of your E/Cs.

Yes, your +3dB assumption would be correct if the source material supports it, that is, if equal LF energy is on both channels.

We fuss over lots of things during design that make little/no audible difference in the end run. IMO, dynamic headroom is worthy of signifigant fuss.

[bfish]

For me, considering dynamic headroom in terms of electrical power simplifies the understanding. Take for example, a high efficiency setup in a home setting that can develop lifelike average levels at the listening position with 1 watt of electrical input. To accurately reproduce an +18dB transient (uncommon in typically over-compressed commercial pop/rock recordings) requires a 64 watt "reserve".

Now move to a larger room or lower efficiency speakers that require 10 watts for that same lifelike average level. That +18dB transient now takes 640 watts...

Viewed in this manner, the requirements become clear. Kinda makes any thoughts of obtaining lifelike reproduction from flea power amps and efficiency-robbing passive XOs somewhat optimistic to say the least.

Since musical transients are often broadband in nature, a specific design requirement for headroom doesn't (necessarily) mean that each component in a multiway speaker has to have the total headroom ability.

Until displacement measurement techniques become practical for the DIYer, we have to rely on sims and specs to determine our limits. Yes, in-room SPL measurements can verify a system's overall dynamic ability. What they don't tell you is how much of that SPL is clean program material, and how much is component-generated harmonics and other (objectively) unwanted noise.

[bfish]

While I'm on the soapbox, the musician in me has something to say.

"Using a wide dynamic range (changing from soft to loud for the newb) is the primary tool I have to make you "feel" the emotion I want to express in my song. Ever since people started writing music to paper, they included little notations on the paper so these changes could be followed by others (crescendo, decrescendo, fortissimo, etc). You use the same technique when you speak to help convey WHAT YOU FEEL. Remove the dynamics, and all that's left are a bunch of freakin' notes that are no more expressive than printed words in a book."

"When I make a recording, the engineer compresses those important dynamics. I hate that, but he insists that it's crucial if I want my music to sell, and explains it in a way that actually makes a little sense, so I let him do it. With the producer looking over the engineer's shoulder, I probably wouldn't have any say in it anyway."

"By the time you buy my recording, the original 15-20dB of dynamic range of my music has likely been reduced to 6-10dB, and there's darn little that you or I could do to change that. What you CAN do is ensure your hifi system can at least handle what's left. I promise you won't regret it."

[martyh45]

Thanks again bfish,

I did look at a sealed enclosure for the 604 but the roll off started high enough that I would have had to move the crossover point upwards to get a good transition. I?m not quite sure how badly a 2 dB dip at 100Hz would effect things but I also wanted the 604s to be usable on their own. I ended up picking about 4 cubic feet vented and F3 about 55 Hz with a source impedance of 3 Ohms. I have heard from people I trust that it is a bad idea to try to get too much bottom end out of the early 604s so at 4 cubic feet they will be the worlds largest mini monitors.

I did think about the 411 as an alternate but I don?t have any in the collection at this point and I am unwilling to pay ebay prices to own a pair. What I have on hand is 416-8A, 416-8B, 515 and 515B. I am leaning towards the 416s because I can re-use the cabinets for a few other projects I have been kicking around for awhile if things don?t work out.

As far as the design maximum spl, GM gave me pretty much the same advice I.E. working from average. Trouble is, I?m not quite sure how to determine what average level is. Adding max and minimum levels of what I consider loud playback and dividing by two gets me to roughly 85dB so 109+3 for double woofers -7db for distance leaves 20dB of headroom. But, looking at the meter bouncing around it seems to be generally reading higher than 85dB, more like 90~92 which would put me in the 10~15dB range. So, if the average needs to be time weighted then I am probably cutting it pretty close.

I wish I had more experience correlating simulations to measurements but I guess there is only one way to get it. So far, the only thing I have to go by in this range is a pair of sealed box Q=7 12? overhung motor subs I built years back. I have played with the tuning/ box size of the 416s to get a similar roll off curve. Using published Xmax as the limit for both I get a predicted 1.5 dB greater maximum output from the 416 above 20 Hz with about 1/5 of the power input required. I remember being happy with the 12s but that was back in my Dynaudio power sucker mini monitor days. The speakers may have overloaded more gracefully but the amps I could afford that were able to drive them sure didn?t.

Regarding distortion, I have not been able to find many published distortion figures. Bowers and Wilkins publishes them but they specify at 90dB. Hiraga measured his 416-8A Onken at 100dB and the figures he published in his original article are about the same as what B&W claims for their flagship 801 at 90dB. IMO, that is pretty impressive when you compare the tools at B&Ws disposal with the slide rulers the guys at Altec were using to design the 416s. I can see though how the Altecs might be prone to more 2nd harmonic distortion when they are pushed beyond their geometric excursion limits. The asymmetric field beyond the gap should (if I understand the theory correctly) result in a rapid rise in 2H.

Musician ?e says! What is your instrument? If you are play electric guitar, I have a small gift to offer in the spirit of community solidarity. Shoot me a PM and we can hash out the details

[bfish]

And from the beatnik at the end of the bar;

"You're talkin' about Twitch Factor, man, yeah, Twitch Factor. Like, you know, if you don't twitch when Keith Moon does a 2 handed crash, like, you ain't got it man. Yeah, man, ya gotta twitch now and then to know you're alive, ya know man?"

<font color="#FFFFFF" size="1">[ January 03, 2008 10:20 PM: Message edited by: bfish ]</font>

[bfish]

You've done your homework Marty.

I know a guy with some nice 411s and 3154s...

Guitars and keys. I recently acquired my granpa's old fiddle and am struggling with that too.

Hmmm...PM to follow.

[bfish]

Originally posted by bfish:
...There are charts...

<font size="2" face="Verdana, Helvetica, sans-serif">http://hyperphysics.phy-astr.gsu.edu...d/dba2.html#c1

[bfish]

Your Q on average levels made me realize I've been sloppy with my use of the terms dynamic range, and headroom. Dynamic range is a plus/minus value, headroom is plus only.

Average levels are what you'd expect if the performance was live. If you know a band, just move your speakers and have them over, but only use acoustic instruments. If you're at the orchestra, (I hear), you wanna be front and center. If it's theater organ, same thing. There are charts, made by curious guys like us that long ago measured the stuff.

The "Twitch Factor" was a weak attempt at humor, but it's also very true, and a reliable subjective indicator of dynamics (unless your nickname's Tweek, or you drink too much coffee).