George (on Topic Only)

Output level would still be higher though, even if "power" were technically the same - because of F=BLi... more current, more force, more output, higher excursions...AND higher distortions. But which came first, the chicken, or the egg?

 

just reading the 4th page makes me thing Whats all this mumbo Jumbo. Lets do some spl. BL. Xmax. Efficency. Cone. Scott's wall. Planet audios power .


I do understand alot of it but i think i will stay in the spl lanes for a while before i go to designing speakers t:

 

You know, I have been wondering the answer to that question for a while (not the chicken/egg thing ). So let's say same situation, but output is the same. So output is the same, but one has more current than voltage. Will the one with more current have more distortion, even though it's power level is lower?

 

Aw, come on Scott. You know it's fun.

It's really interesting to me *on topic* to see how SPL competitors are slowly going back to the "old school" methods. I.E., sand inbetween double enclosure walls / between port curves and walls, massive amounts of bracing, etc. We've known about that for years but I haven't seen anyone implimenting it in the SPL lanes until recently. *off topic* Man I wish I would have had the money to get into SPL earlier. Oh well...I'll just settle for being a SQeer. Besides, I like my hearing.

 

Hi guys,

Stephen and Geo, you're both right! Force truly is BLi, so given all else being equal, you need the same current to generate the same SPL in the drivers you're talking about.

Now, if you had one driver that needed X amps of current to generate Y Newtons of force, and a second driver that needed 0.5X amps to generate that same Y, then the second driver would have lower distortion. Of course, it's also 3 dB more efficient, meaning either BL doubled or mass was cut by 41%.

All else being equal, current is current, BLi=ma, and how you get that current is immaterial. In car audio we tend to have lower impedance because back in the dark ages (the 80s - were some of you even alive then? ) high voltage switching power supplies were expensive. So you used the least amount of voltage possible, meaning that to maximize current flow at that low voltage rail you used low impedance drivers.

Now days, with switching FETs being cheap and plentiful, a 3 kW switching power supply is quite affordable. We stick to low impedance mainly because of market inertia. This is one area that I have a lot of hope for 42V car systems - higher voltage systems allows use with higher impedances.

Why higher impedances? Go back to the top - that whole effiency thing. What is efficiency? BL and Mms. Guess how I can increase BL for really cheap - add more L... Go to an 8 Ohm voice coil, I can double the BL for free (as compared to a 4 Ohm voice coil). Meaning I don't need as much current to generate the given force, but I do need more voltage. Of course, with a 42V rail, we can get a clean +/- 40V into the speaker without a switching power supply, and that is a solid 200W into an 8 Ohm load! Into a 4 Ohm load, we have 400W of power. Imagine a head deck that can do 400Wx4 - yes, that's the benefit we get...

So, bottom line - current is the key, it does increase BL nonlinearities as current increases, but for two drivers, if all else is equal, it doesn't matter if it's a high impedance or low impedance. Key being "all else being equal".

Dan Wiggins
Adire Audio

 

okay, so suppose i were designing a series of subs...a 10, 12, and 15 (im not really but i love all this info!). anyhow, i use the same suspension, motor, and somehow i manage to make the mms the same (ie the only difference is actual cone area).

what T/S are different, and why?

im betting the efficiency is higher, and requires a bigger box, but what beyond that?

thanks,
eric

 

Ready for the answer?

Vas

That's it!

Fs is set by: Mms and Cms.
Qms is set by: Mms, Cms, and Rms.
Qes is set by: Re, Mms, Cms, and BL.
Qts is set by: Qms and Qes (Re, Mms, Cms, BL, Rms).
Vas is set by: Cms and Sd.
SPL is set by: Mms and BL.

You kept the same suspension, so the Cms is the same. BL was the same (same motor), Re was the same, and since it's the same suspension, Rms will be the same. And you stated that you kept the mass the same. That means the only thing that changes was Sd, which only affects Vas.

The Brahma uses the same motor and suspension for all drivers; the only differences are Sd and Mms (we actually have the Mms increase as the driver diameter increases). So you can see what happens to the T/S parameters as you only change those two physical parameters.

You can see that Mms and Cms are pretty key - they affect just about everything. That's why they are often called the critical parameters, because of their importance to all other parameters (sometimes BL is thrown in as a critical parameter, because although it only affects Qes, Qts, and SPL, it is what you need to help with driver efficiency which is often a critical consideration).

You can also see why a subwoofer optimized for small boxes is inefficient. You want a small box, then you need a small Vas. That means Cms must be small. Of course, the lower Cms the higher Fs, so you must add mass to the driver to get the Fs back down. And SPL is highly affected by Mms. So going for that small box can be expensive in terms of efficiency, especially if you want to maintain depth extension.

Dan Wiggins
Adire Audio

 

Here's the relationships for each of the T/S parameters listed above, which may give you a better feeling for what's affected as you change the basic physical parameters:

Fs: Inversely proportional to the square root of Mms and Cms (need to quadruple either value to halve the Fs).

Qms: Proportional to the square root of Mms, inversely proportional to the square root of Cms, and inversely proportional to Rms (quadruple Mms doubles Qms, quadruple Cms halves Qms, double Rms halves Qms).

Qes: Proportional to the square root of Mms, Proportional to Re, inversely proportional to the square root of Cms, inversely proprotional to the square of BL (quadruple Mms double Qes, double Re doubles Qes, quadruple Cms halves Qes, double BL quarters Qes).

Qts: proportional to the sum of the inverse of Qms plus the inverse of Qes.

Vas: proportional to Cms and the square of Sd (double Cms doubles Cms, double Sd quadruples Vas).

SPL: Inversely proportional to the cube of Mms, proportional to BL (double Mms cuts SPL by a factor of 8, double BL doubles efficiency).

Dan Wiggins
Adire Audio

 

Dan, I think you just sent almost everybody a couple days of sitting behind a scientific calculator and a handful of T/S sheets.

 

Well, I do think that was likely the best explanation of T/S specs that I've ever seen, particularly because of the explanations of the relationships.

Personally, I think that explaining them in the scope of "what affects what" isn't confusing, but rather clarifying! B)

 

its confusing to just read, but sit down for about 5 minutes, and play with numbers, and it would all make sense...not that i know this personally, but i understand concepts when i have a real world use for the knowledge.

eric

 

incredible thread guys

keep it coming

I'm soaking it up like a freakin sponge







Jeff

 

Okay, we've all discussed things about the motor, and how the BL curve should be flat.... but what about the suspension?


What are the things involved in choosing and/or making a spider (and surround?) more linear (performance-wise), to complement the linear motor?

Also, how is a spider made in the first place? Whats it made of and how does it keep its shape?

 

good point hobbes

I've wondered lately how Kms effects other specs, and how we can

keep it flat as excursion increases. I got in a pretty big arguement with

DD about EROM sorrounds.......





Jeff

 

I explained this on page two.

 

Spiders used to be, and still are, made of cloth. The materials over the years have broadened, that's for sure. In some speakers, the spiders (where the name actually came from) were plastic "S" shaped pieces attached to the voice coil former. Hopefully Dan will chime in with a description of what material today's spiders are made of.

As far as surrounds go, the half-roll "normal" surround is tough to be for linearity (surround-wise). The new tall and narrow surrounds give up surround linearity for increased surface area while keeping excursion the same. However, most of the drivers suspension linearity (up to about the last 1/3 of stroke) comes from the spider(s), not the surround. With today's new linear suspension technologies, companies are able to taylor the spider(s) to the motor and basic drivers parameters/design goals...which is really cool. What you end up with is one very linear suspension that compliments the motor.

Dan Wiggins, feel free to jump in here. BTW: Did you get my email a few weeks ago? If not I'll shoot you another one or give you a call.

 

I think this is often confused with how electronic componentry or the designed behavior of an amplifier behaves as you approach the limits of the componentry contained within that design.

Within the scope of any single amplifier (JL's RIPS amps being a potential big exception), as you wire the amp to a lower impedance load, the amp makes more power. This is because the amp is simply a voltage source, putting out a certain voltage out it's speaker outputs... the load you wire up determines how much current will flow through the speaker and the amplifier.
It's simply Ohms Law that states that the lower the impedance of the load, the more current will flow through the circuit. And given that the voltage is the same regardless, and power is current times voltage, the increasing current makes for increasing power.
But the more current that flows through the amp, the more stressed the components within become, as they can only handle so much current flowing through them.
Consider each of them in this regard to be like fuses... they'll melt once a certain amperage is reached. :blink:
This is why your amp has a minimum impedance rating... it's not really because of the impedance, per se.. it's because of how much current will flow as a result of the impedance hooked up.

At any rate, the more power the amp puts out, the more stressed it becomes..
All of it's specs begin to suffer... such as distortion.

That being said...
Yes, in the scope of a single amp, as you drop it's impedance, it's distortion will suffer.
However, if you designed an amp to be able to drop to very low impedances, and it's componentry contained was actually designed to drop that low and still stay within design parameters, then there's no reason why you couldn't build an amplifier that ran nice and clean way down low, 1 ohm, 1/2 ohm, whatever.
But amplifiers like that would potentially cost more money to make, also, to handle the current calmly. [/b][/quote]
ok... sorry to bring a question back from the dead, but been reading this thread when i have time, and haven't had time to catch up and be a part of the "current" discussion due to being so busy... anyway.

as for distortion increasing when lowering impedence... Yes, i completely agree with this to a point... BUT only when the current ACTUALLY is increasing. yes if you lower the impedence (by half) you increase (near double) the current that flows... BUT from my understandings from talking to Loyd for awhile on the topic of lowering impedence (below spec)... and amp will only allow so much current to flow.... I'm not talking about fuses popping, but actually amps having a current limiter built in, thus restricting how much current will actually flow, yet not popping the fuse.
Now how does this effect amps output below spec? lets say the amp is 2 ohm stable, draws 30amps @ 2ohms, and is limited to 50amps.
we drop down to 1.5ohm and we're looking at 45amps draw
1ohm and its limited to 50amps tho without the current limiter it would be 60amps
what about if we drop even lower than 1 ohm.... we're still runing 50amps

but whats the difference of lets say 1ohm, and .75 or even .5ohm??? At first glance you'd think nothing other than the actual ohm load.... BUT power would stay the same cause current is the same...... But then you have to factor in the difference in heat thus causing more resistence in the circuit which will actually cut down on the power output (watts).... so its safe to say that .5ohm load might actually have a lower power output than the 1ohm or maybe even 1.5ohm load.

now what does this have to do with the THD levels that Kephart was asking about?
well if current is limited at 50amps... whats the difference of 1ohm drawing 50amps & .5ohm drawing 50amps?

not all amps will shutoff running below ohm loads... some amps won't even shutoff when running WAAAY below ohm loads.... so you can't completely rule out the idea that there is a certain point that an amp is no longer effected THD wise by lowering the impedence.
BUT YEAH, when running an amp in spec... the drop from 4ohm down to 2ohm(or any other drop) will i believe double the THD in the signal

sorry if my point seems trivial... but for those who play with SPL setups... not that THD matters, but i think its a good idea to point out that power just doesn't double by halving the impedence.... and maybe THD doesn't increase much at these much lower ohm loads. not trying to split hairs to prove my genius, i'm a newb and i know it, just trying to understand a bit better. correct me if i'm wrong.

 

ok... now something related to the current topic....
progressive spiders seem to be the best spider from what i've read in terms of linearity.... now take some of the new motors like the XBL2 or the other companies dual gap type motors..... even tho they are very flat (the BL) compared to Under or over-hung designs..... they seem to have some peaks....

take the brahma 12 dumax sheet.... its BL at rest is different than at 7.5mm or even 20mm.... now is it possible to completely cancel out that difference in BL with a completely custom spider.... like.... kinda a progressive.

like, looking back at the brahma 12 dumax... have the inner most ripples(rest position) of the spider being x stiff, and then at 7.5mm have them X stiffness -1 (just slightly less stiff) and then the ripples near 20mm being X again....

make sense? would this work... could it completely make the linearity of the sub almost perfect if you perfected the spider?

 

What i've been thinking of lately is if we know the exact tendancies of a driver, could we process the signal in such a way that would result in zero distortion?

Like say we're playing a test tone. We know the driver's BL curve, so you make the wave misshapen in a way that cancels out the illinearity. Insead of a nice smooth sine wave, it's kinda pointy. It sounds like a good idea, but i'm sure there's a fundamental misunderstanding somewhere

 

It actually doesn't work that way. Here's a quote from page two of this thread by Dan:

Also, the small waves you see in the Brahma DUMAX are so small, that it isn't audible. They could have kept the curve perfectly flat at the expense of some excursion.