Jump to content
007

High BL, high efficency... but why the need for high power?

Recommended Posts

Err, I think you misunderstood the white paper.

Bob makes it clear his alignment is very inneficient, hence his 2700 watt amplifier...

He also personally told me this about 13 years ago when he demonstrated the first prototype of this sub to me and a few others (Dan Wiggins was there, LOL)....

Share this post


Link to post
Share on other sites
Err, I think you misunderstood the white paper.

Bob makes it clear his alignment is very inneficient, hence his 2700 watt amplifier...

He also personally told me this about 13 years ago when he demonstrated the first prototype of this sub to me and a few others (Dan Wiggins was there, LOL)....

let me re-quote myself with character roles and i think what i said will make sense

"so many of you get that you need a pretty big amp for a pretty big woofer, but intuitively this makes little sense right?" - narriator

"a big motor should in fact be more efficient!" -naive person 1

"[actually] bob explains this pretty well why this is not true..." - narriator

more clear?

also, a little known fact about his amp, its closer to 600 watts per standards used to measure amps today. And I'm not claiming his sub was efficient.

Edited by 7

Share this post


Link to post
Share on other sites
He also personally told me this about 13 years ago when he demonstrated the first prototype of this sub to me and a few others (Dan Wiggins was there, LOL)....

I guess I don't understand the connection you are trying to make. His white paper has little to do with the bulk of high the efficiency, large motor subwoofers discussed on this site (BTL, Havoc, etc).

You also know that Bob has a way with words, right?

Share this post


Link to post
Share on other sites

.pdf link not working for me

Share this post


Link to post
Share on other sites
He also personally told me this about 13 years ago when he demonstrated the first prototype of this sub to me and a few others (Dan Wiggins was there, LOL)....

I guess I don't understand the connection you are trying to make. His white paper has little to do with the bulk of high the efficiency, large motor subwoofers discussed on this site (BTL, Havoc, etc).

You also know that Bob has a way with words, right?

i was trying to make a connection as to why people tend to reccomend high power amps with subwoofers like BTL's for example. they are very efficent, but of course, also have high back EMF, so if there is not enough voltage, they wont make enough bass. The example is even more dramatic with a 4HP for example...

Share this post


Link to post
Share on other sites
He also personally told me this about 13 years ago when he demonstrated the first prototype of this sub to me and a few others (Dan Wiggins was there, LOL)....

I guess I don't understand the connection you are trying to make. His white paper has little to do with the bulk of high the efficiency, large motor subwoofers discussed on this site (BTL, Havoc, etc).

You also know that Bob has a way with words, right?

, also have high back EMF, so if there is not enough voltage, they wont make enough bass. The example is even more dramatic with a 4HP for example...

That back EMF is an effect of induction, right ?

Why is it such a big problem ?

Share this post


Link to post
Share on other sites

Yes, it is from induction (inductance) and it becomes a problem when you take a 10" driver with a huge, long coil with tons of Le to get massive stroke and handle tons of power like Bob Carver did with his little sub. With XBl2, Underhung and other low Le, linear drivers, it isn't hardly an issue... Additionally, the subs like the BTLs are very efficient and don't need a lot of voltage to make bass...

Drivers like a Solo-X and other purely over-hung motors with tons of X-max and huge, long heavy voicecoils have tons of Le and back EMF can become a considration...

Back EMF also isn't an issue if you don't put your driver in a box so small and innefficient that it takes 2,700 watts to get it moving to full stroke....

Share this post


Link to post
Share on other sites

no no, back EMF is usually modeled as a dependent voltage source. I'm not talking bout "Le" here...

we're talking about a voltage generated in the voice coil which opposes that of the amp by simply moving the coil thu the magnetic field.. you can physically try this yourself. Its easy: Disconnect your driver from your amp, then take a a short wire and short out your speaker (its easy if its a single coil) Then push the cone down and you'll feel a very high force pushing back.

inductance in the TSP sense is simply a magnetic energy stored inside the steel and magnets and even the air (but to a lesser degree) which then as any inductor acts a current source which lags that of the coil which acts on that inductor which is exactly the voice coil.

mathematically these two concepts are very different.. for example, the AC non-linear inductor is virtually impossible to solve by hand.

95Honda, im not even goin gto argue this anymore, i only came here to lend some knowledge that bob discovered. If you think BTL's need low voltage to move, then why do people constantly say you need lots of power on then? Its got a BL^2/Re of 250? Good luck with the back EMF... a stiff suspension will not save that.

Share this post


Link to post
Share on other sites
If you think BTL's need low voltage to move, then why do people constantly say you need lots of power on then?

You'll notice, I am one of the only people who has consistently said the opposite...

a stiff suspension will not save that

Also, I have always stated the opposite....

Additionally, a driver like the BTL (in the correct alignment) will always produce more SPL at any given voltage than an innefficient alignment such as Bob's little sub...

I have always seen the direct proportionality of Le to back EMF. You traditionally see drivers with permable core metals, shorting rings and short voice coils have lower Le and Back EMF.

I completely understand what back EMF (voltage) is and why it is produced. I know how an armeture works. I also understand what makes it worse. I also know what mittigates the problems associated with it, and what exaggerates the problems...

You don't have to agree with me, that is why this is a forum. We aren't arguing either, we have different points on this. Don't take it personally...

Share this post


Link to post
Share on other sites

Shorting out the terminals with a piece of wire is a great way to ship a driver with looser suspension.

Share this post


Link to post
Share on other sites

Basically, his point was this:

Speakers are highly inefficient.

A moving speaker generates back EMF.

The counter-voltage that is induced (back EMF) subtracts from the voltage supplied to the windings.

Now because of how inefficient a speaker is, the suggestion is that the counter-EMF is especially problematic with low current inputs. This is, to an extent, true, but you must remember that at high-outputs we also lose output from non-linearity and thermal power compression. To be clear, I don't believe in this fallacy of "it requires X amount of power to get moving", and will not until someone shows evidence to the contrary. I have seen several measurements of a particular speaker's response vs frequency vs amplitude where output doubled with every doubling of power up to about 400 watts, after which the increase became increasingly small. If your theory were to be true, I would expect to gain maybe 1-2 dB for the first few doublings, then closer to 3 dB, then falling off again after that, which I simply have not seen.

Further, it is an inductance-related problem (though not necessarily solvable with what we're doing now). More on this a bit later.

Share this post


Link to post
Share on other sites
Further, it is an inductance-related problem (though not necessarily solvable with what we're doing now). More on this a bit later.

This was my point from the begining...

And Bob has a point.... But it is most valid when you put a super high Le driver in a tiny, very inefficient alignment and pour on the power....

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×