How High A BL Figure Is Practical?

[kelticwizard]

Suppose I have 15 inch speaker with a 4 inch diameter voice coil and a 58mm long voice coil. How high a BL product would it be be able to achieve with normal speaker procedures?

How high would the BL product be with a 3 inch diameter voice coil? Soea it make a difference?

A 2 inch diameter voice coil?

Is 30 TM the practical limit? Howabout 40 TM, 50 Tm or higher?

[Steven Kephart]

There is way too many other factors to take into account to give an accurate response. The magnet size, whether the other metal parts are saturating or not, how many layers the voice coil is, size of wire used, how tight your gaps are etc. all will effect your total BL.

Also, high BL isn't necessarily a good thing. You just want as much BL as is needed for the design. Have you ever heard the term "over motored"?

Steven Kephart
Adire Audio

[kelticwizard]

Originally posted by Steven Kephart
There is way too many other factors to take into account to give an accurate response. The magnet size, whether the other metal parts are saturating or not, how many layers the voice coil is, size of wire used, how tight your gaps are etc. all will effect your total BL.

Also, high BL isn't necessarily a good thing. You just want as much BL as is needed for the design. Have you ever heard the term "over motored"?

Steven Kephart
Adire Audio

Your answer seems to be "fit your BL to the other parameters you have already set for your driver/box combo".

What I seek to do is: set my other parameters by the highest BL I can achieve by the usual manufacturing methods.

As we know, all the parameters affect all the other parameters. Somewhere along the line, you have to fix one parameter as "given" and see how it affects the others. My goal? To make the smallest size Closed Box, with highest efficiency, with a 15 inch woofer that has an excursion of (+/-) 25 mm in a Closed box with F3 of 20 Hz.

With the BoxModel freeware of Bullock and White, you can set certain parameters and have them see how they affect other parameters. I believe they use Small's formula.
http://www.hal-pc.org/~bwhitejr/ (Do not use the TL software, Bullock admitted the equations were faulty. For TL, use the freeware at www.quarter-wave.com).

BL is one of those parameters that is determined by other parameters, such as Vas, Fs, etc. At least in Bullock and White's software.

It is all very well and good to set various parameters by Bullock and White's freeware, (or Small's formula), but it is useful to know just how high you can drive that BL figure before it leaves the bounds of practical design. I see PA speakers that have BLs in the high 20s. If I go to a speaker manufacturer with a formula for a speaker that requires a Bl figure of 45, is he going to say to me, "You're out of your mind. That BL figure is simply beyond the means of practical manufacturing methods."

My goal is to find just how high standard manufacturing methods can make the BL.

Originally posted by Steven Kephart
The magnet size

As big a magnet as is necessary to produce the max BL by normal, standard methods.

Originally posted by Steven Kephart
....whether the other metal parts are saturating or not....

Part of my question. How high a BL can you achieve, using standard manufacturing materials and methods, before you run into saturation situation? The diameter of the voice coil is either 4 inches or 3 inches, the excursion is (+/-) 25 mm. Mx power handling would be 500 watts or 1,000 watts.

Originally posted by Steven Kephart
....how many layers the voice coil is, size of wire used, how tight your gaps are....

Fine, give me an idea of an optimized motor system, manufacturable by more or less standard methods in the industry, that takes those things into account. Make your voice coil as many layers as it takes to optimize the BL count. The speaker won't be used over 100 hz, if that is a consideration. If you can optimize the BL even more by not having the speaker useful over 50 Hz, that is fine too.

Whatever priorities a speaker manufacturer uses to arrive at max BL for a 15 inch speaker, with a 4 inch or 3 inch voice coil, with a linear Xmax of (+/-) 25 mm, and which cutoff will be 100 Hz-or even 50 Hz if that makes it better-is what I am after. Achievable by standard manufacturing methods.

Would it be 30 Tm? 40 Tm? 50 Tm?

Thiele and Small tell us that +6 dB = twice the box size =1/2 octave lower, and that if we want to alter these relationships, we have to supply additional amplifier power.

My goal is to use the maximize BL so as to maximize sensitivity and lower box size for my bass goals.

By the way, this is all theoretical, I am not in the business of manufacturing subwoofers. Not yet, anyway. But I sure would like to see what is actually achievable.

[eric180db]

if you wanted to raise bl which raises efficiency you have to trade something off. Thats just physics. If xmax is decreased it will directly raise BL, thats why woofers designed for ported boxes will generally have a higher BL because the xmax requirements are smaller. there are other factors that affect BL but Xmax is the biggest. Higher turns on the voice coil also raises BL but then the speaker is more reactive with higher inductance.

[filgor]

The answer to your question from what I know realy comes down to how heavy a voice coil you can tolerate. and how much excursion you need.

Remember that there are two factors involved in Bl, they are B and l (funny enough!)

You can increase the size of the magnet until the pole pieces are saturated with magnetic flux. Once you have done this B is at its maximum. Actually you will get very slight increases in B with increased magnet strength beyond this point but the stray field becomes very powerful and your extra magnet gets wasted.

Beyond this point you have to incrase the 'l' part of the equation and there are a number of ways to do this each with thier own trade-offs invoving Voice coil resistance, weight and linear excursion.

To best answer your question it is perhaps better to ask how much of this kind of theory you have considered and what kind of stable parameters ie. voice coil resistance, maximum voice coil mass and, if you are restricting yourself to an overhung type motor assembly the minimum linear excursion required.

Essensially if you use an under hung motor system where the entire voice coil is within the magnetic gap you can have a bl which is fundamentaly limited only by the length of the voice coil winding. In reality the length of the voice coil winding is limited by the maximum weight and/or resistance.

Theoretically you can get some rediculously high values of Bl but the motor structures are massive and impractical.

[BillFitzmaurice]

If you want to see the effect of high BL just do some modeling of drivers and vary the Qts; the effect will be the same as far as response is concerned, since Bl and Qts are very connected. Simply put higher Bl=lowerQts=higher SPL= higher F3. The advantage BL gives in efficiency is offset by the loss of bass response; you can overcome that limitation either by electronic means (EQ and a lot of power) or mechanical means (horn loading). In both cases you can get a very powerful package; high BLs, for instance, allow horn loading from far smaller cabinets than previously though possible ( my Tuba subs. for instance) and good bass from very small direct radiators as well if driven with enough power to accomodate the requisite EQ.

[DanWiggins]

How high do you want to go? I've played around with some designs and have pushed BLs into the 50+ range... Not too useful, though. Qes ends up through the floor, so you get no bass extension.

Lots of PA drivers have BLs in the 20-30 N/A range, but none really create "bass". Loud above 50 Hz? Sure. Usable output below 40 Hz? Nope.

You can have a really high BL and raise the Qes to a usable level (in terms of low end extension) by adding a lot of mass to the driver. Of course, this lowers the efficiency of the driver a lot, and you might as well start with a lower moving mass and lower BL - same results, and a LOT lower cost.

The reason you don't see a lot of high BL designs in audio applications is because there's really never a need for it. Unless you're looking for a 15" driver with a lower end limit of 100 Hz or so, BLs above 25 N/A or so are, IMHO, a waste. And for true subwoofers, BLs above 20 N/A are detrimental to performance.

Now, if you're talking about some electrodynamic voice coil driven actuators, well, BLs can reach into the 80s or 90s, but those are for some esoteric applications where the motor is used for non-audio applications. Like shaker tables, pneumatic air valves, etc.

Dan Wiggins
Adire Audio

[Vikash]

Originally posted by DanWiggins
I've played around with some designs and have pushed BLs into the 50+ range... Not too useful, though. Qes ends up through the floor, so you get no bass extension.

You can have a really high BL and raise the Qes to a usable level (in terms of low end extension) by adding a lot of mass to the driver. Of course, this lowers the efficiency of the driver a lot, and you might as well start with a lower moving mass and lower BL - same results, and a LOT lower cost.

Cost issue aside, increase the cone dia. thereby increasing the mass and get more air volume displacement with less required xmax (more linear potential). Thus those high BL values can be useful?

[kelticwizard]

What I am trying to do is increase BL so that I can lower Fs, (which lowers sensitivity), lower Qts, (which raises sensitivity), and increase Vas, (which also raises sensitivity).

Having a given box size becoming a smaller percentage of the Vas is no big deal if the Fs and Qts is low enough. I just wanted to make sure there were no practical limits to raising BL over 25 TM or so, which seems to be the highest I see, mostly in PA speakers.

[Ron E]

I think you are running into Hoffman's law here, You cannot have high sensitivity. low extension and small box size. You can only have two of the three.

Bl is not determined by other parameters. Bl is a fundamental parameter that affects Qes. Vas and Qts are combinations of other fundamental parameters.

You could make a speaker with a Bl of 100 of you used 40 gauge wire and had a resistance of 100 ohms..... IT probably wouldn't be usable. Instead of maximizing Bl, you should take the target F3 and Box volume and design your parameters around that. You will find that the Box size and F3 will determine your efficiency. Then Re will determine the sensitivity from that.

Good luck