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Does adding mass to a voice coil reduce its BL?
Does adding mass to a voice coil reduce its BL?
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Old 17th September 2020, 04:58 AM   #11
wonderfulaudio is offline wonderfulaudio
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Thanks for your comments.
So, BL does not depend on the mass, just depends on Re for the same given magnet and gap.
The takeaway from this discussion is that in order to compare BLs of various motors (with various Re) the BLs must be normalised to a fixed Re say 8 ohms and then compared, just like we normalize sensitivity to 2.83 volts.

The reason why I was looking at competition subs for home use is that they can be put in very small enclosures which reduces quite a bit of real estate and visual footprint especially in case of multi-subs, amplifier power is quite cheap (and compact) these days with plethora of class D amps so the higher power requirement is not a big issue.
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Old 17th September 2020, 07:29 AM   #12
lcsaszar is online now lcsaszar  Hungary
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The price of higher mass is lower sensitivity. It is not an issue at car audio, where one can pump several hundred watts into a high mass / low compliance (-> low box volume) inefficient loudspeaker. These are not meant for home audio. One reason is group delay is high, resulting in boomy bass.
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Old 17th September 2020, 08:30 AM   #13
KSTR is offline KSTR  Germany
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Quote:
Originally Posted by wonderfulaudio View Post
The takeaway from this discussion is that in order to compare BLs of various motors (with various Re) the BLs must be normalised to a fixed Re say 8 ohms and then compared, just like we normalize sensitivity to 2.83 volts.
Actually, efficiency scales with (BL)/Re. That can be easily understood by looking at a dual coil woofer. The conversion efficiency does not depend on how you wire it up (voltage-based sensitivity does, of course). Series vs. parallel doubles BL but resistance quadruples. In the end the efficiency depends on the amount of VC material in the gap and the conductivity of the material. And it depends on total mass of the cone assembly. For compact subwoofers a high mass is beneficial because it makes resonant frequency of the box low enough and the system Q (Qtc) just right to obtain a good voltage-based sensitivity in small enclosures.
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Old 17th September 2020, 09:32 AM   #14
TBTL is offline TBTL  Germany
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Just sprung to mind: car audio subs usually have a large excursion capability. This implies that coil overhang is large. All that extra coil outside of the gap does not contribute to BL, but raises Re. The desired impedance and therefore Re is fixed, so we must use thicker wire and less turns to adjust Re to a reasonably value. Less turns means less wire in the gap and a lower value of BL. Medium sized magnet, low (BL)/Re.

Now BL can be increased back to 'normal' by using a thicker pole plate, exposing more of the wire to the magnetic field. The drawback of a thicker pole plate is that it requires more magnet material and results in a larger magnet structure. Large magnet, medium (BL)/Re.

Last edited by TBTL; 17th September 2020 at 09:36 AM.
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Old 17th September 2020, 10:00 AM   #15
chris661 is offline chris661  United Kingdom
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In the case of an over-hung magnet, increasing the length of the pole plate will directly decrease Xmax while raising efficiency.

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Old 20th September 2020, 06:18 PM   #16
head_unit is offline head_unit  United States
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Originally Posted by bentoronto View Post
The design of "competition" woofers (and extreme power handling speakers in general) [like PA speakers] results in designs that seem at odds with home audiophile purposes.
Yup. That's because they are being used for other purposes.

Car audio is not like home because the "room" volume is very tiny-especially in those cars where almost the whole thing is being take up by enclosures. And the SPL becomes nonlinear. And the "room" is becoming the same size or less as the enclosures. (Note: I'm talking about enclosed competition cars, not the open-up-and-blast kind popular in South America)

To get low resonance in a small enclosure requires high mass:
Fs=1/(2*pi*sqrt(Cms*Mms). However efficiency is proportional to 1/(Mms)^2. But but BUT this is a MID-BAND efficiency. I dimly recall that more mass can be used to get some more efficiency down low, at the expense of reduced efficiency in the midbass and above. I don't have a tool handy to recheck this.

I do recall clearly that adding BL (more motor force) would increase output above resonance. From the midrange, this looks like there is "less bass" but actually the efficiency around resonance was the same, there is just "less bass" since actually now there is more midbass and midrange.
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Old 20th September 2020, 06:23 PM   #17
bentoronto is offline bentoronto  Canada
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Does adding mass to a voice coil reduce its BL?
Good clarifications. Thanks.

Might be mentioned that in shaping loudness, the clever designer uses the speaker resonance as part of the deal. So if you are canny about where that occurs, you do get bass loudness low down, but maybe not quite right to relate that to efficiency in the strict sense of the term.

B.
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Old 21st September 2020, 05:49 PM   #18
tonskulus is offline tonskulus  Finland
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Sensitivity figures are given at constant voltage and it has almost nothing to do with power conversion efficiency, especially at low frequencies.

"is 1000watts enough for this sub?" is actually pretty nonsense question because of the impedance peaks and fluctuating phase angle (inductive/resistive/capacitive load). Even that 1kW amp might be delivering only 100watts at clipping, so at that point amplifier is running out of voltage rather than avaliable power.

Speakers are very complicated electromechanical instruments to work with and leads to many compromises no matter how good a design.

In some cases, highly damped woofers might be actually less efficient at bass than low BL woofers. When speaker cone is electrically damped (as it is more or less), the braking force is wasted as a heat in typical amplifiers output stage?! But who wants to listen speaker resonance peaks over the original music signal?

Last edited by tonskulus; 21st September 2020 at 05:56 PM.
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Old 25th September 2020, 04:16 PM   #19
454Casull is offline 454Casull  Canada
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Quote:
Originally Posted by wonderfulaudio View Post
Thanks for your comments.
So, BL does not depend on the mass, just depends on Re for the same given magnet and gap.
There's no need to bring Re into it, if you're only looking at BL. The whole thing just gets blown up otherwise when you consider alternate conductor materials. e.g. what happens if you use aluminum instead of copper?
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Old 25th September 2020, 04:26 PM   #20
bentoronto is offline bentoronto  Canada
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Does adding mass to a voice coil reduce its BL?
Many knowledgeable posts. But now let's focus the abstract discussion into meaningful conclusions. For fidelity seems to me, the core issue is control of the motion of the cone and especially transient motions.

Let's say you were buying a driver. Wouldn't part of your decision be based on motor strength (just Bl) relative to cone assembly weight (plus maybe air mass varying with diameter)?
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