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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 offline 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.

Chris
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