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Old 26th November 2005, 05:40 PM   #1
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Default Loudspeaker Theory : BL*Sd/Mms

Hello folks/audiogurus at diyaudio

i can't consider myself as a reference in audiotheory, but i interrested myself very much, and read much things about it, in particular, bass reproduction.
I read on a respected french site something about this factor : BL*Sd/Mms, wich would be sort of definitive acceleration factor, like BL/Mms, but with the effective piston area to put things in perspective.
Here is the site : http://pageperso.aol.fr/petoindominique/

With my limited knowledge in audio, this seemed very pertinent to me. Drivers that have a high BL*Sd/Mms do always have a rather high efficiency, and the importance of efficiency in bass reproduction is a hot topic. it seemed common sense to me that a large surface moving air with small displacement, and not going too low, would be more indicate to reproduce low frequencies that a smaller surface moving air violently.

But common sense isn't scientific knowledge, so i wondered what were the views of the different people of this forum about this Bl*Sd/Mms factor ...

thanks !
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Old 27th November 2005, 12:05 AM   #2
Ron E is offline Ron E  United States
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sensitivity is proportional to Sd*Bl/(Mms*Re)

All the people who claim acceleration factor is some big deal are really just specifying sensitive speakers, but they are too ignorant to know it.

Bass reproduction has nothing to do with this factor of merit, other than in how sensitivity fits into Hoffman's Iron Law (that dictates you can pick two of : small box, deep extension, high sensitivity). For example, you can have high sensitivity and a small box, but you must give up low extension to achieve it.
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Old 27th November 2005, 12:22 AM   #3
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Sensitivity is a much more popular name and is measured for almost everything. No need to fancy it up by including BL, Mms, and Sd, as sensitivity is mostly based on these factors.

EDIT: I could have sworn that there were no replies when I opened up this "post reply" window...
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Old 27th November 2005, 12:40 AM   #4
Ron E is offline Ron E  United States
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Quote:
Originally posted by 454Casull
EDIT: I could have sworn that there were no replies when I opened up this "post reply" window...
..and I thought I replied slowly. I must be faster than I think
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Old 27th November 2005, 03:46 AM   #5
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Inductance plays a major role in driver transient response, linearity, and bandwidth.
Cone speed and mechanical settling time play a major role in driver-to-driver integration throughout the crossover region.

Sd * BL / (Mms* Zmax) is a reasonable figure of merit, where the Zmax includes the inductance at the maximum use frequency


There are reasons to use lighter, lower-mass woofer cones and low inductance motors. Smaller or lighter woofers don’t make faster bass, but they do reproduce higher frequencies than larger or heavier woofers can reproduce, and this is an important consideration for integrate between drivers throughout their overlap region. This overlap region is critical to our perception of bass speed that there is little or no tolerance for error. The null tolerance for integration error extends to phase, amplitude, frequency, and time. Introduce even slight variations between any part of the overlap region and you get audible effects in the bass or midbass.

Our perception of bass speed is mainly a function of how ideally the midrange and woofer are integrated. Bass linearity is greatly involved also; you may see a flat frequency-response curve, but the speaker can still sound like it has lumpy bass response because of less-than-ideal phase relationships between the midrange driver and woofer. Phase can often change with frequency. The woofer and midrange drivers can actually veer off in different directions, phase-wise. Large dynamic drivers operating at the top of their range and medium-sized dynamic drivers operating at the bottom of their range can often diverge significantly in their phase response. When phase or other transient errors happen, you get comb-filtering effects. This comb filtering results in the complex response of the loudspeaker to music being quite different than the response of the speaker when the input is something simple like the sine-wave sweep used to measure "frequency response."

To avoid comb-filtering effects that cause beating reinforcement and cancellation effects in the sound, it is imperative for the phase, time domain, amplitude and frequency performance of the woofer and midrange driver to be aligned. Get the midrange or woofer a little ahead of or behind the other driver, and comb filtering starts. Even small errors show up as speed problems in the bass or midbass. Bass detail is also mainly driver integration and not the quality of the woofer itself. Bass detail comes from the midrange driver, but your ear/brain is so completely fooled by this complex interaction of midrange and bass sound that you believe that it is strictly bass-related.
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Old 27th November 2005, 10:56 AM   #6
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Ron E, 454casull : I know that , i more or less said it in my post, but thanks anyway i know that there are high efficiency addicts that swear only by that (kuey) and not only because of tubes amps so i scratch my head

Linesource : I didn't know that, although i had intuitions about this kind of things.
If i understand well, what you describe happens at the driver overlap, my, the big question is, how do you align the phase domain of two drivers, being appart of the well know "time aligned" speakers with speakers drivers centre that are on the same line. ( i suspect you are not totally speaking about that).

my question was, dynamic wise, i didn't build lots of speakers myself, just read lots of things, but i suspect that the sound of the adire extremis driver, that go into the low thirties, will not sound the same as a mid-efficient 15" driver like a focal 15K or something like that. Intuitively (bigger is better ) and by my low amount of knowledge i feel that i would likely prefer the 15" compared to the extremis.
what i'm sure about is that a 6" and a 15" that have the same f3 frequency will not have the same sound even in the 0-100hz region, if anybody can say that i'm wrong or explain this with better word, i would feel better
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Old 27th November 2005, 09:21 PM   #7
Ron E is offline Ron E  United States
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Quote:
i wondered what were the views of the different people of this forum about this Bl*Sd/Mms factor ...
OK, so you know it all, why ask the question?

It appears from the general sense of it that Linesource is an acolyte of the "phasic mystery" cult, assigning every nuance of audio to the mysterious concept known as phase.

From personal experience, some very good sounding speakers are not time aligned or phase coherent in the sense that they reproduce the waveshapes fed to them. While it may seem to be a good engineering goal, transient perfection is not a requirement or a prescription for "fast" bass or good sound. This is not to say that proper attention should not be paid to crossover design.

There is nothing wrong with specifying sensitive drivers, there are a number of benefits, in more realistic dynamics and less power compression. Sensitive drivers tend to be larger and have more ragged frequency response which can make them sound lively - and also for physical reasons they typically are limited in Xmax.

Knowing this, a smaller heavier driver designed to have a response equalling the larger driver will need to have a longer Xmax, which will usually necessitate a higher inductance. More cone excursion equals more distortion of other tones reproduced by the same driver. It will typically have a smoother response and lower sensitivity....

It is however conceivable that a 6" driver could be designed that would reproduce 30Hz better than a 15", but it would probably cost more than the 15"....
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Old 27th November 2005, 10:22 PM   #8
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High sensitivity necessitates neither ragged response nor smooth response.

See:

http://www.precisiondevices.co.uk/as...s/super/17.pdf

http://www.jblpro.com/pub/obsolete/2123.pdf

http://www.jblpro.com/pub/components/2206.pdf

http://www.rlacoustique.com/reps1.htm

http://www.hempacoustics.com/FR810HQ_10-3-05.pdf
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