A Question on Speaker Driver Efficiency

@toddalin

Hi Todd

I've been reading JBL specifications since the late 1970s. I didn't just fall off the turnip truck. The discussion  was about sensitivity changes resulting in parallel versus series arrangement. You got it completely wrong by falling into the "watts" trap - which is a reference to EFFICIENCY- not SENSITIVITY. Paralleling increases sensitivity. Series arrangement typically reduces overall sensitivity somewhat - depending on the driver's response curve and other factors I mentioned.  And as I pointed out, series arrangements increase inductance the amplifier sees which rolls off response - an obvious qualitative judgment you also missed.
As for 16 ohm voice coils and as you alluded to - "the length of the wire in the magnetic gap", did it ever occur to you that voice coil windings can have several layers and in JBL's case, also be "edgewound" aluminum ribbon? The force factor BL is a function of the magnetic field strength within the gap and that of the voice coil with current traveling through it as it traverses said magnetic field. The amount of flux  that is cut and results in  magnetic force on the voice coil is a function of the fixed voice coil gap height, the magnetic field strength within that gap, the magnetic field strength produced by the current running through the voice coil, and a host of factors associated with electrical and magnetic losses of the system. The number of turns of wire of the coil that at any given time is cutting across the gap is but one of many factors that affect overall force strength. A longer voice coil which has the same winding density as a shorter voice coil has no impact on BL - particularly if the Xmax of both designs is similar and both are operated well within their linear region. And if the applied voltages are the same, guess what - the higher impedance coil draws less current which produces a weaker magnetic field which in turn weakens the force exerted. You are clearly way over your head at this point and it might be a good idea for you to quit while you're behind. Voice coil resistance is only one small factor in loudspeaker design which by itself, has no bearing on the linearity or fidelity of the end product - the complete opposite of what you attempted to conclude. Time to pick up a book on electrical engineering or ask an engineer for an explanation. Nothing you have said in this thread has been accurate or useful. Misleading and irrelevant, yes. Useful? No.

Thanks to all for all the input - Once again Agon members have proven to be very insightful.

@erik_squies

- I double checked the sensitivity numbers and 4, 8, and 16 ohm speakers have identical sensitivity - 95db, but it's definitely something to verify going forward - different brands may vary

The XSim Crossover Simulator  seems to be for audio crossovers
- In guitar amps/Speaker cabinets for the majority of cases there is no crossover, so I'm wondering if it would be of any use?

FYI, since I know a few of you play instruments - I have now tried a Celestian 8" mid freq driver and an Eminence Legend 10": bass driver.. Both of these are excellent and significantly better than the original drivers in the areas of  sensitivity, dynamics and clarity.

One last question for everyone...
- In parallel - each driver would move at EXACTLY the same time (pretty much)
- in series - would the second speaker in the "chain" be moving slightly behind the first speaker due to lag time through the voicecoil of the first speaker ?
- also, would one speaker be affected by the other?
- Would the human ear be able to discern this?

Thanks again for all the feedback - it's very useful

Cheers - Steve :-)

@williewonka

Excellent questions

1)  yes (in phase)
2) yes but it should be virtually undetectable
3)  In series, there will be increased inductance which will roll off high frequencies (yes). In parallel, inductance seen by the amp is cut in half just like the resistance is. Also, increased current draw from the amp almost always affects linearity (increased distortion). Whether or not this is noticeable depends on the amp and how hard you drive it.
4) the human ear should be able to discern the increased sensitivity of the parallel arrangement without any problem. All the other aspects - highly debatable and largely dependent on the person.

All of the above - my opinion based on experience - certainly not the end all be all as I'm sure others might have different experiences.

"Speaker sensitivity is measured in decibels per 1 watt per 1 meter, but is usually referred to as just decibels [source: JBL]."

For an 8 ohm load, this equates to 2.83 volts. But for a 4 ohm load it’s 2.00 volts and for a 16 ohm load its 4.00 volts. Because the OP said the speakers have the same "sensitivity," this has been accounted for.

And yes, JBL does put more wire and turns in the gap for their higher impedience speakers leading to a high Bl.

In fact, there are cases where the higher impedience model looses travel (Xmax) because of this difference.

And seeing as how JBLs are the only make of speaker that matter in my household, I stand behind the logic that a higher impedience typically leads to a higher Bl.

As to the effects of inductance, I did note that there is interaction between the two speakers in a series set-up (goes beyond inductance) that does not exist in the parallel set-up, and for these reasons I would choose a parallel set-up.  

@ toddalin

The industry standard for as long as I can remember relies on a fixed voltage to assess a loudspeaker's sensitivity - not power input. In this way, it doesn't matter what the loudspeaker's impedance is. All are judged on the same level playing field. No one has to adjust drive level to compensate for a "nominal" impedance rating. All drivers under test get the same voltage applied and no concern is paid to the current drawn - simply the acoustic output that is generated. If the driver or loudspeaker under test's efficiency is sought, then the unit of power  (watt) is a concern - not the unit of force applied (volt). If you had any electrical engineering experience, you'd know this and would stop quoting a JBL cut sheet as "your Bible".

As to the 16 ohm bit, I give up. I did my best to explain it to you and have obviously failed. Know one thing though. You aren't the only knowledgeable JBL "fanboy". I happen to be listening to a pair of 250ti Limiteds that I've had since new (1992). My first pair of speakers was the L19s I bought - probably before you were born...ugh....that hurts to say.... In any case, please take some heartfelt advice that is not meant to demean you or put you down - there is alot more to learn about electrical, acoustical, and mechanical engineering than what you will find in a JBL cut sheet. I know. Before engineering school, I studied the very same cut sheets you clearly obsess over.