A Question on Speaker Driver Efficiency

Placing woofers in series will double the resistance and inductance.  This will cause a more steep frequency response roll off on the upper end and will show how sensitive the drivers are at half their standard drive level sensitivity rating. In theory, half the voltage applied to a driver will result in half the sound pressure in the output (-3db). But if two drivers outputs are combined, in theory, output should be doubled - thus cancelling out the effect that halving the applied voltage generates. That's the theory. In practice, the summation of separate but identical woofer responses is somewhat complicated because the way sound pressure waves combine in free space depends largely on the size of each sound source relative to the frequency wavelengths of interest, the relative proximity of the sources in space to one another, and the location in space that response is being detected. In general, you will find that the parallel/series arrangement of multiple drivers can improve overall acoustic efficiency and "apparent sensitivity" at lower frequencies. The essential reason for this is that more driver surface area improves acoustic coupling of the energy source (voltage) to the media being acted upon (air molecules). Try connecting four identical woofers with two series pairs in parallel. You should witness increased output at low frequencies over a single driver for the same applied voltage at most voltage levels Again, however, what is detected in response is heavily dependent on the spatial and frequency characteristics outlined above. Electrically, you should have the same nominal net impedance as one driver but acoustical coupling improvements should provide increased output for the same applied voltage and current draw (the series/parallel arrangement of the 4 woofers should restore the net final inductance value to approximately that of one woofer). All of the above ignores issues pertaining to  the driver's inherent non linearity caused by electrical/mechanical energy loss/storage. For the purposes of the above explanation, one should just assume a "linear" driver.

" If the amp puts out x watts at 8 ohms, and you split it between two speakers totalling 8 ohms, each speaker sees the half the power and puts out the same dB level. When you add them together, the net result is the same.

That said, I would think that, if the net volume in dB for the speakers truly are the same regardless of the impedience, two 16 ohms speakers would be the better choice.

Speakers in parallel work independent of each other. Speakers in series work together and can have an effect on each other.

Also, speakers with higher impedience tend to have a high Bl, and therefore follow the signal more accurately. " -toddalin

I'm afraid very little of what toddalin said above is true. Loudspeaker drivers don't "divide up" power. They respond to a driving force we call voltage. Placing them in series means that voltage is divided between them. Placing them in parallel means that voltage is shared. In the former case, the current drawn from the source is cut in half. In the case of the latter, current drawn from the source is doubled. Sensitivity is professionally expressed as acoustic output generated (typically at one meter distance) for 2.83 Volt RMS input.  It is not expressed in acoustic output (db) per watt of input. That technically, is a measure of efficiency - not sensitivity. With 2.83 Volts applied to two identical drivers in parallel, sensitivity is nearly doubled - BUT SO IS THE CURRENT DRAWN. So there's no net improvement in EFFICIENCY. With two identical drivers in series, current is cut in half because resistance is doubled. Sensitivity may be marginally reduced or improved depending on the sensitivity curve of the driver (how linear the driver's output changes with changes in drive level)

As for the 16 ohm explanation toddalin provided - it is completely bupkiss. A loudspeaker's force factor has nothing to do with its electrical impedance. Force factor is the force exerted on a voice coil that is traversing a magnetic field. A 16 ohm resistance voice coil can have the same force factor curve as a 4 ohm voice coil. Voice coil resistance is not an accurate predictor of driver non linearity (accuracy).

I think you missed the point.

He uses two 4 ohm speakers to produce 8 ohms or two 16 ohm speakers to produce 8 ohms.  Nothing else matters!

If the amp put out 100 watts at 8 ohms, it does so into either of the two noted loads..., because either load totals 8 ohms and the amp puts out 100 watts at 8 ohms.

As for the speakers.  There are two and they share the 100 watts between them.  If they have the same impedience, they share this 100 watts equally, so they each get 50 watts, regardless of whether they are in series or parallel.

Do you think that one will get 25 watts and the other 75 watts or some other crazy combination if they have the same impedience and share the power equally?  That's nuts.

As for Bl, wrong again.  

The Bl is the product of magnetic field strength in voice coil gap and length of wire in magnetic field (T•m), and has a direct impact on how well the speaker reacts to the signal.

A higher impedience voice coil will typically put more wire in the voice coil and therefore has the ability to result in a higher Bl.

But, just look at the TS parameters for JBL speakers of the same type in 4 and 8 or 8 and 16 ohms and this becomes apparent.

e.g., 2265G (4 ohms) Bl=13.6, 2265H (8 ohms) Bl=19.5
2268G = 15.2, H=21.5
2269G = 19.2 H=26.4

etc.

@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 :-)