A Question on Speaker Driver Efficiency

I look at PV, Eminence, and others..., and yes, for the same models, the higer impedience version typically has the higher Bl.

So, contrary to what some would say, I guess that JBL is not the only one who does this and the OP is advised to check the specs for the selected speakers for himself.

Obviously, someone else around here hasn't even bothered to look at the various manufacture's similar offerings to confirm or deny their beliefs.

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?

@williewonka , a previous answer was incorrect.

Both speakers will move at EXACTLY the same time regardless of the hookup.

Usually tube amps like higher impedances, so usually its to your advantage to put the speakers in series if you want cleaner sound. In the scenario you described though it will work fine either way if 8 ohms is your goal; two 4 ohm speakers in series or two 16 ohms speakers in parallel. You will not be able to hear any difference other than the differences that might be in the drivers themselves.

To clear up some obvious confusion on this thread, Sensitivity is a voltage measurement and efficiency is a Power measurement. Into 8 ohms both are the same, since sensitivity is 2.83 volts at one meter and that works out to 1 watt.

Into a 4 ohm load, 2.83 volts is 2 watts not 1. So the amp must make twice as much power and thus there is a 3 db increase in sensitivity (but note that the increase in volume is because the amp is making more power, not because the speaker is more efficient). The converse is true into 16 ohms- now the amp makes 1/2 watt to make 2.83 volts so the sensitivity is 3 db less.

So if you have a 97 db 1 watt/1 meter 8 ohm driver, two in series will have the same efficiency (since each is absorbing 1/2 watt) while the sensitivity is 94 db. If you put the two drivers in parallel for a 4 ohm load, the efficiency is the same as 1/2 is absorbed by each driver if 1 watt is applied. However the sensitivity is now 100db.

The trick is that you are using a tube amp and since they can't double power as impedance is cut in half, the efficiency spec is easier to use since it tells you how loud the speaker will play with your amp. 

Hi OP! 

XSim will help you evaluate the impedance and relative output. You can try making up circuits of various speaker drivers, and the output volume AND impedance will change when you do. :) So it gives you immediate feedback to possible choices. 

Notes:

Sensitivity: Volume in dB at 2.83V. 

Efficiency: Volume in dB at 1 Watt. 

For 8 Ohms, this value is the same. 

Best,

E

@atmasphere

atmasphere said - " a previous answer was incorrect.

Both speakers will move at EXACTLY the same time regardless of the hookup. "

Incorrect? Really? In theory, they move at EXACTLY the same time only when no capacitance exists in the circuit. Current through the coils is the same at any given time only when the circuit doesn’t have capacitance. We know this is not the case. All circuits have some level of capacitance and inductance. The question is whether or not the amount of capacitance in the circuit and how it interacts with the series inductance is readily detectable. In practice, it is not. However, because it is not readily detectable doesn’t mean it doesn’t exist. I stand by what I said above - there will in fact be a phase delay between the two loudspeaker drivers but it will be virtually undetectable.

If you had any experience designing or building amplifiers, you’d know that every energy storing element within a circuit contributes to phase lead or lag between voltage and current. Loudspeaker coils are no different. This becomes immediately apparent when trying to maintain a safe phase margin with a typical feedback amplifier. The frequency at which inadequate phase or gain margin results in oscillation might be well over 50khz - certainly much higher than any loudspeaker woofer can deal with - hence undetectable but still there nonetheless.

As for "obvious confusion" regarding efficiency and sensitivity, there was nothing inaccurate or "confused" about my explanation - which you essentially repeated until you said this:

"So if you have a 97 db 1 watt/1 meter 8 ohm driver, two in series will have the same efficiency (since each is absorbing 1/2 watt) while the sensitivity is 94 db. If you put the two drivers in parallel for a 4 ohm load, the efficiency is the same as 1/2 is absorbed by each driver if 1 watt is applied. However the sensitivity is now 100db. " - atmasphere

With that, you clearly blew it. Two identical drivers in series will draw half the current of one driver but acoustic output of the two drivers in series will be essentially the same as one driver - not 3db down or 94 db as you indicated. Their accoustical output will sum depending on the factors I noted in my original post. So acoustical efficiency is not the same as you indicated - but roughly double.

You try to draw a distinction between efficiency and sensitivity but clearly don’t understand the essence of the difference - you aren’t "applying a watt". The only thing you’re applying is voltage. The device(s) draws whatever current the resistance dictates. In the case of two 97db drivers in parallel, you’ll only get 100db output if a pair are in parallel and are drawing current sufficient for two watts - not 1 watt. Go back and read what you said. Two 8 ohm drivers in parallel that normally produce 97db at 2.83v drive level individually, will only produce 100db when they draw two watts in the parallel arrangement. In series, they will draw half the current but each device's acoustic output will sum to roughly 97db. Efficiency is doubled with the series arrangement and remains unchanged with the parallel arrangement.

Since your confidence level appears to be far ahead of your knowledge level, it might be a good idea to say " I THINK what the previous poster said might be incorrect". A little humility goes a long way and alleviates the need to on occasion eat crow.

@erik_squires

Simple and accurate erik - Kudos!