Back EMF Theory...

[weltersys]

Here is an attached snippet from JBL about it. Stereo Integrity and JL audio both make the same recommendation. I have heard this recommendation from many sources over the years.

Wouldn't it be 3dB difference unless you doubled the power to the drivers as well as going parallel?

The circled snippet is wrong, putting voice coils in series does not reduce damping factor to one. As in many myths, they get repeated and quoted.

Good article on damping factor:
http://www.bennettprescott.com/downloads/dampingfactor.pdf

Not sure what you are asking, but doubling cone area (and cabinets) results in a 3 dB increase in sensitivity, a series pair results in a decrease in sensitivity of 3 dB for a net gain of 0 dB.

In parallel, assuming the drive voltage to two drivers remains the same (an assumption which is totally dependent on the amplifier design and may be wrong) you get 3 dB from doubling cone area plus 3 dB from doubling power, 6 dB increase in level.

Art

 

[wg_ski]

Putting these two impedances in series with each other results in something that resembles neither and does bad things for the overall frequency response. Where one Z goes high, it will reduce the output of the other. If the peaks were to align with one another (magnitude and phase), the resulting response would be as flat as either by itself.

 

[Ron E]

EMF (back or otherwise) is electromotive force measured in volts.

Nice, pedantic BS. Next you'll tell us that back EMF is -BL*u and that is ever so helpful to the original poster... 😉

 

[wg_ski]

The OP thought he could measure it with a DMM, or derive it from a single impedance measurement but you can't. It's a quantity that falls out of the math when modeling a motor and is a function of load on a given motor. It sounds more menacing than it actually is, and BS like that JBL paper (which is dumbed down for your average Joe Blow with a boomin' car audio system) don't help.

 

[Ron E]

Here is an attached snippet from JBL about it.

Here is how to test that assertion. Connect one woofer to a powered up amplifier and tap on it. Compare to a powered off amplifier. Then connect several in series and tap on them. IF they sound different than the singly connected one, what they are saying could have some merit.

I am starting to think that your method of measuring the impedance curve of the different connection schemes is the only way to measure anything about back EMF. The only other way I can think of to do this would involve manually moving the cone of a driver and measuring the peak effects but I don't see that being very consistent or accurate.

Mount two woofers front-front, with a gasket if necessary to get a good seal. Drive one and measure the output of the other.... The output is 100% back emf 😉

 

[tb46]

Hi Y'all,

Dick Pierce wrote a nice straight forward paper on the subject: "Damping: Loudspeakers in Series". A number of .pdf sources come right up in Google.

Regards,

 

[sreten]

Here is an attached snippet from JBL about it.
Stereo Integrity and JL audio both make the same recommendation.
I have heard this recommendation from many sources over the years.

Hi,

Yes. And its simply wrong in all respects. It makes no difference.

rgds, sreten.

 

[zettairyouiki]

That JBL snip says you should wire parallel instead of series to keep the damping factor high? Isn't that the exact opposite of what you should do?

I remember reading an anecdote from some large touring sound provider. They had long cable runs and were running the subs in 2ohm groups resulting in a predictably dismal damping factor. They switched to series for 8ohm loads and reported that despite the reduction in power going to the subs due to the increased impedance, the subjective increase in punch and clarity was stunning and made up for it completely.

I'll see if I can dig that up. I think it was on prosoundweb.

 

[SY]

The concept of symmetry might prove helpful in understanding why that blurb is incorrect if two drivers that are reasonably the same are placed in series. Think of the voltage source as being two identical voltage sources in series as well as the two drivers being in series...

 

[weltersys]

They switched to series for 8ohm loads and reported that despite the reduction in power going to the subs due to the increased impedance, the subjective increase in punch and clarity was stunning and made up for it completely.

I'll see if I can dig that up. I think it was on prosoundweb.

It was me, see post #11.

Art

 

[zettairyouiki]

That what I get for just skipping to the end of the thread. I think that might be the same anecdote I'm remembering. Certainly sounds similar enough. Would especially make sense if you've posted the same before in a different thread.

 

[shredhead]

1st run...

Hi guys. I ran some impedance sweeps first with just the 4 dual voice coil drivers that I am using separately. They are all free air measurements.

I then averaged the responses of the drivers and compared them to actually physically hooking up the 4 drivers in series with each other (dual voice coils in parallel)

The cursor above shows the peak (59.2ohms) at 25.53Hz so I decided to measure volts and current at that frequency and at 50 where the curve is around 11.2ohms. Here are the results:

Sorry Welter, I forgot about the noise tests completely. Easy to forget things when you're testing stuff. I don't really see anything that could be that bad about series wiring of drivers in this test which agrees with many of you. Thanks for the input everybody and I am still down to hear suggestions of new things to test. I will probably test a couple other wiring configurations this weekend.

 

[Ron E]

Nicely done! The true test might be to observe the cones at large displacement when installed in the final configuration. Honestly the damping argument from the JBL sheet is pretty much hogwash, as others have mentioned. If you haven't read it the Dick Pierce article is very well done, as usual for he with the painful sounding name 😉

 

[Zero D]

@ wg_ski

Crown have been misleading people with the 5000 tag ! Still, amps with swithching power supplies can sound even worse into low Z's

I know heat affects the T/S & response etc before TC takes place, as i noted ! I mentioned Thermal Compression in the context that driver manufacturers do.

@ shredhead et al

You might be able to see/measure BEMF by this method i've devised, in my screenie 😉 The output devices might not be BJT's, as shown for reference only.

 

[forr]

the driver just looks like an electrical impedance at the terminals

This is in T&S modelisation.
With 0V signal at the input of the amp and some microphonic EMF generated by the voice coil of a connected driver at its output, it looks a bit different.

 

[shredhead]

Here is something that confuses me...

The damping factor of an amp is frequency dependent, different for all amp designs and should show up in these impedance sweeps right? So any back EMF will add to the impedance curve of the driver(s) and show up in these tests if the amplifier cannot dampen it?

I hooked up 3 different amplifiers, matched the voltage out and did these three impedance sweeps:

I hooked up a class A/B, a class H, and then a class D. They all show identical impedance curves. Does this mean that the amps all have the same output impedance? Or does BEMF only start to affect things when a driver is moving 2 inches peak to peak?

@Zero D, I'm too scared to do that.

 

[weltersys]

Here is something that confuses me...

1)The damping factor of an amp is frequency dependent, different for all amp designs and should show up in these impedance sweeps right?
2)So any back EMF will add to the impedance curve of the driver(s) and show up in these tests if the amplifier cannot dampen it?
3)I hooked up a class A/B, a class H, and then a class D. They all show identical impedance curves. Does this mean that the amps all have the same output impedance?
4)Or does BEMF only start to affect things when a driver is moving 2 inches peak to peak?

1)An impedance sweep tells you the impedance of the driver, not the amplifier's damping factor.
2) No.
3) No.
4) As the amplitude is increased, more BEMF will be produced, if it has an affect on the amplifier it will be more pronounced at higher drive levels.

Different amp designs react differently to reactance and impedance variations, which is why I suggested measuring frequency response with the different loads just under clip/limit to determine which works best with your particular amp(s) and speakers.

Art

 

[shredhead]

I don't want to drive the subs to their limits in free air so I can't do that. If more BEMF is produced at higher levels, isn't it still in proportion to the power generated on the amp side of things to control it? Wouldn't that ratio stay the same at lower levels?

Basically I'm curious of the effects of BEMF for series vs parallel wiring. I just can't figure out to to measure this. I thought it would show up on an impedance curve as increased resistance due to the BEMF decreasing efficiency at the most reactive frequency. If not that then what the crap are the effects of back emf and how do I capture a measurement of the thing?

 

[sreten]

Hi,

A speaker designer I cannot recall recounted seeing series drivers
going into collywobbles at high drive that didn't happen parallel.

As a possibility its very speaker dependent and hard to generalise.

rgds, sreten.

 

[NEO Dan]

Use an oscilloscope to observe the output of a dual voice coil of a driver driven by acoustic coupling to a driver using a thick baffle, power one driver with an amp. change the coil wiring configuration from series to parallel to see the difference.