Series vs. parallel sound quality

[slicemaster101]

Hi all, I haven’t been here in a wile but I decided to get back into this community that I was very active in a wile back.

Anyways, I just wanted to ask a fairly simple question about wiring speakers. I was wondering if there is any sound quality difference between wiring speakers in series or parallel and vice a versa?

Signed,
Slice

[bogoes]

There should be absolutely no difference in sound quality as the drivers are still receiving the exact same signal. The only thing I can think of that would affect the sound quality is that when you connect 2 drivers in parallel, you cut the impedance in half, which usually results in a small amount more distortion coming from the amp...but if you can tell the difference between 0.1% THD and 0.2% THD, then you have better ears than me.

[rdf]

Speakers in parallel see the full damping factor of the amplifier, in series they see a high series impedance changing with frequency (damping factor = 1.) High frequency drivers are less affected by drive impedance than woofers, and cabinets can be designed to take it into account in the case of woofers, but unless your amp can't take the load I'd go parallel.

[Svante]

Originally posted by rdf
Speakers in parallel see the full damping factor of the amplifier, in series they see a high series impedance changing with frequency (damping factor = 1.) High frequency drivers are less affected by drive impedance than woofers, and cabinets can be designed to take it into account in the case of woofers, but unless your amp can't take the load I'd go parallel.

If the drivers are identical, this reasoning is wrong. As an example, two driver with a Qts of 0.4 can be seen as one driver with the a Qts of 0.4 (but doubled Vas etc.). Since the drivers have the same impedance, the voltage across each speaker will be exactly half the driving voltage for all frequencies. It does not matter how a voltage is delevered to a given driver, and the voltage alone determines the sound radiated from the speaker.

If the drivers are slightly different, the result will still be about the same. This is because the sum of the sound pressures is proportinal to the sum of the voltages across each of the drivers and this sum is the total driving voltage.

If the drivers are very different, the driver with the largest impedance at a given frequency will "take over" at that frequency. But the sum of the two will still be proprotinal to the driving voltage.

[rdf]

Your reasoning ignores driver back EMF - the voltage generated by the speaker as it's coil moves through the magnetic field - and the amplifier's job damping it. System Q (Qec') is almost completely determined by electrical Q, defined by Qec'=Qec(Re + Rs)/Re where Qec is driver electrical Q, Re is driver resistance and Rs source. Yet your reasoning makes sense, damn if I can resolve the paradox at the Thiel-Small level.

[Svante]

Nope, my reasoning does not ignore the back EMF. If one of the speakers had been a series resistor, I would have ignored it. Since the two driver's electrical impedances and back EMFs are identical, the back EMF of one driver is just right to cancel the back EMF from the other and vice versa. This keeps the voltage over each of the drivers to exactly half the total driving voltage. This would not have been the case with a series resistor or if you were to put the hand on one of the drivers to keep the cone at standstill.
But I think that this way of thinking is more complicated than the simple voltage divider with two identical impedances.

So, the bottom line is; connecting identical drivers in series is OK, it does not affect Qts.

Edit: you can also see it from the equation for Qes:

Qes=2*pi*fs*Mms/Res, Res=(Bl)^2/Re

If you add a driver in series and see the two drivers as one, B and fs would stay the same, Mms would double, l would double, and Re would double. Net effect: Qes stays the same.

[moamps]

Originally posted by Svante
.....Since the two driver's electrical impedances and back EMFs are identical, the back EMF of one driver is just right to cancel the back EMF from the other and vice versa..... [/B]

Back EMFs are in phase. (inductors in series)

What JBL says about this....strange...

[rdf]

No, Svante appears to be correct, though I was going to argue Qes is not the same as Qes'. Here's a 2000 Usenet post from a man who knows speakers, a professional consultant in the field for decades.

Originally posted by Richard Pierce:


In article <kQiX5.1753$qs2.401109@dfiatx1-snr1.gtei.net>,
Bill Whitlock <bill.whitlock@verizon.net> wrote:
>Perhaps I confused you with my answer. To restate, I believe it is OK to
>wire two 4-ohm speakers in series to present an 8-ohm load to the
>amplifier, provided that the speakers are identical.
>
>Before reading the article by Richard Clark, I believed that doing so
>would have a detrimental effect on damping factor (which can affect the
>speaker's frequency response, especially at low frequencies). Others in
>the audio field still believe this is a controversial issue.

And it is only a belief, unfounded by any physics.

The notion is thatputting one speaker in series with another can
adversely effect that damping. This conclusion is based on a
highly flawed model of what the damping in the speaker REALLY
is and what it is due to. To dispel this falsehood, it's
necessary to do a more detailed analysis of what's really going
on using the actual parameters involved.

The damping of the speakers is due to very specific mechanisms,
primarily mechanical (due to frictional losses in the
loudspeaker suspension, and some minor absorbtion losses) and
electrical (due priomarily to the DC resistance of the voice
coil). The amount of damping is measured using the "Q" factors
due to each contributing source.

The measure of mechanical damping or Q, Qms is:

Mms
Qms = 2 pi Fs -----
Rms

where Qms is the Q factor due to mechanical losses, Fs is the
resonant frequency, Mms is the moving mass of the system, and
Rms is the mechanical (frictional) losses. Notice that there is
no implicit term relating how the Qms is affected electrically,
i.e., how it is connected to the amplifier. Now, with two
drivers, we have double the mass, Mms, because we have two cones
instead of one. But we have also have twice as much suspension
and thus twice as much suspension loss, Rms. Thus the ratio of
Mms to Rms remaines the same, and thus the mechanical Qms
remains the same.

The measure of electrical damping, or electrical Q, Qes is:

Mms
Qes = 2 pi Fs ----- Re
2 2
B l

where Qes is the Q factor due to electrical losses, B is the
flux density in the magnetic gap, l is the length of the voice
coil wire immersed in the magnetic field, and Re is the DC
resistance of the voice coil. Hooking two such system in series
does three things:

1. It doubles the moving mass Mms,

2. It doubles the DC resistance Re,

3. It doubles the length of wire in the magnetic field l.

So while the intuitive leap might be to suggest that because the
resistance Re is doubled, and therefore the Qes is similarily
doubled, doing so completely ignores the other two effects.

Let's look at the ENTIRE analysis. Let's double all the
necessary quantities and plug them back into the equation and
see what happens. For Mms, we'll substitute 2*Mms and so forth:

2 Mms
Qes = 2 pi Fs --------- 2 Re
2 2
B (2 l)

and:

And we can simplify by gathering the factors together. In the
numerator, we have 3 instances of factors of 2, for a total of
8, and there is a single factor of 4 in the denominator:

8 Mms
Qes = - pi Fs ----- Re
4 2 2
B l

Simplifying one step further, ye factoring out the common factor
of 4 now in both the numerator and denominator, we end up with:

Mms
Qes = 2 pi Fs ----- Re
2 2
B l

which shows that the electrical damping for two systems in
series is identical to the electrical damping for a single
system alone. Q.E.D.

This contradicts the seemingly intuitive notion that two drivers
in series must be severely underdamped as a result of the
additional series impedance. It's not the first time that
intuition has failed to coincide with physical reality in audio.

--
| Dick Pierce |
| Professional Audio Development |
| 1-781/826-4953 Voice and FAX |
| DPierce@world.std.com |

I went back to the Thiele Small derivations for the first time in twenty years and they bear this out, the components defining this behaviour appear in series in a driver's equivalent circuit and sum in a linear manner as described. Completely counter-intuitive to me and with some interesting consequences, but physics is physics.
Series those puppies up.

[Paradise_Ice]

Originally posted by bogoes
There should be absolutely no difference in sound quality as the drivers are still receiving the exact same signal. The only thing I can think of that would affect the sound quality is that when you connect 2 drivers in parallel, you cut the impedance in half, which usually results in a small amount more distortion coming from the amp...but if you can tell the difference between 0.1% THD and 0.2% THD, then you have better ears than me.

I think there is a difference on the amplifier for a start.

Running lower loads on amplifier is not good for the damping factor for a start, and the distorsion can be much higher than 0.2%, more like 5% at high volume levels, this depends on the amplifiers design but am talking average.

I had an old Creek CAS 4040 and i had 2 sets of speakers on the poor little amplifier in parallel , it sounded loader with 2 sets of speakers but when i went back to 1 pair it sounded better and more focused.

Who wants louder over quality of sound?

[Timn8ter]

May we take this one step farther? I have two identical drivers. I get nearly the same sound quality wired in parallel along with a series resistor that I do when I wire them in series without the resistor. I'm assuming increased Qe. Correct?