4 ohm or 8 ohm speaker pros and cons

[MICHAEL STRO]

doe's one sound better than the other????

[Lex]

I can't say that one typically sounds better than the other. However, perhaps it's more than a coincicence that many of the finest loudspeakers made are in fact 4 ohms, including my Sonus faber Extremas.

The disadvantage as far as I see it is the fact a 4 ohm load does increase the load on an amp, making it run hotter.

Lex

[ThomasW]

The impedance of a loudspeaker is no indicator of it's sound quality.

[Dr C]

Hi Michael - (guys correct me if I'm wrong on this) generally all things remaining equal, if you had 2 speakers and one was rated at 8 ohms and the other at 4 ohms, then the 4 ohm speaker would be capable of twice the power.

The problem is that very few amps are capable of low ohm loads and tend to overheat as they get unstable when the impedence goes too low. Even though a speaker is rated at 8 ohms, it doesn't mean that this is constant - it will go above and below this figure routinely. So most amps operate between 4-16 ohms to safely take this into account.

Some hi end speakers - like the Sonus Fabers Lex mentioned - have a nominal impedence of 4 ohms. Perhaps the manufacturers know that most consumers that buy their product would be able to afford good amps which are capable of going to 2 ohm loads which would be beyond the capability of most mainstream products.

That said, Thomas is right to say that impedence is no indicator of sound quality. May be a coincidence that many hi end brands have such specs but it's not a rule.

[Lex]

Nice one liner Thomas. Dr. C, that's a pretty nice writup you did there and likely very accurate. I can't say why some speaker manufacturers tend to lower ohm speakers. I'd have to defer to Thomas, Jon, others smarter than me on speaker construction and design.

Lex

[JonMarsh]

I can't say why some speaker manufacturers tend to lower ohm speakers.

It generally comes down to apparent sensitivity. This is an area where manufacturers do play some game- and since sensitivity is generally specc'd relative to 2.83 VRMS (1 watt at 8 ohms, but more the lower the impedance), a lower impedance speaker allows drawing more current and developing more power.

Drivers have their impedance minima above the LF resonance, and before voice coil inductance starts kicking in seriously. For typical midwoofers, that's in the 200 Hz area, and an 8 ohm nominal driver (which usually has a DC voice coil resistance of ~6 oms) will be close to the RDC value for impedance. So, paralleling two 8 ohm drivers in an MTM, for example, may result in an impedance dip to 3 ohms, ignoring for the moment the series impedance of the crossover.

Crossovers themselves are another problem- they can contribute additional impedance loading if they're doing funny things in the crossover region in order to equalize the driver repsonse. You may see a further 10-30% dip in impedance. One of my early X1 Klone crossovers versions had this issue- the crossover program "optimizer" did optimize the net acoustic transfer function of the MTM upper module, but resulted in a 3 ohm impedance minima in the crossover region. No problem for the Aragon 8008BB that I was using for the speakers, but it would have been ugly with a typical HT receiver.


The tweeters used in the M8 two way and it's derivatives (M8ta, M8TM, Arvo Part) are all 4 ohm models, becase this allows making some trade-offs for longer Xmax, (longer voice coil), and getting the "sensitivity" back not by using a humongous magnet structure, but by dropping the impedance a bit. As a result, whether I use a Vifa XT25, Hales Transcendence tweeter, or ScanSpeak D2904/SS9800, the crossover mods are minimal. It just takes a little care to be sure the tweeter/crossover load doesn't dip too load, because all of these tweeters have a DCR of about 3-3.5 ohms, so you don't want any crossover induced impedance dips added to that. With a teeny bit of care, I wind up with a near ruler flat 4 ohm Z from 1.25 kHz to 20 kHz.

If you compare the magent motor assembly on a Focal TD120 vs any of these previously mentioned models, especially the Vifa XT25 (the price leader in this crowd), you can see that it's a different optimization. But the Focal won't go as low, either. Differet strokes for different folks...




Focal TD120DX2



Hales Transcendence Tweeter




Vifa XT25



~Jon

[Lex]

Thanks Jon, interesting analysis. I stayed with you as much as I could. But speaker design is not my forte'. Some of it went, whoosh, over my head.

Lex

[aud19]

Only woosh Doug? Most of it went "ZOOM" over mine :rofl:

Jason

[Bruce]

Thomas,

Did you ever (in the past) experiment with Dynaudio 4 ohm drivers and their rather large 3" voice coils.

[JonMarsh]

We've built some systems with the HiVi clones of the Dynaudio's, but not actually any Dynaudio's. I was considering it at one time, but then they withdrew availability from the DIY market.

Regards,

Jon

[hamlyn]

Hi Michael - (guys correct me if I'm wrong on this) generally all things remaining equal, if you had 2 speakers and one was rated at 8 ohms and the other at 4 ohms, then the 4 ohm speaker would be capable of twice the power.

I am no expert in this matter, however, I have always been under the impression that the amp would yield up to twice the power on a 4 Ohm speaker than it would on an 8 Ohm.
A 150W speaker will yield 150W regardless of whether it is rated at 4 or 8 Ohm.