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Why 4 ohm speakers

post #1 of 113
1/19/09 at 3:17pm
Thread Starter 
Don't laugh, but here's a question I have had for a long time.

Why do manufacturer's make 4 ohm speakers? Given that many receivers are not rated for them, it seems like they must have a pretty good reason to make them.

Does anyone have an explanation as to why a designer would build a 4 ohm speaker?
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post #2 of 113
1/19/09 at 3:22pm
I've wondered about this myself.
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post #3 of 113
1/19/09 at 3:44pm
If you look at specifications of raw drivers from the driver manufacturers, you'll sometimes notice 4 ohm and 8 ohm versions of the same units. The 4 ohm versions are usually more sensitive, and not by a little. A speaker system that's 4 ohms will usually cause a good amplifier to deliver more output than it would into an 8 ohm load, too. The net pickup from these two observations can be 3-5 dB more output. More output means less distortion, more headroom, and the capability of playing substantially louder.

My observation has been that about one-third of the receivers out there have trouble with a fairly benign 4 ohm load, even though sensitivity is higher, making it easier to drive a speaker from that standpoint. I call those receivers "crap." If a receiver cannot deliver more output into a 6 ohm or 4 ohm load than into 8 ohms, it has an under-specified output stage and a wimpy power supply. This was done to save money, and not very much money, either. Digital output stages that can handle 4 ohms are becoming very cheap. There is no excuse anymore for a receiver that can't drive a 4 ohm load. The advertised rating of "100 watts per channel into 8 ohms, one channel driven" is ludicrous and insulting.

Some receivers that don't recommend 4 ohm speakers actually handle the low impedance just fine. I mentioned this a few weeks ago, but Jeff Hipps used Triad Speakers to demonstrate a stunning Newcastle receiver at CEDIA last fall. A 4 ohm load is "not recommended," but the receiver jumped all over those Triads, and Jeff says 4 ohms is no problem. I think some manufacturers say don't use a 4 ohm speaker because their legal department told them.

Look at the specs; look at the 6 ohm rating; look at how many channels are driven when they rate the power.
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post #4 of 113
1/19/09 at 4:16pm
Good, thorough explanation. Thanks Paul.
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post #5 of 113
1/19/09 at 4:20pm
A lower impedance driver can relax enclosure requirements, sometimes significantly. You can then engineer the speaker to have a higher sensitivity, a lower bass extension, or use a smaller box than would otherwisse be required.

Interestingly enough, this trick works only for certain enclosure types. This is why you hardly ever see a 4ohm acoustic suspension speaker and never a 4ohm bass reflex speaker.
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post #6 of 113
1/20/09 at 10:30pm
Thread Starter 
Which enclosure types are usually used for 4 ohm speakers?
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post #7 of 113
1/21/09 at 5:41am
Quote:
Originally Posted by MichaelJHuman View Post

Which enclosure types are usually used for 4 ohm speakers?

Normally they're acoustic suspension. Maybe someone more knowledgable can explain why.
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post #8 of 113
1/21/09 at 8:09am
Quote:
Originally Posted by cansp6 View Post

This is why you hardly ever see a 4ohm acoustic suspension speaker and never a 4ohm bass reflex speaker.

Since those are the 2 main types of speakers and any others are variations on those, what else is there?
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post #9 of 113
1/21/09 at 9:13am
Quote:
Originally Posted by Paul Scarpelli View Post

If you look at specifications of raw drivers from the driver manufacturers, you'll sometimes notice 4 ohm and 8 ohm versions of the same units. The 4 ohm versions are usually more sensitive, and not by a little. A speaker system that's 4 ohms will usually cause a good amplifier to deliver more output than it would into an 8 ohm load, too. The net pickup from these two observations can be 3-5 dB more output. More output means less distortion, more headroom, and the capability of playing substantially louder.

My observation has been that about one-third of the receivers out there have trouble with a fairly benign 4 ohm load, even though sensitivity is higher, making it easier to drive a speaker from that standpoint. I call those receivers "crap." If a receiver cannot deliver more output into a 6 ohm or 4 ohm load than into 8 ohms, it has an under-specified output stage and a wimpy power supply. This was done to save money, and not very much money, either. Digital output stages that can handle 4 ohms are becoming very cheap. There is no excuse anymore for a receiver that can't drive a 4 ohm load. The advertised rating of "100 watts per channel into 8 ohms, one channel driven" is ludicrous and insulting.

Some receivers that don't recommend 4 ohm speakers actually handle the low impedance just fine. I mentioned this a few weeks ago, but Jeff Hipps used Triad Speakers to demonstrate a stunning Newcastle receiver at CEDIA last fall. A 4 ohm load is "not recommended," but the receiver jumped all over those Triads, and Jeff says 4 ohms is no problem. I think some manufacturers say don't use a 4 ohm speaker because their legal department told them.

Look at the specs; look at the 6 ohm rating; look at how many channels are driven when they rate the power.

Very nicely put Paul, as usual you are an asset to this forum. Another key aspect though is the Impedance curve. Fluctuations in the impedance curve combined with a lower impedance can put a resistive load on an amplifier causing the amp to operate less efficiently thus reducing the performance of the amp. It is critical to have a linear impedance curve especially with a 4 ohm speaker.
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post #10 of 113
1/21/09 at 9:24am
Often a single driver is 8 ohms but when you wire 2 together in parrallel it becomes 4 ohms. This is why lots of MTM speakers are 4 ohm.
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post #11 of 113
1/21/09 at 9:27am
Quote:
Originally Posted by racineboxer View Post

Often a single driver is 8 ohms but when you wire 2 together in parrallel it becomes 4 ohms. This is why lots of MTM speakers are 4 ohm.

Even then, with the combined resistors in the x-over network, the impedance of the individual drivers is not the sole determining factor in the loudspeaker's final impedance.
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post #12 of 113
1/21/09 at 12:40pm
Quote:
Originally Posted by Easyaspie View Post

Since those are the 2 main types of speakers and any others are variations on those, what else is there?

I was talking only about speaker enclosure types, of which there are more than just 2.

Quote:
Originally Posted by racineboxer View Post

Often a single driver is 8 ohms but when you wire 2 together in parrallel it becomes 4 ohms. This is why lots of MTM speakers are 4 ohm.

Resistance is additive only if the components are in series. In parallel, the increase in impedance is so small as to not matter. Most multi driver speakers are wired in parallel.

Quote:
Originally Posted by Easyaspie View Post

Even then, with the combined resistors in the x-over network, the impedance of the individual drivers is not the sole determining factor in the loudspeaker's final impedance.

Passive crossover networks have capacitors as well as resistors. The increase in impedance due to a passive crossover is less than you think and any non-trivial increase is usually a result of impedance filtering due to mismatched drivers rather than due to extra load on the system.
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post #13 of 113
1/21/09 at 1:29pm
Quote:
Originally Posted by cansp6 View Post

I was talking only about speaker enclosure types, of which there are more than just 2.



Resistance is additive only if the components are in series. In parallel, the increase in impedance is so small as to not matter. Most multi driver speakers are wired in parallel.



Passive crossover networks have capacitors as well as resistors. The increase in impedance due to a passive crossover is less than you think and any non-trivial increase is usually a result of impedance filtering due to mismatched drivers rather than due to extra load on the system.


Sorry dude. That Wiki article is exactly where I got the info that there are 2 types of enclosures. Sealed box and bass reflex. All others are a variation on either of those two. Bandpass is a hybrid of those two, so maybe I could cut you some slack there.

Parallel exactly halves the impedance, series doubles it. Where are you getting this stuff?

Passive crossovers are used all the time to influence the impedance of a loudspeaker. That is their secondary function, after filtering.
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post #14 of 113
1/21/09 at 2:04pm
Quote:
Originally Posted by Easyaspie View Post

Since those are the 2 main types of speakers and any others are variations on those, what else is there?

Horn-loading changes the impedence of a driver significantly. A horn-loaded 4-ohm driver can appear to be 8-ohm after horn-loading.
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post #15 of 113
1/21/09 at 2:14pm
Quote:
Originally Posted by MichaelJHuman View Post

Does anyone have an explanation as to why a designer would build a 4 ohm speaker?

I've read that with the skinnier speakers of today, baffle diffraction step effects need to be reduced by requiring more power in the midbass. I believe that's why a lot of today's speakers have their minimal impedance in this area.

However, I am not an expert, and look forward to responses. Thanks for the thread.
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post #16 of 113
1/21/09 at 2:16pm
Quote:
Originally Posted by Easyaspie View Post

Parallel exactly halves the impedance, series doubles it.

It only halves it or doubles it if you are discussing 2 drivers (or speakers) and the drivers (or speakers) in question have identical impedances to begin with.

Serial impedance is simply additive. That's easy to define and understand. I'm sure there is a way to define parallel impedance, too, but it is more complicated, especially with more than 2 drivers (or speakers). Mathematically, the overall parallel impedance is the inverse of the additive sum of the individual inverse impedances. (Whoa, did I say that right?)

parallel impedance = 1 / [(1/Z1) + (1/Z2) + (1/Z3) ............etc.]
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post #17 of 113
1/21/09 at 4:24pm
Quote:
Originally Posted by paul scarpelli View Post

i call those receivers "crap."

lol
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post #18 of 113
1/21/09 at 4:50pm
Quote:
Originally Posted by cansp6 View Post

Resistance is additive only if the components are in series. In parallel, the increase in impedance is so small as to not matter. Most multi driver speakers are wired in parallel.


I'm a noobie at this but I've been looking at some DIY designs and I've ran into some 4 ohm rated speakers that I have concerns about because I'd really prefer not to buy a different AVR (mines not rated down to 4 ohm).


For example this Dayton MTM has woofers & tweeter that are 8 ohm but the MTM is rated at 4 ohm: http://www.parts-express.com/pe/show...number=302-991

Same with this Usher kit (both woofers and tweeter are 8 ohm, and the bookshelf is 8 ohm, but the MTM is rated at 4 ohm): http://www.parts-express.com/pe/show...number=302-991


So I ASSume these designs are wired in series which is why the impedance drops them to 4 ohm rated?
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post #19 of 113
1/21/09 at 5:03pm
Quote:
Originally Posted by racineboxer View Post

So I ASSume these designs are wired in series which is why the impedance drops them to 4 ohm rated?

You ASSume correctly, in most cases. At Triad, we have lots of mid/tweet/mid designs that are 4 ohms, and the woofer/mid drivers are almost always 8 ohms, used in pairs for a 4 ohm load. We also have versions with a single woofer/mid driver, and we use 4 ohm drivers in those situations. It's a pain, because often we have to stock both 4 and 8 ohm versions of the same driver.

This would not be an issue if output stages of amplifiers and receivers weren't so freaking cheesey. They're more interested in the rating on the spec sheet than the actually performance when powering a speaker. It's like Ford (historically) and their horsepower ratings.
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post #20 of 113
1/21/09 at 5:10pm
Quote:
Originally Posted by racineboxer View Post

So I ASSume these designs are wired in series which is why the impedance drops them to 4 ohm rated?

Wrong way 'round.

Paralleled impedance is always lower, while series impedance is the additive sum.

Note, however, that two (or more) speakers of different types may not follow the basic DC resistance equations WRT the total impedance.

To simply say that two 4 Ohm speakers in series will equal an 8 Ohm load is a bit of an oversimplification, and it may not even be true. The actual impedance of any speaker varies with frequency; the "rated" impedance is generally an average or "nominal" value. Thus, the sum (or inverse sum of inverses) will also vary with frequency. In fact, the two may either compensate for each other for a flatter impedance curve, or may in fact create steep peaks or dips that can really mess up the overall performance.

Here's my point: A nominal 4 Ohm multi-driver speaker system may have an impedance minima at some frequency which results in a lower value. Say, for example, the speaker system has a 3 Ohm point at 35 Hz. Now, if a 100W amp is designed to operate right at its limit at 4 Ohms, the maximum current would be 5 Amps (I*I*R=P); this leaves us with a drive Voltage of 20 Volts. Now, if the load is actually 3 Ohms at 35 Hz, an attempt for the amp to drive to that 20 Volt drive level would result in an output current of 6.33 Amps, which is a significant over-current.

This is one of the reasons why some speakers are referred to as being "easier" to drive than others.
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post #21 of 113
1/21/09 at 5:50pm
Quote:
Originally Posted by MauneyM View Post

Wrong way 'round.

Paralleled impedance is always lower, while series impedance is the additive sum.

So what I said the first time was right? Ugh.


It was my understanding that two 8 ohm drivers wired in parallel would yield a 4 ohm rated speaker. Above I was told I'm wrong.


Me thinks I was right the first time...
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post #22 of 113
1/21/09 at 6:12pm
Quote:
Originally Posted by racineboxer View Post

So I ASSume these designs are wired in series which is why the impedance drops them to 4 ohm rated?

As pointed out, wired in PARALLEL.


Quote:
Originally Posted by Paul Scarpelli View Post

At Triad, we have lots of mid/tweet/mid designs that are 4 ohms, and the woofer/mid drivers are almost always 8 ohms, used in pairs for a 4 ohm load. We also have versions with a single woofer/mid driver, and we use 4 ohm drivers in those situations. It's a pain, because often we have to stock both 4 and 8 ohm versions of the same driver.

Just realize, racineboxer, that the reason an MTM may be 4ohms is not BECAUSE it has 8ohm drivers in parallel. Conversely, it has 8ohm drivers in parallel BECAUSE the manufacturer was specifically trying to achieve a 4ohm speaker. If a company wants to make an 8ohm MTM it is certainly doable. As is, of course, a 4ohm MT.
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post #23 of 113
1/21/09 at 6:15pm
Quote:
Originally Posted by racineboxer View Post

So what I said the first time was right? Ugh.


It was my understanding that two 8 ohm drivers wired in parallel would yield a 4 ohm rated speaker. Above I was told I'm wrong.


Me thinks I was right the first time...

He's replying to what you posted in post#18. Who told you your initial post was wrong?
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post #24 of 113
1/21/09 at 7:40pm
Quote:
Originally Posted by sivadselim View Post

He's replying to what you posted in post#18. Who told you your initial post was wrong?

Post 12. If you had read post 18 you'd have seen what was quoted.
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post #25 of 113
1/21/09 at 9:10pm
I did read it. He was still responding to what YOU wrote.

Quote:
Originally Posted by racineboxer View Post

Often a single driver is 8 ohms but when you wire 2 together in parrallel it becomes 4 ohms. This is why lots of MTM speakers are 4 ohm.

Quote:
Originally Posted by cansp6 View Post

Resistance is additive only if the components are in series. In parallel, the increase in impedance is so small as to not matter. Most multi driver speakers are wired in parallel.

Quote:
Originally Posted by racineboxer View Post

So I ASSume these designs are wired in series which is why the impedance drops them to 4 ohm rated?

Quote:
Originally Posted by Paul Scarpelli View Post

You ASSume correctly, in most cases.

Quote:
Originally Posted by MauneyM View Post

Wrong way 'round.

Paralleled impedance is always lower, while series impedance is the additive sum.

Quote:
Originally Posted by racineboxer View Post

So what I said the first time was right? Ugh.


It was my understanding that two 8 ohm drivers wired in parallel would yield a 4 ohm rated speaker. Above I was told I'm wrong.


Me thinks I was right the first time...

Quote:
Originally Posted by sivadselim View Post

He's replying to what you posted in post#18. Who told you your initial post was wrong?

Quote:
Originally Posted by racineboxer View Post

Post 12. If you had read post 18 you'd have seen what was quoted.



Who's on first?

What cansp6 said in response to you made little sense in that context. And he did TRY to say that serial impedance (he called it resistance) was additive. But what he said about parallel impedance would obviously not be correct for serial OR parallel impedance. And all the subsequent posts regarding impedance indicated that you were, indeed, correct. Then Paul told you you were correct when you were actually incorrect. Laugh.

http://www.youtube.com/watch?v=sShMA85pv8M
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post #26 of 113
1/21/09 at 9:38pm
Quote:
Originally Posted by paul scarpelli View Post

i call those receivers "crap."

lol
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post #27 of 113
1/21/09 at 9:52pm
I was a speaker designer and I can give you the main reason that some speakers are made in 4 ohms.

The answer is so obvious that it seems simple: Amplifier matching. The designer expects that speaker to be driven by an amplifier that is rated for 4 ohms speakers.

Most high quality amplifiers are rated to drive speakers of at least 4 ohms. Regular consumer grade (Walmart, Best Buy, Circuit City) "amplifiers" (i.e. receivers) are not rated to drive 4 ohms speakers.

The other reasons besides that are insignificant.

Quote:
Originally Posted by MichaelJHuman View Post

Don't laugh, but here's a question I have had for a long time.

Why do manufacturer's make 4 ohm speakers? Given that many receivers are not rated for them, it seems like they must have a pretty good reason to make them.

Does anyone have an explanation as to why a designer would build a 4 ohm speaker?
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post #28 of 113
1/21/09 at 10:08pm
Quote:
Originally Posted by biomed_eng_2000 View Post

..............I can give you the main reason that some speakers are made in 4 ohms.

The answer is so obvious that it seems simple: Amplifier matching. The designer expects that speaker to be driven by an amplifier that is rated for 4 ohms speakers.

The other reasons besides that are insignificant.

The designer may expect a 4ohm speaker to be driven an amplifier that is capable at 4ohms, but that is not at all WHY there are 4ohm speakers.
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post #29 of 113
1/21/09 at 10:23pm
My head hurts
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post #30 of 113
1/21/09 at 10:58pm
That's my point! The engineer made the speaker to be 4 ohms BECAUSE he wants it to be driven by a 4 ohm capable amplifier!

Which came first, the chicken or the egg?

Was a 4 ohm capable amplifier designed to drive a 4 ohm speaker or was a 4 ohm speaker designed to be driven by a 4 ohm capable amplifier?

or perhaps he is asking why there are chickens (or eggs)?

Quote:
Originally Posted by sivadselim View Post

The designer may expect a 4ohm speaker to be driven an amplifier that is capable at 4ohms, but that is not at all WHY there are 4ohm speakers.
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