4 Ohms VS 8 ohms

munk

Is there any engineering reason Polk top of the line, LSi 15 and 25 speakers are 4 ohms instead of 8 ohm?

There seems to be advantages for 4 ohms over 8. A 200 wpc amp at 8 ohms provides even more power into 4. I still miss my Polk bookshelf monitors which were 4 ohms. I hesitate to conclude 4 'sounds better', but those little speakers had a great sound not entirely rediscovered in the 8ohm SL 50's which replaced them.

As the son of a former engineer, I've managed to go through Life woefully ignorant of many of the laws binding our physical universe. If someone could simplify the 4ohm vs 8 ohm I'd sure appreciate it.

If an amp produces more power at 4 instead of 8, why aren't more speakers 4 ohm?


munk

Bernal

1.- http://forums.audioholics.com/forums/showthread.php?t=396
2.- http://www.audioholics.com/education/frequently-asked-questions/connecting-4-ohm-speakers-to-an-8-ohm-receiver-or-amplifier
3.- http://www.audioholics.com/education/amplifier-technology/impedance-selector-switch-1
4.-http://www.ehow.com/facts_5913837_speakers-ohm-vs_-ohm.html
5.- http://archive2.avsforum.com/avs-vb/showthread.php?t=622902

newbie308

Sniff Sniff! Hey what's cooking? Smells like well done voice coil! Mmmm mmmm! What vintage is that speaker so I can select a nice opamp to go with it?

munk

Thank you for the links. This is going to take me awhile to understand.

for equally efficient systems. the 4 ohm speaker should be 3 decibels higher than the 8 ohm having identical efficiency.

LS series is 88 decibels efficient; RiA9's are 90
Yes, I'm going to be confused for awhile.

There is some reason Polk goes 4ohm with their top of the line. I don't know what it is, but it must be something.

The well done voice coil-

steveinaz

munk wrote: »

If an amp produces more power at 4 instead of 8, why aren't more speakers 4 ohm?

munk

More power does not equal higher (SPL) output--it just means more current flow to the amp. 4 ohm loads pull more current, so require heavier duty amps, most people won't buy better-built high-end amps to support a 4ohm load.

Example, if you had 2 sets of speakers that were otherwise identical in specification--but one set was 4ohm, the other 8ohm---driven by the same amp, they would produce the exact same sound pressure level (SPL) at any given wattage. But the amp would need to draw more current to "support" the 4ohm rated speaker.

munk

Would you be able to detect that on your electric bill?

And, why then would a manufacturer make a 4 ohm speaker?

I think I'm starting to get this. Is this correct- there's less resistance with 4 ohms so an amp must push more to achieve similar music-work? So there is no free lunch.

thuffman03

munk wrote: »

And, why then would a manufacturer make a 4 ohm speaker?

So guys like me can say we are driving 1200 watts per channel.

newbie308

steveinaz wrote: »

More power does not equal higher (SPL) output--it just means more current flow to the amp. 4 ohm loads pull more current, so require heavier duty amps, most people won't buy better-built high-end amps to support a 4ohm load.

Example, if you had 2 sets of speakers that were otherwise identical in specification--but one set was 4ohm, the other 8ohm---driven by the same amp, they would produce the exact same sound pressure level (SPL) at any given wattage. But the amp would need to draw more current to "support" the 4ohm rated speaker.

Well said! Power is power. If the amplifier power supply cannot supply the increased demand due to lower impedance, the output will be limmited by the power supply. The KVA rating of the transformer supplying power to the amplifier is the limmiting factor for any sustained current draw. Of course very large fully charged capacitors will enable instantaneous current to reach levels limmited only by the resistance of the downstream components in the amplifier. This is defined as peak Watts.

steveinaz

munk wrote: »

I think I'm starting to get this. Is this correct- there's less resistance with 4 ohms so an amp must push more to achieve similar music-work? So there is no free lunch.

Correct, less impedance (total resistance) = more current flow; but the caveat is, the amp has to be robust enough to handle high levels of current. This is why shorted speaker leads (zero impedance) blows fuses or worse in amps or amplifier sections of receivers/integrateds. It's also why your better quality amplifiers are usually HUGE and HEAVY, if they are of conventional design. There's no getting around large transformers/capacitors/heatsinks being needed for high current capability.

Have you ever wondered why a receiver rated at 100 watts is markedly cheaper (and lighter) than a seperate amplifier at the same wattage? Now you know; high CURRENT capability is probably far more important than watts per channel.

I don't understand enough about speaker design to know why some systems end up being 4 ohms nominal; you'd have to query some of our DIY speaker guys; but the reason 8ohm speakers are so common is because they can be driven easier with lower grade (read that: cheaper) electronic components.

steveinaz

Think of a garden hose as EMF (electromotive force OR potential voltage). Think of current as how much water can flow thru a given hose diameter. If you put a nozzle on the end, and close it slowly, you're introducing or impeding the flow. This is what impeadance is. Total resistance to the flow of current. Now, connect that common garden hose to a fire hydrant and close the nozzle---POP, the hose will split. You need a far larger, and better built hose to withstand the flow (current) of water from a high pressure source like a fire hydrant. You guessed it, those hoses cost alot more...

If you're a car guy, think of it like this:

Horsepower=Voltage potential
Torque=Current
Vehicle weight/friction/aerodynamics=Resistance

You can have all the potential (voltage) in the world, but if you have only limited current--not much work can be done before the circuit fails to deliver when resistance is met.

munk

Alright- This may sound like I'm leaving the topic, but why are the 8 ohm RTiA9's rated for 500 watts? There's not many amp's that can deliver 500 watts in 8 ohms. What was Polk anticipating? Longevity? Unless you put a distorted signal through them, they aren't going to be overdriven.

The standard in the industry is to measure output at a low minimum draw. That's not a very usefull number. There must be paper watts and real world watts. Nad says of the 275 Bee, "150 watts minimum continuous power, 4 or 8 ohms, all channels driven" I thought that an attempt to be realistic.

steveinaz

Power handling ratings on speakers are fairly useless, though sometimes you'll see a specification for maximum SPL--that's sort of useful once you do the math on what your amp can do given the efficiency/impeadance of your chosen speakers; and the size of the room you need to fill with sound.

RMS (root mean square) is suppose to represent 72% of the waveform (continuos signal) power when clipping at the peak of the waveform---though the industry is all over the map on how these specs are given--or how they're derived.

At the end of the day, good high quality amplification will rarely blow a speaker of good quality, regardless if it's power specs--at least not before it runs you out of the room. You get what you pay for applies in audio.

tonyb

" You get what you pay for" applies to just about everything Steve.:)

falconcry72

tonyb wrote: »

" You get what you pay for" applies to just about everything Steve.:)

I don't know about that; I paid for a chick once and definitely got something I DID NOT pay for!!!:eek: