Ohm Question (1 Viewer)

[Daniel_M]

I'm a little confused about

the relationship between ohm ratings

for amplifiers and speakers.

Specs say an amp is rated for say,

400 watts at 8 ohms and 800 for

4 ohms etc. Are there 8 ohm and

4 ohm speakers or can the same speaker

be rated at both 4 and 8 ohms?

Thanks

 

[Wayne A. Pflughaupt]

Daniel,

With speakers, the ohm rating you see is nominal, i.e., kind of an average. In reality, a speaker’s impedance will vary with different frequencies. For instance, it can drop as low as 1-ohm (or even less) at some frequencies, with a high well into the double digits at other frequencies. Typically though, the ohm variance is not as severe as that. A speaker’s nominal ohm rating is fixed, but it can be altered interally by modifying the crossover. A job for the design engineers, not you or I.

With amplifiers, the ratings refer to how much power can be delivered with a certain load (ohms). In theory, halving the load (i.e., 8-ohms to 4-ohms) will draw twice the power from the amp, but in reality that is seldom the case. If you see an amp rated for 200 watts @ 8-ohms and 275 @ 4-ohms, that means it is not as “robust” as one that will deliver say, 375 watts @ 4-ohms. It would be safer to use 4-ohm speakers with the latter than the former.

Hope this helps.

Regards,

Wayne A. Pflughaupt

 

[Saurav]

Are there 8 ohm and 4 ohm speakers or can the same speaker be rated at both 4 and 8 ohms?

Yes, there are 8 ohm speakers and there are 4 ohm speakers. The same speaker cannot be rated at two impedances, AFAIK. However, like Wayne explained, these are nominal ratings, and the actual impedance fluctuates around this nominal value, sometimes quite a bit.

 

[Daniel_M]

Interesting........another question.

You don't see alot of speakers (nominaly)

rated at 4 ohms. Is there a reason most speakers

are rated at 8? If you get a better punch

from your amp with speakers rated at 4 ohms

it seems this would be a better speaker to

buy. Is it a price thing? Are the 4 ohm rated

speakers more expensive?

Dan

 

[Chu Gai]

The best speaker to buy is the one that'll sound best to you in the environment you intend to play it in. To that end, nothing beats being able to audition the speaker in your own place. That said, it may be that you're mistaking the nominal ohm ratings of speakers with efficiency which basically states just how much sound pressure (loudness) a speaker will produce given one watt of power at a 1 meter distance. what is it that you're trying to achieve and what it is that you presently have?

 

[Saurav]

If you get a better punch from your amp with speakers rated at 4 ohms it seems this would be a better speaker to buy.

No, because the amount of "punch" required is something most amps cannot give, and you'll likely end up blowing your amp if it's not designed to handle 4 ohm speakers.

Anyway, like Chu Gai says, the 4/8 ohm thing is incidental, it shouldn't be part of your speaker picking decision. Unless you have an amp which cannot handle 4 ohm loads, in which case you have to pick an 8 ohm speaker. The point is, there are many other factors that are much more important when picking a speaker than its impedance.

 

[BenS]

I must be missing something here.

Wayne states:

likely end up blowing your amp if it's not designed to handle 4 ohm speakers

So this backs up the last statement by Wayne. If ohm rating varies how can an amp blow up? I am not trying to buy speakers just understand ohm load it has always been a mystery to me.

Thanks

 

[Saurav]

So this backs up the last statement by Wayne. If ohm rating varies how can an amp blow up?

If an amp only provides a wattage rating at 8 ohms, that doesn't mean it'll blow up if the speaker's impedance falls to 7.99 ohms. Again, the amp's rating means teh same thing - it can handle a speaker whose impedance stays in the 8 ohm ballpark. Sure, at some frequencies the speaker's impedance will drop below 8, and it may even drop all the way to 4, or 3 or 2. Yes, such a speaker will be a little hard on the amp. However, if you use a 4 ohm speaker, the impedance of that will almost always be around 4 ohms, and will rarely rise above 6. So, that's a more constant stress situation for the amp, whereas the 8 ohm speaker would fall to 4 only sometimes, which is easier for the amp to handle.
I hope that's clearer now

 

[BenS]

Yes, thank you a lot. I appreciate the information I get on this forum. Again thanks for the info.

 

[Chris PC]

As far as ohm rating go, it works like this as far as I'm concerned. If your AMP can handle it, you do benefit from lower impedance, lower ohm rated speakers because an amplifier that puts out 100 watts into an 8 ohm speaker, can, if it has a good enough power supply, and can handle the current that low impedance demands, then you get 200 watts into the 4 ohm speaker. That is an over simplification, but it is generally true. I have 8 ohm speakers and 6 ohm speakers. My amp can handle the 6 ohm speakers, so I do get more dB's out of them because they are able to 'extract' more watts from my amp/receiver.

What ohm ratings are good for is this. You must pay attention to them in order to match the speakers to the amp. You don't want to use 4 ohm speakers on a receiver that tells you "Do not connect speakers less than 6 ohms" for instance. There is some room to move with that, but generally, don't hook up 4 ohm speakers to an amp that you know is designed only for 8 ohm loads.

Amplifiers can give you voltage for volume gain, but it is harder for them to put out that gain at higher current demands, those of lower impedances. It is also a bit more expensive to provide lots of current, so generally, unless the amp is designed for high current loads, you will find that it puts out its max power at 6-8 ohms. Going to lower ohm speakers won't necessarily give you more watts because the amp can't deliver the current.

 

[Daniel_M]

OK, so, if I have an amp and the specs say

it's rated at 400 watts into 8 ohms and

650 watts into 4 ohms, am I getting

more bang for my buck if I buy speakers

rated at 4 ohms?

Are we talking more loudness

at a given mark on the volume control for the

4 ohm speakers as opposed the the 8

or is the amp manufacturer just letting me

know the power is there if I need it?

Dan

 

[Saurav]

Are we talking more loudness at a given mark on the volume control for the 4 ohm speakers

I've wondered about that, and I don't know what the correct answer to that is. Another factor here is the speaker's sensitivity - the wattage you're talking about is electrical power, that doesn't translate directly into loudness or SPL. You'll see another parameter specified for the speaker, which is its sensitivity - let's say, the sensitivity is 90 dB. I believe this means that the speaker produces 90 dB of sound output for 1 watt of input power, when measured at a certain distance (I think it's measured at 1 meter from the speaker). Anyway, so now if you have two speakers one of which is 87 dB and the other is 90 dB, you can immediately see that the 90 dB speaker will play louder with the same amp at the same volume setting. In other words, both speakers are being fed the same amount of electrical power from the same amp at the same volume level, but one produces more sound power from it. The math starts to get a little complicated here, but basically, 3 dB = twice the power. So, if you have a 90 dB speaker playing at a certain wattage, you'd have to double the wattage to get an 87 dB speaker to play at the same loudness.
So, to answer your question, which speaker will play louder depends on the sensitivity. Assuming you have two speakers with identical sensitivity and they only differ in that one is 8 ohms and the other is 4 ohms, what happens at the same volume level? I don't know

 

[Wayne A. Pflughaupt]

Ben,

Saurav’s description of the crossover in speakers was very good, but for additional clarification:

Actually the crossovers are comprised of coils, capacitors, and resistors. The resistors are there for a number of reasons, like “padding down” a tweeter to make its lever better match the woofer. The resistors used also affect the speaker’s overall impedance rating.

For instance, consider some speakers I have: two way, woofer and tweeter each are 4-ohms. Yet, the binding posts measure 8-ohms. How can this be, since the speakers have parallel connections? Obviously the resistors in the crossover are doing it. Actually I removed the tweeter, and I thought that would give me 4 ohms at the binding posts. Wrong! It was still 8 ohms!

That’s why I said that the design engineers can change a speaker’s impedance rating.

Chris,

The reason you “get more dBs” from the 6-ohm speakers has more to do with the speaker’s efficiency than the impedance rating. For instance, an 8-ohm speaker with an efficiency rating of 105dB will deliver higher SPL than a 4-ohm speaker with an 80dB rating.

Daniel,

The example I gave Chris should be convincing that a speaker’s impedance rating has little to do with the “bang for the buck” it delivers. Bottom line, a 4-ohm speaker is going to make an amp work harder. Just make sure your amp can handle it before you bring home 4-ohm speakers

Regards,

Wayne A. Pflughaupt

 

[Thomas_A]

One thing to consider...normaly an amp rated at 8ohms,6ohms and say 4ohms...the lower you drop the ohm rating..normaly you will see the distortion rate go up.
say an amp is rated at 8ohms 100w @ .02% thd... at 4ohms.. it may be 200w but at .1% Thd... Now in reality... anything below 1% is not noticed by human ear..but imo felt by the speaker itself. One thing to do is to check out all aspects of th situation and understand before proceding

 

[Daniel_M]

Thanks Guys,

Actually I'm not buying speakers.

I have Paradigms which I am very happy

with and they ARE rated at 8 ohms.

I am amp shopping however, and I was

just curious about what the ohm ratings

in amp specs meant. Now I know. Thanks.

Dan

 

[Bryan Acevedo]

One thing that is interesting, is that most home speakers are rated at 8 ohms, while almost all car audio is rated at 4 ohms. I never understood why there is a difference - anyone know? My guess is because a car system is only 12V, and, therefore, has to have a lot of current to get the power out of an amp and a 4 ohm load is more appropriate and the amps in a car system can handle it. If I remember correctly, it takes more current (amperage) to drive a lower load, and that is why some amps (that aren't high current) have trouble driving a lower ohm load. And most of the time you are fine, it is only when you start cranking the volume, for long periods of time, and all speakers are set to large, that you will most likely have a problem. The size setting is important, because generally the dips in the ohm rating are at lower bass frequencies, and setting your speakers to small, cuts out the bass, and the hardest load the amp sees.

Bryan

 

[Mark_R]

Thanks to all for the questions and replies. I have found this thread very enlightening.

 

[Kevin Peak]

I too would like to add a little nugget of knowledge (that is, if i am right, which i'm probably not... hah)

When taking into consideration the sensitivity of a speaker, there are a few things to remember. If a speaker rated at say, 90db @1W that means that with one watt it is 90 db loud one meter away from the speaker. If one doubles the watts, you yield three more db headroom at the spot one meter away.

1w - 90db

2w - 93db

4w - 96db

8w - 99db

16w - 102db

32w - 105db

64w - 108db

128w - 111db

256w - 114db...

From the above, one could imagine a situation where if they have an amp that is still very robust and has lots of watts at 4 ohms, you could get a 4ohm speaker that has a sensitivity of say 86-88 and still have more db headroom than the 8 ohm speaker with sensitivity of 90db.

Now, the db level when sitting on a coach across the room, the db levels are going to be lower than listed, but as you can see, without alot watts, one can get reasonable db levels... at least in smaller and medium size rooms like most of us have. There are always other factors like room size, wall coverings, etc. that can affect the situation.