Impedance/Sensitivity relationship

[kay]

Say you have a speaker of 6 ohms nominal impedance with a 88 db sensitivity. How would that compare to one of 8 ohms impedance and 91 db sensitivity?

My logic tells me it will be potentially just as loud if not louder (because it's an easier load to push, and the amp will be able to unleash more power, hence similar dB's despite the lower sensitivity) but draw more current through the amp and so put more strain on it (so it will run hotter).

Some very (overly?) simplistic calculations. Say we test with two pairs of speakers, one is a very efficient 8 ohm load - 92 db/w @ 1m, another is a 6 ohm load - only 86 db/w @ 1m.

Let's call loudness L, sensitivity S, power P, impedance R.

If we keep the current constant (power is irrelevant, remember, current is!), an amp that is rated at 95w RMS will be doing putting out:

I = sqrt(P/R) = sqrt(95w/8ohm) = 3.4 amps

So that same amp could push a 6 ohm load with:

P = I^2*R = 11.875 * 6 = 71w

On the other hand, most amps are rated as pushing about 150% their 8-ohm power-into 4 ohms. That means MUCH higher current.

Any comments on the above?

 

[markw]

There is no realtionship between those two specs

Sensitivity is how mush sounbd is produced with one watt of electrical input.

Impedance is the average "resistance" of the speaker, referenced in ohms, over the audiable range. It may fluctuate up and down according to he frequency so that represents a (very) rough average.

There is no basis for a direct corrolation between the two.

 

[kay]

Of course they're directly related. The above might be a major oversimplification but is not unreasonable. The impedance and power fluctuate the whole time, that's why you take the "nominal" impedance and the "RMS" power. Just trying to paint a picture in my head of how the different speaker parameters affect the overall setup...

 

[annunaki]

This is a better example:

Speaker A - 92db 1w/1m full range pink noise 8 ohms

Speaker B - 92db 1w/1m full range pink noise 6 ohms

The only difference here is that speaker A requires a higher voltage from the amplifier than Speaker B to reach the same volume/wattage level. i.e. "The Facts"

Watts = Voltage^2/impedance

Speaker A) 2.83V^2/ 8ohms = 1 watt

Speaker B) 2.45V^2/6 ohms = 1 watt

In the case of this post:

Speaker A - 88 db 1w/1m full range pink noise 6 ohms

Speaker B - 92 db 1w/1m full range pink noise 8 ohms


Say we want each speaker to reach 100db at 1m

Speaker A - will require 16 watts (9.80 volts at 6 ohms) to reach 100db at 1m

Speaker B - will require about 7 watts (7.50 volts at 8 ohms) to reach 100db at 1m


Just to check, let us say both speakers are 92db 1w/1m with full range pink noise

Speaker A - 6 ohms will require about 7 watts or (6.49 volts at 6 ohms) to reach 100db at 1m

Speaker B - 8 ohms will again require about 7 watts or (7.5 volts at 8 ohms) to reach 100db at 1m

This proves what I originally stated the only thing that goes up is voltage, which proves the fact that as impedance increases so does voltage.

As one can see there is no relevant connection between the two. The one with lower sensitivity requires more power to reach the same volume regardless of impedance. Markw is correct

 

[kay]

Ok, I've not expressed myself properly then. I understand that a lower sensitivity is a lower sensitivity, preiod. You need X watts to get Y decibels at 1m.

However.

Is the fact that one pair of speakers has a nominally lower impedance make the amp's life any easier, asking it to deliver more power to achieve the same dB levels? E.g. amps are usually rated to push more power into 6 ohms, even more into 4 ohms, etc.

 

[Francious70]

the lower the resistance, the shorter the lifespan of the amp. Reason being that lower impedances will allow more current to flow thru the amp, producting more heat.

What I think kay is asking is if the impedance drops on a speaker, does the sensitivity rise?? And the answer would be, no.

Paul

 

[annunaki]

The lower the impedance the closer it (an amplifier) gets to a short circuit. So, the lower the impedance the harder the amplifier works. The reason one gets more power as the impedance drops, is that electricity flows more freely with less resistance.

As for your first question from your last post, an amp will NOT have to deliver more power into a lower impedance, ONLY into a lower sensitivity. So in saying a speaker with a low impedance and low sensitivity, yes it would work harder. However, (assuming the amp has no issues with say a 4-6 ohm load) a 6 ohm speaker with a 95db sensitivity would not work an amplifier as hard as an 8 ohm speaker with an 87db sensitivity. The 6 ohm speaker would have an 8db advantage in sensitivity with 1 watt. To reach 100db, the 6 ohm speaker would only require about 3.5 watts, where as the 87db 8 ohm speaker would require 22 watts or so. Obviously one is demanding more from the amplifier.

 

[korgoth]

what are the advantages and disadvantages to ohm ratings and sensativity ratings?

other than.. high sensativity gets loud easier right?.. why are ohms all different.

would you ever want a low sensativity or low ohm rating

 

[mulester7]

"Say we want each speaker to reach 100db at 1m

Speaker A - will require 16 watts (9.80 volts at 6 ohms) to reach 100db at 1m

Speaker B - will require about 7 watts (7.50 volts at 8 ohms) to reach 100db at 1m"

.....Kay, great thread....you want to know more about what's going on....you need to take what Annunaki said to the bank....I especially italicised from what Annunaki posted, as to how many watts are required to produce 100 decibels of sound pressure....7 watts for an 8 ohm speaker to produce 100 db's of sound pressure....and that's pretty LOUD....7 watts.....

.....moral of this post....don't part with 500 bucks upgrading to a receiver that has 300 per channel when yours has 250....

.....and that would be ""TWO"", channels, Gentlemen.....

 

[Buckeyefan 1]

8 ohm for home

Has anyone noticed that most everywhere except the US rates amplifier power at 6 ohms and 1kHz as opposed to 8 ohms and 20-20,000kHz. This is especially true in the UK. The same goes for speakers. Just something I've noticed on a few foreign forums (say that fast 5x).

When shopping for speakers, try to keep a few things in mind:

* Try to buy speakers with a sensitivity ratings of 91db or higher
* Always shoot for 8 Ohm whenever possible
* Set a budget, then demo within your budget
* Consider your room acoustics. Don't buy horn tweeters for a live room.

I think I've mentioned this once before. If you don't believe the effect of 4 or 6 ohm speakers on a home receiver, try hooking up a 3 way car 6x9 with a 4 ohm rating and a high spl of say, 92db, to your home stereo. It never gets loud. Stick with 8 ohm for home.

 

[kay]

The lower the impedance the closer it (an amplifier) gets to a short circuit. So, the lower the impedance the harder the amplifier works. The reason one gets more power as the impedance drops, is that electricity flows more freely with less resistance.

As for your first question from your last post, an amp will NOT have to deliver more power into a lower impedance, ONLY into a lower sensitivity. So in saying a speaker with a low impedance and low sensitivity, yes it would work harder. However, (assuming the amp has no issues with say a 4-6 ohm load) a 6 ohm speaker with a 95db sensitivity would not work an amplifier as hard as an 8 ohm speaker with an 87db sensitivity. The 6 ohm speaker would have an 8db advantage in sensitivity with 1 watt. To reach 100db, the 6 ohm speaker would only require about 3.5 watts, where as the 87db 8 ohm speaker would require 22 watts or so. Obviously one is demanding more from the amplifier.

annuaki, good points!

Likewise, a 6ohm/90db speaker and a 8ohm/90db impedance will require the same amount of power to reach X dB, but the 6 ohm speaker will cause more current to be drawn through the power supply and put more heat, strain, on the amp.

Bottom line, I've just convinced myself that, per Buckeyefan 1's advise, rather spend a bit more on >90dB sensitive 8 ohm speakers than an amp that delivers 10-20w more per channel.

I started wondering about this lot becase of the Wharfedale Diamond 8 specs - many people report them to be rather loud yet they're only rated at 86dB. I knew that the bottleneck in amps is not absolute power per se, but the amount of current that the power supply can provide.

I think I have all my questions answered now

 

[annunaki]

The first part of your post is more or less correct. However, it (stress and current draw) is not tremendously more by any means. I think that the heat differences would be very minute as well. (when talking 6 ohms vs. 8 ohms).

 

[ducker]

So my Athena AS-R1's aren't that great? or is this not as much of an issue with surrounds?
their Sensitivity is 89db.

insidently: AS-C1 center 90db, AS-F2 fronts 93db

 

[Shinerman]

So my Athena AS-R1's aren't that great? or is this not as much of an issue with surrounds?
their Sensitivity is 89db.

insidently: AS-C1 center 90db, AS-F2 fronts 93db

Nah, your surrounds are fine. 89db is NOT bad. Plus, they are surrounds and usually are not producing very demanding sound. Your receiver is more than capable of driving them. The difference on paper between a 89db surround and a 93db surround can be seen but it in real life listening, I seriously doubt you or anyone else could ever tell a difference or should even care for that matter.

Shinerman

 

[annunaki]

Well said Shinerman. The real world listening differences are not as drastic as the numbers make it seem sometimes.

 

[Buckeyefan 1]

Try this experiment

So my Athena AS-R1's aren't that great? or is this not as much of an issue with surrounds?
their Sensitivity is 89db.

insidently: AS-C1 center 90db, AS-F2 fronts 93db

Ducker,

If you have the time, do a quick experiment. Try an R1 in the front (unhook one AS-F2 and compare the db level) along with one AS-F2. I bet the AS-F2 is twice as loud as the R1. Now plug both R1's in to the front speaker wires. Play a cd at a set volume. Now swap the R1's for the AS-F2's. (Use only a 2 channel mode - no sub). See if there's a large difference in total volume. I bet there is. I wouldn't worry about it. Athena probably varies the sensitivity levels in their smaller speakers to protect them. You can adjust your receiver's db level for the speakers with the lower sensitivity ratings up a hair to compensate. It's pretty typical of mfg's to do this. Let us know the results. If I'm wrong, egg on my face.

 

[warnerwh]

The most important thing to shop for in speakers is the SOUND. Having 4 ohms speakers just means you should have an amp that can handle 4 ohm loads and I understand even most home theater receivers can handle this. I'm running a cheap Kenwood receiver I think I paid 400 bucks for to power my home theater. The mains are Legacy Classics which are large floor standers that are rated at 4ohms and I've had no problems. Anything under about 86db sensitivity is low and you'll most likely need extra power if you like the sound loud at all. Everybody I know has speakers that are rated at 4ohms. There's alot more to it than impedance. Your first priority should be that you like the sound for the money you're spending. 95% of speakers will not have severe phase angles at low impedances nor low enough sensitivities to worry about. I've been in the audio hobby since the early seventies and the above poster's first two recommendations are pretty off base imo.

 

[Shinerman]

The mains are Legacy Classics which are large floor standers that are rated at 4ohms and I've had no problems.

How do you like your Classics? They are probably my next upgrade. I have listened to them quite a bit and I love them. Do you have a Legacy center or surrounds as well? How do they compare?

Sorry for all the questions, you are the first person on this board I have seen that has them.

Sorry for the hijack!

Shinerman