Ohm load and sensitivity questions.

[corysmith01]

This may not be able to be answered...I may have to just try it an see, but I guess I'm looking for "theoretical" answers.

I'm currently contemplating getting new speakers. I'd like to move to floorstanding. I'd like to buy internet direct. So, with those thoughts in mind, I've decided on two sets that include left and right speakers paired with a center. They are:

Axiom M60ti / Axiom VP100

-or-

Onix Rocket 550's / Rocket ELT center special edition (i've checked with av123 to make sure this would voice/pair properly with the 550's..they say it should, no problem)

I'm currently using: B&W dm601 s3 / B&W LCR60
Receiver = Marantz SR8400

My question is this; How much of a difference is the ohm/sensitivity difference in these speakers going to make? Just looking at specs, it's pretty significant. You get different ohms and different sensitivities. I guess I'm wondering how taxing or how "not-taxing" the Rockets will be compared to the Axioms. Here are the specs:

Axiom M60ti: 93 db / 8 ohms
Axiom VP100: 93 db / 8 ohms

vs.

Rocket 550: 88 db / 6 ohms
Rocket ELT Cse: 87 db / 6 ohms

I've checked my receiver and it's rated to 6 ohms. At 8 ohms, it's rated @ 110w per channel, while at 6 ohms, there's a slight boost to 135w per channel. Just for comparison, my B&W's have a mix of the two specs above; 88 db sensitivity, but with an 8 ohm rating and the Marantz seems to have no trouble.

So, hopefully that isn't too confusing. I'm very interested in both sets of speakers and am ready to pull the trigger...I just want to make sure what's going to be best for my receiver, understanding that I don't have a dedicated amp to power these. Of course, I could also be over-analyzing these specs and perhaps they will make very little difference...I'm also open to hearing that...that would actually put my mind at ease.

Any thoughts on the above scenarios? Thanks a ton.

[markw]

...each increase of 3 db in sensitivity means that, for the same apparant volume, a speaker will need one half the power. Another three db increase in sensitivity will again halve the required power.

so, all else being equal, the Axiom's should require one quarter the power of the rockets to reach the same volume level.

Of course, there are many other factors, such as tonal balance, to think of but from what you've offered, that's about all I can say.

[agarwalro]

The first figure is called the Sensitivity and means that if you feed 1 Watt of power to the speaker and measure the loudness usually called Sound Pressure Level (SPL) and measured in Sound Decibles (dB), under perfect conditions (no reflections) or in an anechoic room (the walls of which reflect sound from the speaker in a way that no reflections go to the mike), 1 meter away from the speaker you will receive 93dB SPL in case of the Axioms and 88dB SPL in case of the Onix. Basically, the Axioms play louder than the Onix. All things being equal the higher the sensitivity the better. But nothing in life is free, higher sensitivity comes at the expence of accuracy of overall response, extension of bass response, and physical size.

The 8 ohm or 6 ohm rating is the Impedence (electrical load) that the speaker represents when connected to an amplifier. Roughly speaking, an 8 ohm speaker will draw half the current compared to a 4 ohm speaker. The lower a speaker's impedance drops, the more important it is for the amplifier to be able to supply higher current and remain stable. This means bigger elements like torroids, capacitors, high current transistors etc are needed in the amplifier, better circuitry etc and therefore cost increases. Generally you should have no problem if you mix 8 ohm and 6 ohm speakers and just leave the amplifiers speaker setting at 8 ohms (never use the 6 ohm setting on the amplifier even if you have 6 ohm speakers anyway since current limiting circuitry kick in and may effect sound quality). But connecting a 4 ohm speaker to an amplifier designed for 8 or 6 ohm speakers will cause it to overload and the protection circuit will disconnect the speaker from the circuit or whorse you might blow something and cause unrepairable damage.


The combination of the two figures can be interpreted as follows, the Axioms sound louder than the Rockets when connected to the same amplifier keeping volume knob in same position, but the Onyx can be driven as loud as the Axioms with a less powerful amplifier.

Hope this helps out.
Cheers.

[markw]

The combination of the two figures can be interpreted as follows, the Axioms sound louder than the Rockets when connected to the same amplifier keeping volume knob in same position, but the Onyx can be driven as loud as the Axioms with a less powerful amplifier.What I think you meant to say is "...but the Onyx can be driven as loud as the Axioms with a MORE powerful amplifier."

The higher the sensitivity figure, the less power is needed.

[corysmith01]

Just to make sure...so taking in what's been mentioned above, the receiver I have shouldn't preclude me from buying either of these sets of speakers. As stated, the Rockets will be harder to drive, but since it hasn't been mentioned, I'm gonna assume that my receiver is enough...yes, no?

Thanks.

[Buckeyefan 1]

Cory,

Based on the specs alone, the Axioms would be more friendly to your receiver. Who knows which sound better? Without forking over shipping charges, you're completely out of luck. That's the problem with mail order speakers. You "gotta have faith."

[agarwalro]

What I think you meant to say is "...but the Onyx can be driven as loud as the Axioms with a MORE powerful amplifier."

The higher the sensitivity figure, the less power is needed.

Yikes, I must be on drugs ;-). Sorry about that. After all the facts I made a small mistake, but a blunder to someone reading. Mark, thanks for keeping me on my toes.

Let me give you the rest of the math. Say you have a 100 Watt amplifier. Using the relationship Power = Resistance x Current squared...

The 8 ohm speaker will draw current = sq. root (100/8) = 3.536 Amperes
The 6 ohm speaker will draw current = sq. root (100/6) = 4.082 Amperes

So to drive the 6 ohm speaker as loud as the 8 ohm speaker you need a more power since any amplifier will be connected to the same 120 Volts AC mains.


Cory,

Based on the specs alone, the Axioms would be more friendly to your receiver. Who knows which sound better? Without forking over shipping charges, you're completely out of luck. That's the problem with mail order speakers. You "gotta have faith."

I wish I could have been as consice, but I thrive on details and recondite text... ha ha ha. But, I completely agree.