Ohms FAQ

[ThisBass]

I wouldn't trust too much in manufactures power ratings.

These power rating values are most of the time of theoretical nature and can therefore be very different for one and the same loudspeaker.

To tell an example. The Audio Engineering Society AES recommends to calculate the power rating by the Zmin of a loudspeaker, however for all the "RMS" ratings (as far as I know) the power dissipation values are calculated by Z nominal. So far as I know is this done by Eminence and many other well known manufactures.


Example:
a well known popular 8 Ohm 12" loudspeakers Zmin runs down below 6 Ohm (5,7 Ohm measured in free air).

The loudspeakers RMS power is rated to 250 Watt.
This 250 Watt RMS power consumption is calculated by using Z nominal (8 Ohm).

The AES specification will lead to a significant different value cause of using Zmin for the power calculation. Let's say 350 Watt AES then for the same loudspeaker.


I think there is a lot of discussion in forums if a 350 Watt amplifier is capable to burn off a 250 Watt RMS loudspeaker cause the amplifiers RMS power is significant above the loudspeakers rating.
No problem!
Consider the AES recommentation and calculate the loudspeakers power dissipation by Zmin, and so everything is fine.


Fortunately there are some professinal manufactures on the market which provide us engineers with some additinal specs we need to make proper estimate how much amp power we can provide to a given loudspeaker system.
Depending on the nature of audio signal, cause different signals cause different force to the system.

Unortunately there are a lot of manufactures leaving us poorly abandoned. That's very poor cause without additional specs we do not know anything about the cab we are playing with.
The only specs given by most manufactures are Ohm and Watt. And in addition to it some doubtful value for the lower frequency limit.
It would help a lot if at least the real -3dB lower limit would be given. Most of the time we read in the manufactures specs something like a theoretical -10dB lower limit that says nothing.
Compared this practice to my own 212 custom cab I could perform a SPL measurement direct to the vents and would get a plenty value of SPL even at 40Hz.

By the other hand, If I know the realistic -3dB lower limit and this limit is at 65Hz for example and the rated power is something about like 500 Watt RMS.
Then I know I have to take care a little bit with to much amp power cause the critical point starts already at 65Hz.
On the other side I'm a good bit more on the save side with a 500 Watt RMS cab when the -3dB lower limit says 55Hz or 50Hz.

But it's curios to spec a cab's lower limit to 35Hz for example. Although it is often done by many manufactures.
Specs like this are meaningless.

[PauBass]

After much reading I'm still a bit confused.

Quick question, possibly silly but want to make sure

Is there much audible difference in volume between a 700 watts 4 ohm cab and the same cab in 8 ohms?
My amp is rated 750 watts at 4ohms and 400 watts at 8 ohms

[B-string]

Audible yes, useful is doubtful. 700 watts of bass frequencies is a tall order for a single cab unless it has multiple stout speakers (in which case it would probably be rated higher).

[PauBass]

Originally Posted by B-string

Audible yes, useful is doubtful. 700 watts of bass frequencies is a tall order for a single cab unless it has multiple stout speakers (in which case it would probably be rated higher).

Sorry, I forgot to mention it's a 410 cab, an Aguilar DB410, rated at 700 watts and 101 dB sensitivity.
It will be paired with a DB750 amp, rated at 750 watts at 4 ohms and 400 watts at 8 ohms.

So, will there be much difference in volume between the 4 and 8 ohm versions?

[Vandy]

Originally Posted by PauBass

. . . a 410 cab, an Aguilar DB410, rated at 700 watts and 101 dB sensitivity.
It will be paired with a DB750 amp, rated at 750 watts at 4 ohms and 400 watts at 8 ohms.

So, will there be much difference in volume between the 4 and 8 ohm versions?

Apparently, one (1) cabinet showing an amp 8 ohms is not as "loud" (slightly > about -3db) as the same amp pushing the 4 ohm version of the same cab, BUT . . .
If you get two (2) of the 8 ohm cabinets (showing 4 ohms to that very same amp), you will get tons(?) more air moving (loudness) . . .

[PauBass]

Originally Posted by Vandy

Apparently, one (1) cabinet showing an amp 8 ohms is not as "loud" (slightly > about -3db) as the same amp pushing the 4 ohm version of the same cab, BUT . . .
If you get two (2) of the 8 ohm cabinets (showing 4 ohms to that very same amp), you will get tons(?) more air moving (loudness) . . .

Thanks.

I only want to use one cab on my rig, two 410 is too much, so I want to make sure I make the right choice with the 8 or 4 ohm cab so I can have as much volume as possible with just that one cab.

So, much difference between those two cabs?

[RickenBoogie]

No, not really much difference. certainly not enough to choose a 4 ohm cab, which limits future expansion.

[PauBass]

Originally Posted by RickenBoogie

No, not really much difference. certainly not enough to choose a 4 ohm cab, which limits future expansion.

Great, thanks for the advice...8 ohms it is then.
Here in the UK we don't have many options, 4 ohms is usually especial order and with that a price increase over the 8 ohms version.

[WorldCarver]

I'm still a bit hazy on understanding the formula. My head doesnt have a way to set how many ohms I'm running but it allows 2,4, and 8 ohms (Carvin b2000). I'm running this head through an ampeg pr810h that can run 8 or 4 ohms and a peavey 215 cab that runs at 8 ohms. How many ohms am I running? How can you tell? Below are links to my gear and their specs.

http://www.carvinguitars.com/products/B2000
http://www.ampeg.com/manuals/PR-810H.PDF
Couldn't find a manual for the 215.

Any light shed on the situation would be most appreciated.

[B-string]

If you setup the 810 for 4 ohm operation and add the 8 ohm 215 the combined load to the amp is 2.667 and the amp will be fine. SS amps (in most every case) don't have a switch for output loads, that is more a tube amp thing.

[Downunderwonder]

Back up. I think you'll find the ampeg cab can run as one 8 ohm cab or 2 4 ohm cabs in the same box. You'll have to to run it as 8 ohms, otherwise it's going to be 2 ohms just for the 8x10.

[Thor]

Originally Posted by Downunderwonder

Back up. I think you'll find the ampeg cab can run as one 8 ohm cab or 2 4 ohm cabs in the same box. You'll have to to run it as 8 ohms, otherwise it's going to be 2 ohms just for the 8x10.

That is not correct. The manual says that each 4x10 unit is 8 ohms. They can each by driven by a separate amplifier carrying a 8 ohm load. OR they can be connected together externally by a cable from the top jackplate out to the other jackplate in, presenting the amp at that point with 4 ohms.

If the 2x15 is 8 ohms then the final load is 2.667 ohms.
OR
If the 2x15 is 4 ohms, the final load when that is connected in parallel is 2 ohms.

Either will work fine BUT if the 2x15 is connected is an 8 om cab, and the loads are connected in parallel, then the 2x15 will get 1/3 the POWER. 1/3 will go to the top 4x10 and 1/3 will go to the bottom 4x10.

If the 2x15 is 4 ohm, it will get 1/2 the power.

Open up the Peavey and see what speakers are in it. Note comments from Bill Fitzmaurice in this thread if it is old or has similar spec speakers.
Help with Peavey 215 Cab

[HolmeBass]

Originally Posted by IvanMike

Volume (...) Doubling your watts results in a 3 dB increase in volume, which is generally the smallest change in volume a person can hear. (...)

3 dB is not the smallest change in volume a person can hear. This is a canard that keeps getting passed along and I feel it is quite misleading. Reference the link below, you will see, and hear, that 1 dB is close to the Just Noticeable Difference (JND), sensibly enough. A 3dB difference, while still not a lot, is clearly audible.

Even better in the link below are sound samples, about 1/4 of the way down, that demonstrate white noise in decreasing volumes; first by 3 dB steps, then by 1 dB steps, and finally by 0.3 dB steps. You can clearly hear for yourself that a 3dB difference is easily audible, well above the JND.

http://www.animations.physics.unsw.edu.au/jw/dB.htm

[Thor]

Point well taken. And having studied some psychophysical auditory phenomena along the way, I beleive that is completely correct from the POV of the human auditory system, though I am not sure at which SPLS that may not be true. Does that article deal with anything over 100db for example. You know, metal band time?

IvanMike is off in cyberspace somewhere, and I will make the appropriate edit. Or, if you would like to edit that paragraph and PM it to me, I will insert it after looking it over. I would like to review the link you sent as well.

(I have a jam I am about to be late for, but thank your for pointing out the canard, we will marinate it now and cook it tomorrow.)

As always, we welcome helpful suggestions and corrections to posted material.

[Downunderwonder]

I believe Bell labs defined 3dB along the lines of the smallest "significant" difference where average listener didn't have to prick up his ears, so to speak, to identify a change.

I've done the tests too. 6dB is "now we're cooking" kind of a bump. 3dB "isn't anything to write home about", if I'm bumping myself up it's got to be at least that.

If I'm upgrading 300w to 500w, less than 3db extra headroom, "I payed how much?, I was ripped off".

[Frost.Blast]

Very, very useful post. Thanks a bunch, cleared up MANY things.

[hiflyer22]

I have done a bunch of reading here but can't find a really specific answer to my questions.

I picked up a used Genz Benz Shuttle 6.0 a couple of weeks ago and last Friday I picked up an Eden Nemesis 410 4ohm cabinet.

After getting the 410 home and dialling in a happy tone (with a Sadowski preamp pedal), I decided to "experiment" with my old 8ohm 115 cab.

Yes that's right I did bad. Plugged in the 4ohm 410 (500 watts) AND the 8ohm 115 (300 watts) into the Shuttle 6.0 rated at 4ohm minimum load, and played for around 20 mins with the master volume on less than 1/2 of 1 (less than 0.5 on a scale of 0-10). The two cabinets were plugged in separately from the two Shuttle speaker outputs.

Of course, I only read this thread the next day..

There were no strange noises or smells at the time, and the next day the happy tones seemed to remain (although it is very hard to tell, I have fiddled around with both shuttle eq/gain/enhancement controls and sadowski controls a lot, dialling in different tones).

My questions are:
1) How do I really know if I have damaged the Shuttle, without taking it to the amp mechanic?
2) Is there any way to know for sure using my ears?
3) What are the chances of having damaged the Shuttle at such low volumes, or does the master volume setting make no difference in this case?

P.S. Talkbass rocks and the amount of information and opinion here is astounding, I have learnt so much over the past couple of months. As a guitarist who has suddenly become a bass player, this forum is priceless. Thank you to all the people involved.

Cheers

[Jim Carr]

Don't sweat it. It would have gone into thermal shut-off before meltdown.

[hiflyer22]

So the danger here is that using two mismatched cabs can potentially heat-up the power amp inside the Shuttle to the point of meltdown? And the Shuttle has built-in thermal shut-off protection to stop it from cooking? So basically you're saying the amp knows when it is getting too hot?

Do you think this thermal shut-off protection is reliable?

I just want to be sure...it's very hard to tell soundwise as I had only just gotten the cabinet home and am not that familiar with it's tone...

[RickenBoogie]

You got away with it once, I wouldn't risk the amp doing it again. It's rated 4 ohm minimum for a reason.