Watts and Ohms... help, I'm lost!

Strat_a_tat_tat

So... if I've got a bass amp that puts out 300W @ 4 Ohms (and that's it's preferred mode of operation)... but it also puts out 165W @ 8 ohms... is it going to sound quieter if I swap out the current 4 Ohm cab for an 8 Ohm cab?  Are there any other affects by swapping from 4 to 8 Ohms?

I guess the efficiency of the two speakers (SPL??) will have some bearing... so I'll assume both speakers are similarly efficient.

The amp in question has a Mosfet power section and valve pre amp (it's a Mesa Boogie Walkabout). Not sure if that makes any difference.

Thanks.

ecc83

If the two loudspeakers were of identical sensitivity the 4R at 300W would be 2.6dBSPL louder, that is, not a lot!

Of course calculated loudness levels do not always match subjective experience but I doubt there will be much in it.

In theory again, the amp will have an easier time on an 8 Ohm load but in practice again, assuming it is a good design (ICBM?) no matter.

Dave.

ICBM

Agreed. The amp will be slightly - I would say noticeably, but not drastically - quieter, although it does depend on the speaker sensitivity... it's not impossible that if the 8-ohm cab is more sensitive the result could be the same volume, or just possibly even slightly more.

The amp will also be under less stress and will run cooler, but any well-designed amp won't have a problem running at its minimum impedance anyway.

jpfamps

Speaker efficiency is usually measured at one watt.

We did  some measurements on guitar speakers a while back and found that there was not a linear relationship between power input and efficiency; so some speakers were more efficient at low volume, but had lower maximum output than some less efficient speakers. A good analogy would be the difference between acceleration and top speed.

Conclusion, it's virtually impossible to second guess what the real world perception of loudness well be.

Strat_a_tat_tat

Thanks @ecc83, @ICBM and @jpfamps.

The replacement cab is quoted as having a sensitivity of  99.9 dB 1W 1M. Is that efficient or not?

I can't get the figure for the Boogie speaker.

Thanks jpfamps... interesting point.

ecc83

Strat_a_tat_tat said:

Thanks @ecc83, @ICBM and @jpfamps.

The replacement cab is quoted as having a sensitivity of  99.9 dB 1W 1M. Is that efficient or not?

I can't get the figure for the Boogie speaker.

Thanks jpfamps... interesting point.

It won't be anywhere near as sensitive at bottom E! Good figure for midband though . Yes, agreed, Pin is not linear with noise out, Aka distortion!

Dave.

ICBM

jpfamps said:

We did  some measurements on guitar speakers a while back and found that there was not a linear relationship between power input and efficiency; so some speakers were more efficient at low volume, but had lower maximum output than some less efficient speakers.

And it also depends on voicing and cabinet loading/tuning, especially for perceived low-end volume.

A well-designed cab properly tuned for a less efficient speaker may easily be louder than a more efficient speaker in a simpler box (AKA typical guitar and many bass cabs).

The car analogy is good, it's a bit like trying to correlate engine power to acceleration as well as top speed. They are related, but not directly or always completely predictably.

slacker

The size and number of speakers will affect the volume as well. What if you swapping a 4ohm 1x10 for a 8ohm 2x15?

Strat_a_tat_tat

slacker said:

The size and number of speakers will affect the volume as well. What if you swapping a 4ohm 1x10 for a 8ohm 2x15?

I'll be swapping a 1x12 (that also has a passive, unwired 1x10 in it) for a different 1x12 (without the unwired speaker)

slacker

As stated previously it's down the efficiency of the two speakers. BTW whats wrong with the 10/12 cab? Also whats the resistance of the 12 in the 10/12 cab?

Strat_a_tat_tat

slacker said:

As stated previously it's down the efficiency of the two speakers. BTW whats wrong with the 10/12 cab? Also whats the resistance of the 12 in the 10/12 cab?

The 12 / 10 cab is fine... just wanting to reduce weight.

The current 12 is 4 Ohm (the replacement cab is likely to be 8 Ohm)

slacker

If you are using the single 8 ohm 12 with the 10 unplugged your cab is running at 8 ohm. Replacing it with a single 12 8ohm cab should make little difference to the volume.

I think, I'm getting confused.

Strat_a_tat_tat

slacker said:

If you are using the single 8 ohm 12 with the 10 unplugged your cab is running at 8 ohm. Replacing it with a single 12 8ohm cab should make little difference to the volume.

I think, I'm getting confused.

Whoops... my mistake!  I meant the old cab is 4 Ohm and the potential new one is 8 Ohm

paulmapp8306

But if the old cab has an un-connected 10" speaker in it, that will change the ohm rating, assuming the 4 ohm rating is for a fully wired up cab

I think I'd agree, that you existing cab is probably loading your Amp with 8 ohms without the 10 connected.

ICBM

paulmapp8306 said:

But if the old cab has an un-connected 10" speaker in it, that will change the ohm rating, assuming the 4 ohm rating is for a fully wired up cab

I think I'd agree, that you existing cab is probably loading your Amp with 8 ohms without the 10 connected.

The 10" isn't "unconnected", it's a purpose-designed passive radiator. The 12" speaker impedance is 4 ohms.

Strat_a_tat_tat

ICBM said:

paulmapp8306 said:

But if the old cab has an un-connected 10" speaker in it, that will change the ohm rating, assuming the 4 ohm rating is for a fully wired up cab

I think I'd agree, that you existing cab is probably loading your Amp with 8 ohms without the 10 connected.

The 10" isn't "unconnected", it's a purpose-designed passive radiator. The 12" speaker impedance is 4 ohms.

Yes, @ICBM is totally correct. By design, the cab has one 4 Ohm 12" speaker that's connected... the 10" speaker just flaps around in sympathy with the 12" speaker's movements.

I've just realised an obvious thing that I could do as a bit of a half-arsed experiment... attach the amp to a 12" 8 Ohm guitar speaker cab and see what different that makes (I've got a Boogie guitar cab with an EV 1x12 Black Shadow... that should be tough enough to put up with a bit of bass for a few minutes). Obviously, the different speaker sensitivities (I don't have any measurements) will make a difference... but I might learn a little about the affect of different impedances (and sensitivities). Not very scientific I know!

ICBM

Yes, the EV should handle 165W of bass with no problem - they're rated at 200W and were originally designed as PA bass drivers. I used one in a Marshall 1912 1x12" guitar cab for a while with a Trace Elliot AH150 and didn't blow it.

Strat_a_tat_tat

Thanks @ICBM.