The physics of cabs/speakers and perceived volume

imalrightjack

Hi all,

I've been switching between two cabinets (by going into standby, switching cables - although @ecc83 has a better solution for me lined up - thanks!) and trying gauge how I feel about them both.  One is compact, open-backed and pretty small with a Vintage 30; the other is an over-sized cabinet and closed back with a Celestion Creamback Neo.  The smaller one sounds louder to me, despite the speakers having the same dB rating.

Does anybody have a good explanation of how cabinet size/open or closed back affects perceived volume?

I've searched around online but couldn't find a great deal of good information and thought you reliable chaps might have some thoughts - or even have discussed this before.

Thanks!

ICBM

They're not the same dB rating - the Creamback Neo is 97dB, the V30 is 100dB. That makes the V30 substantially louder. It also has a very pronounced upper-mid peak which makes it seem perceptively even more than that, I think.

There is a difference in the cabs too though - generally in a small-ish room, an open-back will sound louder because you'll hear reflections from the walls of sound which has come out of the back of the cab, as well as directly from the front.

imalrightjack

Ah, I wonder if the search I did took me to the wrong page?  It definitely came up as matched.  Maybe it's the normal Creamback?

How does cab size affect things?  I appreciate bass is going to increase.

imalrightjack

By the way, I'm asking only out of curiosity.

ICBM

imalrightjack said:

Ah, I wonder if the search I did took me to the wrong page?  It definitely came up as matched.  Maybe it's the normal Creamback?

The Creamback G12H-75 is 100db, although like the G12H-30 it doesn't sound as loud as a V30 to me. The G12M-65 and the Neo are both 97dB.

That said, the M65 and the H75 sound exactly the same volume when in a cab together! So exact that even with your head right in front of the cab you cannot perceive any difference in volume, the sound appears to come from a spot exactly between the two speakers.

Cab size makes a difference too, because a small cab - even open-back - damps the speaker movement more than a larger one does, but I think it has more effect on the tone than the volume.

ecc83

The problem is, all the theory and maths about speakers and cabs is based on "hi fi" drivers and (as IC is always at pains to point out!) guitar speakers are not like and do not behave like hi fi drive units. Add to this that the theory assumes the cone acts as a rigid piston over the frequency range of interest, a rigid piston a V30 cone certainly is not!

But to generalize, box volume will not affect midband sensitivity hardly at all, a larger frontal area than normal might give a bit of  a boost at certain frequencies but guitar cabs are rarely any bigger than they need be!

All things being equal a smaller, closed box with have a higher resonance than a bigger one but, paradoxically that CAN give the impression of more bass! Having TOO big a box "unloads" the cone and reduce VLF power handling but that is rarely an issue for guitar cabs.

Dave.

ICBM

ecc83 said:

The problem is, all the theory and maths about speakers and cabs is based on "hi fi" drivers and (as IC is always at pains to point out!) guitar speakers are not like and do not behave like hi fi drive units.

As far as I can see, the theory and maths of guitar cabinet design is based on the following:

Dimensional ratios which ensure that your cabinet is different enough in each direction that it won't fit under a head from any other maker without looking wrong, and that if it accidentally does then it must be the wrong impedance or power handling, or preferably both.

Large enough to take an integer number of 12" speakers (no other sizes are allowed by law) but small enough to fit in the boot of an average hatchback, and also just large enough that you can't get a guitar case on top or next to it as well.

Either light enough to just about get away with having only a top handle but heavy enough to make carrying it that way really awkward - extra height and/or depth helps here too - or conversely have only end handles and yet be easily light enough that it would be possible to pick it up with one hand if it had a top handle, and the end handles placed so trying that means it hangs at an angle guaranteed to catch on the ground.

Expensive enough that it's more than double the price of essentially the same product made in China but without the brand name to match your amp; bonus points for the speakers having been shipped halfway across the world twice, making it four times as expensive.

Speakers must either be the cheapest you can buy in multiples of 1000, or V30s - no other choices are allowed.

Sound? Doesn't matter - the user will change the speakers anyway, thus throwing any other calculations out of the window.

RiftAmps

@icbm You've forgotten about the rear descriptive plate stating impedance/power handling, but that can only be correctly deciphered by the Enigma machine.