Tube wattage in relation to decibels

[byoung]

If I have two tube guitar amps, both are identical components except that one has a 15 watt power amp, and one has a 30 watt power amp. The 30 watter is a 212 box, and the 15 watter is a 112 box, but they are both using the same type celestion speaker. I'm curious technically speaking if there is a way to tell how much louder the 30 watter is in dB? The builder told me that there is only about a 3db difference in output between the two, I thought it was more. I know it isn't a linear scale but out of curiosity I was trying to figure out what the difference is.

 

[gyraf]

Wouldn't the easiest be to measure it?

 

[Mendelt]

Theoretically (if i'm not mistaken) twice the sound means about 3dB more.

If you had two of the 15W amps they would be about 3 dB louder than the 30W amp.
So logically the 30W amp with an extra 12" speaker could be 3dB louder.

In practice this might not be the case. Maybe a bigger box will make the speakers less or more efficient. Maybe the combined impedance of the two speakers is a more difficult load to drive efficiently and so on...

So if you really want to know you should listen to Gyraf and measure the output.

 

[Walrus]

In sound pressure level terms, you need 10dB difference for twice the perceived loudness.

 

[Kamel]

are you sure?

 

[Walrus]

[quote author="Kamel"]are you sure?[/quote]
No :?
Seems I may have mixed my measurements, but I knew what I meant... :wink:

Quote/
Perception of Loudness (20dB = 4x)
Interestingly, our perception of loudness is not the same as sound pressure level. Although the actual formulae
is somewhat complex, as a rough rule of thumb, an increase of 10db SPL is perceived to be approximately twice as loud.

Thus a 20 Db gain would seem to be about 4 times as loud.
And a 40 Db gain would seem to be about 16 times as loud.
/Unquote.

 

[NewYorkDave]

Twice the power is 3dB, but it takes a 10dB increase to sound "twice as loud" to our ears.

All other things being equal, a 30W amp is 3dB louder than a 15W amp. But all other things are not equal in your case--so if you really need to know, do like Jakob says and measure the SPL.

 

[owel]

I don't think there is a power (watts) to SPL (loudness) conversion formula.

Measure the SPL directly.

 

[JohnRoberts]

Not to confuse any comparison but in guitar amps the power output is often specified at several percent distortion. Be sure that power levels are measured at similar distortion levels as that can make a difference too.

JR

 

[NewYorkDave]

The (purely arbitrary, but apparently common) figure I used when testing guitar amps was 5% THD with a sinewave input. This tended to correspond with manufacturer's nominal output specs.

 

[JohnRoberts]

Yup, AFAIK there is no standard, and a bunch more watts at 5% than say .1%. Just compare apples to apples.

JR

 

[pstamler]

Just to clarify: It's entirely possible that when you set the two amps to the same volume level that they will have the *same* output and therefore the same SPL from the speaker. That's a matter of total voltage gain in the amplifier, not power. What the power specs (15W and 30W) mean is the maximum powers available from the amp -- as Dave says, at something like 5% harmonic distortion. Maximum power from the 30W is 3dB greater than the 15W, so maximum SPL will be likewise 3dB greater from the 30W amp. All things being equal, of course.

Peace,
Paul

 

[JohnRoberts]

All things equal a 2x12 will couple to the room differently than 1x12 so that's another variable.

JR

 

[moamps]

The mutual coupling between the drivers will also have a role here
so I espect about 6dB gain in SPL.

Regards,
Milan

 

[byoung]

I was just more curious than anything. When the builder asked me if I wanted it in 15 or 30 watts, he said 30 watts is about 3db more than the 15 watts and that the speakers added a little more punch. I just thought 3db is all it is for double the wattage. But now I'm seeing that there are many different variables in measuring how loud the difference will truely be.

 

[ebartlet]

[quote author="moamps"]The mutual coupling between the drivers will also have a role here
so I espect about 6dB gain in SPL.

Regards,
Milan[/quote]

But each driver is now only getting half the power... theres no free lunch here... (although bass response is improved)

 

[JohnRoberts]

Depending on the driver size/spacing there will be some acoustic gain due to coupling between 2 drivers but it will be in a limited frequency range, not wide band.

Guitar amps are musical instruments in their own right so there are sundry important interactions.

JR

 

[Mbira]

From a non-technical standpoint, I'd say the 15 and the 30 watt amps are going to seem "about as loud as eachother", but if you play clean and/or with a drummer, you probably want the 30 watts.

Joel

 

[Guest]

Your builder is right; you will get more of character from the same speaker with 30W than with 15W amp, if you use a dumb peace of a wood to play. However, if you use an acoustic guitar, the thing is contrary different: use better kilowatt speakers with 30 Watt amp.

 

[PRR]

> one has a 15 watt power amp, and one has a 30 watt power amp

3dB

3dB is twice the power but 1.4 times the acoustic pressure and 1.4 times the acoustic velocity (1.4*1.4=2). At "medium" acoustic loudness, 3dB is "1.4 times as loud". 6dB would be "twice as loud". However when we say "loud", we often mean "LOUD". Up past 80-90dB SPL, your ear-muscle tenses-up almost half as fast as the pressure rises. So while 6dB is twice the pressure in the air, you need 8dB or 10dB more air pressure to get 6dB more pressure at the ear nerve after 2dB-4dB of de-sensing in the ear muscle. "10dB" is the figure large hi-fi amp makers like to use.

But remember that music is played from 72dB SPL to 120dB SPL, a 63,000:1 range of Power or a 250:1 range of acoustic pressure. So in a broad sense, 3dB is an insignificant difference. Non-electryfied musicians almost never perform the same piece twice within 3dB.

> 2*12" box, ... 1*12" box ... both using the same type ...speaker.

Two adjacent Twelves will double efficiency below 500Hz. They will have some phase addition (and subtraction!) above 1KHz, roughly +/-5dB max, but this may be inaudible on top of +/-10dB typical raggedness in large light cones.

Two speakers makes a more complicated, and often "more interesting", sound field. However when you carry a 2*12" up stairs to a gig, sound field complexity is not worth the strain. And at most better paying gigs, the customers are too drunk/stoned to appreciate sonic subtleties.

So +6dB in the lows and +3dB to possible +5dB in highs, assuming you use every last Watt available (at whatever THD you accept or find musically interesting).

> I just thought 3db is all it is for double the wattage.

Right. On the same speaker, one which does not "break-up" at these powers, a 5W Champ and a 50W Twin are 10dB different. In the same club, the Twin is barely more than "twice as loud". (The Twin is known for its twin 12" speakers: to be fair, the Champ must feed the Twin's speakers. The Champ's own 8" speaker is much smaller and lamer, which is why a stock Champ is much less than a stock Twin.)

The specific difference in this specific case is: a healthy VOX AC-30 clone with two hot Twelves has a slight chance of keeping up with a modern drum-set. A one-12 AC-15 is loud alone but can be overwhelmed by nearly any rock/pop and most jazz drummers.

A drummer can peak far over 10 Acoustic Watts (AW) but will typically average near 1AW. A "15W" in one good typical gitar-12" will make around 0.5AW. A "30W" in two 12" is more like 1.5AW. 3dB (0.5AW gitar and 1AW drums) can be significant in a direct fight, whereas it would be insignificant by itself with no reference.

In the Big Geetar Amps' heyday, PA systems sucked and the g-amps were expected to fill the room without help. That's why 2*100W + 2*8*10" Marshall Stacks happened: they will fill a football stand with level the PA systems of the day would not touch. Now that everybody and his kid brother has a 4,000 Watt PA system, the custom is for guitarists to carry a 5W-20W amp with "the right sound", and mike it to the PA system for kilowatt delivery.

> a matter of total voltage gain in the amplifier, not power.

Yes, but gain is cheap and power is costly/heavy. Since 1940, most guitar amps have ample reserve gain.

> maximum powers available from the amp -- ... something like 5% harmonic distortion

In fact I bet a buck that "15 versus 30" means "two or four EL84" with appropriate transformers. So it IS "the same amp" only doubled-up. Truth may be 10 Watts at 1% and 25W at 25%THD for one pair, with 20W and 50W on the 4-jug build. (And yes, 25% THD is common on many plucks in many e-guitar stylings.) So at the amp-power POV, it is right on 3dB difference, whatever other qualifications you may apply.