In many years of playing out, doing repairs and being asked to perform mods to people's old tube amps, there is nearly always a single misunderstanding that everyone shares from bar owners to FOH engineers to guitar players to service techs: volume, and what causes it.
You know the thread, "I like this amp but (too loud/not loud enough) so I need (more/less) wattage". That amp is too (big/small) for this room. Sound familiar? It seems like it should be intuitive; amps are marketed and sold by their power output after all? It would be nice, but it's wildly incorrect. The truth is it is anything but intuitive or simple, which is probably why you've been mislead all this time. Truth is sometimes inconvenient, and inconsistent. Welcome to reality! (We have coffee)
What do we hear?
First, ask yourself a foundational question: what is it that we actually hear?
We perceive some air pressure changes as sound; waves of air pressure moving past us. The threshold of hearing represents the smallest pressure change we can detect as sound. We call that 0dB and it is less than 1 billionth of atmospheric pressure. The loudest possible sound on Earth is also knowable, based on available atmospheric pressure. 194dB SPL (Sound Pressure Level) is where the trough of a sound pressure wave is about to become a vacuum at sea level. (14.7lbs/sq in)
Every 3dB represents a doubling of power, it is a log scale. It's also considered the 'threshold of perception" - the minimum amount that most people can accurately identify.
A 10 dB increase is perceived as twice as loud, and takes 10X the power.
70dB is a normal listening level at home.
100-110dB is "live band loud".
150 dB will burst your eardrums and 185-200 dB is considered to be capable of killing someone. And 194 is our theoretical maximum at sea level air pressure.
Then you have the the Saturn V rocket which hit about 203 decibels and I have not found a great explanation of how just yet; my assumption is there is a lot of pressure. There are water jets around the bottom to absorb the sound so that it doesn't damage the rocket. That is loud.
A good band in a good club with a real FOH engineer will hover in the 90-100dB/SPL range. You are seeing a live band but not getting your head blown off. So how do we get there? (practice practice practice)
Now note that nowhere in the formulas will you see wattage required to produce that sound pressure level, or dB/SPL. You also won't see speaker size. You often won't see room size or distance specified either. It's all about the changes in pressure where you are. How you get there is separate and distinct, and as they say on social media, "it's complicated".
Efficiency:
You may have seen some speakers express sensitivity* as "dB/SPL at 1W/1M". This is dB Sound Pressure Level (SPL) in an anechoic chamber. This is not in a 'real room'; and the speaker here is treated as a point source; a theoretical thing. It's an infinitely small point where all sound originates. It's still a good measurement, but it's not the same as saying it's going to take your listening room up to X dB of SPL at 1W of input. Volume is 3 dimensional, and if you took 2 speakers, say a 10" and a 12" that had the same efficiency, the 12" would be louder. It is moving more air. We use a point source because it simplifies the math. Not surprisingly, sound in a real room is a whole 'nuther animal with a lot of details. (there are some great books about acoustics out there if you are interested) Suffice it to say (very generally) that your amp is louder in a small room with reflective surfaces and quieter in big room that has been acoustically treated, and quietest outdoors (why bandshells exist).
* correction - not efficiency, but sensitivity. Why? Because we are talking about a 2 dimensional point source. Actual efficiency is how much sound that speaker makes in 3 dimensions - in every direction.
Speaker sensitivity can vary wildly - far more than your amplifier wattage in many cases. Recall above we said a doubling of power (wattage) is good for about 3dB more level.
To get 3dB more out of your tube amp you generally need to:
Double the number of output tubes
Install a bigger power transformer (to handle the doubled current requirements) and
Install a larger output transformer that can pass this amount of power at the lowest frequency of interest.
Those are just the basics, (can your speakers handle 2X the power?) and now you have spent a lot of money, killed your resale value and wound up with a very different amp for a paltry 3dB? This is why I won't do this for clients. It's a terrible approach, born of a complete misunderstanding of how to get what the client really wants.
Once again, 3dB is considered the 'threshold of perception", a minimum interval that most people can accurately identify. Trained engineers can hear down to 1dB differences, but the average person can't. We talk about perception specifically because how you perceive things may not align with how others perceive them for lots of reasons. We're all different. Age, gender, frequency, hearing damage/sensitivity, etc. can all contribute to how you hear. I think it's loud but you don't. That is pretty normal. But the dB SPL scale is a good starting point to compare things in an apples to apples manner.
Speaker Basics
To dig in more, hit the post about speaker basics, but here we will just summarize:
1. They have poor sensitivity, 3-5% is not uncommon. (well designed horns can hit 25-50%)
2. They don't handle high or low frequencies well (luckily we only care about 80Hz-5kHz)
3. Their load impedance varies dramatically and changes based on their cabinet (!)
4. They can introduce a lot of distortion. (double digits are not uncommon)
5. The overall level is directly impacted by the room and where they are placed
They are the worst solution, except for all the others.
But what does that tell us about volume and watts?
Wattage Ratings:
Wattage is the simplest so let's bang that out first. The "wattage" rating of a speaker is primarily focused on the heat the voice coil can withstand. It has almost nothing to do with volume. (* the ability of the motor to handle the motion incurred when driven by some amount of power is a thing, of course, as is the ability of the amp to power the speaker to full excursion, but let's not overcomplicate this...) And remember that we are taking those industry standard sensitivity readings at 1 watt, so it is probably fair to say everyone wants their speakers to work well with as little as 1 watt of power.
There is a common misconception that wattage is some kind of volume rating; it's really not.
The voice coil is comprised of wire wrapped around a very thin tube which is suspended in an intense magnetic field. We send it an audio signal which makes it move in that magnetic field. The voice coil is attached to the cone and the cone moves back and forth and sends out compression waves that we can hear. (It's a dynamic microphone, backwards.)
Early speakers used a tube of paper. Later on different materials were used including Kapton (a high temp plastic), Nomex (a fireproof paper-like material), Copper and Aluminum. High coil temps would sometimes set the paper formers on fire. Too much current can also melt the wire. The speaker's power rating is taking these things into account and you may see various air venting strategies or heat sinks on high powered drivers to help raise this limit. On the other hand, it is a compromise; bigger wire or heavier voice coil material to handle more power/heat will change how the device responds. There's no free lunch.
It is important to note though, that power rating is at RMS watts of a clean (sine wave) signal. If you heavily distort the amp, your amp will be putting out waves that are squared off - or nearly so. This greatly increases the load on the speakers by both delivering more apparent power and increasing the duty cycle (how much time is it "on" and how much time is it "off" which affects how well it can get rid of heat) net net, a 20 Watt tube amp can blow an 80 Watt guitar speaker under the right circumstances. I've done it more than once. Pro audio folks will lament that it is easier to blow speakers with lower powered amps; and this is why.
Volume:
Speakers can vary more than 6dB SPL in their own sensitivity (4X the power), so you need to think about how many speakers you have and how efficient they are to really understand your amp's potential volume. Say you were to add 2x12" drivers that are each 6dB more efficient than the single 12" you are replacing. This will potentially add 12dB of output which is nearly 20 times more power. (13dB is 20X) An extreme example, but not out of the question.
And of course there is also size: a 12" moves more air than a 10", and a 10" moves more air than an 8" etc. Smaller speakers lose sensitivity at low frequencies but can sound "tighter" because they are not reproducing lower frequency information. Once the speaker diameter is smaller than the wavelength, you start rapidly losing sensitivity. This is part of why an 8" sounds so thin next to a 15", and why woofers are usually big. Physics. This is also why we need to much more power to recreate bass - because our speakers are small and have a poor impedance match with the atmosphere.
Measurements Taken from Dave Hunter's excellent "The Guitar Amp Handbook"
8" speaker moves 50.27 square inches
10" speaker moves 79 square inches (2x10 = 158 sq in, 4x10 = 316 sq in)
12" speaker moves 113 square inches (2x12 = 226 sq in, 4x12 = 452 sq in)
15" speaker moves 177 square inches
* these numbers are conservative (low) they assume the speaker is a flat piston, so A = Pi*R^2. If possible, find the Sd of the speaker you're evaluating; that is its actual conical area less the dust cap. Also if the surround is not part of the cone you should subtract it. But these are ballpark and fine for basic use. The key idea is that a 15" is moving more air than 3x8". A 4x10 is moving a lot more air than a 2x12 etc. You can visualize which setups are the loudest (all else equal).
I know of a guy who did a national tour while on a major label (CBS) with a Fender Champ plugged into a Marshall 4x12 cab. How did that work? Going from a single 8" to a 12", and then doubling that - twice (1x12 to 2x12 and then 2x12 to 4x12) gives you ~12dB on top of the increase from an 8" to a 12" and the 12's are much more efficient than the 8. That 3W amp was now putting out the equivalent of a 60W amp (very roughly) without changing a single part of the circuit. It's a clever solution, as long as you don't have to carry the 4x12 yourself.
The room matters?
There is a concept called full space, half space and quarter space when you have a speaker system operating. This primarily affects the low frequency response of the system since lows are omni-directional. High frequencies are already directional.
"Free Space" simply means there is nothing around the speaker - it is suspended in mid air, somewhere high above the ground. There is nothing affecting the speaker's frequency response. It's somewhat imaginary but still important as a baseline.
"Half Space" means there is a single plane acting on the cabinet, in our reality, this is going to be the floor of whatever room you are in. Sound tends to reflect off of the surface of this plane and so the system will produce more acoustic level (all else equal)
"Quarter Space" now there are 2 planes acting on the cabinet - the amp is sitting on the floor near a wall. This is going to be perceptibly louder than the half space location since we are reflecting more of the acoustic energy in the direction we want.
"Eigth Space" is your amp in a corner. 2 walls and a floor reflecting back. This is generally going to be the location that yields the most acoustic volume.
Here's a handy chart: (credit: Genelec)
Added: Dynamics or "Punch"
I was speaking with a friend the other night, and he is a pro sound engineer. We were at rehearsal and both impressed with how loud our tweed deluxe repros were. 12W and a single 12" speaker, keeping up with a 4 piece band with drums. I blurted out my usual, "watts aren't volume", and he added, "watts are punch". And he's right, and I left it out of this article just as part of keeping things "simple" but I think now that I over-did that.
With the dB vs power info up above, you can infer a few things. First, music isn't all one volume, right? You have quiet parts, & you have loud parts. The difference is typically called 'dynamic range'. Modern music has about 10dB of dynamic range, so the normal parts are about 1/2 as loud as the loudest parts, and the loudest parts will take about 10X the power of the average parts. And this is where your "punch" is. So, if you have a 30W amp and you are listening at 3W, you have 10X on tap when the big crescendo hits. You're ok. If you are however listening at 20W, you are not going to get those dynamics; you are probably going to clip. How many watts it takes to listen at a comfortable volume depends on the sensitivity of your speakers and the room they are in, and your personal preferences. All these things tie together. And the same goes for your guitar, do you want clean punch or do you want smooth clipping when you hit the guitar harder? As a related aside, human hearing has a practical dynamic range of about 140dB. 16 bit audio can handle 96dB, and 24 bit audio can handle 144 dB - in theory.
Additional Considerations: what should your guitar sound like in the mix.
What is the end goal here? Boosting lows will eat power (and headroom!) and get lost in the "mud" as it competes with the rhythm section. A good FOH engineer will probably EQ it out. You need to think about all of this.
Are you indoors or outdoors? Do you have a PA to go through or does stage volume have to carry you?
You will get the hang of it, but it is a lot.
Stage Volume vs PA
You should also think about your stage volume and how hard you want the amp working. If you play clean pedal steel, you will want to hit your stage volume easily and have plenty of reserve power so your low notes don't distort. If you are playing Texas blues on your Strat, you may want the amp running on the edge of melting down. In the first case you may want a Twin Reverb, in the second, a Deluxe Reverb or even a Princeton. The point is to think about the goal, and where you want to wind up. There is no one "right" answer. The blues player when handed a twin will be too loud by the time the Twin is getting dirty. A little planning can go a long way.
Lets see if we can summarize this into some tactical realities that can feed into your overall strategy:
1. Perceived volume is a function of compressed waves of air. More volume requires more air moving &/or at higher pressure.
2. Playing in bigger rooms dissipates the sound more - there is more air and more space to send your compression wave into.
3. Speaker sensitivity varies widely and can have far more impact than just amplifier watts
4. Adding more and/or larger speakers to move more air is a very effective strategy to get more volume.
5. Larger speakers handle lows better, while smaller speakers tend to sound tighter due to reduced bass response. This carries over into arrays, so a 2x12 usually has better bass response than a 1x12, all else equal. Perhaps the greatest array ever, the 8x10" cabinet used by the mighty Ampeg SVT, gets the best of both worlds: tight 10's and big bass. It also means that each driver only deals with about 38 watts of power. Smart kids over there in NJ. ;)
6. Optimizing your guitar rig for lows is probably futile due to realities of the mix, and will severely impact your apparent "headroom". Understand your end goal. Guitar trio tone will be a lot different than 4+ piece band tone. Trios can handle a much chunkier tone because there is more space.
7. Playing indoors tends to give you more volume, depending on the actual room and where you are set up in it. Playing outdoors tends to give you less as all the indirect sound energy is lost to space. You will probably need 2-4X the output outdoors to carry the same level. If you have a full PA, you can ignore these issues. Yay!
[Modern class D amps skew this a bit; in a world where it is trivial to supply 600 or 1000W of power to even a single monitor we do not need to worry nearly as much about driver sensitivity. But here we are focusing on classic tube amps for guitar and bass. And conversely the reason we need 1,000 watts is generally because our drivers have such poor sentitivity.]
References
Speaker size vs Freq response: http://hyperphysics.phy-astr.gsu.edu/hbase/Audio/spk2.html#:~:text=of%20the%20loudspeaker.-,For%20an%208%2Dinch%20speaker%2C%20diameter%20of%20speaker%20equals%20wavelength,enclosure%20to%20produce%20good%20sound.
More dB SPL goodness http://www.sengpielaudio.com/calculator-soundpower.htm
Sound Intensity vs distance: https://www.qlight.com/productdata/techdata/en/08_Intensity_of_Sound.pdf
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