When it comes to volume, wattage is only part of the equation
By Phil O'Keefe
There seems to be some confusion when it comes to how "loud" an amplifier can get. When it comes to "volume", many musicians only consider the amplifier's power or wattage rating, and in general, more watts does mean "louder". But while wattage is an important consideration, the efficiency of the speaker(s) that are connected to the amplifier are also an important factor in the loudness equation.
DECIBELS AND LEVELS
Decibels (abbreviated "dB") are a logarithmic unit of measurement that pertain to a ratio between two numbers. Okay, I can see eyes rolling and glazing over, so I'll simplify things, and attempt to keep the "math" to an absolute minimum. With a logarithmic scale, you can't just add numbers in the usual way - a doubled number isn't "twice as much", but rather, many times more. For example, 100dB is many times greater than 50dB, not just "twice as much". When it comes to "loudness", which is measured in Sound Pressure Level, (or SPL), a 10dB increase in level is roughly equivalent to a "doubling" of perceived loudness. In other words, if one amp is generating 90dB SPL and another amp is hitting 100dB SPL, the second amp will generally be perceived to sound about twice as "loud" to the typical listener.
WATTAGE, POWER AND SPL
So how many watts does it take to get twice as loud? Let's imagine two amps - one of ten watts, and a second of twenty watts. The twenty watt amp is double the power of the ten watt amp, but doubling the power only translates to an increase of 3dB SPL. Remember, in order to sound "twice as loud", you need an increase of 10dB, so while a twenty watt amplifier will sound noticeably louder than a ten watt amp, it will not sound twice as loud. The same thing holds true at higher wattages - a 100W amp is not going to sound twice as loud as a 50W amp; assuming identical speakers, it will only be 3dB louder, which is noticeable, but definitely not a doubling of perceived loudness.
SPEAKER SENSITIVITY RATINGS
Speakers have specifications in terms of their sensitivity and efficiency - their ability to convert the incoming electrical energy into acoustical energy. Dynamic, moving coil speakers (the type found in most guitar and bass amps) are notoriously inefficient, and most of the incoming power is actually converted into heat, and not sound. Normally, speaker sensitivity is measured in a anechoic chamber (non-reflective, soundproof room) and expressed something like this:
90dB @ 1W / 1m
Translated into English, that means "ninety decibels (SPL) with one watt of power, and measured at a distance of one meter from the speaker." A more efficient speaker will have a higher number, and a less efficient speaker will have a lower number. All other things being equal, a more efficient speaker will make your amp sound louder than if it has a less efficient one installed. (Fig. 1)
Figure 1: While the two amps pictured are nearly identical in power (20W vs 18W), the one on the left is significantly louder due to its much more efficient speaker
PUTTING IT ALL TOGETHER
So let's assume we have a speaker with a sensitivity of 90dB @ 1W / 1m and a power handling capacity of up to 100W. If we power that speaker with 1W of power, it will generate 90dB when measured at a distance of 1 meter. If we double that power to 2W, the SPL measurement will increase to 93dB. If we increase the power to 10W, then the SPL measurement will increase to 100dB, which is "twice the perceived loudness" when compared to 1W. So it actually takes ten times more power to give us a perceived doubling of volume level. Since this imaginary speaker is rated to safely handle up to 100W, we could double that volume level yet again, and in theory, hit up to 110dB SPL by increasing the power all the way up to 100W. One watt = 90dB. One hundred watts, or 100X more power = 110dB. That's a huge increase in power but only a "doubled double" (4X) increase in terms of perceived volume levels!
As you can see, it takes considerable increases in power - in the wattage of the amplifier - to "double" the perceived "volume". This is where speaker sensitivity / efficiency comes into the equation. If we replace that 90dB @ 1W / 1m speaker with a model that has a sensitivity of 100dB @ 1W / 1m, the numbers change dramatically. For starters, 1W of input power will give us 100dB SPL. Remember, the first speaker required 10W to achieve that same volume level! So by installing a more efficient speaker, we can get the same perceived volume level from a 1W amp as we could from a 10W amp that is coupled to a less efficient speaker. Again, this applies all the way up to the maximum power handling capacity of the speaker. Assuming our 100dB @ 1W / 1m speaker can also handle up to 100W, it can give us up to 120dB SPL; again, that's double the perceived "volume level" of the 90dB @ 1W / 1m 100W speaker's maximum level of 110dB SPL.
AMPLIFIER POWER PLUS SPEAKER EFFICIENCY AND POWER HANDLING = MAXIMUM VOLUME
So remember, while increasing the amplifier power can make you louder, increasing the speaker sensitivity will make more efficient use of the available power from any amplifier. This means it's impossible to make generalizations about the "loudness" of any amplifier based solely on its wattage. You simply must factor in the power handling capacity and sensitivity of the speakers in order to know "how loud" it will be capable of getting. If your 15W amp has a relatively inefficient speaker installed, but is still "almost" loud enough for your needs, you may not need a higher wattage amp - simply installing a more efficient speaker, such as the Electro-Voice EVM 12L in Fig. 2, may give you all the increase in volume you seek, without having to replace the entire amplifier. Similarly, you may not need a 100W amp; replacing the stock 95dB @ 1W / 1m speakers in your 50W amp with new speakers that are rated at 101dB @ 1W / 1m will more than make up the difference in terms of the "volume levels" you will be able to generate... it will actually be capable of "sounding louder" than that 100W amp will when it is running into the less efficient speakers.
Figure 2: Replacing inefficient speakers with a highly efficient speaker model, such as this E/V EVM 12L, will make any amp sound louder
Of course, if you really want to get loud, then the answer is to couple a high power amplifier with high efficiency speakers that are rated to handle the power...
Phil O'Keefe is a multi-instrumentalist, recording engineer / producer and the Associate Editor of Harmony Central. He has engineered, produced and performed on countless recording sessions in a diverse range of styles, with artists such as Alien Ant Farm, Jules Day, Voodoo Glow Skulls, John McGill, Michael Knott and Alexa's Wish. He is a former featured monthly columnist for EQ magazine, and his articles and product reviews have also appeared in Keyboard, Electronic Musician and Guitar Player magazines.