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Tech Tip: Wattage, Speaker Efficiency, and Amplifier "Loudness"

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When it comes to volume, wattage is only part of the equation

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 10 watts, and a second 20 watts. The 20-watt amp is double the power of the 10-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 20W amplifier will sound noticeably louder than a 10W 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 actual-ly converted into heat, not sound. Normally, speaker sensitivity is measured in an anechoic chamber (non-reflective, soundproof room) and expressed something like this: 90dB @ 1W/1m.

Translated into English, that means "90 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.

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 10 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.

Tags: Amplifiers Speakers

Comments  

# Joe 2014-05-26 16:16
Good article however I have 2 120w amp heads Names unmentioned.Amp head a sounds great vol has to be turned up to at least 5 to really let loose. Amp head b sounds very good also but will blow your face out at 3. Same 4-12 cabinet here guys.
Reply
# Scott 2014-05-23 06:25
The Ah-Ha light did indeed go on.
Reply
# Cesar Cardenas c. 2014-05-23 00:19
Very good article, it really helped me to understand more, speakers and relation with power and sound level.

Thanks.
Reply
# Daddy D 2014-05-22 23:38
Well as far as guitar amps go, the more weight you can carry into the gig, means your guitar amp can be louder.

My biggest concern is clarity and a smooth response at all freqs I can generate. I guess my amp is loud enough and I don't have to buy a heavier one.
Reply
# duke 2014-05-22 19:42
Always ask your amp manufacturer what they found best during bench tests. If you are using a mass produced amp don't worry about it, use what it came with, a speaker won't fix a poor quality amp.
Reply
# matt 2014-05-22 18:42
A good rule I heard is. Ideally the watts of your speakers be twice the wattage of your amp. So with a 25 watt amp. Have 50 watts for your speakers. this way you get the best tone. And you are not underpowered so you won't blow your speakers.
Reply
# Ben 2014-05-22 19:40
Matt, if i have a 100W 2x12, would getting two 100W speakers be optimal? or two 200W ones? you said speaker capacity should be double the amp, but does the wattage split evenly between the speakers or no?
Reply
# M. Underwood 2014-05-22 17:23
Good article. Let's remember though, any amplifier has a peak efficiency, typically 50 to 60 percent of its rated power. If you want reliable service from your amp, keep this in mind. This will give your amp plenty of headroom but not over draw current through the circuits. Just FYI.
Reply
# grubbs 2014-05-22 17:21
This clears a lot of things up, thanks.
Reply
# XwpisONOMA 2014-05-15 11:32
Sorry, I tried, I could not understand anything of what you're saying. I came here trying to answer a simple question: If I have -say- a 1Watt amp what is the power rating of a speaker I should most efficiently connect it to ? Efficiently means the least amount of wattage producing a good, non-distorted sound. In other words is there a rule of thumb: So many watts from the amp require so many watts (min-max) capacity from speaker.
Reply
# pbhales 2014-08-18 13:29
Once again, there seems to be confusion about wattage and sensitivity and loudness. The power-handling capacity of a speaker is demarcated in watts-- a "100watt" speaker will handle the full-on signal from a 100watt amp without blowing. You use that spec when you're trying to mate a speaker to an amp so you don't promptly ruin your investment. Why, then, do speaker companies make guitar speakers with different wattages rather than just one high-wattage speaker? Because many people want the sound of "soft breakup" that comes with pushing a speaker at something close to its max. If you're looking to get max perceived loudness out of a rig, you go for the highest-sensiti vity speakers around-- typically topping out at 102-103 db. That makes for a substantially louder rig than the same head run through a typical lower-efficienc y speaker, say with a 95 db. sensitivity.
Reply
# Batelec II 2014-05-08 03:18
Wow..interestin g!
Reply
# Bobb Woods 2014-03-29 11:06
This is where the "Ah-HA!!" light goes on... very informative.
Reply

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