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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  

# clueless 2015-04-14 16:07
I've a QSC RMX2450 and wonder if it will a good mate for JBL PRX425 speakers. Can someone please give advice?
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# clueless 2015-04-14 16:17
additional details -
RMX 2450 specs:
Continuous Power Output Wattage:
- 8 Ohms: 500 W
- 4 Ohms: 750 W
- 2 Ohms: 1200 W
- 8 Ohms Bridged Mono: 1500 W
- 4 Ohms Bridged Mono: 2400 W
- Current Consumption: 10 Amps

JBL PRX425:
Frequency Range: 55 Hz - 19 kHz
- Frequency Response: 75 Hz - 16 kHz
- Coverage Pattern: 90 degree x 50 degree nominal
- System Sensitivity: 100 dB SPL (1w@1m)
- Power Rating, Pink Noise: 600 W / 1200 W / 2400 W (Continuous/Program/Peak)
- Rated Maximum SPL: 134 dB SPL peak
- Nominal Impedance: 4 ohms
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# Bob Dole 2015-04-14 16:36
Those JBL (nice SPL btw) are 600w @ 4 ohms. The QSC is 750w @ 4 ohm. The 1200w "program" and 2400w "peak" seem way exaggerated. I'd be VERY cautious with the master pre-amp volume and avoid trying to floor them given their price. While I don't doubt the RMS (continuous) rating of the speakers, I wouldn't expect them to tolerate a doubling of that on regular basis let alone 4 times. At 4 ohm, the amp puts out 150w more RMS which is what would concern me. Keep volume reasonable and it should be OK. But accidentally have full 750w from the QSC hit them at once and I suspect you'll loose a tweeter or crossover cap, etc. The QSC is an OK amp. I run old AB amps which are similar to the RMX line. I try and up the ohms to avoid over powering my speakers while lessening the burden on the amp. With such a high SPL anyways, you don't need a ton of wattage to get them going. Consider adding polypropylene metal caps to the amp board to clean up it's sound if it's running green polyester caps ;)
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# Devin 2015-04-13 10:28
My 4x12 cabinet has 4 12" 16
Ulm 30-watt speakers. Would it be acceptable to replace those with higher wattage speakers? If so what would be the outcome
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# Bob Dole 2015-04-13 11:00
@Devin Speaker wattage is just a rating of what they are supposed to be able to handle safely before self-destructin g. You can put 100 watt speakers on a 15 watt amplifier, and suffer no issues. You need to watch the SPL of the speaker though as some companies will tout a high wattage, but it'll only have say an 83 db rating, meaning it'll need the higher wattage to hit the same db as a higher SPL speaker at far less wattage. Speaker wattage is more or less a safety feature so you can hopefully couple it to something that won't smoke it on the 1st bass note :) If you had a 500 wpc amp and tried powering a 50 watt woofer, it'd cook the poor thing, but without a rating you'd never know. Car audio is an exception as they aren't bound by law and often exaggerate, but speakers are typically accurately rated. Again, just make sure the speaker has a high SPL and matches the tonal qualities you are looking for and it'll be OK.
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# Bob Dole 2015-04-13 11:12
Also, avoid pushing the speaker to the point of clipping, as that can still damage it. A woofer or rather an actual subwoofer that's typically over-built, can probably survive it, but a mid or tweeter might not tolerate it.

And sorry, to answer your question, higher wattage won't change anything. If you want to experiment with different sounds, look at paper, polypropylene, or other materials. I find paper makes for louder mids and somewhat louder full range, whereas polypropylene makes cleaner bass in the lower freq. range, especially when driving high wattage. Although I used to run some Radio Shack Realistic 50 watt polypropylene woofers in the early 90's and they made really good bass when matched to a capable amp. Most car audio subs are polypropylene or some other synthetic. Just experiment if you can afford it, or ask around about specific speakers on appropriate forums to narrow down your selection and go from there. Good luck.
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# Bob Dole 2015-03-13 21:22
Also, keep in mind 30wpc to 100wpc is more than adequate (referring to vintage solid state gear from late 70's, 80's, high-end PA stuff, etc) for a house system or really small church, etc. Whereas 1000wpc to 2000wpc would be needed in large venues. If you want the best sound (to your ears), don't focus on wattage. Just use the wattage of the amp to ultimately dictate how loud the overall system can be, then match speakers that can handle it. I have an old late 70's MCS 3233 (a JCPenny house brand no less) in my collection that's rated at 33 wpc and peaks around 65 wpc. It'll walk all over a modern 100 wpc stereo in sound quality and even loudness. In my teens I even ran 2 12" woofers in a 3-way and it sounded amazing at full volume. In a small bedroom, you couldn't even hear someone talking and the bass was knocking the clock off the wall in another room and rattled plates, despite the fact the wattage was split across 6 speakers, and woofers were rated around 50w. Just experiment!
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# Bob Dole 2015-03-13 21:05
"Double speaker watts vs. amp wattage", isn't always needed anymore and depends on the actual amp and what you are doing. Typically, you want the speaker wattage to at LEAST be what the amp puts out if running 1 speaker per amp. Most vintage SS amps from late 70's, 80's, HAD higher dynamic wattages, so a 33 wpc amp could easily hit 60 wpc peaks at full volume. Average modern home stereo amps aren't built as rugged, and stay closer to their rated RMS as sound quality is NOT a concern like it once was, hence old gear when taken care of sounds better. Remember, if your AMP is putting out 200 wpc x2 channels, and each channel has a 3-way speaker cab with say a horn, piezo, and woofer, that 200w is evenly split to each speaker. 200w/3 =66w seen per speaker. If each speaker is rated at 100w, you'd want a 300 wpc amp to get the most out of it, though less wattage can power it fine. Subs can sound better if overpowered slightly, but caution is needed. Aim for highest SPL and match watts.
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# Bob Dole 2015-03-13 20:36
For those asking wattage questions, speaker questions, etc. here's my take. Tweeters, horns, mids, are typically higher SPL like 97dB @ 1W/1m to 100dB @ 1w/1m, whereas subs are often in the low to mid 80's, but typical woofers are in the low 90's. Paper cone speakers are typically louder sounding than polymer speakers, hence why many PA and guitar speakers are still made of that, but they seem to often introduce unwanted effects at very large volumes whereas polymer can stay more controlled and doesn't have that "ringing" effect. Subwoofers, with car audio subs typically being heavily over-built with over-sized magnets with pole venting, dual voice coils, and extremely rigid baskets handling anything from 100w up to a genuine 1000w+, seem to sacrifice SPL in favor of actual wattage, though some subs are 89 dB with high wattage, and those will sound "louder" than a 85 dB running from same amp. Running mult-amps, you can often get away with less watts for highs but need more for bass.
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# John Alfieri 2015-02-09 07:08
Should have added this to last question. The QSC 153
claim 1000watts continuous and the DXR15 claims 700watts continuous if that helps
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# John Alfieri 2015-02-09 06:50
I appreciate your advice. Here is our situation. I have a pair of QSC 153 3way speakers 134db {spl } We played in a larger hall the other night and felt we needed more power to cut through to the back. I purchased two DXR15 133 db{SPL} to connect to the QSCs. Leaving out any discussion about subs, how much louder will this set up be ? Will it be noticeably louder and have a longer throw ? Thanks
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# cayde 2015-01-22 08:18
I was just wondering why a lot of times when it comes to guitar heads and speaker cabinets, that the speaker cab will say 200 or 300 watts @8 ohms, but the head that is displayed to go with that cab says 40 watts @ 8 ohms output. So the question is is why are most heads between 30 to 100 watts output, but the cabs go up to like 200, 300, 400 or even 500 watts handliong? I was allways under the impression that you can do damage to your system if the watts are not enough for your speakers. Thanx for any answers.
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# Emmanuel 2014-12-16 15:37
Hi, I am searching for an amplifier that can really make a difference in terms of output and durability!
Can anyone who knows, advice on the wattage I need and the type of speakers than can harmoniously work with it without much of a headache? What I need is far from a home theater....this is for commercial purposes! Grateful for any help!!!
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# 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.
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# Scott 2014-05-23 06:25
The Ah-Ha light did indeed go on.
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# 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.
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# 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.
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# 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.
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# 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.
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# 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?
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# 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.
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# grubbs 2014-05-22 17:21
This clears a lot of things up, thanks.
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# 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.
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# 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.
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# Batelec II 2014-05-08 03:18
Wow..interestin g!
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# Bobb Woods 2014-03-29 11:06
This is where the "Ah-HA!!" light goes on... very informative.
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