Simple guide to power amp requirements...

steveinaz

Many ask questions about how much power they will need for a given room/speaker combination. It’s a common question, so I’m making this post for future reference. Keep in mind this is not an article for the "techno-weenies" or electrical engineers, it's a simple guide, and written in that light.

Three factors come into play when determining how much power you need:

1. Room size
2. Speaker efficiency
3. Desired loudness level

Since room size is often the variable we cannot change, (at least not easily), let’s talk about speaker efficiency and desired loudness level. Speakers have a specification for “efficiency” or “Sound Pressure Level” (SPL); some also have a “max SPL” specification. Some definitions:

1. Efficiency: Typically the level in decibels the speaker produces at 1watt, measured at 1 meter from the baffle, usually at 1kHz. A typical measurement might be 89dB for a relatively high efficient design.
2. Sound Pressure Level: SPL/Efficiency are used interchangeably.
3. Max SPL: Some manufacturers provide this number as a “warning” to not exceed this output level.

To determine how much power you need, you need to know your speakers efficiency specification. It takes approximately a 3dB increase (for most people) to notice a change in level; you have to double wattage in order to gain 3dB in level. Given this, and easy way to figure out how much output you need is to create a chart. Using the example above of the 89dB efficient speaker, lets see how much power is required to attain certain output levels, then lets add speaker “b” with a specification of 92dB for SPL:

Watts……SPL (spkr “a”)….....SPL (spkr “b”)
1………..…89dB…………......……92dB
2………..…92dB…………......……95dB
4………..…95dB……………......…98dB
8………..…98dB………….....….…101dB
16……....101dB…………….......104dB
32……..…104dB…………….......and so on
64……....107dB
128……..110dB
256…..…113dB
512……..116dB
1024…...119dB

For reference:

60dB….normal conversation
90dB….lawn mower
110dB….car horn
120dB….jet aircraft at take-off/Rock concert
140dB….gun shot

Note: As you double wattage, you increase output by 3dB. In the above example, the 89dB @ 1watt/1mtr specified speaker would require 16 watts to attain 101dB SPL, fairly loud. Note that speaker “b” only requires half the power to attain the same output loudness. Also note, there is no distinction for impedance; SPL is SPL, regardless of impedance. You just need to be aware that 4ohm speakers require a more robust power amplifier due to the higher current demands. Efficiency is not necessarily a measure of quality; it’s a function of the speakers’ design. There are many fine speakers that are relatively inefficient, and also many highly efficient designs that are not of good quality.

tcrossma

Practically speaking, does this mean anything? I don't like to listen to music very loudly, and i'm pretty sure I don't listen at the 100db level. The chart indicates that I only need a 16 wpc amp.

tim

heiney9

tcrossma wrote: »

Practically speaking, does this mean anything? I don't like to listen to music very loudly, and i'm pretty sure I don't listen at the 100db level. The chart indicates that I only need a 16 wpc amp.

tim

That is correct only if you listen to your speakers using a 1kHZ test tone exactly 1 meter away. The chart is a guide if all things are equal. Music is never just one sustained tone. The tones are variable and a lower note, say 45Hz, will need much more power to reproduce accurately with out clipping depending on room size and how far away you listen.

steveinaz

tcrossma wrote: »

Practically speaking, does this mean anything? I don't like to listen to music very loudly, and i'm pretty sure I don't listen at the 100db level. The chart indicates that I only need a 16 wpc amp.

tim

It would mean that if you only listen at 100dB, and you're running, say 100 watt amp, you've got a good cushion of at least 6dB for dynamics.

Again, it's a guide. If you like LOUD music, you might want to be in the 128-512 watt realm based on a 89dB speaker SPL.

In your scenario, you're probably right. If you listen at less than 100db, in a small room, a 20 watt amp would do the job.

Gaara

I was under the impression that 8ohm speakers are measured at 1 watt to get efficiency, which is equivalent to 2.83v but 4 ohm speakers 2.83v would equal 2 watts. This would mean that if a 4ohm speaker is rated at 2.83v then the efficiency should be = to the spec - 3db to make up for the 2 watts as opposed to 1.

Don't know if it is beyond the scope of this thread but I feel that it is important.

venomclan

You guys are all way off. An amp has to have 200 watts to sound good. Room size, speaker efficiency or amp type make no difference
V

fredv

Hee hee, I am kind of moving backward. From 3 digits of SS power to 2 digits of tube power, and now I am looking at Nelson Pass's F4 - a unity gain flea amp

dudeinaroom

you do not need a 200 watt amp to sound good. You just have to have a good one. But what can I say, my amp puts out 200 into 8 ohms.

venomclan

dudeinaroom wrote: »

you do not need a 200 watt amp to sound good. You just have to have a good one. But what can I say, my amp puts out 200 into 8 ohms.

I know Dude, I was just poking a little fun at Cfrizz...

cfrizz

Says the man with a 250wpc Krell! HYPOCRITE!!!:p

venomclan wrote: »

I know Dude, I was just poking a little fun at Cfrizz...

pearsall001

Thanks for the info Steve. Here's another way of doing it. This site does video & audio calculations. All you do is plug in the numbers. Pretty neat stuff. http://www.myhometheater.homestead.com/viewingdistancecalculator.html#anchor_13194

Early B.

A powerful amp is like driving a muscle car -- you probably won't use all of that extra power, but it's nice to have it under the hood just in case. If you prefer to cruise the 'hood in a moped because it's more fuel efficient, I suppose a flea amp will accomodate your needs.

venomclan

cfrizz wrote: »

Says the man with a 250wpc Krell! HYPOCRITE!!!:p

I am replacing it with a 1 watt tube amp
Dr. Evil laugh Muhahahahahahaha

Sami

So LSi15's with 88dBm efficiency would result to 105dBm with 200W amp at 9ft, versus 125W and 103dBm. I think both are more than plenty for me, I don't think my ears could take more.

At work when I speak of power levels and mention dB, I always get myself corrected, "you mean dBm or dB?" Damn RF Engineers...

cfrizz

I don't give a rats **** about all the numbers! My ears tell me the 200wpc ROCKS!

And a good many other people have agreed once they got powerful amps.

Early B.

cfrizz wrote: »

I don't give a rats **** about all the numbers! My ears tell me the 200wpc ROCKS!

And a good many other people have agreed once they got powerful amps.

You tell 'em, Cathy. Curse 'em out, dammit.

steveinaz

cfrizz wrote: »

I don't give a rats **** about all the numbers! My ears tell me the 200wpc ROCKS!

And a good many other people have agreed once they got powerful amps.

Exactly, that's why the 150-250 watt amps have such fierce competition. They're large enough to handle most size rooms with serious output.

Face

I have to go past halfway on my 2 ch. setup. Once the acoustic panels are done and the window is covered, I'll give it a try. 5(out of 12) is audible in the backyard as is.

Btw, great thread!

dudeinaroom

10 o'clock is my listening level, 11:30 and all of my neighbors complain(I live in an apartment). would really like to get in a house where I could separate the 2ch and HT. So I'd have to get a new set of speakers for the HT, and A sacd player for the 2ch, but I guess for know they will have to play nice and live together and share the amp and dvd player.

madmax

heiney9 wrote: »

That is correct only if you listen to your speakers using a 1kHZ test tone exactly 1 meter away.

Sometimes I get crazy and switch to a 1.5kHZ test tone.
madmax

WilliamM2

Also keep in mind that efficiency ratings are for one speaker, each additional speaker adds 3db. And efficiency ratings are usually done in an anechoic room. In the real world, there is also room gain.

For example, my system at 200 wpc calculates to 115db (one speaker), but I have measured 122db from the listening position (10ft. from speakers), with even higher peaks with 2 speakers.

Phil Dawson

It's really very simple - all things being equal more power is better than less power and you can almost never have too much power.
Phil

billbillw

Gaara wrote: »

I was under the impression that 8ohm speakers are measured at 1 watt to get efficiency, which is equivalent to 2.83v but 4 ohm speakers 2.83v would equal 2 watts. This would mean that if a 4ohm speaker is rated at 2.83v then the efficiency should be = to the spec - 3db to make up for the 2 watts as opposed to 1.

Don't know if it is beyond the scope of this thread but I feel that it is important.

I know I'm dredging this up from the past. This thread actually came up in a Google search I was doing.

I don't think Gaara is around anymore, but if he is, thanks bud! After all these years in the hobby, I never realized that they consistently rate 4-ohm speakers differently. When I looked back at my SS-M9 review, sure enough, it says B-weighted sensitivity is 89dB @2.83V/m. Sure enough 2.83V is 2W.

So, I DO really need the ~400W into 4 ohms that I have (or maybe more?).

I had been seriously considering down-scaling my system to something that made 200-300W into 4 ohms, but I think I will not settle for anything less than I have now, or perhaps I just need to keep what I have.

To determine the approximate power to create a given volume, I quickly dumped it into a spreadsheet and it spit out this:
89dB 2W
92dB 4W
95dB 8W
98dB 16W
101dB 32W
104dB 64W
107dB 128W
110dB 256W
113dB 512W
116dB 1024W

So, to get dynamic peaks above 110dB, I really do need in excess of 250w. 400W doesn't even get me to 113dB? I mean, who wants to settle for 'car horn' loudness when you are cranking some Tool or Guns N Roses?

zingo

I read somewhere the Musical Fidelity suggests no less than 350wpc for uncompressed dynamics and to avoid clipping. That would be right in line with your 400wpc amp, especially with 4 ohm speakers.

tonyb

Harder to drive speakers usually don't sound as good at lower volumes. Some of this is speaker design, or amp design or both. Think of sitting at light revving your engine. The you pop it into 1st and dump the clutch. An amp needs to be under load to sound good or make an audible difference. Lower volumes don't present that much of a load to an amp so all that energy stays stored in the caps. You turn up the volume and it's like dumping that clutch. More efficient speakers are designed to run on less power but even more efficient speakers with multiple drivers still need some current to control all those drivers. My Focus are 95 efficient but have so many drivers that dynamics suffer at lower volumes because the amp is under very low loads.

In another thread, someone asked why audiophiles seem to gravitate to 4ohm speakers. Because they present a heavier load to the amp, the amp wakes up....better sound. Overall sensitivity, load on the amp, everything synergizes, comes together. It's that synergy thing again, but understanding how amps work, power flows, and how speakers ask for it is paramount to putting together a system that synergizes.

tal288

The person way up top is very smart. Speaker specs I'm familiar with: SENSITIVITY IMPEDANCE AMPLIFIER POWER RECOMMENDATIONS. I'VE LISTEN TO LOUD ROCK AND > 116 dBL's is LOUD!!!

tal288

I don't want HEAVIER LOADS FOR MY AMP! < 4 Ohms requires more current which means EXPENSIVE AMPS. I'm not an EE But more amplifier power means more money!!! In addition impedances' below four ohms will cause overheating for my amplifier which will shut down. Finally Your amplifier will go into clipping if you try to play it too loud. low sensitivity, difficult speaker loads will wreak havoc on those who own receivers. I don't know maybe I am wrong

tal288

Somebody above said that when they try to drive speakers with four to six ohms but that doesn't sound as Good. Of course. When speaker loads drop below 4 ohms, that creates extraordinary demands on an amplifier let alone a relatively inexpensive amplifier or Worse, a receiver. My B&W NAUTILUS 801 SPEAKERS OWNERS MANUAL SPECIFICATIONS READ:"SENSITIVITY 91 DB spl (2.83V, 1m)" "NOMINAL IMPEDANCE. 8 ohms ( minimum 3.0 ohms)." Altghough SENSITIVITY is relatively amplifier friendly, the NOMINAL IMPEDANCE, particularly the 3 ohm minimum (btw ALL SPEAKERS impedance levels "float" up and down at different frequencies), poses power problems for my BRYSTON. 14B SST...600 WPC AT 8 OHMS AND 900 WATTS AT 4 OHMS, BUT DECREASES AT 2 OHMS!! FINALLY SUBSCRIBE TO "STEREOPHILE" MAGAZINE WHERE, IN ADDITION TO A REVIEW OF A PRODUCT, JOHN WILL SUBMIT IT TO A TECHNICAL ANALYSIS ON HIS BENCH

tonyb

tal288 wrote: »

I don't want HEAVIER LOADS FOR MY AMP! < 4 Ohms requires more current which means EXPENSIVE AMPS. I'm not an EE But more amplifier power means more money!!! In addition impedances' below four ohms will cause overheating for my amplifier which will shut down. Finally Your amplifier will go into clipping if you try to play it too loud. low sensitivity, difficult speaker loads will wreak havoc on those who own receivers. I don't know maybe I am wrong

Not necessarily. 4ohms does not require expensive amps. Also a receiver is a different animal than an amplifier dedicated to one purpose. There is an amplifier made for every purpose, for every speaker. Harder to drive loads do not dictate more money per say, it's more so a right tool for the right job sort of thing. The problem comes in with some not knowing what tool is needed, or trying to get the job done with a lesser tool.

cfrizz

tal288 wrote: »

I don't want HEAVIER LOADS FOR MY AMP! < 4 Ohms requires more current which means EXPENSIVE AMPS. I'm not an EE But more amplifier power means more money!!! In addition impedances' below four ohms will cause overheating for my amplifier which will shut down. Finally Your amplifier will go into clipping if you try to play it too loud. low sensitivity, difficult speaker loads will wreak havoc on those who own receivers. I don't know maybe I am wrong

You are correct that basically more current usually means more watts per channel which equals more money spent to get the power that you want. That is how it is with everything in life, more costs more money.

I would rather have more power because I then know that my speakers are operating to their full potential, and there is much less risk of them clipping at higher volume because the power is there for them to access when they need it. So for ME the money is well spent on buying a separate amplifier dedicated to that task.

Receivers try to be all things to all people, but the sacrifice they have to make is in lack of power to properly drive speakers. Those of us on here know this and so we encourage people to spend more money to make sure that they have a system that will keep them happy for many years. Which means receivers with pre-outs to add a power amp to get the most out of your speakers.

The bottom line is either you are willing to spend the money on a new or used amplifier or you're not. If not then enjoy what you have just watch the volume, and realize that you are not hearing everything your speakers are capable of producing because they don't have enough power to do so.

Welcome to Club Polk Tal!

Erik Tracy

Just to add to the discussion, I believe that listening distance will also factor into any power estimates. I recall reading someplace that for every doubling of distance SPL drops by 4dB. So sensitivity ratings are measured at 1m, so at 2m you are down 4dB, and at 4m (12 ft) you are down another 4dB for a total drop of 8dB from the speaker.