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]

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]

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 :D
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 :D

[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]

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]

:D :D :D :D :p

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

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.c...l#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]

:D :D :D :D :p

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

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 ass 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.]

I don't give a rats ass 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]

I don't give a rats ass 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]

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. :D
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