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Old 25th January 2004, 07:14 PM   #1
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Default Why can 8W sound as loud as 100W?

I have been reading around and have noticed that many tube amps have output ratings very low (8W for example). However, from what I have read, they still put out sound that is loud and accurate (if not too loud at full volume). Yet speakers hooked up to a 100W amp is also loud yet it has more than 10times the power rating. How can 8W be as loud as 100W? Is the signal more efficient? Please let me know, as I am always curious and am thinking about building this tube amp as a first DIY amplifier. http://www.s5electronics.com/gpage1.html

Thanks for your time.

-Ed
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Old 25th January 2004, 07:22 PM   #2
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The only way efficiency comes into play is with your speakers efficiency at moving as much air for the same power...

A 94dB efficient speaker would produce x more amount of noise for 10 watts than would a 80dB efficient speaker for 10 watts
or something like that, I have no idea how the mathematics are done...

Other than that, a 50 watt amp is only 1/4 louder than 25watts, No?

Same as a 400watt amp producing 2x of the output power of a 100 watt amp or something like that.
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Old 25th January 2004, 07:49 PM   #3
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I'll tell you one thing for certian tho

My cheapo Magnavox 10 watt mid-range all-range with whizzers can fill this entire room and corridor with sound without any boxes on a 20w per channel transistor amp while my $150 ones in boxes inside a 3 way give HEAPS of bass that you can hear for a mile, but have NO mid-range to speak of when compared to these beauties :P

May be a bit confusing but it's all in there somewhere. :P
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Old 25th January 2004, 07:56 PM   #4
Morse is offline Morse  United States
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Hi guys;

>>>...Other than that, a 50 watt amp is only 1/4 louder than 25watts, No? Same as a 400watt amp producing 2x of the output power of a 100 watt amp or something like that...<<<

Double the output power and you get +3dB increase in perceived volume, all else being equal. Quadruple the output power and you get +6dB increase in perceived volume. +10dB is generally regarded as being "twice as loud", perceptually.

Now, as for the original question, it comes down to the use of the term RMS (Root Mean Square, or "average"). Music is produced as transient peaks, so two amps that have identical RMS ratings (their ability to produce output power on average for a long time period) may not sound equally loud if one can produce more PEAK watts. Where this gets dicey is that there's no agreement on just HOW to go about measuring those peak watts. Marketing depts used to play all manner of games with output wattage ratings giving inflated, meaningless, numbers. That's why the FTC stepped in and set standards for measurement (which are now routinely sidestepped by doing things like using 8 ohm static loads, 1kcps sine waves, and only driving one channel - all of which is legit, so long as it's in the fine print).

Perhaps a better way of dealing with the issue of "peak watts" is to measure "peak transient voltage" for a time period determined to represent most musical transients.

One other consideration is the slew rate - a "slow" amp may not be able to produce a rapid transient signal at all to it's full peak. Thus a "faster" amp can tend to sound louder than a "slow" amp. The speed is called "slew rate" and is a measure of rate of change of volts with respect to time - thus if an amp can't "swing" up to the full output voltage in the short time that a transient signal is present, it won't pull the same number of output volts for that transient (it's like it rounds the top off, get it?).

Hope this helps.
All the best,
Morse
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Old 25th January 2004, 07:56 PM   #5
Jax is offline Jax  Sweden
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Our ears are logarithmic. We can barely hear an increase of 3dB.
3dB = twice the power.

8W to 100W is just over 10dB, just a little louder. 10dB = 10 times the power. The 100W amp is just a little louder.

An 8W amp with 100dB speakers will sound exactly as loud as a 80W amp into 90dB speakers.
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Old 25th January 2004, 07:57 PM   #6
sobazz is offline sobazz  Denmark
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Ok... to examples. We have speaker rated 86db/1w/1m. Everytime you double the power applied to the voice coil your speaker will put out 3db more SPL. In order to reach 92db you'll need to apply four times the power - 4w. In order to reach 110db you'll need to apply 256 times the power - 256w.

Imagine a speaker rated 98db/1w/1m - a very efficient speaker. In order to reach 110db you'll only need to apply 16 times the power to the voice coil.

That explains why a low wattage tube amp can easily destroy your ears when used with sufficiently efficient speakers.
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Old 25th January 2004, 08:10 PM   #7
Bricolo is offline Bricolo  France
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Jax, I think that doubling the power/volume is a 6dB increase, yet to confirm...
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Old 25th January 2004, 08:43 PM   #8
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Thanks for the info everyone. Makes perfect sense now. Thanks again.

-Ed
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Old 25th January 2004, 09:03 PM   #9
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Quote:
Jax, I think that doubling the power/volume is a 6dB increase, yet to confirm...
Sorry, no confirmation. Try again with voltage
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Old 25th January 2004, 09:19 PM   #10
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Default Morse, interesting question

Got an interesting question for you or anyone else for that matter.

"+10dB is generally regarded as being "twice as loud", perceptually."

I certainly agree.

On "millionaire show" I believe the question was asked:

"How much louder is a 30db signal than a 10db signal?"

The choices given were:

A) 3
B) 200
C) 100
D) 1000

What would be your answer?
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