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RMS Watts?

Millennium

Millennium

pfm Member
Hi there
Hope everyone is safe and well. Suggest some caution regarding covid, but that's off topic.

So, say I have 92 dB/W/m speakers (3 to 8 nominal ohm impedance speakers)

about 1m from each speaker in a stereo pair is the listening and measuring position.

amplifier is IN THE END (effectively) producing about 88db of sound before distortion. It's warmed up.

amp has RCA in and fed by 2.25V RMS output of DAC/pre. Might be a little hot, but it's close to 2v RMS.
DAC/pre is maxed and is showing about -1db on the meters peak, maybe upto 0 (max)

let's try an intellectual exercise. You're building the first amp ever. You have to prove it's max output and it's as described above.

Is it ACTUALLY LESS THAN A WATT?

I'm shook.

(PS I have an update to this post but I think this is something that is worth asking stand alone!)
 
Why the surprise?

The way that I have always looked at it is to think of how hot a 5W/20W/whatever watt heater is and imagine that as sound. Maybe not so easy for anyone with no science background............

No different to lighting. The old standard GLS lamp was something like 0.001% efficient turning electrical energy into light. That is around 10 lumens per watt. The equivalent commercially available LED (which has to have some kind of gear to run it off 240V mians) is around 100 lumens per watt, so you use only one tenth the electricity for the same light output - energy efficiency in turning electrical energy to light - around 0.01%

(The major problem online in terms of lighting is that people know too little and efficacy and efficiency get used inter-changeably - they are NOT the same by a country mile.)
 
I'm sorry I'm gonna need a moment lol

Which is the most grossly overblown spec? The dB/W/m of the speakers or the proposed 15W output of the amp in question? From your response I tend to guess that the speaker specs are more "optimistic" than the amp RMS ones (?)

I am one of those people who thought I was clever looking at RMS measurements rather that PMPO (rubbish) in the Argos catalogue LOL...

if the amp was replaced with another more powerful amp, and that amp produced an effective say 110DB (concert hard level) output at my 1m listening position from my pair of 92 dB/W/m speakers, how many watts would that be? Like, 6 ?! Can any decent amp actually do this?

this seems like a crazy situation where a sorta decent amp can be performing like 12x below spec. I am MORE shook...
 
Speaker voice-coils get hot/dissipate a lot of heat, which is where the majority of the energy goes - just like in a lamp - way more heat than light as opposed to way more heat than sound, in terms of watts/joules.

You'd need a conversion of dB sound to watts sound to see where the balance lies, but just as with light, this will vary with wavelength/frequency and figures are in some way "averaged" (maybe there is an industry/international standard, as with lamps, but I somehow think that that is unlikely).
 
Yeah Vinny I really appreciate the reply. I had no idea that quoted speaker specs are for the coils / internals rather than the external output SPL. This whole dB/W/m measurement unit I think I will ignore for now - seems like pure FUD

I am not clever. Others have noticed this before me, perhaps or likely with other low wattage class A amps and similar speakers.

What's the practical solution to get towards 105 - 110 DB SPL? With a decent amp? Which isn't a 600W heater?!
 
Just remember speaker impedance and what sort of voltage drives a speaker - watts (heat) will be V^2/R.

So far as I am aware, put one watt into a 100db/W/m and there you have it.

davidsrsb said:
According to this article, lighting is better than that, LED can theoretically reach 40% and in practice 25%
https://en.wikipedia.org/wiki/Luminous_efficacy
Click to expand...

That hits the nail on the head - THEORETICALLY. That is also only the actual LED, and almost certainly monchromatic. It skips the running gear for starters.
Theories are the most wonderful of things :)
A quick look at the table and it seems to be garbage anyway. I'll dig out my lamp design books and quote a really accurate efficiency (W/W) for incandescent lamps..........


My off-the-shelf LED lamps here are around 100lm/W - p-poor efficiency.
 
If only the OP made any sense....

Anyhow... Log Watts x 10 gives you power of amplifier in dBW. Add this to dB/W figure for speaker to get theoretical max output. One or two speakers, direct or reverberant sound field, correlated or uncorrelated noise etc etc all complicate matters.

It puts the relative volumes of amplifiers of increasing power into perspective as well! 10W = 10dBW, 50W = 17dBw, 100W = 20dBW, 200W = 23dBW etc etc
 
Perhaps worth pointing out just in case: With music the peaks tend to be somewhat higher than the rms level. With decent recordings of acoustic music can easily be well over 10 times the power. So light-bulb rms levels can mislead.

LEDs are, of course, non-linear loads as well... 8-]
 
Arkless Electronics said:
10W = 10dBW, 50W = 17dBw, 100W = 20dBW, 200W = 23dBW etc etc
Click to expand...

In that the dB refers to sound/volume, presumably this follows the add ten and the volume doubles rule?

Vinny said:
I'll dig out my lamp design books and quote a really accurate efficiency (W/W) for incandescent lamps..........
Click to expand...

The books are going to be buried somewhere. Finding anything that looks right online, and even two figures that agree is extremely difficult. Working figures for standard GLS, tungsten filament lamps - 1.5 - 2.5% efficient. Which would tie in with LEDs of 25% efficient, but that ignores the losses in the running gear for LEDs (and losses in phosphors for anything but monochromatic LEDs).
 
Vinny said:
LEDs work at a defined voltage drop with not too great a margin for error. Are you talking about dimmed ones? If so, the load is nothing to do with the LED, it is the running/dimming gear.
Click to expand...
LED driver ICs convert mains ac to drive a string of LEDs with a constant current. The driver can reach 90% efficiency. Dimming is done by controlling the current.
But this is off topic
 
davidsrsb said:
LED driver ICs convert mains ac to drive a string of LEDs with a constant current. The driver can reach 90% efficiency. Dimming is done by controlling the current.
But this is off topic
Click to expand...

Hmm. But white (actually UV) LEDs have a voltage drop of 3.4V, typically. An LED either runs or it does not, it drops 3.4V or not (or dies if significantly more than 3.4V is applied).

Dimming is via current - granted - it chops it up just as a triac dimmer on incandescent lamps.
 
Arkless Electronics said:
If only the OP made any sense....

Anyhow... Log Watts x 10 gives you power of amplifier in dBW. Add this to dB/W figure for speaker to get theoretical max output. One or two speakers, direct or reverberant sound field, correlated or uncorrelated noise etc etc all complicate matters.

It puts the relative volumes of amplifiers of increasing power into perspective as well! 10W = 10dBW, 50W = 17dBw, 100W = 20dBW, 200W = 23dBW etc etc
Click to expand...

If I am not making sense please educate me. My A level physics suggests that an item specced with a unit of dB/W/m provided with 1 watt at 1 meter should produce the stated number of decibels.
Please don't say I'm not making sense without explaining why, in the context of my OP and the first reply. I get it, I'm just shocked that the stated specs of the products of 92 and 15w would produce < 90 DB SPL at max
you see surely how a layman might assume that this is 15x factor off?

I didn't realise that (quote) "Speaker inefficiency comes from the impedance mismatch between air and the cone material" I think. Sound is air pressure - if the cone gives 10 DB, the air gives 0.5?!

Arkless, I'm not an amp designer nor an industry insider, do you not think there is something amiss here?
 
I think I missed the obvious Arkless

"Log Watts x 10 gives you power of amplifier in dBW"

any FAQ that covers this? BBS? lol
edit: is it due to the extra distance measurement factor of m that the watts needs to be converted to Log(w) * 10 ?
 
additional quick question for Vinny:

quote 'So far as I am aware, put one watt into a 100db/W/m and there you have it.'
does this mean that to get 100 db at 1 watt you would need to be infinitely close to the cone?
 
I am out for now. Enjoy Audio PFM

I will just go with demos and Reputation

{please listen to London Grammar's new stuff}
 
Vinny said:
LEDs work at a defined voltage drop with not too great a margin for error. Are you talking about dimmed ones? If so, the load is nothing to do with the LED, it is the running/dimming gear.
Click to expand...

Erm... My point really was that an LED is nothing like a loudspeaker so fas the load behaviours. So the comparisons aren't going to help anyone much.
 
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