advertisement


RMS Watts?

Millennium said:
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?
Click to expand...

The value quoted for the speaker will be for a specific distance, room acoustic, assumed impedance, and spectrum, etc, of test sound. Your home check may differ in every way. This also ignores the question of your meter being accurate.
 
Millennium said:
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?
Click to expand...
Your question is not quite clear. However, if you want to conjure with amp power, speaker sensitivity and loudness the first thing is that you really need to do two calculations. One for average level (I assume RMS in the title is equivalent) and one for peak level.

We also these days need to deal with nominal watts (i.e. watts into 8 ohms) when it comes to 'speaker sensitivity. Calculating real power in watts from real 'speaker impedance does not lead to helpful results.

Average power:
  1. Simplifying, take one 'speaker [0] at your sensitivity of 92 dB SPL / 1 nominal W / 1 metre [1].
  2. Take your average listening level of 88 dB SPL at 1 metre [2].
  3. The average power level is (88 - 92) = -4 dBW. That converts to 0.4 W nominal.
Yes, 0.4 W is all the average power needed for very loud average sound level in this case.

Peak power:

But music has peaks that are much larger than its average. You don't want your amplifier to clip those peaks. So you actually need an amplifier that is rated at more than that 0.4 W average. Here you need a figure for peak to mean ratio of the music you want to play. Modern pop may have 6 dB. Classical music with 15 dB is common. I have music in my collection with well over 20 dB. I generally choose 20 dB for planning purposes which converts to power x100.

Practical conclusion:

So 0.4 W average goes up to 40 W to not clip any of my music's peaks. That's what I would design (or select) as a power amplifier for this simple situation. I am assuming that's the question you are asking. And this power must also be available into the actual impedance of the 'speakers (although perhaps not continuously). But this may be too much or too little for your tastes. I can't tell.

[0] A pair in a real room will be louder at 1 m but then quieter as you move the listening position back further. This can be estimated but not for now.

[1] This is quite sensitive - the average these days according to Stereophile figures is about 87 dB SPL / 1 nominal W / 1 metre.

[2] This is very loud - even a mixing/mastering studio probably averages 82 dB SPL and it's maybe more like 78 dB SPL average level at home for many.
 
Millennium said:
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?!
Click to expand...
Quoted loudspeaker sensitivity figures are for real sound pressure 1m away but often measured by manufacturers in ways that significantly overstate them. But they are far from FUD if you apply a suitable discount factor.

If you want very loud, like 105 - 110 dB SPL (although you didn't specify if this is average or peak so I have no practical answer) then first of all you need 'speakers that are capable of that sort of output. Quite a few consumer loudspeakers - especially fairly small ones - will never produce that sort of output (assuming average) either cleanly or at all, however much power you feed them. Manufacturers of consumer 'speakers rarely quote any maximum for the SPL they can produce.
 
I totally concur with @John Phillips anaylses.

I've 15yrs with a pair of Impulse H2s and yes, watching the voltage waveform across them on a 'scope/ rms ac meter while playing at the levels I want in- room showed-up that average power levels required were tiny; under 250mV or so, 10-20milliwatts averaged into a 95db/W/8ohm load, easily yielded the 75-78dB at the sofa, c2.4m away; and that's fine by me.

I also have/use/love a pair of Quad 989 electrostats, perhaps 8-10dB less sensitive, and still- with an Avo8 Ac voltmeter hooked across the speaker terminal like a giant VUmeter readable across the room - rarely exceeds c. 2Vrms. I've no shortage of dynamic range at that... and also - since so much of the listening experience is spent down at such low average levels - why noise performance of the entire chain up to the loudspeaker terminals matters so much.

tl;dr: I couldn't care much less about a new bargain 400+w amp. Show me how it behaves below, say 5w ...
 
martin clark said:
I totally concur with @John Phillips anaylses.

I've 15yrs with a pair of Impulse H2s and yes, watching the voltage waveform across them on a 'scope/ rms ac meter while playing at the levels I want in- room showed-up that average power levels required were tiny; under 250mV or so, 10-20milliwatts averaged into a 95db/W/8ohm load, easily yielded the 75-78dB at the sofa, c2.4m away; and that's fine by me.

I also have/use/love a pair of Quad 989 electrostats, perhaps 8-10dB less sensitive, and still- with an Avo8 Ac voltmeter hooked across the speaker terminal like a giant VUmeter readable across the room - rarely exceeds c. 2Vrms. I've no shortage of dynamic range at that...
Click to expand...
Indeed. And I really think the OP's 88 dB SPL listening level (if I understood it correctly - maybe I didn't) is way too loud for reality. If the OP's average listening level is a more reasonable 78 dB SPL then the amplifier in his example is averaging 0.04 W and he needs just 4 W peak to handle very dynamic music as long as it isn't at "party" level.
 
What this really means is that we don't need 250W muscle amplifiers, with maybe x100 voltage gain unless we live in enormous American houses with ML speakers.
One of the common problems with efficient speakers is background noise and hum at normal listening levels.
There is a market for 10W amplifiers with maybe x5 gain
 
I try to keep in mind that many people enjoyed ELS57 with 84 dB/W efficiency and the 15 W Quad II. Customers for these probably had bigger houses than we generally have now
 
davidsrsb said:
What this really means is that we don't need 250W muscle amplifiers, with maybe x100 voltage gain unless we live in enormous American houses with ML speakers.
One of the common problems with efficient speakers is background noise and hum at normal listening levels.
There is a market for 10W amplifiers with maybe x5 gain
Click to expand...

The problem is that situations vary wildly from case to case.

Music varies in the sound level you'd choose and in its peak/mean ratios. (Further confused by people playing games with things like reconstruction filters in DACs.)

Speakers aren't 8 Ohm resistive loads.

Power amps don't all have rigidly clamped power rails. Nor will deliver x2 the power into 1/2 the load impedance. Or, indeed, regardless of load phase angles.

The effect of rooms, which is significant. Plus listening position, etc.

So it become a case of "usually" having to preceed any general comment.

So "usually" an amp that may be rated at about 50wpc rms should be fine *if* it can then do voltage peaks equivalent to 100W or more into 8Ohms into your speakers. And go on playing at these levels without eventually overheating, etc.

I have the scars on this one. :) The old Armstrong 600 amps could do the above in many cases if you got the later versions. But if you had a low impedance high phase angle speaker, or played with the average level high for some time, you'd hit a problem. The heatsinks would get very hot indeed because they were tiny. Fine for "normal" (assumed!) use. But more "radio 2/3" than "radio 1" assumed.

So one of the points of a higher power amp rating is to cover more situations. Not simply to deafen you to the people banging on your door to complain about the 'noise'. :)
 
I thought RMS was meaningless, predominantly found quoting nonsense ratings like 300/400/500w on micro/midi/mini systems decades ago.
 
eevo1969 said:
I thought RMS was meaningless, predominantly found quoting nonsense ratings like 300/400/500w on micro/midi/mini systems decades ago.
Click to expand...

No, those were the peak power ratings. They might have been based on the amount of power that could be delivered into, say, a 1 ohm load for the tens of microseconds before it blew up - or more likely just made up by the marketing dept.
 
Certainly the range of required power levels can vary enormously depending on situation.

As has been discussed above 10WPC is fine for many people. Probably a majority in fact.

If you want genuinely realistic levels, especially in a large room, then huge amounts of power are usually needed.

The logarithmic nature of human hearing is the killer here as although 10WPC is enough for most people to find it loud, complaints from other family members or neighbours etc etc type loud even, as soon as you think "yeah it's pretty loud but it's nowhere near as loud as live music" and start turning it up louder still you find that "louder still" very quickly demands 50 and 100 or more Watts... To the human ear 100W is only around twice as loud as 10W!
 
Remember when the Quad 405 launched with the headline 100 W and then they had to make the limiter to stop it frying ELS63s

When you reach 100 W you quickly find out how little power tweeters can survive.
 
You must log in or register to reply here.


advertisement


Back
Top