A Test. How much Voltage (power) do your speakers need? - Page 15 - diyAudio
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View Poll Results: I measured the test tone at:
2 volts or less 140 37.74%
Between 2-5 volts 126 33.96%
Between 5-10 volts 48 12.94%
Between 10-20 volts 21 5.66%
Over 20 volts. 36 9.70%
Voters: 371. You may not vote on this poll

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Old 28th January 2012, 02:05 PM   #141
Pano is offline Pano  United States
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Thanks Mooly.
Quote:
Originally Posted by Mooly View Post
I then played Pano's tracks and they came out at 0.271 volts RMS which calculates out to -9db relative to the test disc 0db reference.
Right. My tone will be 9dB below the reference tone on your CD because the CD reference tone is actually at -3dB from peak. Why? because you can't record a sine wave at 0dB, it would be a flat line! If the peaks of the sine are at 0dB, then the RMS value is -3dB. See images below. Confusing? Kinda.
The Phillips CD calls that sine "0dB" because it is the loudest possible sine wave.

Quote:
The white noise test shows the level going "over" the 0db point.
I noticed that on your screen shots. Why is that happening? Did you measure at the CD output, or at the speaker terminals? When I scoped, I saw almost all peaks at 4V, max. But one track I saw 1 or 2 peaks at 6V. Kick back from the speaker? I don't know.
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File Type: png full-wave.png (10.0 KB, 250 views)
File Type: png minus-12-wave.png (9.9 KB, 253 views)
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Old 28th January 2012, 02:11 PM   #142
AndrewT is offline AndrewT  Scotland
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Pano,
your use of -12dB and -9dB is at odds to all my previous understanding of 0dBfs.

That was why I had to ask the question way back, you had confused me with ref to -12dB when in fact your were referring to -9dB.
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Old 28th January 2012, 02:13 PM   #143
AndrewT is offline AndrewT  Scotland
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Quote:
If one takes the AES style pink noise used for testing, one finds the peak to average ratio is 6dB, that is the peak Voltage is twice what the average Voltage is and so if one wanted to test a loudspeaker at a 100W AES pink noise rating, one needs at least a 400Watt amplifier to deliver that signal unclipped.
TD told us why.
Did you forget his message because you didn't like parts of it?
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Old 28th January 2012, 02:22 PM   #144
Pano is offline Pano  United States
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Sorry Andrew, that quote above means nothing in a digital recording. 0dBFS is as high as the signal can go. Everything else is below that.

However I do agree that if you use a pink noise signal with an average to peak ratio of 6dB, then your peaks will be twice the voltage of the average. That means 4X the power. I don't see any problem with that. If the pink noise had an average to peak ratio of 12dB, then peaks would be 4 time the average voltage, or 8X the power. Pretty much what I've been saying all along.

Why is my use of -12dB and -9dB at odds with your understanding? Don't you understand the difference between Full Scale and RMS values? Certainly you do. Did you see the waveforms I posted above?
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Old 28th January 2012, 02:48 PM   #145
Pano is offline Pano  United States
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OK, thinking about it a bit more, I can see where the confusion might come from. If a CD has a tone at "0dB" then one would suppose that the peaks are at 0dB full scale.
Click the image to open in full size.
If you measure that tone with a volt meter, you would see 3dB less than peak value, because the meter reads the RMS value, not peak. A 0dB peak sine has an RMS value of -3dB.

My test tones have an RMS value of -12dBFS That means 12dB below full scale. That will put them 9dB (RMS) below a "0dB" sine wave.
Click the image to open in full size.

I have tried to make that abundantly clear throughout the thread, but it looks like I failed. "What we have here, is failure to communicate."

I'm off to work and will be tied up all week on a show. Don't know if I'll have time to check in. Have fun without me, try not to tear up the place too much!
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Old 28th January 2012, 05:13 PM   #146
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by Pano View Post
I noticed that on your screen shots. Why is that happening? Did you measure at the CD output, or at the speaker terminals? When I scoped, I saw almost all peaks at 4V, max. But one track I saw 1 or 2 peaks at 6V. Kick back from the speaker? I don't know.
The scope was measuring line out from the player. I don't think anything in the 2-20KHz CD domain could provoke a measurable kickback. You need to reverse the direction of current flow really fast in inductive components to see that and I don't think anything off disc could do that.

I'll have think on all this

The great thing about a thread like this is that it really gets you thinking.

As I mentioned earlier, 16 bit = 65536 discrete steps. The Philips disc also say this

"Level Definition"
The reference level of 0db refers to a sinewave whose most positive and negative value corresponds to +32767 quantisation steps and whose negative value corresponds to -32767 quantisation steps

So... and this is something I have never considered... is that saying the sinewave is "biased " to the midpoint so that it can swing equally in each direction. A kind of digital midpoint ? Two times 32767 is 65534 discrete steps. I am sure there is some sound technical reason for it not being 65536.

I honestly don't know

I will try the white noise on another player and see although surely it can't be an effect of the player.

Enjoy the show
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Old 28th January 2012, 05:16 PM   #147
rongon is offline rongon  United States
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OK, I just followed directions and performed the test. AC voltage at the speaker terminals measured 3.4V.

Equipment is a 6W per channel PP 2A3 amp into Snell Type J/III speakers.

--

Sorry, I forgot some of the other info requested.

- The room is 12' x 24' with an 8' ceiling.
- The speakers are touted to have 91dB/1w/1m sensitivity. On the high side of "standard."

So 3.4^2 = 11.56

I need a 12 watt per channel amp? Hmmm...

--

Last edited by rongon; 28th January 2012 at 05:37 PM.
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Old 28th January 2012, 06:27 PM   #148
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Hi
I think what was confusing at least for me was the title suggesting this indicated how much power your speakers need (like with music).

What it tells you is how much Voltage it takes to be subjectively pretty loud with a mid bass sine wave.
Depending on the music and recording, it may take 4, 10, 100, 1000 or more times that much (power) to encompass what is in some recordings if played at the same subjective average loudness.

It is like Pano said, digital zero is a point you cannot / should not pass, that level is the loudest / largest Voltage sample that can be recorded on that medium.

For a moment, pretend loudspeakers are dynamically linear, if you put in twice as much power, it puts out the signal 3dB louder and so on (which they do up to between about 1/10 and 1/8 rated power when they begin to compress).

Lets say one had speakers that were 85dB 1w 1m and one sat in an absorptive room (to make it simple) and were 4 meters from each speaker. As this is “stereo” we will take a worst case and say you have a signal on one channel.

Lets say you’re comfortable loud level was only 80dB SPL in that sine wave test.
AS your 4 meters away, due to the inverse square law we know the level is +6dB greater at one meter and we know that at one meter, the speaker was 86dB or 1dB more than 1W or 1.26 Watts.

To get the same SPL reading on a sound level meter using broad band AES pink noise at couple octaves or more wide, it takes an amplifier than can put out 55 Watts (RMS) or twice the Voltage as the signal has a 6dB peak to average ratio.

Similarly with a recording that has a 30dB peak to average ratio, one finds the peaks require 1260 Watts while the average level for the same SPL is still only 1.26 Watts.

Now, +30dB over 80dB sounds like it would be loud, and it would be if it were also a continuous sine wave but if used to reproduce a snare drum or other percussive instrument, it will fall FAR short of the real thing.

It is the very difficulty in capturing and reproducing large dynamic ranges, particularly in a high noise background, that lead to compression and then the low bit MP-3’s so you can hear music when your jogging or with the car top down.

Thankfully one can’t hear instantaneous clipping like traditional “ugh turn it down” clipping, this only saps the dynamics. The only way around this is to have efficient speakers and power or sit very close, or choose your music to fit the system’s capability.

Ultimately, with so many variables in equipment and program material, the ONLY WAY to tell if you have enough Voltage or Power (either view) is to examine what is coming out of your power amplifier with an oscilloscope. If you have one and know how to use it, DO try this, examine the highest peaks and see if any are flat topped.

More often than I would have thought, one finds instantaneous clipping and not just from a power amplifier but any stage can clip, the arrangement of this is “optimizing gain structure”.

Also, it’s worth pointing out that in the recording and engineering areas what we see as dynamic range can be referring to sort of different things.
For example, a pure sine wave has NO dynamic range, the amplitude of the envelope does not change, the signal’s dynamic range is zero. Conversely, a pure sine in a 5 cycle long Gaussian amplitude envelope like Don Keele uses occupies about 1/3 octave bandwidth but is short enough to test and stress a speaker at very high power without sounding loud at all.
Best,
Tom
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Old 28th January 2012, 07:27 PM   #149
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Tom Danley View Post
..................
AS your 4 meters away, due to the inverse square law we know the level is +6dB greater at one meter and we know that at one meter, the speaker was 86dB or 1dB more than 1W or 1.26 Watts. .............
Could someone review and confirm or otherwise?
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Old 28th January 2012, 07:30 PM   #150
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When I was debating how much power, or rather rail voltage I would need for my HT I hooked up my 2430A and played a low passed and unfiltered signal as loud as I ever would go. Wasn't much so I cranked it up to almost rail voltage, 20V P-P or so, RMS was around 6V I think. A bit too loud and the various squealing noises around the room felt almost louder than the speakers themselves.
Conclusion was that a Class-D amp powered by 24V PSU would do just fine.

Active speakers are a bit harder to calculate for as the voltage of a low or high passed signal can be greater than the input signal. Probably due to phase shift.
(A Hilbert transform of a square wave is probably the best example of this)
Hilbert transform - Wikipedia, the free encyclopedia
That is probably the reason for the previous images showing a greater P-P voltage of pink noise than a sine wave of full amplitude. As all digital to analog converters (sort of) comes with a low pass filter.
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