VU meter scale explanation for a caveman

[b-rider]

I KNOW this has been discussed ad-nauseum on this site, so forgive me for asking again:

Can someone please explain, or direct me to a previous discussion, as to why my amplifier VU meters are scaled so that when I turn up the volume pot, the meter readings go UP from a -20 dB level to a 0 db level, with red (clipping?) beyond.

As a caveman, with the unit shut off, I'm hearing 0 db sound level. When I turn it on and turn the volume pot UP, I'm hearing (what I think is) LOUDER, or more, dBs.

What gives? For the rank consumer who buys fat amplifiers for the WPC rating, don't we want to see WATTS output? Isn't there a way to translate, meter-wise, an average WATTAGE output in a meter?

Can't wait for the snarky responses, let 'em fly!

 

[rothwellaudio]

Decibels are a way of measuring ratios with respect to an arbitrarily chosen, or universally agreed, reference. Sound Pressure Level decibels (dB SPL) are referenced to an agreed level of sound which is just about audible.
Your VU meters aren't reading dB SPL. They're reading decibels with respect to a particular voltage within the amplifier. 0dB is when the signal is at that reference level. Signals below that reference level are -XdB. Signals above that reference level are +XdB.
There are various standards, such as dBm, dBV, dBu, dBSPL... You can look up those definitions if you're really keen.

 

[audiotemp]

For the rank consumer who buys fat amplifiers for the WPC rating, don't we want to see WATTS output? Isn't there a way to translate, meter-wise, an average WATTAGE output in a meter?

A VU meter is intended for targeting a signal to a a particular reference level. It's generally not very accurate for measuring other levels. A power meter is not a VU meter.

 

[toddalin]

I KNOW this has been discussed ad-nauseum on this site, so forgive me for asking again:

Can someone please explain, or direct me to a previous discussion, as to why my amplifier VU meters are scaled so that when I turn up the volume pot, the meter readings go UP from a -20 dB level to a 0 db level, with red (clipping?) beyond.

As a caveman, with the unit shut off, I'm hearing 0 db sound level. When I turn it on and turn the volume pot UP, I'm hearing (what I think is) LOUDER, or more, dBs.

What gives? For the rank consumer who buys fat amplifiers for the WPC rating, don't we want to see WATTS output? Isn't there a way to translate, meter-wise, an average WATTAGE output in a meter?

Can't wait for the snarky responses, let 'em fly!

0 dB means that there is 0 db (no) attenuation applied to the power/volume. -20 dB means that the power/volume is attenuated (reduced) by 20 dB or to one 100th of the power.

 

[Erik Tracy]

As I understand it, 0dB would correlate to rated output of the amp in question into a standard 8ohm load.

If the amp is rated at 100watts/channel, then 0dB on the meters would mean that the amp is outputting 100watts

You could then work backwards to get an estimate for watts against the meters bouncing to dBs
a 3dB difference 'down' would mean 1/2 the power, so -3dB would be 50 watts for the amp above
just keep halving the power for every 3dB down from there
-6dB = 25 watts
-9dB = 12.5 watts
-12dB = 6.2 watts
-15dB = 3.1 watts
etc

 

[highend64]

Decibels are a way of measuring ratios with respect to an arbitrarily chosen, or universally agreed, reference. Sound Pressure Level decibels (dB SPL) are referenced to an agreed level of sound which is just about audible.
Your VU meters aren't reading dB SPL. They're reading decibels with respect to a particular voltage within the amplifier. 0dB is when the signal is at that reference level. Signals below that reference level are -XdB. Signals above that reference level are +XdB.
There are various standards, such as dBm, dBV, dBu, dBSPL... You can look up those definitions if you're really keen.

And dBFS if digital.

 

[audiotemp]

As I understand it, 0dB would correlate to rated output of the amp in question into a standard 8ohm load.

If the amp is rated at 100watts/channel, then 0dB on the meters would mean that the amp is outputting 100watts

You could then work backwards to get an estimate for watts against the meters bouncing to dBs
a 3dB difference 'down' would mean 1/2 the power, so -3dB would be 50 watts for the amp above
just keep halving the power for every 3dB down from there
-6dB = 25 watts
-9dB = 12.5 watts
-12dB = 6.2 watts
-15dB = 3.1 watts
etc

Perhaps for a pure sine wave input. But for anything else, the "Watts" indicated will typically be neither peak Watts nor average Watts, even into an 8 Ohm load. And of course the speakers are not 8 Ohm resistors, so...

 

[WaynerN]

I build VU meters. They are calibrated with my Soundcraftsmen all band pink noise to 0 db. While the VU meter according to some may not be very accurate, I found a problem with a proto-type phono stage that had a throbbing rhythmic pattern to it, because it showed up as micro movements on the meters. They also let the user know that pretty much all is well. Most meters can be used as power meters as they usually have adjustable input levels, or you can put some resistance into the mix or both.

 

[b-rider]

As I understand it, 0dB would correlate to rated output of the amp in question into a standard 8ohm load.

If the amp is rated at 100watts/channel, then 0dB on the meters would mean that the amp is outputting 100watts

You could then work backwards to get an estimate for watts against the meters bouncing to dBs
a 3dB difference 'down' would mean 1/2 the power, so -3dB would be 50 watts for the amp above
just keep halving the power for every 3dB down from there
-6dB = 25 watts
-9dB = 12.5 watts
-12dB = 6.2 watts
-15dB = 3.1 watts
etc

THIS is GOLD, Jerry, GOLD! This is what I was looking for, and it synchs with the double-power/per 3dB SPL rise I've heard before. Thanks, Erik, I'm good!

 

[ymarko]

Got it.

 

[Sandman1221]

Got it.

Shot in the dark here, an amp rated for 100 watts puts out 100 watts without exceeding the THD rating. So my Marantz Model 50 is rated for 70 watts at 8 ohm, but only 100 watts at 4 ohm instead of 140 watts, because with a 4 ohm load above 100 watts the actual THD exceeds the specified THD rating. The Model 50 was tested and shown to put out 135 watts at 4 ohm, by a pro reviewer, but it then exceeded the rated THD.

So when you turn it up and meters go into the red zone, the actual THD is exceeding the rated THD, just as the output watts are exceeding the continuous watts RMS rating.

Just like the RPM dial on a car, it has a red zone, you can rev the engine into the red zone, but it is not recommended!

 

[twiiii]

It all depends on the manufacturer and what the rules they chose. Mcintosh first series of SS amps with Meters were originally calibrated with 0 db as being full power. They changed that to +3 db being full power to keep the amps from being over driven into hard clipping. ( MC 2105 & MC 2505). Later on the meters became more of watt meters displaying at first both watts and DB. Then later on the scales were expanded to 50 db and then to almost 60 db with watts being the primary reedings. Mac added time retention circuits so the human eye and brain could more accurately see what the peaks were shown by the meters. Some amps had accumulation selection , too, so that owner of the amp could see the heating be applied to the amp and to the speakers by the amplifier output voltage.

Here is a picture of my Meter from the left channel mono MC 830 SS power amp. Its rated at at 300 watts across 8 ohms and 480 across 4 ohms . The power guard turns on at 560 watts with 1% distortion at 560 watts continuous output at 1000 HZ. But playing great dynamic LP's or CD I have seen the amp meters swing over 600 watts with out the power guard light and circuits operating. The amp has about a 60 % peak capability. or 480 X 1.6 appox = 750 watts up to 40 milliseconds. As you can see from the photo the measurements are given in watts shown in 10 db increments. 0 being 600 for 4 ohms, then 60 watts would be -10 db and then 6 watts would be -20 db and so on. I normally run to some where between 6 and 30 watts for pop music and and between 0.6 watts and 120 watts for classical music. My live tapes and direct to disc Lp's and some very dynamic CD's can push 200 to 300 watts across 4 ohms. But for only very very short periods of time. My speakers are rated at 89 db 2 watts 1 meter across 4 ohms. But because they are line arrays they only drop around 4 db every time you double the difference. So 100 watts to each speaker gives me about 104 db at 4 meters which is considered classical music concert level and movie theater maximum film levels. So my amps and speakers have about 7.5 db left in reserve. The speakers have SS protection for the drivers which will shut down the drivers if over driven. About 564 watts for the each speaker for a continuous level depending on the the past heating effect of the previous signals. I have pegged the meters about 8 times very briefly from the famous Telarc recording of the 1812 Overture when the cannons are fired during the finale. But thats only two times. The first 4 blasts of the cannons are significantly higher than the rest of the of the blasts on the CD recording. When the Cellos start at the beginning of the piece the meters just barely move not touching the 6 number of the 6.0 milliwatt indicator reading. So thats over 60 db of dynamic range on that very unique recording. The previous famous Mercury LP recording was not as dynamic . Requiring less than 6 to 8 times the power of the CD. You can the tube filaments ring each time a cannon is fired for the Mercury LP.

 

[KentTeffeteller]

A VU meter is intended for targeting a signal to a a particular reference level. It's generally not very accurate for measuring other levels. A power meter is not a VU meter.

Also note true VU meters are not found in consumer audio equipment, with true VU meter ballistics. The meters in consumer equipment are more peak meters. True VU meters are more averaging, and slower response.

 

[Blue Shadow]

True VU meters are more averaging, and slower response.

The meters on my Crown OC-150a are exactly as you say, slower responding. But Crown built it with true VU meters reading in dB or peak reading in volts. I almost don’t even understand how to extract watts from their mathematical manipulations to get there.

Home gear has metering designed to entertain as much as inform.

 

[twiiii]

I was at a Syn aud-com school with Don And Carolyn Davis and during a break one morning he explained how professional VU meters with typical ballistics were in accurate by 18 db and is why professional tape recorders used +6db at 3% distortion as the reference point at 185 nWBs. Then Scotch and Ampex and TDk and Maxell started the tape wars. Where better signal to noise and more sensitive tapes were developed and where higher levels of signals could be used as the O level. I think the last time I calibrated a R2R tape machine it was at or above 524 nwb for 3% distortion at 15 ips for a 2 track machine. Any way old tube Ampex Professional machines could almost gain 12 db better signal to noise with tapes being offered in the 70's using AME equalization and maybe another 5 or 6 db using Dolby A. DBX deci-linear system would give close to 100 db signal to noise. We really didn't need digital. Can you imagine the sound of FM modulated audio Discs to replace LP's. That would have been the way to go.

 

[KentTeffeteller]

The meters on my Crown OC-150a are exactly as you say, slower responding. But Crown built it with true VU meters reading in dB or peak reading in volts. I almost don’t even understand how to extract watts from their mathematical manipulations to get there.

Home gear has metering designed to entertain as much as inform.

Very true. Crown is also true pro gear (and no nonsense). Agree home gear metering is designed to entertain as much as inform (for me more entertainment)