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db conversion makes doubt

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mic5

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i got the value of sound in watts.
How can i do to make it in db?

whether the constant multiplier 4.343 is correct ?

but a calculator value mismatched with multiplied value.(log value)

which is correct to convert a number in db?
 

MikeMl

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audioguru

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First you need to know how many real RMS Watts are peoduced by your amplifier into the exact impedance of your loudspeakers. Many amplifier manufacturers quote phoney Whats instead of real Watts.

Then you must measure the distance from your speakers to your sound level meter and subtract dB's for the distance. The acoustics of the room affects certain frequencies.

Then you calculate the sound pressure level by using the spec'd sensitivity of your speakers.
 

mic5

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thanks for your response! i think of it.
 

bountyhunter

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bountyhunter

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First you need to know how many real RMS Watts are peoduced by your amplifier into the exact impedance of your loudspeakers. Many amplifier manufacturers quote phoney Whats instead of real Watts.

Then you must measure the distance from your speakers to your sound level meter and subtract dB's for the distance. The acoustics of the room affects certain frequencies.

Then you calculate the sound pressure level by using the spec'd sensitivity of your speakers.

It's been a couple of decades, but what stuck in my brain is that if you know actual acoustic power, you convert to dBA by ratioing the acoustic power to one micro Watt, then doing the Ten log thing. I seem to recall that one micro Watt was the reference for the dBA scale, but you do have to use acoustic power (not amplifier power or anything else). Anyway, a scale of determined dB values has to be referenced to something since dBs are (by definition) a ratio od one thing to another thing.
 
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audioguru

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Never mind dBA which uses A-weighing. A-weighing is a filter used to remove most low frequencies and some high frequencies from low level sounds because our hearing's sensitivity does that.
At loud levels the filter should not be used because the response of our hearing is almost flat.
 
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bountyhunter

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dBA is not only specific frequency weighting, it has to be magnitude referenced so that all dBA meters will read the same loudness values for a specific sound level. That means there has to be a reference value to set the 0dB point on the scale.
 
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bountyhunter

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Never mind dBA which uses A-weighing. A-weighing is a filter used to remove most low frequencies and some high frequencies from low level sounds because our hearing's sensitivity does that.
At loud levels the filter should not be used because the response of our hearing is almost flat.
That's not really accurate. There is still a heavy frequency change for human hearing even at relatively loud levels of SPL. Attached is the standard "equal loudness" curve for human hearing I have always seen used for reference. You see large magnitude changes from low frequency to high even at the highest SPL level on the chart which is 120 dB. That is a pretty loud level, most people would not like to hear it for long. At SPL levels which are nearly deafening, the frequency contour does continue to get flatter but it has to be dangerously loud.

Equal Loudness Curves
 

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audioguru

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I don't know whose curves are correct.
 

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bountyhunter

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The one on the right looks to me like an approximated copy of the real one, but the one on the right still shows a 20dB difference between lowest frequency and the SPL at 4K Hz so there is still a significant variation with frequency at 120 dB.

I remember from back in school (35 years ago) seeing the ones on the left in our book except the family of curves extended upward to higher SPLs like 140 - 150 range and those were getting flatter. I remember asking the teacher how they got volunteers for the testing at those levels because their ears would probably be damaged. I looked yesterday and I didn't see any curves above 120 posted anywhere anymore probably because they aould be too dangerous to take the data.
 
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Externet

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mic5: Find out the efficiency of your speaker. It will be a figure like 90dB at 1 metre at 1 Watt.

If your amplifier is delivering 50 Watts into such speaker, assuming 90 dB SPL as its efficiency , the SPL at 50 Watts is

90 + (10 log 50) = 90 + (10x1.7) = 90 + 17 = 117 dB SPL at 1 metre distance.
 
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