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Author Topic: possible to find relationship between physical levels and electronic levels?  (Read 2148 times)

stickman

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ok odd and perhaps idiotic question:

is it at all possible to convert from electronic dB levels to physical SPL levels?

my thinking is that the only way it can be done is with some kind of calibration method. say, a reference tone before the actual recording that had a known level. could it also be done if the gain and levels were known at the input of the recording by way of some kind of unity output? any maths or experimental trickery is ok, just is it possible?


also, is electronic dynamic range related to physical dynamic range in SPL?
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Ethan Winer

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> is it at all possible to convert from electronic dB levels to physical SPL levels? <

Sure - get a $50 Radio Shack SPL meter. If you want more accuracy there are better units available, though of course they cost more. Now, I'm sure your next question is how does one calibrate such a device? Very Happy

> is electronic dynamic range related to physical dynamic range in SPL? <

Yes, it's a one-to-one relation. If you double the voltage coming out of a preamp the electrical signal increases by 6 dB, and so does the sound from the loudspeaker.

--Ethan

stickman

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i have good SLM's... what i meant was, what is the best process for finding out what the physical SPL levels were before they went to tape if all you have is the tape to go by?

does a calibration tone have to also be recorded at a pre-defined level so that there is a reference point or is there other ways?


Quote:

> is electronic dynamic range related to physical dynamic range in SPL? <

Yes, it's a one-to-one relation. If you double the voltage coming out of a preamp the electrical signal increases by 6 dB, and so does the sound from the loudspeaker.



so what your saying is, if i have 40dB of dynamic range in the computer (dBFS) then the SPL dynamic range was also 40dB?
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Ethan Winer

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stickman wrote on Wed, 01 November 2006 20:35

what is the best process for finding out what the physical SPL levels were before they went to tape if all you have is the tape to go by?


In that case there's no way to determine the original SPL.

Quote:

so what your saying is, if i have 40dB of dynamic range in the computer (dBFS) then the SPL dynamic range was also 40dB?


If you have only 40 dB dynamic range in your computer, you need a new sound card! Very Happy

Seriously, again there's no way to know. I can tell you this: Whenever I've recorded a live orchestra concert the ambient noise in the room has always been louder than the inherent backgroun noise of my DAT recorder. So from that you can tell that the recording medium has more dynamic range than the orchestra did in the venue. But that's about all you can know for sure.

--Ethan

stickman

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thanks ethan, i see your point but perhaps there is still some confusion as to what i'm saying. i will use an example:


1. say you had a source on tape/disk that varied between a -5dB and -20 dBFS signal - just say you did. without knowing the actual SPL levels that went to tape, would this information at least still represent a 15dB dynamic range in the original SPL?

for example: the SPL might have been between 60dB and 75dB if it was measured with an SLM originally?

so what i am saying is does a 1dB SPL difference = a 1dB difference on tape/disk. if not, is there some calculation that tells me the difference between a 1dB difference on tape in relation to that difference in SPL?

therefore, i am not talking absolute levels here, just ratios i guess.


2. back to absolute levels, i understand that there is no way to know the SPL levels that went to tape, but i still imagine it could be done with some kind of calibration tone or something that had a pre-defined level? would this be possible and if so, how?  
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Ethan Winer

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> without knowing the actual SPL levels that went to tape, would this information at least still represent a 15dB dynamic range in the original SPL? <

Yes, as I already explained there's a one-to-one relation.

> i still imagine it could be done with some kind of calibration tone or something that had a pre-defined level? <

Right. You get a calibrated SPL meter and play something, then read the meter and say into the microphone "68 dB" or whatever.

--Ethan

stickman

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Quote:

Right. You get a calibrated SPL meter and play something, then read the meter and say into the microphone "68 dB" or whatever


ah, i see. but what i mean is this (sorry to keep bringing it back up):

you have an electronic calibration tone on tape/disk. then the actually recording follows. is there a way to convert that electronic calibration tone back to some reference SPL level so that then the actual signal that follows can be known also?

so say you had a 1k tone at the start of the tape at 0VU and then the recording comes after.

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franman

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I'm sorry guys.. this thread is just silly... can you explain the purpose for wanting to convert things on tape back to their original SPL levels??

Set and use a reference monitor level, and don't move your volume knob......
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stickman

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i thought this place was to ask silly questions?!?

it's really not the silly anyway as i see it. programs like smaartlive wouldn't exist without these types of needs.

all i am saying is i understand how there is a need to calibrate a microphone through audio hardware and software to get a reference level (because gain and other parameters could not be known). all i was wondering was if there was a way to calculate a known VU reference tone into a known SPL level. then anything that came after the tone would be known?

not really that silly. could be quite useful in playback of recordings where an estimation of SPL was needed. guess not though.
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