| halfgaar |
Hi,
I've been wondering for a while what the correct gain drop is when playing a mono channel over two speakers, maintaining the same SPL; -3 dB or -6 dB.
Dolby specifies a gain of .707 (-3 dB) when mixing a mono channel in two channels. But, when you were to sum that with a standard summer, the result would be louder than the original.
The power consumption of the two speakers when playing the source with -3 dB per channel is the same as playing the original source on one speaker. But then there's something about increased efficiency I don't know how to factor in.
So, which one is it? |
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| dangus |
I think you'll only see that increased efficiency if both speakers are very close together.
It should be easy to verify with an SPL meter and a test disc. |
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| halfgaar |
I tried to use an SPL meter, but this room is not accousticly dead, so that's kind of difficult. Moving the meter a few CM and the reading is different. I tried with non-directional sounds as well, like 30 Hz, and with that, the correct drop was -6 dB.
But, I can vaguely remember something about low frequencies requiring -6 dB and mid to high -3 dB. Perhaps this has to do with what you say, that the speakers need to be very close to eachother to see increased efficiency, except for low frequencies. |
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| AndrewT |
Hi,
I would expect +3db when you sum the two channels.
So you should insert -3db on each signal before you sum or -3db on the output after the sum.
The efficiency calc only works at +3db when the wavelength to be reproduced is greater than 4times the distance between the speakers.
If you had a pair of speakers mounted beside each other and they measure 500mm across the pair of cones then the efficiency increase is for frequencies below 340/4/0.5<170Hz.
There is progresively less gain between 170Hz and 340Hz when all the efficiency gain is lost.
However, another factor comes into play. Beaming due to interference effects throws more sound forward and can sound louder even though total sound power is the same. This lobing is very uneven and will destroy the stereo effect (and the dual mono) |
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| halfgaar |
When summing two signals, the output voltage becomes the two inputs combined, right? So, summing two 3V peak-to-peak signals becomes 6V peak-to-peak. Twice the amplitude is +6dB, so why do you expect + 3 dB?
Something that is related to this: in a mixing panel, when you pan a signal from left to right (keeping the SPL intact), how much gain difference is there between center and 100% left? |
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| AndrewT |
Hi,
I think a mixing desk only adjusts the relative volume when doing it's psuedo pan.
It should have the phase corrected as well to try to maintain the true stereo (location) information, but I don't think they do that.
A digital desk may do more clever processing. |
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