80db + 78db = 85db WTF?

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JohnA

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I'm a bit perplexed about this one.
Martin logan pair playing on it's own registers 80db at the listening position.
Musical Fidelity pair playing on it's own registers 78db (on purpose, so it feels like all of the sound is coming from the Logans)
Both pairs playing together register 85db.

Why? If either one registered 80db, both should be 83db, right?
Now that one is below 80db, both should be 82db or even a tad less. Why is it so high?:confused:

Each pair is hooked to the same signal via totally separate amps.
 
JohnA said:
I'm a bit perplexed about this one.
Martin logan pair playing on it's own registers 80db at the listening position.
Musical Fidelity pair playing on it's own registers 78db (on purpose, so it feels like all of the sound is coming from the Logans)
Both pairs playing together register 85db.

Why? If either one registered 80db, both should be 83db, right?
Now that one is below 80db, both should be 82db or even a tad less. Why is it so high?:confused:

Each pair is hooked to the same signal via totally separate amps.
Click to expand...

Hola JohnA...I think that the add that you are doing is logarithmic (the sound is logarithmic), and not arithmetical...it is a very complex add to do, and that is why your results...don't worry about them, and enjoy your superb sound!, happy listening,
Roberto.
 
Last edited:
roberto said:
Hola JohnA...I think that the add that you are doing is logarithmic (the sound is logarithmich), and not arithmetical....
Click to expand...

Yes, 80db + 80db = 83db (doubling of power)

Here I get a lot more in real-life measurements, hence the question.
If it is 5db more with the second pair playing, then I'd need to readjust the surround speakers and the sub every time I add the second pair (for rock music only)
If it were less than 3db (as it should theoretically be) then I'd keep my standard surround settings.
 
I think you would be correct, if the sound was coming from the same source. Since there are multiple sources, at different positions, room acoustics come in to play. I think.
 
JohnA said:
I'm a bit perplexed about this one.
Martin logan pair playing on it's own registers 80db at the listening position.
Musical Fidelity pair playing on it's own registers 78db (on purpose, so it feels like all of the sound is coming from the Logans)
Both pairs playing together register 85db.

Why? If either one registered 80db, both should be 83db, right?
Now that one is below 80db, both should be 82db or even a tad less. Why is it so high?:confused:

Each pair is hooked to the same signal via totally separate amps.
Click to expand...

See my earlier post. If you measured the sound with a SPM than the meter may be poorly calibrated or you measured at a room mode peak. Physically it is impossible for the sound pressure to change more than three dB from the combination of two sound sources regardless of their levels. If you average a number of responses around the room you'll find that the SPL should be very close to 82.1 dB. I'm not clear if the dipolar nature of MLs should change the calculation, but I don't think so.
 
Are you playing exactly the same music at the same time? In that case, I'd say you are experiencing room acoustics - waves are combining and the amplitude is increasing accordingly. Try playing pink noise through each system and you might well see results that match the theory more closely.
 
amey01 said:
Are you playing exactly the same music at the same time? In that case, I'd say you are experiencing room acoustics - waves are combining and the amplitude is increasing accordingly. Try playing pink noise through each system and you might well see results that match the theory more closely.
Click to expand...
Yes, of course it is the same music, in fact the 1KHz tone from the 1st Stereophile test disk.
 
risabet said:
...or you measured at a room mode peak. ...
Click to expand...
That is my fear:eek: that my listening position is at a room mode peak.

Due to the shape and orientation of the room I can't really change it, and the sofa is against a wall (a big no-no if we want to avoid combing effects and room modes:eek:)

Scary...
 
JohnA said:
That is my fear:eek: that my listening position is at a room mode peak.

Due to the shape and orientation of the room I can't really change it, and the sofa is against a wall (a big no-no if we want to avoid combing effects and room modes:eek:)

Scary...
Click to expand...

No - when you're playing the same music at the same time the waves are combining - increasing the amplitude. Try equalised pink noise. Let me know the results. I'm very interested........
 
amey01 said:
No - when you're playing the same music at the same time the waves are combining - increasing the amplitude. Try equalised pink noise. Let me know the results. I'm very interested........
Click to expand...

yes, that's what this article (blog, whatever..) says.
I'm not exactly sure how twice the number of sources would produce 3db or 6db more depending on waves combining. They are all very close to each other and are in phase anyway.

I'll try pink noise and different frequencies as well, I am intrigued now
 
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