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Old 3rd January 2007, 12:52 AM   #41
Wizard of Kelts
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Join Date: Sep 2001
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Default Re: Crossovers and response deviations

Quote:
Originally posted by forr
A few months ago, I experienced what Kelticwizard suggested in post #4 :
--- This is because once one driver is outputting 12 dB less than the other in a speaker system, it is considered as not being able to be heard. Just for an experiment, you can try this yourself with your computer speakers. . Download a freeware tone generator from David Taylor's website here: www.satsignal.net => Audio Tools. Choose any convenient frequency-say 500 Hz. Fill in 500 Hz in both boxes where is says sweep frequency. Set the right speaker at 0 dB, the left speaker at 12 dB down. Then check and uncheck the left speaker to see if you can detect when it is playing and when it is not. It will be hard to tell a difference.---
That is indeed what I wrote.



Quote:
Originally posted by forr
But I have done it entirely with analog systems, sine signals of different frequencies feeding :
- two similar speakers through a twin channel amplifier on which I slowly vary the volume control on one channel
- one speaker through a mono-amplifier which input received a mix of two same sinewave, one set at different levels.
- same as above but with headphones.
This was done at comfortable levels to avoid well known loudness effects. All three arrangements gave similar results. I encourage anybody to duplicate one of them.

I wanted to check the just noticeable differences of levels which can be detected. This should decide which crossovers to use in an uncompromised system.


The result of my experiences are in total disagreement with 12 dB differences said to be just noticeable. Whatever the frequency, I noticed differences vanish only when the weak signal is at a level about -30 dB under the main signal.
Once again, I applaud your effort in taking this further, as the answer will inform us all. If I am wrong, I certainly do not mind being shown where I went wrong.

However, I am afraid that you have gone off-track here, from my understanding of your post. What you have seen to have proven is that if two different frequencies are being played, there has to be a much, much greater difference than 12 dB for one frequency to drown out the other.

I agree with this completely. If a mere 12 dB was enough for one frequency to drown out a different frequency, then there would be no use for music sources with a dynamic range greater than 12 db, let alone 60 dB, (phono) or 100 dB (CD). I think we all can agree that we do indeed need music sources with a dynamic range greater than 12 dB-or even 30 dB.

However, we are dealing with the question of not the decibel difference where one frequency drowns out a different frequency, but what decibel difference is required for one source to drown out a second adjacent source playing the same frequency.

Both the woofer and the tweeter are playing the same frequency, or combination of frequencies, throughout the crossover region. It is just at various points throughout the crossover region, each are playing them at different decibel levels from the other, (except at the crossover frequency, when both are playing equally). For this reason, I fail to see why the point where one frequency is at such a decibel level that it drowns out a different frequency is relevant to this crossover issue. What is relevant is the decibel difference where one source drowns out a second adjacent source playing the same frequency.

For that purpose, I would propose using the decibel difference where one cannot tell when the softer source is switched on or off.
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