Multiple Small Subs - Geddes Approach

[fredk1]

Originally posted by gedlee

Remember, I don't "cross" the mains to the subs, they overlap.

That was my initial impression, but comments by others along the way lead me to wonder.

The idea of overlap seems counterintuative. I would expect that each additional source in the overlap region would add to the perceived volume in that region. If there is an overlap between 80-120Hz, I would expect to see a hump in that area in any frequency response measurement. What am I missing?

Thank you for taking the time to post here and on other forums. This approach is ideal for those of us who have neither the money nor the space for the large powerful subs that are so popular today.

[Sheldon]

Originally posted by fredk1
The idea of overlap seems counterintuative. I would expect that each additional source in the overlap region would add to the perceived volume in that region. If there is an overlap between 80-120Hz, I would expect to see a hump in that area in any frequency response measurement. What am I missing?

You are doing a crossover. You are just using the natural roll off of the main speaker as the HP filter for the mains (a HP filter is a HP filter, doesn't matter how you get there - electronically or mechanically). You adjust the subs electronically to blend into that so that there is no hump (or at least none that you don't want). This is done by measuring the in-room response with the speakers sited in their use position.

Sheldon

[gedlee]

Sheldon is basically correct. There is no bump because its all taken into consideration. What people just don't seem to understand is that below about 200 Hz the room dominates everything. Concepts on paper just don't apply. You don't know what the HP of a main is just because you set it at 80 Hz. or what the LP of a sub is, or where its f3 is just because your tune it "thus and so". That stuff is all irrelavent for the most part. But if you place the subs and mains in the room and measure them in-situ then you do know precisely how they work "in the room!" You can then optimize the setup for multiple seats and multiple subs to get the smoothest reponse obtainable in that room, with those subs and mains.

[xpert]

Originally posted by gedlee


In it he shows how a mode in steady state will vibrate at the driven frequency even if its not precisely on that frequency, but, and this is the NON-minimum phase part, the decay occurs AT the resonant frequency. For example, lets say we have a mode at 51 Hz and we drive it at 49 Hz. In the steady state the enclosure has 49 Hz in the sound field, but when I turn off this source, the sound field slowly (relative to the decay rate) changes from 49 Hz to 51 Hz. No electronic system can do this and to call this kind of phenomina "minimum phase" is certainly not correct. Acoustics is not electronics and behaves quite differently.

Hi Earl,

What is written above would not stand a further analysis. A resonator is a resonator irregardless it is realized as a mechanic or electronic system. The clue here is that decay is a change of the systems state. That implies a broader spectrum of Fourier components than the initial spot frequency.

I'm convinced You just forgot about o'l Fourier. I'm with You, discussing all that basics with the an eagerness as people are in for world first class stereo is quite tiresome.

no offence

[xpert]

Originally posted by markus76
New spectrogram with findings that might surprise some people here. This time the DVD "Slumdog Millionaire" (chapter 4, H4 min)�C, L and R:
...

Best, Markus

Hi Markus,

Would You mind to check the windowing of Your spectrograph? Is it uniform or weighted (e/g Hanning), and how wide is it? Is it floating or stepping on the time axis?

thanks a lot

[markus76]

Hanning, size 32k

http://www.sonicvisualiser.org/doc/reference/1.5/en/

Best, Markus

[john k...]

Originally posted by xpert


Hi Earl,

What is written above would not stand a further analysis. A resonator is a resonator irregardless it is realized as a mechanic or electronic system. The clue here is that decay is a change of the systems state. That implies a broader spectrum of Fourier components than the initial spot frequency.

I'm convinced You just forgot about o'l Fourier. I'm with You, discussing all that basics with the an eagerness as people are in for world first class stereo is quite tiresome.

no offence

Absolutely agree. Any resonant system that has finite Q will oscillate at the driven frequency in steady state. When the input is removed the system will respond according to its natural (transient) response from the state it was in when the input was removed and decay, ultimately oscillating at its natural frequency, if the damping is small (i.e. the system rings). This does not preclude the system being minimum phase.

[gedlee]

Originally posted by xpert


Hi Earl,

What is written above would not stand a further analysis.

Originally posted by john k...


Absolutely agree. Any resonant system that has finite Q will oscillate at the driven frequency in steady state.

Well you are both wrong. Morse is one of the formost Physicists of the 20th century and doesn't get things like this wrong. Read it in his book - Morse, P.M., "Vibration and Sound" - last page of the book pg. 429 Do a "further analysis" if you dare! If you have the capability to do that! Show its wrong or recind your comments!

[john k...]

Originally posted by gedlee





Well you are both wrong. Morse is one of the formost Physicists of the 20th century and doesn't get things like this wrong. Read it in his book - Morse, P.M., "Vibration and Sound" - last page of the book pg. 429 Do a "further analysis" if you dare! If you have the capability to do that! Show its wrong or recind your comments!

Gees Earl, you said

In it he shows how a mode in steady state will vibrate at the driven frequency even if its not precisely on that frequency, but, and this is the NON-minimum phase part, the decay occurs AT the resonant frequency.

Now that is exactly what I said, except for the non-MP part BECAUSE MP has nothing to do with the system decay. MP is a steady state relationship between amplitude and phase. Decay is the transient part. Systems are either MP or non-MP. Two systems with identical amplitude response but one is MP and the other in non-MP will have different transient response. But any MP system that has an underdamped natural frequency will do exactly as I said. That a system oscillates at the driven frequency and decays to zero after the input is removed with potential osscilation at the system's natural frequency has nothing to do with MP or non MP.

[gedlee]

John

Your comment then wasn't clear because it sounded like you were objecting to the whole claim, but at any rate, I don't agree with your analysis, unfortunatelky I don't have time to refute it right now. Morse never used the term MP, he never does, no one in acoustics does, so I suppose my mistake was ever using it in the first place as I still contend, as I always have, that it is ill applied to acoustics.