[Me]

Hi everyone. 

I hope I'm posting this in the right spot.

It is my understanding that a speaker with multiple drivers each doing their own thing (tweet, mid, woof, etc.) can be phased so that the the drivers all produce sound at the same time.

Quick question:  This is only possible at either a low volume or at a volume where all the drivers are phased to work in unison, right?  Once a speaker hits higher volumes, the drivers would no longer be working in unison, therefore causing the drivers to cancel or reinforce each other.  The Frequency response of the speaker would always be changing, right?

Is it possible to create an active filter that adjusts itself so that the drivers are always in phase, regardless of volume?

Maybe if someone were to use drivers, all electromagnetically driven, hook them up to an active filter that adjusts power output to the electromagnets depending on volume (or something else), maybe the phasing could be consistent. 

Also, when a speaker diaphragm moves forward,  it compresses a certain volume of air in front of it, creating the sound wave.  If one were to try to imagine what the sound wave looked like, what would it look like?  How deep would the wave be (from front to back), and does that matter?  Is it the front of a sound wave that is produced that matters in phasing or is it somewhere else?  Does this make Any SENSE???

Thanks.

[*Scotty*]

The SPL that a speaker system is operated at does not affect the phasing between the drivers assuming that the drivers are operated within their linear region, the inter-driver phase relationship is set by the physical placement of the drivers on the baffle and the cross-over design. An active cross-over could be designed to compensate for the drivers placement on the baffle and the drivers electrical characteristics. Dunlavey and Thiel loudspeaker brands had passive cross-overs that produced a phase coherent wave-front at a predetermined point in space in front of the loudspeaker. Dunlavey achieved this by applying his expertise in antenna design to build a phased array which focused at precise location where the listener was intended to be seated.  The phase coherency of the speaker would be dependent on the listeners location relative to the loudspeaker. Because there is more than one driver on the loudspeakers baffle the summed output of the multiple drivers will only be phase correct at one location in space in the front the speaker system. A digital crossover could theoretically produce a phase coherent wavefront at any location in front of the speaker system by adjusting the phase relationship between the left and right speakers.
Scotty

[JohnR]

The volume level won't change whether the "drivers are working in unison", bar gross distortion. Apparently the Thiele-Small parameters are not constant with excursion but that doesn't seem to be what you are asking about here.

If you could see the sound wave, it would look like ripples moving through the air. From the source it expands outwards, like when a pebble is dropped into a still pond. At low frequencies and at a distance from the driver, it expands outwards in a sphere (or it would if hanging from a crane in mid-air). At higher frequencies, the baffle "limits" the wave to a hemisphere, speaking very roughly, and/or the size of the driver makes the waveform more like a cone. Is that what you are asking?

[Edit: cross-posted with scotty]

[Me]

To Scotty & JohnR:  I appreciate the response, makes sense.

To JohnR:  I'm talking about the physical characteristics of a single sound wave, and not the propagation of the wave or waves.  Is it the front of the waves that get measured, or is it somewhere within the center of the wave?  Would the air density of the sound wave be greater not at the front, but more towards the center? 

Please correct me if I am wrong (since I'm guessing), but wouldn't a lower frequency and or higher spl sound wave create a deeper wave from front to back compared to a higher frequency and or lower spl wave?  Does this need to be taken into consideration when trying to get multiple drivers to be phase coherent?

If this still doesn't make sense, please let me know, and I will try to explain more clearly.

[JohnR]

The SPL that you measure is the variation in pressure at the location of the microphone. The SPL (in the absence of a room or other reflections) drops by 6 dB each time you double the distance from the source (assuming it can be treated as a point source).

Lower frequencies have a longer wavelength, higher frequencies have a shorter wavelength. However there is no direct connection between wavelength and SPL.

[Me]

Ok, I see what you are saying. Thank you.