Sloped baffle

"If an 18kHz sound left its' source and a 30Hz sound wave left its source at the same time. would they both get to the listener at the same time?"
Yep, its not the speed of frequency, its the speed of sound. All frequencies travel at around 1000 ft per second... I'd have to look it up to be exact, but it also varies by sea level. The difference is how many times a wave will hit you.

Let's assume we have a single dynamic cone speaker with a pass band of 35Hz to 20K Hz. Let's forget about high frequency beaming and cone breakup. Just assume this hypothetical speaker has a flat frequency response within its pass band, as measure on axis.

Bifwynne, I agree with Ngjockey here that if your hypothetical speaker has a flat freq response between 35Hz & 20KHz then all signals in this frequency region will pass thru minimally unaltered. That's the meaning of "pass band" - frequency passes thru minmimally altered. This, of course, means that in the 35Hz-20KHz the effect of the speaker coil moving inside the magnetic field poses no issues. So, there should be almost zero phase shift in the 35Hz-20KHz region.

Is there a frequency range where a speaker is phase coherent

yes, its phase coherent inside its pass-band. In the case of your hypothetical speaker it's phase coherent within 35Hz - 20KHz.

or does phase nonlinearity increase as a function of frequency ... period??

yes, it does. And, in the case of your hypothetical speaker, phase coherency degrades below 35Hz & above 20KHz both of which are outside the pass-band of the speaker/driver.

If the answers to all of these questions are -- yes, then it seems to me using 1st order X-overs and sloped baffles is at best a rough justice engineering response to a problem that is inherent with dynamic speakers that use voice coils.

Bifwynne, I'm not sure that you realize what the benefit is of using 1st-order x-over? The benefit of 1st-order x-over is that the PHASE DIFFERENCE (not talking about the absolute phase of a certain frequency) among all the signals in the audio band (20Hz-20KHz) is constant.
So, you have a music signal coming into the speaker. This music signal is a complex mixture of many frequencies. All these frequencies have some absolute phase that is different from each other. Further, each frequency has some non-zero phase difference with another frequency in this complex music signal. So, this whole complex music signal now goes into a time-coherent speaker as an electrical signal & comes out as a sound pressure wave. The phase difference amongst all the frequencies in this complex music signal do not change (i.e. remain the same) if the speaker used a 1st-order x-over. This means that the timbre & harmonic structure of the music remained unchanged as it passed thru the speaker. No other higher order x-over can achieve this i.e. higher order x-overs change the phase diference among the many frequencies of the music signal as it (music signal) passes thru these higher order x-overs.
So, ifffffff, the solution is a moving target (as you wrote) a time-coherent, first-order x-over speaker is the least damaging (IOW, the best compromise solution to a moving target problem).
hope that this helps some.....

07-06-14: Sounds_real_audio

I have a question. If an 18kHz sound left its' source and a 30Hz sound wave left its source at the same time. would they both get to the listener at the same time?

No, they would not.
If you have the driver creating the 18KHz signal & the other driver creating the 30Hz signal mounted on a perfectly vertical plane, the acoustical center of the 18KHz driver would be in front of the acoustical center of the 30Hz driver. Due to this, the 18KHz signal would get a head-start & would reach your ear 1st.
You hear this all the time at shows - the music is always "tipped up". You hear way too much high freq & the bass seems to be missing. The speakers are not time-coherent & often the drivers are not time-aligned.
If I remember Roy Johnson's paper, the woofer driver has a 90 degree phase lag in its pass band meaning that it starts to produce the 30Hz signal 1/4 wavelength of the x-over frequency later than the tweeter driver.
That's why you see sloped baffles with the tweeter on top - the furtherest away from the listener's ear. This aligns the acoustical centers of the tweeter, mid & woofer drivers to give them a chance to arrive at your ear at the same time.
hope this helps.....

Bombaywalla, yes, your post is responsive and I get it. I still wonder out loud whether speaker inductance as a function of frequency response in fact remains constant within the speaker's pass band. Indeed ... even if speaker inductance remains constant as a function of frequency, wouldn't that also impact phase coherency?

I gather from your prior posts that the answer is "no" as long as inductance doesn't change. Then there will be no impact on phase coherency. Instead, phase coherency is effected only when there is a change in X-over reactance, albeit whether it is capacitive or inductive.

Al ... if you're catching any of this, please chime in. I think this is an important issue. Put it to you this way, my sense is that even if proponents and opponents of the importance (or not) of phase coherence want to argue yay or nay on the issue, it seems to me that phase shifting can't be good factor ... at best neutral.

BIF

Bombaywalla, sounds_real_audio didn't ask if the were on a sloped front or a flat front, He simply asked, If they leave the source at the same time, would they end up at the listener at the same time...
I believe his real question is "do all frequencies move at the same speed"
I'm sorry if I mis understood the question, but as it was proposed, the answer is Yes.... I'm not try to start an argument, only to head off confusion.
Tim