Figure 18 is a prime example of what you would hear if you stand exactly between two speakers playing the same signal (i.e. mono) with one speaker out of polarity. The bass will disappear. But, there will always be a difference in distance between you and the speakers due to the spacing of your two ears and probably a slight overall difference in distance between you and each speaker.
A difference in distance means a difference in the time arrival and thus there will be phase shifts between the sound from the two speakers. The amount of shift will vary with frequency. Because of the shorter wavelengths at high frequencies, the phase shifts allow most of the highs to be heard. They may be out of polarity but the effect is like what is shown in Figure 8.
Also, in a room you would also hear sound reflections from the floor, walls, and ceiling. You would only hear something like the red line in Figure 18 outdoors away from any reflective surfaces or in an anechoic chamber.
The small distance between your ears and any small difference in distance from you to each speaker do not cause appreciable phase shifts at low frequencies. This is because of the considerably larger wavelengths. The difference in your distance from each speaker might be only 1 inch (25 mm). However, the wavelength of even a 1 kHz sound is roughly 1 foot (300 mm) and at 100 Hz roughly 10 feet (3 m).
At the lower frequencies the polarity difference predominates because the phase shifts due to the difference in your distance from the speakers is very small compared to the wavelengths of the low frequencies. Thus the lower frequency signals, being nearly in phase but out of polarity, will cancel like in Figure 4. The lower the frequency the less the phase shift between the two speakers and the greater the cancellation.
A POLARITY / PHASE FIELD TRIP!
As with all physical exercise, check with your doctor first, who might not recommend you do this for some reason.
Find two railroad tracks, lie across them, and wait. Two trains, one on each track, come along. Both are right side up and both hit you at exactly the same time. The trains are in polarity and in phase.
The same thing happens again and both trains hit you at exactly the same time. However, this time one train is upside down. That is a polarity reversal.
The third time both trains are right side up but one hits you first and the other hits you shortly after the first. That is a phase shift.
The last time the second train is upside down and hits you later than the first. That is both a polarity reversal and a phase shift.
SUMMARY
So there you have it. Although this has only touched on a few areas concerning phase and polarity issues, it is hoped you better understand the difference between the two and a few of the effects of each. Remember that the audio frequency range covers wavelengths of over 30 feet (10 meters) at the lowest frequencies to less than an inch (under 25 mm) at the highest frequencies.
While a reversal of polarity will affect all frequencies identically, a difference in time arrival between two otherwise identical signals will have very different effects on the phase between them. The amount of phase shift will be different at different frequencies and this will depend on how much time difference there is between the arrival of the two signals.