On 11/14/2024 1:15 PM, Bob McGraw wrote:
I find the physical position of my head and ears related to the position
of the two speakers can produce a null in the sound, especially with a
tone.
Because it's a single tone, this is the same phenomena that causes what
we call "selective fading" on the AM band and picket fencing at VHF-UHF.
It's all about the travel time from the two speakers to our ears; if
we're dead center between them, the wavefronts from the two speakers add
because they're precisely in phase (zero degrees difference); as we move
closer to one than the other, the phase difference (measured in degrees)
increases and the don't add as perfectly and eventually begin cancelling
each other; when that difference reaches 180 degrees, they cancel
perfectly.
At radio frequencies, the fading between peaks and cancellations occurs
because we're receiving the signal over different paths, thus with
different travel times. The difference in speed at which the signal
peaks and dips between what happens at the AM BC frequencies and the
much higher VHF/UHF frequencies is wavelength.
In the world of pro audio, we were forced to learn the difference
between phase (a continuously valued function that varies both with
frequency and time) and POLARITY, which is REVERSING the signal, like
reversing a pair of wires carrying the signal, or running it through an
inverting gain stage. In the world of audio, the "signal" is not only
one frequency/wavelength, but thousands of them. So the cancellation or
addition varies from one note to the next. If were to plot that on a
graph, it would look like a comb pointing upward, and it can impart a
certain un-natural-ness to the sound.
73, Jim K9YC
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