This is just slightly off topic but I need someone smarter than me to answer this. Is there any truth to the story that two low-frequency sound sources placed in close proximity can begin to cancel each other out?
Another "yes and no" reply.
Everything in audio is about TIME. Time is a measure of chronology - it's 5 seconds later than when I started typing this sentence; a measure of *distance* because sound waves, generated by time, have a physical wave length; and time is a measure of cyclical events - like frequency
In a general sense, loudspeakers placed less than 1/2 wavelength, using the *acoustic centers* of the loudspeakers, will tend to combine in SPL. The question is "less than 1/2 of WHAT wavelength".
This leads us to the
Phase Wheel, and as you travel clockwise around it, at 3 o'clock you're 90 degrees out of
phase, at 6 o'clock you're 180 degrees out of
phase (full cancellation) and at 9 o'clock you're 270 degrees out of
phase, but coming back to 360 degrees, where you'll be in
phase but 1 period late.
https://books.google.com/books?id=MOnKiy8qRt4C&pg=PT82&lpg=PT82&dq=phase+wheel+wavelength&source=bl&ots=higSmUiXf_&sig=Dqd_C13_o7l9Y06HVSWJfFn4Whg&hl=en&sa=X&ved=0ahUKEwjt0MKLy5bYAhUN1mMKHVyqAXEQ6AEIPTAG#v=onepage&q=phase wheel wavelength&f=false
Those pages are from Bob McCarthy's "Sound Systems - Design and Optimization" (hint - the gifting holidays are fast approaching); not a simple read but an invaluable resource.
At any rate the above should lead you to this conclusion - most of us have been using them "
phase" to describe a DC phenomena called "
POLARITY".
Polarity is absolute, regardless of frequency (hence the "DC phenomena" moniker) - if you flip a dual
banana plug around you change
polarity.
PHASE is the relationship of 2 wave forms in time, relative to each other. A change in frequeny will, by definition, change the
phase relationship if both wave forms do not remain the same. Google for "summation of sine waves" as
phase relationships are the heart of summation and you'll find some nice illustrations.
So back to your question about cancellations - what frequencies are your worried about? We can safely eliminate frequencies above 100Hz (for the most part). If you're concerned about 60Hz (the thud part of a kick drum sound), then you'd want to find the wave length of 60Hz, then make sure your subs are less than 1/4 wavelength from any boundary (reflective) surface because once it reflects, the distance is doubled and you're now 1/2 wavelength LATE, which will create a cancellation when the acoustic energy combines. If you separate the subs by 1 wavelength or more, you will get the "
subwoofer power alley" down the middle of the room, with radially symmetrical nulls of cancellation and lobes of summation. Note this happens whether the subs are flown or on the
ground, they only need to be separated by distance. Further note this occurs in both the horizontal plane and vertical plane and will happen if you have both flown subs and subs on the floor (turn your
power alley sideways
).
Probably more info than necessary, but I hope this helps. If we need to continue this topic I suggest a new thread.
Have fun, good luck.
Tim Mc