A thought just occurred to me. When you are using speakers, the sound from the left speaker will affect the sound from the right speaker, so if the same signal is present in both channels but it's delayed in one, the phase difference can cause the signals to be added or subtracted.

In the "sweet spot" between the two speakers, if the two channels are out of phase, you will hear certain frequencies being amplified or attenuated.

If I am listening with headphones, I can hear the signals independently in each ear, so even if I'm listening to two sine waves that are 180 degrees out of phase, I will be able to hear both independently, and I will interpret the difference in phase as a delay that will effect my spatial imaging of the sound.

If I were listening with speakers instead, wouldn't this same situation with two sine waves that are 180 degrees out of phase cause the sound to be completely attenuated (depending on where I am seated in the room)? Does this mean that mixing with headphones can lead me to overlook some issues that I would normally hear with speakers?

Sorry, I guess this isn't really a PD-specific question; it's more of a general audio production thing that will be important no matter what applications I'm using.

You should try it for yourself. The phase-inverted signals coming from two speakers will not vanish in most cases, but listening to them on the cans will sound very different, usually like the sound is coming from behind your head. Also, check some HRTF localization tricks (like the famous barbershop clip) - they work only on cans because they use phase shifting.

I don't think they would be completely attenuated because even at the sweet spot, because your ears are two separate receivers. If you stuck a mic at the sweet spot, you'd hear it more because they would mix at the exact same spot. Some interference is likely to happen, of course. After all, each ear will pick up something from both speakers. But it's not as dramatic as summing two out-of-phase signals to a mono one.

You should always check your mix on both, anyway. You can definitely hear things in one that you can't hear in the other. I've personally found that mixing the low end on headphones is unreliable (I've been burned several times by this) and that ear fatigue can creep in faster.

Thanks for your insight. I had never heard that barbershop clip, but I listened to it earlier and it was really interesting how well the recording worked. I guess it's really just important to test and to listen extensively using different equipment if possible before making any final judgments.