Comb filter effects are a popular topic of discussion among audiophiles (and many others, of course). However, some audiophiles may not have a good concept of what they look like, why they may (or may not) be bad, and what they can do about them. Or even if they have them and need to do anything about them.
Picture the signal from the speaker as a single sine wave. Two, speakers, two sound waves. If they are in phase, then adding them together simply doubles the amplitude at every point. If you have two speakers playing the same signal at the same level, and are equidistant from them, then you will hear the signal twice as loudly as if a single speaker was playing. Or will you? It’s complicated…
The figure below shows two 1 kHz sine waves generated by two speakers, but one is 3” further away from the listener. The red line is from the closer speaker and the dashed blue line is the signal from the farther speaker. Clearly there is a difference in phase between the two signals, and if we add them together at each point in time (t is relative in this plot), we will not get a doubling in amplitude.
Remember wavelength is related to frequency and lower frequencies have longer wavelength. If we change the frequency to 100 Hz, there is less phase shift, since the distance is smaller compared to the wavelength. Their sum would still not quite double, but it would be close.
Take that same 3” difference and look at 2 kHz, and now the waves are nearly 180 degrees shifted in phase. Add them together and they will nearly completely cancel. Ouch!
So, if there is a path difference between two speakers, and we sum their outputs at the listening position, what we hear depends upon the difference in distance and the frequency. The figure below shows three plots: red is two identical 1 kHz signals summed from two equidistant speakers, dotted blue is the result when one speaker is 3” farther away, and dashed brown is the result with the same 3” difference in distance but at 2 kHz. You can clearly see how the amplitude changes.
(To be continued...)
Picture the signal from the speaker as a single sine wave. Two, speakers, two sound waves. If they are in phase, then adding them together simply doubles the amplitude at every point. If you have two speakers playing the same signal at the same level, and are equidistant from them, then you will hear the signal twice as loudly as if a single speaker was playing. Or will you? It’s complicated…
The figure below shows two 1 kHz sine waves generated by two speakers, but one is 3” further away from the listener. The red line is from the closer speaker and the dashed blue line is the signal from the farther speaker. Clearly there is a difference in phase between the two signals, and if we add them together at each point in time (t is relative in this plot), we will not get a doubling in amplitude.
Remember wavelength is related to frequency and lower frequencies have longer wavelength. If we change the frequency to 100 Hz, there is less phase shift, since the distance is smaller compared to the wavelength. Their sum would still not quite double, but it would be close.
Take that same 3” difference and look at 2 kHz, and now the waves are nearly 180 degrees shifted in phase. Add them together and they will nearly completely cancel. Ouch!
So, if there is a path difference between two speakers, and we sum their outputs at the listening position, what we hear depends upon the difference in distance and the frequency. The figure below shows three plots: red is two identical 1 kHz signals summed from two equidistant speakers, dotted blue is the result when one speaker is 3” farther away, and dashed brown is the result with the same 3” difference in distance but at 2 kHz. You can clearly see how the amplitude changes.
(To be continued...)