A Study on Crossover Alignment
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March 2025
When performing crossover alignment, it has become common practice to use a Dual FFT-based measurement tool to observe the phase responses of both the main and sub speakers and determine the appropriate delay that aligns their phases near the crossover frequency. In some cases, inverting the polarity of the subwoofer allows phase alignment with a minimal delay time. However, is using inverse polarity the correct approach? This article examines crossover alignment in an ideal system where the main and sub have perfectly matched phase characteristics when they are equidistant. The discussion focuses on various main and sub configurations, assuming an ideal phase response.
Note: The discussion considers only direct sound, as reflections complicate the conditions significantly.
The speaker system used has a crossover frequency of 90 Hz between the main and sub. When equidistant, a fourth-order Butterworth crossover ensures perfect phase alignment across the entire frequency range. At the crossover frequency, the level decreases by 3 dB relative to the out-of-band level. When the high-box and sub are combined, their levels increase by 6 dB at 90 Hz, making it 3 dB higher than the out-of-band levels.
1. Hi-box is half a wavelength away from the Sub at 90 Hz
When the high-box is positioned half a wavelength (5.5 ms delay) farther than the sub at 90 Hz, applying a 5.5 ms delay to the sub aligns their arrival times at the observation point. This configuration yields the same result as the equidistant setup when levels are matched.
What happens if the polarity is inverted? Since the distance difference is half a wavelength, polarity inversion aligns the phase at the crossover frequency. However, at frequencies above and below the crossover, phase deviation occurs increasingly with frequency. Due to the arrival time difference, there is a 180° phase shift at 180 Hz (twice 90 Hz). As a result, the level response is lower than when using delay to align timing, and at 160 Hz, the level drops by approximately 1 dB compared to the high-box alone.
2. Hi-box is one wavelength away from the Sub at 90 Hz
When the distance difference is one wavelength, phase aligns at the crossover frequency. However, phase deviation is even greater at other frequencies. At 135 Hz (1.5 times 90 Hz) and 45 Hz (half of 90 Hz), a 180° phase shift occurs. At 125 Hz, the level drops by more than 2 dB compared to the high-box alone.
Of course, if a one-wavelength delay is applied to the sub, there will be no phase deviation.
3. Hi-box is 1.5 wavelengths away from the Sub at 90 Hz
When the distance difference is 1.5 wavelengths, inverting the sub's polarity once again aligns the phase at the crossover frequency. However, the phase deviation becomes even more pronounced. At 120 Hz (4/3 times 90 Hz) and 60 Hz (2/3 times 90 Hz), a 180° phase shift occurs. Since the level difference between the high-box and sub is smaller near the crossover frequency, cancellation effects become significant, resulting in a level drop of more than 3 dB at 120 Hz compared to the high-box alone.
Conclusion
In real-world applications, reflections and considerations for the entire listening area make the situation more complex than this theoretical discussion. However, in cases where the high-box and subwoofer have well-matched phase characteristics, using delay to align their arrival times can be a viable approach to crossover alignment.
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