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The Modern Analyzer, Part 2

May 30, 2014 2:19 PM, By Bob McCarthy

Understanding the transfer function analysis.

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Figure 4: An introduction to coherence as applied to sound system optimization. See larger image.

Putting the Transfer Function Analyzer to Work

Transfer function amplitude answers the question of gain or loss both as a whole and over frequency. Phase answers the question of what frequency arrived when. The impulse response can precisely identify direct sound and reflection timings. Coherence tells us whether our data is contaminated by noise.

The impulse response is the tool for setting delays, and tells us what surfaces are causing the most trouble. Hopefully we will be allowed to treat them. The phase response is the key for joining speakers together at crossover—subs to mains, mid-drivers to HF drivers, and more. Phase response also reveals the compatibility between different speaker models. Maybe they meet at the highs but what about the mids? There are specialized phase filters that can help with this.

Transfer function amplitude helps us find the best aim, splay angles, and spacings. We measure the level on axis and then compare it to the level at the other key locations. For a single speaker we compare the on-axis response to the outer edges of coverage and move the speaker until minimum differences are found. For splay angle, we compare the individual on axis levels with that at the intersection between the speakers. The angle can be adjusted to minimize the difference. Spacing is done much the same way. Move the speakers apart until the level at the mid-point matches the level on axis to the individuals.

Equalization uses the coherence function to select which frequencies are good candidates for equalization and which are better off trying other solutions. Transfer function amplitude then shows us the peaks and dips that need treatment and how well they have been equalized.

Hopefully this two-part article will help you to get the best out of your FFT analyzer. Much more information on this topic can be found in my book Sound Systems: Design and Optimization on Focal Press.

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