Subwoofer Phase Delay and Equalization
III. Phase / Delay Adjustment
9. You may find that adjusting the phase setting (typically done at the subs themselves or at the processor feeding them, if necessary) further smoothes an otherwise uneven main/sub splice. Experiment will quickly show whether an improvement occurs or not: you may hear more or less bass for a given phase setting. As simplistic as that sounds, altering phase has a truly complex effect on the perceived characteristics of a room's acoustic and thus the sound you perceive at the listening position. Making phase adjustments must be approached with the understanding it can do as much harm as good.
Having said that, with you listening from a variety of positions within the seating area, have an assistant, adjust the phase or polarity setting for one sub (leaving the other off)and listen for what setting sounds best. Then do likewise for the other sub, with the first one shutoff. Finally, with both subs on, adjust further, but only if necessary. You may find repeating this test with a variety of test media of further help.
Although delay adjustment can serve a variety of purposes, I include it with phase adjustment simply because as you alter delay, you alter phase. Delay can also be thought of as an electronic means of establishing subwoofer placement symmetry or minimizing path differences between the subs and the listening position.
Delay can effectively be used for a variety of purposes, in both consumer and pro audio. Some find that ignoring phase settings altogether and focusing instead on delay adjustment works best. In the context of subwoofer integration, delay can be used to ensure that the direct sound from both subs & mains arrives at the listening position at the same time. Knowing the speed of sound, you can quickly determine the time of flight differences (and thus the amount of delay) between signals for many subwoofer arrangements, but how would you calculate for arrangements such as those showing in Fig 39a?
For that particular case, assume the front sub & LCR are arranged (or calibrated) such that their signals arrive at the seating position simultaneously. Calculate the time (in ms) or distance (in feet or meters) of flight from the front sub & LCR locations to the listening position as well as that from the rear sub to the listening position. Subtract the latter from the former, then apply the resulting delay value to the rear sub.
For example, suppose your LCR main/front sub cluster is 5 meters from the listening position and the rear sub is 1meter from the listening position. 5 m - 1m gives a flight distance difference of 4 meters. You would therefore need 4 meters delay on the rear sub. In that way, direct sound from the LCR main/front sub cluster and the rear sub reach the listening position at the same time.
10. Equalization. Approach this one with caution: it can create as many sonic problems as it solves, especially when misapplied. There are some things this processor can do very well and some it does poorly. And there are things it simply can't do at all. Be aware of its limits and remember, it's the direct sound of the subwoofer you're equalizing, not the room!
Before proceeding, here are a few cautions worth keeping in mind. An equalizer is a frequency-domain electrical device that cannot correct time-domain acoustical problems. As well, no analog EQ can correct for steep, high-Q dips in a response, so you'll be focusing on flattening or otherwise lowering any response peaks.
Over the years I've seen a variety of techniques used for setting up an equalizer without any sort of measurement gear at hand. Here's one of the more efficient approaches I've seen. I'll assume for this segment of the set up process you're using a parametric equalizer. A parametric equalizer (PEQ) is preferable when fine-tuning your sub's performance as you can adjust gain, Q (or reciprocally, Bandwidth) and frequency. Together, they allow you to more effectively target those portions of the low frequency audio spectrum that need alteration.
By this point in the setup process, you've already done quite a lot that hopefully has improved the integration of your subs with your system's mains. You've gotten to know the space quite well and if you've kept notes along the way it should be clear at this point what acoustical problems remain. With your PEQ bands zeroed out, begin playback of your test material and listen for any noticeable remaining peaks in response. (If needed, repeat this test for every seat in your listening area). If you haven't already, at this point you'll likely identify one particularly noticeable resonance, followed by a few others, present, but less pronounced.
We'll focus on the most noticeable resonance first. This part of the procedure should be done with both subs & the mains on. Select an appropriate band pot and dial (or key in) in a modest boost, say +3 to +4dB, and a fairly sharp Q. Then slowly sweep up, then down, the frequency scale; you're listening for that frequency where the boost is most noticeable. You may have to do this a few times.
Once that frequency of max. boost effect has been identified, you've targeted the resonance frequency. Slowly lower the gain at that frequency until the peak disappears and instead assumes a balanced, natural sounding position within the test media's spectral mix. Now vary the Q value until the sound you are hearing sounds even more balanced or natural. Re-adjust the gain as necessary. Do the same adjustment process for the other, lower-amplitude resonance frequencies until you are satisfied. Now listen from all the other seats in the listening area, starting with those nearest your first listen test location.
The purpose here is to determine a consensus or acceptable compromise in perceived response across all the seats in the listening area. This may not be at all possible to achieve . Indeed, you may find that an acceptable level of performance is achievable at only one seat within the listening area. Following the above approach, however, maximizes the probability of finding the placement\settings solution that optimizes subwoofer performance. It uses a bare minimum of test media, your ears and if you like, appropriate measurement gear.
In the Part II of Place For Bass, I'll look at a hardware-based measurement approach to optimizing subwoofer performance that will assume the end user has at hand both signal-generation as well as appropriate measurement hardware.
If this setup sequence has helped bring your system to a level of performance you're happy with, its time to raid the snacks cupboard, grab a collection of your favorite beverages, cue up your favorite flicks and enjoy!
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