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Why do subs sound bloated or slow?

The use of subs in 2 channel audio is controversial around A’gon. Detractors argue that subs usually make a system sound bloated or slow.

IME, the two biggest challenges for integrating a sub into a 2 channel system are optimizing frequency response and optimizing transient response. When frequency response isn’t flat, the bass sounds bloated. When transient response isn’t time aligned, the bass sounds slow.

Here is my pet theory about why systems that use subs often sound bloated or slow: Under many circumstances, optimizing frequency response and optimizing transient response is a zero sum game. In other words, getting one right usually means you get the other wrong.

Thoughts?

Bryon
bryoncunningham
cbw723
89 posts
 
Marty, you mention in your response that you had your speakers/subs coplanar. That might not actually result in optimal time alignment depending on a number of factors. In particular, if there is EQ on the subs, that can delay the signal by anywhere from one to several milliseconds. That means your subs might need to be one to several feet closer to your listening position than your speakers for proper time alignment. If you have a means to delay your mains, that's easiest, but otherwise you're stuck moving the subs. There may be other sources of delay, too. But even if there is no delay, the subs may need to be a bit forward or back from the mains. In my case, the optimal position put the subs an inch or so in front of my mains.

Some people may already know this, but there's a neat trick for getting your time alignment close: You flip the polarity of either your mains or your subs (whichever is easier), then play a tone at the crossover frequency (e.g., and 80 Hz tone for an 80 Hz crossover). You then adjust the delay (or sub position) to *minimize* the SPL at the listening position. (You're effectively maximizing destructive interference.) When you're done, you flip the polarity back to normal, and you should be very close to time aligned. From there you can listen to a music passage that spans the crossover frequency and move the subs an inch or so at a time and see what sounds best. If you've moved your subs a lot from their original position, you should probably re-EQ before this last step.

It's a fun Saturday project, even if you can't possibly leave the subs in their final position. And you can let your ears tell you if time alignment matters for your system. In my case, it was a huge difference. As Marty says, YMMV.
martykl
3,578 posts
 
Cbw,

Good point re: time coherence and co-planar siting, and an interesting test excersize that makes some sense. Never tried it, but - if I get the energy - I will.

I spent well over a year screwing around with my subs daily (during a long early retirement/extended unemployment stretch) which allowed for pretty much unlimited indulgement (other than wife imposed constraints) in such activities. Now that I'm working again, time constraints are a limit, but the idea is appealling and I hope to give it a go at some point.
bifwynne
2,206 posts
 
Marty and CBW: Interesting posts. On Jan 23rd, I mentioned above:

"I . . . run two channel fronts and I have a Paradigm Signature Servo subwoofer. This Paradigm sub has its own Class D amp to power the woofer and three controls knobs to control phase, cut-off and loudness.

Because I only have one sub, I also bought a Paradigm X-30 crossover unit. The X-30 has the same controls and also allows me to combine left and right channels without shorting my pre-amp Mains. I feed my main fronts off the preamp's Main 1 output to the power amp. I run my fronts straight out. I feed the X-30 off of the pre amp's Main 2. output As stated, I use the X-30 to combine the two channels into one channel for the subwoofer.

My front speakers are Paradigm Signature 8 v2. The fronts roll off at roughy 35-40HZ. Of course room placement also affects base contour. However, as a general matter, I cross the sub over at 40-50HZ. Based on my ear, that sounds about right. Too high a cross-over and the bass sounds boomy. Too low, and bass is thin.

I also adjust phase by ear. I listen for the best sounding bass response. Loudness often varies by source material, but as I got used to working with the sub controls, I don't move the adjustment controls around too much."

Does this sound like a reasonable approach to adjusting the subwoofer? Also, as mentioned, I can adjust phase from 0 to 180 degrees. My sub is placed about 6 inches behind the fronts. For some reason, it seems like a 40 60 degree phase adjsutment sounds best to my ear.

Would you guys explain the relationship of (a) phase degree adjustments (i.e., 0 to 180 degree) to (b) coplanar placement and (c) time coherence??? My sense is that in a perfect world, if the sub and mains are placed equidistant on a fron plane with the mains from the listener, there is no need for a phase adjustment. However, if the sub is moved forward or behind the mains, or placed along a different wall, then a phase adjustment is needed.

One other complication: As stated, I can adjust phase, cut-off and loudness from BOTH the sub and the X-30 control box. Because it is more convenient to use the X-30 control box, I set the sub controls at zero degrees for phase; 3/4 gain for loudness; and 180 HZ (max) for crossover cut-off. However, as stated, I use the X-30 box to control the foregoing adjustments. Phase: 30-40 degrees; Crossver 35-40 HZ; Loudness -- whatever sounds about right, less is generally more. Does this approach make sense to you??

Thanks for your advice. BIF
bryoncunningham OP
893 posts
 
BIF - IME, the two simplest and most effective techniques for subwoofer placement that require no equipment other than a SPL meter are:

1. Optimizing frequency response by placing the sub at the listening position, then walking around the room and listening for the location where the bass is *consistent* across low frequencies. You can do this by ear with music, or you can use a SPL meter and low frequency test tones. Once you find the location in the room that has the smoothest frequency response, you place the sub in that location. Now the bass response at the listening position should be in pretty good shape.

2. Optimizing time alignment by maximizing constructive interference at the crossover frequency. This technique was described by Cbw in his post on 1/27:

You flip the polarity of either your mains or your subs (whichever is easier), then play a tone at the crossover frequency (e.g., and 80 Hz tone for an 80 Hz crossover). You then adjust the delay (or sub position) to *minimize* the SPL at the listening position. (You're effectively maximizing destructive interference.) When you're done, you flip the polarity back to normal, and you should be very close to time aligned.

Both of these techniques are simple and highly effective. Unfortunately, IME, they don't always give you the same answer about the best location for the sub (which is the point I've been trying to make throughout the thread). So, if I were you, I would start with the first technique to find a general location for the sub, then use the second technique to refine the placement.

Hope that helps.

Bryon
martykl
3,578 posts
 
Personally, I'd recommend an RTA unit, preferably with on-board PEq.

Adjust the PEq and phase for flat on-axis FR at and around your usual listening position. There may be some variation as you move the mic, but -in my case- there was little since my room is set up for solo listening and "window" is narrow.

Screw around a bit with the PEq bands (i.e. use them as tone controls) and/or placement - changing one variable at a time - and try to note all correlations between audible results and the shape of the RTA's FR output. If you have control of the sub's Q, adjust that to different settings as well.

Eventually, you'll figure out what the best sound "looks" like on the readout, and you'll be able to re-locate the subs and dial in a close replica of your preferred response very quickly. Tweak from there by ear.

For me, the best SQ is yielded by flat (or flat to +/- 35hz with slighly and gently rising output below) with subwoofer Q set for medium damping. This has been pretty much true, regardless of where the subs have been physically placed.

I find the SQ remarkably consistent (and remarkably good) when using my Rythmiks/NHT and Velo SMS-1 when dialed in with this technique, which basically holds on-axis FR constant. I would, however, be the first to acknowledge that, for other listeners, differences may be more audible.

Marty
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