L1® versus B1 Bass Module Fall Off Over Distance

Originally posted by Chris-at-Bose on the Bose Musicians Forum

L1^® versus B1 Bass Module Fall Off Over Distance

Hi folks, As many of you know first-hand, nearby direct sound from the L1 falls off with distance more slowly than most other speakers, including all bass speakers, and including B1 Bass Modules. We rarely notice this difference indoors, because room modes in the bass impose large changes from place to place that dominate over the different rates of fall-off of the direct sound waves from the L1 and B1 Bass Module.

But outdoors, there is nothing to prevent us from noticing that the bass falls off more rapidly with distance than the L1 's sound does. This will happen no matter how many B1 Bass Modules we have, no matter what EQ or presets we use, and it will also happen if we use other subs, no matter how big they are. The only ways to fully prevent it would be to either give up the slow fall-off of the L1 :( or make a line array of bass speakers at least 15 feet tall (wow!), but, aside from the expense, such a thing would be extremely dangerous. :eek: So we have to find a way to optimize the effect because we can't make it go away.

Before I explain how to do that, I want to describe the effect in a different way that makes the solution more clear. Since the bass and mid-highs are falling at different rates with distance, there can be only one distance where the balance between bass and mid-highs is "perfect". To my ears, the L1 and B1 achieve this balance point outdoors somewhere in the 10-20 foot range, call it 15 feet. But how much closer or farther must I go to hear a small change in the balance? Human hearing is not very sensitive to small changes in bass level, so 3dB counts as a small change in this frequency region. If I go to either half the distance (7.5 feet) or double the distance (30 feet) I hear a small change of 3dB in the bass balance compared to the ideal. That's because the L1 changes by 3 dB per doubling or halving of distance, while the B1 changes 6 dB, so the difference between them changes 3 dB either way. So there is a large region of distance where the bass is very close to the ideal level and that region covers about a 4 to 1 distance range, in this case about 7.5 to 30 feet.

Now what if I boost the level of the bass by 3dB with a tone control? (A tone control isn't perfect compensation for this effect at all frequencies, but it's not bad.) Now the point of "perfect balance" moves out from about 15 feet to about 30 feet, and the range over which the balance is close to ideal becomes about 15-60 feet, still about a 4 to 1 range. If I boost the bass control even more, the range moves out further, but keeps the 4 to 1 ratio. So now you see how to optimize this effect. Simply apply modest bass boost until the tonal balance is not quite thin at the largest distance you want to cover. Then the balance will be very close to correct from that distance to about 1/4 of that distance. Inside that range, the bass will be modestly stronger than normal, but we all tolerate a little extra bass better than we tolerate too little. :)

There is one thing to keep in mind when applying this outdoor optimization. You are asking for more bass from your B1s and they may not have more to give at the level you want to deliver. If you try to eliminate the thinness with a bass control and it doesn't increase the bass you hear, then your B1s are already giving all they've got and you will need more B1s (for this job) to get the bass control to have any effect. (Remember that more B1s will not replace the need to use the bass tone control here, because the PS1 keeps the tonal balance the same as you add more B1s.)

But before you invest in more B1s, make sure you are getting all the bass you are entitled to from the ones you have. For stereo music playback, you can get a little more bass by stacking your left and right B1s tight together. Outdoors, if there is a hard wall nearby, you can get a good deal more bass by placing all the B1s right at the wall. If you can't get right to the wall, then keep the B1s at least 7 feet away from the wall. If you place B1s outdoors anywhere from 2 to 7 feet from a hard wall, you will get a big hole in the bass response--bass notes will be very weak at some pitches while other pitches will be okay. The reduction in bass in that case will be significant. Try to avoid such placement if you can.

Getting the Most from the B1s

Overall, outdoor settings demand more from your sound system in the bass than indoor settings, because outdoors there is no bass reinforcement from a room. Also, people are often farther away outdoors. Both factors lead to your often needing more B1s outdoors than indoors. Please try these other ideas before getting out your wallet.

Hope this is helpful.

Chris

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