Bass Null

[Jaycan]

Hi
I am experimenting with different 2 ch setups. Currently I have Axiom M80s driven by Levinson 336, BAT preamp, various sources, and a Vandersteen 2wq subwoofer in a 22 x 16 x 8(H) room. I have the axioms about 9 feet apart along one of the short sides of the room, and 3 feet from the backwall. I have moved the sub to different positions along the same wall. The room is untreated. The problem is that from the listening position (13 feet back) the bass is sometimes pretty thin, yet if I go to the garage or upstairs, the whole house is vibrating with subsonic energy. Somehow I can't seem to get the same response from in front of the speakers. I will eventually go to Legacy speakers and maybe another subwoofer, but I'm sticking with this setup for another month or so. Any ideas?

 

[savelife]

Dr. Bob

Your sub is creating a bass suck out mode due to its position in your room and its relationship with the dimensions of your room. This can only be fixed by moving your subwoofer to a desirable location and change its phase in relationship with the room and main speakers. If you can't do that, simply move the woofer to the immediet (within 3 feet) left, right, or rear of your seating area. This will put you in the near sound field of the woofer. Since you cannot determine the location of bass at a frequency below 80 Hz, you will not realize the sub is "near" you. Since you are sitting in the subwoofers near field pattern, the bass frequencies will reach your ears before they are screwed up with reflections from walls and bad room dimensions. You will enjoy reltively clear, flat, and more dynamic bass.

Finally, if you want to know the very best place to put your subwoofer, put it in the middle of your listening room (dead center). Then play a 60 Hz bass tone from a Stereophile test CD. Next walk around the room with a Radio Shack sound decible meter set on C weighted scale, 75 db setting. While the bass frequency warble tone is playing, walk around the room and observe the meter. Where ever you get the loudest (highest) decible reading, that is the best place to place your woofer for maximum decible output. Once you have found that location, check it at 120 Hz, 100 Hz, 80 Hz, 62 Hz 50 Hz 40 Hz 33 Hz 25 Hz and 20 Hz. If the readings remain with + or - 3 db of each other, you have the golden spot to place your woofer. However, if you see a giant suck-out or boost at any of the frequency, retest your area as previously described and find the second loudest spot in your room and try that again as discribed herein.

Good luck with your woofer location hunting project...it will solve your problem.

Best Regards; "Dr. Bob"

 

[Rob Babcock]

A much better method is to place the subwoofer where you would normally listen and follow the above instructions. You don't really care what the bass is doing in the exact center of the room unless that's where you listen at. And be advised that on a centerline is usually an extremely poor location.

Placing the sub behind you in the nearfield will sometimes work. While it's true in theory that an 80 hz tone can't be localized, many subs will be producing sounds far above that for two reasons: one, a crossover isn't a brickwall filter but rather a sloping one. Depending on how steep it is and what freq it's X'd over at, some highs will pass to the sub. And two, many subs produce harmonics and/or port noises (if ported) that can betray the subs location.

By all means, though, give it a try. Most experts recommend corner placement, but not all. Russ Herschellman in particular feels that's the worst place for a sub. You can find many "experts" that can't agree, so take their advice as a starting point and experiment.

BTW, in my particular system, I have a pair of subs both located in the same left front corner of the room. This was the best practical location, and loads the room pretty uniformly (at least after I judiciously apply some parametric EQ ).

 

[Rob Babcock]

Note: Room modes are independant of our transducers & cannot be changed by moving the speakers. Room modes are a function of the room dimensions and cannot be changed without changing the room physically. When we move our subs or speakers, we do so to move them out of a particular peak or null (and of course usually into another, hopefully less objectional mode). This is why EQ alone can't fix all problems, and why room treatment can be a powerful tool.

It also explains the common, if mistake, belief that large rooms "support" deeper bass. A large room will spread room modes around more than a small one, but in practical terms a room would have to be immense to approach an anechoic state where there're no modes are reflections. All rooms will have the same inherent number of nulls and peaks, but in a small room there are less locations, so many modes will meet in the same spot. This creates a very uneven response.

You will have to address the issue thru placement and (ideally) room treatments. If you have a bad null, you will never be able to pour enough bass power into it to fill it, no matter what speakers you buy. Luckily, yours is a fairly large room, and very small changes in speaker location can get you a big change in bass. You may be able to get good results moving your sub just a few feet.

 

[Jaycan]

Thanks savelife, and Rob. You both have provided very usefull info that I'll implement over the next week or so. I'll let you know how it turns out.
Best regards