"I'm confused. How can you reduce a "null" with a bass trap? Nulls are where the frequency don't exist, not where they're over prominent." Most EE's aren't confused by this.
A null is a negative peak... the energy is negative of the zero energy state... that doesn't make it less energy... unless you insist
biasing (that's an inside joke or "pun" for the EEs) your understanding by looking in one direction.
Nulls can be observed when peaks are mixed with phase incoherency. Peaks can be observed when nulls are mixed with phase incoherency. In context, those two statements refer to "standing waves" rather than direct sound.
You can shoot a high energy null directly out of your speaker if you want to. For example; most snare drum hits are initiated with a swing towards the "negative" side of zero energy. Some my say that the "peaks" have reverse polarity and some may simply wait for the signal to return to positive to before calling the apogee of the signal a "peak". It hardly matters what you call it.. it's happening anyways.
Words, names, labels et al are meant to enlighten rather than confuse.
It seems to me that often times, people choose to be confused so that they can exercise criticism of others.
Everyone, that want's too, knows that standing wave "nulls" can be minimized by minimizing the peaking (positive or negative) that creates the circumstance.
Broad band sound absorbers can be thought of as "bass traps" when they are placed in locations that have a greater proportion of bass energy than mid and upper energy.
Placing sound absorption in the corners of any room will create a circumstance where the absorbers effect relatively more bass than mid and upper frequency energy.
Mid and upper frequency energy will be reproduced by a sound system at relatively low amplitude and the energy dissipates quickly. The first and perhaps second reflections in a listening environment include appreciable amounts of mid and upper frequencies but then it falls off and there is relatively little left in the corners.
The bass energy, however, persists and eventually wallows in the corners of the room. If you can absorb the sound energy in the corners of your room you will absorb relatively more bass than the mids and upper frequencies... because there is almost none of the mid and upper range energy left to be absorbed by the time the sound collects in the corners.
If you can clean up the bass in the corners you will enjoy sound with less comb filtering in the bass frequencies.
This is why people who are not as flippant as flippant people seem to be, refer to corner mounted, broad band sound absorbers, as "bass traps".
With regards to a 8x7x8.5 room.
It is easy to predict that there will be a problem at 80hz.
80.714285hz to be more particular.
Bass traps can help to minimize the issue but they will not solve the issue. The room is poorly suited for
listening to sound.
The 80hz null is being caused by reflections and interference that occur before the bass ends up wallowing in the corners.
Bass trapping can only do so much in a room that is so poorly proportioned for acoustic activity.
One could, perhaps, install a Helmholtz resonator in the ceiling or some adjacent room and let the bass leak out of the 8x7x8.5 room but the results will seem to offer diminishing return on effort because the real problem is being caused by the earliest reflections that the bass signal encounters.
The wavelength of the bass signal is, as Matt mentions, so long that in a small room that there are no tangible first or second reflections of the bass signal... so I refer to the perception of the earliest reflections encountered. I don't know how many it takes to get to tangible bass... but it's a lot.
It seems, to me, as if the OP has already ready realized most of the benefits that can be expected with room treatment.
With regards to 1/4 wave standoff placement. It is indeed correct to consider that the 1/4 and 3/4 wavelengths of any sound wave have the greatest energy and there fore placement of absorption at these distances will be most effective.
It may be useful to recall that considering the 1/4 and 3/4 wave intervals with regard to stand off placement from a wall is an attempt to get double duty out of a sound absorber.
In other words the absorber is already working on the direct sound that hits it and it will also work on the the sound that has passed through it and is being reflected back through it.
It's good to get the extra benefit of the second pass and you can *tune* the absorption capability with careful placement.
However, the primary absorption of direct sound has the potential to be the most effective at reducing comb filtering effects.
A wall may have zero energy for some specific frequencies... but all the other frequencies will arrive and reflect off the wall with energy that is relative to their specific wavelengths.
Don't think of the wall as a zero point... at least not as a generalization, as there are too few examples where that is actually the case.
Absorbers will encounter sound from all sides. The energy reflected off the wall behind the absorber can be absorbed as well as the sound that hits the front.
You don't have to have the absorber standing off a wall for it to be effective but you can get extra duty out of it if you do.
The bottom line, the room is too small to locate an absorber at a direct or first reflection of a 1/4 or 3/4 wavelength of the bass frequencies so all you can do is wait to absorb the later reflections as the signal ends up in the corners.
If you can't absorb the stuff effectively, you will end up experiencing annoying peaks and nulls.
It's probably an appropriate time to consider using something like ARC... regardless of how "controversial" it seems.
best regards,
mike
edited spelling and some grammar
post edited by mike_mccue - 2012/09/10 13:40:25