I have my home studio in a small room (8 feet by 7 feet, height, 8½ feet   /  2.4m by 2.1m, height 2.6m).  I added 6 rockwool 5 inch panels and there was a definite improvement in sound quality.  But, I still a big null at 80Hz.   I'm thinking, if I can locate where this frequency is loudest in the room, I could make sure it will then be covered by a panel, which will hopefully reduce the null.    How can I go about finding the place in the room where 80Hz is the loudest?

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.

also, what we flippantly call "bass traps" are not really bass traps.  they're actually broadband traps.  most absorption occurs between 500Hz and 4kHz.  there is "decent" absorption between 125Hz and 500Hz, but it's not as great as above 500Hz.

absorption below 500Hz is still done by the traps, but not nearly as much, the coefficients diminish the lower you go.  check this table for the absorption coefficients
http://www.bobgolds.com/AbsorptionCoefficients.htm

in order to "trap" 80Hz you'd have to build a Helmholtz resonator at that frequency.

Even tho Ethan's information seems to be under fire on these forums by some, there is a wealth of information here;
http://www.ethanwiner.com/acoustics.html

he briefly talks about Helmholtz resonators.  personally I thought about trying to add some to my studio, but I don't think it's worth it.  broadband traps - and lots of them - are really the best way to go. 

for frequencies below 125hz where the traps don't really do that much, I use ARC (another controversial subject).

Well, 80Hz is possible to hit with bass traps, BUT, it's just hard. The wavelength of 80Hz is about 4.25m (14 ft). Correct my if I am wrong, but I believe you want the quarter wave thickness to catch 80Hz standing waves. The reason is at the wall, the velocity of the air is zero. At the half way point, the velocity is also zero. But at 1/4 and 3/4, the velocity of the standing wave air particules is a maximum. Thus a bass trap positioned right here in this max velocity space will have maximum effect. So you would need a bass trap that is about 1m out from the wall with substantial thickness to slow down the air. Doesn't mean you need 1m thick, but it's gonna have to be darn thick for significant effect. So prob close to.

Nulls are where the room is an extra half a wavelength bigger or smaller, I think, so when it bounces back it's negative. So putting any good bass traps or helmholtz resonators aiming for 80Hz should kill the negative reflection and allow the 80Hz (energy which does actually still exist) to be heard again.

So long and short of it, with a room that small, you can't but putting 1m thick traps in. You're pretty much screwed... Some tri-corner traps will help a little, but I wouldn't like your chances of doing much to that 80 null. Get some good headphones to check the low end on is probably your safest and most realistic option there. Your room is just not big enough.

"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

Try opening the door to the room when you mix. That will let some of the low end out of the room and possibly reduce the null in the listening position. You just need to shift it not necessarily remove it. 80Hz will be loudest in the corners of the room by the way.

I agree that it's too small a room to mix in as mentioned above.

Thanks for the replies.  Some interesting stuff in there.  

I know its not good, but I'm stuck with the room unfourtunately, no other option other than a corner of the TV room, which would be a bigger room, but stuck in an odd-shaped corner and no symmetry.  

Probably I will use a combination of a better set of headphones, and ARC.   And make the best of it.   

Some inconvenient truths...

With those room dimensions, you're going to be stuck with problem low-frequency resonances. It's just something you're going to have to live with, and if you want your finished product to be commercial grade you'll have no choice but to send it out for mastering. That's hard reality #1. 
 
ARC or any other kind of room equalization won't help you. A null occurs when sound waves meet their own reflections 180 degrees out of phase. It's a physical phenomenon for which there is no electronic cure.

Bass traps effective down to 80Hz are not practical in your small room. They'd have to be enormous. A Helmholtz resonator is the only solution, and even that wouldn't fix the problem completely. Read up on DIY Helmholtz resonators and see if you're feeling ambitious enough to attempt building a couple. (BTW, you can hang them from the ceiling instead of placing them in corners to avoid losing floor space.)

There is, however, one thing you can do, and that is to experiment with different speaker and desk placements. It's possible you can find a layout that helps mitigate the resonance. It won't be a total cure, because the longest dimension of your room is shorter than the wavelength of 80Hz. You'll likely have to move your speakers by 3 feet or so to get an appreciable improvement (that's a quarter-wavelength of 80Hz). Try to avoid sitting in the geometric center of the room if you can, which is going to be tough in an 8' room.

If I were in your position, I'd frankly invest in some good headphones.

why can't you build a false wall, at 45 degrees, or even 30 degrees, across a couple of corners, to deal with nulls?


i mean, i'm asking because i don't know.


why wouldn't this work, assuming it's well constructed drywall, floor to ceiling, with even some little tricks in there (staggered studs, or maybe some vinyl sound barrier, etc.....?

Maybe some day some one from IK or Audessey will elaborate and explain how ARC uses a customized convolution file, which is amalgamated by using "fuzzy logic" to combine a series of impulse samples unique to your room so that it does actually have some effect in the time domain.

ARC isn't just EQ... but it seems to be a big mystery as to what it is going on that is above and beyond being just EQ.

I am curious about that but rarely see it discussed.




Hi Bit,
 Thanks for reminding us that the 1/4 wave of 80hz is 3.425 feet.

 I was thinking, (or rather wasn't thinking) that it was longer.

 Thanks for making the ideas more concrete by stating an actual number! I should have thought that through earlier this morning.


best regards,
mike

Bitflipper, thanks for the input.   

I made 6 basic broadband absorbers, and to be honest, making a Helmholtz Resonator is probably beyond my limited DIY abilities 


I wonder if, by any chance, a move to another room would be a good idea?


In fact, I moved from the bigger TV room, into the smaller room (which is a storage closet really :-)) recently.   Then made the broadband absorbers ... maybe the bigger room, with the traps, would work anyway better.


Its such an odd shape, and there is only one corner possible -- diagram of both rooms can be seen here.  Top diagram is the bigger room I moved from, with placement of my desk and monitors (which I could move around within the corner area...).   Lower one is the closet room, which feels a helluva lot more small now with 6 broadband absorbers :-)