hello kind gs acoustics experts and aficionados,

it's been a little while since I've been on here, although yall's wealth of knowledge were instrumental in helping us get a nice sounding space circa 2010.

As it turns out, we're in a new place, and I'm having to review some of this knowledge to get the new space sounding mad decent again. The new space is fairly large and rectangular-ish, but we have a corner that is a 45deg angle sliding door, another one that is a stairwell landing, and another one that is 3 doors to other rooms. Obviously, this isn't ideal, but where there is a will there is a way!

For the most part, my dementia isn't too far gone, so we've hit the available vertical corners with soffits and monster traps, and we've hit the ceiling to wall corners with a monster trap and minitraps (mixed media ftw). We also have a cloud made up of one 242 .. we were going to make it 3 242s, but mounting directly onto the ceiling is out of the question, and we're still working out the safety of a tight wire going from wall to wall with that much weight. Due to the size of the room, we're consolidating the treatment near the listening position.. at least until the resources aren't as scarce.

Anyway - with everything the way it is now, we've got a great stereo image, but the bass region is less than ideal.

The question I have regards the nulls and sums created by the room reflections / modes. I've read countless times that bass traps go in corners because bass bunches up in the corners. I've also read several times that one should walk around the room and listen for unevenness in the sound to make decisions on where to place the treatment.

That being said, if there is a null, regardless of it being in a corner or not, will sound absorption be effective at that place? Simultaneously, if there is a sum, will absorption be more effective there?

My intuition makes me believe that where there is a null, the out of phase sound waves are essentially the same thing as no sound waves at all. At the same time, the sound waves are there, so even if null, one could argue that you would be absorbing two sound waves with a panel placed at the null.

Your help and advice is, as always, greatly appreciated. Cheers!

Weeeelllllll.... Sort of! The issue is room modes, or standing waves. Those are resonances between the walls of the room, and are therefore directly related to the dimensions of the room. Every single mode will have a peak against each of the the entire walls that affect it, as well as other peaks and nulls at various fixed points in the room. And since a wall extends all the way into its corners (doh!), you can therefore summarize things easily: "All modes terminate in corners". For every single mode in the room, it will have a peak in two or more corners. That is ALWAYS the case. So, if you put treatment in all the corners, then you are guaranteed of hitting all the peaks. And if you reduce a peak (knock it down a bit), then you also reduce the corresponding null (raise it up).... because modes are standing waves! Each mode is one single wave, so whatever you do to PART of that wave, you have also done to the REST of the wave. Damp the peak, and you also damp the null.

That's not necessary for treating modal issues: because ALL modes ALWAYS terminate in corners! So all you need to do is to treat the corners properly, and you have treated all the modes. It really is that simple. You might want to walk around the room to listen for other issues, but there's not much point doing that for modes. It's like walking around your kitchen with your hand stretched out, trying to find the location where the hot water is coming from.... it would be far better to just go right to the tap, because that's ALWAYS where the hot water comes from... There's no point in looking inside the fridge, or in the knife drawer, or behind the light switch... Just look at the place where it always happens. Treat the corners, and you treated the modes.

The same rule applies: treat the corners, and you automatically treated the peaks, and therefore the nulls too. If you damp the peak of any wave, then you damped the entire wave... which means the nulls to.

In theory, for some higher-order modes you can probably find spots out in the room where there are peaks, and you could, indeed, put treatment at those spots to damp the modes... but who wants to work in a room where there are panels hanging all over the place, at the half, quarter, one-eight, and one sixteenth points in the room? (Because that's where the other peaks and nulls will be: always in the same locations in the room: half of the length/width/height, or a quarter of it, or 1/8, 1/16, etc.) So if your room is 24 feet long, it would look rather strange to see large panels hanging out in the open at the 12 feet mark, and the 6 foot mark, and the 3 foot mark... You'd find it hard to walk around the room like that! If you put the trap in the corner instead, then ALL of those other spots will get the benefit.

The above is why studios are normally treated in this way: with major bass trapping in the corners, and possibly also against some of the walls. Because that's where all the action is. If you put trapping in any corner, it is already 12 dB more effective than placing it out in the room, simply because you get a 6 dB boost in effectiveness from each of the walls, as compared to empty space. And if your trap is in a "tri-corner", where three surfaces meet (eg, two walls and the ceiling), then you get an 18 dB boost in effectiveness from that location, as compared to empty space. What's not to like about getting such large boosts for free?

With a modal null, there is no "out of phase" pair of waves! It's just one single wave. The null is just that: the lowest intensity of one aspect of the wave. But it is only one wave. It is a standing wave, so the peaks and nulls for that specific wave will always occur at the exact same location in the room, but there's no phase cancellation going on: its just the minimum point of that wave, in exactly the same way as the peak is the maximum point of the wave.

However, that being said, its also important to understand that that null is not really an absolute zero of energy for the wave, and neither is the peak the absolute maximum. There's two aspects two a wave; one is pressure, the other is velocity. When we speaker of "nulls" we are normally talking about pressure nulls, since that's what we here, and what most types of mics detect. But there's also velocity. Where the wave pressure null falls in a room is also the same spot where the air molecule velocity peaks, and the point where the velocity is at zero is also where the pressure peaks. If you add up the pressure energy and the velocity energy for any given point on the wave, you will always get one single answer for that wave, which is the total energy carried by the wave.

Think of a rubber ball that you throw against the wall: as it flies through the air, the velocity is at maximum, and there's no pressure inside the rubber, but at the point where it hits the wall, just before it bounces back, the velocity obviously drops to zero, and the pressure inside the rubber is very high: The ball is not moving at all when it is squashed up against the wall. Velocity is zero. All of the energy from that velocity has been converted into "rubber compression": the ball is squashed up nearly flat against the wall. And as that energy is released, the ball starts accelerating back towards you: velocity increases, pressure decreases. When it is in full flight again back towards you, there is zero pressure inside te rubber, and the velocity is at maximum again. If you could see that all in slow motion, you's see how the ball gradually slows down as it touches the wall and starts to compress the rubber, tbe slows down more and more while the rubber compresses more and more, until at one point is is completely stopped, while hte rubber is complete compressed. The the opposite happens, as the rubber starts pushing back, decompressing, and moving the ball the other way.

The exact same happens with a sound wave: as it approaches the wall, pressure is low but velocity it high (we are talking about air molecule velocity here, not the velocity of the wave itself), but exactly at the wall surface the velocity is zero while the pressure is maximum... So at any point on the wave, there is some combination of velocity and pressure that, together, add up to the total energy.

Thus, a null is not REALLY a null! It is just a PRESSURE null, but it is also therefore a VELOCITY peak! Even though you would not hear that tone if you put your ear there, a ribbon mic would still "hear" it, because ribbon mics are velocity based, bot pressure based. And there are two types of acoustic treatment as well: ones that work on the pressure component of the wave, and ones that work on the velocity component. Thus, you need to place the right type of treatment at the right spot on the wave....

It all sounds rather complex (and it is!) but the one thing you can always be sure of, is that treatment in the corners will always affect room modes, because that's where they start and end.

- Stuart -

Hello Stuart, Thanks for that lengthy explanation!

I have a question related to what you wrote. I have an L-shaped room that serve as a studio, workroom for my wife and as an occasional guest room (please see attached image). I have started building floor-to-ceiling bass traps (front corners) and started treating the room to create an RFZ. This first attempt is to my knowledge pretty risk free, as you cannot (so I hear) overdo bass trapping and you want to reduce early reflections at the listening position. What you cannot see in the figure is that the front wall (332 cm) is slightly tilted from 2 meters onward, so no direct reflection will hit my ears from this wall. For now I haven't planned in a ceiling cloud. Flutter echo is currently not one of my main concerns...

As I plan the rest of the treatment I have a question regarding an idea I have. Instead of treating the rest of the corners in the room (this would affect the guest area quite a bit unfortunately), do you think it makes sense to divide the room into two partitions (denoted A and B in the image)? The structure I plan to build is a floor to ceiling gobo (on wheels, fold-able, open-back) filled with 5000 rayls/m glasswool (30 cm thick), and in the middle of it (ear height) integrate a flush mounted DIY 1D QRD. I haven't picked a prime yet but I guess it will be somewhere in the range 43-71. I reckon the glasswool will act on the velocity component of the sound and since the front of the room is going to be quite dead, the diffusion will bring some life back to it. There will be some gaps between the structure and the walls (marked with red arrows in the image). Do you see a fundamental problem in my reasoning here? I have a measurement microphone already and new speakers are on the way, but I'm not sure how measurements will help me here, since I'm effectively changing the geometry and size of the room when I integrate this new barrier. This build will be very complex so a heads-up would be nice before I start... :-)

Thanks again for all the input you provide here on this part of the GS forum! It's fun and very informative to read!

Sorry if I hijacked the tread. Maybe it would have been smarter to start a new one...

/Cheers, Mark

Hi Stuart,

Thanks very much for the detailed post. That has cleared up quite several misconceptions that were floating around in my head! Indeed, it feels like I was not only barking up the wrong tree, but the tree was actually a cactus.

So my key take-aways here are as follows:

- reflections in the bass region manifest themselves differently than in upper frequencies. The lower sound wave is monolithic, so to speak, with the same wave nulling or peaking throughout based on the size of the room. In a way, the sound wave is "static" throughout the room, similar to how an oscilloscope would display it.

- the highest point in velocity for the sound wave equates into the most attenuated sound, as perceived by the human ear. On the other hand, the highest point in pressure equates into the loudest perceived audio.


If I can pick your brain, or anyone else's for that matter, I do have one more nagging thought in my brain. Specifically, what defines the room?

If all the walls around the room were fully sound-proof, then (I think) it would be obvious - the room is defined by those walls.

In an apartment, especially those built in the all-american dry-wall style, sound continues to travel beyond those magnificently thin walls. I would imagine this affects the physics around the modes of the room.

Say there are two rooms adjacent to each other, should the full space's end-to-end length define one set of modes, whereas the inner room's end-to-end define another set of modes, or is it better to ignore adjacent rooms?

@ stuart , once again, thanks for your insight!