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The Most Underrated Aspect Of Bass Absorption?

Brandon Drury —  January 5, 2011 — 5 Comments

bass absorption

The stock setup for most home control rooms tends to be bass trapping the corners (via superchunk or corner straddling 703/Roxul). I believe this is total underkill in terms of taming low frequency problems, but that’s another topic and not everyone has the budget/facility/situation to treat their acoustics with an industrial-strength solution.

After we’ve fired our bb gun at the Dreaded Mud Monster by treating only the corners, most people move on to treating the direct reflections. I’ll define these as the reflections that hit a single surface and then smack you upside the head, but unfortunately have a territorial dispute with other waves of the same frequency causing politician-like truth telling from your studio monitors. The conventional wisdom is to just slap something up that’ll suck up the top end. My new reading says this is the MOST CRITICAL time to absorb low end.

I’ll start my claim off with a quote from Low-Frequency Optimization Using Multiple Subwoofers
This friendly-to-cyborgs (and DanTheMan SMILEY)) article is on the Audio Engineering Society website, so it’s safe to say this paper has clout.

Quick Glossary (According To Sweetwater)

Axial Mode = Axial Modes involve just two parallel surfaces – opposite walls, or the floor and ceiling. In other words, an Axial mode consist of waves resonating only along one dimension such as the length, width or height of the room.

In English: Problems caused by sound bouncing once or twice between a couple of parallel walls.

Tangential Mode = Tangential Modes involve two sets of parallel surfaces – all four walls, or two walls the ceiling and the floor.

In English: Problems caused by sound bouncing all over the damn place.

Oblique Mode: Oblique Room Modes involve all six surfaces – four walls, the ceiling and the floor.

In English: Problems caused by sound REALLY bouncing all over the damn place.

For most rooms the axial modes dominate the low-frequency performance……. Experience suggests that the remaining tangential and oblique modes are rarely significant.

So according to this quote (assuming I’m not royally screwing up my conversion from Greek to English) it seems that direct reflection are BY FAR the most import aspect of low end treatment. The sound that bounces all over the place is “rarely significant”. Those reflections that everyone says to treat by using a mirror and looking for your monitors seem to “dominate low-frequency response”. :eek:

This is info that rarely gets passed around.

OBVIOUS QUESTION: So why treat the corners first?

My understanding: No idea. There may be more too this. While corner traps are a great way to treat a large chunk of linear feet in the room, if the quote above is dead on, are the corner bass traps going to knock down these axial modes?

It seems that the quote above is telling us that we need to take the low end absorption of our direct reflections quite a bit more seriously as these are the deadly modes.

The general view when dealing with these direct reflections is to wimp out and use studio foam. This does take care of everything above 1k or whatever, which doesn’t hurt, but totally giving up on the low end seams like a wretched mistake to say the least. Again, “ axial modes dominate the low-frequency performance”.

Why not go through the extra hassle and SAVE some money by doing something that’s gonna make REAL impact. Why not place 703 or Roxul that’s at least 4” thick and at least 6” from the wall in all direct reflection points? At worst, use pink stuff insulation spaced accordingly.

This does present a construction challenge that mostly eludes me. It also is about as close to full-range absorption as we can get. It’s a GOOD way of doing it and well worth the extra effort to anyone so inclined.

This Foam Business

I’ve done my usual bitching and whining about studio foam this week. Some people have mentioned that foam is cheaper. That surprised me because the foam I see everywhere is pretty damn expensive. (One dude did mention the cost of screws in the UK, which in my mind is screwing Brits a lot harder than it’s screwing wood.)

Unless I have my head way up my mixer, it seems that this foam business (particularly mounted directly to the wall) isn’t the same as MXL vs Neumann. I never give anyone crap for the tool they select. Grammy Awards don’t fall from the sky when you use Neumann, anyway. It seems to me that there is something convenient about the foam even if it’s BAD for your room and your craft. How making a room muddier and duller is “convenient” when you need your monitors to translate is beyond me.

Note: I went on Ebay and searched for “studio foam”. Check this out. Notice how many of these have “sound proofing” in their title? These are FLAT OUT lies as this stuff has no capacity to sound proof a room. No wonder new guy are so confused.

The Truth About Studio Foam

Exhibit A
Here’s some foam. If glued directly to the wall, it’s optimized to absorb at 3.4Khz and up. You may as well be drinking battery acid. This would have some use in a reverb tank to suck the “liveliness” out of.

24sq foot for $55 bucks.

Exhibit B
This is where I got the article picture above with the green dolla’ sign. I wonder if Massenburg uses this?
It’ll absorb 1.7Khz and up. Yuck! It sure looks “money”. :CONFUSED SMILEY:

8 sq foot, Buy It Now $39.

Exhibit C
This is pyramid foam. It looks interesting. They took a 3” hunk of foam and cut it in half. This is NOT as good as standard 2” foam. The best analogy is imagine you are STARVING and you want a steak. Imagine they cut 50% of out to make a pretty imprint which basically leaves you the bone. Optimized for maybe 2.5Khz and up if we are lucky.

It comes with glue so you can be sure these are “designed” to do NOTHING for the mud in your room.

96sq foot, $145

They clearly have no intention of even providing you accurate information about fiber-based absorption (foam counts) being effective at the ¼ wavelength. Then again, who’s buying a $ foam “decoration” and wants to hear such information.

Exhibit D
The highly in demand guitar art foam.

Obviously designed to look like a mother-in-law would buy it….much like all those “Guitar Is Life” t-shirts she’s bought me for Christmas. It beats a bunny suit, I guess.

2” thick (mostly) with holes. Probably optimized for about 2k.

4sq foot. $44 Buy It Now.

All of these products are the kind of thing you might see a Chinese person selling 3 blocks from Times Square in a shopping cart. So let’s move to something official.

Exhibit E
Here our friends at Sweetwater are selling Auralex. At least they are honest.

“SonoFlat Panels are not only stylish, but offer great absorption of mid and high frequencies”

That means they look pretty and don’t do anything for the low end. These guys know what an axial mode is so their honesty is appreciated even if it’s a hair optimistic. What I can’t figure out is where it would be useful? Is there EVER time when you’ve cleaned up the low end too much and now have a disproportionate amount of high end? The answer is no home recording land.

2” thick and optimized for absorbing 1.7Khz and up.

$110 for 14sq foot.

So where should I get my 703/Roxul?

In 2005 I bought a truck load (I believe it was 37 bags of (4) 24”x48”x4” Roxul 4lb/cubic ft) for about a grand by searching for “industrial insulation” in the yellow pages. It ended up being less than $30 per bag and there were no shipping fees. Granted, I did buy in bulk and prices are surely higher now.

Even if you absolutely had to order it and pay WAY more than I did, this auction has it for $100 for 24 sq ft (doubled up to make 4” pieces) and that includes a $40+ shipping fee. That’s the absolute worst it’s gonna get.

Don’t be shy about researching alternatives to standard 703 and Roxul. There are quite a few other options that you may find locally.

Even better, if you have the cash and want it done RIGHT, give Ethan at Real Traps or Glen at Gik a yell and tell ‘em I sent you.

Conclusion

Direct reflections appear to be dramatically more important to accurate low end response in our monitoring than has ever really been pushed in home recording land.

While the effort is greater, properly treating direct reflections with full-broadband absorption that easily SLAUGHTERS anything you can do with foam is actually cheaper, particularly if you can find a local supplier.

Lastly, foam sucks.

Saved Comments

Convectuoso – 01-10-2012, 01:05 AM Edit Reply
Don’t ask me why, but bass tends to build up in the corners, hence trapping the corners. Probably to do with two surfaces being so close together.

Now I could be out of my depth here. But as long as you don’t go crazy with reflection based absorption, it shouldn’t really dull your room that bad. The problem is where you hear both your direct speaker sound and the reflection at the same time, causing comb filtering. But if you are absorbing the reflections and breaking up modes, then you are still hearing your speakers direct sound, uncluttered by reflections.

Take this with a grain of salt as I have no idea if I’m 100% right.

FWIW, unless you are going for an RFZ (reflection free zone, read: expensive) or completely building the control room from scratch, I would go with a LEDE (live end, dead end) approach. Simple as and will do for most project studios. Basically make your end with speakers pretty dead, and then at the back the room use diffusion or absorbers to break up the reflections hitting the back of the room. It kind of sends all the problems to the back of the room.

nulldevice – 01-10-2012, 08:30 AM Edit Reply
Bass builds up the corners, that’s why we trap them first. They’re the intersection of the major reflective modes, if my memory is serving me correctly, so by trapping them you’re actually doing quite a bit towards reducing room mode resonance.

What you really want to do to reduce the effect of room modes on your mix is reduce the RT60 at the lower frequencies to about .2 – .4 seconds. This can generally be accomplished y covering about 30% of your reflective surfaces with rockwool/OC70x/Roxul/etc. With a slight air gap you’ll get better frequency coverage but short of purpose-building the room and putting enormous hemholtz resonators everywhere, there’s just some LF modes you’ll have to deal with. I mean you’d need to have a QRD two feet deep to diffuse anything below 120hz, and god only knows how much rockwool or how big a hemholtz resonator you’d need.

Basically your goal, especially in a project studio, is to improve the room, not make it an acoustically perfect space, because you will go completely insane trying that. Get a decent measurement mic and a frequency chirp analyzer program and treat what you can for your listening and recording positions.

Bjorgvinben – 01-10-2012, 08:43 AM Edit Reply
Did you just reference a Christmas Story in a post about bass absorption?

Yeah, I’ve been looking into finding good material for bass trapping. Sadly, whenever I go into Home Depot looking for Rockwool they go huh? I guess it’s somehow not the same as fiberglass, but I’ve been getting mixed reviews on that. Isn’t that a good substitute?

rangrdan – 01-10-2012, 08:58 AM Edit Reply
At Lowe’s you can special order online or at the store Roxul products and pick them up in a week or less. I just bought Safe N Sound, 8 panels 2x4x 3″ for about $42 per bundle.

stainlessbrown – 01-10-2012, 09:05 AM Edit Reply
Almost every insulation manufacturer makes a RFG that is very similar/identical to Dow 700 series (Certainteed CB 220/300, Knauf (sp?)…and I don’t remember the #, etc). The hassle still continues to be purchasing directly. As it was originally designed as insulation for heating/AC applications there’s still those distributors who will only sell to licensed HVAC contractors. I work for a mechanical contractor and I had to contrive a “use” for them to get them to sell it to me.

About the only difference I see is that some of the brands have a painted side meant to keep the compressed fibers from rubbing onto you if you bump into an air handle/duct wrapped with this . It’s common to see 1/2″ on home AC units. Everything seems to come in cartons 24x48x 12″ so 4″… you get 3 panels, 2″ you get 6, 1″ you get 12. Best price I’ve gotten is $55 a carton, shipping was free providing I waited for the “company truck” and then went and picked it up.

The RFG takes more time to install and make ‘cosmetically”pleasing, but there’s no comparison to the foam…. I spent a lot of bucks early on buying Aurlex… I managed to kill the room acoustics, such that everything needs some amount of echo… and LF still plagued me.

After reading this I may ‘beef” of the wall treatment with some more RFG. As to the corner taps, I cut the RFG into triangles and stacked in my corner traps so they are solid, with a bare minimum of 4″ into the corner… and most have a greater depth. It did make a difference and I wish someone wouldhave forced me to treat my rooms before allowing me to buy any recording hardware/software…

i would have saved a bunch-o-money

bozmillar – 01-10-2012, 10:26 AM Edit Reply
my greek translator may be different than yours, but I didn’t come to the same conclusion from that quote as you did. Axial modes aren’t first reflections, they are resonances from back wall to front wall, or side to side, or ceiling to floor. I’ve never heard anyone say that they are the most damaging, but it doesn’t seem far fetched to me.

Why corners? The way I understand it, you want to have your traps as far away from the wall as possible to get the max low freq absorption. The easiest and least obtrusive way to do this is on a corner because you can get 12 inches of space between the wall and your panel without eating into your room.

bozmillar – 01-10-2012, 10:35 AM Edit Reply
my greek translator may be different than yours, but I didn’t come to the same conclusion from that quote as you did. Axial modes aren’t first reflections, they are resonances from back wall to front wall, or side to side, or ceiling to floor. I’ve never heard anyone say that they are the most damaging, but it doesn’t seem far fetched to me.

Why corners? The way I understand it, you want to have your traps as far away from the wall as possible to get the max low freq absorption. The easiest and least obtrusive way to do this is on a corner because you can get 12 inches of space between the wall and your panel without eating into your room.

userissteve – 01-10-2012, 10:42 AM Edit Reply
Roxul “Safe-n-Sound” – their acoustic stuff – can now be ordered through Lowes or Home Depot. Go to the Roxul web site to be directed if you need help finding it. Just bought a bag of 8 pieces 24″ x 48″ x 3″ (64 s.f.) for $50 to get me started.

2dogs – 01-10-2012, 10:55 AM Edit Reply
Firstly I agree with Brandon that Roxul is not adequate on its own as a bass trap. Its only part of the recipe. I have a pro grade studio and I can categorically attest to that. A bass trap is a treatment that consists of many parts. Here’s a link to SOS (Sound on Sound) which shows you how its made and how it works. Even though they used it in the context of a vocal booth and on a flat wall the results are evident. http://http://www.soundonsound.com/sos/mar06/articles/studiosos.htm

If you wonder why corners are best for bass traps, turn on your favourite tune and stand in various places in the room. When you get to the corners you’ll understand why.
My studio is equipped with an array of Primacoustics treatments, which their engineers designed for my studio, Primacoustic Acoustic Solutions including an array of different corner and flat mount bass traps. But that’s not cheap. Nevertheless, my bass traps are wonderful and they look fabulous. The important part is that they are built on the same principle as the SOS traps and they are invaluable in tuning your rooms. Take a minute to watch the Primacoustic MaxTrap video its very informative and accurate.

The material that actually kills the most low frequencies is the polymer layer because it converts sound into heat. The best one out there, can be obtained from Acoustiblok. A layer of that in your trap, along with the rest of the materials will give you a high performance bass trap. If its too expensive right now, be patient and save. Don’t go cheap on this. Best to have less gear and a good sounding room than the other way around.

If you’d like to learn the physics and how acoustics are calculated I highly recommend reading “Recording Studio Design” by Philip Newell. This 711 page book is expensive($100 +/-) but it will give you an exceptional understanding of acoustics. Sharpen your pencil, its not for the faint of heart.

Best of luck. Drop me a line if you have any questions.
Gilles Roger
2Dogs Productions
Fredericton, Canada.

cporro – 01-10-2012, 11:35 AM Edit Reply
my take on this.

why corners? every boundary in a room has a bass boost. go listen close to the walls. a corner is the intersection of 2 boundaries. it has major boost. imagine your room as pool of water. drop a stone in the middle. you will get the biggest/highest waves bouncing into the corners. you can try this in your kitchen with a square pan and some water if you want.

yes, axial modes (the ones that just involve 2 walls, not 4 or 6) are the ones people actually need to be worried about in the real world. get a room mode calculator (man free online) or figure out the math yourself. then go listen to your room. you will find your room demensions really do relate to boosts/cuts (standing waves) are certain frequencies. the same way an acoustic guitar’s size and shape affect it’s tone.

bass in rooms is the big problem. why? small rooms have all kinds of problems but the most expensive one, the one that takes the most material and construction is bass. treating reflections from a wall that causes comb filtering at your listening position is as easy as some foam on the reflection point. cheap, fast, and a place where foam works.

bass requires lots of mass or sealed cavities.

for the price compressed fiberglass or rock wool etc kills foam. you need some high density foam and lots of it to compete and that’s expensive. no wonder all the diy stuff avoids foam.

a little confusion here about terms i think. direct reflections are what i think of as causing comb filtering. like if a hard surface reflects (console, close wall) the music to you 3ms later then your speaker. that causes comb filtering you can really hear. the problems with bass (standing waves related to your room size) are called room modes or first order room modes. i think that’s right. but i believe we are all talking about the same stuff.

i used a “green” insulation call ultrasoft. there are few posts on the whole process.

Ultra Touch-ed by Bonded Logic | BlueDustStudio / Chris Porro you have major bass issues you not only get major peaks and roughs that don’t exist but the bass rings on. your whole room rings.

i’ve heard man mixing guys monitor at very low volumes and i can’t help wonder if this is because room modes are minimal. your room won’t ring nearly as much at low volumes. this should be something easy to measure with some free acoustic software.

one thing that i heard that interest me is the way different bass treatments work. some opperate based on reducing speed others by reducing pressure. sealed pressure devices like panel traps are placed against a wall where pressure is greatest. and things like foam, rock wool, compressed fiberglass should in theory work best places off the wall. in fact you do see a lot of these with stand offs so they aren’t mounted right on the wall.

tricky topic.

cporro – 01-10-2012, 11:41 AM Edit Reply
there are other things you can try if bass if an issue in your monitoring space. use meters to gauge the first 2 octave. 31hz, 61hz. monitor with headphones (takes some time to get used to). stop using your sub. don’t forget a decent reference mix for bass.

and when treating make sure you take measurements to see if things are moving the right way. room eq wizard was what i was using. and don’t move your speaker or test mic between measurements.

DanTheMan – 01-10-2012, 12:17 PM Edit Reply
Brandon, I nearly peed! Love this article. I certainly have a more than healthy appreciation for acoustics and loudspeakers. The broadband absorption is a very good idea for 1st reflections(depending on your goals of course)–including ceiling, floor, and front wall as they also work for other reasons(SBIR and imaging). Rear wall is debatable, but it seems to help a lot in my small room. In my HT, it feels weird for some reason–in a creepy sort of way. You might find it difficult to get into your primary modes range with porous absorption, but those can be effectively killed with EQ at the listening position so no matter IMO. They may not be a big issue anyway. In my bedroom studio they are b/c of the square(ish) shape, but cooperate well with my 5″ speakers. In my HT, they just bring up the bottom as you’d hope for. Measurements are king to figuring this out and in the bass range you either need an Earthworks or a calibrated omni mic as far as I’ve seen. There may be other mic options that will work. Your conclusion is dead on BTW.

This guy has some nice drawings and basic algebraic calcs of modes: Acoustics Crash Course 1 – Modes

I’m still a little unsure about how you feel about foam. Could you clarify that a bit? Ha ha,

Dan

xjbear – 01-10-2012, 08:31 PM Edit Reply
Brandon just so you know, and sometimes i hate to give away my secrets, but the cheapest way to get the best OC203-205 equivelant is something called DuctBoard from your local HVAC supply house. I use Locke supply in springfield, MO. they can get it in thicknesses of 1.5 inches and it comes in a 10′x4′ board for $37. thats 40sq ft for less than $40 and it absorbs down to 125hz for 1.5″. you can double or triple it to drop it all the way down to 80 and 40hz +/-. it does have a foil face on one side that peels off super easy but i have had better luck sandwiching two pieces together with the foil in the middle and hanging it 2-3″ away from the wall. just mount a 10×4 on the wall in a wood frame, cover with a canvas drop cloth from a hardware store and your lightyears ahead of anyone with a foam collection.
Ps shop local, boycott chinese products.
Bear…

SOULMAN – 01-11-2012, 07:05 AM Edit Reply
Originally Posted by convectuoso
don’t ask me why, but bass tends to build up in the corners, hence trapping the corners. Probably to do with two surfaces being so close together.

Now i could be out of my depth here. But as long as you don’t go crazy with reflection based absorption, it shouldn’t really dull your room that bad. The problem is where you hear both your direct speaker sound and the reflection at the same time, causing comb filtering. But if you are absorbing the reflections and breaking up modes, then you are still hearing your speakers direct sound, uncluttered by reflections.

Take this with a grain of salt as i have no idea if i’m 100% right.

Fwiw, unless you are going for an rfz (reflection free zone, read: Expensive) or completely building the control room from scratch, i would go with a lede (live end, dead end) approach. Simple as and will do for most project studios. Basically make your end with speakers pretty dead, and then at the back the room use diffusion or absorbers to break up the reflections hitting the back of the room. It kind of sends all the problems to the back of the room.

i think that the catch is: In order to trap low frequency build up one could resolve this problem by building, according to the studio room sizes, a specific double (secondary) wall behind the inner studio wall and simply let bass content escape by acoustically adjustable panels integrated into this, maybe not cheaper, but more effective way.

Thomas Hartkop – 01-11-2012, 08:11 AM Edit Reply
The best way to decrease the bass and other frequencies is to use owens-corning 703. Get on ebay and check this out. It is fiberglass in 1 and 2 inch sheets. 2 x 4 feet in size. It is best for gobos, ceiling sheets covered with cloth and it works great. I have been using this for years. This is the material used in large concert halls and can be purchased already covered with cloth of your choosing. Check this out. There is nothing better!

Tom

nulldevice – 01-11-2012, 08:28 AM Edit Reply
Mr 2dogs – re:the acoustiblok stuff, are there other alternatives to it? I’ve seen a number of “noise damping” floor underlayer vinyl layers and I’m wondering if they’re functionally similar or the same for building a trap like that. My local building supply has something like “db4looring” or some similar silly name, that supposedly does something akin to acoustiblok…wondering if that’s a workable alternative.

Thanks.

Audio~Geek – 01-11-2012, 11:40 AM Edit Reply
Why corners – bass builds up in corners. Play some bassy music and stand in a corner – MORE bass!.
I hypothesize that low frequencies hit one of the walls, some is reflected and some moves across the surface (like ripples in a pond), reaches the end and changes direction until it loses all energy. Corner bass traps will absorb some of the direct sound (the air gap increases absorption remember) and wall traveling waves on 2 surfaces. (just a guess at why its a better location).

For most rooms the axial modes dominate the low-frequency performance……. Experience suggests that the remaining tangential and oblique modes are rarely significant.
This is because once you treat the axial mode the tangential and oblique modes are also absorbed.

Bass problems aside, the most audible acoustic issue (in my opinion) in standard boxy drywall rooms is bright, springy flutter echo and blurring of transients.
Treating these mid and high frequency issues with fiberglass or foam products instantly improves recording quality, though mud will remain of course. For this you need to treat the majority of the wall surface, I’ve found only one of the parallel walls requires treatment with even foam to audibly reduce flutter.

Let’s say you cover all your walls with 703 floor to ceiling. First of all you’ve likely taken care of most of your low frequency issues. Secondly you’ve probably now gone completely overkill on mid and high frequency absorption and made the room completely dead and dry and likely a very uncomfortable or disorienting space. no reflections can be a mind fuck!

There has to be a balance of control and mid to high frequency attenuation. Absorb broadband in the corners, mid to high frequency absorption for the rest. Not enough? Thicker or denser bass traps in the corner. Not enough? Tuned bass traps.

bozmillar – 01-12-2012, 11:13 AM Edit Reply
Originally Posted by xjbear
Brandon just so you know, and sometimes i hate to give away my secrets, but the cheapest way to get the best OC203-205 equivelant is something called DuctBoard from your local HVAC supply house. I use Locke supply in springfield, MO. they can get it in thicknesses of 1.5 inches and it comes in a 10′x4′ board for $37. thats 40sq ft for less than $40 and it absorbs down to 125hz for 1.5″. you can double or triple it to drop it all the way down to 80 and 40hz +/-. it does have a foil face on one side that peels off super easy but i have had better luck sandwiching two pieces together with the foil in the middle and hanging it 2-3″ away from the wall. just mount a 10×4 on the wall in a wood frame, cover with a canvas drop cloth from a hardware store and your lightyears ahead of anyone with a foam collection.
Ps shop local, boycott chinese products.
Bear…

according to a duckboard white paper I just read, the absorbtion coefficients down at 125Hz are very poor (0.07). Is this the same stuff you are referring to? http://www.jm.com/insulation/perform…ts/AHS-447.pdf

DanTheMan – 01-12-2012, 10:29 PM Edit Reply
Perhaps this needs rehashed: where you hear the bass is where there is high pressure–exactly where porous absorption doesn’t work. Where you don’t hear the bass is the areas of high velocity–exactly where porous absorption does work.

Dan

the evil – 01-14-2012, 10:20 AM Edit Reply
im going to try to put my knowledge of ultrsound in here as i think it relates its just referred to differently and i never see it being brought up in these discussions. i will try to keep it as non-technical as possible. In this article Acoustics Crash Course 1 – Modes
they are talking about how when sound moves through the air it compresses and expands (this is known as a compression or longitudinal wave) as it hits an interface (a surface with different acoustic properties) some of the waves penetrate the surface and some of the waves reflect (lower frequencies have more penetrating ability and less attenuation over distance). Depending on the angle of which the waves hit the interface a couple of things happen. In the article they talk about oblique modes and talk about the speed being half (this is know as shear waves or transverse waves). what happens is when the angle is high enough the compression waves change to this wave mode and actually propagate differently. Instead of compressing they oscillate at a right angle(or transverse) to the direction of wave propagation. This is why the speed is halfed but the good news is this wave mode is not as strong as a compression wave. If the transverse wave hits another interface at the right angles it can change back into a longitudinal wave. The other thing that happens is if the angle is even higher, the wave does not reflect hardly any energy and it actually rides along the interface surface (these are called surface waves) these waves have even less energy than shear waves, but they also can change back into shear waves or compressional waves if the angle is right. This is my belief for the corner traps with bass absorbtion. Imagine the bass riding the wall along the surface until it hits the corner and then guess what… it refracts back into a compressional wave. i think that is partially why bass seems louder in the corners, so these corner traps are trying to keep it from escaping. The angles of incident for wave mode change is approximately 30-80 degrees and you will get shear waves and 80 and higher and you will get surface waves, so the more parallel your monitors are to a wall i bet there are a bigger build up bass in your corners. also the more perpendicular your monitors are to the back wall the more of a chance of standing waves, comb filtering and even frequency cancellation. I think this is why they recommend monitors at 30 degrees, as they hit a reflective surface the mode changes into a shear wave and less energy is put back into the room and it has less energy. These are all theories and i havent really got into acoustical engineering principles but im sure these thing s apply.

It seems louder b/c the way we actually hear physiologically speaking. It’s complex and boring.

The bass frequencies are omni. The direction your monitors are facing have nothing to do with it.

30 degrees just has to do with the isosceles triangle and it’s for imaging purposes. Technically speaking it’s not a good system.

Better systems for imaging are here:
Ambiophonics Tutorials
3D3A Lab at Princeton University

Dan

the evil – 01-14-2012, 07:30 PM Edit Reply
The bass frequencies are omni. The direction your monitors are facing have nothing to do with it
yes i understand this as we dont hear stero under 300hzish either. what im talking about is the the lower frequencies have less attenuation over time and can travel further than higher frequencies. one cycle at 10K has attenuated long before 100Hz. so when lower frequencies hit an interface at a high angle it can travel down the surface of the interface until it reaches another interface and reflects or refracts depending on the angle of incidence. i just think that this is a reason for bass to build up in the corners instead of other (higher) frequencies. to me it sounds like a logical theory… it also just seems funny to me that 30 degrees is the number always used. i understand about an isosolece triangle but it also happens to be the same angle as wave mode changes. the question is, is this a coincidence? if at 30 degrees the reflections lose energy due to mode conversion couldnt that also be a reason for this particular angle? i mean you can have an isosolece triangle with higher angles, but 30 is always referenced. But this is where my knowledge of sound waves is high and room acoustics is low, but i cant help feeling that there is a relationship between the two.

Doing ultrasound we are generally using one frequency at time but we know how it reacts and we use this to steer the beam to where we want it. We use the time and attenuation to our advantage to measure things (similar to sonor) but with much more accuracy. In the medical field it is accurate enough to actually show pictures of what is inside a pregnant belly. this is all based off the principles of sound and how it travels though materials. i cant help but think about all this going on within my room and how it affects what i hear and how the room responds differently to the placement of speakers and its reflectors.

Edit:
Sorry Dan, i read a few of the articles as those pretains to pyschoacoustics and how we percieve audio. im actually talking about the relfections once the leave the speakers and what they do in the room and how those reflections/ refractions move and attenuate within a room. i saw this article blog being abou the relfections and the absorbtion within a room and i was more concerned with what these wave modes are doing within the room.

DanTheMan – 01-15-2012, 08:32 AM Edit Reply
We do hear stereo below 300Hz.

US imaging is essentially a different world than audio. I’d try to separate them. Bass ‘build up’ is well understood and well documented. You can find any level of depth(pun) on the subject in texts on acoustics. It’s too complex to go through in a post, but the angle of the monitor is not going to radically change this. You can twist your monitors, measure and see for yourself. I have mine on lazy susan bearings for all sorts of tests–I’m a nerd.

The triangle is also supposed to be equilateral–that’s where the angle comes from.

Dan

the evil – 01-15-2012, 09:12 AM Edit Reply
equilateral now i got it… i just kept thinking about reflections/ refractions and how they move around the room and not the optimum listening position. I figured if you know where and what this is doing you would be able to better understand where certain types of absorption are needed. ive been slowing getting into room acoustic principles but a lot of what im reading is very technical and doesnt seem to explain much. thanks

This seemed like a pretty basic article on the subject and was an easy read
Acoustic Sciences Home Theater Article

rodneyc9 – 01-19-2012, 04:21 PM Edit Reply
In the old days in Nashville(pre-digial), we used large blocks of wood. Beams of pine or cedar 6 to 8 feet long 6 x 6, 8 x 8. Don’t use treated lumber. Yes, starting in the corners. They are dense enough to absorb starting @ 300HZ and below. Relatively inexpensive and very effective.

Jack_Death – 01-19-2012, 04:25 PM Edit Reply
Originally Posted by bozmillar
according to a duckboard white paper I just read, the absorbtion coefficients down at 125Hz are very poor (0.07). Is this the same stuff you are referring to? http://www.jm.com/insulation/perform…ts/AHS-447.pdf
I think the OP said 1.5″ thickness, your link is to the 3/4″ stuff. Maybe that accounts for the difference?

IMF OnSite Recording – 01-21-2012, 08:12 PM Edit Reply
Im going with Mineral Wool 1260: MINERAL WOOL 1260 (6 lbs/ft) 48″x24″x2″

works like OC705 but WAY cheaper.. and the coefficients aren’t lacking in the high range either like some people imply 705 does.

2 Cases will give me 2 4′x4′x6″ thick first order reflection panels, spaced from the wall.

JohnyMoreira – 01-27-2012, 01:41 PM Edit Reply
I’m gonna add my 2 cents here and try to explain in a simple and visible way why bass builds up in the corners (from what I’ve been taught).

As someone already stated here, sound waves behave immensely similar to ripples on a pond since both are mechanical waves and thus propagate through the vibration of actual particles unlike light waves which do not have the need of particles to travel (sadly this means if you explode a spaceship with your lasers on outer space the sound would be none and thus all of your dreams as a die hard Star Wars fan are shattered by this mere physics fact… by the way you wouldn’t see the lasers hitting the spaceship either… doh!).

With this in mind lets go to fact number 2. Sound waves interact with each other all the time. What happens here? Well most of us have had the experience of hearing an out of phase recording (if not just try to record the same sound simultaneous with two microphones flipped upside down and at the same distance of the sound emitting object aka the instrument or whatever you wish to use) and it’s not very pleasant. What’s happening there? Well several waves (if you’re recording a multi timbral instrument, aka anything that’s not a teramine or a tone generator) are interacting and meeting each other exactly inverted. What happens in this case is something very similar to adding up 9 with -9, which means the final result is null and thus the final wave disappears (kinda like fighting fire with fire).

Now getting to the wall corners part. As we all know (unfortunately) our common walls are not very absorptive and for most frequencies they act exactly as the opposite, as reflectors. So if you think of your sound leaving your speakers just as if they are the center of the pond ripples but now imagine it in 3d and the ripples being huge spheres, sound propagates to all directions from there and reaches the walls, ceiling and floor that acting mainly as reflectors will make this waves come back in other directions and so on until all the energy of the wave disappears (which depending on the frequency and magnitude of the wave will vary in duration). Well the problem here is that all frequencies are equal to a wave length of inverted proportions.

In a simple way, imagine a bass drum which has its main “thump” at around 50 hz, this means we’re talking about a sound wave with a length of approximately 6,86 meters (~22 feet). Along this course of 6,86 meters this wave will be interacting not only with your walls but with the reflections of itself after hitting them. This is we’re it gets tricky but also pretty cool.
The way it will interact with itself will depend on its angle.

Now you say “What the f***?”

Well in physics we (puny humans…) tend to read the world in a mathematical form, so we define the sound wave as a function which writes a sinusoidal graphic (aka the waveform we all know form our daws when the zoom is really really big and we see something that appears to be a sea wave) and translates the up and down motion of the wave we see represented when we record any kind of audio. So at 0º the wave is on the horizontal axis, at 90º the wave reaches the top on the positive side of the vertical axis, at 180º it goes to the horizontal axis again, at 270º it reaches the bottom and thus the top value of the negative side of the vertical axis and finally at 360º it’s the same s*** as the 0º.

With all this graphic mumbo jumbo all you need to retain is that if two sound waves of the same frequency find themselves they’ll have a good ol’ royal rumble and the resulting waves will be the remains of the two waves and that is calculated by knowing if the wave is on the positive side from 0 to 180º or the negative side from 180º to 360º.

So imagine two waves meet at 90º and 270º respectively. They’ll be like Godzilla fighting the King Kong, there will be no winner and no remains, they are exactly opposed and will thus be obliterated completely.

That’s why in a Big Live Venue in order to reduce the bass build up on stage that can complicate a lot the work of the sound engineer it is common to use sets of 3 sub woofers, in which 2 of them are facing the audience and a third one is facing the stage. This placement gets that stage facing speaker to kill a good part of the bass being emmited by the other two since the cone of the speaker is producing the wave in exactly the opposed form of the other speakers thus having the angles of the waves flipped over and becoming out of phase.

Now imagine the opposite, two waves find themselves on exactly the same degree and thus at the same phase, this would be like Homer Simpson meeting Peter Griffin on a bar at Happy Hour, both would be extremely happy and inflated (and drunk of course).

The same happens in this case, two waves meeting like that will cause the resulting wave to double its magnitude and thus become louder and more noticeable in a room.

Now for the corner part. Just imagine the number of times a 2 wall corner can augment this interactions and being that most bass waves are far bigger than most parts of the corner you can guess which kind of interaction happens the most: augmentation.

Now after all of this you may think “wait but then how come we use the 1/4 of the wave length for our acoustic treatments, shouldn’t we use the 1/2 length instead?”. The answer is NO but nice thought!

This is due to the fact that when a sound wave hits a wall it behaves like a beam of light on water. A small part is absorbed by the water creating the refraction and the other is reflected, that’s why you can see your beautiful face reflected on a pond. The key here is the world REFLECTION. So the wave after being reflected and losing some of its energy comes back as a simetrically reflection of the initial wave. Plus, acoustic absorption is more effective when the particles are at the highest velocity which happens when the compression is the highest which means at 90º and at 270º. This means you if you’re lucky you can get the same wave twice at the point of the highest rate since 2*1/4 is 1/2 of the wave and that’s exactly the distance between the negative and positive highest value or (repeating again) the 90º and 270º mark.

Of course there’s other important variables here but I was after a simple answer (and I guess I might have overcomplicated it).

Hopefully it’s helpful for anyone and please be free to bash my head out on a guitar amp or something if I’ve written something utterly wrong in all scientific ways.

Sorry for the long text.

John More over and out.

webster7 – 02-07-2012, 08:47 AM Edit Reply
The bass sounds that are objectionable are the standing waves produced from the parallel walls and ceiling and floor. It
would help diminish the unwanted bass if you changed the angle of one of the parallel walls say 25 degrees. An angled
square foot of a soft material like wood covering one whole wall would change the direction of the sound so that standing waves were not produced.
Another culprit is the back sound hole in most sub woofers. This hole should be well damped with foam or cotton wool.
The back sounds coming out of the hole are not in phase with the front bass sounds (unless they are traveling through a
long labyrinth channel). Out of phase low frequencies tend to muddle up the clean sound from the front of the speakers.
Of course, acoustical tile on the ceiling and carpets on the floor help absorb the reflected bass sound.

Kimgul – 02-21-2012, 05:01 PM Edit Reply
I have lots of basstraps made of rockwool and fin my controlroom sounding ok but have anyone here experience using MLV as membrane behind the basstraps? Would something like these help even better when they turn the low frequencies into heat (or so).

Wonder if I should buy something like SoundAway Soundproofing Mass Loaded Vinyl Noise Barrier , but not shure if I`m getting something that I do not need.

Brandon Drury

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Brandon Drury quit counting at 1,200 recorded songs in his busy home recording studio. He is the creator of RecordingReview.com and is the author of the Killer Home Recording series.
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5 responses to The Most Underrated Aspect Of Bass Absorption?

  1. Hi All… :
    Open cell foam absorbs acoustic energy directly in relation to the speed of the sound wave, or it’s velocity… 344 metres per sec at 21 degrees C. Unfortunately at a boundary, i.e. a wall the speed of your sound wave is 0. Because it stops before it comes back (reflection)… like throwing a tennis ball at the wall. If you have the foam or absorbent away from the wall, you catch the sound wave while it still has some velocity, which is then converted to heat within the open cell matrix. Due to the nature of sound, all sound waves (all frequencies) exist in the corners of a room, so if you treat the corners first, you help the room overall. Naturally you need to reduce reflections at the listening position as well… they can come from the walls each side of your ears or the top of your mixing desk…

  2. Mischa Shashumshkavich July 22, 2013 at 2:18 pm

    i am currently working/recording in a round room……where should i start? j/k, i am glad i found this site, as i am in the process of setting up a recording space,and i’ve learned quite a bit here

  3. Circular rooms are tough. I’ve seen this encountered in books, but I honestly don’t know where I’d start. The concepts are basically the same with round rooms and rectangular rooms in that low end that bounces around will null with itself in some places and add together in others. You are going to need real deal bass traps for sure, but beyond that vague recommendation I’m a bit out of my element. You may want to try asking on the forum: http://forum.recordingreview.com/f8/

  4. Mischa Shashumshkavich July 24, 2013 at 9:54 am

    i’ve actually had situations where bare corners and or reflective surfaces have enhanced the sound,but that was live with no recording/mixing involved, jamming with djembes where the added bass seemed to help the sound…..btw, i was just joking about the round room, not thinking that people have actually built them…..or even tried recording in them…(i will try and keep rhetorical questions to a minimum from now on,but can’t promise anything).but like i said, i am very happy i found this site,because the room i AM dealing with is less than ideal,but i like a challenge…. and i’m sure i will be back for much needed help

  5. Ha! You got me. I bit hook, line, and sinker.

    Round rooms do exist…at least in drawings in acoustics books. SMILEY

    The whole room mode thing is a tricky bird as the nulls created when out-of-phase waves collide will cancel quite a bit of the low end. It’s very common to hear MORE low end in a room that has the low end reflections sucked up. With that said, some room do have a thing to them without a single bit of treatment. This is generally a creative tracking thing more than a mixing thing.

    Brandon

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