Bass resonance problem

pinksoir

Ok,

I've had this really bad resonance problem in my room for a long time. It happens only on one note, B on the low E string on a bass. It also happens on any other instrument on the same note (synth etc). I'm not sure exactly what frequency that is, but it's just that one note. Actually, I used a sine wave on a synth to check and it also happens to a degree on the C above it too, but not so much.

So I've done a couple of things to remedy the situation. I built rockwool bass traps. It was still happening. The speakers were sitting on top of some other speakers which were directly on the desk. I copped on that what was happening was the desk was amplifying all sound coming from the monitors and when I played the B it was amplifying it like crazy causing a ton of resonance. So then I went ahead and bought some isolated monitor stands so's the monitors would no longer be in contact with the desk. The problem is greatly reduced but it is definitely still there, especially when I have everything turned up loud, just this big big resonance every time I hit a B.

The room I'm in is rectangular, about 2.5m wide by 5m long. There is a window at one end (it's a converted garage so the window is basically the whole wall) and my desk is in front of the window. I can't move my desk anywhere else as the door is down the other end. My desk is about 2 feet from the window. Could it be a reverberation from the glass? I have heavy enough velvet curtains which I keep closed most of the time. I'm not sure if it's making any difference...

The monitors I have are Rokit 5s. I have stuck foam into the holes beneath the woofers but I can't hear any difference. When I listen through cans there is no resonance at all.

I haven't yet treated the first reflection points in the room, that'll be the next thing I do. I'm waiting for foam to come back in stock in the shop.

Any ideas what's going on here? I googled and found people with similar problems but no real solutions...

Cheers.

SonasRec

Sounds like you have a standing wave at 60 or 120 Hz. The glass isn't causing this, it's the dimensions of the room. Check this article to find out more about room modes & standing waves http://www.audioholics.com/education/acoustics-principles/listening-room-acoustics-1

Or google 'room modes' or ''standing waves', there's lots of info out there.

i57dwun4yb1pt8

window is basically a bass trap as well , but you may have a glass vibration
in it.

i always have a towel or curtain against the glass - touching it .

if this dont stop it , then a larger amount of bass trapping is needed in the corners . especially at the desk end .




also put some stuff in the ceiling or on the ceiling from the attic side .


http://www.bobgolds.com/Mode/RoomModes.htm

pinksoir

Cheers dudes. The bass traps I built are floor to ceiling and are 200mm thick. As it's a converted garage, there's no attic above the ceiling, just solid concrete.

I read that stuff about standing waves and room modes but I don't really understand what I can practically do to combat the problem. If I understand it, is it the waves bouncing between, for example, the two side walls? And the distance between those walls is equal to the length of the wave form?

Would putting in diffusers make a difference? When I get my reflection point foam will that make any difference? I presume not seeings as it's a pretty low frequency and the foam is for mids and up...

I'm at a loss now.The isolated stands have improved the problem a lot, but it is still extremely annoying. Well, niggling mostly.

I'll check for window vibration.

EDIT: Nothing from the window. If I stand half way down the room the resonance is way less noticeable to the point of being almost undetectable. Come to think of it, it's not really a resonance. More like a massive boost in volume. If I play a sine wave on a synth it is the most pronounced (again at B but also at C - that would be an octave below middle C and the B below it). It is even making the keys in my keyboard vibrate.

EDIT 2: I built my traps with regular rockwool, not the RW3 panels. I used the rolls which were lengths 10cm thick and I doubled them up to 20cm and pulled them pretty tight. Did I screw up? Is RW3 the way to go for traps?

fitz

Are your traps just panels, or are they wedge shaped?
If it's just a panel creating a triangular space behind it with the corner, it's not really going to do what you need. That triangular space is what you need to have your absorption in.
Some of the more acoustics-savvy guys will correct me if I'm wrong?

madtheory

Fitz is right. You need to trap the reflection points. If, as SonasRec says, the resonance is at 60Hz or 120Hz, that confirms it's the reflection points (he used the room dimensions you gave to calculate those tones).

From what you're saying, you've only done the tri-corners, is that correct? It might help to put some absorption in the cavity formed by your desk- but there's a bunch of stuff you need to do first: [url=]http://www.ethanwiner.com/acoustics.html[/url]

Foam won't do anything for bass, it can't absorb that much energy. In fact, foam is over rated and is often a waste of money if you haven't done enough trapping.

You're right, diffusion will help. It has the same effect as absorption, but it's more complicated and expensive to make it as effective as a large amount of rockwool. The trick is to get a balance between the two methods so you have a room that's pleasant to work in while being as acoustically neutral as possible.

pinksoir

Thanks again.

The traps are indeed just panels with a space behind. So should I be buying some more rockwool and filling up that cavity behind? Just reading that EW article tells me that the traps I've built are pretty much ineffective as the rigid stuff 1" is equal to the fluffy stuff 4-6" thick. So I've the equivalent of 1" of the rigid stuff, and he recommends a minimum of 4". Would it be better to replace the traps I have with RW3 instead of filling the cavity behind them? I could use the fluffy stuff for something else, I suppose, like fist reflection points maybe? It definitely works in deadening the sound right in front of it...

The resonance definitely seems to be at 120Hz as it's the B an octave below middle C as opposed to another octave lower. When I use a sine wave (which has no harmonics right?) it happens on both that B and the C above, so I guess it's happening at 120-130Hz.

Anyway, thanks a lot. I really appreciate the help.

EDIT: madtheory, what do you mean by the desk cavity? My desk is literally just a table top on legs.

madtheory

Re: desk cavity (a.k.a. resonator)- I assumed you had one of those modular studio desks, these are often great resonators. If it's open on all sides then it's not a resonator! Of course anything solid resonates to some degree, but for your purposes those are trivial.

A great solution for the corners is to stack a couple of rolls of RW3 in each one. Really simple and inexpensive. It will probably give you some "free" extra bass trapping. Handy for you, you can re use your panels at the main reflection points.

You probably don't need to trap 120Hz specifically, you can try Ethan's low bass absorbers by modifying the panels you have. But try just positioning the panels you have right now, and see if that improves the situation.

pinksoir

Ok, so you reckon I can use my fluffy rockwool panels for first reflection pojnts? That's handy enough.

And do you mean the rigid RW3 panels in the corners, or the rolls of the fluffy stuff?

pinksoir

Ok. So I went out and bought some RW3 and replaced the fluffy rockwool with in my traps. The traps are now good and solid and have definitley tightened up the bass all round.

But the volume/resonance problem is exactly the same! Bummer. I have no idea what is going on. It is starting to bug me a lot though. It's definitely 120-130Hz, when I EQ that out (about -9db) everything sounds fine.

It couldn't be my table, could it? It's made of particle board and when I put my ear to it I can here it resonating at the problem frequency. But that could just be the standing waves causing it to do so. I can hear all frequencies through the table, it's just the problem freqs are amplified.

Tomorrow, so long as the right shops are open, I'm gonna go ahead and build some first reflection dealies out of the fluffy rockwool I have left over. If that ain't working I'd welcome any other suggestions anyone may have.

madtheory

The RW that comes as flat slabs is best for panels because it's more dense than the rolls- so takes up less space for a given amount of absorption. But the rolls are cheaper, corners are deadly, so you might as well fill them up with rolls. Some people cut slabs into triangles and stack them up, the yanks call them "superchunks". A lot of effort for a minor gain. The rolls are just as good really.

So ya deal with the reflections first. Half the time you can get away without tuned absorbers.

Then you can tell properly if the desk is a problem.

Tis good friday- no builders providers will be open...

pinksoir

Cheers. I don't have any absorption on the ceiling yet. Should I be looking to build a trap there with RW3 or would the regular fluffy stuff do any good?

I have tried adjusting the monitors in both height and distance but it's the same deal. There is actually a load of resonance and it even makes stuff rattle, like the locks on boxes etc. be they on the floor or on the table.

CrimsonSounds

Have you ever tested your room at all ? REW (Room EQ Wizard) is a free app on the hometheatershack site I think, but you can google it. Once you find the offending waves, you can play around with some mobile traps while playing those frequencies and you may be able to tame it somewhat. Given how low the frequency is, you may want to consider a higher grade of rockwool for it - RW5 has a higher density than RW3 for lower frequency absorbtion so you might want to think about that also. Irish Insulation in Parkwest can get it if you need it, though they don't keep it on stock for some reason - plenty of RW3 up there though !

madtheory

If you read the EW article, he recommends absorption on ceiling and wall, as well as how to position the monitors (they're the links to other articles on his company's site) that's part of the "bunch of stuff" I referred to earlier.

There's no real need at this stage to run tests, given that you're not yet trapping specific frequency ranges. Although it would be nice to have before and after measurements at the mix position so you can see what's changed, hopefully for the better. But you absolutely have to treat the main reflection points first. You can play around with positioning other traps after.

i57dwun4yb1pt8

concrete ceiling eh ? how high ?

as Niall said :
i think you may need to float a thin ceiling filled with rock wool about 4 inches thick or likely more.
you could be sitting in a large ceiling to floor reflection area.

and fill the corners wall t oceiling with rockwool maybe up to 300 - 400 mil thick .
i think they call this super chunking - but you fill the whole corner very deeply , not panels.

concrete rooms need huge treatment for low end as they basically reflect everything in the low end all over the place. -
need aload of focussed deep absorbers.

we are talking 30 to 350 hertz in the main.

and a concrete ceiling - id say this is the main issue

you will need to absorb the **** out of a concrete room


iv turned a 7 by 7 by 7 foot concrete cube into a pretty decent mix room , in terms of low end control and mid to high liveliness
but its not pretty - the corners are hevliy filled
but the floor and ceilings are thin - so are bass traps themselves.

CrimsonSounds

I cut the slabs into 4 big triangles and stacked them all the way up. They're 2.5 foot wide, and at the deepest they're 16cm I think. Takes quite a few packs of them ! I did the 2" suspended cloud hanging about 2" off the roof - it's really low - and it works wonders. The mix position is almost a trap from the floor all the way around to the other floor with about 10% space. Extremely tight once you walk into the mix position. Definately worth measuring before and after as if the after situation needs to be tweaked, at least you'll understand what the first set of traps have changed.

TroutMask

Check the desktop for mechanical resonances first. Check if 2 of the room dimensions are the same - if they are, you'll get extra reinforcement of that room mode.
f=c/w (where c = speed of sound 1130 f/s, f is frequnecy and w is your wavelength)
w = c/f =1130/113 = 10 feet (the 113 is an approx. freq in the ballpark of your lo-mid cited freq.)
if your walls (short length) and your floor-ceiling is about 10 feet - there's your
mode problem

pinksoir

Thanks again folks.

@DaDumTish: Yeah, the room is pretty much concrete. It does have plasterboard on the walls and ceiling, though obviously that's not got any affect on the acoustics.

I can't do too much work on the place as I rent. Best I can do is build a cloud but I'm not so sure how I'd attach it to the roof securely (plasterboard).

@madtheory: I have my speakers positioned as EW recommends but yeah, I haven't treated the first reflection points yet so that's next.

@CrimsonSounds: I may have to do a similar thing...

@TroutMask: The distance between the walls and the floor to ceiling height is pretty similar alright. Around about 8ft 2. So the mode that gives me is around about 138Hz. That is outside the problem frequency range I'm experiencing, just about.

I'm going to take out my desk and see if that makes any difference. At least then I can remove it from the equation.

madtheory

pinksoir wrote: »

So the mode that gives me is around about 138Hz. That is outside the problem frequency range I'm experiencing, just about.

That's because it's not a solid concrete room. Accounting for the what the room is actually made of gets complex very quickly. Plasterboard is actually a good LF trap.

But ya, stuff those corners and trap the main reflection points, then you can see if you need to throw more science at it.

TroutMask

yeh - we have not even got into tangential modes yet. But having 2 dimensions the same is definitely a major problem w/ any room so it might never come right. You could look for a giant vase with a resonant frequency of around 130 hz

pinksoir

I only have a giant vase with a resonant frequency of 140 unfortunately

pinksoir

Right so. I took out the desk and the problem persists. At least now I know it's not that.

So next up is the first reflection points. I really hope they work. I have a load of the fluffy rockwool left from replacing it with RW3 in my traps. Though I'm thinking I should buy RW3 for the first reflection traps too.

Any idea for any uses I could put the fluffy stuff to?

CrimsonSounds

Hi There, What exactly is the "fluffy stuff" ?!!! I think I know what it would look like, but do you know what it is ? Appart from being a saggy bag mess to work into any asthetically pleasing shape, and given that it's probably low density with a lot of air in there, it may be more trouble than it's worth.

pinksoir

It's the rolls of rockwool, you know for insulation rather than acoustics. I had built my traps out of it but replaced it with RW3. I now have the roll left over. It definitely absorbs, especially when pulled tight, but more for mids to highs I reckon.

pinksoir

Disregard.

CrimsonSounds

Hi Pinksoir,

What exactly are the dimensions of that room in feet and inches?

Is that test done without any traps in the room at all, or is that with your original trapping in there ?

As for the fluffy rockwool, you could make panels out of it - though you would need to re-enforce them well to stop the sagging out the front. It's not going to be any good for anything other than mid-high freq trapping, so panels around the room would be where you would use it.

pinksoir

Howdy,

I uploaded a new image as the last one was a screwy reading (I had a feedback loop going on as I hadn't muted the line input).

Yeah, that's the reading with the RW3 bass traps in the corners.

I couldn't give an exact measurement, but it's close to 8ft 2" tall by 8ft 5" wide by 18ft 4" long.

madtheory

There's a very good reason several people have asked you about the room dimensions... have you read the entire EW article?

pinksoir wrote: »

It's the rolls of rockwool

This is the fourth time I've said this: stack the rolls in the corners! Assuming it's still in rolls? It is certainly dense enough to act as a good bass trap in this position. And why are you measuring, if you haven't got all the basic trapping and reflection points done?

The graph is weird. It looks like you've a buffer error, or very strange clipping. What mic did you use? You have to use a measurement mic at the mix position.

pinksoir

The measurements I gave are as close as I can get. I don't have a measuring tape, just a metre stick so they won't be perfectly exact, but they are extremely close (within a few mm). Indeed I did read the whole article.

Sorry, I missed your reply where you said to roll up the rockwool in the corners. It's all been cut up so I'll have to put it in something so bits of it doesn't float around the room! I'm gonna go buy the stuff for my reflection points today so I'll hopefully have them done by tomorrow.

I stuck up that measurement... dunno why actually. But anyway, it's a crappy one as I was still working out how to do it properly.

For my reflection points. I'm going to have 4 slabs of rockwool but I only need two reflection point absorbers and possibly one on the ceiling. Any ideas for the fourth slab? Or should I cut one in half and make two 90cmx120cm reflection point absorbers? Then have one 60cmx120cm for the roof if needs be.

Also, for the fluffy stuff in the corners. Can I use black sacks or should I use fabric to contain it? Does it make much of a difference?

Thanks again for the help...

EDIT: Actually, is an open A string on a bass 55Hz? In that case I'm looking at the B and C just above it which is ~61-65Hz. So the problem I have isn't 123-130Hz at all.

madtheory

Exactly what kind of RW is this "fluffy stuff"? So it's been unwrapped? You could just wrap it up again so it's the same as when you bought it, good and tight. Alternatively, google super chunks, that will give you an idea of what to do with the tricorners- these are usually made from the slabs though.

If you've extra slabs you can either:
1. Make the panels double thickness. That will certainly be fine on the ceiling, might use up too much precious space at the sides. If it's 50gm then there's not much benefit in doubling thickness it's already quite absorbent.
2. Put several on the front and back walls.
3. Put them on the bicorners.

The EW article has a picture of all the main reflection points. It'll also help you decide which of those 3 options is best, but I would favour 3.

pinksoir

Well I have the corners done floor to ceiling with RW3 traps so both bi and tri corners are covered. The front wall is a window so I can't really do that. I could maybe put a trap on the back wall, but half of that wall is a window, and the other half is shelving with books and boxes and stuff on.

The left over 'fluffy' rockwool I have is the stuff you get in rolls, 100mm thick. I'll roll it back up and stick it behind the RW3 corner traps I have. The question I have is if it matters what I use to hold it together with; fabric or black plastic bags. Does it need to breath or what?

I haven't decided yet, but I might make my first refection points 90cm wide instead of the usual 60cm I did for my corner traps, unless this is a bad idea?

madtheory

USE THE CORNER PANELS FOR THE REFLECTION POINTS. No need for panels and rolls/ fill in, that's a waste of good panels. Fill the corners simply with the rolls. It will work fine in the packaging it comes in. But given that you've unwrapped it, you would do a nicer job building a super chunk of some kind and stuffing it tight with what you call the "fluffy stuff". You'll get slightly better LF absorption that way, without the plastic wrapping. The "breathable" thing is only a very rough guideline, it's not hugely important, especially at the level you're at.

And...I did say this several times already: take out your existing corner panels and use them for the reflection points.

For the size of room you're in, a bigger panel is better (within reason). So build whatever size is most convenient for you.

BTW if you've only done the corners, you've only done the tri corners. There's a bicorner everywhere you've a right angle!

pinksoir

Ahaaaa. Bicorners. Of course.

I didn't understand that you were telling me to take out my existing traps and replace em with the rolls. Got it now. I did go off earlier and bought more RW3 though...

Do you think the rockwool rolls turned into sort of superchunks like you described would be better than the RW3 panels I have now or about the same? Cos if they're better I'll build them, but if it's negligible I'll leave it as it could be fiddly enough.

Thanks again.

madtheory

Eureka!

The rolls are more effective than panels. And there's no building as such- just stack em in the corners and cover with something pretty. As you say- "sort of superchunks". They measure only slightly less effective than actual superchunks, with zero effort.

A normal room takes two with a slight gap at the top. You can also cut a roll with a saw to make a small fill for the gap at the top.

pinksoir

Alright so. I only have one roll of fluffy rockwool though so I wouldn't have enough to superchunk without buying more. I presume it has to be a certain thickness (ie as thick as a roll) for it to be effective?

I'm gonna go ahead and build 4 more panels from the RW3 I got earlier anyways, treat the FR points and go from there.

BTW having a listen to your tune-skis at the mo (vol II). Good stuff.

madtheory

Thanks

Just to re-iterate: leave them in their original packaging and just stack them. Normal rooms take 2 in each corner.

pinksoir

Good stuff.

I do have the RW3 panels I bought today though. I don't really fancy spending more cash on 3 more rolls of rockwool to do the corners. At this point I guess I have a few options;

1) Stuff what fluffy rockwool I have (one roll) in behind my RW3 panel corner traps to fill the gap. I think what I have will be enough. This'll leave me with 4 RW3 panels to build absorbers from.

2) Build 60cm-faced triangular superchunks from the RW3. This'll leave me with 2 RW3 panels to make absorbers from.

3) Buy 3 rolls of rockwool and stick them in the corner. This'll leave me with 8 RW3 panels to use as absorbers.

As I said I'd rather avoid parting with any more cash so for me number 3 is kinda out. Between 1 and 2 what would be best?


Obviously I'll be building my first reflection points first before I do anything else.

i57dwun4yb1pt8

you need to take the wrapping OFF
and cover them in either cloth that allows air in but no fibres out , or two layers of decorators dust sheets.( cloth )

the plastic wrapping makes them near useless- the air needs to get in .

pinksoir

I'm not so sure about that. From what I've found online both EW and the dudes at GIK advise to leave them in the wrapping.

But anyway, I'm not going to be using the rolls so it's irrelevent

i57dwun4yb1pt8

ethan winer said that ?

hmm, surprising - well he's the man , so i stand corrected.

madtheory

I guess you missed the extended debates about OC with or without foil backing, and the use of cling film for a membrane absorber? Endless fun...

pinksoir

Okaaaay. I built my FR absorbers. The resonance thing is still there, exactly the same.

On the plus side, the FR absorbers are really great.

What now?

madtheory

You've done the main reflection points including floor and ceiling? And the bicorners? Have you got exact room dimensions and put them into a mode calculator? I'm not clear on your room size- do you have two equal dimensions? Because that will require a Helmholtz absorber. Have you tried moving the mix position slightly?

pinksoir

Okay, here's a breakdown of what's going on.

My room is:

255cm wide (8ft 4")
248cm tall (8ft 2")
559cm long (18ft 4")

I have calculated the modes using EW's mode calculator. They are as follows:



The problem frequencies I am experiencing are:

61-65Hz

123-130Hz

Basically both C an octave and two octaves below middle C, and the Bs below them.

The trapping I have done thus far is:

-Front two corners, floor to ceiling with 100mm (4") RW3 panels.
-Left and right First Reflection points with the same, but not floor to ceiling.

I have two RW3 panels left so I will treat the ceiling tomorrow. I haven't done the bi-corners as I don't have enough panels/cash to do them right now. I can't easily treat the back corners really as there's a ton of stuff there - bookshelves etc.

Looking at the modes from the ModeCalc it looks like the problem frequencies are occurring in the modes from the room length... though I don't really know what I'm doing so I could be wrong.

Cheers

madtheory

So let me get this straight... you still haven't done the superchunks made of rolls of RW? You still need to do all four tricorners? And you can move the front corner panels to the ceiling which SAVES YOU BUILDING MORE PANELS. And put a rug on the floor. Then come back to us...

pinksoir

I am not going to buy 3 more rolls of rockwool to stick in the corners as they are nearly 30 euro a roll and I have no money left. I explained to you numerous times that I only ever had one roll of rockwool. I don't have 4 rolls to stack in the corner. I do however have the one roll rolled back up into black sacks and packed behind the RW3 panels. It reaches about half way up, stacked, on each side.

If the rear tricorners are the next most important thing to do, then I'll do them today. It will be extremely difficult. I explained earlier in the thread that my back wall is half window, half shelving. And the door into the room is where the back wall meets the side wall. I will find a way to do it.

I have a rug on the floor and it's carpeted anyway.

Thanks again for the help.

i57dwun4yb1pt8

FIGHT ! FIGHT ! FIGHT ! FIGHT !

fitz

pinksoir wrote: »

I am not going to buy 3 more rolls of rockwool to stick in the corners as they are nearly 30 euro a roll and I have no money left. I explained to you numerous times that I only ever had one roll of rockwool. I don't have 4 rolls to stack in the corner. I do however have the one roll rolled back up into black sacks and packed behind the RW3 panels. It reaches about half way up, stacked, on each side.

If the rear tricorners are the next most important thing to do, then I'll do them today. It will be extremely difficult. I explained earlier in the thread that my back wall is half window, half shelving. And the door into the room is where the back wall meets the side wall. I will find a way to do it.

I have a rug on the floor and it's carpeted anyway.

Thanks again for the help.

I may be wrong (and I'm sure I'll be corrected if I am), but if you're not going to fill the corners fully, I suspect you'd be better off having those black sacks in the top half of the cavity behind your panels.

I'd also say that the corners near the listening position are more important than the rear corners, if on limited budget.

pinksoir

Naww, madtheory's aiight.

I'm looking at treating that tricorner above where the door is. Any ideas on any designs to use? With my RW3.

I can think of a triangular shaped one suspended in the corner, or superchunking, or a 120x30cm (half size panel) suspended lengthways across the bi corner and into the tri corner.

madtheory

Yes, fitz is right, the front corners are the most critical, but not by much.

pinksoir wrote: »

I am not going to buy 3 more rolls of rockwool to stick in the corners as they are nearly 30 euro a roll and I have no money left.

OK, but you can't get around the physics of it. If you want get rid of the resonance, you need more absorptive mass in the room.

fitz

Would agree with MT, there's no cheap way to treat a room to a high standard.