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Huge standing wave, help needed

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RobinKidderminster

New member
May 27, 2009
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Rockwool is floppy like foam of the same thickness roughly so it needs support. Bass traps is the first to try - cut triangles of RW and pile them as high as possible - this will give you an idea of effectiveness before covering them properly and a little research will tell you sizes etc. For a panel I used thin panelling held at the corners with brackets to make a 6x2 box and stretched wire to hold the rockwool in place in the frame. Covered properly they are a cheap solution and if u don't like them before buying commercial panels it will tell u if its gonna work well.
 

Benedict_Arnold

New member
Jan 16, 2013
661
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RobinKidderminster said:
Suggest throwing some quilts into corners just to see if things improve. Rockwool is fire resistant I think. 4kg is not a great weight over a decent area.
Rock wool is completely fireproof. It's made by dripping molten rock onto a spinning half-grapefruit sized steel cup, upside down. The molten rock spins off as small fibres which instantly harden. The fibres are then collected and formed into mats, batts, whatever.
Good sound insulation as well as good loft insulation, but can be "dusty" if not faced off or sealed behind plasterboard.
I've been to their factory in South Wales and seen it for myself.
 

Benedict_Arnold

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Jan 16, 2013
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They're saw toothed plastic spikes to grip the insulation with big plastic feet (hence the name) for sticking to walls or ceilings. Kind of like huge plastic drawing pins.
 

sarged

New member
Jun 6, 2016
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Benedict_Arnold said:
Rock wool is completely fireproof. It's made by dripping molten rock onto a spinning half-grapefruit sized steel cup, upside down. The molten rock spins off as small fibres which instantly harden. The fibres are then collected and formed into mats, batts, whatever. Good sound insulation as well as good loft insulation, but can be "dusty" if not faced off or sealed behind plasterboard. I've been to their factory in South Wales and seen it for myself.
Well if they are really dusty as you say, I can order foiled boards as well.

Here is how they look like, btw it's called ThermalRock Slab. they have 120 kg foiled 0.6Mx1.2Mx2inch panels for $20 /piece.

 

thewinelake.

New member
Jan 22, 2016
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Just reading this site: http://www.gikacoustics.com/understanding-different-bass-trapping/

Makes me wonder if you could somehow build a pressure trap tuned to your standing wave?

Another fascinating article here: http://arqen.com/bass-traps-101/placement-guide/

Also http://audioundone.com/do-it-yourself-bass-traps

And, blimey, there's whole bloomin' forum dedicated to the things here: https://www.gearslutz.com/board/bass-traps-acoustic-panels-foam-etc/
 

BigH

New member
Dec 29, 2012
97
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sarged said:
Benedict_Arnold said:
Rock wool is completely fireproof. It's made by dripping molten rock onto a spinning half-grapefruit sized steel cup, upside down. The molten rock spins off as small fibres which instantly harden. The fibres are then collected and formed into mats, batts, whatever. Good sound insulation as well as good loft insulation, but can be "dusty" if not faced off or sealed behind plasterboard. I've been to their factory in South Wales and seen it for myself.
Well if they are really dusty as you say, I can order foiled boards as well.

Here is how they look like, btw it's called ThermalRock Slab. they have 120 kg foiled 0.6Mx1.2Mx2inch panels for $20 /piece.
Foiled will be no good it will reflect some sound waves, probably cause you more problems. You want to get acoustic insulation that is for heat, insulation in cold countries.
 

sarged

New member
Jun 6, 2016
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RobinKidderminster said:
I think that stuff is too hard? But it certainly must be acoustically transparent so foil backing useless
Well harder is better. 2 inch of 120 kg density is roughly equivalent to 4 inches of 60 kg density absorbents.
For rockwool this will mean around 40% abroption at 125 hz and 100% absorption at higher frequencies.
As of foil, contrary to what you say, I think it will offer further dumpening. Maybe it can reflect some very high frequencies, but this could only be a problem if the panel is facing the tweeters. If placed behind or above the speakers, I don't see how it will hurt.
 

BigH

New member
Dec 29, 2012
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sarged said:
RobinKidderminster said:
I think that stuff is too hard? But it certainly must be acoustically transparent so foil backing useless
Well harder is better. 2 inch of 120 kg density is roughly equivalent to 4 inches of 60 kg density absorbents.For rockwool this will mean around 40% abroption at 125 hz and 100% absorption at higher frequencies.As of foil, contrary to what you say, I think it will offer further dumpening. Maybe it can reflect some very high frequencies, but this could only be a problem if the panel is facing the tweeters. If placed behind or above the speakers, I don't see how it will hurt.
If you know the answers why are you asking us questions?
 

sarged

New member
Jun 6, 2016
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BigH, don't get offended because I've also read that foil helps with dampening. I don't pretend that I know really, but this is something that seems logical to me, so I just expressed 'what I think' . Of course, I will highly appreciate if you can give me some arguments or references that will prove my point wrong.
 

Benedict_Arnold

New member
Jan 16, 2013
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I have the Rock wool product book somewhere - but my copy is 20 years old so it may be a bit out of date...

Look at the products on line and do your homework.

I would have thought thicker less dense material would do a better sound absorption than thin dense stuff, but I am more than open to being edjumacated to the contrary.

While I'm on, however, are you sure this is the way to go? Check out the posts on avsforum.com, not avforums.com, about home theater (sic) builds. Lots of experts (ahem) posting there. If you're not careful you could end up just moving your standing wave problem up or down a few Hertz. What you need to do is stop the reflections all together or disperse them so they don't set up a standing wave. Fake Persian carpets hung on the walls might be a better solution technically and aesthetically.
And yes, back in the days when the World was black and white and people walked funny, they really did stick egg boxes to the walls of recording studios. My paternal grandfather was the Chief Engineer at BBC Bristol until 1968.
 

BigH

New member
Dec 29, 2012
97
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sarged said:
BigH, don't get offended because I've also read that foil helps with dampening. I don't pretend that I know really, but this is something that seems logical to me, so I just expressed 'what I think' . Of course, I will highly appreciate if you can give me some arguments or references that will prove my point wrong.
How can foil help with dampening bass?

Maybe you should just get a load of egg cartons instead.
 

Benedict_Arnold

New member
Jan 16, 2013
661
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BigH said:
sarged said:
BigH, don't get offended because I've also read that foil helps with dampening. I don't pretend that I know really, but this is something that seems logical to me, so I just expressed 'what I think' . Of course, I will highly appreciate if you can give me some arguments or references that will prove my point wrong.
How can foil help with dampening bass?

Maybe you should just get a load of egg cartons instead.
The foil is actually aluminized ripstop plastic (i.e. plastic sheet, with nylon cords at 1 cm x 1 cm grid (approx.) embedded, with a microscopically thin coating of aluminium on the outside.

The plastic is thin enough to trasmit sound through to the damping / absorbing layer of rockwool beneath without causing too much reflection (it's not drum tight or on a hard surface after all), but is there mainly as a dust and water barrier. Rockwool doesn't work well as a thermal insulation if its saturated with water. Trust me, we tested the snot out of this stuff when we were contemplating using it to insulate "pipe in pipe" underwater pipelines back in the 90s. (Pipe in pipe uses two concentric pipes. The inner one carries the oil and / or gas, the annulus is filled with thermal insulation, and the outer pipe stops the insulation from becoming saturated by seawater and / or crushed by the water pressure. My then employer makes a killing selling such pipelines for deep water oil and gas production.)

But this does bring up a point. I'm not sure Rockwool will stop the standing wave unless its surface is exposed to the sound. Hidden behind plasterbaord it will only reduce sound (or noise, depending on what's playing, what time of night it is, etc.) being transmitted into other rooms. The plasterboard will reflect the sound just as well as the original wall does. And unless you want your living room looking like a well insulated attic....
 

NSA_watch_my_toilet

New member
Aug 24, 2013
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Your bass is already -10 dB under your actual highs, many more if we look at the mediums. So I would not insert corner bass traps, IMO it will worsen the problem. What you need, is to tamper this hugly medium jump. For this, you will need some absorbers in THIS frequency (they are different densities of foam). Looking the intensity of the problem, you will need more than one.

What I would try to do, is to damper the room to tame the mids and highs for coming back at the same volume of the bass frequencies. It will although have an incidence on this "waving" of frequencies that you have.
 

sarged

New member
Jun 6, 2016
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Actually I've just tried applying DSP, by manually lowering the hyped frequencies using my ears and a variable graphic equalizer.
I only lowered the frequnecies that sounded too loud (56, 124, 197, 271, 650 hertz) so that it overall sounds smooth from my listening position.

This produced interesting results, a good deal of unwanted boominess has gone and even as I walked around the room it sounded better. The bass is still there - in more or less ok proportions, but some songs just became thinner sounding.
I haven't tested it much, but let's see where this gets. Maybe I should try applying some broader EQ on top of that?


I wonder if -10 db for my bass has to do with my room, or maybe it is a problem of measurement.
I believe that cheap mics can roll off the low end, (as well as high end).
Edit: I just tried applying +10 db boost for low end and it sounded like **** lol... the sound has still got plenty of bass, and even me being greedy for bass, I don't find it lacking, so maybe it just doesn't show on that graph.

Here is how my eq looks right now:

I also rolled off 30 hz because they have little impact on sound while still producing tough load on the amp.
That onkyo receiver is shittier than I thought, it clips prematurely. Never again will I buy a receiver lol...
 

steve_1979

Well-known member
Jul 14, 2010
231
7
18,795
sarged said:
... I did some research and converted this frequency to wavelength resulting in 2.74 meters...
Are your speakers exactly half way between the floor and the ceiling by any chance? This can often cause standing wave issues.

Even if they're not half way between the floor and the ceiling, try raising or lowering your speakers by a few inches to see what effect it has.
 

andyjm

New member
Jul 20, 2012
15
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0
Benedict_Arnold said:
RobinKidderminster said:
Trust us. Foil will negate any positive effect. :-(
Only if you line your hat and underpants with it....
A quick primer on the way stuff works.

Soundwaves are in reality air molecules moving back and forth. An acoustic panel works by having these molecules move back in forth in a region where there is a lot of friction (the twisted fibres in a rockwool mat for example). This slows the air molecules, dampening the sound, with the sound energy being given off as heat.

Sealing the panel, either by having a foil cover, or by covering the panel with material that has too tight a weave will make the panel useless.
 

andyjm

New member
Jul 20, 2012
15
0
0
sarged said:
Actually I've just tried applying DSP, by manually lowering the hyped frequencies using my ears and a variable graphic equalizer.I only lowered the frequnecies that sounded too loud (56, 124, 197, 271, 650 hertz) so that it overall sounds smooth from my listening position.

This produced interesting results, a good deal of unwanted boominess has gone and even as I walked around the room it sounded better. The bass is still there - in more or less ok proportions, but some songs just became thinner sounding.I haven't tested it much, but let's see where this gets. Maybe I should try applying some broader EQ on top of that?

I wonder if -10 db for my bass has to do with my room, or maybe it is a problem of measurement.I believe that cheap mics can roll off the low end, (as well as high end).Edit: I just tried applying +10 db boost for low end and it sounded like **** lol... the sound has still got plenty of bass, and even me being greedy for bass, I don't find it lacking, so maybe it just doesn't show on that graph.

Here is how my eq looks right now:I also rolled off 30 hz because they have little impact on sound while still producing tough load on the amp.That onkyo receiver is shittier than I thought, it clips prematurely. Never again will I buy a receiver lol...
I had suggested DSP earlier in the thread. You seem pretty computer savvy. Have a look at 'Room Equalisation Wizard' (REW). Partnered with the right hardware, this will allow you to effectively construct a filter that is the inverse of your room, allowing you to negate room effects as much as possible. It is not without its challenges, and it can't fill in nulls, but for a specific listening position it can work quite well.

Even if you don't want to go down the FIR/convolver route, REW provides sharp notch filters that will allow you to do a more tailored job on getting rid of the peak.

Edit:

Get yourself a decent measurement mic, the -10dB LF is almost certainly the mic. MiniDSP out of HongKong sell a decent USB calibrated measurement mic.

Don't bother pouring power into a null, its a null. All you will do is fry the amp or the speakers. The only way to fix a null is do something physical - move stuff around, use a different room.
 

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