DIY bedroom treating for mixing - how to?

bassmothership

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Hi, greetings from Japan! this is my first post here.:jump:

I'm a bedroom mixing engineer. Absolute noob in acoustics. I really need help with sound absorption (specifically to tame the range below 200Hz) and learn ways to DIY my own panels but without using any of that Mineral Wool stuff. I wanna learn about the options of "healthy" materials that can make my bedroom stop fooling my ears too much. :sarcastic:
You can see from the picture that I deliberately placed futons on walls and covered the corners with curtains, also spread cases and gear to get some "treatment" to the room.

So I did my very first measurement with REW from my mixing position, using a Focusrite 18i20 (1st gen) and a Behringer ECM8000 through a pair of Yamaha H50M speakers.

Now that I get my room measurements, what's next? How do I actually get started? What should I look for? I'm so completely clueless...

To make things much worse for me, I live in Japan and it seems this is a country in the stone age of acoustic treatment.
There's not much happening here regarding home studio DIYers, (there are no companies coming up with effective Bass Traps solutions, credible audio forums, etc...)
People here basically follow the trend and instead of DIYing acoustic panels and bass traps, they spend money with foam materials. (Auralex stuff is everywhere here). There are some DIYers working with mineral wool here and there, and that's basically it.

In other words, I'm totally on my own here.

Please, help me. :whew:
 

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MeJ

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I'll post some reply tomorrow... have to write it up.
 

bassmothership

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Thank you for your reply.

Since when I started this thread I changed my room a bit. On the corners I stacked those cases filled with clothes and duvets and push the monitors really close to the door window. And did the measurements attached below.
 

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MeJ

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Good, you are still there and thanks for posting the plots, that helps.. I'll write up a response then and post it tonight or tomorrow: there's some things you can do, but recording and mixing in a very small space involves inevitable compromise, as no doubt you are aware :)

Overall, AVNirvana is more a Home Theatre forum. There's a lot of expertise here re: REW but less so for recording and mixing. You might also want to check out John Sayers' Recording Studio Design Forum. I'll list a few more resources as well.

MeJ
 

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Sorry for delay, got stuck with stuff. Will post today.
 

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So, what I'm going to do is tell you what I would do with your room and why. I'm actually recording this in a very similar sized room which has very similar problems. One advantage I have here is that the ceiling height is quite a lot greater so I've been able to put lots of stuff on the ceiling, that’s harder for you.

Let's look first at a few of the problems in your room. I see these as being, first, your room is asymmetrical at low frequencies. This is because you have one wall that is solid and one that is plasterboard (as we say in the UK) or dry rock as the US calls it I think. That wall is transparent to low frequencies, say below 150 Hz. This means that as far as the audio is concerned, mid frequencies reproduced by the speakers are seeing a room that is more or less symmetrical, but at low frequencies it's not, that wall is transparent to low frequencies, so your speakers are not symmetrically placed within the actual space at low frequencies. I have exactly the same problem here in this room and there's nothing (easy) you can do about it. Fortunately it's not going to be a really big issue, and it does have the slight benefit that the room will reproduce lower frequencies than it should given the apparent dimensions. What's more important is the second problem, again an asymmetry, and in this case that is the L-shaped stub at the end of your room with the door in it which will resonate, causing a slight “honk” at some frequency. We’ll come back to that soon.

The main requirement for any small room to control sound is bass trapping. Mid and high frequencies are rarely a problem and if they are a problem they are really pretty easily tackled. Because of the wavelength of the sound involved, a couple of centimetres or inches of absorbent will tackle high frequencies. Five or six inches or 15 cm of absorbent will handle mid frequency. For bass you need 4-ft, or over a metre, of absorbent as a minimum and that's never easy to accommodate. Note that it is possible to use resonant absorbers for bass, but they need to be bought or made. Not simple.

I’d do the following things:

  1. we need to add some bass trapping.
  2. we need to deal with wall reflections from the speakers.
  3. we need to deal with speaker placement and comb filtering (SBIR).
  4. we need to make the room as symmetrical as possible at mid frequencies and above
  5. Then measuring what we have now and adjusting, dealing with reflections at high frequencies etc.

Let's start with the bass trapping. You don't have a lot of room but I can see one place you could do some trapping and that is over the large glass door/window behind your mixing area.

I really like using thick (“winter”) duvets for cheap sound treatment. They are readily available, conveniently packaged, cheap, fairly light weight, and you can easily buy nice covers to brighten things up a bit. They also avoid making serious structural alterations. Of course, it’s not as good as “real” soundproofing but everything is a compromise.

See the photo 3 attached here. This shows the thick duvet that I have hanging down the wall behind the speakers. There's actually a chimney breast here, so in fact there’s around 1 foot or 30 cm of depth behind the duvet for half of its width, with bookshelves and books.

You could do the same thing, and this would help kill the reflections and interactions at this end of the room. Of course you have the window/door there, but what I did was sew curtain rings to the duvet and hang it on a curtain rail (image 2) so it can fairly easily be moved for access to the books, the same would work for you.

However I’d recommend a variation. You have a low ceiling. I’d drape a long duvet from a position say 4ft/120cm out from the wall, and across another rail just above the windows, forming a rough and ready bass trap and also killing the wall where the windows are - see image 4. This may take a bit of creative thought to make it work for getting light in too, I’m 6000 miles away so can’t guess at that :)

In the second photo you have moved stuff in the fill in the corners of the room by the speakers. This will have some effect but it could be improved.

A point to bear in mind with sound treatment. Walls, so a first approximation anyway, do not move! So any sound treatment that is in contact with a wall is not doing anything and is largely wasted. It’s much better to space it away from the wall, it then is doing some work.

I would be inclined to hang two more duvets (possibly part doubled over to double thickness) across the corners to make the whole of the corner into a bass trap. They need to have a depth of at least 3 ft to do anything. There’s no reason futons could not stay in there.

We could include item 2 while we are at it. If you look at my photo 1, you can see that I have two panels set so as to absorb the reflections from the walls adjacent to the speakers.

You could do this, or use a hanging duvet to do the same thing. Both would work fine. The idea is to put something on the wall to block the reflection that would be there if you put a mirror on the wall. The exact placing depends on where the speakers and listening position are.

As a bass trap bonus, at the end of your room you show a wardrobe. Is this stuffed full of clothes? If so, good, if not then I would stuff a duvet or two into it. The point is, then leave the door open while recording. Instant sort-of bass trap. It will also serve as a sort-of vocal booth. Place the mic so that the singing is going into the wardrobe and it will minimise reflections from behind the mic.

As a final bonus, there’s the L shaped stub at the end of the room. If you hang another duvet to seal off the L, with yet another on the wall opposite, you will effectively be creating another simple bass trap while also rendering the room somewhat more symmetrical at higher frequencies. This will also stop the stub causing resonance effects.

For this to work, the duvet needs to be ceiling to floor and completely cover the stub.

Now, I prefer duvets to futons as they are lighter and more easily moved. If you are going to put something over a door or window access it needs to be easily moved, but there’s no reason that the futons can’t be used on the side wall opposite the stub and door, or at the side of the speakers, as they will not need to be moved.


That’s it for part one, more in part two…

PLEASE NOTE this is all “in principle”, there’s a whole lot of ways of achieving the objectives, this is just to give the idea, adapt as necessary!
 

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MeJ

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Part 2

OK, so the plan:

  1. we need to add some bass trapping.
  2. we need to deal with wall reflections from the speakers.
  3. we need to deal with speaker placement and comb filtering (SBIR).
  4. we need to make the room as symmetrical as possible at mid frequencies and above
  5. Then measuring what we have now and adjusting, dealing with reflections at high frequencies etc.


We have covered 1 and 2. Now let’s look at the rest, and start using REW to help.

As far as I can tell, you have a pair of Yamaha HS5 speakers - could be wrong, they could be 7’s but I think not. Let’s look at the frequency response of these to set a baseline, see the two images below… one of these is the “factory” response from Yamaha, the other much less flattering one is from the Sonarworks blog and their anechoic chamber (all references at end). I trust the Sonarworks data more.

Their curve shows a big boost at 1 kHz and a strong roll-off after a smaller bump above 10 kHz.

How much of your room response is from the speakers? Well, all your curves show a broadly similar picture, of a boost around 1 kHz and a bump and strong roll-off above 10 kHz, so it is feasible that these features are largely down to the speakers. If you want to change that, you may have to mess with the settings for HF on the back, or ultimately, change the speakers for something flatter. These HS5 speakers are well regarded it seems, but they are not NS10s which have many superior characteristics, while sounding like **it for general music playback - for views see the references at the end. Anyway they are what you have.

What you emphasised was concern for response issues below 200Hz. These speakers don’t really give much below 70 Hz. The HS7 has more response down to say 50Hz, another half octave or so. So we are discussing the lowest octave and a bit of the speaker's response in the room.

How can this region be improved? Here’s Point 3.

Take a look at the diagrams I've done using the REW room modelling facility—this is a really useful tool for getting an idea of what changes moving stuff around can make. Unfortunately it's no feasible to model the stub in REW so I have ignored it, this means the modelling is very approximate, though some of the eignetomes will be identical, there will be other ones involving the stub that are at lower frequencies.

Note the settings of the model: I have not used subwoofers (the usual purpose of this) but main speakers, and I have in the first case placed them at my best-guess at your current positioning - but it’s just a guess. Also I have used the sealed box setting - this is not true, and gives a slower rolloff than reflex would, but I suspect it is more representative in practice of your room given various room gains.

Note the dreadful response below 120Hz.

Now look at the second model. All I have done is move the speakers back against the wall.

The response below 120Hz is very much better.

There’s two reasons for the change. First, the speakers are much better loaded by the room as they are closer to a room boundary, but also we have got rid of boundary interference. This is a form of comb or notch filtering which will kill certain frequencies by direct cancellation. In real life it won’t be quite as clear-cut as in the model, but it’s still very real.

If you move the speakers very close to the wall behind them then you almost certainly will improve the response below 120Hz or so.

If we look at your response measurements, they all show a huge hole at around 100Hz which is also visible in the model of your current speaker placement.

Rather than go on for ever here about this, have a look at the article here:

http://arqen.com/acoustics-101/speaker-placement-boundary-interference/

  • Which explains it all really well. Note that your speakers are definitely at the small end of the scale, so note the warnings that are given in that article. If they were 4” I’d be worried, but with HS5 I’d just carry on as recommended.

With your speakers, they are rear-vented bass reflexes, so you can’t get too close to the wall. If you hang a duvet close to the wall to kill the effects of the comb filtering, and get the rear of the speaker within an inch or two from the duvet and so within 6” of the wall, you can remeasure and see if it affects the 100Hz hole, as in theory (and the model) it should. It’s always down to suck-it-and-see, and adjustment will make some things better and some things worse, the trick is to balance these with what you want to achieve. REW measurements are a great tool to see what effects changes produce.

Point 4: Room symmetry. I can’t see the rear of your room, but I can see a lot of guitar cases on one side behind the digital piano of whatever it is. Id’ try to distribute this lump of solid reflection more evenly around the room. Although you are using “near-field” monitors, your ear still uses the far-field reflections to judge space, so it’s a good idea to try to make that reflection reasonably symmetrical. Just try and make the two sides of the room reasonably similar. It won’t affect the measured response much, but will help your ear especially with spatial and reverb judgements.


Point 5. Back to the measurements.

If we look at the first three simple response measurements, there’s a peak at 55 Hz or so and another at 75 Hz or so. If you look at the modelled room response, the peak’s at 50Hz. All of this is down to the lowest eigentone of the room - the lowest direct wave that fits into the room. I’d bet that the low one is due to the length into the stub where the door is, the higher one being down to the other corner of the room, and that’s why there are two instead of the single peak in the model. It’s not inherently bad, and in any case there’s nothing you can do about that without changing room dimensions, but putting the duvet over the stub should smooth it out somewhat. Any bass trapping you can do will smooth that out, and also help with the 100Hz notch, though I reckon that’s mostly the SBIR issue as mentioned above.

You may be able to put another bass trap duvet above the wardrobe, or just hang a duvet right across the whole of the rear wall in front of the wardrobe with the door open. That should clean up the room response quite a bit.

Looking at the waterfall responses, these speakers are pretty clean above 500 Hz or so with respect to resonances - much better than the original NS10s were.

I don’t know what the nasty hole at 300Hz is. It could be the ceiling notch (same logic as SBIR) as it involves a distance of around 45 inches. If so, then the duvet hanging from the ceiling over the speakers in the first drawing I did might well help with that. But the quarter-wavelength is around 11 inches. What’s 11 inches from the speakers?

Right, that’s it for now, HTH.

MeJ

References

Wavelength for frequency
http://www.mcsquared.com/wavelength.htm

NS10 stuff
https://www.soundonsound.com/reviews/yamaha-ns10-story

Sonarworks
https://www.sonarworks.com/blog/gear-reviews/yamaha-hs5-studio-monitor-review/

SBIR, again
https://www.genelec.com/monitor-placement

Various speakers FR
https://www.gearank.com/guides/best-studio-monitors
 

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bassmothership

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I'm back!

I'm reading carefully and digesting slowly.

You're extremely kind for putting this wealth of information here my friend. Cannot thank you enough.


Before anything, what are duvets made of in your country? I ask that cuz here nothing is made of cotton, Here is basically polyester or polystyrene. Do you think that polyester duvets are worth it?
 

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Polyester is good. The Master Handbook of Acoustics by Alton Everest is excellent. Cloud, ie. Overhead treatment is good. RealTraps and GIK websites have detailed guides as what to put where.
 

MeJ

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Re duvets: I, like you, am keen on 'natural' materials, especially against my skin. Bulk cotton (like futons) can be useful in audio, wool, and particularly longhair wool, more so, especially in speaker design. But polyester duvets are very effective if thick and dense enough, which is why I specify winter weight. I'm in Europe, and the winter ones here are very suitable. I'll post later on when I have time a bit of an explanation on how to use duvets (or any absorber!) effectively...
 

bassmothership

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Polyester is good. The Master Handbook of Acoustics by Alton Everest is excellent. Cloud, ie. Overhead treatment is good. RealTraps and GIK websites have detailed guides as what to put where.

Thank you for the book suggestion! I've just got it now, gonna study it thoroughly. I'm also reading Floyd Toole's Sound Reproduction.

Re duvets: I, like you, am keen on 'natural' materials, especially against my skin. Bulk cotton (like futons) can be useful in audio, wool, and particularly longhair wool, more so, especially in speaker design. But polyester duvets are very effective if thick and dense enough, which is why I specify winter weight. I'm in Europe, and the winter ones here are very suitable. I'll post later on when I have time a bit of an explanation on how to use duvets (or any absorber!) effectively...

Since I cannot find any cotton or denim, if I can use polyester, that's some good news at last. :jump:
 

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Many countries, notably not the USA are adopting Polyester . Caruso Isobond, Autex.
It does not need fabric covering and is fire certified and is a great absorber.
 

MeJ

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Bass treatment in a small room: How to achieve the desired effects.

First, what are bass frequencies? I can’t find an agreed definition. Wikipedia says 16-256 Hz, in other words the musical bass definition. For me, bass mainly is less than 100Hz, that is low bass instruments, bass drums, and so on. Let’s compromise and say below 200 Hz or so. So how do we treat the region below 200Hz?

First, why do we need to? It’s because in a small room the bass response will initially be defined by the eigentones. Eigentones, “own tone” or better “self tone” from the German, are the resonant frequencies defined by the room dimensions.

In a very large room - say over 120ft/35m in its largest dimension - there are many eigentones in the 20-200Hz region: see image 1

In a large room, say 40 ft/12m still there are many resonances in the bass region: image 2

In a small room, there are very few: image 3.

This means that in a small room the eigentones in the bass region have a tendency significantly to relatively emphasise the frequencies at which they occur, distorting the listening experience.

So, what can we do in the small room? We can’t change the frequencies of the eigentones without changing the room dimensions.

What we CAN do is smear out the resonances, so as to reduce their effect on the room.

There’s at least two main ways to do this, but both involve changing acoustical energy - movement of the air molecules - to another form of energy, usually heat, via adding resistance.

All methods of doing this are called bass trapping, but they work in different ways.

Wavelength issues

The problems with altering low frequencies come from their wavelength. For example:

40Hz (about low E on the bass guitar): wavelength is about 29 ft or 8.6 m.

120 Hz: 9.4 ft/2.9m

4000Hz: 3.4 inches/9cm

If you put 4” of foam on the wall it will completely dampen incident sound at 4000Hz and above. It will not noticeably affect 120 Hz or below. Let’s see why.

Sound waves, nodes and anti-nodes, pressure and velocity

We usually represent a sound wave as a vibrating string, like this:

See image 4 (from https://www.acousticsinsider.com/why-bass-traps-in-corners/)

But we should remember that really this vibrating string representation is indicating the velocity with which the air molecules are vibrating, somewhat more like this:

See image 5 (from https://www.semanticscholar.org/paper/Time-conservative-finite-volume-method-with-for-Aybay/9d91ae2dfbf99ecf355f6a7816e29b276ced982c)

In image 4, note that there is also an arrow pointing at the wall, and indicating that there there is maximum pressure and minimum movement. The minimum movement is obvious: the wall does not (to first approximation anyway) move, so there can be no movement of the air molecules.

Less intuitively obvious is the fact that the air movement of sound therefore produces maximum pressure at the wall.

The wall is a node. The point of maximum movement is an anti-node. But these are not just at the wall: there is a node at the half wavelength as well, and at the full wavelength, and so on indefinitely. And there is an anti-node of maximum movement at the quarter and three quarter wavelength, and on indefinitely.

But this image does not represent the lowest eigentone. Look at image 6. This shows symbolically the three lowest eigentones - as pressure, not velocity (which is the usual half-sine wave). There is maximum pressure at the wall (minimum velocity) and minimum pressure in the centre (maximum velocity).

We can now understand the two main ways of damping down, smearing out, this resonance.

Method 1, we can absorb the energy of the moving molecules. This CANNOT be done at the wall - as the molecules there are NOT moving. Back to image 4: the movement is at a maximum at a quarter wavelength. If we put something at that anti-node to damp the movement it will smear out the resonance and stop it being sharp.

Unfortunately, at the low E string on a bass guitar say, usually 41 Hz about, the quarter wavelength is about 7 ft/2m and that takes up a lot or space in any room, and is usually impossible in a small room. Back to image 4 - if we put the absorber around half way between the anti-node and the wall we still have near 70% of the movement going on and can damp it there while only needing a depth of 3 ft 6/1m. Usually even that is too much. But I hope it is clear that for low frequencies damping material near the wall does nothing. Those foam things in corners may look nice but at 85% useless, and the small part that does anything is usually too close to the wall.

Even top quality “bass traps” often do little to affect the response a at true bass frequencies.

For example, here’s some good panels:

https://gikacoustics.co.uk/product/freestand-bass-trap/

If you check the absorption, it falls off very rapidly at bass frequencies. I’d call them wideband absorbers myself, though enough of them would have some effect at bass frequencies - but not enough effect.

Which brings us to method 2: absorbent damped panels.

Back to diagram 4: the highest PRESSURE is at the wall node (and other nodes). So what if we let the wall move? The high pressure will move the wall around. If we then put some resistance into that movement, then we have absorbed and broadened, smeared, the resonance peak again. This is how diaphragmatic bass traps work. They don’t need to be a quarter wavelength deep at the frequency of operation, so they are very suitable for small rooms. However they are quite expensive to buy.

You can make them: here’s one good guide:

https://www.acousticfields.com/how-to-build-a-diaphragmatic-absorber/

There’s more info here:

https://www.audionirvana.org/forum/the-audio-vault/room-acoustics-and-treatment/101463-diaphragmatic-bass-traps

And many other places round the net.

Finally, there are other things that can be done. There's a thing called a Helmholtz absorber. The idea here is that a closed volume with a small hole in it will resonate, think of blowing over a beer bottle. If you put some resistance in the neck of the bottle it will absorb energy at that frequency. This is useful for “spot” frequency fixes, where there is one nasty resonance to tame, but it has been used more broadly in the past. (I have one in my room here).

There’s also active correction, but this is quite costly and I’ve not heard it in action myself. Should in principle work though:

https://www.psiaudio.swiss/avaa-c20-active-bass-trap/

Summing up

In a big studio where there’s room for it, deep (>1m/4ft) simple absorption is usually used because it is guaranteed effective.

In a small studio you can usually treat the corners of the room, but the absorber (it can be anything - rock wool, polyester, or a duvet!) needs to be out from the corner as far as is feasible. It needs to be where the air is moving at the problem frequency. Packing stuff into the corner is less effective the closer it is to the walls.

Also in a small studio you can use diaphragmatic absorbers but it is best to do this after you know what your problem frequencies are, as they need to be adapted or tuned accordingly.

Finally, spot frequencies can be treated using tuned Helmholtz absorbers, but that is definitely a final step as it’s hard to get right (ask me how I know :)

If you are wealthy active absorption may be an option.

Just remember, all the passive methods work by turning movement of air molecules into heat in two principal ways, by absorbing where it is moving most of by making the room boundaries move a little.

Finally, as I guess you are a bass player, try damping the bottom E on your guitar. The whole string at bottom E is a half wave. If you damp it just below the half way point (quarter wave, maximum movement) it will not produce a harmonic, it will be very damped. If you try to damp at the bridge that is much less effective. That’s the principle in action for absorbent materials at low frequencies.

HTH.


Reference

A classic paper: https://www.acousticsciences.com/media/articles/room-acoustics-and-low-frequency-damping-aes-1986

More: https://www.gearslutz.com/board/bas...s-diaphragmatic-resonators-amp-limp-mass.html
 

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MeJ

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DanDan

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wow Some test data. Highlighted is the 4" 703 sample mounted on the boundary.
In the 125Hz Octave it has an absorption of 88%. So nearly full absorption at only about 7% of the Wavelength. On the Wall.
1:4 Wave Nonsense.png


This behaviour of simple fibre batts is enhanced by a 1:1 spacing from the wall. Also by adding a thin membrane to increase the resonant absorption, and a vast boost of absorption in corners.
The historic quarter wave academia has long been superseded by empirical results. Acousticians often use simple fibre absorption of say, 1/10 Lambda.
Boggy achieved full EBU spac results using only 60cm fibre.
Also simple google research will show a lot of disappointed Limp Membrane experimenters.
BBC R&D have made freely available the designs of their modular LF absorbers, using simple cheap available materials such as fibreboard instead of MLV.
 
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MeJ

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Thanks for your comments, DanDan. You have highlighted a rigid panel absorber with a treatment on one face which means it does not absorb much at high frequencies (note the 0.23 figure @ 4KHz) but is effectively a membrane absorber at lower frequencies (ie, @ 250 Hz. And yes, of course, standing this off the wall will improve performance at lower frequencies to offset the otherwise deteriorating results as frequencies go down to 40Hz. Personally, I'd go for the 701 plain 4" with 16" spacing which is better across the board. I don't see where I mentioned limp membranes, though I have used them, and they can work. There's much better solutions.

However, first, the OP here pointed out in the initial post "there are no companies coming up with effective Bass Traps solutions, credible audio forums, etc..."

And second, I'm attempting to explain how he can get results that work with what is available to him IN HIS BEDROOM STUDIO. That inevitably means going back to basics and simple methods. They still work, as they have for the last 50 years.

Perhaps you'd like to offer some thoughts for how the OP can achieve their desired results? At low cost? With available materials? And limited expertise?

MeJ
 

DanDan

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The same semi rigid FRK batt can be reversed to present no HF reflection. Also the membrane remains effective when an airgap is introduced.
That aside, my bad, I highlighted the wrong one. The plain 703 4" has an absorption of 85%
The overall point being that historic quarter wave theorising creates a false impression. i.e. that the simple batt traps cannot work at LF. They do.
1:4 Wave Nonsense.png

The best Bass Traps in the thinnest space are obviously RPG Modex Plates or PSI AVAs. Putting filters such as cost, available materials, limited expertise in place does not rule out the very simple design of the Modex Plate. Nor the BBC design which is just teenage level woodwork.
Even Rolls of Attic Insulation stacked in the corners would be very helpful.

Recommendations for Pressure Zone Treatment are typically accompanied by MLV and other tuned (as in Acoustic Fields) techniques. This can lead to a lot of disappointment. The BBC design is proven, but do note that many, particularly in the UK mistakenly think the BBC used a form of MLV. They did and it was abandoned in favour of simple fibreboard.
 
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MeJ

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Thanks DanDan for your correction. I have not yet seen any point on which we disagree. I can't see anywhere that I recommended using quarter-wave techniques: I am simply attempting to explain first principles; hand out a fishing rod rather than a fish. OTOH, if you want to maintain that quarter-wave treatments don't work, I would have to cordially disagree. They do work: they just take up a lot of room, which is why we don't use them nowadays. Panels are better - but you have to buy, or make, them.

I hope you don't mind me pointing out that you still haven't offered much advice to benefit the OP in their circumstances - the rolls of attic insulation is one I've often used, but he has used assorted stuff for that, if you read the thread. Would you mind giving him some hints, in context of his needs (I'm assuming 'him')? I'm always pleased to learn from younger acousticians.

The point being, this is not a commercial spec that is being drawn up. I don't believe the OP is asking for advice here because they are rolling in money to purchase off-the-shelf solutions, however good. Even if they were available, which apparently they are not.

MeJ
 
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DanDan

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"But I hope it is clear that for low frequencies damping material near the wall does nothing."
That is utter nonsense. Once again 4" 703 on the wall has an Alpha of 0.84 in the 125Hz Octave.
"If you put 4” of foam on the wall it will completely dampen incident sound at 4000Hz and above. It will not noticeably affect 120 Hz or below."
Again utter nonsense. Good foam has very similar absorption to fibre. http://forum.studiotips.com/viewtopic.php?f=8&t=536
"I hope you don't mind me pointing out that you still haven't offered much advice to benefit the OP in their circumstances"
Actually I do mind your continued posting of falsehoods. See post 12, MHOA and Polyester.

Your later comments regarding the OP are unhelpful and walking a line which I find tedious.
Lastly, believe me " I'm always pleased to learn from younger acousticians" What do you mean by this?
 
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bassmothership

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@MeJ Thanks a lot for your patience and laying out the basics here.

In a very large room - say over 120ft/35m in its largest dimension - there are many eigentones in the 20-200Hz region: see image 1

In a large room, say 40 ft/12m still there are many resonances in the bass region: image 2

In a small room, there are very few: image 3.
This means that in a small room the eigentones in the bass region have a tendency significantly to relatively emphasise the frequencies at which they occur, distorting the listening experience.

so that seems a key concept to understand small rooms I guess! the space between resonances

it's having these wide spaces between just a few resonances in the low range that screws the listening experience in small rooms, right?. There will always be resonances anyway, any room gonna have. it's just that by them existing really closed, (like in big rooms) they don't skew our perception. And since from the middle upper range above, the resonances are all already so close to each other that it's not too difficult to shape them with absorption. Correct?




We usually represent a sound wave as a vibrating string, like this:

See image 4 (from https://www.acousticsinsider.com/why-bass-traps-in-corners/)

But we should remember that really this vibrating string representation is indicating the velocity with which the air molecules are vibrating, somewhat more like this:

See image 5 (from https://www.semanticscholar.org/paper/Time-conservative-finite-volume-method-with-for-Aybay/9d91ae2dfbf99ecf355f6a7816e29b276ced982c)

In image 4, note that there is also an arrow pointing at the wall, and indicating that there there is maximum pressure and minimum movement. The minimum movement is obvious: the wall does not (to first approximation anyway) move, so there can be no movement of the air molecules.

Less intuitively obvious is the fact that the air movement of sound therefore produces maximum pressure at the wall.


I don't really get image 4. If the minimum velocity starts at the wall (0º of the wave length), isn't the pressure at the same place supposed to be at 90º (which is the peak amplitude of that sound wave)?
Image 4 shows pressure starting at the wall at 270º. Isn't it the opposite? Did I get this wrong?

Like in image 6, the sound pressure wave is at 90º at the wall, is that correct? Is that what image 6 is telling, right?



The wall is a node. The point of maximum movement is an anti-node. But these are not just at the wall: there is a node at the half wavelength as well, and at the full wavelength, and so on indefinitely. And there is an anti-node of maximum movement at the quarter and three quarter wavelength, and on indefinitely.

But this image does not represent the lowest eigentone. Look at image 6. This shows symbolically the three lowest eigentones - as pressure, not velocity (which is the usual half-sine wave). There is maximum pressure at the wall (minimum velocity) and minimum pressure in the centre (maximum velocity).

Node = maximum sound pressure, minimum velocity,
anti-node= minimum sound pressure, maximum velocity

Got it!



In a small studio you can usually treat the corners of the room, but the absorber (it can be anything - rock wool, polyester, or a duvet!) needs to be out from the corner as far as is feasible. It needs to be where the air is moving at the problem frequency. Packing stuff into the corner is less effective the closer it is to the walls.

Now you guys started a discussion about the efficiency of materials against vs not against the wall. I gotta study more about this, baby steps...


Finally, as I guess you are a bass player, try damping the bottom E on your guitar. The whole string at bottom E is a half wave. If you damp it just below the half way point (quarter wave, maximum movement) it will not produce a harmonic, it will be very damped. If you try to damp at the bridge that is much less effective. That’s the principle in action for absorbent materials at low frequencies.

Havent tried that, let's see how it goes.



This is very interesting.


Once again 4" 703 on the wall has an Alpha of 0.84 in the 125Hz Octave.

So does that mean that https://www.acousticsinsider.com/why-bass-traps-in-corners/ is giving the wrong information? I.e.: According to the Corning's data, panels can absorb lows right against the wall where the velocity is at its lowest... 0.88 at 125 Hz. So the closer this number gets to 1.0 the more it effectively absorbs the given frequency, right?
This topic is really a mine field :dizzy:



Good foam has very similar absorption to fibre. http://forum.studiotips.com/viewtopic.php?f=8&t=536

I read the post but except from the graph in sabins I dont find any more detailed data about the MegaLENRD, like these detailed OC data sheets. I find it strange these expensive MegaLENDR without any easily accessible information to compare... What am I missing here?


Maybe one thing I didnt make clear, I really dont mind the costs if that's what's going to help solve my issue with the low end in my mixing room.


Thanks a lot guys for the discussion! I'm learning a lot here!
 

DanDan

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I can feel your enthusiasm bassmothership. However I thinks study and comprehension are probably best done privately. As you can see there are seeming conflicts. But deeper science, much deeper, actually shows how both are entirely possible, which agrees with tested reality, and indeed understandable, if you have a brain the size of Mount Fuji.
Mej's theories are textbook, fully Pro world class studios 4 Metres high do go to such depths. But there are more complex explanations as to why panels of 1/10 Lambda are almost 100% effective when near a boundary. Yet only a third of that effect when sitting far from a boundary. But a full AREA boundary, such as a suspended ceiling, is good down to 50Hz with absorptive tiles only a couple of cm thick.

Modes seem to command the majority of attention, but they are not necessarily the biggest factor to worry about https://realtraps.com/art_modes.htm

Stick with the MHOA, except for the miraculous Helmholtz thing on page 228. A bit of gut instinct, common sense, that single tub trap simply did not achieve what is presented in the graph. That chapter is by a guest writer.
But always and ever, for one thing to be right does not mean another has to be wrong. More and ultimately, enough science makes the bridge.
You will find bits and pieces of the real deal in posts by Eric Desart on studiotips.com etc. Then take a look at Galaxy Studios.....

Toole is um, a real academic. There is a book by a legendary Acoustican, who also recorded Tubular Bells. Recording Studio Design by Philip Newell.
Take a look at http://www.newellacousticengineering.com
The websites of RealTraps, GIK, are excellent for explaining theory and immediately using it. You must have more local operators in your part of the world? Ethan derives his science from testing, so his conclusions automatically concur with reality. https://ethanwiner.com/basstrap_myths.htm https://realtraps.com/art_modes.htm

Finally for more inspiration, take a look at one of the world's best and perhaps the only innovator in recent decades. Boggy, Bogic Petrovic, sadly RIP way too young.
His stuff is visible on FaceBook, Instagram. Lots of pictures showing his studios starting from simple wooden framing filled with cheap fibre then covered in fabric and finally on the visible surface lively wooden mathematical patterns.
Here's one of mine, a CR squeezed into a horribly narrow space, probably narrower than yours. It is Boggy Lite. Simple cheap frames contain deep amounts of cheap attic insulation. Fabric and a random pattern of wooden reflectors, which keep students from falling asleep in the soft warm walls......
IMG_20171020_141005718.jpg


Then the Back wall is about 30cm deep on one side and say 45cm at the other.
IMG_20171006_123652540.jpg
 
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MeJ

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Sorry bit busy. Will try and answer at w/e
 
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