My control room build!

Document your build here: All about your walls, ceilings, doors, windows, HVAC, and (gasp!) floated floors...
FeedTheStudios
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My control room build!

#1

Postby FeedTheStudios » Tue, 2019-Oct-15, 15:09

Hello everyone! My name is Abraham, I'm building a mix control room to be my main mixing studio. The design was done by Soundman2020 (Stuart) and the main challenge is the dimension of the room. It's a 3.45m x 2.80m x 2.50-2.40m (roof slant towards window) room, so it's difficult to treat correctly due to all the room modes.

Here is the initial freq response:
Image

This is the floor plan
Image

So far the treatment looks like this
Image

I have been gathering materials and dusting off my carpentry skills so process is a bit slow. I have built the frame of the ceiling cloud:
Image

I haven't gotten to the soffits because the homasote is not sold in my area and can't find any replacement materials. The mass loaded vinyl for the baffles is less hard to encounter but it's only sold in big quantities and, because of the high price, I'm also looking for replacement materials or less quantity sold.



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Soundman2020
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Re: My control room build!

#2

Postby Soundman2020 » Tue, 2019-Oct-15, 16:41

Hi Abraham, and WELCOME to the forum! Glad you signed up OK, and started posting your build thread.

Just to add a bit more detail for those following the thread, about the two graphs that Abraham posted. The first one is the predicted empty room response, and the second one is that actual measured empty room response. You can see that it's a pretty close match, with some differences that are due to things like the window, door, etc.

It's also a rather small room! At just under 10m2 (107 ft2), it's one of the smallest rooms I've designed, and it's a real challenge. But the soffits should do a lot to improve things (soffits in small rooms are a great solution for several acoustic issues), the cloud is steeply angled and hard-backed to help with stuff in the vertical direction, and the treatment on the rear wall and side walls (still to come: not designed yet) will do still more.

Abraham has had a hard time sourcing some of the materials in Mexico, but he has managed to find everything so far... except the Homasote for the hangers. Fortunately, one of our forum members is Steve Gleason, who just happens to be the Technical Director at the Homasote company, so I'm hoping he'll drop in on this thread, and see if he can give Abraham some help in finding Homasote in Mexico. If not, then we'll have to switch to another material for the hangers. Homasote is the best, but if you can't get it, then that's OK. There are other options too.

Abraham also just alerted me by email that I messed up one of the dimensions in the SketcUp mode, so I'll fix that, then he can carry on building his place.

- Stuart -



FeedTheStudios
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Re: My control room build!

#3

Postby FeedTheStudios » Wed, 2019-Oct-16, 11:39

Wow, that's amazing. I didn't know Mr. Gleason was on the forum!

Yes, some of the acoustical materials are quite hard to find. The OC fiberglass is available on order, so does the mass loaded vinyl but seems that only large quantities.
I gotta say it's a pretty interesting and rewarding journey to be able to try my hand at building (thanks to Mr. Allsop's design) a mixing studio.



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Re: My control room build!

#4

Postby sgleason » Thu, 2019-Oct-17, 15:22

Hey guys....sorry to be late to the party. Unfortunately, There are no dealers handling Homasote in Mexico so it will have to come from the US.

FeelThe Studios.....if you reach out via a PM, Abraham, and tell me where you are I'll see what's closest to you.

Great to be a part of this forum, Stuart.


Please don't confuse me with data!

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Soundman2020
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Re: My control room build!

#5

Postby Soundman2020 » Thu, 2019-Oct-17, 15:29

Welcome Steve! It sure is great to have you here. I use your products in many of my designs, but you guys sure do make it hard to find your stuff in some places around the world! :)

- Stuart -



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Re: My control room build!

#6

Postby Kpgushue » Thu, 2019-Oct-17, 19:27

Looks like its going to be a fun little room. My question is, is it possible to plot out a predictive frequency response curve for the full audio spectrum?



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Re: My control room build!

#7

Postby Soundman2020 » Thu, 2019-Oct-17, 20:46

Kpgushue wrote:Looks like its going to be a fun little room. My question is, is it possible to plot out a predictive frequency response curve for the full audio spectrum?
That's a good question! The answer is yes, it is possible... but actually it isn't really necessary. The mid and high frequencies won't be nearly as bad as the lows, in terms of frequency response. It's only the low end, with all the modal stuff, SBIR, and things like that, where the major issues are.

This is all related to something called the "Schroeder frequency". I'll go into that a little, but before I do, I should point out that the "Schroeder frequency" isn't really a valid concept in a small room, because in order to have a true Schroeder frequency you would also need a true diffuse sound field in the room, which cannot happen in a small room for complex reasons: so, in the strictest technical sense, "Schroeder frequency" isn't the right term here... but people still use it anyway! It would be more correct to call it the "transition zone".

OK, so now that you know it isn't valid, but it's there anyway, what exactly is it?

In simple terms, the "Schroeder frequency" marks the spot on the audio spectrum where the dominant type of acoustic response changes from modal, to diffuse. Once again, there isn't really a diffuse field in a small room, in the strict technical sense: that can only happen in a large room. But here too people use the term wrongly all the time... In simple terms, a diffuse sound field is one where the ambient sound of the room is equally likely to come at you from an direction, regardless of where you stand in the room. Or in other words, the ambient sound is all around you, equal in all directions and at all locations, with no variation. By "ambient sound", I mean the sound that does not come directly from the speakers: it's the sound that did originally come from the speakers, but then it hit the walls, floor, ceiling, furniture, people, etc., bounced around all over the place, bounce after bounce, getting scattered with each additional bounce, and it did that many times over, so that it is now moving pretty much at random... and therefore it seems to come from all around you, not from any specific direction. It's more complex than that, though, in the strict technical sense, but that's the basic idea.

So the Schroeder frequency tells you what the lowest frequency is where the sound field is "diffuse". Above Schroeder, its smooth and gentle (sort of...), below Schroeder, it's a mess! Above Schroeder, the frequency response is a wiggly line that doesn't stray too far up or down from the median. Below Schroeder, it looks like the mountains of the moon, blended with the Grand Canyon!

But WHY?

That's actually not too hard to understand either! It's all about modes... This is a little long, but important if you want to understand what Schroeder is really about, and why it isn't necessary to worry about the overall frequency response above the Schroeder frequency for a room. So here goes! I wrote this a while back, somewhere else, so I'm just doing a "cut and paste" job here, with some editing, but it saves me typing the same thing all over again!


---

Room ratios is a whole major subject in studio design. It works like this: The walls of your studio create natural resonances in the air space between them, inside the room. (This is totally different from the MSM resonance of the walls themselves: this is all about what happens within the ROOM, not what happens inside the walls. Two totally different things.)

So, you have resonant waves inside the room. We call those "standing waves" or "room modes". Those "modes" (resonances) occur at very specific frequencies that are directly related to the distances between the room boundaries (walls, floor, ceiling). They are called "standing waves" because they appear to be stationary inside the room: they are not REALLY stationary, since the energy is still moving through the room. But the pressure peaks and nulls always fall at the exact same points in the room each time the wave energy passes, so the "wave" seems to be fixed, static, and unmoving inside the room. If you play a pure tone that happens to be at the exact frequency of one of the "modes", then you can physically walk around inside the room and experience the "standing" nature of the wave: you will hear that tone grossly exaggerated at some points in the room, greatly amplified, while at other points it will sound normal, and at yet other points it will practically disappear: you won't be able to hear it at all, or you hear it but greatly attenuated, very soft.

However, the peaks and nulls fall at different places in the room for different frequencies. So the spot in the room where one mode was deafening might turn out to be the null for a different mode.

Conversely, if you have a mode (standing wave) that forms at a specific frequency, then playing at a slightly different frequency might show no mode at all: for example, if a tone of exactly 73 Hz creates a standing wave that is clearly identifiable as you walk around the room, with major nulls and peaks, then a tone of 76 Hz might show no modes at all: it sounds the same at all points in the room. Because there are no natural resonances, no "room modes" associated with that frequency.

That's the problem. A BIG problem.

Of course, you don't want that to happen in a control room, because it implies that you would hear different things at different places in the room, for any given song! At some places in the room, some bass notes would be overwhelming, while at other places the same notes would be muted. As you can imagine, if you happen to have your mix position (your ears) located at such a point in the room, you'd never be able to mix anything well, as you would not be hearing what the music REALLY sounds like: you would be hearing the way the room "colors" that sound instead. As you subconsciously compensate for the room modes while you are mixing, you could end up with a song that sounds great in that room at the mix position: the best ever! But it would sound terrible when you played it at any other location, such as in your car, on your iPhone, in your house, on the radio, at a club, in a church, etc. Your mix would not "translate".

And you also don't want major modal issues in a tracking room, for similar reasons: As an instrument plays up and down the scale, some notes will sound louder than others, and will "ring" longer. The instrument won't sound even and balanced.

OK, so now I have painted the scary-ugly "modes are terrible monsters that eat your mixes" picture. Now lets look at that a bit more in depth, to get the real picture, and understand why they look bad, but aren't so bad in reality.

So let's go back to thinking about those room modes (also called "eigenmodes" sometimes): remember I said that they occur at very specific frequencies, and they are very narrow in bandwidth? This implies that if you played an E on your bass guitar, it might trigger a massive modal resonance, but then you play either a D or an F and there is no mode, so they sound normal. Clearly, that's a bad situation. But what if there was a room mode at every single frequency? What if there was one mode for E, a different mode for D and yet another one for F? In that case, there would be no problem, since all notes would still sound the same! Each note would trigger its own mode, and things would be happy again. If there were modes for every single frequency on the spectrum, and they all sounded the same, then you could mix in there with no problems!

And that's exactly what happens at higher frequencies. Just not at low frequencies. Because of "wavelength"...

It works like this: remember I said that modes are related to the distance between walls? It's a very simple relationship. Remember I said the waves are "standing" because the peaks and nulls occur at the same spot in the room? In simple terms, for every frequency where a wave fits in exactly between two walls, then there will be a standing wave. And also for exactly twice that frequency, since two wavelengths of that note will now fit. And the same for three times that frequency, since three full waves will now fit in between the same walls. Etc. All the way up the scale.

So if you have a room mode at 98 Hz in your room, then you will also have modes at 196 Hz (double), 294 (triple), 392 (x4), 490(x5), 588(x6), 686(x7) etc., all the way up. If the very next mode in your room after that one, happened to be at 131 Hz, then there would also be modes at 262 Hz(x2), 393(x3), 524(x4), 655(x5), etc.

That's terrible, right? There must be thousands of modes at higher frequencies!!! That must be awful!

Actually, no. That's a GOOD thing. You WANT lots of modes, for the reasons I gave above: If you have many modes for each note on the scale, then the room sounds the same for ALL notes, which is what you want. It's good, not bad. In simple terms: "Many modes close together on the spectrum" = good. "Just a few modes, far apart" = bad.

But now let's use a bit of music theory and math and common sense here, to see what the real problem is.

If your room has a mode at 98Hz, and the next mode is at 131 Hz, that's a difference of 32%! 98 Hz is a "G2". So you have a mode for "G2". but your very next mode is a "C3" at 131Hz. That's five notes higher on the scale: your modes completely skip over G2#, A2, A2#, and B2. No modes for them! So those four notes in the middle sound perfectly normal in your room, but the G2 and C3 are loud and long.

However, move up a couple of octaves: ...

There's a harmonic of your 98Hz mode at 588 Hz, and there's a harmonic of your 131 Hz mode at 524 Hz. 524 Hz is C5 on the musical scale, and 588 Hz is a D5. They are only two notes apart! Not five, as before.

Go up a bit more, and we have one mode at 655 and another at 686. 655 is an E5, and 686 is an F5. they are adjacent notes. Nothing in between! We have what we want: a mode for every note.

The further up you go, the closer the spacing is. In fact, as you move up the scale even higher, you find several modes for each note. Wonderful!

So at high frequencies, there is no problem: plenty of modes to go around and keep the music sounding good.

The problem is at low frequencies, where the modes are few and far between.

The reason there are few modes at low frequencies is very simple: wavelengths are very long compared to the size of the room. At 20 Hz (the lower limit of the audible spectrum, and also E0 on the organ keyboard), the wavelength is over 56 feet (17m)! So your room would have to be 56 feet long (17 meters long) in order to have a mode for 20 Hz.

Actually, I've been simplifying a bit: it turns out that what matters is not the full wave, but the half wave: the full wave has to exactly fit into the "there and back" distance between the walls, so the distance between the walls needs to be half of that: the half-wavelength. So to get a mode for 20 Hz, your room needs to be 56 / 2 = 28 feet long (8.5M) . Obviously, most home studios do not have modes at 20 Hz, because there's no way you can fit a 28 foot (eight meter) control room into most houses!

So clearly, the longest available distance defines your lowest mode. If we take a hypothetical dimensions as an example (typical of a very small home studio), let's say the length of the control room is 13 feet (4m), the width is 10 feet (3m), and the height is 8 feet. (2.5M) So the lowest mode you could possibly have in that room, would be at about 43 Hz (fits into 13 feet or 4M perfectly). That's an "F1" on your bass guitar.

The next highest mode that you room could support is the one related to the next dimension of the room: In this case, that would be width, at 10 feet / 3M. That works out to 56.5 Hz. That's an "A1#" on your bass guitar. Five entire notes up the scale.

And your third major mode would be the one related to the height of the room, which is 8 feet /2.5M, and that works out to 71 Hz, or C2# on the bass guitar. Another four entire notes up the scale.

There are NO other fundamental modes in that room. So as you play every note going up the scale on your bass guitar (or keyboard), you get huge massive ringing at F, A# and C#, while all the other notes sound normal. As you play up the scale, it goes "tink.tink.tink.BOOOOM.tink.tink.tink.tink.BOOOOOM.tink.tink.tink.BOOOOOM.tink.tink...."

Not a happy picture.

Sure, there are harmonic modes of all those notes higher up the scale. But in the low end, your modes are very few, and very far between.

So, what some people say is "If modes are bad, then we have to get rid of them". Wrong! What you need is MORE modes, not less. Ideally, you need a couple of modes at every single possible note on the scale, such that all notes sound the same in your room. In other words, the reverberant field would be smooth and even. Modes would be very close together, and evenly spread. And this is where Mr. Schroeder comes into the picture: he figured out that if you have three modes for any given note on the scale, you are fine (sort of... he phrased it a bit different, but that's the idea). So, as you go up the scale, the modes get closer and closer, more and more dense, until eventually there are at least 3 for each note. That point, where you first get three modes per note, is the Schroeder Frequency.

So trying to "get rid of modes" is a bad idea. And even if it were a good idea, it would still be impossible! Because modes are related to walls, the only way to get rid of modes is with a bulldozer! Knock down the walls... :shock:

That's a drastic solution, but obviously the only way to get a control room that has no modes at all, is to have no walls! Go mix in the middle of a big empty field, sitting on top of a 56 foot (17 M) ladder, and you'll have no modes to worry about.... 8) :roll:

:shot:

Since that isn't feasible, we have to learn to live with modes.

Or rather, we have to learn to live with the LACK of modes in the low end. As I said, the problem is not that we have too many modes, but rather that we don't have enough of them in the low frequencies.

Obviously, for any give room there is a point on the spectrum where there are "enough" modes. Above that point, there are several modes per note, but below it there are not.

There's a mathematical method for determining where that point is: Schroeder was the guy who figured it out, years ago, so it is now known as the Schroeder frequency for the room. Above the Schroeder frequency for a room, modes are not a problem, because there are are lots of them spaced very close together. Below the Schroeder frequency, there's a problem: the modes are spaced far apart, and unevenly. (The Schroeder frequency is a bit more complex than just that, since it also considers treatment, but this gives you an idea...)

In fact, there's a very simple equation for calculating what the Schroeder frequency is for any give room:

f(sch) >= 2000*sqrt(T60/V)

Where:
T60 is the 60 dB decay time for the room,
V is the cubic volume of the room.

It's that simple.

This next part is more for people who are able to design their room from scratch, where they can change their dimensions a bit for the walls and ceiling height. It's not relevant to your question here, nor to Abraham's room since his walls are fixed and he can't move them, bit it does help to understand modes better.

So if modes are a big problem, what can we do about that?

In reality, not much! If you can vary the dimensions of your room, then you have the luxury of having SOME control... but not much. In fact, all we can do is to choose a "room ratio" that has the modes spaced out sort of evenly, and NOT choose a ratio where the modes are bunched up together. For example, if your room is 10 feet long and 10 feet wide and 10 feet high (3m x 3m x 3m), then all of the modes will occur at the exact same frequency: 56.5 Hz. So the resonance when you play an A1 on the bass, or cello, or hit an A1 on the keyboard, will by tripled! It will be three times louder. The nulls will be three times deeper. That's a bad situation, so don't ever choose room dimensions that are the same as each other, if you can avoid it.

You get the same problem for dimensions that are multiples of each other: a room 10 feet high (3m) by 20 feet wide (6m) by 30 feet long (9m) is also terrible. All of the second harmonics of 10 feet will line up with the 20 foot modes, and all of the third harmonics will line up with the 30 foot modes, so you get the same "multiplied" effect. Bad.

In other words, you want a room where the dimensions are mathematically different from each other, with no simple relationship to each other. It turns out that as long as they are different by 5%, you are fine. So that room with the 10 x 10 x 10 dimensions would be much better if the width was 5% narrower, at about 9'6", and the ceiling were 5% higher, at about 10'6". That's a better ratio.

That brings up the obvious question: What ratio is best?

Answer: there isn't one! :)

Over the years, many scientists have tested many ratios, both mathematically and also in the real world, and come up with some that are really good. The ratios they found are named after them: Sepmeyer, Louden, Boner, Volkmann, etc. Then along came a guy called Bolt, who drew a graph showing all possible ratios, and he highlighted the good ones found by all the other guys, and predicted by mathematical equations, plus a few of his own: If you plot your own room ratio on that graph, and it falls inside the "Bolt area", then likely it is a good one, and if it falls outside the "Bolt area", then likely it is a bad one. Sort of.

So, there are no perfect ratios, only good ratios and bad ratios.

It is impossible to have a "perfect" ratio in a small room, simply because that would require enough modes to have three modes for every note on the musical scale, (ie, the Schroeder frequency would have to be lower than 20 Hz...) but that's the entire problem with small rooms! There just are not enough modes in the low end. So you can choose a ratio that spreads them a bit more this way or a bit more that way, but all you are doing is re-arranging deck chairs on the Titanic, in pleasant-looking patterns. The problem is not the location of the deck chairs; the problem is that your boat is sunk! :) Likewise for your studio: the problem is not the locations of the modes: the problem is that your room is sunk. No matter what you do with the dimensions, you cannot put a mode at every note, unless you make the room bigger. It is physically impossible.

But that does not mean that your room will be bad. That's the common perception, and it is dead wrong.

All of this leads to the question you didn't ask yet, but are probably heading for: What can I do about it?

Here's the thing: Modes are only a problem if they "ring". The wave is only a problem if the energy builds up and up and up, with each passing cycle, until it is screaming, and then that "built up" energy carries on singing away, even after the original note stops. That's the problem. If you stop playing the A1 on your guitar, and the room keeps on playing an A1 for a couple of seconds, because it "stored" the resonant energy and is now releasing it, then that's a BIG problem! The room is playing tunes that never were in the original music! :shock:

If a mode doesn't ring like that, then it is no longer a major issue. (It is still an issue for other reasons, just not a major one....)

So how do you stop a mode? You can't stop it from being there. But you CAN stop it from "ringing". You can "damp" the resonance sufficiently that the mode dies away fast, and does not ring. You remove the resonant energy and convert it into heat: no more problem! A simple analogy: it's not good if you own a large angry dog that barks all the time and bights your visitors, but it's fine to own a large angry dog with a muzzle on his mouth, so he cannot bark and cannot bight!

You do that with "bass trapping". A bass trap is like the dog muzzle. It doesn't get rid of the problem, but it does keep it under control. You use strategically placed acoustic treatment devices inside the room that absorb the ringing of the mode, then it cannot ring. There are several ways to do that, with different strategies, but the good news is that in most rooms it is possible to get significant damping on the worst modes, so that they don't ring badly, and don't cause problems. Note that bass trapping does not absorb the mode: it just absorbs the ringing. Some people don't understand this, and think that the bass trapping makes the modes go away: it doesn't. All it does is to damp them. The modes are still there, and still affect the room acoustics in other ways, but with good damping, at least they don't "ring" any more.

And that is the secret to making a control room good in the low end! Choose a good ratio to keep the modes spread around evenly, then damp the hell out of the low end, so modes cannot ring. It's that simple.

The smaller the room, the more treatment you need. And since those waves are huge (many feet long), you need huge bass trapping (many feet long/wide/high/deep). It takes up lots of space, and the best place for it is in the corners of the room, because that's where all modes terminate. If you want to find a mode in your room, go look for it in the corner: it will be there. All modes have a pressure node in two or more corners, so by treating the corners, you are guaranteed of hitting all the modes. Take a look at the design for Abraham's room, and you'll see some enormously massive bass trapping in the front corners. There will be more trapping in the rear corners too (not designed yet), but those guys in the front will have a useful effect on some of the modal issues.

As I said, there is no single "best" ratio, but there are good ones. You can use a "Room Mode Calculator" to help you figure out which "good ones" are within reach of the possible area you have available, then choose the closest good one, and go with that. And stay away from the bad ones.

Arguably, Sepmeyer's first ratio is the "best", since it can have the smoothest distribution of modes... but only if the room is already within a certain size range. Other ratios might be more suitable if your room has a different set of possible dimensions. So there is no "best".

But that's not the entire story: So far, all the modes I have mentioned are only related to two walls across the room, opposite from each other. I mentioned modes that form along the length axis of the room (between the front and back wall), others that form along the width axis (between left and right walls), and others that form on the height axis (between floor and ceiling): Those are the easiest ones to understand, because they "make sense" in your head when you think about them. Those are called "axial modes", because they form along the major axes of the room: length axis, width axis, height axis.

However, there are also other modes that can form between four surfaces, instead of just two. For example, there are modes that can bounce around between all four walls, or between the front and back walls as well as the ceiling and floor: those are called "tangential modes". And there are other modes that can form between all six surfaces at once: they involve all four walls plus the ceiling and the floor. Those are called "oblique modes".

The complete set of modes in your room consists of the axial modes, plus the tangential modes, plus the oblique modes.

That's what a good room mode calculator (a.k.a. "room ratio calculator") will show you. There are bad calculators that only show you the axial modes, which is pretty pointless, and the good ones show you all three types. Yes, it is true that tangential modes are lower in intensity than axials, and that oblique modes are lower still, but they can still cause you trouble. If you look at the very top graph that Abraham posed, you can see some faint vertical lines: those mark the predicted frequencies and intensities for the modes in his room. The faint red lines are axial modes, the faint green lines are tangential mods, and the faint blue lines are obliques. Those red lines are longer because the axials are more powerful: the blues lines are shorter, because they are the least powerful obliques. If you compare the predicted modes to the actual modes, you'll see that, indeed, the axials are very strong.. but there still some sign of tangentials, and even obliques in there. So you can't ignore those. If you find a calculator on the internet that only calculates axial modes, forget it: its no use.

Use one of these Room Ratio calculators to figure out the best dimensions for your room:

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

http://amroc.andymel.eu/

Both of those are very good, and will help you to decide how best to build your room. They give you tons of information that is really useful to help figure out the best dimensions.

However, after having said all of that, modes aren't that important, despite all the hype they get: Modes are just one aspect of room design, but there are many more. It's wise to choose a ratio that is close to one of the good ones, or inside the Bolt area, but you do NOT need to go nuts about it! There's no need to nudge things around by millimeters or smalls fractions of an inch, hoping to get a "better" ratio. Just stay away from the bad ones, get close to a good one, and you are done. End of story.

----


Whew! End of long waffling rant about modes!

But now I can answer your actual question better! The Schroeder Frequency marks the spot on the spectrum where the modal issues stop dominating, because there are enough modes per note to smooth things out. So, above the Schroeder frequency, there won't be any wild swings in frequency response (no huge peaks or enormous valleys): just somewhat milder variations, that aren't so important usually. So you don't need to predict that. It is possible to predict it, but it isn't really needed. And there's one other issue: the higher you go up the scale, the more modes there are, so the slower your prediction process goes... at the top end, there are thousands of modes for each note, so calculating all of those then summing them, is mathematically intensive. And thus, not often done.

To show you more graphically WHY it isn't needed, here's a frequency response diagram for a typical room:

CRFKUS-REW-FR-20..20k-1..24-BASELINE.png
You can see what I'm talking about, visually there. At about 350 Hz, the peaks and valleys stop. Below that, it's all "mountains of the moon", but above 350 Hz, the line gets a hell of a lot flatter, with only minor variations. Thus, you could assume that the Schroeder frequency for this room is somewhere around 305 Hz. ... and you can see why it isn't really necessary to predict the modal response above that, because it's mostly flat. The remaining variations are not even modal in nature, but rather from things like reflections, comb filtering, and suchlike, so even trying to predict them is non-trivial. It's easier to measure the actual response of the room after it is built, then just treat the issues that are really there.


If you are still awake after reading through all of that, then hopefully you found something useful in there! And if you are asleep, then I'll shut up and let you carry on sleeping, peacefully .... :)

- Stuart -



Kpgushue
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Posts: 7
Joined: Mon, 2019-Oct-14, 00:49
Location: Boston, USA

Re: My control room build!

#8

Postby Kpgushue » Fri, 2019-Oct-18, 12:49

Thanks Stuart, thats one of the clearest, most practical explanations of room modes ive ever read, and ive read many.

I have several questions and comments, but i will start a thread specifically for acoustic predictions. I don't want to sidetrack (hijack) Abrahams thread, or shift focus from his project.

I will say its difficult for an intermediate journeyman studio builder/designer, it seems that that the process of applying acoustic theory to design, in a methodical way seems to be evasive, even in best of books or forums. The knowledge base seems to have "modes" at the entry level, and advanced level, with a "null" at the intermediate level. Thats the area to take a person from applying the known "best practices" and "rules of thumb" ie the fundementals, which go a long way, to being able to apply, manipulate, and predict the sound of the room at the design phase. This is where scratch builds become a bit more mysterious, since existing construction defines many things for you.

Anyway thanks for the reply, and best of luck to Abraham, ill be enjoying his build!

-Kyle



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Re: My control room build!

#9

Postby Soundman2020 » Fri, 2019-Oct-18, 13:40

Kpgushue wrote:Thanks Stuart, thats one of the clearest, most practical explanations of room modes ive ever read, and ive read many.
Thanks for the kind words, Kyle. I do try to explain things as clearly as I know how, even it it takes a few extra words to do it... :shock: I think is critically important for studio builders to understand WHY they are doing things in their room, rather than just WHAT to do. If you understand the reasons, then you'll do a better job building, in my experience.

I have several questions and comments, but i will start a thread specifically for acoustic predictions. I don't want to sidetrack (hijack) Abrahams thread, or shift focus from his project.
Cool! I look forward to seeing that, and hopefully being able to answer your questions.


I will say its difficult for an intermediate journeyman studio builder/designer, it seems that that the process of applying acoustic theory to design, in a methodical way seems to be evasive, even in best of books or forums. The knowledge base seems to have "modes" at the entry level, and advanced level, with a "null" at the intermediate level. Thats the area to take a person from applying the known "best practices" and "rules of thumb" ie the fundementals, which go a long way, to being able to apply, manipulate, and predict the sound of the room at the design phase. This is where scratch builds become a bit more mysterious, since existing construction defines many things for you.
Right. Acoustics is a BIG subject, even if you only look at the part related to studio design. It does take a while to get your head around all of the concepts, and start putting them together. Even when you do get the basics of each part, it still isn't easy to mesh those bits and pieces into a coherent "big picture". But keep at it! Sooner or later, the light bulb will come on, and you'll start making sense of things.

For a while I've been planning to write a thread about the basic process for studio design, to guide first-time studio builders. Sort of a basic "How to...." or maybe a recipe ("Take two mature speakers, insert in identical soffits, add one mix position, and several random electronic boxes. Do not stir! Cook slowly over low heat generated from overloaded power amp for three years. Lather on thick acoustic treatment, and sprinkle liberally with lots of money. Then add even more money. Finally, baste on one more layer or money. Do not allow to cool! Serve immediately, even before fully cooked. Serves one to many, but most of them will complain about the flavor anyway. Ignore complaints"). OK, so maybe a bit more serious than that! What you mention is generally the case with most first-time builders. That "null" in the middle is rather deep! That's probably why many home studio builders decide to hire a studio designer to do the heavy lifting of putting all the concepts together, rather than doing it themselves. It does take a lot of time and effort to figure it all out, then put it into practice, so it's easier to just hire someone to do it. There's also the issue of: Does it even make sense to invest all that effort and time in learning a skill that they will only ever use once or twice in their entire life? For some people, the answer is yes: they want to learn how to design a studio just out of curiosity, or for satisfaction in learning something new. But for many people, it's not so clear that it is worthwhile, so hiring a designer or acoustician or consultant makes more sense.

Anyway, please do start that thread, with your questions! Do that in the "acoustics and treatment" section of the forum, here: viewforum.php?f=14

- Stuart -



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Re: My control room build!

#10

Postby Kpgushue » Fri, 2019-Oct-18, 17:22

Great! Started the thread.

So, if i am interpreting things right the stuff above the Schroeder Frequency, are more suited to general amplitude and decay time, predictions and analysis, where modal response its amplitude, decay time, and frequency that you predict and anylize. Just in a general sense, or am i off?

Also im un-clear on how whats meant by having more than one mode per note? Im assuming its different from the generally bad situation of having coinciding modes due to the same dimensions on two boundaries. i.e, if the length and with have the E2 dimension, 13.6ft (4.14m) 82.4hz, thats a bad thing, what is meant by having more than one mode @ E2?

For reference heres some quotes from above:


So, what some people say is "If modes are bad, then we have to get rid of them". Wrong! What you need is MORE modes, not less. Ideally, you need a couple of modes at every single possible note on the scale, such that all notes sound the same in your room. In other words, the reverberant field would be smooth and even. Modes would be very close together, and evenly spread. And this is where Mr. Schroeder comes into the picture: he figured out that if you have three modes for any given note on the scale, you are fine (sort of... he phrased it a bit different, but that's the idea). So, as you go up the scale, the modes get closer and closer, more and more dense, until eventually there are at least 3 for each note. That point, where you first get three modes per note, is the Schroeder Frequency.



---------


No matter what you do with the dimensions, you cannot put a mode at every note, unless you make the room bigger. It is physically impossible.

------


The Schroeder Frequency marks the spot on the spectrum where the modal issues stop dominating, because there are enough modes per note to smooth things out. So, above the Schroeder frequency, there won't be any wild swings in frequency response (no huge peaks or enormous valleys): just somewhat milder variations, that aren't so important usually.

----


Thanks again.



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Re: My control room build!

#11

Postby Soundman2020 » Tue, 2019-Oct-22, 00:04

if i am interpreting things right the stuff above the Schroeder Frequency, are more suited to general amplitude and decay time, predictions and analysis, where modal response its amplitude, decay time, and frequency that you predict and anylize. Just in a general sense, or am i off?
In very broad terms, .... sort of! :) The Schroeder frequency is just a calculated number that marks a spot on the spectrum. The frequency response above that point should be smoother in general, reasonably close to flat: The frequencies below that point will be a mess.... unless you add treatment to the room.... HOWEVER! ... adding treatment to the room also changes the Schroeder frequency! The "deader" the room becomes, the further down the scale that magical spot moves. So, a room with nothing but hard, solid concrete walls would have a higher Schroeder frequency than the same room where all the walls are covered in thick porous absorption, and/or with heavy diffusion. It's related to the decay time (RT60).... Which of course, also implies that, strictly speaking, it isn't valid for small rooms! :)

Also im un-clear on how whats meant by having more than one mode per note?
"Three modes per note" is just a simple way of explaining what Schroeder actually said. In reality, it's about how much adjacent modes overlap each other, but I find it easier to get your head around the concept of a piano keyboard and how many modes "fit onto each key".

Here's an image that sort of helps to clarify what this means:

Modes-per-note.jpg
That's a room mode prediction from Bob Gold's calculator, for a hypothetical room measuing 17 feet long, 13 feet wide, 8 feet high. On the right hand side, each colored line represents a mode. Roughly, yellow means "too far apart", light green means "almost there", and dark green means "OK.". There's a piano keyboard up the right side. You can see that, for low frequencies, there's some keys that have no mode at all. As frequency increases, above about 65 Hz there's at least one mode for each key, and above about 130 Hz, there's several modes for each key. The Schroeder frequency for that room, is 119 Hz, and if you look at the graphic you can see that that is, indeed, roughly where you get at least 3 modes for each key on the keyboard. Below that, the room is dominated by modes. Above that, things get smoother and other issues are more dominant.

Another way of looking at is with a Bonello plot: That shows how many modes there are per 1/3 octave band. I don't recall the exact guideline here, but I think it might be "more than 10 modes per1/3 octave band" is what you are looking for.

But in any case, the Schroeder frequency doesn't really say much, for the average studio builder. Knowing it, or not knowing it, won't make a big difference to how the room should be treated.

For a typical home studio builder, it's better just test the empty room with REW to see how it is actually performing, then design treatment to deal with the issues. Start with the biggest problem first, then work your way up through the smaller ones, until you get to the point you are happy with the room... or you run out of money! :)

- Stuart -



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Re: My control room build!

#12

Postby darcy123 » Tue, 2019-Nov-19, 07:51

This is useful.



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My control room build!

#13

Postby lostandfound » Tue, 2020-Apr-07, 13:26

Great post, Stuart

thanks :yahoo:

Hi everyone, it's my first post here, have clemency ...
Google translation is helping me.
I also hope not too off-topic.

I am struggling to give Stuart the conscious vision of a complete professional, I have read his posts since he started posting on the John forum very humble at the beginning, as we all try to be when we are inexperienced, then slowly ....

I have to recognize him of great merits, also considering that I have not been able to establish a collaborative relationship with him for a question in principle, I know how to appreciate his availability and I perceive the passion that drives him in this scope .... makes him very close to all of us.
The meaning of my thought is to try to sensitize acoustic designers more by bringing them back to "real" reality.
Any of them grew up with the community, asking, confronting others, participating in discussions, etc etc.
Slowly acquiring information and experiences that others probably had already faced and share it, fantastic !!
This is what a community to be !!

The thing that bothers me is when is many of them convert their previous profession into acoustic designers becoming more and more slaves to a simple economic logic.
I break a spear in favor of Stuart, he is perhaps one of the few who has shown himself available to discuss everything with everyone, also tackling broad training topics but, like all of them, equally capable of bringing us to the crossroads and ... making us understand that without a good designer you won't get anywhere.
Recently I got to contact a famous acoustic designer, i don't know if he is a member of this forum, I would prefer to read him too.
Exquisite person ... nothing to complain about in this, but, at the first exchanges of e-mails he tells me: .... do not think in the least of realizing your acoustic design yourself .... it will be a failure at 99,9%....encouraging!! what to add?
Nor did he ask me what my knowledge of acoustics was, what I wanted to achieve! As well as a series of statements like ... your measurements are not going well, the window, the door and the view towards the live room are not going well and you must be ready to upset everything.
My feeling was.... I acquired the client ... now I leave him in his underwear, let's see how much he cares that I plan it!
... if this is the approach to a true professional .... then it is better that you speak about his sympathy.

I'd like not to mention his fee .... calling a world-renowned surgeon for a surgery would cost less!!

But ... there is someone who has money to afford it and is convinced that he has chosen the best designer (maybe it will be too), perhaps he will only have to admit it to himself because he will never know if others, probably even cheaper, could have done same ... or maybe better or with less expense for the achievement.
Besides, and this is the real point, I will never know if would have made a difference in the result by choosing him since I did not commission his advice.
Obviously, and these are the rules of the market, everyone makes their own choices and ... if they think are worth more and have customers, they certainly must not account for my logic, in any case I wish prosperity and long life to all.

My poor moral is:
if we are here it is, in addition to sharing experiences regarding acoustics or whatever, to try to get information from those who know more than we are well aware that we will not have an acoustic project of our control room as a gift, we are aware of it !!
But I would like that those who know more about us have the same humility to be sincere in admitting immediately how far they are willing to help us with their knowledge, Stuart nobody wants to steal the secret of how you calculate the Sorbothane or your soffits, many hours of work and study, it is right that they are paid! It will remain your secret!....as no one will know if the results that we can achieve by ourselves will be less happy than those obtainable with your advice .... the real point is this, everyone loses !! we will not have a designer even just for the basic layout and you will not have new potential customers :(

However, I have a good consideration of you for how you propose yourself, certainly much better than others.
I just hope it doesn't get censored


All the best

Luc




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