Flutter Echo Dilemma

[br??]

It's been about a year now since I made the biggest upgrade to my system so far -- acoustic treatment. Since then, I've become better at critical listening and identifying the flaws of my room, and I think it's time to remedy the flutter echo between the two side walls, which I left completely bare. My room is small-ish, slightly rectangular with a low-ish ceiling, and I've used DIY bass traps and diffusers for the other surfaces.

The dilemma arises because I'm not sure how to go about dealing with the flutter echo. Absorption? Diffusion? Both? Where? How much? If I've learned one thing from reading the articles by Gene and Dr. Floyd Toole it's that there's no easy way to answer this question. My speakers have excellent off-axis performance, so I'm leaning towards leaving the first reflection points bare and treating the rest of the side walls with as little absorption and/or diffusion as I can; however, the room is very 'ringy' and I wouldn't want to under-treat these walls.

 

[yepimonfire]

You could get some of those foam acoustic panels in 12“x12“ size and sparingly treat the side walls and back walls. Foam panels aren't much good for bass but do well at higher frequencies where the flutter echo is a problem. 1" pyramid foam is great for reducing but not deadening reflections. If you sit close to the back wall I'd treat that too. I've noticed if I get too close to it imaging and localization suffers.

 

[br??]

Hey, thanks. Yep the back wall is treated, and some pyramid foam sounds like it could be a good idea. Could I achieve the same effect using thinner and similarly dense sheets of insulation? Would diffusion (something like this) work too?

 

[yepimonfire]

It depends on what exactly you're trying to achieve here. It seems to me you want to keep some liveliness to the room, and that's probably a good thing. Dead rooms sound unnatural. Insulation is an extremely good sound deadener. If I am correct in my thinking that's not what you want.

If cost is the reason you're wanting to go with insulation, you are looking at the wrong vendors. Auralex is extremely overpriced. I've had very good results in the past with foam factory. With 1“ pyramid panels you could treat even the first reflection points and still keep some reasonable amount of first reflections,whilst taming those third, fourth, and so on reflections that cause the fluttering. The important part would be making sure that your brain perceives the sound coming from the speakers as the originating source. Part of this relies on timing, but even more of this relies on volume. In small rooms, the timing differences between the sound coming from the speakers and the sound coming from reflections is reduced, since there is less time for the reflected signal to decay there is less volume difference as well. Utilizing the aforementioned panels will simply reduce this but still allow you to experience natural sounding reflections. Thin insulation is likely to deaden anything at or above 1/4 wavelength, effectively killing off most midrange and highs. The pyramidal shape and porous nature of acoustic foam still allows some reflection, though at a lower volume level, and would be more appropriate for what you're trying to do. Placing them parallel to each other in the room would effectively kill the flutter echo while still leaving much of those initial reflections intact. If it were me and my room was untreated or treated only with bass traps, I would go for 8 24x48 panels, two behind the speakers at the front wall, centered at ear height, two parallel to each other on the side walls at ear height, equidistant to each other, preferably at the first reflection points, and two on the back wall, parallel and equidistant. You might also consider 4 12x12 panels evenly space on the ceiling.

You said the back wall is treated already? What else is treated and where? What is it treated with?

 

[br??]

Aesthetics was more what I was thinking about. If I could use insulation it would blend in with the rest of the treatment. There are floor to ceiling bass traps in the front corners, and half height ones in the rear at ear level, with two more traps on the ceiling. Spanning the space between the two rear traps is a skyline diffuser, also at ear level.

As you noted, I'd like to keep as much liveliness to the room as possible. Also, unrelated(?) to the flutter echo, I was hoping that treating the side walls would help tame some of the resonances of the room. I don't think 1" pyramid panels are going to be thick enough to control room modes in the midrange, but insulation might; however, like you pointed out, it might deaden the sound too much.

 

[yepimonfire]

It's effective after about 500hz. You could always go with 2", but that might deaden the room too much.

 

[br??]

Hmm. Some food for thought. Would diffusion be pointless due to the size of the room? If I need to go below 500 Hz, could I use insulation or thicker pyramid foam on the front and/or rear of the side walls without deadening the room too much?

Also, do you think I'm overdoing it with bass traps on the ceiling? Would I be better off with some pyramid foam instead?

 

[highfigh]

It's been about a year now since I made the biggest upgrade to my system so far -- acoustic treatment. Since then, I've become better at critical listening and identifying the flaws of my room, and I think it's time to remedy the flutter echo between the two side walls, which I left completely bare. My room is small-ish, slightly rectangular with a low-ish ceiling, and I've used DIY bass traps and diffusers for the other surfaces.

The dilemma arises because I'm not sure how to go about dealing with the flutter echo. Absorption? Diffusion? Both? Where? How much? If I've learned one thing from reading the articles by Gene and Dr. Floyd Toole it's that there's no easy way to answer this question. My speakers have excellent off-axis performance, so I'm leaning towards leaving the first reflection points bare and treating the rest of the side walls with as little absorption and/or diffusion as I can; however, the room is very 'ringy' and I wouldn't want to under-treat these walls.

Room dimensions?

You did nothing for treating the first reflections?

What is allowable, WRT cosmetics? If you have book cases, they do a great job of reducing unwanted reflections. If you might want artwork on the side walls, this would be a good time to think about using panels that have artwork covering.

 

[yepimonfire]

Hmm. Some food for thought. Would diffusion be pointless due to the size of the room? If I need to go below 500 Hz, could I use insulation or thicker pyramid foam on the front and/or rear of the side walls without deadening the room too much?

Also, do you think I'm overdoing it with bass traps on the ceiling? Would I be better off with some pyramid foam instead?

You could get down to as low as 100hz with 4“ pane
panels but that would deaden the room. I don't know enough about diffusers to answer that.

 

[br??]

The room is very approximately 4x3 meters, probably slightly less, and there aren't any cosmetic restraints. That being said, bookshelves would probably be too deep and intrude too far into the room. And yep, I did nothing for the first reflections. With so many differing opinions I thought it better to put all of my time and money into treating the areas that everybody agreed needed treating, and sort out the side walls later if they became a problem.

I hadn't really thought about using artwork coverings, but it doesn't look like I'm going down the route of insulation panels anyway.

 

[yepimonfire]

Also keep in mind most flutter echo occurs at high frequencies, you don't need something to extend below 500hz. Lower frequencies build up in a much more uniform way to where it adds to the sound, such as a room mode giving a big 12dB boost at whatever frequency the dimensions likes to amplify. If you've got and pc download rew and take a measurement of the room. More than likely you will notice that it begins getting chaotic well above 500hz. Clap your hands and listen to the reverb from it, it's almost entirely high frequency.

I'm of the opinion that first reflections and reflections in general are a good thing when controlled. Obviously if you have a concrete room it's going to sound like crap, but normal reflections that don't impede imaging only enhance the sound. I've had a dead room before and it was not very good sounding, especially with music which is already recorded and mixed in near dead rooms. A few thin foam panels will do the trick at taming excessive flutter while still allowing the room to still Interact with the speakers enough to sound natural. At this point you are overthinking things.

BTW, I'm curious, how well did bass traps work at taming low frequencies in your small room? I'm in an almost square room myself, 11x12x8 approximately. I have a 15dB boost at 52 hz id like to get rid of.

 

[br??]

Yeah, I think you're right in saying that I'm overthinking things. As for the bass traps, I can't tell you quantitatively because I didn't have a measurement microphone until after I added the traps, but the low frequencies are well tamed at the moment, at least in terms of frequency response. Surprisingly flat below 100 Hz, save for a few dB suckout at 70 Hz. Qualitatively, though, the difference was astounding. There's still some slight ringing here and there, but I plan on attempting to remedy that by replacing the insulation with better stuff. 15 dB is a lot though, so maybe have a look around to see how much trapping you'd need. Bass traps definitely do the job, though, and I highly recommend them. Our rooms are pretty similarly sized, so if you'd like me to send you some measurements or something let me know.

 

[yepimonfire]

Yeah, I think you're right in saying that I'm overthinking things. As for the bass traps, I can't tell you quantitatively because I didn't have a measurement microphone until after I added the traps, but the low frequencies are well tamed at the moment, at least in terms of frequency response. Surprisingly flat below 100 Hz, save for a few dB suckout at 70 Hz. Qualitatively, though, the difference was astounding. There's still some slight ringing here and there, but I plan on attempting to remedy that by replacing the insulation with better stuff. 15 dB is a lot though, so maybe have a look around to see how much trapping you'd need. Bass traps definitely do the job, though, and I highly recommend them. Our rooms are pretty similarly sized, so if you'd like me to send you some measurements or something let me know.

Any mic will do, even a laptop one gives me relatively accurate results surprisingly. How much space do the traps take up? This isn't a dedicated listening room so if they're relatively large or require drilling into the walls I won't be able to do it. Might be better to just use a sub eq or audyssey once I get mic replaced.

 

[highfigh]

The room is very approximately 4x3 meters, probably slightly less, and there aren't any cosmetic restraints. That being said, bookshelves would probably be too deep and intrude too far into the room. And yep, I did nothing for the first reflections. With so many differing opinions I thought it better to put all of my time and money into treating the areas that everybody agreed needed treating, and sort out the side walls later if they became a problem.

I hadn't really thought about using artwork coverings, but it doesn't look like I'm going down the route of insulation panels anyway.

If you won't be using panels, is this because you don't want to put anything on the walls? If so, your flutter problems won't be going away any time soon.

Flutter is inevitable when the walls and floor/ceiling are close to parallel unless the surfaces don't allow reflections that are strong enough to be heard.

If you have an iPhone, download an app called ClapIR and use it to see which frequencies are worst and start there. If the frequencies are higher and caused by the shortest dimensions, it would be from the floor and ceiling (typically 8' apart). Put a rug in front of the speakers if the floor is bare. That will make a big difference. Move a chair to block the first reflections, if possible.

You're not dealing with phase cancellations, you're dealing with flutter- losing a bit of energy isn't a problem in a small room, so 1" or 2" panels won't be as bad as something that's 4" thick. They work and making a few to test for what you need won't cost much. I have made these with Owens Corning 703 rigid fiberglass panels that I attached to pegboard with contact cement and then covered with muslin. You can get the fiberglass, pegboard and contact cement at any big box store like Home Depot and the muslin can be found at any fabric store. If you decide to cover them with something else, go ahead- the muslin can stay and it will help to prevent shedding fibers. A little time and less than $30 should get you 4 panels.

 

[highfigh]

Any mic will do, even a laptop one gives me relatively accurate results surprisingly. How much space do the traps take up? This isn't a dedicated listening room so if they're relatively large or require drilling into the walls I won't be able to do it. Might be better to just use a sub eq or audyssey once I get mic replaced.

WRT the computer mic- maybe. Mine doesn't work well and it has a dropoff in sensitivity around 2KHz and above- I saw this and used a frequency sweep to verify that it was the mic/audio card and not my speakers.

 

[br??]

How much space do the traps take up? This isn't a dedicated listening room so if they're relatively large or require drilling into the walls I won't be able to do it. Might be better to just use a sub eq or audyssey once I get mic replaced.

To tame your 52 Hz peak they would need to be quite thick, probably about 8", but they can be as large as you have the room for and should still be beneficial to an extent. I didn't want to drill into my rear wall, so I just built some small legs to prop the traps up at ear level.

And highfigh, I'm all for putting something on the walls, I just can't decide what exactly. yepimonfire recommends 1" pyramid foam at the first reflection points to tame the reflections without removing them, and I like that this will preserve the liveliness of the room. He said that insulation panels will deaden the room too much -- do you disagree? I'm happy to use either, but insulation will match the rest of the treatment nicely.

 

[br??]

Alright, so I moved the two traps on the rear wall around to the first reflection points. This dramatically reduced the flutter echo and moved most of what remained to the rear quarter of the room. To my surprise the detail, clarity and tightness of the sound improved considerably, albeit at the cost of a small fraction of soundstage. It also helped to squash some of the resonances I've been struggling with, which is good news. This makes me think that I need broadband absorption on the side walls, rather than thin pyramid foam. At the same time, if the foam does a better job within its frequency range while still keeping the room lively, I might be able to compensate for the rest of the issues when I replace the insulation in the existing traps with more broadband-friendly stuff. Although there'll probably be two 4" sheets of insulation left over, making it more convenient and cheaper to use those instead of buying some foam.

 

[highfigh]

Alright, so I moved the two traps on the rear wall around to the first reflection points. This dramatically reduced the flutter echo and moved most of what remained to the rear quarter of the room. To my surprise the detail, clarity and tightness of the sound improved considerably, albeit at the cost of a small fraction of soundstage. It also helped to squash some of the resonances I've been struggling with, which is good news. This makes me think that I need broadband absorption on the side walls, rather than thin pyramid foam. At the same time, if the foam does a better job within its frequency range while still keeping the room lively, I might be able to compensate for the rest of the issues when I replace the insulation in the existing traps with more broadband-friendly stuff. Although there'll probably be two 4" sheets of insulation left over, making it more convenient and cheaper to use those instead of buying some foam.

That "loss of soundstage" is from those reflections and in addition to the reflections from the right speaker on the right wall, you also have reflections from the left speaker on the right wall and vise-versa. These cause sounds to appear to come from places that would be impossible and it's not only unnatural, it's not supposed to happen, if you really want to hear what is in the recording.

I would download the apps and Room EQ Wizard, learn to use them and look at the response, rather than trying to determine what's needed without them. It's possible to see what has changed and this makes it much easier to find solutions. The low end was really annoying me in my system and since I had made some demo panels for a theater I was working on around the same time, I looked at the Room EQ Wizard (usually referred to as 'REW'), I saw a deep notch in the response, due to energy in the corners. I watched the response change using pink noise as the source while I placed the first panel, watched it change more with the second panel, etc- I no longer have a problem with the notch in the low end, the loss of energy isn't a problem in a smaller room and unless the SPL is high, I don't have any "phantom" sounds coming from the right side (where I have a kitchen cabinet that really can't be treated).