GIK Acoustics 244 Sound Panel Review

[Glenn Kuras]

Vaughan,

> what I was finding hardest to come to grips with is the fact that insulation material in a wall can affect the wall-to-wall spacing vibrational frequencies <

Here's another way to look at it: A wall is not a single entity. It has thickness. Sound passes through the "skin" on one side, through the middle material, then to the skin on the other side. Since you (now) know that sound slows down as it passes through insulation, adding insulation inside the wall means sound gets to the other side later than it would have otherwise. So the outer skin is effectively farther away, thus lowering that dimension's mode frequency.

--Ethan

You know if we could only "see" sound then it would make all of this stuff easier to understand. I remember when I did a lot of sailboat racing we would string several light weight 36 foot strings to the front of the boat during practice so we could better understand how the wind was reacting to the sails. It was incredible what you would learn. Which in return made you a better sailor. To bad we can not do that with sound. Or is there a way?

Glenn

 

[Vaughan Odendaa]

Ethan, got it ! Thanks. Now from here, I guess that as Scott was saying, wall resonance doesn't matter much because 1" or 2" thick insulation won't overly affect the room modes.

Correct ?

--Sincerely,

 

[Ethan Winer]

Glenn,

> when I did a lot of sailboat racing we would string several light weight 36 foot strings to the front of the boat during practice so we could better understand how the wind was reacting to the sails. <

Indeed. My partner Doug and I have pondered for a long time how to get sound waves to show. We've thought of fog/smoke with strobe lights, plastic sheets photographed from an angle to show the light changing as the sheet flexes, and so forth. There are some good animations on the web, but those are all simulations - not the real thing. The system setup DVD Digital Video Essentials has a good animation of sound waves in a room, but that too is just a simulation.

--Ethan

 

[Ethan Winer]

Vaughan,

> as Scott was saying, wall resonance doesn't matter much because 1" or 2" thick insulation won't overly affect the room modes. <

The only way to know how much a few inches of insulation shifts the room's modes is to measure with and without. I'll guess a percent or two.

Again, the main point is it doesn't really matter if the modes shift lower or not. That won't change the treatment strategy, so to me it's more a curiosity than something of practical value.

--Ethan

 

[Glenn Kuras]

Glenn,

> when I did a lot of sailboat racing we would string several light weight 36 foot strings to the front of the boat during practice so we could better understand how the wind was reacting to the sails. <

Indeed. My partner Doug and I have pondered for a long time how to get sound waves to show. We've thought of fog/smoke with strobe lights, plastic sheets photographed from an angle to show the light changing as the sheet flexes, and so forth. There are some good animations on the web, but those are all simulations - not the real thing. The system setup DVD Digital Video Essentials has a good animation of sound waves in a room, but that too is just a simulation.

--Ethan

Simulations are not a good thing because you can only learn from the person who "THINKS" they know things about acoustics (the person writing the program).

 

[Savant]

Vaughn,

• The resonance of a wall cavity has nothing to do with room modes. (I wasn't sure if that was a conclusion or not - thought I make it clear just to be safe.)

• The construction of a room will affect its resonant frequencies. The equation used to calculate room modes (referenced in Everest above) assumes completely rigid boundaries. This is difficult to achieve in reality. Several feet of solid concrete comes close, but still isn't perfect. An assumption of rigidity is an assumption of zero damping. A more complete version of the above referenced equation would include (among other things) a damping factor. A single layer of drywall over studs (no insulation) has damping associated with it. Insulating the wall restricting drywall's ability to damp. Dead-sheet (like mass-loaded vinyl, or visco-elastic compounds) or even more drywall will also decrease damping. The less damping there is, the closer the measured natural frequencies will be to the predictions using the equation referenced above.

• Adding fuzz or foam to a wall increases damping in the room and, therefore, also changes the room's resonant frequencies.

• The differences between the frequencies calculated and those measured - regardless of damping - are not usually significant, with one exception: If there are plans to build Helmholtz (resonant, tuned) absorbers to address specific low frequency problems, it can only be done in situ. I.e., "off-the-shelf" Helmholtz absorbers can never be an economical reality since it's a complete crap shoot whether the center frequency of the device will be anywhere near the problematic resonant frequency. Tuned devices can work extremely well, but they must be built, tweaked, and fine-tuned in the room to address the room's specific problem(s).

• All of this is moot if the ultimate solution being planned is broadband low frequency absorption. When using this "brute-force" approach, most problems down to a certain frequency will be more or less attenuated equally.

 

[Savant]

how to get sound waves to show.

I would suggest reading up on the experiments of Helmholtz, Rayleigh, Tyndall, etc. - the pioneers of modern acoustic science. They had much more rigorous and exciting experiments, illustrations, and explanations of basic acoustical phenomena than we do. Our technology tends to stifle our imaginations.

 

[Vaughan Odendaa]

Savant, but all that I have been hearing from people lately is that wall resonance will affect room modes to some degree. Even if it's small.

So then it is not related but it still affects room modes ?

Alright. Savant, in your own words, please explain the reason why room modes exist. My understanding is that it is caused by frequency excitation in air between walls (wall to wall spacings). The volume of air between walls will resonate when frequencies whose wavelengths fit into those spacings.

Resonance in air basically. As opposed to wall resonance where it is resonance in a cavity. Is that right ?

--Sincerely,

 

[Savant]

Vaughn,

Your thoughts on the concept of room modes is not flawed. Much like the air trapped in the room, the air trapped between two sheets of drywalls mounted on either side of wall studs has resonances associated with it. However, the two resonant events are mutually exclusive of each other. It is the rigidity of the (dry)wall that will alter the room (and stud cavity) resonances.

I can appreciate how this might be confusing. The resonance of the wall could be audible in the room. It is perceptible as an annoying "ring" in many large spaces such as houses of worship. However, this resonance will not change the room's resonances. The damping of the drywall will, but the wall cavity resonance will not.

Does that help?

 

[mike c]

my room is made up of all concrete walls EXCEPT for a wall cavity where a door I don't use is, I covered it up with 3/4" plywood leaving a 6" gap between the door and plywood. is this affecting my frequency response?

 

[Ethan Winer]

Mike,

> is this affecting my frequency response? <

Pretty much everything affects the frequency response in a room. Do you have a way to measure this accurately?

--Ethan

 

[ScottMayo]

my room is made up of all concrete walls EXCEPT for a wall cavity where a door I don't use is, I covered it up with 3/4" plywood leaving a 6" gap between the door and plywood. is this affecting my frequency response?

You have bare concrete walls, except for a sheet of plywood over a door?

The plywood won't have much effect - 3/4" plywood is pretty heavy stuff - but an all-concrete environment should be giving you huge frequency response issues. If it's a typically sized rectangular room, you would't have to ask - just play a series of sustained bass notes and walk around the room. You should hear very audible dips in the volume when you wall through a null.

When my room was still just an unfinished, untreated cube, I did rattle tests with a subwoofer, running the sub at high volume at sustained frequencies I knew would wake the room's resonances. There was no furniture in the room, nothing but bare, featureless walls and floor and the open metal framing of a soffit. The modes were unbelievable - I could have billed the room as a science demonstration for school kids. When my wife and I both stood in different nulls, we could talk to each other normally and the sound was little more than a thin, vague hum. If either one of us stepped out of the null, conversation became impossible, and if we stepped into a mode things became physically unpleasant, even with hands over ears. The test meter showed nearly 120dB in the corners (kids: DON'T try this without real ear protection) and well under 50dB in nulls. Very freaky. You don't get these swings in normal rooms because furniture breaks up modes and so on, but they can be startling otherwise.

 

[mike c]

plywood closed

plywood opened

what i'm worried about is the gap in between, could it be the cause of my 57hz dip? should I stuff that gap with insulation?

 

[STRONGBADF1]

plywood closed

plywood opened

what i'm worried about is the gap in between, could it be the cause of my 57hz dip? should I stuff that gap with insulation?

What if mike c made a accustic pannel door? Whould the air gap be an issue then? (assuming that it is now.) I guess stuffing it with some sort of insulation would be easier though.

Just curious and thinking "out loud",
SBF1

Hey and when mike c opens it he would have a bass trap...

 

[Vaughan Odendaa]

Savant,

However, this resonance will not change the room's resonances. The damping of the drywall will, but the wall cavity resonance will not.

Thank you for confirming that my understanding of the subject is not completely erroneous. If you put damping in the wall it will act as a wall spaced further apart (much like when you absorb reflections on walls, you make the walls seem further away).

But you say that the resonance itself will not actually do anything to do the room modes itself. One last question for you and then I think I'll just shut up. Why is it that the resonance itself does not change or influence the room modes ?

Savant, thank you for all your explanations. I appreciate it so much. The same goes to Ethan Winer and all those who contributed. Thank you guys ! And I apologize to the thread starter, I know this thread is not about pesky questions concerning room modes. My apologies.

Thanks again.

--Sincerely,

 

[Ethan Winer]

Vaughan,

I assume Jeff has more sense than me and is off having a good time on this (USA) holiday weekend. So I'll try to answer.

> Why is it that the resonance itself does not change or influence the room modes ? <

Because the wall's resonance is a separate system. It's like putting a timpani drum in a room. The drum has a resonance related to its own dimensions and other properties, even though it's in a room. So just because a wall is attached to a room, its own internal resonance is still a separate system.

--Ethan

 

[ScottMayo]

what i'm worried about is the gap in between, could it be the cause of my 57hz dip? should I stuff that gap with insulation?

Um.. is the ceiling height just about 10'? That would be your 57Hz dip.

If that room is what it looks like - bare sheetrock everywhere - then you should be getting astonishing modes and dips, based almost entirely on the length, width and height of the room. That would dwarf any possible effect from the plywood.

 

[Savant]

Vaughn,

Ethan's timpani example is valid and appropriate. To elaborate on it for further illustration:

• Fill the timpani with fuzz and the timbre of the drum becomes a not-quite-so-sonorous "thud."

• Play the resonant frequency of the timpani on another instrument in the room, and the timpani will absorb some of the energy and resonate in sympathy. Akin to exciting the wall cavity resonance in a large room and hearing the "ring." (A broadband excitation in the room - such as a handclap - will elicit a similar sympathetic ringing, though probably not as strong.) This is excitation of the cavity/timpani mode(s), not the room modes.

• Whether or not the timpani is in the room, the room has the same modal resonances. Similarly, whether or not the room is finished with drywall, the modes will be the same. The only exception to the latter is the damping caused by the non-rigid boundary, which will shift the modes to a small degree.

Hope things are getting clearer, unlike the weather outside on this holiday - t'storms for me!

 

[mike c]

Um.. is the ceiling height just about 10'? That would be your 57Hz dip.

If that room is what it looks like - bare sheetrock everywhere - then you should be getting astonishing modes and dips, based almost entirely on the length, width and height of the room. That would dwarf any possible effect from the plywood.

my ceiling heights is 9 feet 4"

if that dip is caused by the ceiling height, it should be the same wherever I put my sub right?

so how do I treat for this?

 

[twisted3]

I was wondering if it is better to have bass traps straddling the corner walls or having a gap from the corner walls to the sides of the bass trap? If so, how big of a gap?