GIK Acoustics 244 Sound Panel Review

[Ethan Winer]

Folks,

> I am talking about the fundamental resonant frequencies. Will they be changed by simply putting insulation material in the walls ? <

Yes!

It may not be intuitive, but adding insulation inside a standard sheet rock wall makes it act more like a bass trap at whatever frequency the unsupported spans tend to resonate at. This absolutely lowers the resonant frequency a very real, if small, amount. Likewise, adding bass traps in a room also lowers its mode frequencies.

Click the link below to see two graphs taken in my approx. 16.5 by 11 by 8 foot "lab" room. The first graph is for the room empty, and the second is with a bunch of bass traps. You can see that with traps, every one of the peaks has shifted down a few percent. The shift is most evident at the higher end of the frequency range, where the bass traps have the most affect. But even for the peak just above 56 Hz, the frequency is lower in the treated room. The highest peak shown in the untreated room is off the right edge of the graph, above 200 Hz, and in the treated version it's clearly a bit below the 200 Hz marker. Same for the peaks just above the 164 Hz marker.

Okay, so now I'm sure you're asking, "Why does this happen?" The answer is simple: Sound waves slow down as they travel through absorption, so it takes longer for them to come out the other side. This is equivalent to having traveled a longer path, or as if the room were made larger. This is also why acoustic suspension loudspeakers are stuffed with insulation. It makes the cabinet appear larger, thus extending the usable response to a lower frequency.

--Ethan

 

[Vaughan Odendaa]

So if I have an axial mode with a fundamental frequency of 40hz, will putting 2" of insulation material in the walls where the axial mode is calculated lower that modal frequency ?

--Sincerely,

 

[Ethan Winer]

Vaughan,

> So if I have an axial mode with a fundamental frequency of 40hz, will putting 2" of insulation material in the walls where the axial mode is calculated lower that modal frequency ? <

Probably, but it depends on the wall construction and whether it is even able to vibrate at that frequency. A very thin (hypothetical) wall might vibrate around 80 Hz, so in that case it would shift the length mode's second harmonic down but not the fundamental.

--Ethan

 

[ScottMayo]

Vaughan,

> So if I have an axial mode with a fundamental frequency of 40hz, will putting 2" of insulation material in the walls where the axial mode is calculated lower that modal frequency ? <

Probably, but it depends on the wall construction and whether it is even able to vibrate at that frequency. A very thin (hypothetical) wall might vibrate around 80 Hz, so in that case it would shift the length mode's second harmonic down but not the fundamental.

--Ethan

So we're back what with I started with. The primary reverberation, from what bounces off the wall, won't change. The reveberation of the wall itself, which it will radiate back out after the note stops, so to speak, will plummet. The really LF reveberation that gets through the wall and bounces on whatever is behind it, will be affected, measurably.

Usually the first effect listed is what matters and the other two aren't especially relevant. But it depends on the size and shape of the room, the space behind the walls, the construction of the walls, and so on. I remember being in a room which had been hacked together with unevenly spaced 2x4 studs, and thin wood panelling nailed right to the studs, with concrete somewhere behind it. (This was long ago: it probably wouldn't pass code anywhere, these days). The room had the oddest acoustics I remember - the walls themselves would vibrate just from people talking, and it colored speech noticably. Stuffing insulation behind those walls would have made a huge difference. In modern construction, such things matter much less, but I wouldn't say it doesn't matter.

 

[Ethan Winer]

Scott,

> The primary reverberation, from what bounces off the wall, won't change. <

It really will change, at least a little. In small rooms, "reverberation" is a bit of a misnomer. At bass frequencies anyway, it's pure single-frequency ringing. And the frequency of the ringing will indeed be shifted lower, as shown in the ETF waterfall plot I posted.

> Stuffing insulation behind those walls would have made a huge difference. <

I'm sure it would have!

--Ethan

 

[Vaughan Odendaa]

As I said before, I am getting confused with cavity resonance from the wall itself and the room modes caused by the wall-to-wall spacing.

Can someone please put these two things into perspective for me ? What are the major differences/consequences of each and how do they affect the sound.

Thanks.

--Sincerely,

 

[Vaughan Odendaa]

Room modes, as I understand it, determine the natural resonating frequencies in a given room determined by the spacing of the walls. So if a room is 25 foot long, it will have a resonating frequency at 22.6 hz. Correct ?

So if a sound wave from the source corresponds to this frequency to will vibrate quite a lot. Now if a wall is thick, it will have a lower resonating frequency compared to a thinner wall. If a wall has insulation material in it, it will just shift the natural resonating frequency of the room modes as per the wall-to-wall spacing. So a wall with insulation material will essentially act as if it were thicker to begin with.

So instead of having a fundamental resonating frequency at 22.6 hz, the insulation in the wall cause the room modal frequency to be shifted to 20 hz (hypothetically speaking). Is this correct ? But putting insulation material in a wall, by and large, doesn't really help matters that much ?

As per the above post, I just need some clarification on these two things (wall resonance, and room resonance as it applies to the spacing between walls). Thanks.

--Sincerely,

 

[Vaughan Odendaa]

So we're back what with I started with. The primary reverberation, from what bounces off the wall, won't change. The reveberation of the wall itself, which it will radiate back out after the note stops, so to speak, will plummet. The really LF reveberation that gets through the wall and bounces on whatever is behind it, will be affected, measurably.

It seems you two don't agree on this. How the heck am I supposed to understand it then ? I don't quite understand what you mean by the above. Could you elaborate a little more and explain what you mean ?

Thanks.

--Sincerely,

 

[Vaughan Odendaa]

And yet another question for Ethan (sorry about all these questions). I now understand that bass traps can lower the natural room resonance in a wall, the room modes, now my question to you is, how many bass traps would it take in order to lower the natural room modes in a given room by a significant degree ?

Thanks.

--Sincerely,

 

[ScottMayo]

And yet another question for Ethan (sorry about all these questions). I now understand that bass traps can lower the natural room resonance in a wall, the room modes, now my question to you is, how many bass traps would it take in order to lower the natural room modes in a given room by a significant degree ?

Thanks.

--Sincerely,

It depends on the traps - and other things.

A rough and ready answer is, if you put a bass trap in each corner, you are very likely to hear a positive difference. If you use 2 in the back corners, you might get some audible improvement. It depends on a number of things, but that's a simple answer that works for many people.

 

[Vaughan Odendaa]

Thanks, Scott. Please answer my other questions.

Thanks.

--Sincerely,

 

[Vaughan Odendaa]

Scott, I know that bass traps will give me an audible improvement. But I would like to know how many traps would be required to change the natural frequency of the room modes.

Isn't that what Ethan said the traps could do ?

--Sincerely,

 

[ScottMayo]

So we're back what with I started with. The primary reverberation, from what bounces off the wall, won't change. The reveberation of the wall itself, which it will radiate back out after the note stops, so to speak, will plummet. The really LF reveberation that gets through the wall and bounces on whatever is behind it, will be affected, measurably.

It seems you two don't agree on this. How the heck am I supposed to understand it then ? I don't quite understand what you mean by the above. Could you elaborate a little more and explain what you mean ?

Thanks.

--Sincerely,

We probably do agree if we keep our terms straight.

When sound hits a wall, some goes through, a bit gets lost as heat, and some reflects off. This depends on frequency. Low frequency sound goes through wallboard fairly easily, though some is always reflected. High frequency sound is almost all reflected and absorbed, mostly reflected.

The reflected portion isn't much affected by what's behind the wall, since it is reflected before it gets there. The part that goes through is, of course, affected by whatever it finds back there.

So. High frequency sound is affected almost entirely by the walls themselves. Insulation behind won't affect that. That reflection contributes to comb filtering, messed up imaging, and overall harshness.

Low frequency sounds are part reflected and part pass though, and insulation will affect what passes through and back, by absorbing energy and lengthening paths. Those reflections result in modes.

In a lot of modern construction, the wall's drywall is just 4" from concrete or some other reflective surface. In a 15' room, that 4" doesn't amount to much, and the room's modes tend to act as if the distance is about the same as the width between the walls, more or less. In other rooms, like mine, with double wall construction and deliberately large airgaps, the situation is more complicated. Ethan's right in that reflection from behind the wall do matter, and usually shows up as a fatter, less peaky mode, especially if you insulate the space. Nonetheless, the room's dimensions are what you use to find the rooms basic, most audible behaviours, and make the most important starting point.

 

[Ethan Winer]

Vaughan,

> I would like to know how many traps would be required to change the natural frequency of the room modes. <

The purpose of bass traps is not to lower the room's resonant frequencies. That's just a side effect, and if it happens or not is irrelevant. What does matter is that bass traps flatten the response by reducing peaks and raising nulls, and broaden the peaks (lower Q) so they are less frequency specific, and tighten up the low end by reducing ringing.

--Ethan

 

[Vaughan Odendaa]

So the walls resonating frequency is mostly affected at high frequencies ? And a rooms modes, the wall-to-wall spacings is all low frequencies ? And how does the one affect the other ?

I know how to calculate room modes but if the wall composition plays a factor and you don't know the specifics of the construction but you know the distance between the walls, are the calculations for modes not techically correct ?

I don't know. I don't know how the walls own resonance affects the rooms resonance as it is defined by the wall spacings. I'm getting hung up on these two things and I can't seem to differentiate between the two. There is something not clicking.

Please bare with me guys.

--Sincerely,

 

[ScottMayo]

So the walls resonating frequency is mostly affected at high frequencies ? And a rooms modes, the wall-to-wall spacings is all low frequencies ? And how does the one affect the other ?

I know how to calculate room modes but if the wall composition plays a factor and you don't know the specifics of the construction but you know the distance between the walls, are the calculations for modes not techically correct ?

I don't know. I don't know how the walls own resonance affects the rooms resonance as it is defined by the wall spacings. I'm getting hung up on these two things and I can't seem to differentiate between the two. There is something not clicking.

Please bare with me guys.

--Sincerely,

A wall's own resonant frequency doesn't usually matter very much. If yu want to know what it is, pick a spot between two studs and give the wall a whap with your knuckles. If you get back a discernable pitch, that's a frequency that the room is going skew, but the effect is usually *minor*. If you get back a dull thunk with no particular pitch, you've got a well insulated wall or thick plaster, and then the effect really is negligable. None of this is related to the size of the room - it's all about the studs and the wall thickness and composition. There are music studio with deliberately reverberant, wooden walls, to add warmth and liveness. (Audiophiles: don't try this at home.)

The space between walls is what creates modes. It's about air, not plaster. The caveat is that for many rooms used for HT - often basements - that you have two sets of walls - the plaster, and the concrete right behind the plaster. Low frequency sound can get through the plaster and bounce off the concrete. So in theory, you'd get two separate modal behaviours, slightly apart in freqency. In practice, two frequencies reveberating in a fixed space tend to align if the difference between them is small, so it's very reasonable to treat the effect as a single mode. Usually.

If that doesn't explain it, I think I'm just not going to get it across.

 

[Vaughan Odendaa]

Thank you for attempting to explain this to me. I hope I haven't come across as a nuisance. But I think I'm starting to understand this now, or at least a bit better.

Strange how I never come across this information on Everast though. I have looked online and there is precious little to differentiate these two things. I guess 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 (room modes ).

I simply didn't know why that was. Because in my mind, if the distance between walls have not changed, then insulation cannot affect anything. And this made me even more confused as I didn't know how the walls resonating frequencies would affect the room modes frequencies.

Sometimes these things can get complicated because there isn't enough information out there to give you easy explanations.

--Sincerely,

 

[Buckeye_Nut]

I tend to use the KISS approach because I dont care if I can write a detailed dissertation about soundwaves & how bass traps effect them to improve the sound.

All I care is if I hang the darned things, they'll do their job so I can hear the difference

Just 'slap' them up there & enjoy, and dont sweat the technical BS. Your ears will tell you they're working.

 

[Glenn Kuras]

I tend to use the KISS approach because I dont care if I can write a detailed dissertation about soundwaves & how bass traps effect them to improve the sound.

All I care is if I hang the darned things, they'll do their job so I can hear the difference

Just 'slap' them up there & enjoy, and dont sweat the technical BS. Your ears will tell you they're working.

I guess a great way to say it is "hang and play"!

Glenn

 

[Ethan Winer]

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