Hi,
Here's my condition:
My studio & control room, without any acoustic treatment, are already double plastered brick wall constructions, so the soundproofing is already good.
Right now I'm going to treat the room with rockwools covered with fabrics.
The questions:
- Rockwool comes with different density, 60, 80, and 100. Which one should I use, considering that the room itself is quite soundproofed, so I figure that I won't need the 100 density to soundproof the room? Is my assumption right or wrong?
- I'm going to cover the rockwools with fabrics, so I went to some stores which provides sofa covers, and the available fabrics are a mixture of olefin & polyester which is not very expensive, and the expensive 100% cotton fabrics. Are they the right fabrics for covering the rockwool?
Thanks for any info.
Ari
Fabrics & Rockwool Question
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bolehnggak
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John Sayers
- Site Admin
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Well to start with Ari the rockwool isn't for soundproofing - it's for sound treatment. It will absorb the sound within the room - it has nothing to do with the sound that gets out of the room. The higher the density the more absorption you will get.
The cloth is purely cosmetic - nicer to look at than rockwool so use what you like to cover it. I prefer natural fibers like cotton myself.
cheers
john
The cloth is purely cosmetic - nicer to look at than rockwool so use what you like to cover it. I prefer natural fibers like cotton myself.
cheers
john
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Michael Jones
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knightfly
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When placed INSIDE a sound wall, the rockwool helps act as a trap and improves sound attenuation. The recommended density when used inside a wall is 3 PCF, same as 703. About half the air cavity taken up with insulation seems to be optimum. Heavier than 3pcf improves bass loss slightly, at the cost of higher freq's getting through. Lighter than 3pcf and the opposite happens (bass performance worse, highs better)
Soooo, when a Musician dies, do they hear the white noise at the end of the tunnel??!? Hmmmm...
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bolehnggak
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What density do you recommend for the room walls?knightfly wrote:When placed INSIDE a sound wall, the rockwool helps act as a trap and improves sound attenuation. The recommended density when used inside a wall is 3 PCF, same as 703. About half the air cavity taken up with insulation seems to be optimum. Heavier than 3pcf improves bass loss slightly, at the cost of higher freq's getting through. Lighter than 3pcf and the opposite happens (bass performance worse, highs better)
-
knightfly
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That would depend entirely on what you're trying to absorb. Usually, you would want to absorb down to as low as you could if you're using the treatment to minimise standing wave problems in a parallel walled room for example - the heavier the density, the lower frequency absorbed - but thickness and distance from the wall/ceiling also enters in. Wavelengths of lower frequencies require several FEET of distance from the wall (boundary) in order to be at the most effective distance (1/4 wavelength)
Other things enter into this which make it nearly impossible for me to tell you that - what kind of furniture/how much is in the room, is there carpet on floors, what shape is the room, how dead do you want it, what is the total volume of the room, the list is longer than I have time to type... Steve
Other things enter into this which make it nearly impossible for me to tell you that - what kind of furniture/how much is in the room, is there carpet on floors, what shape is the room, how dead do you want it, what is the total volume of the room, the list is longer than I have time to type... Steve
Soooo, when a Musician dies, do they hear the white noise at the end of the tunnel??!? Hmmmm...
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Eric_Desart
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If interested:
Copied this message from another group.
I'm not sure you can access all references.
All absorption values in this message:
125 Hz 250 Hz 500 Hz 1000 Hz 2000 Hz 4000 Hz
All quotes originates from:
http://www.recording.org/users/acoustics
Date June 13 2003
1) QUOTE 1
...... "It's not difficult to understand why 705 fiberglass is so
much more absorbent than foam at low frequencies. One of the most
important properties of any acoustic absorbing material is its
density. The denser it is, the more it absorbs and the lower in
frequency the absorption extends to. Acoustic foam has a density of
less than 2 pounds per cubic foot (pcf). For example, one popular
manufacturer specs their foam at 1.7 pcf. Compare that to 705
fiberglass, which has a density of 6 pcf. So for a given panel size
and thickness, 705 is more than three times more effective at low
frequencies than foam.".......
IS THE ABOVE STATEMENT TRUE?
Let's look at the Crowing Corner's published data:
http://tinyurl.com/dv13 hit "submittal sheet".
Owens Corning 701... 24 kg/m^3 - 1.5 pcf
25,4 mm - 1.00" --- 0.17 0.33 0.64 0.83 0.90 0.92 0.70
51.0 mm - 2.00" --- 0.22 0.67 0.98 1.02 0.98 1.00 0.90
Owens Corning 703 ... 48 kg/m^3 - 3 pcf
25,4 mm - 1.00" --- 0.11 0.28 0.68 0.90 0.93 0.96 0.70
51.0 mm - 2.00" --- 0.17 0.86 1.14 1.07 1.02 0.98 1.00
Owens Corning 705 ... 96 kg/m^3 - 6 pcf
25,4 mm - 1.00" --- 0.02 0.27 0.63 0.85 0.93 0.95 0.65
51.0 mm - 2.00" --- 0.16 0.71 1.02 1.01 0.99 0.99 0.95
Conclusion:
There is no reason to link higher low-frequent absorption
systematically to higher density of the absorptive material.
MELAMINE RESIN FOAM a.k.a. BASOTECT a.k.a. WILLTEC a.k.a. WHATEVER,
It has rather unique fire properties (of course still inferior to
mineral wool) and due to its lightweight and acoustic properties is
also used in airplanes and airplane seatings. Also used a lot for
industrial purposes.
http://www.isomo.be/idis215.htm
This is FLAT unsculpted naked Melamine Foam.
Measured as per DIN 52212.
European Labs give systematic somewhat lower values than US labs
(slightly deviating standards)
The Leuven Lab gives in mid and high somewhat lower values than lots
of other European Labs.
Note: the author assumes that the values of the 60 mm type originate
from another lab, or at least are not measured in comparable
circumstances.
Look at the extreme low foam density:
8-11 kg/m^3 - 0.50 - 0.69 pcf
30.0 mm - 1.18" --- 0,14 0,28 0,51 0,75 0,89 0,89
40.0 mm - 1.57" --- 0,18 0,37 0,72 0,92 0,97 0,97
50.0 mm - 1.97" --- 0,24 0,49 0,83 0,94 0,98 0,98
60.0 mm - 2.36" --- 0,27 0,72 1,07 1,09 1,04 1,02
100.0 mm - 3.94" --- 0,49 0,89 0,97 0,98 1,02 1,04
Compare those 50/60 mm values with the 50 mm glassfiber values.
While Melamine has a relative low gasflow resistance, clearly also
here the link between density and low frequent absorption is NOT
supported.
This material is averaged 0.62 pcf versus the 6.0 pcf of the OC 705
type
While rock wool and glass wool have undoubtful extremely good and
hard to beat acoustic absorption properties, the exact how's and
why's are a complex combination of factors.
GENERAL CONCLUSION:
The statement that increasing density can systematically be related
to better low frequent absorption is WRONG.
In function of price versus low frequent absorption it will often
prove a costly and inefficient manner, mostly easily substituted by
adjusted mounting.
Absorption is defined by a complex of factors as entrance impedance,
gas flow resistance etc. and is angle of incidence dependent.
At sharp angled incidence (grazing) high densities will diminish low-
frequent absorption.
For splitter silencers laboratory test showed that lowering density
could improve low frequent absorption, which was the contrary of the
results for the same material in reverberation room (random
incidence) measurements.
For Control Room applications this can be important since absorption
is often used to control first reflections to the mix position, which
mostly are angled to sharp angled sound incidences.
Following quotes are located under the heading:
MIDRANGE AND HIGH FREQUENCY ABSORBERS
QUOTE 2:
......" Without question, the most effective absorber for midrange
and high frequencies is rigid fiberglass. Owens-Corning 703, or an
equivalent from other manufacturers, is the standard material used by
professional studio designers." ......
Comment:
Fiberglas and equivalents is as well effective for low frequent
absorption. If properly used, it has a relative smooth behavior with
easier to control side effects unlike most types of resonators.
QUOTE 3:
......"The velocity of any wave is greatest at a distance equal to
1/4 of its wavelength - about four feet from the wall for a 70 Hz.
wave"......
Comment: This is true for STRAIGTH incidence of a DISCRETE frequency
of a TRAVELING PLAIN wave (that's a lot of boundary conditions),
which in practice rarely occurs. With Room Modes a lot of the
velocity happens parallel and/or in the direct neighborhood of the
wall. Also traveling waves are subject to COMPLEX wave behavior and
the angle of incidence influence (theorem of Pythagoras).
Look at the animation (is Dutch site but animation itself is
important which is language insensitive)
http://www.kuleuven.ac.be/bwf/onderwijs ... akoe_1.htm
The green dots are the pressure antinodes. The black and red dots
represent the particle velocity. The red dot at the left wall shows
the particles move parallel with the wall.
One can easily notice that there is a lot of velocity against and in
the neighborhood of the walls, showing that even for a simple 4,2,0
mode that the 1/4 wave approach is much to simplified to be related
the absorptive material thickness.
Also look at:
http://ceae.colorado.edu/~muehleis/clas ... waves.html
The Green colored parts of the 3,2,0 mode are the maximum velocity
areas. They are as well located close to the wall than in free space.
Also look at the shift towards the reflective boundary of the
velocity maximum for traveling waves, when waves are combined
resulting in COMPLEX PLAIN WAVES in MS Word (138 kB):
http://tinyurl.com/ectc
An alternative view on air particle velocity.
How does absorption behaves in function of wavelength (is the 1/4
wavelength approach OK?). A temporary simulation document.
Let's calculate real life absorption measurements in function of the
alpha value versus the % of the wavelength defined by the frequency
versus material thickness :
List 1:
based on absorption values from Owens Corning to be found in list
archive, entered by Scott Foster.
OC 703 · 2.0" · 50.8 mm
0125 Hz · 0.22 · 001.8%
0250 Hz · 0.82 · 003.7%
0500 Hz · 1.21 · 007.4%
1000 Hz · 1.10 · 014.8%
2000 Hz · 1.02 · 029.6%
4000 Hz · 1.05 · 059.2%
OC 703 · 3.0" · 76.2 mm
0125 Hz · 0.53 · 002.8%
0250 Hz · 1.19 · 005.5%
0500 Hz · 1.21 · 011.1%
1000 Hz · 1.08 · 022.2%
2000 Hz · 1.01 · 044.4%
4000 Hz · 1.04 · 088.8%
OC 703 · 4.0" · 101.6 mm
0125 Hz · 0.84 · 003.7%
0250 Hz · 1.24 · 007.4%
0500 Hz · 1.24 · 014.8%
1000 Hz · 1.08 · 029.6%
2000 Hz · 1.00 · 059.2%
4000 Hz · 0.97 · 118.4%
OC 703 · 6.0" · 152.4 mm
0125 Hz · 1.19 · 005.5%
0250 Hz · 1.21 · 011.1%
0500 Hz · 1.13 · 022.2%
1000 Hz · 1.05 · 044.4%
2000 Hz · 1.04 · 088.8%
4000 Hz · 1.04 · 177.6%
List 2:
Current Owens Corning site:
OC 701 · 2.0" · 50.8 mm
0125 Hz · 0.22 · 001.8%
0250 Hz · 0.67 · 003.7%
0500 Hz · 0.98 · 007.4%
1000 Hz · 1.02 · 014.8%
2000 Hz · 0.98 · 029.6%
4000 Hz · 1.00 · 059.2%
OC 703 · 2.0" · 50.8 mm
0125 Hz · 0.17 · 001.8%
0250 Hz · 0.86 · 003.7%
0500 Hz · 1.14 · 007.4%
1000 Hz · 1.07 · 014.8%
2000 Hz · 1.02 · 029.6%
4000 Hz · 0.98 · 059.2%
OC 705 · 2.0" · 50.8 mm
0125 Hz · 0.16 · 001.8%
0250 Hz · 0.71 · 003.7%
0500 Hz · 1.02 · 007.4%
1000 Hz · 1.01 · 014.8%
2000 Hz · 0.99 · 029.6%
4000 Hz · 0.99 · 059.2%
In order to get a picture of the relationship : % of wavelength
versus absorption one must use much more measurements, and in 1/3
octavebands (as they are measured in the laboratories).
However the above commonly known standard materials give a clear
picture that the 1/4 wave approach equaling 25 % to reach absorption
maximum is just a theoretical approach with little relationship to a
real life sound field.
This 1/4 wave length approach should certainly not be used (as seen
on so many sites) to support the idea that extreme thickness' are
needed (or extreme cavities for that matter) to absorb low
frequencies, thereby assigning wool for mid and high frequency
absorption.
For practical general engineering purposes and a random sound field,
one can estimate:
1) A material thickness (including cavity) of 5% of the wavelength
will exceed 80 % absorption. The material itself must not exceed 1%
of the wavelength. In practice for random incidence the 80% will often
be surpassed. See this as a rule of thumb engineering approach.
2) A material thickness (including cavity) of 10% of the wavelength
will about approach the maximum absorption which in real life will never
exceed ca 95 %. For critical applications one can increase the
material thickness to 2 % of the wavelength.
For most real life circumstances any further increase of the above
values, IF any, will only result in very minor improvements.
To decide in function of discrete frequencies, angle of incidence or
any other special sound field one must try to understand the sound
field and the mechanisms defining the absorption of a material (which
often is a hybrid of a theoretical approach and empirical experience).
The most important to remember is:
NONE of the existing absorption measurements one will ever find will
confirm the 1/4 wave approach.
This is plain theory (looking good) only valid within lots of rarely,
if ever, occurring boundary conditions. So while explaining the
simplified theory versus velocity, one should NOT link this to real
life use.
This 1/4 wave theory is often used on commercial sites as opposed to
the superior (?) properties of other low frequency absorbers, where
one often does not explain the disadvantages and/or side effects of
those absorbers.
Real life absorption values are a complex thing, where the MATERIAL
PRPOERTIES ARE ONLY 1 PARAMETER IN A COMPLEX OF LOTS OF acoustic
PARAMETERS.
Several years back I went into an official University lab with:
12 identical rock wool boards (1200*900*120 mm - 47.2*35.4*4.72" --
density ca 60 kg/m3 - 3.75 pcf) surrounded by a lightweight metal
frame (25*121*25*0.7mm - 1x4.76x1x0.028"). So the board edges where
reflective.
This was part of a huge study to investigate lots of different
phenomena.
The idea (for this part) was to check the influence on the absorption
of different positions of those rock wool boards within the lab.
It's hard to imagine that all those measurements result from the same
12 boards, measured in the same lab, the very same day. (Note that
the lab is certified, as most are, down to 100 Hz).
Also important is to note that such a lab is a controlled acoustic
environment, designed to exclude as much as possible undesirable room
acoustic effects. Now try to imagine how such values must become when
applied in real live acoustic circumstances with all kinds of
arbitrary properties.
One measurement simulates ca diagonal absorption. As one can see
those values are incredibly high. There is just no substitute for
this principally approach.
You can see a related COPYRIGHT protected picture (ca 22 kB) here:
http://tinyurl.com/ecnw
This picture is also included in a more extended MS Word doc (ca 355
kB)
http://tinyurl.com/ecql
QUOTE 4:
...... "As you can see in Figure 5 above, absorption for a given gap
depth reaches a maximum at only one frequency, in this case 100 Hz.
It then falls at a higher frequency where 1/2 the wavelength equals
the gap depth. It rises again when the gap matches 3/4 of the length
of the next higher frequency, and so forth. You can avoid the
reduction in absorption by filling the entire gap with material
instead of using a thinner piece spaced away from the wall or
ceiling. When the entire depth is filled, material is available to
absorb all of the frequencies whose 1/4 wavelengths fall within that
depth." ......
Comment:
This relates to the previous point and is ONLY true in strict
theoretical (non-real-life) circumstances, leading to a wrong and
costly acoustic treatment advice (unless it should be cheaper to fill
the cavity, or use thicker boards rather than making a construction
to create this cavity), contradicted by almost any standard
reverberation room laboratory measurement based on absorptive
material mounted on a cavity.
Only an experienced acoustician will notice the described (and then
only the one at 1/2 wavelength, not the higher ones) dip when the
material thickness is relative thin (<= 1/10 of cavity + material
thickness) and the reflective background and cavity is about 100%
symmetric. The resulting minor dip (mainly noticeable in 1/3 octave
absorption measurements), IF existing and noticeable, is
insignificant and in 99% of real live cases can be ignored.
If this should not be the case, ANY tradition acoustic ceiling (99.9
% of the absorption market) should show gaps in the absorption right
in the middle of a frequency range significant for speech
intelligibility.
QUOTE 5:
...... "Likewise, fiberglass placed exactly at a rigid boundary does
nothing because the air particles are not moving there. And since
there's no velocity, the fiberglass has very little effect. As
fiberglass is spaced further from the wall, the air particles passing
through it have greater velocity. But they are slowed down as they
pass through the fiberglass, making the transfer of energy less
efficient which attenuates the wave. The velocity of any wave is
greatest at a distance equal to 1/4 of its wavelength - about four
feet from the wall for a 70 Hz. wave - though it goes back down to
zero at half the wavelength." ......
Comment:
This relates to the several previous points, assuming plain waves
with discrete frequencies at straight incidence.
A real live sound field is completely different. Velocity is defined
by the result of complex waves
QUOTE 6:
...... "In practice, you don't necessarily have to measure
wavelengths and calculate air gaps. Most people are not willing to
give up two or more feet all around the room anyway, so just make the
gap as large as you can justify. If you can afford to fill the gap
entirely with material, all the better because that helps avoid holes
in the range of frequencies absorbed. And even though the velocity is
indeed highest at 1/4 wavelength, there's still plenty at 1/8th of
the wavelength too. You could also use a few different gap sizes, or
make some panels much thicker than others, or install the fiberglass
at an angle so the top of a 2x4 panel mounted vertically is touching
the wall and the bottom is spaced away half a foot. I'm sure you get
the idea." ......
Explained in previous comments
QUOTE 7:
..... "BETTER BASS TRAPS" ...
My favorite type of bass trap is the membrane absorber, also called a
panel trap because it's made with a wood front panel.
Comment:
Basically the http://www.recording.org/users/acoustics FAQ indeed
reflects the preference of the Author for panel traps in function of
low frequency absorption, at the expense of accuracy.
This is related by the fact that the author, has some DIY designs for
panel traps, but also owns a company SELLING commercial panel traps.
While a panel trap can certainly serve acoustic purposes, mainly as
space saving absorbers, the site nowhere explains the related
mechanisms, not the side effects of using narrow banded absorbers,
nor the acoustic leaks it creates on the insulation of drywalls right
at the (frequency spots) where modes occur. As such a panel trap is
a bit contradictory:
One tries to design a drywall with an "as low as possible" mass-
spring-mass, preferably below the frequency range of interest,
resonance which is defining for the insulation of this cavity wall. A
panel trap (certainly the low frequent heavy ones) alters this mass-
spring-mass resonance back to the higher frequencies, diminishing the
insulation of the drywall (all depends on mass and spring ratios). A
panel resonator is a mass-spring system as a cavity wall is, as such
creating a triple leave system on cavity walls, and depending on the
mass and spring ratios will alter resonances as well for the
absorption of the panel resonator as the insulation of the wall.
More details about the acoustic effects of other resonators as
Helmholtz & panel traps will maybe discussed in later messages.
Just some info for completeness
This is a list once entered by Scott Foster in a list posting.
Source unknown to me
Owens-Corning 703 - 1"
0.03 0.22 0.69 0.91 0.96 0.99 0.63
Owens-Corning 703 - 2"
0.22 0.82 1.21 1.10 1.02 1.05 0.90
Owens-Corning 703 - 3"
0.53 1.19 1.21 1.08 1.01 1.04 1.01
Owens-Corning 703 - 4"
0.84 1.24 1.24 1.08 1.00 0.97 1.06
Owens-Corning 703 - 6"
1.19 1.21 1.13 1.05 1.04 1.04 1.11
Illbruck/Sonex once published a lot of absorption data on their
industry site, but removed this data and makes industrial data
difficult accessible.
The Foam Selection Guide below will give you a quick idea of the type
of foam solution illbruck might suggest for your application
http://www.customfoamfab.com/files/selectionguide1.html
Acoustic absorbers: Open cell urethane, willtec®, and willmid®
foams.
In fact the market for FLAT acoustic foams is MUCH LARGER than the
market for SCULPTED acoustic foams.
They are used in the automotive industry, airplanes, compressors and
lots of other technical applications in enormous quantities.
The main reasons however are not to be found in the superior acoustic
qualities but in technical properties in function of the behavior
versus vibrations (wool degenerates when used in e.g. a compressor
enclosure) and lots of other chemical, mounting, vibrational reasons.
The magic surrounding the sculptured foam in acoustics, certainly in
the studio world, seems to have more impact on the American than the
European market.
Maybe go deeper into that in later messages.
With kind regards
Eric
This COPYRIGHT picture further down shows the importance of how absorption material is used in function of positioning within the room. Sabine values are only 1 parameter. All those measurements are made with the same rock wool boards framed to exclude edge absorption influence, in the same lab, the very same day.
Copied this message from another group.
I'm not sure you can access all references.
All absorption values in this message:
125 Hz 250 Hz 500 Hz 1000 Hz 2000 Hz 4000 Hz
All quotes originates from:
http://www.recording.org/users/acoustics
Date June 13 2003
1) QUOTE 1
...... "It's not difficult to understand why 705 fiberglass is so
much more absorbent than foam at low frequencies. One of the most
important properties of any acoustic absorbing material is its
density. The denser it is, the more it absorbs and the lower in
frequency the absorption extends to. Acoustic foam has a density of
less than 2 pounds per cubic foot (pcf). For example, one popular
manufacturer specs their foam at 1.7 pcf. Compare that to 705
fiberglass, which has a density of 6 pcf. So for a given panel size
and thickness, 705 is more than three times more effective at low
frequencies than foam.".......
IS THE ABOVE STATEMENT TRUE?
Let's look at the Crowing Corner's published data:
http://tinyurl.com/dv13 hit "submittal sheet".
Owens Corning 701... 24 kg/m^3 - 1.5 pcf
25,4 mm - 1.00" --- 0.17 0.33 0.64 0.83 0.90 0.92 0.70
51.0 mm - 2.00" --- 0.22 0.67 0.98 1.02 0.98 1.00 0.90
Owens Corning 703 ... 48 kg/m^3 - 3 pcf
25,4 mm - 1.00" --- 0.11 0.28 0.68 0.90 0.93 0.96 0.70
51.0 mm - 2.00" --- 0.17 0.86 1.14 1.07 1.02 0.98 1.00
Owens Corning 705 ... 96 kg/m^3 - 6 pcf
25,4 mm - 1.00" --- 0.02 0.27 0.63 0.85 0.93 0.95 0.65
51.0 mm - 2.00" --- 0.16 0.71 1.02 1.01 0.99 0.99 0.95
Conclusion:
There is no reason to link higher low-frequent absorption
systematically to higher density of the absorptive material.
MELAMINE RESIN FOAM a.k.a. BASOTECT a.k.a. WILLTEC a.k.a. WHATEVER,
It has rather unique fire properties (of course still inferior to
mineral wool) and due to its lightweight and acoustic properties is
also used in airplanes and airplane seatings. Also used a lot for
industrial purposes.
http://www.isomo.be/idis215.htm
This is FLAT unsculpted naked Melamine Foam.
Measured as per DIN 52212.
European Labs give systematic somewhat lower values than US labs
(slightly deviating standards)
The Leuven Lab gives in mid and high somewhat lower values than lots
of other European Labs.
Note: the author assumes that the values of the 60 mm type originate
from another lab, or at least are not measured in comparable
circumstances.
Look at the extreme low foam density:
8-11 kg/m^3 - 0.50 - 0.69 pcf
30.0 mm - 1.18" --- 0,14 0,28 0,51 0,75 0,89 0,89
40.0 mm - 1.57" --- 0,18 0,37 0,72 0,92 0,97 0,97
50.0 mm - 1.97" --- 0,24 0,49 0,83 0,94 0,98 0,98
60.0 mm - 2.36" --- 0,27 0,72 1,07 1,09 1,04 1,02
100.0 mm - 3.94" --- 0,49 0,89 0,97 0,98 1,02 1,04
Compare those 50/60 mm values with the 50 mm glassfiber values.
While Melamine has a relative low gasflow resistance, clearly also
here the link between density and low frequent absorption is NOT
supported.
This material is averaged 0.62 pcf versus the 6.0 pcf of the OC 705
type
While rock wool and glass wool have undoubtful extremely good and
hard to beat acoustic absorption properties, the exact how's and
why's are a complex combination of factors.
GENERAL CONCLUSION:
The statement that increasing density can systematically be related
to better low frequent absorption is WRONG.
In function of price versus low frequent absorption it will often
prove a costly and inefficient manner, mostly easily substituted by
adjusted mounting.
Absorption is defined by a complex of factors as entrance impedance,
gas flow resistance etc. and is angle of incidence dependent.
At sharp angled incidence (grazing) high densities will diminish low-
frequent absorption.
For splitter silencers laboratory test showed that lowering density
could improve low frequent absorption, which was the contrary of the
results for the same material in reverberation room (random
incidence) measurements.
For Control Room applications this can be important since absorption
is often used to control first reflections to the mix position, which
mostly are angled to sharp angled sound incidences.
Following quotes are located under the heading:
MIDRANGE AND HIGH FREQUENCY ABSORBERS
QUOTE 2:
......" Without question, the most effective absorber for midrange
and high frequencies is rigid fiberglass. Owens-Corning 703, or an
equivalent from other manufacturers, is the standard material used by
professional studio designers." ......
Comment:
Fiberglas and equivalents is as well effective for low frequent
absorption. If properly used, it has a relative smooth behavior with
easier to control side effects unlike most types of resonators.
QUOTE 3:
......"The velocity of any wave is greatest at a distance equal to
1/4 of its wavelength - about four feet from the wall for a 70 Hz.
wave"......
Comment: This is true for STRAIGTH incidence of a DISCRETE frequency
of a TRAVELING PLAIN wave (that's a lot of boundary conditions),
which in practice rarely occurs. With Room Modes a lot of the
velocity happens parallel and/or in the direct neighborhood of the
wall. Also traveling waves are subject to COMPLEX wave behavior and
the angle of incidence influence (theorem of Pythagoras).
Look at the animation (is Dutch site but animation itself is
important which is language insensitive)
http://www.kuleuven.ac.be/bwf/onderwijs ... akoe_1.htm
The green dots are the pressure antinodes. The black and red dots
represent the particle velocity. The red dot at the left wall shows
the particles move parallel with the wall.
One can easily notice that there is a lot of velocity against and in
the neighborhood of the walls, showing that even for a simple 4,2,0
mode that the 1/4 wave approach is much to simplified to be related
the absorptive material thickness.
Also look at:
http://ceae.colorado.edu/~muehleis/clas ... waves.html
The Green colored parts of the 3,2,0 mode are the maximum velocity
areas. They are as well located close to the wall than in free space.
Also look at the shift towards the reflective boundary of the
velocity maximum for traveling waves, when waves are combined
resulting in COMPLEX PLAIN WAVES in MS Word (138 kB):
http://tinyurl.com/ectc
An alternative view on air particle velocity.
How does absorption behaves in function of wavelength (is the 1/4
wavelength approach OK?). A temporary simulation document.
Let's calculate real life absorption measurements in function of the
alpha value versus the % of the wavelength defined by the frequency
versus material thickness :
List 1:
based on absorption values from Owens Corning to be found in list
archive, entered by Scott Foster.
OC 703 · 2.0" · 50.8 mm
0125 Hz · 0.22 · 001.8%
0250 Hz · 0.82 · 003.7%
0500 Hz · 1.21 · 007.4%
1000 Hz · 1.10 · 014.8%
2000 Hz · 1.02 · 029.6%
4000 Hz · 1.05 · 059.2%
OC 703 · 3.0" · 76.2 mm
0125 Hz · 0.53 · 002.8%
0250 Hz · 1.19 · 005.5%
0500 Hz · 1.21 · 011.1%
1000 Hz · 1.08 · 022.2%
2000 Hz · 1.01 · 044.4%
4000 Hz · 1.04 · 088.8%
OC 703 · 4.0" · 101.6 mm
0125 Hz · 0.84 · 003.7%
0250 Hz · 1.24 · 007.4%
0500 Hz · 1.24 · 014.8%
1000 Hz · 1.08 · 029.6%
2000 Hz · 1.00 · 059.2%
4000 Hz · 0.97 · 118.4%
OC 703 · 6.0" · 152.4 mm
0125 Hz · 1.19 · 005.5%
0250 Hz · 1.21 · 011.1%
0500 Hz · 1.13 · 022.2%
1000 Hz · 1.05 · 044.4%
2000 Hz · 1.04 · 088.8%
4000 Hz · 1.04 · 177.6%
List 2:
Current Owens Corning site:
OC 701 · 2.0" · 50.8 mm
0125 Hz · 0.22 · 001.8%
0250 Hz · 0.67 · 003.7%
0500 Hz · 0.98 · 007.4%
1000 Hz · 1.02 · 014.8%
2000 Hz · 0.98 · 029.6%
4000 Hz · 1.00 · 059.2%
OC 703 · 2.0" · 50.8 mm
0125 Hz · 0.17 · 001.8%
0250 Hz · 0.86 · 003.7%
0500 Hz · 1.14 · 007.4%
1000 Hz · 1.07 · 014.8%
2000 Hz · 1.02 · 029.6%
4000 Hz · 0.98 · 059.2%
OC 705 · 2.0" · 50.8 mm
0125 Hz · 0.16 · 001.8%
0250 Hz · 0.71 · 003.7%
0500 Hz · 1.02 · 007.4%
1000 Hz · 1.01 · 014.8%
2000 Hz · 0.99 · 029.6%
4000 Hz · 0.99 · 059.2%
In order to get a picture of the relationship : % of wavelength
versus absorption one must use much more measurements, and in 1/3
octavebands (as they are measured in the laboratories).
However the above commonly known standard materials give a clear
picture that the 1/4 wave approach equaling 25 % to reach absorption
maximum is just a theoretical approach with little relationship to a
real life sound field.
This 1/4 wave length approach should certainly not be used (as seen
on so many sites) to support the idea that extreme thickness' are
needed (or extreme cavities for that matter) to absorb low
frequencies, thereby assigning wool for mid and high frequency
absorption.
For practical general engineering purposes and a random sound field,
one can estimate:
1) A material thickness (including cavity) of 5% of the wavelength
will exceed 80 % absorption. The material itself must not exceed 1%
of the wavelength. In practice for random incidence the 80% will often
be surpassed. See this as a rule of thumb engineering approach.
2) A material thickness (including cavity) of 10% of the wavelength
will about approach the maximum absorption which in real life will never
exceed ca 95 %. For critical applications one can increase the
material thickness to 2 % of the wavelength.
For most real life circumstances any further increase of the above
values, IF any, will only result in very minor improvements.
To decide in function of discrete frequencies, angle of incidence or
any other special sound field one must try to understand the sound
field and the mechanisms defining the absorption of a material (which
often is a hybrid of a theoretical approach and empirical experience).
The most important to remember is:
NONE of the existing absorption measurements one will ever find will
confirm the 1/4 wave approach.
This is plain theory (looking good) only valid within lots of rarely,
if ever, occurring boundary conditions. So while explaining the
simplified theory versus velocity, one should NOT link this to real
life use.
This 1/4 wave theory is often used on commercial sites as opposed to
the superior (?) properties of other low frequency absorbers, where
one often does not explain the disadvantages and/or side effects of
those absorbers.
Real life absorption values are a complex thing, where the MATERIAL
PRPOERTIES ARE ONLY 1 PARAMETER IN A COMPLEX OF LOTS OF acoustic
PARAMETERS.
Several years back I went into an official University lab with:
12 identical rock wool boards (1200*900*120 mm - 47.2*35.4*4.72" --
density ca 60 kg/m3 - 3.75 pcf) surrounded by a lightweight metal
frame (25*121*25*0.7mm - 1x4.76x1x0.028"). So the board edges where
reflective.
This was part of a huge study to investigate lots of different
phenomena.
The idea (for this part) was to check the influence on the absorption
of different positions of those rock wool boards within the lab.
It's hard to imagine that all those measurements result from the same
12 boards, measured in the same lab, the very same day. (Note that
the lab is certified, as most are, down to 100 Hz).
Also important is to note that such a lab is a controlled acoustic
environment, designed to exclude as much as possible undesirable room
acoustic effects. Now try to imagine how such values must become when
applied in real live acoustic circumstances with all kinds of
arbitrary properties.
One measurement simulates ca diagonal absorption. As one can see
those values are incredibly high. There is just no substitute for
this principally approach.
You can see a related COPYRIGHT protected picture (ca 22 kB) here:
http://tinyurl.com/ecnw
This picture is also included in a more extended MS Word doc (ca 355
kB)
http://tinyurl.com/ecql
QUOTE 4:
...... "As you can see in Figure 5 above, absorption for a given gap
depth reaches a maximum at only one frequency, in this case 100 Hz.
It then falls at a higher frequency where 1/2 the wavelength equals
the gap depth. It rises again when the gap matches 3/4 of the length
of the next higher frequency, and so forth. You can avoid the
reduction in absorption by filling the entire gap with material
instead of using a thinner piece spaced away from the wall or
ceiling. When the entire depth is filled, material is available to
absorb all of the frequencies whose 1/4 wavelengths fall within that
depth." ......
Comment:
This relates to the previous point and is ONLY true in strict
theoretical (non-real-life) circumstances, leading to a wrong and
costly acoustic treatment advice (unless it should be cheaper to fill
the cavity, or use thicker boards rather than making a construction
to create this cavity), contradicted by almost any standard
reverberation room laboratory measurement based on absorptive
material mounted on a cavity.
Only an experienced acoustician will notice the described (and then
only the one at 1/2 wavelength, not the higher ones) dip when the
material thickness is relative thin (<= 1/10 of cavity + material
thickness) and the reflective background and cavity is about 100%
symmetric. The resulting minor dip (mainly noticeable in 1/3 octave
absorption measurements), IF existing and noticeable, is
insignificant and in 99% of real live cases can be ignored.
If this should not be the case, ANY tradition acoustic ceiling (99.9
% of the absorption market) should show gaps in the absorption right
in the middle of a frequency range significant for speech
intelligibility.
QUOTE 5:
...... "Likewise, fiberglass placed exactly at a rigid boundary does
nothing because the air particles are not moving there. And since
there's no velocity, the fiberglass has very little effect. As
fiberglass is spaced further from the wall, the air particles passing
through it have greater velocity. But they are slowed down as they
pass through the fiberglass, making the transfer of energy less
efficient which attenuates the wave. The velocity of any wave is
greatest at a distance equal to 1/4 of its wavelength - about four
feet from the wall for a 70 Hz. wave - though it goes back down to
zero at half the wavelength." ......
Comment:
This relates to the several previous points, assuming plain waves
with discrete frequencies at straight incidence.
A real live sound field is completely different. Velocity is defined
by the result of complex waves
QUOTE 6:
...... "In practice, you don't necessarily have to measure
wavelengths and calculate air gaps. Most people are not willing to
give up two or more feet all around the room anyway, so just make the
gap as large as you can justify. If you can afford to fill the gap
entirely with material, all the better because that helps avoid holes
in the range of frequencies absorbed. And even though the velocity is
indeed highest at 1/4 wavelength, there's still plenty at 1/8th of
the wavelength too. You could also use a few different gap sizes, or
make some panels much thicker than others, or install the fiberglass
at an angle so the top of a 2x4 panel mounted vertically is touching
the wall and the bottom is spaced away half a foot. I'm sure you get
the idea." ......
Explained in previous comments
QUOTE 7:
..... "BETTER BASS TRAPS" ...
My favorite type of bass trap is the membrane absorber, also called a
panel trap because it's made with a wood front panel.
Comment:
Basically the http://www.recording.org/users/acoustics FAQ indeed
reflects the preference of the Author for panel traps in function of
low frequency absorption, at the expense of accuracy.
This is related by the fact that the author, has some DIY designs for
panel traps, but also owns a company SELLING commercial panel traps.
While a panel trap can certainly serve acoustic purposes, mainly as
space saving absorbers, the site nowhere explains the related
mechanisms, not the side effects of using narrow banded absorbers,
nor the acoustic leaks it creates on the insulation of drywalls right
at the (frequency spots) where modes occur. As such a panel trap is
a bit contradictory:
One tries to design a drywall with an "as low as possible" mass-
spring-mass, preferably below the frequency range of interest,
resonance which is defining for the insulation of this cavity wall. A
panel trap (certainly the low frequent heavy ones) alters this mass-
spring-mass resonance back to the higher frequencies, diminishing the
insulation of the drywall (all depends on mass and spring ratios). A
panel resonator is a mass-spring system as a cavity wall is, as such
creating a triple leave system on cavity walls, and depending on the
mass and spring ratios will alter resonances as well for the
absorption of the panel resonator as the insulation of the wall.
More details about the acoustic effects of other resonators as
Helmholtz & panel traps will maybe discussed in later messages.
Just some info for completeness
This is a list once entered by Scott Foster in a list posting.
Source unknown to me
Owens-Corning 703 - 1"
0.03 0.22 0.69 0.91 0.96 0.99 0.63
Owens-Corning 703 - 2"
0.22 0.82 1.21 1.10 1.02 1.05 0.90
Owens-Corning 703 - 3"
0.53 1.19 1.21 1.08 1.01 1.04 1.01
Owens-Corning 703 - 4"
0.84 1.24 1.24 1.08 1.00 0.97 1.06
Owens-Corning 703 - 6"
1.19 1.21 1.13 1.05 1.04 1.04 1.11
Illbruck/Sonex once published a lot of absorption data on their
industry site, but removed this data and makes industrial data
difficult accessible.
The Foam Selection Guide below will give you a quick idea of the type
of foam solution illbruck might suggest for your application
http://www.customfoamfab.com/files/selectionguide1.html
Acoustic absorbers: Open cell urethane, willtec®, and willmid®
foams.
In fact the market for FLAT acoustic foams is MUCH LARGER than the
market for SCULPTED acoustic foams.
They are used in the automotive industry, airplanes, compressors and
lots of other technical applications in enormous quantities.
The main reasons however are not to be found in the superior acoustic
qualities but in technical properties in function of the behavior
versus vibrations (wool degenerates when used in e.g. a compressor
enclosure) and lots of other chemical, mounting, vibrational reasons.
The magic surrounding the sculptured foam in acoustics, certainly in
the studio world, seems to have more impact on the American than the
European market.
Maybe go deeper into that in later messages.
With kind regards
Eric
This COPYRIGHT picture further down shows the importance of how absorption material is used in function of positioning within the room. Sabine values are only 1 parameter. All those measurements are made with the same rock wool boards framed to exclude edge absorption influence, in the same lab, the very same day.
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John Sayers
- Site Admin
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- Location: Australia
- Contact:
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knightfly
- Senior Member
- Posts: 6976
- Joined: Sun Mar 16, 2003 11:11 am
- Location: West Coast, USA
Eric, whether you do or don't accept John's offer (Please do Please do Please do Please do Please do Please do Please do Please do Please do Please do Please do Please do) thank you for that VERY informative post. It clears up several things I was wondering about, and confirms my suspicions about said panel traps. I noticed some of the dialog on that subject at Yahoo, and was glad that someone with VASTLY deeper credentials than mine (several years of part-time self-study ONLY) had taken Mr. Bass Trap to task on some of the claims.
Please at least stick around a while - I have some questions about proposed wall construction, but will first have to find the time to draw several examples to clarify what I'm asking.
And yeah, just in case I wasn't clear, I am DEFINITELY in favor of Eric becoming a member of the "family" :=) Steve
Please at least stick around a while - I have some questions about proposed wall construction, but will first have to find the time to draw several examples to clarify what I'm asking.
And yeah, just in case I wasn't clear, I am DEFINITELY in favor of Eric becoming a member of the "family" :=) Steve
Soooo, when a Musician dies, do they hear the white noise at the end of the tunnel??!? Hmmmm...
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Eric_Desart
- Senior Member
- Posts: 760
- Joined: Wed Jun 18, 2003 6:09 pm
- Location: Antwerp/Belgium
- Contact:
Many thanks guys,
I'm blushing !!!!!!!!!!!!
I'm struggling with myself.
If doing this, it's a commitment which can prove time consuming, creating expectations by the visitors.
Not that I don't like that, but I'm only me, with my means, posibilities, priorities and time. I feel I can't guarantee always being available.
So I wondered (but have no explicit thougths about it), checking the Acoustics forum, a lot of topics overlap annd/or are of common interest.
Does shared moderation excists or can be a thougth?
But I'm honored by the welcome experienced in this forum for which my sencere thanks.
Best regards
Eric
I'm blushing !!!!!!!!!!!!I'm struggling with myself.
If doing this, it's a commitment which can prove time consuming, creating expectations by the visitors.
Not that I don't like that, but I'm only me, with my means, posibilities, priorities and time. I feel I can't guarantee always being available.
So I wondered (but have no explicit thougths about it), checking the Acoustics forum, a lot of topics overlap annd/or are of common interest.
Does shared moderation excists or can be a thougth?
But I'm honored by the welcome experienced in this forum for which my sencere thanks.
Best regards
Eric
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bolehnggak
- Posts: 95
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- Location: Indonesia
- Contact:
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Eric Best
- Senior Member
- Posts: 311
- Joined: Fri Feb 14, 2003 1:51 am
- Location: Lansing, MI USA
- Contact:
Eric,
I have read things that you have written before and always been impressed. Thank for what you have written. I would love to have you as a moderator here. We all understand that you can do what you do when you have time so we all will have reasonable expectations. I think your expectations of yourself will far outweigh our expectations of you.
The only request that I have is no more than one post a day like that last one. I just re-read it for the third time and will probably do it twice more before the day is over. Give me science!!!!!
Thanks,
Eric
I have read things that you have written before and always been impressed. Thank for what you have written. I would love to have you as a moderator here. We all understand that you can do what you do when you have time so we all will have reasonable expectations. I think your expectations of yourself will far outweigh our expectations of you.
The only request that I have is no more than one post a day like that last one. I just re-read it for the third time and will probably do it twice more before the day is over. Give me science!!!!!
Thanks,
Eric
"It don't get no better than this"
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Eric_Desart
- Senior Member
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- Location: Antwerp/Belgium
- Contact:
Hello Eric,
Here Eric.
Many thanks, for your complement and any encouragment here.
I'll decide end next week (sitting a bit in a strange personal situation).
To Ari:
If you use rock wool as absorption material as John explained, it basically doesn't matter that much what you use to cover it with.
Anything you can blow trough easily will do. It must have an open structure.
If not, as PVC foil with some mass, it starts acting as a membrane damper, which can be good if intended.
But any open fabric (carpet or not), won't influence your wool in negative sense (unless you get a head as a tomatoe trying to blow trough it, but can do it rather smoothly in stead). So depends of type of carpet.
The range is really very broad, no need for magic things.
Best regards
Here Eric.
Many thanks, for your complement and any encouragment here.
I'll decide end next week (sitting a bit in a strange personal situation).
To Ari:
If you use rock wool as absorption material as John explained, it basically doesn't matter that much what you use to cover it with.
Anything you can blow trough easily will do. It must have an open structure.
If not, as PVC foil with some mass, it starts acting as a membrane damper, which can be good if intended.
But any open fabric (carpet or not), won't influence your wool in negative sense (unless you get a head as a tomatoe trying to blow trough it, but can do it rather smoothly in stead). So depends of type of carpet.
The range is really very broad, no need for magic things.
Best regards
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John Sayers
- Site Admin
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- Joined: Mon Jan 27, 2003 12:46 pm
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