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Acoustical Testing of Windows
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RALPH T. MUEHLEISEN, PH.D., P.E., LEED AP
INSTITUTE OF NOISE CONTROL ENGINEERS BOARD CERTIFIED,
FELLOW OF THE ACOUSTICAL SOCIETY OF AMERICA
Principal Building Scientist and Building Energy Research Section Leader Energy Systems DivisionArgonne National [email protected]
Sound Transmission Loss of a Single Glaze Window
Standard Methods of Testing
Testing of Small Samples
References
Sound Transmisson Loss of a thin single glaze window
The sound transmission loss (TL) curve of nearly all single glaze windows looks
a bit like the curve below with three regions: stiffness, mass, and damping
controlled with a dip in the coincidence region
The curve below is one for diffuse field incidence, i.e. waves hitting the panel
from many directions not just one
Mass Controlled: 100Hz – 2 kHz
The low-to mid frequencies are mass controlled where the TL rises 6 dB per
every doubling of frequency and the amplitude is directly proportional to the
mass of the window (density *thickness).
– For a 6mm window this is about 100 Hz – 2kHz
The only way to increase TL is to increase the actual mass or use acoustic
metamaterial properties to increase the effective density of the glass+coating
𝑻𝑳𝒎𝒂𝒔𝒔 𝒍𝒂𝒘 ≈ 𝟏𝟎 𝐥𝐨𝐠𝟏𝟎 𝟏 +𝝎𝝆𝒉
𝟑. 𝟔𝝆𝟎𝒄𝟎
𝟐
Damping Controlled: >2 kHz
Above about 2 kHz there is a large dip in the TL called the coincidence dip.
𝑓𝑐 =12.7
ℎ
ms
, ℎ = thickness in m
Here TL is damping controlled and laminates or thin films can help increase TL
𝑻𝑳𝒅𝒂𝒎𝒑𝒊𝒏𝒈 ≈ 𝟐𝟎𝒍𝒐𝒈𝟏𝟎𝝎𝝆𝒉
𝟐 𝝆𝟎𝒄𝟎+ 𝟏𝟎 𝐥𝐨𝐠𝟏𝟎
𝟐𝜼
𝝅
𝒇
𝒇𝒄− 𝟏
Coincidence is a phenomenon when the
bending wave in the glass has a wavelength
that matches air
air = bending cos
At that freq, sound travels easily through
panel, especially if damping is low
All panels have a coincidence dip, – Well damped materials like thick gypsum, concrete
and wood, the dip is shallow
– In low damping materials like glass the dip is deep.
– Window films/coatings can increase damping and
reduce coincidence effects
To see coincidence, we must excite the
window with sound from *many* angles and
not just normal incidence
Coincidence
Sound Transmission Class: STC
A TL curve created from diffuse incidence measurements can be converted to a
single number measure of sound isolation called Sound Transmission Class
(STC) using ASTM E413 which compares the TL to a reference curve.
ISO has a similar but not equivalent
measure called sound reduction
index Rw defined by ISO 140-3
STC requires that you use E90 to
find TL to ensure that any
coincidence effect is excited.
Measuring Window Transmission Loss
Measuring Window TL
ASTM E90 is a standard way of testing TL of windows and walls.
You put a big sample of material between two reverberation rooms that create a
diffuse sound field and measure Lp on both sides of the wall/window.
This method creates waves hitting
window from many directions and
so excites coincidence effects
Requires side-by-side reverberation
rooms, i.e. a large acoustical testing
facility
Requires a full size window (or at
least a large material sample of at
least 0.75m square)
– ASTM E1425 Defines the
preferred sizes
Measuring Normal Incidence Window TL
ASTM E2611 uses an impedance tube to measure the normal incidence TL
Uses a round sample typically of 30mm diameter for high frequencies and
100mm diameter for lower
This method only finds normal incidence TL so will not show a coincidence effect
– i.e. you cannot not compute an STC from this because it cannot excite
the coincidence effect.
– This also over estimates the diffuse field mass law effect by about 5 dB
Designed for porous materials with low TL not glass with a high TL
Measuring Small Samples
So, how do I effectively test the acoustic effects of a thin film/coating on a small
sample using standards?
SHORT ANSWER: YOU CAN’T MEASURE THEM DIRECTLY
LONG ANSWER: Instead of measuring the TL, you measure the damping
coefficient of the coating – that is something you *can* do on smaller samples of
materials. Then you estimate the effect on window TL using theory
– ASTM E756
– Resonant Ultrasound Spectroscopy (RUS)
LONG ANSWER CAVEAT: The ASTM E576 for measuring damping still requires
pretty large samples - you generally need to be able to coat a piece of glass
about 1cm wide and at least 10 cm long
ASTM E756 measures frequency response and extracts damping from
resonance peak width.
This can be used for a sandwich retrofit pane as well as a thin film
Needs samples about 10cm x 1cm to make sure we excite the bending waves
that would get excited by the coincidence effect and measure that damping
Testing Small Samples: ASTM E756
Resonant Ultrasound Spectroscopy can be used on small samples but it only
estimates the elastic constants from which damping must be extracted.
– This is doable in theory but I’ve never actually seen anyone extract damping
from changes in the elastic constants
– I don’t know how well this would work for a retrofit sandwich pane
Need only mm size samples but these need to be polished into nice
parallelpipeds
Testing Small Samples: RUS
Estimating Window TL from Damping Coefficient
If you have the damping coefficient and the rest of the info about the glazing you
can use the simple equations developed by Sharp to estimate TL from the
damping coefficient, density, bending stiffness, and thickness
B. H. Sharp, “Prediction Methods for the Sound Transmission of Building
Elements,” Noise Control Engineering Journal, vol. 11, no. 2, pp. 53–63, 1978.
This will work with laminated retrofit glazing too if you can measure or estimate
the damping coefficient, density, and bending stiffness of the laminate assembly.
These data can be obtained from additional analysis of the resonance curves of
E756 or by other ASTM dynamic material testing (E1876, C1198, etc) that use
similar simple resonance excitations
Where can I get testing done?
ASTM E90:
– NVLAP Certified Acoustic Testing Labs (Riverbank Labs, Intertek, NGC, ETS,
Western Electro, NWAA, Orfield, Elemen.)
– Universities with STC testing setup (Penn State, U. Hartford, etc. not certified)
ASTM E2116:
– Argonne (not certified)
– Certified Acoustic Testing Labs + other NVLAP certified labs.
– Lots more Universities (Penn State, BYU, Purdue, U. Hartford, basically
anyone who has someone doing acoustics, not certified)
ASTM E756:
– Argonne (soon - not certified)
– Many NVLAP certified Labs (not just the acoustic labs that do E90)
– Many universities who have civil, mechanical, or materials departments will
have the equipment to test
RUS:
– A handful of universities with material science departments
Time and Costs
ASTM E90 Tests:
– The window or frame assembly must be mounted in a special mount and then
put in the test chamber. Tests take several hours. This is overall a week+ in
time (not including shipping) and scheduling can be difficult
– Costs: Upwards of $20k-$50k per test
ASTM E2116:
– Mounting sample takes a couple hours, tests take a couple minutes
– Costs: Probably about $5k at a testing lab. BUT DON’T DO IT
ASTM E576:
– If you provide proper samples, the tests take only a few minutes.
– Costs: Probably about $3-5k per sample – but I’m not sure on that.
References
References
ASTM E90-09(2016) Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements
ASTM E2611-17 Standard Test Method for Normal Incidence Determination of Porous Material Acoustical Properties Based on the Transfer Matrix Method
ASTM E413-16 Classification for Rating Sound Insulation
ASTM E1425 - 07 Standard Practice for Determining the Acoustical Performance of Windows, Doors, Skylight, and Glazed Wall Systems
ASTM E756-05: Standard Test Method for Measuring Vibration-Damping Properties of Materials
B. H. Sharp, “Prediction Methods for the Sound Transmission of Building Elements,” Noise Control Engineering Journal, vol. 11, no. 2, pp. 53–63, 1978.
Migliori, Albert, and J. D. Maynard. "Implementation of a modern resonant ultrasound spectroscopy system for the measurement of the elastic moduli of small solid specimens." Review of Scientific Instruments 76, no. 12 (2005): 121301.