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 Laboratoryrmuehleisen@anl.gov630-252-2547

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.