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By David S. Cmar

September 2005

Interior design trends in recent years have embraced many concepts that were once only part of luxury developments such as movie screening rooms and floor coverings made from exotic woods and tiles.  This has required architectural and interior designers, consultants and engineers to address acoustical issues surrounding building design and construction.  While much of the focus has been on improving the acoustic qualities on the inside of buildings, an area that tends to be overlooked (or under funded) involves the use of acoustical windows.

What is an Acoustical Window?

Windows are designed into structures to provide viewing, to transfer light and outside air, and for aesthetic value.  When a window must also provide significant acoustical isolation, the design, construction and installation of window assemblies must be carefully controlled for a successful result.  Glass panes (and typical residential and commercial windows) are not inherently good acoustical barriers.  Such windows provide various levels of acoustical isolation (STC ratings) depending on the specific construction of the assembly.  When "typical" assemblies do not provide adequate isolation, acoustical windows can be considered.  Acoustical windows  are typical windows that have been acoustically upgraded by increasing the thickness (weight) and number of glass panels, the air space between the glass, improving the glazing system for the glass, and improving the sealing system between the glass frame, window frame and structure.

STC and Coincidence Dip

Acoustical windows come in various configurations and with differing noise reduction capabilities.  The Sound Transmission Class (STC) rating describes the acoustical capabilities of windows.  This rating is determined by measuring the transmission loss through the window at frequencies ranging from 125 to 4000 Hz.  The transmission loss in these frequencies is compared to STC curves to determine its value.  In order to accurately determine the STC of a material, it should be tested in an approved chamber in accordance with ASTM E-413-87, ISO 717-1 or equivalent.


Figure 1. STC Rating for a Typical Double Glazed (Non-laminated) Window

In general, the STC ratings for windows (and other materials) increase proportionally with the window’s mass.  Increasing window thickness is an obvious way of increasing mass.  However, there is a phenomena that occurs that limits the improved reduction due to a ‘dip’ in performance at certain frequencies.  This is referred to as coincidence dip.  The coincidence dip is dependent on the material’s stiffness and thickness and occurs at the point where the sound transmitted through the window matches the natural frequency of the glass. This phenomenon occurs at higher frequencies in glass.

Window construction can vary greatly depending upon factors such as glass thickness, window pane spacing, and framing materials.  The following chart provides a reference for some typical window constructions and their expected STC ratings.  If significant noise reduction is required, e.g. emergency generator installed directly outside window or broadcast/recording studios, then specially designed and installed acoustical windows may be necessary.  Generally these windows have STC ratings of 40 or better.

Window Construction

Typical STC1

1/8” Double Glazed Window with ¼” air space


¼” Single Glazed Window (Caulking)


¼” Laminated Glass Single Glazed Window


¼” + 1/8” Double Glazed Window with 2” Airspace


¼” + 1/8” Double Glazed Window with 4-3/4” Airspace


¼” Double Glazed Laminated Glass, 4” Spacing, Steel Frame, w/Interior Absorption

45 - 48

½”+ Double Glazed Laminated Glass, 4” Spacing, Steel Frame, w/Interior Absorption


Applications for Acoustical Windows

When is it necessary to choose an acoustical window over standard window construction?  Acoustical windows (ie. high STC window assemblies) are needed when an analysis using low STC windows and the appropriate source level predicts that resulting noise levels will be higher than recommended by the design criteria.  This question unfortunately is not assessed properly in many new construction situations and leads to the costly task of replacing existing windows with acoustical versions.  In order to determine the necessary transmission loss for noise through window openings two factors need to be assessed:

1)  What sound level (source leve) currently exists (or may exist) outside of the window?

2)  What is an appropriate sound level (design criteria) for the space use on the inside of the window?

Room noise requirements can vary considerably, even within the same building.  Offices and meeting rooms, (though not strictly evaluated using A-weighted broad-band sound levels), should generally have sound levels no greater than 35 – 50 dBA inside the room.  This will allow for adequate voice communication, especially when using telecommunications devices.  Levels above 50 dBA will lead to decreased productivity, higher stress levels and interference with normal conversation.

In hotels and residences, sleep disturbance is a common side-effect of high noise levels.  Many studies have shown that human sleep patterns are disturbed when interior levels exceed levels as low as 45 dBA.  

Normally, achieving these sound levels is not difficult, even when minimum construction standards are met.  However, noise sources such as aircraft, HVAC systems and increased traffic may force a re-evaluation of window effectiveness.

During the design phase, an evaluation of both intended room use and potential external noise sources should be done by the architect or engineer.  This will ensure that the windows will adequately attenuate noise inside the building to levels and allow spaces to be utilized as intended.  Buildings intended for uses such as residential, commercial/industrial buildings, hotels, schools and libraries may require acoustic windows to reduce noise caused by ‘nuisance sources’ such as:  

·       Road and rail traffic (45 – 65 dBA)

·       Aircraft flyovers (45 – 60 dBA)

·       HVAC systems (40 – 55 dBA)

·       Industrial Sources (40 – 70 dBA)

Other Uses

In addition to preventing ‘nuisance’ noise, other common applications for acoustical windows include:

Anechoic Test Chambers, Broadcast and Recording Studios.  Because the sound quality of recordings can easily be affected by even minor sounds, it is critical that these rooms be soundproofed.  Many of these studios incorporate acoustical windows to allow people involved in production to view the activities inside the studio.  Typically, ambient sound levels inside studios need to be no more than 20 to 30 dBA.  Although the sound level requirements differ, similar applications occur in product testing labs, correctional facilities, and Broadcast & Recording Studios. 

IAC Non-parallel Acoustic Window for

Broadcast Studio Application

Dynamometer, Engine & Product Test Cells.  Windows used for product test cells such as dynamometer rooms require both sound reduction and protection from explosions or part and component failures.  This protection aspect usually requires the use of thick laminated glass and a substantial window frame.  This is also positive for noise reduction.

Churches/Places of Worship.  Churches and places of worship utilize acoustical windows both outside and inside to provide privacy and quiet for worshippers utilizing different parts of the building.


IAC STC 53 Acoustic Window Assemblies at Cry and Prayer Rooms

Mt. Hope Church, Lansing, MI

IAC Window Construction ¼” Laminated Safety Glass - 4” Airspace - Absorptive liner - ¼” Laminated Safety Glass.  Frame #11ga steel, 8" thick. Weight 12psf.)

Architect: Hobbs & Black, Inc., Lansing MI, CM: Wieland-Davco, Corp. Lansing, MI

Acoustical Analysis & Design:  Environmental Concepts, Inc., Grand Rapids, MI 


Private Office & Conference Rooms Offices and conference rooms often require acoustical windows to able to provide privacy for sensitive discussions and meetings.

Other uses may include: Control Rooms, Lunch Rooms, Counseling/Therapy, Music Practice rooms, Security offices and Air/Bus/Train terminals.


Key Factors in Acoustical Window Design

Designing the optimal acoustic window requires a thorough understanding of acoustical environment, building construction, as well as customer requirements for functionality, a material’s esthetic value and of course, budget.  This causes great confusion when attempting to evaluate different window systems.  

Generally, single glazed windows have limited use in applications where significant noise reduction is required (STC of 30 or higher).  As is discussed below, large improvements in window performance generally require some form of double glazed window to take advantage of benefits provided by the spacing between window panes.  Doubling or tripling the thickness of single glazed windows provides only marginal improvement in transmission loss, while adding significant weight and cost to the window. 

Double glazed windows are the most common form for acoustical windows.  The most critical factors in improving the performance of double glazed windows include:

a.            Use of laminated glass

b.            Increased airspace width

c.             Proper construction and installation

d.            Proper frame sealing

e.            Proper window glazing

“When discussing various types of glass assemblies that may provide superior acoustical performance, people are almost always surprised that insulated glass (thermo-pane) is not on the list.”, says Ralph Balck, P.E.. Mr. Balck is an acoustical engineer and President of Environmental Concepts, Inc. (ECI).  ECI is the Western Michigan representative for IAC Acoustical Company (IAC), a manufacturer of high performance acoustical barriers and absorbers. Mr. Balck has designed and supplied acoustical window assemblies for numerous projects, almost all unique applications.  “People assume that because insulated glass does a better job of reducing heat transfer, it will also do a better job of reducing the transfer of noise.  This is almost always not the case.  In fact the small air gap that is typically found in insulated glass and is so useful for its thermal properties, is usually a disadvantage acoustically.  The air in the gap couple (not isolates) the two sheets of glass, thereby improving the ability of a thermo-pane window to transfer sound energy." 

According to Balck, “Using laminated glass (two or more layers of glass bonded together by a visco-elastic damping layer between them) the STC of a window assembly can be significantly improved.  The damping layer, although very thin and not visible, improves the overall glass performance (increases STC) by eliminating or greatly reducing the losses that inherently occur at coincidence frequencies in single panes of glass.   It should be noted that in colder regions the improved STC provided by laminated glass will be reduced as the damping layer is substantially cooled.

The air space between glass sheets in commercial windows can be as little as 0.25” and is generally no more than 1”.  The acoustical performance of such assemblies is inherently limited by such small air gaps.  Generally, acoustically rated windows have spacing of 2” – 4” between panes.  The airspace between the panes should not be less than four inches if an STC above 45 is desired.  

Acoustical windows can be purchased factory-assembled or they can be constructed locally (in a shop or on-site).  Any window purchased from a factory should have been tested acoustically and should come with a guarantee of performance.  It is not likely that a local window producer could include such features.

The ultimate acoustical performance of a window assembly can easily be compromised by errors that may occur during design, fabrication and installation.  “Finding contractors that are skilled and experienced in the design, fabrication and installation or windows with high acoustical performance can be a challenge”, says Ralph Balck. “Even when the contract documents provide a detailed window design, the owner would be wise to provide a high level of supervision and quality control to minimize risk."  Factors such as window depth, weight, frame materials and construction can all have a significant impact on the effectiveness of window installations.” 

It will be difficult to obtain a guarantee of acoustical performance from a contractor who constructs windows assembled in his shop or on-site.  The STC of contractor-constructed windows will certainly depend on the training, skill, workmanship and supervision of the installers.  Those qualities may be difficult to discern.

For applications requiring high STC windows it may be desirable to use pre-designed and pre-tested window assemblies.  The installation of such pre-fabricated window assemblies is straightforward with little risk of compromising the factory guaranteed performance.

As in the case of all windows, proper sealing is extremely important.  There are three aspects of sealing windows: 1) The seal of the glass into the frame (glazing), and 2) the sealing of the frame to the window opening, 3) gasket seal for operable windows.   To achieve an STC above 40, double-glazed windows should be permanently sealed.   Sound transmission through cracks around openable windows reduces their Transmission Loss relative to that for sealed windows.  In general, the reduction in TL tends to be greatest for the high frequency bands.  For a window with good weather-stripping, the STC is usually from 3 to 5 lower that that for an equivalent sealed window.  The higher the STC of a sealed window, the more it is decreased by a given sound leak.  To further insure isolation of noise between double-glazed panes, the panes could be of different thicknesses, different weights, and slightly non-parallel to each other.  This prevents acoustical coupling and resonance of sound waves.2

Additional Improvement Techniques

More modest improvements to window performance can be made by:

·        Adding absorptive material to cover the surface of the frame in between the panes of glass can provide some additional attenuation.  This increases the STC only slightly, although substantial reductions can be achieved for higher frequency noise (up to 10 dB).

·        Increased glass thickness.  Increased glass thickness provides additional mass, therefore increasing the ability of the window to block low-frequency sound.  Note:  The use of laminated glass is generally a better option as it can provide higher transmission loss than a single pane of identical thickness (3 dB or higher).

·        The use of slightly dissimilar thickness of the panes.

·        The use of slightly non-parallel panes.  The use of slightly non-parallel panes is a technique employed when extremely high sound insulation is required,   such as in control rooms of television studios.

·        Heavier window frame construction such as steel or aluminum vs. vinyl and wood.


Selecting Acoustical Windows

Many companies make claims as to the noise reduction capabilities of their window products with claims such as STC 55, 95% noise reduction, etc.  In general, most manufacturers of standard windows have little expertise in acoustic design and do not actually test individual windows to determine STC ratings (this would put their windows at a cost disadvantage).  This requires purchasers of windows to understand and control the selection of window performance, materials and installation.  Acoustical consultants can be invaluable in specifying window construction and installation based on evaluation of site specific acoustical factors such as source frequency spectrum analysis, room use criteria and available transmission paths.  


A careful consideration of noise transmission potential through windows on buildings prior to construction will lead to more satisfied, owners, and tenants. It will also ensure that every dollar spent on acoustical improvements will be spent wisely and effectively.




  1. Egan, David, Architectural Acoustics, 1988, pg. 223
  2. Quirt, J.D. Sound Transmission Through Windows, National Research Council Canada, Division of Building Research, Canadian Building Digest 240, 1988


Ralph Balck, is President of Environmental Concepts Inc. a supplier of  architectural, industrial and environmental acoustical products including IAC Acoustical Windows and Doors.  He can be reached at (616) 235-2595 rbalck@environmentalconcepts.com or visit the website www.environmentalconcepts.com.


Mr. Cmar is President/CEO of Phase To of Canada, Inc. and also in charge of Business Development for Phase To, Inc.  Mr. Cmar has over 15 years of experience in noise engineering.  He can be reached at (519) 734-7001 or email davecmar@phasteo.com


Phase To, Inc. (PTI) does not represent, sell, or market any noise control materials or hearing conservation products.  References to any specific products in this or any other PTI article does not express explicit endorsement by Phase To, Inc.  PTI focuses specific attention on innovative items or products that we find to be superior in the noise control or hearing conservation marketplace.


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