Factors Affecting STC Ratings

System Integration

Sound Transmission Class (STC) ratings quantify the barrier-loss rating or sound-insulating properties of a particular partition, wall, door or other building element. When a privacy door is built in a factory, each door or a sample door is subject to a controlled Transmission Loss (TL) and/or STC test to measure the sound attenuation performance of the door. When the door is delivered to the new owner’s facility and has been installed by a contractor, the door should be field tested. This type of on-site testing is commonly referred to as a “field STC” test. However, a field STC measures not only the door but other installation and material characteristics around and near the door. For any STC test — a laboratory test or field condition test — the higher the number, the better the wall or door is at reducing sound through the partition.

Testing only the door in an installation disregards important aspects of the overall installation, such as light switches, outlets, walls and gaps, which potentially deteriorate the STC rating of the installed door if not tested properly.

The concern is not so much the selection of a properly rated STC door — it's easy to find a door with an STC rating of 45 to 50 — it often matters most how the door is installed; i.e., is the door installed to manufacturer’s specifications and does it integrate and work well with surrounding walls, vents, fixtures, and other nearby objects.

STC-rated walls, doors and windows help keep audio inside the room from passing through the partition and outside the room. Often, integration and installation errors drive the STC rating lower. If testing a door to determine its actual STC rating once it is installed in a room or office, testing the door alone is a common mistake. The STC field test is a measurement of a single point — not of a particular object. Instead, the STC test quantifies all barrier loss items, building elements and objects near the test point. So while the intent is to test the door for its sound isolation performance, the actual testing required goes far beyond just the door — tests must include the adjoining wall, the spaces above and below the door, the impact of nearby vents and other egress mechanisms such as outlets, light switches and other hardware.

Thus, when testing a new STC-rated door, it is important to remember that a field STC test measures the acoustical attenuation performance of all structures in the vicinity of the door and not just the door itself. An STC-50–rated door can be installed in a wall having an STC rating of only 45. In such an example, the door will never achieve its listed STC-50 rating because of the lower STC value of adjoining walls. Nearby vents or electrical outlets may also reduce the STC testing result if the vent is not acoustically baffled or if the electrical outlet is not acoustically treated. Regulations, architects, and clients often specify an STC system performance of STC-45 or STC-50, but they often fail to pass the STC final field test due to dozens of potential factors that can dramatically impact the STC performance of the area being tested.

Material Selection

The material used to create an STC-rated wall or door can vary dramatically. For example, a wall might be poured concrete, standard metal stud construction, or triple-leaf construction (using a total of three layers of drywall) with varying types of gypsum board, fire barriers, foils and insulating materials. Each wall type can affect the STC performance of the space.

Acoustical engineers and STC testing firms have tested and documented hundreds of wall types and have detailed, “as-built” diagrams to aid program managers, general contractors and clients in their decision to achieve a desired STC performance goal for their office or facility. Often it’s not just the material being used that matters most, but rather the layering technique, wall transfer function, how it's affixed, and how the wall is built. There are hundreds of wall types that have varying STC ratings from 10 to over 70.

Stud Type and Spacing

The type of studs used and the spacing of wall studs (16" or 24" on center — or "OC") can affect the STC performance of a wall. Acoustical energy can couple through wall studs, so paying particular attention to this topic can affect the STC performance of the wall.

Type of Insulation

The amount and type of insulation used inside the wall can vary in terms of quality, density and noise reduction rating. Fluffy fiberglass insulation may not work as well as wool-based, semi-rigid batt insulation or mass loaded vinyl solutions using viscoelastic polymer materials. There are dozens of insulation types that are specifically designed for soundproofing. Each insulation type has different sound absorption characteristics. Most insulating materials have varying coefficient ratings, which specify their ability to block sound at certain frequencies. These coefficients and performance specifications must be considered when selecting the right insulation for your particular project or STC objectives.

Flanking Paths

Flanking paths may allow unintentional egress of audio from the secure space; these flanking paths must be identified and corrected to ensure complete protection.

Flanking paths are air gaps, spaces, or other paths that allow audio generated inside a room to escape. These flanking paths may be caused by:

  • Slight gaps under or around a door
  • Air gaps around the electrical outlets (or even the prong holes themselves)
  • Keyholes
  • Micro-gaps where the door frame meets the wall
  • Gaps where the locking bolt meets the door strike plate
  • Supply or return vents, and/or cable troughs above the drop ceiling or below the raised floor
  • Structural channels
  • Holes drilled into the lower or upper slab for cable or wire runs

Additionally, any other path, channel or space, no matter how small or large, visible or hidden, is a potential flanking path that allows audio to leave the area.

Wall Penetrations

Once a wall is built to an STC standard, the first thing subcontractors and industry trades (plumbing, electrical, HVAC, audio/visual, alarm companies, etc.) often do is drill or cut holes into the walls for adding:

  • Electrical boxes
  • Light switches
  • Access control readers and/or alarm components
  • Fire speakers and smoke detectors
  • Life safety equipment
  • Push or Request to Exit (REX) buttons
  • Doorbells
  • Or one of dozens of other fixtures

These fixtures are needed to make a facility functional and operational. In almost every instance, the various holes and cutouts needed for these fixtures negatively affect the STC rating of the walls. Even in instances where an outlet is surface-mounted inside a room to prevent damage to the wall, the screws used to attach the outlet to the wall penetrate into the drywall, thus reducing the physical and STC integrity of the wall. This is a common problem affecting the STC performance of a room. Often, many of these cut outs or screw holes are not even visible to human eye or to the facility inspectors.

Door Installation

STC-rated doors can be purchased by themselves and hung on an existing frame (not recommended) or they can be purchased as a door-and-frame assembly to be installed into an existing or new rough opening (recommended). There may often be gaps were the frame meets the walls, and these air gaps create paths for audio to escape. The door frame must be properly attached and sealed to the wall using one of many proper installation methods available. Generally speaking, door manufacturers that make STC-rated doors can provide certified and qualified installation technicians or recommend certified installers to ensure the door assembly (door and frame) is installed in such a manner to achieve the desired field STC rating.

Installation Errors

Many common installation errors can lead to a diminished STC rating, including:

  • Two pieces of drywall may not be properly aligned, causing a slight gap at the base or top of the wall. Tape and drywall mud may be applied to fix these gaps, but often the drywall mud is not applied uniformly or deep enough into the crack to ensure the desired STC performance.
  • Complex framing methods can create unforeseen gaps or spaces when metal structural studs meet standard studs — often in areas the client cannot inspect.
  • The method of how a contractor deals with a structural pillar near the perimeter walls can impact the STC performance near that pillar — how acoustical energy interacts with the boxed pillar and the perimeter wall.
  • Large, heavy STC doors are often properly affixed to structural side studs but the top door header may be missing or improperly installed, causing audio to be transmitted directly through the wall above the door.
  • Lighting soffits, transom windows, and various architectural features can have a dramatic impact on the STC performance of any given space.

True Floor/True Ceiling

Often, “true floor to true ceiling walls” are specified in secure areas. This helps ensures both the physical and acoustical integrity of the space. There are numerous examples of “true floor to true ceiling walls” being specified, but the reality of how they are erected and installed can be quite different. Often, the ceiling slab can have a corrugated composite steel floor deck, which has a ribbed profile designed to interlock with the concrete slab, creating a strongly reinforced concrete slab on each floor. The ribbed profiles of these decks often allow audio to escape at the ceiling between the ribs. Even when acoustical foam is used to seal these gaps, the overall STC performance level of the room can be much lower than anticipated. Close attention must be paid to these common flanking paths, and carefully selected remediation techniques must be applied to ensure the overall integrity of the walls.

Factors Affecting Speech Privacy

When a client decides to install a sound- or speechmasking system, they often have a wall or door that does not have a high STC rating. Words can be heard and understood outside of the room. When a person inside a room speaks to a listener outside of the room, they are in fact performing what is called a “voiced intelligibility test.” As crude as a voiced intelligibility test may be, it has some value in understanding the degree of the problem. In a perfect world, all of these such tests would be performed with specialized hardware to quantitatively measure the Speech Transmission Index (STI). STI is discussed at length in the International Electrotechnical Commission (IEC) standard 60268-16. STI is a measure of intelligibility whose value varies from 0 (completely unintelligible) to 1 (perfect intelligibility). When such intelligibility tests are performed, the typical intent is to determine whether someone talking within a room can be heard outside the room. The degree to what can be understood is important. Maybe all words spoken at a loud level can be heard; maybe only 20 to 50 percent of the conversation can be understood when the talker speaks at normal conversational levels; maybe only muffled voices can normally be heard, but if that same person talks into a sound-amplified lectern system within the room, many more words may be understood. The point is that there are dozens of variables in play that affect someone’s ability to hear and understand what is being said from inside a room. When words can be understood from outside the room, it's time to consider installing a speech-based soundmasking system.

Speech-based soundmasking systems normally have three elements: a generator, an amplifier, and speakers. How these three elements are designed, implemented, and integrated are key to having an effective system versus an annoying, ineffective system. Remember, if the wall or door has poor acoustical attenuating properties, audio will leave the room. The job of a speech-based soundmasking system is to ensure no one can understand what is being said. If the audio can't be prevented from leaving the area, it must be made valueless by someone trying to listen (or record) the conversation from outside.

Below are some of the factors that impact the effectiveness of a speech-based soundmasking system’s ability to ensure a person speaking inside a secure room cannot be understood (completely unintelligible) outside the room.

Speaker Density and Placement

Speakers or emitters properly mounted — but dispersed or spaced too far apart — create acoustical hot and cold spots along the wall. Energy from the speaker must be distributed as uniformly as possible with the appropriate spacing to create an effective mask — both horizontally and vertically along the walls — to ensure conversations made inside the facility are properly masked around the perimeter of the room. Each of many wall types often dictates the optimum speaker density and spacing. This is true for emitters placed inside the wall below the drop ceiling, as well as volumetric speakers placed above the drop ceiling. Together, TS and ID have spent years studying and rigorously testing proper speaker placement, density, and spacing to ensure the most effective results possible.

Speaker Selection

Volumetric speakers (airborne) and structure-borne emitters (surface) must be designed to deliver the correct power and frequencies needed specifically to mask speech to effectively reduce intelligibility. A speaker with too small of a driver to properly replicate the bass frequencies of the human voice will not suitably mask human speech. The speaker must be capable of delivering the proper amount of wattage, bandwidth, and dynamic range to be considered useful as a speech masking candidate speaker. All of our speakers and emitters are purpose-built to achieve the proper performance characteristics required to mask speech correctly.

Power Delivery and Amplitude Adjustments

A high-quality speechmasking system must deliver the correct amount of wattage and allow every amplifier channel and connected speakers or emitter to be adjusted independently. Precise amplitude control can make the difference between a properly designed system and one that is distracting and uncomfortable to be around long term. Our SpeechMask® hardware allows for amplitude adjustments in 0.1 dB increments so the proper amplitude can be achieved throughout the room or facility. Each segment or section of a wall can react differently to sound stimuli, and abnormal acoustical amplitude variations may be detected in an improperly adjusted system. An incorrectly adjusted system can result in serious acoustical security vulnerabilities. The amplifier, distribution channels, and speakers must have the appropriate wattage rating to accomplish the speechmasking task effectively and efficiently.

Spectrum Adjustments

The audio spectrum generated to mask speech must be adjustable in frequency and shape due to the various transfer functions found throughout the hardware, structures, and the air or surface mediums. Often, unwanted high-frequency sounds can manifest themselves inside the walls, resulting in a thin or tinny sound being propagated outside of the room. A generator must have built-in spectral and filter shaping adjustments to remove the unwanted effect of a wall's transfer function. Our SpeechMask® generator can perform a host of functions to properly correct for unwanted transfer effects.

Continuously Variable Generator

Generating a proper speech mask requires sound engineering, science, research and experience. There are many studies that describe optimal methods to mask speech. All of them point to numerous variables that must be considered and properly implemented to make the speech categorically unintelligible. Spectral density, quantity, level effects, resonance, articulation functions, amplitude and frequency variations, content, quality and fidelity, speech fragmentation, spectrum shaping, periodic pauses and interruptions, tonal and pitch qualities, and other improbable vocal effects all affect speech intelligibility. As a result of rapid advances in speech filtering and voice enhancement technologies, the ability to continuously change the source generator’s output in time, amplitude, pattern, and content is fundamentally critical to preventing extraction of the target voice inside the room. Years of algorithm development and hundreds of test hours were applied to create the most comprehensive and secure speechmasking generator ever produced. Our proprietary SpeechMask® technology is the most advanced speechmasking technology in the world.