Ever wonder what the STC Ratings of Doors means and why it’s important? Street noise, people and pet noise, TV, radio, everywhere we go there is noise. Wouldn’t you just love to shut the door on it all? Did you realize you can literally shut the door on noise? We use doors for privacy, security, protection, and to keep the weather out. Doors can keep sound from moving in or out too. However, some doors are much better at it than others.
Sound Transmission Class Rating for Doors determines how successful it will be at blocking sound transfer. The STC rates the ability of different door materials, construction, and assemblies at preventing sound movement over a range of 16 standard frequencies. The higher the rating, the better it blocks transmission loss (TL).
In this article, I’ll discuss how different door construction stands up to sound and what to look for when choosing a door.
The Basics of Sound Transmission
The basics of sound transmission use specific terms as references, so others know what is happening, or going to happen.
Decibels (dB) are used to measure how loud a sound is compared to silence. The rustle of leaves is approximately 20 dB or 100 times louder than total silence, and 50 dB – 60 dB is a conversation level – 100,000 to 1,000,000 times louder than silence.
Transmission loss (TL) is the amount of sound loss from the source as it passes through a door, wall, ceiling or floor, or some other obstruction. It is measured in decibels. If we have a noisy room full of teenage boys creating 70 dB of noise, and when we shut the door the noise on our side drops to 50 dB, we have a transmission loss of 20 dB.
It is important to understand that pitch affects transmission loss too. If that room is full of pre-adolescents producing 70 dB of sound at a higher pitch, the noise heard after the door is closed may be 65 dB, so a transmission loss of only 5 dB.
Frequency is measured in Hertz (Hz) and represents the tone of a sound, similar to a musical note. A Piccolo has a high pitch that reaches 3800 Hz, while a double Bassoon can have a low pitch of 33 Hz.
Absorption refers to how a material handles or controls sound waves or energy. The more vibrational sound energy the material traps and turns into heat, the better it is at absorbing sound. The converted sound waves are not reflected or transmitted. They dissipate as heat.
Transmission is the movement of sound energy between and through different materials, like doors, walls, ceilings, floors, and even the air. The more a material impedes different frequency ranges, the greater the transmission loss.
Understanding Sound Ratings
Sound ratings are used to identify how well a product or material blocks the movement of sound. Different terms or abbreviations are used when discussing soundproofing, so understanding the terms are important.
Sound Transmission Class (STC)
To measure how well a door, window, wall, or other material or assembly blocks the movement of sound, they are tested in laboratories and also in the field. The test involves “firing” 16 fixed frequencies from 125 Hz to 4,000 Hz at the assembly or material and measuring the amount of sound on both sides in decibels (dB). The results are graphed, and the curve compared to a reference graph; the better the results, the higher the STC rating.
Sound Transmission Loss (TL)
The decrease of sound, or blocking of sound movement, is known as the Transmission Loss (TL) and is measured in decibels (dB). It represents the decrease in the intensity of sound through a barrier material.
The TL is identified with a number value between 0 and 1 and rounded to the nearest tenth (0.1). Zero (0) means no loss, and one (1 or 1.0) means 100% of the sound is blocked or absorbed.
STC Class: What Can We Hear?
The average human ear begins life hearing sounds between 20 Hz and 20,000 Hz. Age, illness, and loud noises can reduce that over time. The STC (Sound Transmission Class) rating of a wall, floor, door, window, barrier, or partition identifies how well those materials reduce or prevent soundwaves from penetrating another space.
The higher the STC rating of a door or wall, the less sound will move through to the other side. Here’s what you could expect for different STC values:
- 25 Normal talking can be heard and understood through a wall.
- 30 Loud talking can be easily understood, and normal speech can be heard but usually
- 35 Loud talking can be heard but isn’t understandable.
- 40 Loud talking sounds like a murmur; it’s considered the beginning of “privacy.”
- 42 Loud talking sounds like a whisper.
- 45 Loud talking isn’t heard. However, music and heavy traffic may still be heard.
- 50 Very loud sounds like musical instruments and stereos barely heard.
- 60+ Excellent soundproofing, even loud sounds are barely heard.
What is an STC Rated Door?
A door that has been sound rated usually means the door panel, jambs/frames, and seals have been tested in a lab for sound transmission loss (TL) and earned an STC rating. In some cases, the rating applies only to the door panel as it has been tested without the door frame assembly.
Acoustic doors normally include the seals and frames and have an STC value between 42 and 52. The assemblies have been tested under ASTM E90-09 guidelines for transmission loss of airborne noise. The higher STC rating usually comes at a higher cost too. Door panels or assemblies with lower ratings are less effective at controlling sound movement and cost less.
STC Ratings for Doors
We often purchase doors based on aesthetics, price, and location – interior or exterior. When looking for doors that control sound transmission, we need to look at the door panel, frame assemblies, and seals too.
Door construction is very important when selecting a sound controlling door for your home or office. You don’t have to give up the aesthetic element, but the budget may take a hit.
Doors that are intended for use inside a home are not normally weather protected or sealed and are commonly 1-3/8” thick. They are intended for locations that are protected from the elements.
Hollow core doors are inexpensive, lightweight, and easy to install. Two veneer or Masonite panels are glued to a wood frame perimeter to form the door panel. The inside is called hollow, even though there is often a web of corrugated material inside to add support and help secure the two faces together.
The sound transmission class of hollow core doors is low, between 20 and 25. A hollow core is like a drum; the two faces vibrate and resonate sound through into other spaces.
These doors are commonly used in homes as bedroom and passage doors. They provide visual privacy but allow conversations to be heard and understood through them.
Solid wood doors are heavier and denser than hollow core doors. Often constructed of rails, stiles, and 2 to 8 wood inserts or panels, they block more sound.
Stiles and rails can be made from single planks, from laminated thinner planks, or finger-jointed wood pieces. The panels may be made the same way, or from MDF with a veneer overlay.
Some woods are denser and have a higher STC value. Softwoods are not as effective as harder woods at decreasing sound movement.
Solid wood doors have an STC between 30 and 35, so loud talking can be heard and understood, but quiet conversation won’t be understandable. Thicker doors often have ratings up to 45 which is enough to mute even loud voices to be indistinguishable.
Solid core doors are solid all the way through; they are heavy and dense. Many today are made of wood particles mixed with glue, heated and pressed into a molded door form or panel, which is then finished with a wood look coating. Other solid doors are made with two veneer sheets sandwiching a core made of wood blocks glued to form a slab.
The denser the core, the less sound transmitted through. Wood particle filled core doors rate between 30 and 35 on the STC scale, which won’t make intense conversations private. The denser wood block cores can reach values of 55 to 60, which can mute even musical instruments.
Hollow Metal Doors
Hollow core metal doors are commonly made of galvannealed steel. Galvanneal is a process of galvanizing the steel and then annealing it to make it softer and easier to shape when cold.
Strips of steel are run through a 300-ton press to form frames and the perimeter framework for the door panel. The steel plates are welded to the perimeter with a corrugated metal web in between the plates.
The panel plates can be flat or pressed to have different profiles. They are then primed for painting.
Hollow steel doors have an STC rate between 20 and 29 and won’t block sound enough to make a quiet conversation private. Current practice is to fill the hollow core with polystyrene foam to improve environmental efficiencies which raise the STC to the 30 to 34 range. Loud speech will still be decipherable, but the quiet conversation is much more private.
MDF doors are made of recycled wood fibers and glue that is heated and pressed into flat panels. The doors are solid and denser than wood particle filled doors, and also won’t warp like other wood doors can.
The doors may have a flat profile or look like stile and rail construction. They don’t have a wood grain finish, so are smooth, which is easier to paint.
A solid 1-3/8” MDF slab has an STC of 33. The door could be made as a stile and rail with smaller insert panels, inlaid with molding, layered panels, or routered to look like a colonial or multi-panel door.
The sound rating of MDF doors depends upon the thickness of the material and how the door panel is constructed. The sound rating can range from 33 to 50 based on how the door panel is manufactured.
Glass doors can be made of a glass panel inserted into a wood or steel door. The dimensions of the glass and frame material of the door panel surrounding the glass depend upon construction methods of the manufacturer.
Some are wood stile and rail with glass insert; others can be MDF or solid core with a cut out for the glass. Steel doors with glass are similar to wood panels with a blank cut out into which the glass is fitted. Some doors are solid glass panels with little or no framing.
The size of the glass, its thickness, and whether it is single or double ply will impact the soundproofing values, as will the material of the framing around the glass. Standard glass doors can have an STC between 28 and 35, laminated between 35 and 40, and soundproof glass from 48 to 55 or higher. Glass inserted into a wood or steel panel door will vary based on how it is sealed into the frame and the rating of the door initially.
Doors intended for exterior use on homes or buildings are normally sealed and weatherproofed to keep winds and heat or cold out. They are usually 1-3/4” or thicker. The doors are designed and finished to withstand the elements for the environment they are exposed to.
Steel Exterior Doors
More than 50% of all exterior doors in North America are steel construction, and they are often less expensive than other materials. Made of 24-gauge galvannealed steel they should last a lifetime.
The door panels are primed and painted to suit current trends. Steel exterior doors commonly have a high-density foam polyurethane or wood product core. Some doors have an exterior steel panel for durability and security, and an interior wood face for aesthetics.
The transmission of sound through an exterior steel door is less than that of an interior door. The thicker and denser the core, the better it blocks sound. However, almost as important are the seals that prevent weather and sound movement between the frame and door.
A well-sealed door will have an STC between 40 and 50, or better. It will stop most mid to high-frequency sound movement, and lessen low range sound transfer.
Wooden Exterior Doors
Exterior wooden doors are made using thicker materials than interior doors. Doors are often 1-3/4” to 2-1/4” or thicker. They are more elegant than steel but often require more maintenance too.
The doors may be made of solid wood, stile and rail construction, or have a veneer over a wood block, particleboard or MDF core. The doors can be heavier than steel doors
The thicker wood used in exterior doors decreases the sound transmission. A solid wood door will range from STC 35 to 49; the heavier the door, the better the sound suppression.
The overall effectiveness of an exterior door at preventing sound intrusion includes the seals between the door and frame.
Fiberglass Exterior Doors
Fiberglass doors can be manufactured to look like wooden doors, even with the wood grain. They are durable and withstand weather and human interaction better than wood or steel.
The doors are available in different thicknesses, plus multiple profiles, colors, and stains. The fiberglass forms a molded veneer with a polyurethane foam core and commonly has an aluminum reinforced frame.
The STC rating of fiberglass doors depends on the core material used, and the thickness of the door. Its overall transmission loss with better quality weather seals too. A fiberglass door with polyurethane core is rated between 22 and 32, while the particleboard core is 30 to 38 – the heavier and denser the material, the more sound it will interrupt.
Acoustic entry doors are door assemblies that are manufactured to block or absorb sound frequencies. The doors can be used for entry into homes, buildings, or as interior doors to more effectively soundproof a room or office.
The doors may be made from wood, metal or fiberglass, with laminated layers of insulation and MLV. The panels are thicker than many, and the jamb or frame is molded for a tighter fit, and often has magnetic or improved seals.
An acoustic door has a superior STC value of 60 to 70, although some claim any door over 50 is acoustic. Sound suppression rates of 60 or better offer excellent soundproofing and mean that even loud noises are barely audible.
How to Improve STC Rating
Doors may have adequate soundproofing to meet your requirements. Unfortunately, the door jambs, seals, and even walls may be the problem. Before you go out and spend $100.00s or even $1000.00s, here’s a quick list of things to check.
- Make sure the wall is adequately insulated. An uninsulated wall has an STC of 33 to 35, so a good door won’t help. Insulate the wall to prevent sound transfer.
- Check behind door trim to ensure it is properly insulated, if it isn’t, fill in the gaps with appropriate material.
- Check the weather-stripping and make sure it seals the door, if not replace or install it.
- Seal the gap between the door and the floor to prevent air and noise movement.
For more information on ways to improve door soundproofing check out the article How to Soundproof a Door.
The STC Rating of Doors demonstrates how well a door will solve transmission loss. The higher the rate, the better the door blocks sound transfer. Hopefully, you found the article of value.
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