News

The Mechanics of Sound Transmission

When sound comes in contact with a barrier, such as a door, some of the energy from the vibrations transfers to the door. The resulting vibrations in the door itself then set the air in motion on the other side of the door—creating more sound vibrations.  

 

The mass, damping and stiffness of the barrier determine its resistance to the passage of sound waves. The greater the mass, the less sound is transmitted through the barrier. Mass is especially important for blocking sound at lower frequencies

Sound vibrations can be reduced using damping materials, which are typically limp-mass materials.  Damping material is sometimes used as core material in doors designed to provide the highest levels of sound control.

 

The stiffness of the barrier is also a factor in sound transmission. Although more flexible barriers transmit less sound, for practical reasons sound-control doors are generally made from very dense, stiff materials. Unless they contain inner layers of damping material, some sound will inevitably be transmitted through the door.  On the other hand, those dense, stiff materials also work well at reflecting sound back to its source.  Most acoustical doors are constructed of wood or steel with stiffness and barrier batts added to any hollow cavity inside the door

Naturally, the effectiveness of sound-control doors varies with different combinations of materials. With so many variables, how can we determine how well a particular door will block sound?  And how can we compare the effectiveness of different doors?

 

Sound Transmission Loss (TL)

A door's ability to reduce noise is called its sound transmission loss (TL) effectiveness.  TL is a value given in decibels, which is determined by measuring sound pressure levels at a given certain frequency in the source and receiving rooms.  The calculation also factors in the area of the partition shared by the two rooms, and adjusts for the receiving room's acoustic "liveness" (known as "reverberation time").  The adjusted difference between the two levels is the TL of the door.  The higher the TL, the better the result.

 

Leaving out the adjustments to illustrate using a simple example, if the source room measurement is 100 dB at 300 Hz and the receiving room measurement is 60 dB at 300 Hz, the TL of the barrier is 40 dB at 300 Hz. TL is measured in test laboratories according to ASTM E90 "Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions."

 

Sound Transmission Class (STC)

TL measurements for a door are taken across a range of frequencies, which makes it difficult to compare the effectiveness of different doors. Sound transmission class (STC) ratings solve that problem by giving a single value to acoustical performance for a door. STC is determined by a weighted average of TL values taken over 16 frequencies, which are fitted to a curve in a method defined by the ASTM E413 Classification Standard for Rating Sound Insulation. The higher the STC value, the better the rating—and the better the performance, as shown below:

 

Sound Transmission Class (STC) Table

STC

Performance

Description

50-60

Excellent

Loud sounds heard faintly or not at all

40-50

Very good but not understood

Loud speech heard faintly

35-40

Good

Loud speech heard but hardly intelligible

30-35

Fair

Loud speech understood fairly well

25-30

Poor

Normal speech understood easily and distinctly

20-25

Very poor

Low speech audible

 

 

STC values are used to define the performance requirements for achieving a specified reduction in sound transmission from a source room to a receiving room. The STC rating of an installed door also determines how much noise reduction is possible between a given source room and receiving room.

 

The maximum values that can typically be achieved with various types of doors used in commercial construction.  It also shows the STC effects of using glass in door lite kits.  In general industry practice, doors with STC values of 30 or higher qualify as acoustical doors. 

 

Representative STC Values

The values shown are typically the best available ratings (tested as panels) for the type of door and glazing materials used. For comparison, 12 inches of reinforced concrete provides an STC of 56. On the other hand, if you have a standard mineral core 1 3/4” door without any sound absorption material, sound control performance will almost certainly be very poor.

Wood Doors

STC

Particle Core (1-3/4”)

30-31

Particle Core (2-1/4”)

33-34

Mineral Core (Fire-Rated)

35-37

Sound Core

36-45

Hollow Metal Doors

Honeycomb Core

35-37

Styrene Core

34-35

Steel-Stiffened with Sound Core

46-55

Glass

¼” Plate

29

¼” Laminate

34-36

3/8” Plate

30

½” Plate

33

5/8” Plate

33

 

There are two different scenarios for testing a door as a sound barrier to determine its STC value.  When tested as a panel, the door is sealed into the test wall, and the resulting STC rating is actually the rating for the entire wall. Alternatively, the test can be conducted with the door operable, yielding an STC rating that is generally more representative of what can be expected when the door is installed.  In practice, manufacturers test using both scenario, and sometimes both.  It is important to note what kind of rating is published and be aware of the differences.

 

Because sound-control doors are tested under optimal laboratory conditions, it is equally important to understand that performance in the field will almost always be less than the door's published STC rating under either of those scenarios.  The key to achieving an STC rating for the opening that is as close as possible to the published rating for the door is using appropriate acoustical gasketing. Without acoustical gasketing, even openings with high-rated doors perform very poorly at blocking sound. And results can be equally disappointing with acoustical gasketing that is poorly designed, constructed or installed.    

 

STC Testing for Zero’s Sound Trap-52 System

Band No.

1/3 Octave Band Centre Frequency Hz

Sound Transmission Loss in dB

Door Sealed

Door Operable

(1)

125

40

42

(2)

160

43

43

(3)

200

49

48

(4)

250

47

45

(5)

315

48

46

(6)

400

47

46

(7)

500

49

46

(8)

630

51

48

(9)

800

52

50

(10)

1000

55

52

(11)

1250

58

55

(12)

1600

61

57

(13)

2000

62

58

(14)

2500

64

58

(15)

3150

66

59

(16)

4000

62

59

Sound Transmission Class

55

52

 

The values in the table on top show the derivation of Sound Transmission Class (STC) from testing a ZERO SOUND TRAP-52 system.

 

System Components:

 

• #770 head and jamb seal,

• #119 head and jamb spring seal,

• #367 automatic door bottom,

• #564 saddle.

 

The acoustical system was installed on an STC 55 door fitted in a standard 3’ x 7’ test opening between two reverberating rooms