Factors Affecting Sound Transmission Loss

Front 1

STC

Back 1

Sound Transmission Class

Front 2

What are some factors that may affect the STC of an assembly?

Back 2

Depth of air spaces

Presence or absence of sound absorbing material

Degree of mechanical coupling between layers

Mass

Stiffness

Front 3

What are the three interacting physical factors important when determining if an occupant is bothered by noise from adjacent spaces?

Back 3

Sound transmission losses of party walls/floors

Level of noise generated in adjacent spaces

Level of background sound in the occupants space

Front 4

What should the effective STC rating for dwelling units be to achieve a reasonable degree of acoustical privacy?

Back 4

at least STC 55

Front 5

True or False:

Values in excess of STC 60 can be achieved with typical building materials without resorting to extreme designs.

Back 5

True,

but care is required during the design and construction process.

Front 6

Mass Law

Back 6

is a theoretical rule that applies to most materials in certain frequency ranges.

TL = 20 log10 (ms f) - 48 (1)

TL = the random incidence transmission loss of the layer; ms is the mass per unit area (kg/m squared); and f is the frequency of the sound wave (Hz)

Front 7

What is the most important physical property controlling the airborne sound transmission loss through an assembly?

Back 7

The mass per unit (ms) area of its component layers

Front 8

To increase the sound transmission loss of a partition by 12 dB at all frequencies what must be increased?

Back 8

The mass per unit area must be increased by a factor of 4.

The Mass law equation predicts that each time the mass per unit area of a single layer wall is doubled, the transmission loss increased by about 6 dB. Mass per unit area can be increased by thickness or by selecting a more dense material.

Front 9

What is the coincidence effect?

Back 9

The stiffness of a material create dips ("coincidence dips") in transmission loss graphs in certain frequency ranges which appear below those expected from the mass law. It often leads to a reduced STC rating.

Front 10

Why are the critical frequencies of concrete and plywood important to building acoustics?

Back 10

Because they are particularly vulnerable to STC reductions due to the effects of Coincidence.

Front 11

What must be used when lightweight construction and high STC values are desired?

Back 11

Double layer constructions.

Front 12

What are the important factors to consider when developing a double layer assembly?

Back 12

Masses of the component layers

Depth of the air space

Use of sound absorbing materials (within the airspace)

Rigidity of the mechanical coupling between layers (ideal assembly has no rigid mechanical connection between its two surfaces)

Front 13

In a double layer wall or floor the air trapped between the two layers acts as a spring and a resonance "mass-air-mass resonance" occurs. What can be done to mitigate this resonance?

Back 13

Adding sound absorbing material inside the cavity and increasing the air space between the two layers so that the mass-air-mass resonance is below 80Hz.

Note: adding sound absorbing material also helps for standing wave resonances between layers of a double layer wall.

Front 14

True or False:

Increasing the thickness of sound absorbing material beyond 75mm has little effect on the STC rating.

Back 14

True,

But in significantly thicker walls it becomes more important to use thicker layers of fibres in the insulation. (glass fibre and mineral wool are most common)

Front 15

What are some ways to reduce sound transmission due to mechanical connections in a Gypsum wall board assembly?

Back 15

Staggered wood studs

Separate rows of wood studs

Single row of wood studs with resilient metal furring strips to support the GWB layers intependently

Front 16

True or False:

Non-load-bearing steel studs are not resilient enough to provide adequate mechanical decoupling between GWB layers.

Back 16

False,

Good results have also been obtained using 150mm structural studs in conjunction with resilient channels.

Front 17

What is the STC rating of a typical gypsum wood stud partition (aka family home partition) and how much would the rating raise if sound insulation was added in the airspace?

Back 17

33 and only by 3 because of the transmission between the GWB to stud to GWB

Rigid mechanical connectors are the acoustical equivalent to a thermal bridge and should be avoided.

Front 18

What should the airspace depth be to meet the 80 Hz criteria for a single layer of GWB attached to a concrete block wall? And, What should the airspace be if the GWB layer is doubled?

Back 18

greater than 60mm. If the airspace is too small the sound transmission losses can be reduced relative to the unfinished wall.

If the GWB is double the airspace can reduce to 35mm

Front 19

What is the most common cause of reduced sound transmission losses in concrete block wall assemblies?

Back 19

Mass-air-mass resonance, an increase in the air gap can increase the STC considerably.

Front 20

What are the less obvious paths for the sound called?

Back 20

Flanking paths or Flanking Transmission

Front 21

Regardless of what STC is selected, what should be carefully controlled and sealed?

Back 21

All air-gaps and penetrations. Even a small air-gap can degrade the isolation integrity of an assembly