When sound reaches a barrier, three things can happen, as shown in Figure 1:
Absorption – The sound is absorbed and dissipated as heat.
Transmission – Sound can pass through the barrier.
Reflection – Sound can be reflected back off the barrier.
Figure 1: Sound at a barrier can be absorbed, transmitted, or reflected.
Sound transmission loss (STL) is a quantification of how much sound energy is prevented from traveling through an acoustic treatment. Transmission loss quantifies the effectiveness of acoustic treatments for an engineering application.
This article explains sound transmission loss, considerations for effective design to maximize transmission loss, and test methods to quantify transmission loss: 1. What is Sound Transmission Loss (STL)? 2. Design Considerations for Sound Transmission Loss 2.1 Effect of Coverage on Transmission Loss 2.2 Effect of Holes on Transmission Loss 3. Applications and Measurements 3.1 Impedance Tube Methods 3.2 Room Methods 3.2.1 Sound Intensity and Sound Pressure 3.2 2. Sound Pressure and Sound Pressure 4. Conclusion
1. What is Sound Transmission Loss (STL)?
Sound transmission loss can be defined as a ratio of the sound energy transmitted through a treatment versus the amount of sound energy on the incident side of the material as shown in Equation 1:
Equation 1: Sound Transmission Loss equation
Where:
Wi is the incident sound power
Wt is the transmitted sound power.
Note: These quantities are pictorially represented in Figure 1 above.
Sound transmission loss is a function of frequency. The transmission loss performance of a certain material will differ greatly with frequency (see Figure 2, below). The y-axis of a plot represents how many dB the acoustic absorber reduces the incident energy.
Figure 2: Sound transmission loss as a function of frequency. At 2050Hz, the acoustic absorber reduces the incident energy by 10dB. The data used in this plot was calculated using the matrix method on a muffler.
For example, in the plot above, it is clear that the transmission loss value is highly dependent on frequency. At 2050Hz, the muffler reduces the incident energy by 10dB (green dotted line). However, at 3500Hz, the muffler does not reduce the incident energy at all (purple dotted line).
2. Design Considerations for Sound Transmission Loss
When trying to reduce the sound passing through a barrier, it is common to apply a layer of acoustic treatment material. Coverage of the barrier as well as any holes in the acoustic treatment will affect the sound transmission loss.
2.1 Effect of Coverage on Transmission Loss
If trying to reduce the sound through an object (such as a square panel) one may apply some acoustic material onto the panel. It is important to note that the percentage of coverage of the object will have an effect on the sound transmission loss.
In Figure 3 below, transmission loss is plotted for three scenarios: the square panel being completely covered by acoustic material, 3% of the panel remaining exposed, and 25% of the panel remaining exposed.
Figure 3: When the area of the acoustic material does not completely cover an object, the sound transmission loss decreases.
Notice that even with a small area of exposure (just 3%), the transmission loss can decrease dramatically. Increasing the size of the exposure (even as large as 25%) does not have as great of an effect as introducing a small initial hole. The effects are especially notable at higher frequencies.
2.2 Effect of Holes on Transmission Loss
If there is a hole in the acoustic treatment, it will cause a decrease in the effective transmission loss. Note that this effect is especially noticeable at higher frequencies.
In Figure 4 below, transmission loss is plotted for three scenarios: full coverage, a small 1% leak, and a larger 5% leak.
Figure 4: Introducing leaks and holes in the acoustic barrier reduces transmission loss, especially at higher frequencies.
It is important to recognize that even small holes and small areas of missing coverage can greatly reduce the transmission loss of a barrier. As the hole gets larger, the effect is not as dramatic as introducing that first initial hole. This is especially apparent at higher frequencies.
3. Applications and Measurements
Sound Transmission Loss (STL) can be a good metric for benchmarking the acoustic performance of products.
Common testing applications include:
Determining the effectiveness of a muffler in a system
Determining the transmission loss of various ducting systems
Determining how well a building panel or partition attenuates sound energy
Determining how well an instrument panel insulates a cabin from engine noise
Knowing the sound transmission loss helps to determine and improve acoustic properties of materials.
There are two major test methods for determining STL:
Impedance tube method
Two room method
Transmission loss is independent of the source meaning that STL can be measured using a source such as a loudspeaker (it does not need to be measured in-situ).
3.1 Impedance Tube Methods
The impedance tube method is useful for determining the STL of components like ducting and mufflers as well as small material samples. Impedance tubes are typically made of straight “sound proof” tubing (typically thick steel) as shown in Figure 5.
Figure 5: Top: Impedance tube. Bottom: Side view of impedance tube to show sample location in the tube.
One end of the tube is connected to a sound source which outputs a broadband range of sound waves. The element to be tested is mounted in the middle of the tube. The sound waves approaching the sample are direct incidence and normal to the sample.
Care must be taken to ensure a tight connection between the sample and the tube. For example, if testing a muffler, there must be a tight connection between the outlet of the first tube and the inlet of the muffler as well as the outlet of the muffler and the inlet of the second tube. An example impedance tube setup to measure a muffler is shown in Figure 6.
Figure 6: The engineer ensures a tight fit between the ends of the muffler and the ends of the impedance tube.
NOTE: Conical adaptors may be used to account for a difference in diameter between the sample outlets and the impedance tube diameter. Conical adaptor corrections can be factored in using Simcenter Testlab.
Figure 7: The cone correction screen in Simcenter Testlab.
There are a few methods to determine STL in an impedance tube. Siemens Testlab uses the four-microphone transfer matrix method which assumes a lumped parameter model of a 3D acoustic cavity.
In this equation set, p1, v1, p2, and v2 can be measured. The unknowns are T11, T12, T21, and T22. Because there are four unknowns and two equations, there must be two different loading conditions to create four equations to solve for the four unknowns.
To get the two different conditions, it is possible to either run the test under two loading conditions or change the source location between test runs.
The two load method is recommended over the two sources method because:
It is easy and quick to change the end condition.
There are no cables moved between conditions.
Inexpensive objects like tubing is moved (not fragile/expensive objects like sources).
Typically, the two loading conditions are a rigid termination and an anechoic termination.
Figure 8: The result of a sound transmission loss test in tube is the sound transmission loss of the material vs. frequency.
The above figure is taken from the "Simcenter Testlab Sound Transmission Loss Measurement Using Impedance Tube" software.
3.2 Room Methods
It is also possible to calculate Sound Transmission Loss (STL) using the two room method. This method is most appropriate for larger samples and even complete components like doors, windows, and vehicle components like a dash panel.
Using this technique, the engineer not only learns the overall Sound Transmission Loss of the component but also gains insight as to where most of the noise is coming through the component. The intensity testing will result in an intensity map of the product in which transmission paths are highlighted as well as a plot of STL vs frequency.
In the reverberant room, an omnidirectional source is used to create a diffuse field. The anechoic room is used to create a free-field condition to avoid any reflections that would show up as incorrect localization spots. The material sample is placed between the two rooms.
In the anechoic room, the sound intensity can be measured either using a sound intensity probe.
Figure 9: Rooms setup for a sound transmission loss test using intensity.
When measuring STL using two rooms and intensity, Equation 1 can be modified such that STL is calculated as follows:
Equation 2: STL for two room method using intensity
Where:
Lpi is the incident sound pressure
LIt is the intensity measured on the material sample in the anechoic chamber
Si is the area incident
St is the area transmitted
6.18 is a constant accounting for a few things like air density and speed of sound
When measuring flat samples, Si = St ---> Si/St = 1
Therefore, when measuring flat samples:
Equation 2 Simplified for Flat Samples
A common automotive STL application with this technique is measuring a vehicle dash panel as seen in Figure 10, below.
Figure 10: A sample being measured using the two room intensity method. The sample is mounted between a reverberant room and anechoic room.
Before testing, a sound map of the dash panel can be created which lends information as to where the leaks are is shown in Figure 11 below:
Figure 11: The intensity map of the object can be viewed to learn which locations are transmitting the most sound.
Checking for leaks with tools like the Simcenter Sound Camera helps ensure that the test reflects the transmission loss of the treated dashboard rather than problems with the test fixture.
At the end of the test, a standard STL value vs frequency plot is generated. An example for a homogeneous material is shown in Figure 12.
Figure 12: The sound transmission loss can be viewed on a per-octave basis.
The intensity method is common for investigating dash panel, doors, and instrument panels.
In both rooms there is either a microphone on a rotating boom or a collection of microphones in various locations. The function of the rotating microphone / multiple mics is to measure and calculate the average sound pressures of both rooms. An acoustic source is placed in the sending room.
Figure 13: Setup for measuring sound transmission loss using two reverberant rooms.
The sound pressure must be measured in both rooms.
Sound Transmission Loss (STL) is calculated as a pressure difference between the rooms with a correction for the receiving room.
Equation 3: STL for two room method using pressures.
Where:
L1 is the average SPL in the sending room
L2 is the average SPL in the receiving room
S is the test object area
A is the equivalent sound absorption (A = 0.16V/T) where V = receiving room volume and T = reverberation time of the receiving room.
Figure 14: A material sample is being prepared to be inserted in the opening between the two rooms.
Again, with this method the sound transmission loss is measured versus frequency. An example of the results sheet in Simcenter Testlab from the two room method is shown in Figure 15 below.
Figure 15: The result of a sound transmission loss test using the two room method.
Sound transmission loss can be a good metric with which to measure the efficacy of a barrier or duct system to reduce the transmission of sound energy. Remember that small gaps in coverage, and small holes can greatly reduce transmission loss. Keep in mind that the results from bench testing can be slightly different than how the material performs in-situ because of boundary conditions and environmental conditions.
