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Sound Transmission Loss
When sound reaches a barrier, three things can happen, as shown in Figure 1:
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.
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.
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.
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).
Effects on 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
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 the figure 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.
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.
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 the figure below, transmission loss is plotted for three scenarios: full coverage, a small 1% leak, and a larger 5% leak.
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.
Applications and Measurements
Sound transmission loss can be a good metric for benchmarking the acoustic performance of products.
Common testing applications include:
Knowing the sound transmission loss helps to determine and improve acoustic properties of materials.
There are two major methods for determining STL:
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).
Impedance tube method:
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).
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.
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.
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:
Typically, the two loading conditions are a rigid termination and an anechoic termination.
Simcenter Testlab offers a single page workbook for determining Sound Transmission Loss.
The above figure is taken from the Simcenter Testlab Sound Transmission Loss Measurement Using Impedance Tube software.
It is also possible to calculate sound transmission loss 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.
There are a few methods using rooms:
Sound intensity and sound pressure:
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.
When measuring STL using the two room intensity method, Equation 1 can be modified such that STL is calculated as follows:
When measuring flat samples, S_i = S_t ---> S_i/S_t = 1
Therefore, when measuring flat samples,
A common automotive STL application with this technique is measuring a vehicle dash panel as seen in Figure 10, below.
An intensity map of the dash panel can be created with the resultant data which lends information as to where the leaks are.
In addition, the standard STL value vs frequency plot is also generated.
The intensity method is common for investigating dash panel, doors, and instrument panels.
Sound pressure and sound pressure:
The two room method is based on a pressure difference between a sending room and a receiving room (both of which are reverberant rooms). The sample is placed in an opening connecting these two rooms.
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.
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.
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 below.
It is possible to do this testing with Simcenter Testlab Sound Transmission Loss Testing using Rooms software.
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.
Questions? Email Scott MacDonald (firstname.lastname@example.org) or check out the on-demand webinar: How to identify acoustic material testing properties.