Simcenter Testing Solutions Measuring Sound Transmission Loss Using Rooms

Video: Measuring Sound Transmission Loss using Rooms

This article explains how to use Simcenter Testlab program “Sound Transmission Loss Testing using Rooms” for material sample testing. Standards for this type of testing include ASTM E90, ISO 140, and SAE J1400.

The article has the following sections:

1. Introduction
2. Starting Testlab
3. Setup
4. Background Noise
5. Measuring Reference
6. Measuring Transmission Loss
7. Additional Options
     7.1 Broadband versus Band-by-Band
     7.2 Standards
     7.3 Intensity Based Transmission Loss

1. Introduction

When testing material samples for their absorption or transmission loss acoustic properties, different approaches can be used depending on the size of the material sample, and the desired sound field for the test (Figure 1):

Figure 1: Top Row – Normal incidence sound field absorption and transmission loss tests with impedance tube for small samples, Bottom Row – Random incidence absorption and transmission loss sound field tests with rooms for large samples

For large samples of materials (approximately 1 meter by 1 meter in size), large rooms are used for testing.  For example, for transmission loss, the material sample is placed between two reverberant rooms as shown in Figure 2.

Figure 2: Left – Reverberant room with diffuse sound field, Right – Reverberant receiving room.

Sound is generated in the sending reverberant room.  Every attempt is made to create a diffuse sound field that ensures a uniform sound level over the desired frequency range for testing the material.  The sound is forced to travel through the material separating the two rooms as shown in Figure 3. 

Figure 3: Material sample placed between the two rooms for the sound transmission loss test.

The sample is carefully sealed at the edges so the sound of just the material is tested. Leaks prevent the proper measurement of the materials, since the sound bypasses the sample being tested thru the leak.  Using a Simcenter Sound Camera can be an excellent way to determine if such a sound leak exists before performing a test.

The difference of the sound level between the two rooms is the transmission loss, which is expressed as an octave spectrum versus frequency as shown in Figure 4.
 

Figure 4 – Sound Transmission Loss (STL) result

The higher the transmission loss, the less sound that transmits from the source room into the receiver room.  Materials with higher transmission loss are more effective at blocking sound passing between the two rooms.  The material might prevent sound from passing by reflection or by absorbing sound via air molecule friction.

The unit of measure for STL is decibels.  The decibels represent the reduction in sound level through the material.

2. Starting Testlab

From the Simcenter Testlab folder, open the Testlab Acoustic subfolder.  Open the “Sound Transmission Loss using Rooms” icon as shown in Figure 5.

Figure 5: In the Testlab Acoustic folder, open the “Sound Transmission Loss using Rooms” icon.

After the software opens, go the Channel Setup worksheet.  Here the microphones used in the source and receiving rooms need to be defined in the software based on the setup shown in Figure 6.

Figure 6: Microphone arrangements in source sending reverberant room (left) and receiving reverberant room (right).

The microphones in each room will be averaged together.  Ideally, multiple microphones would not be needed if the sound field in each room was perfectly diffuse.  If the field was perfectly diffuse, the sound level measurement would be the same, regardless of where the measurement is taken in the room.  Because in real life there will be some differences, multiple microphones are used and averaged together.

The microphones in the reverberant room with source are labelled “Send1”, “Send2”, etc in the Channel Setup.  The microphones in the receiving room are labelled “Receiver1”, “Receiver2”, etc as shown in Figure 7.

Figure 7: In the Channel Setup worksheet, source room microphones contain “Send” in the Point Identification while receiving room microphones contain the work “Receive”.

Set the Channel Group to Acoustic, and the Input Mode to ICP if ICP style microphones are being used.

To calibrate the channels, see the Knowledge base article “Simcenter Testlab Calibration”.

In the Scope worksheet, set the Signal Type to Random and set a Voltage level for the output to the source as shown in Figure 8. 

Figure 8: Set Signal Type to Random

*** Be sure the amplifier to the sound source in the reverberation room is turned down before starting the source. ***

Now go the Transmission Loss worksheet as shown in Figure 9.

Figure 9: In the Transmission Loss worksheet, go through the steps on the left side in order.

There are some steps that can be followed clicking through the left side of the Transmission Loss worksheet from top to bottom.

First press “Check Setup” in the upper left corner a shown in Figure 10. The test settings will be checked against the relevant standards and any differences will be flagged.
 

Figure 10: Warning if the averages are set to low according to standards.

The warning shown in the previous figure was given because the number of averages was reduced below the recommend 70 averages recommended by the standards. 

If everything meets the settings required by the relevant standards, a “Setup OK” message should be shown (Figure 11).

Figure 11: Press “Check Setup” in the upper right of the Transmission Loss worksheet before performing any measurements.

If the message “Setup OK” appears in a green box, then the background noise of the two rooms can be measured.

4. Background Noise

The next step is to measure the background levels of both the sending and receiving rooms.

To make an accurate measurement of the transmission loss, the sound level of the measurements should be higher than the levels inherent to the source and receiver rooms (Figure 12). 
 

Figure 12: Ideally, there is a 15 dB separation between measurement and background levels.

There should be a good separation of the background and measurement levels. This is particularly important in the receiver room where the levels will be low if the tested material is effective. Ideally, this difference should be 15 dB or greater in every octave band of interest.

If the separation between the background and measured levels is between 10 and 15 dB, the measured levels are corrected for the background.

If the difference is 10 dB or less for any octave, than the measurement is not valid (Figure 13).

Figure 13: If any octave does not meet the required separation between background and measurement, the software will not calculate the STL.

If this happens, either the offending octave band must be eliminated from the measurement, or the background levels reduced relative to the measurement.

Click on the “Measure Background Noise” button on the middle left side of the Transmission Loss worksheet (Figure 14).

Figure 14: After the background measurement is performed, the Send and Receive status turns green.

The final check will be done when the STL calculation is done.  A global indication at this point is given by the message, but a per octave band will be performed ultimately.

5. Measuring Reference

Some standards, if selected, require that a known homogeneous material be tested before doing the actual measurement.  This is the case for SAE J1400.  This serves two purposes: a general check on the test setup, and determination of small corrections to be applied to the measurement.

After installing the known material sample between the two rooms, select “SAE J1400-REF” in the lower let corner of the Transmission Loss worksheet (Figure 15).

Figure 15: The reference material dialog box for SAE J1400.

There are two parameters that must be entered when selecting the SAE J1400 reference measurement: diagonal distance of the sample and surface density of the homogeneous material.

The diagonal length of the sample determines the lowest frequency that can be measured in the test (Figure 16). 
 

Figure 16: The larger the diagonal length of the sample, the lower in frequency the STL measurement.

The wavelength of sound is larger the lower in frequency, which dictates the lowest frequency.  In the standard, this is a recommendation, but not a requirement.

The known sample is made of a single, known material (hence the term homogeneous).  Based on the mass density of the material the theoretical Sound Transmission Loss (STL) can be calculated.  After clicking on the “Measure Transmission Loss” button, the actual and theoretical STL measurements will be compared (Figure 17).

Figure 17: Theoretically calculated STL (blue) based on mass density compared to actual measured STL (green).

These differences are then used as corrections during the actual measurement.  This is in addition to any correction applied for background noise.

6. Measuring Transmission Loss

With the background noise measurement finished, and any reference material measurement done, the actual measurement can take place.

Press the “Measure Transmission Loss” button on the middle of the left hand side.

After the measurement is done, press the “Process STL” measurement located in the upper middle of the screen.  A single average and maximum STL from all the microphones will be calculated. 

Display the average STL by highlighting it in the list.  Press the “Display” button to view it at the bottom of the screen as shown in Figure 18.

Figure 18: The average STL of the material can be displayed.

7. Additional Options

There are some additional options that can be used.

7.1 Broadband versus Band-by-Band

If desired, instead of a single broadband measurement across the entire frequency range, the source can be excite one octave band at a time.  This creates a greater concentration of energy in a single band which may help reduce the chances of failing the background noise criterion.

This can be set in the middle of the left side of the Transmission Loss worksheet as shown in Figure 19.

Figure 19: Switch to band-by-band excitation if needed for greater separation between measurement and background levels.

7.2 Standards

There are several different standard that are used in testing materials: ISO140, SAE J1400, ASTM E-90.  These can be selected on the lower left side of the Transmission Loss worksheet as shown in Figure 20.

Figure 20: Select Standards in lower left of Transmission Loss worksheet.

As previously mentioned, the SAE J1400 standard requires a reference material to be tested.  The “SAE J1400-REF” must be selected and tested first (with a known material in the room), before the actual test can be performed with the “SAE J1400” selection.

The ASTM E-90 and ISO140 require some T60 measurements that can be selected in the upper right of the menu.

7.3 Intensity Based Transmission Loss

Another common configuration for testing the STL of a material sample is to use an Intensity probe, rather than microphones, in an anechoic receiving room as shown in Figure 21.

Figure 21: Sound material test for transmission loss using intensity probe.

In this case, the Simcenter Testlab Sound Intensity program should be used.

Questions?  Email peter.schaldenbrand@siemens.com or contact Siemens Support Center.

Related Links

• Index of Testing Knowledge Articles
• History of Acoustics
• Microphone Basics
• Sound Pressure
• What is a decibel?
• What is A-weighting?
• Octaves and Human Hearing
• What is Sound Power?
• What is Sound Intensity?
• Decibel Math
• What is the Acoustic Quantity called Q?
• Sound Absorption
• Sound Transmission Loss
• Noise level certification, how to select the right standard?
• Sound pressure, sound power, and sound intensity: What is the difference?
• Sound fields: Free and diffuse field, near and far field
• Loudness and Sones
• Auditory Masking
• Tone-to-Noise Ratio and Prominence Ratio
• Fluctuation Strength and Roughness
• Critical Bands in Human Hearing
• Kurtosis
• Sound Quality Jury Testing
• Fundamentals of Sound Seminar - Part I
• Fundamentals of Sound Seminar - Part II
• Acoustic Testing YouTube Playlist