Using Transmission Loss
Transmission Loss (TL) is a tool for
measuring the insulating properties of walls, floors, or doors
separating two rooms.
It is based several international standards, including ISO 140-4,
"Acoustics —
Measurement of sound insulation in buildings and of building elements",
ISO 717-1. "Acoustics — Rating of sound insulation in buildings and of
building elements — Part 1: Airborne sound insulation", ASTM E336,
"Standard Test Method for Measurement of Airborne Sound Attenuation
between Rooms in Buildings", and also ASTM E-90 for Transmission Loss
(TL), E413-87 for Sound Transmission Class (STC), and E1332-90
for Outdoor-Indoor Transmission Class (OITC).
Refer to the specifications that apply
in your use to case to determine what to analyze, and for more
information about setting up and performing the tests to meet the
appropriate standards.
Implementation
Our implementation uses the filter-based
Real Time Analyzer (RTA) and the impulse-response based Room Reverb
modules to collect the data required for these calculations. Depending
on the results that you require, you may not need to measure the RT
time using the Room Reverb module.
TL uses the concept of a "source room", where the test noise is
present, and a "receiving room", where the level of the test noise that
propagates into the room is measured to determine the noise reduction
properties of the separating element, such as a wall, floor, or door.
The basic procedure involves generating
pink noise in the source room, and then measuring the levels in both
the source and receiving rooms, and in some cases measuring the reverb
decay time (RT) in the receiving room.
The standard suggests doing several
tests at different locations in the room, and averaging these tests
before computing the results. This may be done either by moving the
microphone around in the room, while the RTA is in Average mode, or by
doing several discrete tests and saving and recalling them into one
composite test.
Operation
The
test is done in two distinct steps, source room tests and receiving
room tests. The selector on the screen changes the mode. When both
tests are complete, switch the selector to Results to view the results
of the calculations.
Make sure that you have at least 10dB greater level in the sending
room, compared to the receiving room. If the receiving room ambient
noise level is high in some bands, this may mean that you need a very
loud noise source in the sending room.
Settings
First, open the Settings page by tapping the wrench icon on the bottom
of the screen, and select the desired test format, Rw or Dntw for ISO 717, STC
for ASTM E336, or OITC for ASTM E1332.
Also on this page, enter the room dimensions if you will be using the RT results, which are recommended for the ISO 717 tests.
Sending Room Levels
You will need a pink noise source in
this room that can also be used when you move the microphone to the
receiving room. There are several ways to do this.
- Wired connection
- Wireless link
- Remote Generator
- Other pink noise source
Set the selector to "Send".
Turn on the pink noise, and start the
RTA running. The RTA runs in average mode (Leq), so it is constantly
linearly averaging results. In this mode, you can move the microphone
around the room, and the RTA will do the averaging for you
automatically.
You can start a test over at any time by tapping the Reset button in
the Average box, and you can pause and start the test using the Play /
Pause button in the bottom toolbar.
When this test is complete, tap the Play / Pause button to stop the
test. The levels shown on the red RTA bars will be used as the 1/3
octave SPL levels for the source room.
Receiving Room Levels
Leaving the pink noise generator in the source room, move the microphone to the receiving room.
Change the selector to "Receive".
With the pink noise running, measure the
levels in this room. Use the Reset button on the Average box to start a
new measurement, and use the Play/Pause button if you need to pause the
test while moving the microphone.
When this test is complete, tap the Play / Pause button to stop the
test. The levels shown on the red RTA bars will be used as the 1/3
octave SPL levels for the receiving room.
Turn off the pink noise generator in the source room.
Receiving Room RT Time
If you wish to also measure the room reverb decay time (RT time),
change the selector to "RT".
Next, tap the "Test RT" button to bring up the recording screen. If the
room dimensions of the receiving room, or the size of the insulating
panel (wall, floor, or door) are not correct, enter the proper values
at this time.
You will need to generate an impulsive sound that contains a broad
range of frequencies. Often a balloon pop, or even a starter pistol is
used for this purpose, although we recommend dropping a heavy book onto
a hard floor surface.
Tap the Record button, wait for the countdown to reach 0, and create
the sound. The room should be as quiet as possible during this test,
for best results. Wait a few seconds after the sound, perhaps 5
seconds. Tap the Stop button.
Tap the Done button on the upper toolbar, and the calculations will be
completed and the 1/3 octave response shown on the screen. When you are
satisfied that you have a good test, tap Done to return to the Results
screen.
Ambient Noise Levels
If you wish to correct for the ambient noise levels in the receiving
room, first enable this option on the TL Setup page. If you believe
that the ambient noise levels are at least 10dB below the levels with
the pink noise generator turned on, you can skip this step and proceed
to the Results page.
Change the selector to "Ambient".
With pink noise turned off, measure the levels in this room. Use the
Reset button on the Average box to start a new measurement, and use the
Play/Pause button if you need to pause the test while moving the
microphone around the room.
Now you are ready to view the test results.
Viewing Results
Change the selector to "
Results".
The test results will be shown here. The main plot, in green, is
the level difference at each octave band. The reference curve that
applies to this level difference plot is shown in grey. This curve is
determined as per the spec, using a sum of deficiencies.
If an RT test was done, these results are shown as 1/3 octave red bars, referred to the scale on the right side of the screen.
The values box will show any computed single-value numbers, per the appropriate specification, such as Rw or STC.
Save / Recall
A number of different things can be saved and recalled, depending on what screen is active.
Saving and Recalling Level Tests
You can save a level test for either of the rooms, or the ambient noise levels, to recall later, or to add to a sum of tests.
From the Source or Receiving room page, tap the folder icon to bring up
the settings page. From here, you can save the current test, which is
the state of the red RTA 1/3 octave level bars, or you can recall a
test that was saved previously.
When you recall a test, you will have the choice of letting this
recalled test fully replace the 1/3 octave levels, or having this test
averaged into the current 1/3 octave levels. You can average as many
tests as you like into the current test.
After you have created a composite test, by averaging several
individual tests, you can simply use those levels in the current
evaluation, or you can save this composite into it's own test, to make
it easier to recall later.
Saving and Recalling RT Results
From the RT screen, you can tap the folder icon to being up the
Save/Recall page. There you can save or recall RT time results. Note
that RT results are not averaged, they always replace the current
values.
Saving and Recalling TL Results
From the Results page, tapping the folder icon brings up the
Save/Recall screen. From here, you can save the current results, which
also saves a tab-delimited file that you can open in a spreadsheet
program. All of the results, including the source and receiving room
levels and RT test, if completed, are also stored here.
You can also recalled a previously saved set of results. Note that this
will also recall the source and receiving room levels that were in
effect when that set of results was saved.