ACOUSTICS - Reverberation Time
As discussed in the Sound
Behaviour topic on the Square One website, reverberation time (RT) is the
simplest and most commonly used objective measure of the acoustic performance
of a space. It is defined as the time taken for the sound level of a steady
source to drop by 60dB after it is abruptly turned off and is given as a value
The RT is basically a ratio of the weighted sound absorption coefficients of
materials within a space to its volume. The quickest method of calculation is
simply to weight each material by its surface area - which is known as a statistical
RT. This method takes no real account of the actual geometry of a space, just
the materials within it and an overall form factor. However, it is usually a
good predictor and is very widely used.
An alternate method is to actually trace many thousands of acoustic rays randomly
sprayed within the space and then to weight each surface by the number of ray
intersections. This method usually produces different results to the statistical
method as it focusses on the more acoustically significant surfaces and ignores
those the sound can't actually get to.
As is usual with these things, the performance of the real room is likely to
be somewhere between these two extremes. In this tutorial we are going to look
at both calculations and modify some materials within a space. It is assumed
here that you have already been through at least the introductory tutorials
and are reasonably familar with the ECOTECT interface.
Loading the Example Model
- First open the ReverberationTime.eco
model from the Tutorial Files folder located in your main ECOTECT Install
The zone we are interested in here is called MainSpace. You will notice that
it is a thermally complete zone, meaning that its envelope is entirely defined
and that planes or surfaces do not project outside the space in any way.
This zone represents a small lecture room with concrete walls, a partially
exposed concrete ceiling and a timber-clad concrete floor slab. The only real
absorption in the room comes from a plywood panel bulkhead running the length
of the room.
- Now select the Statistical Reverberation... item in the Calculate menu.
This will prompt you to calculate interzonal adjacencies. This is a necessary
step as it allows ECOTECT to pre-process your model, checking plane equations,
surface areas and looking for adjacencies between zones. The prompt should
look like the following:
Usually this calculation is very quick, however it is the overshadowing analysis
that really takes the time. As we are not going to use this model for thermal
analysis yet, we can simply turn off shading calculations.
- Click the Settings... button in the prompt.
This will display the Inter-zonal adjacencies dialog box. We basically need
to select the None option in the Overshadowing Accuracy selector,
as shown below.
- After selecting None, click the OK button.
This will display the Graphical Results dialog box. You will notice from the
settings section immediately below graph that the volume of the zone has been
already calculated at 333.6m³ and that it has been assigned 35 cloth-covered
seats, which are currently 80% occupied.
From this information, recommended reverberation times for speech and music
can be derived. This is shown as a faint blue band running the full width
of the graph.
The RT graph of the MainSpace shows values for each of the 9 octave bands
that represent the Ear's hearing range. The coloured lines represent three
different equations for performing the statistical calculation, denoted as
Sabine, Millington-Sette and Norris-Eyring (see the Sound
Behaviour topic for more details). The bold line, in this case the blue
Sabine, represents the equation whose results will be stored within the zone's
You will notice that the RT for speech frequencies (500Hz - 4kHz) are well
above that recomended for speech, being closer to the upper limit recommended
for music. The longer RT values are also biased towards the lower frequencies.
Sound recording engineers would refer to such a room as 'boomy' or having
a lot of 'bottom end'. This in itself may not be a problem as speech does
not contain too many low frequencies, however recorded material such as a
video may have, so we should really know how often such material may be used
before we can assess it as a problem.
However, at speech frequencies the recommended RT for speech is around 0.7-0.8sec
whereas the room currently has an RT at these freqencies of 1.2-1.4 sec, almost
Adjusting the RT
We can adjust the RT of any space by simply changing the materials assigned
to any of its surfaces. As an obvious first experiment, we are going to play
with the material assigned to the area of dropped ceiling running through the
centre of the room.
- Click inside the main application window and then select the three objects
indicated in red below.
You will note that they are currently assigned the PlywoodPanel material.
To compare, we are going to assign them as SuspendedAbsorber and
see what changes that makes to the RT graph.
- Once the three objects are selected, click on the SuspendedAbsorber
material in the Material Assignments panel and then click the Apply button
at the bottom of the panel.
These are two quite different materials. The PlywoodPanel is relatively
rigid but without significant mass. Thus it allows low frequencies to pass
pretty much straight through creating an enclosed space within the ceiling
that acts like a bass trap. Mid-to-high frequencies are reflected back into
the space with relatively little absorption. The SuspendedAbsorber
material on the other hand contains thick fibrous material with significant
mid-high frequency absorption, as shown in the comparative animation immediately
You can view these graphs by double-clicking on the material in the Material
Assignments panel to display the Element Properties dialog, and then choosing
the Acoustic Data tab.
- Next return to the Graphical Results dialog and click the Recalculate
This should display the following graph.
As you can see, this has significantly reduced the RT in the mid frequencies,
basically the speech band - perhaps a little too much as it is now a little
below the recommended RT for a space with that volume. Unfortunately, acoustic
design is never as cut and dried as this. It may be that the potential occupiers
of the space would prefer it a little 'dry'.
As a simply exercsie, you should try to reduce the low frequency RT of the
space. There are really very few materials that absorb low frequency sound.
However, the ECOTECT Help File contains lists of the absorption coefficients
of a wide range of different materials and absorbers. To access this list, simply
open the Element Properties dialog and click the Help... button, or navigate
within the help file to Modelling > Material Assignments > Material
You can add absorber to the space by either assigning it to an existing surface
or by adding moveable partitions, which can be removed or adjucted to control
the overall response.