- You can start with a premade recording, such as sample_data.wav or HawleyBathroom.mp3 (or HawleyBathroom.m4a) that I made by clapping and banging a book in my bathroom.
...Or...
- You can record one yourself:
- The easist way is to provide some impulse sound such as clapping, popping a balloon or banging a book.
- Or, the "proper" way is to digitally contruct an impulse response from sine sweeps; if you want to do that, see the tutorial "Creating Impulse Responses with SHAART".
Load the audio file for your recorded decay as File A. You can also load a second one as File B. We will use the simple case of one decay given by sample_data.wav for what follows. For multiple decays in one recording, see Part III, below.
- Initially you see the decay itself, shown without any octave-band filtering:
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Using the drop-down menu in the upper right, select the octave band you want to filter in, such as 1000 Hz, and release:
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With your mouse, click near the top of the graph (but ignore the initial peak) and while holding the button down, drag the resulting red line such that it is parallel to the overall shape of the decay (it doesn't have to be on top of the decay, just parallel to it). Also, it doesn't matter how long you draw the line, all we care about is the slope.
- In the top center of the graph, the text "RT60 = 1.24 s" appears. That is the reverb time you measured.
- Re-measuring: To re-draw the line, simply click and drag again. To switch to different octaves, simply change the octave drop-down.
In this case we will measure multiple days, contained in HawleyBathroom.m4a. In this case, we will use the "Zoom" feature provided by the magnifying class icon in the bottom left.
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First, we need to load the file and choose and octave; let's do 4 kHz:
Ooops! Notice there's still a red line from the previous measurement, and even though the Octave filter says "1000 Hz", it's really not: THIS IS A BUG. It's actually showing broad-band (unfiltered) signal.
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But when we select the 4000 Hz octave, the display becomes correct:
In this example there are 6 decays: the first 3 impulses are from hand claps, and the last 3 are from backing a book on the wall. -
Now click the magnifying glass icon near the bottom left of the window, decide on one of those 6 decays too zoom in on, and click-and-drag a box around it. We'll choose the last decay:
When you release the mouse, they display will be zoomed in on that decay: -
If you want to zoom out and try again, click the "Home" icon in the bottom left. If you want zoom in even more, click the magnifying class again and draw a new box. -
At this point, simply & click and draw a red line along the decay, as in the "Single Decay" example above:
Note that this small bathroom room has a double-valued reverb time: This can be seen a bit more obviously in the zoomed-out view, showing a "kink" in the middle of each decay. This is probably due to the bathroom door having a large opening under it, so that the bathroom is acoustically coupled to the hallway and kitchen outside. You may notice similar effects for your rooms.
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Bug/Feature: When you select a new octave band, SHAART will zoom back out to the full display. For example, if we choose 125 Hz octave next, we get this:
...and every time we change the octave, it will zoom out. (Note that the 3 hand claps on the left have much less energy at low frequencies than the 3 book-bangs on the right.)
This un-zooming behaviour is a "feature" of clearing the display each time we draw a plot; in order to keep the zoom window the same when filtering, it would take a fair amount more coding work -- perhaps a future version of SHAART will be able to do that.
Author: Scott Hawley