This app is designed to assist acousticians when designing environments such as home cinemas. It calculates the acoustic absorption curve of a variety of absorption devices mounted against a rigid backing such as a brick wall.
The porous absorber is typically made from some material such as Rockwool or glass fibre insulation. You need to know the flow resistivity of this material in order to get the best results from this tool.
This app is the reimplementation of an Excel spreadsheet I wrote in 2004. It was rewritten as part of an on-going exercise in learning Rust and cross-compiling to Web Assembly using wasm-pack and wasm-bindgen.
When the app starts, the "Rigid Backed Porous Absorber" tab will be selected by default.
If this is the first time you have run this calculator, then all calculations will be performed using default values.
If you have used this calculator before, then each of the curves will be plotted using your previous values.
As you move the mouse pointer over the chart, cross-hairs will appear and the absorption value at a particular frequency will be displayed when the mouse hovers over a plot point.
Four absorber device types have been implemented:
There is also a configuration screen on which you can change less frequently altered values such as air temperature and pressure.
An online version of this tool is available here
The graph maintains an aspect ratio of 21:9 as the browser screen resizes and has an arbitrary minimum width of 1000 pixels.
When switched on, a cross-section of the absorption device will be displayed on the left of the chart.
The graphic shows the wall on the far left, then the air gap, then the porous absorber layer, and finally if relevant, the slotted/perforated panel on the right.
As you change the dimensions of the air gap and porous absorber layer, the diagram changes to show the device dimensions in proportion to each other.
For devices using both a panel and a porous absorption layer, the diagram is split in half. The top half shows the absorber against the panel, and the bottom half shows the absorber against the rigid backing.
If desired, the "Smooth curve" checkbox can be switched on. This will connect each plot point using Bézier curves; however, it should be noted that this feature was added for its aesthetic appeal and does not imply that the actual absorption between the plot points follows the line drawn on the screen
The graph always plots an 8 octave range starting at the specified start frequency. Normally, this should be left set to 62.5 Hz in order to see the standard analysis range (i.e. up to 16 KHz). However, should you wish to, you can set the start frequency to be as low as 20 Hz, in which case, you will still see an 8 octave range, but the upper limit will now be 5.1 KHz
All inputs are made using the range sliders. The sliders can be moved either by dragging the button with the mouse, or for more precise input, select the slider and use the left/right arrow keys.
I decided to use sliders as the input UI element instead of simple input fields for two reasons:
- It prevents invalid or out of range values from being entered, thus ensuring that the calculation engine always receives valid input
- It creates an "animation" effect whereby you can see how the absorption curve changes dynamically as you move a slider
This app uses the browser's local storage to remember your device parameters. If at anytime you wish to clear these cached values, select the "Configuration" tab and press the "Clear Cache" button. Only the values pertaining to this application are cleared from local storage.
It is possible that after a new version of this app is released, old values in the local storage cache might cause you to see an empty chart. If this happens, clear the local storage cache and refresh your browser page.
These instructions assume you have already installed Rust and wasm-pack, and that Python3 is available to act as a Web server.
- Clone this repo
- Change into the repo's top-level directory
- Ensure that you have installed wasm-pack as per the instructions behind the link
- Compile using
wasm-pack build --release --target web
To test this app locally, run the shell script ./test_server.sh then visit http://localhost:8000
The purpose of this shell script is simply to ensure that the Python Webserver serves files of type .wasm with the correct MIME type of application/wasm.
Running this shell script is not a requirement; alternatively, you could simply start the default Python Webserver using the command python3 -m http.server.
Using this approach however, you will see the following non-fatal error in your browser console:
porous_absorber_calculator.js:242 `WebAssembly.instantiateStreaming` failed because your server does not serve wasm with `application/wasm` MIME type.
Falling back to `WebAssembly.instantiate` which is slower.
Original error: TypeError: Failed to execute 'compile' on 'WebAssembly': Incorrect response MIME type. Expected 'application/wasm'.
Debug/trace output is enabled on a per-module basis.
If you wish to see the debug/trace output for a given module, set the Boolean flag for the selected module in config::trace_flags, recompile using wasm-pack and then perform a hard refresh of the browser page.
Debug/trace output will then be visible in the browser console whenever control passes through that particular module.
For instance, if you switch on debug/trace for the module calc_engine::perforated_panel, you would see a large amount of output in the browser console similar to this:
WASM ---> calc_engine::perforated_panel.calculate()
WASM calc_engine::perforated_panel.calculate() End correction delta = 0.4056544608234196
WASM calc_engine::perforated_panel.calculate() End corrected panel thickness = 0.014056544608234196
WASM ---> calc_engine::perforated_panel.do_perforated_panel_calc()
WASM calc_engine::perforated_panel.do_perforated_panel_calc() Wave number = 1.1436219656127162
WASM calc_engine::perforated_panel.do_perforated_panel_calc() Angular frequency = 392.6990816987241
WASM calc_engine::perforated_panel.do_perforated_panel_calc() Characteristic impedance = 1787.9810938318747-1860.0733055079756i
WASM calc_engine::perforated_panel.do_perforated_panel_calc() Complex wave number = 6.010782579685053-5.340728208625488i
/// SNIP ///
WASM calc_engine::perforated_panel.do_perforated_panel_calc() No air gap z1 = 707.6641216476378-2491.209277813604i
WASM calc_engine::perforated_panel.do_perforated_panel_calc() No air gap z2 = 707.6641216476378-2436.6102350779433i
WASM calc_engine::perforated_panel.do_perforated_panel_calc() No air gap reflection = 0.871126497297388-0.2800858595510562i
WASM calc_engine::perforated_panel.do_perforated_panel_calc() No air gap absorption = 0.16
WASM <--- calc_engine::perforated_panel.do_perforated_panel_calc()
WASM <--- calc_engine::perforated_panel.calculate()Nothing so far
I realise that this app is an implementation of the theory of how acoustic absorption can be calculated. I can make no guarantees that reality will match up to the results of these calculations!
I have also taken every reasonable step to ensure that the calculations are accurate to the equations and methodology documented in the book "Acoustic Absorbers and Diffusers. Theory, Design and Practice" by Trevor Cox and Peter D'Antonio (First Edition). However, this book is now in its third edition and therefore certain calculations may have been modified or revised; consequently, the graphs plotted by this tool may vary from those plotted by a tool based on the most recent version of this book.
For best results, view this app use Brave, Google Chrome or Firefox Quantum browsers from a desktop machine.
This app has not been optimized for display on mobile devices.
I can provide a limited level of support; however, I cannot guarantee a prompt response...
Chris Whealy chris@whealy.com
This project is licensed under the Apache Software License, Version 2.0 except as noted otherwise in the LICENSE file.