A PDF version of this online manual is available here.
When AudioTool is started, you will see the main screen for the Real Time Analyser (RTA). The screen is divided into three regions:
The labels on the buttons indicate the current state of the control.
The bottom row of buttons, in pink, select major functions of AudioTool.
To hide the buttons from the screen simply tap in the top region of the display. To restore the buttons, tap anywhere in the display.
An option in the Menu allows to select/toggle between the front microphone, for those phones that have one, and the main microphone (usually at the base of the phone). The front microphone is the one that is typically on the front of the phone used for video recording. The setting is remembered between sessions. For phones that do not have a front microphone, then the menu option is still available, but all it will do is select the main microphone's AGC stream - which is probably not what you want :-)
The frequency resolution of AudioTool is governed by the sample rate (typically 44100 samples/sec) and sample size (typically 4096 samples) by the relation:
Frequency resolution = Sample Rate / Sample Size
So, the typical resolution is around 10Hz. You can improve the resolution via the Menu option called "Fine Freq.(slowest)" - this simply increases the sample size to 16384. The downside is that the frame rate will slow down (the display will not react as quickly as before). The advantage is that the frequency resolution will typically rise to around 2Hz: this will be very noticeable in the RTA "Full Res." mode, for example.
Selecting <CANCEL> will return you to the main display, selecting <CLEAR> will remove an existing loaded file from being displayed. Selecting one of the files results in it being shown on the main display thus:
The AudioTool .at files have the following format. There is a file header line, containing the date/time etc., followed by rows of measurement data. For each row:
Note: for versions prior to v6.0.3 you can calculate the third column from the second (call it X) using the formula:
Sqrt(dBcorrection + dBcalibration + dBvonHann + 10log(X*X))
Where dBcorrection is 20, dBvonHann is 4.2 and dBcalibration is shown in the file header. For v6.0.3 and above, remove the sqrt in the formula above.
The real time analyzer (RTA) has several modes, described below.
Frequency response is shown in the ten ISO 1 octave bands.
Frequency response is shown as a function of time: the spectrum at each moment is drawn as one row of a bitmap. The colour of each pixel indicates the intensity of the sound at that position in the spectrum. The most recent time is at the bottom of the display.
You can change the speed of the spectrogram (the rate at which new rows of the bitmap are drawn) between Fast and Slow using the option available in the Menu.
In this mode, AudioTool shows the results of the continuous Leq calculation. Leq is the average of the power sum of the measured instantaneous dB SPL levels over an interval of time. For more details about Leq see, for example, Leq at National Instruments . It is the "equivalent continuous sound level".
The Leq calculation will continue to run indefinitely, but it can be stopped by pressing the RTA "Running" button (top left) - if you press that button again, a new Leq calculation is started afresh.
Shown below is a screenshot of the Leq display. The broadband Leq, and the elapsed time for the calculation is shown, as well as the target duration (in this case 1 minute). (The duration can be adjusted using the Menu option "Set Leq Duration".)
The table shows the 1 octave Leq measurements: the current calculation is shown in the "Now" column. The other columns show prior results ... so the T-1 column is the result from the calculation that last finished, the T-2 the one before that, and so on.
(Normally, Leq measurements are made using the A Weight filter.)
AudioTool stores up to 5,000 Leq measurements internally every time the app is started. The measurements can be saved to an AudioTool .leq file - use the "Store" button for this purpose. The stored file can be opened in a text editor, and imported into e.g. Excel. Here is an extract from an Leq file:
AudioTool Leq File recorded at Sat Jun 15 11:42:18 PDT 2013
MilliSeconds now: 1371321738067 Columns: Time Start, Time Finish, Duration(s), LEQ, LEQ octave1, LEQ octave2 ... 40 LEQ measurements in file
2013-06-15T11:41:13 2013-06-15T11:42:13 60 74.4 47.4 50.9 60.7 65.8 66.6 65.1 64.3 65.9 68.8 58.1
2013-06-15T11:40:13 2013-06-15T11:41:13 60 69.0 43.9 47.0 58.9 63.7 61.8 59.3 53.2 54.1 62.5 51.2
2013-06-15T11:39:13 2013-06-15T11:40:13 60 65.8 43.3 46.4 57.8 60.0 58.7 55.8 49.5 50.4 58.4 51.2
2013-06-15T11:38:13 2013-06-15T11:39:13 60 66.1 43.1 47.0 57.7 62.3 58.4 55.7 49.4 49.2 55.0 51.1
2013-06-15T11:37:13 2013-06-15T11:38:13 60 69.3 44.0 53.5 58.1 63.3 61.0 59.0 54.2 55.6 64.2 52.1
2013-06-15T11:36:13 2013-06-15T11:37:13 60 69.1 43.2 50.3 58.8 63.5 59.4 57.5 51.1 52.6 64.8 52.2
2013-06-15T11:35:13 2013-06-15T11:36:13 60 67.9 44.9 50.2 56.8 62.3 59.2 59.1 53.2 54.7 61.9 51.8
2013-06-15T11:34:13 2013-06-15T11:35:13 60 69.0 45.9 51.0 58.9 64.1 60.1 60.5 55.3 54.9 61.4 52.7
In the RTA and Spectrogram modes, the user may choose to have AudioTool use a logarithmic or a linear scale for the frequency (Hz) axis. The choice is made via the menu, shown below:
Choosing "Set Lin Hz" menu option will change to the linear scale.
The frequency limits for the Hz scale can be adjusted, using the menu option in the "More" menu. In the example screenshot above, the user has set limits of 20Hz to 10kHz. Here is what the linear scale looks like, for the same limits:
Note that the Hz scale limits, and the dB scale limits, are saved by AudioTool when the user terminates the app: they are restored when the user restarts AudioTool.
You can adjust the dB and frequency markings in the RTA and the dB marks in the Chart Recorder in the following way:
(Scroll and Pinch of the scales in this way is disabled when the Cursors are visible.)
AudioTool will store and remember the calibration number(s) you enter, and will also remember whether you are using the single value or octave based method.
For the 1/3 octave based method you will see a screen similar to that shown below:
Each of the 1/3 octave band's current calibration values (deciBels correction, positive or negative) is shown in a white disc - when the you first calibrate, all values will be zero. The red disc shows the band currently being adjusted. If you double tap on the left hand side of the screen, the selected disc moves to the left. If you double tap on the right side of the screen, it moves to the right.
To adjust the selected band, place your finger anywhere on the screen and drag the disc up or down, until you see the correction value you want. Take your finger off the screen when you have adjusted the disc, and then double tap to select the next band, and so on.
Your aim is probably to move the discs so as to obtain a flat response, and match the overall SPL to that of an external meter, or to correct for any odd peaks or troughs your Android phone may exhibit in its frequency response.
When satisfied with the results, choose "Finish Calibration" from the Menu, and confirm whether you want to save the values. These values will be retained on your phone and will be loaded every time AudioTool starts.
By default, the dB calibration values in each band are all 0dB. You can apply a global offset to these values using the "Set Cal. Offset" Menu option (which appears when you are using the 1/3 Octave calibration method.)
The "Set Cal. Offset" option will bring up a dialog where you can enter the desired value:
Now, when calibrating, the dB levels in all bands will be offset by the amount you entered:
You can adjust the levels in each band in the way described above. Remember to select "Finish Calibration" from the menu when you are done!
(The observant reader will notice that the current global offset value is shown in the Mic display, as O=5.0)
AudioTool supports loading a calibration file for the Dayton Audio iMM-6 microphone. These files have names like "99-0101.txt" when downloaded from Dayton's website. For use in AudioTool, you will simply need to rename the file so that it has the .cal filetype (e.g. "99-0101.cal") and save it in the AudioTool directory on your Android phone. Then, in AudioTool, make sure that in the Menu you have the 1/3 octave calibration method selected. You can then select the "Load Cal" option from the Menu, and choose the Dayton file from the list. The calibration data will be loaded, power summed to the 1/3 octave bins used by AudioTool, and saved in your Preferences - there is no further need to load the file whenever you start AudioTool, unless you change the calibration method or alter the calibration values manually.
Please note that the "AudioTool" directory is created the first time you Store an RTA, Chart, RT60 file or store a 1/3 octave calibration. If you haven't stored one of these before, please do so - then the AudioTool directory should be easily found on the phone's storage, and the Dayton file can be placed in it.
Rex has provided a handy Excel tool and instructions for how to calibrate your Android device.
The time history of SPL readings is shown: the chart recorder "pen" at the right hand side of the screen draws the latest value. The axis is labelled in seconds in the past. At most 10,000 SPL readings are stored, corresponding to a few minutes of history (this value varies from one Android device to another). The chart data can be stored using the Store button, if so desired. (Note the use of the cursors in the example below.) The grey text at the bottom left of the chart indicates the current length of the buffer, in seconds.
You can measure room reverberation times in RT60 mode, selected using the RT60 button.
If the Signal Generator is Off when RT60 mode is started, the system waits for a loud impulse signal, such as a clap to trigger the measurement. This sound will energise the room, and how quickly the sound pressure falls off with time will be measured. The longer the sound takes to die away, the more reverberant is the room.
After the clap is detected, the measurement starts. Since the RT60/reverberation time for a room will be different depending on frequency, AudioTool makes a wideband (all frequencies) measurement, as well as measurements in each of the ISO octave bins. To do this, the data are passed through a bank of IIR octave filters centred on the desired frequencies. The results are displayed in tabular form, and may be exported to a file using the "Store" button.
RT60 measurement can also be made using the signals (e.g. White Noise) from the generator to energise the room. If the Signal Generator is On when RT60 mode is started, then the Trigger button:
can be pressed when the RT60 measurement should be made. When the Trigger button is pressed, the output from the Signal Generator is stopped, and the RT60 measurement of the decaying sound intensity is made.
Technical notes: the RT60 uses a Schroeder integral to work backwards from the noisefloor to the detected peak intensity. The Schroeder integral is shown as a red trace on the RT60 display. The RT Early number is the time for the intensity to drop from the peak to 10dB below the peak, multiplied by 6. The 15-20 number is the time for the intensity to drop from 5dB below the peak to 20dB below the peak, multiplied by 4.
The screen shot below shows an example of an RT60 measurement.
The RT60 results table can be hidden or shown by using the Menu option:
Hiding the RT60 table allows closer inspection of the RT60 curve data:
For the periodic signals, like Sine, Square etc., the frequency of the wave may be set using two methods:
To turn the generator output on and off, use the large button so labelled.
Technical note: The precision with which AudioTool can generate a given frequency depends on the internal samplerate being used and the sample buffer length. Typically, you can expect AudioTool to generate a frequency within 10Hz or less of the value you specify.
When the Sweep button is selected, AudioTool will generate swept sine waves.
To adjust the start and stop frequencies, and the duration of the sweep, use the "Set Sweep Frequency Limits" button to bring up the dialog box shown below:
In the example above, the settings will cause a sine wave starting at 100Hz to be swept up in frequency to 2000Hz, over a duration of 10 seconds.
If the generator is in Sweep mode, then the current sweep frequency is stored for every value of SPL measured by AudioTool. The current sweep frequency is also displayed on the main screen. In addition, if Chart Recorder mode is selected, and the chart saved using the Store button, then the SPL readings coupled with the generator frequency will be saved in the file.
The chart file may then be read in to e.g. Excel and a frequency response curve created by plotting the SPL readings versus the frequency values. Here is an example:
Each row of the Chart Recorder file contains the time, the measured dB SPL, the detected peak frequency, and optionally the sweep generator frequency at that time if sweeps were running at the time.
There are two cursors that can be enabled via the main menu:
In 1/1 Octave mode you can display the Noise Curve family, via the main menu, and AudioTool will calculate the prevailing NC value.here.