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POWhat does Android's getMaxAmplitude() function for the MediaRecorder actually give me?
 

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  1. POWhat does Android's getMaxAmplitude() function for the MediaRecorder actually give me?
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    <p>The Android MediaRecorder has a function <pre>.getMaxAmplitude();</pre> which, as the <a href="http://developer.android.com/reference/android/media/MediaRecorder.html#getMaxAmplitude%28%29" rel="noreferrer">API</a> tells me, <i>"Returns the maximum absolute amplitude that was sampled since the last call to this method."</i> but I can't find what amplitude this is? Is it in pascal or watts?</p> <p>I have found on several pages on the web that you can calculate a value closely corelated to decibels using (as suggested <a href="https://stackoverflow.com/questions/7189275/how-to-calculate-microphone-audio-input-power-in-decibell-unit">here</a>). </p> <pre><code>double db = (20 * Math.log10(amplitude / REFERENCE)); </code></pre> <p>which would let me assume that the returned value is in some linear scale (probably something like milipascal...) </p> <p>REFERENCE=0.1 (I am aware that this should be something like 2*10^(-5) Pascal ((20 uPascal)), but that returns strange values... 0.1 strangely works better.)</p> <p>Right now I measure the MaxAmplitude() using the <pre>getMaxAmplitude()</pre> and put this into the variable <i>amplitude</i>.</p> <p>This is the method:</p> <pre><code>public double getNoiseLevel() { //Log.d("SPLService", "getNoiseLevel() "); int x = mRecorder.getMaxAmplitude(); double x2 = x; Log.d("SPLService", "x="+x); double db = (20 * Math.log10(x2 / REFERENCE)); //Log.d("SPLService", "db="+db); if(db&gt;0) { return db; } else { return 0; } } </code></pre> <p>This is done 5 times in half a second, which gets kind of an average</p> <pre><code>for(int i=0; i&lt;5; i++) { try { Thread.sleep(100); } catch (InterruptedException e) { e.printStackTrace(); return 0; } level = level+getNoiseLevel(); if(level&gt;0) { counter++; } } level=level/counter; Log.d(LOG_TAG, "level="+level); </code></pre> <p>I get something that kinda looks like decibel but I'm not sure its actualy decibel at all...</p> <p>So, could anyone help me on this? It seems very odd that the API does not specefy at all what is returned...</p>
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    1. USLukas Ruge
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      1. POWhat does Android's getMaxAmplitude() function for the MediaRecorder actually give me?
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    1. COThis worked, thanks for explainig what getMaxAmplitude does. But I'm not sure if these are accurate dB values. When I test in some cases and try to compare values to this: http://www.newton.dep.anl.gov/askasci/phy99/phy99405.htm , there is a missing 30dB. Do you have any idea why?
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      1. POWhat does Android's getMaxAmplitude() function for the MediaRecorder actually give me?
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    2. COThere may be several reasons: 1. This algorithm uses getMaxAmplitude as a basis for calculation which means all the less loud events in the period are ignored. This leads to results that can be above the actual dB level (and usually in natural environments they are) 2. Microphones of phones are diferent. Some are more sensibel then others. This algorithm does not take this into account, doing the same calculation on any phone. Due to these diferences some phones may give substantially higher or lower values.
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      1. POWhat does Android's getMaxAmplitude() function for the MediaRecorder actually give me?
      1. USLukas Ruge
    3. COIn addition, due to the limiting factor (the Value only gets up to 32767), very loud noises are not detected accuratly. Usually the cutoff will be at around 100 dB.
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