I had build an application using following source code (following the steps).

package com.decibal.level;

import androidx.appcompat.app.AppCompatActivity;

import android.media.MediaRecorder;
import android.os.Bundle;
import android.os.Handler;
import android.util.Log;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

    TextView mStatusView;
    MediaRecorder mediaRecorder;
    Thread runner;
    private static double mEMA = 0.0;
    static final private double EMA_FILTER = 0.6;

    final Runnable updater = new Runnable() {
        @Override
        public void run() {
            updateTv();
        };
    };
    final Handler handler = new Handler();
    
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mStatusView = findViewById(R.id.statusView);

        if (runner == null){
            runner = new Thread(){
                public void run(){
                    while(runner!=null){
                        try{
                            Thread.sleep(1000);
                            Log.d("Noise","Tock");
                        }catch (InterruptedException e){
                            e.printStackTrace();
                        }
                        handler.post(updater);
                    }
                }
            };
            runner.start();
            Log.d("Noise","start runner()");
        }
    }

    public void onResume()
    {
        super.onResume();
        startRecorder();
    }

    public void onPause()
    {
        super.onPause();
        stopRecorder();
    }

    public void startRecorder(){
        if (mediaRecorder == null){
            mediaRecorder = new MediaRecorder();
            mediaRecorder.setAudioSource(MediaRecorder.AudioSource.MIC);
            mediaRecorder.setOutputFormat(MediaRecorder.OutputFormat.THREE_GPP);
            mediaRecorder.setAudioEncoder(MediaRecorder.AudioEncoder.AMR_NB);
            mediaRecorder.setOutputFile("/dev/null");
            try{
                mediaRecorder.prepare();
            }catch (java.io.IOException ioe){
                android.util.Log.e("[Monkey]", "IOException: " + android.util.Log.getStackTraceString(ioe));
            }catch (java.lang.SecurityException e){
                android.util.Log.e("[Monkey]", "SecurityException: " + android.util.Log.getStackTraceString(e));
            }
            try{
                mediaRecorder.start();
            }catch (java.lang.SecurityException e){
                android.util.Log.e("[Monkey]", "SecurityException: " + android.util.Log.getStackTraceString(e));
            }

            //mEMA = 0.0;
        }
    }

    public void stopRecorder(){
        if (mediaRecorder!=null){
            mediaRecorder.stop();
            mediaRecorder.release();
            mediaRecorder = null;
        }
    }

    public void updateTv(){
        mStatusView.setText(Double.toString((getAmplitudeEMA()))+"dB");
    }
    public double soundDb(double ampl){
        return 20 * Math.log10(getAmplitudeEMA()/ampl);
    }

    public double getAmplitudeEMA() {
        double amp =  getAmplitude();
        mEMA = EMA_FILTER * amp + (1.0 - EMA_FILTER) * mEMA;
        return mEMA;
    }
    public double getAmplitude() {
        if (mediaRecorder != null)
            return  (mediaRecorder.getMaxAmplitude());
        else
            return 0;

    }
}

I am listing decibel meter result below :

59.28dB
72.12480000000001dB
81.13996800000001dB
71.78239488000001dB
66.2051831808dB
73.232829308928dB
64.03725268942848dB
66.16596043030856dB
66.50655366884938dB

When I speak

101.3610485870159dB
354.1377677739226dB

When i don't speak anymore

56.662042843827614dB
9.06592685501242dB
1.4505482968019872dB

When I was trying to check decibel meter using the sound meter.

The meter was at 60-71(When I speak also). The source code is correct but, the equation is wrong(that's what I think). When I don't speak than, I just can hear fan rotation sound (noise) which is not too much. Phone can detect the sound also. But, I worry when I stop speaking than decibel meter drops. How can I manage the decibel meter?