full code

#include <LiquidCrystal.h>
#include <dht.h>

/*---- DHT -----*/
dht DHT;
const int DHT11_PIN = 12;

/*--------Display----------*/
// initialize the library by associating any needed LCD interface pin
// with the arduino pin number it is connected to
const int rs = 5, en = 6, d4 = 7, d5 = 8, d6 = 9, d7 = 10;
LiquidCrystal lcd(rs, en, d4, d5, d6, d7);

/*----Sound Sensor-----*/
#define SOUNDANALOG_PIN A0
const int SOUNDDIGITAL_PIN = 11;
boolean soundDigitalVal = 0;
int soundAnalogVal = 0;

/*-------Dust-------*/
const int DUSTANALOG_PIN = A5;

unsigned int samplingTime = 280;
unsigned int deltaTime = 40;
unsigned int sleepTime = 9680;

float dustVoMeasured = 0;
float calcVoltage = 0;
float dustDensity = 0;

void setup(){
  Serial.begin (9600);

  pinMode(SOUNDANALOG_PIN, INPUT);
  pinMode(SOUNDDIGITAL_PIN, INPUT);

  //Setup of the LCD's number of column and rows
  lcd.begin(20, 4);
  //Displaying welcome to the LCD
  lcd.print("Welcome!");
}

void loop (){
  lcd.clear();
  //delayMicroseconds(samplingTime);

  /***-------- DHT11 Sensor -----------***/
  int chk = DHT.read11(DHT11_PIN);
  lcd.setCursor(0,0);
  lcd.print("Temp: " + String(DHT.temperature) + ((char)223) + "C");
  lcd.setCursor(0,1);
  lcd.print("Humidity: " + String(DHT.humidity)+"%");
  Serial.println("Temp: " + String(DHT.temperature) + ((char)223) + "C");
  Serial.println("Humidity: " + String(DHT.humidity)+"%");
  /*-----------------------------------*/


  /***-------- Dust Sensor ---------***/
  dustVoMeasured = analogRead(DUSTANALOG_PIN);
  calcVoltage = dustVoMeasured*(5.0/1024);
  dustDensity = 0.17*calcVoltage-0.1;
  if ( dustDensity < 0)
  {
    dustDensity = 0.00;
  }
  lcd.setCursor(0,2);
  lcd.print("Dust: "+String(dustDensity)+"ng/m3");
  Serial.println("Dust: "+String(dustDensity)+"ng/m3");
  /*-----------------------------------*/


  /***---------- Sound Sensor ----------***/
  soundAnalogVal = analogRead(SOUNDANALOG_PIN);
  soundDigitalVal = digitalRead(SOUNDDIGITAL_PIN);

  double soundAnalogValDB = 20 * log10(soundAnalogVal);
  //double soundAnalogValDB = (soundAnalogVal + 83.2073 ) / 11.003;

  //Serial.println (analogVal);
  //When the sensor detects a signal above the threshold value, LED flashes
  if (soundDigitalVal==HIGH) {
    lcd.setCursor(0,3);//set the cursor at first row
    lcd.print("Sound: "+String(soundAnalogValDB)+"db" + " (High)");//Sound Intensity level
    Serial.println("Sound: "+String(soundAnalogValDB)+"db" + " (High)");
  }
  else {
    lcd.setCursor(0,3);//set the cursor at first row
    lcd.print("Sound: "+String(soundAnalogValDB)+"db" + " (Low)");//Sound Intensity level
    Serial.println ("Sound: "+String(soundAnalogValDB)+"db" + " (Low)");
  }
  /*-----------------------------------*/

  Serial.println();
  delay(1000);
}