int SPEAKERPIN = PIN_PA02;
const int BUFFER_SIZE = 4;
char buf[BUFFER_SIZE];
int bufIterator = 0;
int frequency = 100; // beep every x ms
int distanceBuf[5];
long prevBeep = 0; 

void setup() {
  delay(2000);
  pinMode(SPEAKERPIN, OUTPUT);
  delay(500);
  tone(SPEAKERPIN, 349, 200);
  delay(200);
  tone(SPEAKERPIN, 392, 200);
  delay(200);
  tone(SPEAKERPIN, 523, 200);
  delay(500);
  Serial.begin(9600);
  Serial.write("Serial ready..");
  Serial1.begin(9600);
  Serial1.write("Serial1 ready..");
}

void loop() {
  delay(20);
  if (Serial1.available() > 0) {
    Serial1.readBytesUntil('\n', buf, BUFFER_SIZE);
    int distance = atoi(buf);
    clearBuf();    
    distanceBuf[bufIterator] = distance;
    bufIterator = (bufIterator + 1) % 5;
    determineFreq();
    Serial.println(frequency);
  }
  determineBeep();    
}

void determineFreq() {
  int result = 0;
  for (int i = 0; i < 5; i++) { 
    result += distanceBuf[i];
  }

  if(result > 0) {
    frequency = (result / 5);
  }
}

void clearBuf() {
  for (int i = 0; i < BUFFER_SIZE; i++) {
    buf[i] = 0;
  }
}

void determineBeep() { 
  long cFrequency = frequency * 10;
  if(millis() - prevBeep > cFrequency) { 
    tone(SPEAKERPIN, 700, 75);
    prevBeep = millis();    
  }  
}