int const buzzer = 11;

int freq;

void readInput(){
  while(!Serial.available());
  if(Serial.available()){
    String input = Serial.readString();
    freq = input.toInt();
  }
}

void setup() {
  Serial.begin(9600);
  pinMode(buzzer, OUTPUT);  
}

void loop() {
  if(Serial.available()){
    readInput();
    tone(buzzer, freq);
  }
  Serial.print("Current frequency : ");
  Serial.println(freq);
}