Louder piezo suggestion?

I’m working on a small project with an Arduino Pro Mini that will monitor and drive a piezo buzzer if a door is left open.
I have a few small piezo elements but they are not loud enough to be heard from a different room. Is it a matter of finding a larger sized piezo? I pulsed the pin using LED PWM code I had played with earlier:

My piezo is connected to pin #11 for pwm so I just changed one of the LEDs to 11

#define RED_LED 3 //RED LED pin
#define GREEN_LED 5 //GREEN LED pin

int i = 0; //count up and down

void setup(){
  pinMode(RED_LED, OUTPUT); //LED is an output
  pinMode(GREEN_LED, OUTPUT);
  Serial.begin(9600);
}

void loop(){
  for (i=0;i < 255; i++){
    analogWrite(RED_LED,i); //set the RED LED brightness
    analogWrite(GREEN_LED,255-i); //set the GREEN LED brightness to be opposite of red
    delay(1); //wait 1ms otherwise the change is instant
    /*   Serial.print("RED LED: ");
     Serial.println(i);
     Serial.print("GREEN LED: ");
     Serial.println(255-i); */
  }

  for (i=255;i>0;i--){ //now go backwards
    analogWrite(RED_LED,i); 
    analogWrite(GREEN_LED,255-i);
    /*   Serial.print("RED LED: ");
     Serial.println(i);
     Serial.print("GREEN LED: ");
     Serial.println(255-i); */
    delay(1);
  }
}

check the newTone library from teckel - IIRC it has more power!

Another way to increase the volume is to mount it on a rigid board or in a box with a hole just like a speaker case.

newTone library did help a bit but I still need something a bit louder.

Essentially the problem I am trying to solve is that on occasion one of the doors doesn't get shut well enough and over time it actually opens as if on it's own. The alarm (if door is open for longer than 5 minutes) should be loud enough to be heard over a TV set and through a wall or two. The little piezo I have is not annoying enough to buzz through a wall and over a TV set. So I need something with more oomph but not loud enough to leave your ears ringing like a burglar alarm.

I am thinking something like this piezo should be loud enough without being deafening?

Two words. Mallory Sonalert.