void ShieldUp(Powerup powerup)
 {
     // Only display shield image if there isn't one active already (prevents duplicates)
     if (!Powerup.GetShield())
     {
         shieldImage = Instantiate(shieldPrefab, transform.position, Quaternion.identity) as GameObject;
     }
     Powerup.SetShield(true);
     AudioSource.PlayClipAtPoint(shieldUp, transform.position);
     powerup.Collected();
 }
 void SpeedBoostOn(Powerup powerup)
 {
     // Player cannot stack speed boosts
     if (Powerup.GetSpeedBoost() != true)
     {
         Powerup.SetSpeedBoost(true);
         globalTime = Time.time;
         speed     *= 2;
         fireRate  /= 2;
         AudioSource.PlayClipAtPoint(speedBoostOn, transform.position);
     }
     powerup.Collected();
 }