increase speed every spawn wave

Hello, how to implement an wave system? Like in every 30 seconds the speed of the falling objects will increase in certain amount. The spawning works fine but i find some inconsistencies regarding the increasing speed every 30secs.

here is the my coroutine code:

 IEnumerator objectGenerator(){
         yield return new WaitForSeconds (delay);
         if (active) {
             var sr = GetComponent<Renderer> ();
             GameObject clone;
 
             var x1 = transform.position.x - sr.bounds.size.x / 2;
             var x2 = transform.position.x + sr.bounds.size.x / 2;
 
             var spawnPoint = new Vector2 (Random.Range (x1, x2), transform.position.y);
 
             var randNum = Random.Range (0, 99);
 
             if (randNum >= 89) {
                 clone =    Instantiate (lifePrefab, spawnPoint, Quaternion.identity) as GameObject;
                 Rigidbody2D rbody2d = clone.GetComponent<Rigidbody2D> ();
                 rbody2d.velocity = fallingSpeed;
             } else {
                 clone = Instantiate (prefabs [Random.Range (0, prefabs.Length)], spawnPoint, Quaternion.identity) as GameObject;
                 Rigidbody2D rbody2d = clone.GetComponent<Rigidbody2D> ();
                 rbody2d.velocity = fallingSpeed;
             }
 
             ResetDelay ();
         }
         StartCoroutine (objectGenerator ());
     }

and this is my increase speed function..

 void IncreaseSpeedPerMins () {
         totalTime += Time.deltaTime;
 
         if (totalTime >= timeInterval) {
             foreach (Spawner value in spawnObj) {
                 value.fallingSpeed += velCounter;
             }
             timeInterval += intervalMultiplier;
         }
 
     }