a code example for https://medium.com/scry-llc/applying-explosion-knock-back-1f394f7160e8

closeportal.cs

private void Close()
{
    ParticleSystem particleSystem = GetComponent<ParticleSystem>();
    particleSystem.Stop();
    Destroy(gameObject, particleSystem.main.startLifetime.constantMax);
    StartCoroutine(FadeTime(particleSystem.main.startLifetime.constantMax));

}

private System.Collections.IEnumerator FadeTime(float timeToFadeIt)
{
    float timeRemaining = timeToFadeIt;
    while (timeRemaining > 0)
    {
        audio.volume = timeRemaining / timeToFadeIt;
        timeRemaining -= Time.deltaTime;
        yield return null;
    }
}