How to let my program detect and change frequency?

How do I let my music visualizer detect the frequency of the song when its high it display a smiley face(which i already did) and a frown face when the song is slow with low frequency. I have already able to draw the faces but dont know how to make my code detect the beat and change

import ddf.minim.*;
import ddf.minim.analysis.*;

Minim minim;
AudioPlayer groove;
AudioMetaData meta;
BeatDetect beat;
float yoff = 0.0;
float slowChange;
int screen=0;

void setup()
{
  selectInput("Select a file to process:", "fileSelected");
  size(640, 360, P3D);
  ellipseMode(CENTER);
}

void fileSelected(File selection) {
  if (selection == null) {
    println("Window was closed or the user hit cancel.");
    screen = 1;
  } else {
    println("User selected " + selection.getAbsolutePath());
    minim = new Minim(this);
    groove = minim.loadFile(selection.getAbsolutePath(), 2048);
    groove.play();
    beat = new BeatDetect();
    screen = 2;
  }
}

void draw()
{
  if (screen ==2)
  {
      background(#7A9EB9);
      fill(#FFA00F);
      ellipse(320, 180, 300, 300);
      beat.detect(groove.mix);
      stroke(#40FAFF);
      strokeWeight(2);

        if(beat.isOnset()==true)
        {
           fill(#FF0808);
           ellipse(270, 120, 60, 60);

          fill(#2CE823);
          ellipse(370, 120, 90, 90);

        }
        else
        {
           fill(#3B0D37);
            ellipse(270, 120, 60, 60);

          fill(#2754CB);
          ellipse(370, 120, 90, 90);

        }

        if(keyPressed)
        {
          //sad
            noFill();
           for(int i = 0; i < groove.bufferSize() - 2; i++){
             slowChange = lerp(slowChange, groove.left.get(0), 0.2);
             //Left right, up down, (how high)
             arc(310, 320 + (30 * slowChange), 100, 300, PI+QUARTER_PI, TWO_PI, OPEN);
           }
        }
        else //happy
       {
             noFill();
            for(int i = 0; i < groove.bufferSize() - 2; i++){
                 slowChange = lerp(slowChange, groove.left.get(0), 0.2);
               arc(320, 180 + (50 * slowChange), 300, 200, 0, PI, OPEN);
            }
       }



    fill(#0220D8);
    beginShape();
     float xoff = 0;
      for (float x = 0; x <= width; x += 10) {
        float y = map(noise(xoff, yoff), 10, 1, 0, 300);
        vertex(x, y);
        xoff += 0.10;
      }
      yoff += 0.10;
      vertex(width, height);
      vertex(0, height);
      endShape(CLOSE);
    }
    else
    {
      rect(10,50,60,20);
    }
  }