Only react to low frequencies

programming flash
1 

// SpectrumAnalyzer.as
package {
    import flash.display.*;
    import flash.media.*;
    import flash.net.*;
    import flash.utils.ByteArray;
    import flash.events.*;
    class SpectrumAnalyzer extends Sprite {
        // Settings
        private var lineThickness:Number = 2;
        private var lineColor:Number = 0x993300;
        private var circleSize:Number = 75;
        private var scaleOnBeat:Number = 1.1; // 120%
        private var reactToBeat:Number = 30;
        //
        private var music:Sound = new Sound;
        private var ba:ByteArray = new ByteArray();
        private var __width:uint;
        private var __height:uint;
        function SpectrumAnalyzer(mp3:String, _width:uint, _height:uint) {
            __width = _width;
            __height = _height;
            x = __width/2;
            y = __height/2
            music.load(new URLRequest(mp3));
            music.play(0, 999);
            addEventListener(Event.ENTER_FRAME, processSound);
        }
        private function processSound(ev:Event):void {
            SoundMixer.computeSpectrum(ba, true, 0);
            graphics.clear();
            graphics.moveTo(0, -circleSize);
            graphics.lineStyle(lineThickness, lineColor);
            var vol:Number = 0;
            for (var i:uint = 0; i <512; i++) {
                var lev:Number = ba.readFloat();
                vol += lev;
                var a:uint = i;
        var j:Number = i;
                if (i <256) a += 256;
                if (i == 256) graphics.moveTo(0, -circleSize);
                graphics.lineTo(-Math.sin(i/256*Math.PI)*circleSize*(lev+1), Math.cos(a/256*Math.PI)*circleSize*(lev+1));
            
            }
    if (vol> reactToBeat) {
            scaleX = scaleY = scaleOnBeat;
        }
    else {
                scaleX = scaleY = 1;
            }
            
        }
    }
}

I am thinking that

private var reactToBeat:Number = 30;

is completely dependant on song.mp3. If the song was processed at a different volume level, then you may need to lower the value. (e.g. 30 would be lowered to 19)

I ultimately want to have this work with any song, because I will be making a playlist and have song.mp3 replaced with the var.

To start, however, I would at least like to get this spectrum to only respond to low frequencies.

I think for start, I would need to change:

SoundMixer.computeSpectrum(ba, true, 0);

to

SoundMixer.computeSpectrum(ba, false, 0);

so that it reads it as a frenquency spectrum rather than a complete wav file (according to the as3 ref doc)
With that, the first 256 values should be low freqs and 257 to 512 would be high freqs.

How can I utilize that to only scaleOnBeat to the low frequencies?
Also, I didn’t know if it was best to use a select case for a certain range of values.

Thanks