update-vu-meter.js

var updateVuMeter = function (peak_data, $elements, old_leds_total) {
    var leds_total = Math.ceil((peak_data / 20) * 1.7); // between 1 and 5
    var is_going_up = leds_total > old_leds_total;
    var start_led = is_going_up ? old_leds_total : leds_total; // default is 0
    var end_led = is_going_up ? leds_total : old_leds_total;

    // lights on or off the leds
    for (var i = start_led; i <= end_led; i++) {
        $elements.eq(i)[is_going_up ? 'addClass' : 'removeClass']('active');
    }

    // save old value
    return leds_total;
};

// et dans ma fonction qui tourne en meme temps que le son j'appelle comme ça :
var peak_data_left = parseInt(this.peakData.left * 100);
var peak_data_right = parseInt(this.peakData.right * 100);
save_peak_data_left = updateVuMeter(peak_data_left, $left_leds, save_peak_data_left);
save_peak_data_right = updateVuMeter(peak_data_right, $right_leds, save_peak_data_right);