public function generateNoise($duration = 1.0, $percent = 100)
    {
        $numChannels = $this->getNumChannels();
        $numSamples  = $this->getSampleRate() * $duration;
        $minAmp      = $this->getMinAmplitude();
        $maxAmp      = $this->getMaxAmplitude();
        $bitDepth    = $this->getBitsPerSample();

        for ($s = 0; $s < $numSamples; ++$s) {
            if ($bitDepth == 32) {
                $val = rand(-$percent * 10000, $percent * 10000) / 1000000;
            } else {
                $val = rand($minAmp, $maxAmp);
                $val = (int)($val * $percent / 100);
            }

            $this->_samples .= str_repeat(self::packSample($val, $bitDepth), $numChannels);
        }

        $this->setDataSize();  // implicit setSize(), setActualSize(), setNumBlocks()

        return $this;
    }