public function insertSilence($duration = 1.0)
    {
        $numSamples  = (int)($this->getSampleRate() * abs($duration));
        $numChannels = $this->getNumChannels();

        $data = str_repeat(self::packSample($this->getZeroAmplitude(), $this->getBitsPerSample()), $numSamples * $numChannels);
        if ($duration >= 0) {
            $this->_samples .= $data;
        } else {
            $this->_samples = $data . $this->_samples;
        }

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

        return $this;
    }