These artificial echoes have the same structure as natural echoes but are placed in such a way that devices can selectively extract the artificial echoes to determine the watermark.

Advantages

• No Bluetooth, WiFi or other connectivity required
• Battery-efficient
• Run on old and/or slow hardware
• No cost to increasing the user base
• Less sensitive to frequency and timing shifts
• Highly reliable with a low false positive (false trigger) rate

This method looks at hiding the sound outside the human auditory spectrum. humans tend not to perceive sounds above around 16kHz, and this threshold further lowers as they age. Therefore, the audio spectrum beyond 16kHz is sometimes called the ultrasound range.

Advantages

• It’s simple to implement, since we don’t have to worry about impacting the audio quality

Disadvantages

• The watermark can easily be removed without impacting the content
• Ineffectual for forensic purposes
• It can often be removed in compression/ (compression algorithms), for instance uploading to Social Media video platforms and even TV broadcasting

The idea for this technique is to distribute the watermark over the whole audio spectrum. This implies that the watermark becomes intertwined with the actual content, rendering it hard to remove the watermark. And since the watermark exists in the same frequency range as the content, it won't be removed by compression algorithms. A term often used to describe this technique is ‘white noise’.

Advantages

• Can be easily calculated and added

Disadvantages

• Doesn’t work when the devise is in movement
• Adds an often-audible “hum” to the audio
• Detection can be slow and battery-consuming
• High false-positive rate (due to doppler effects).