“Sound of silence”: a secure indoor wireless ultrasonic communication system

According to a report from the World Health Organization, over 1.4 million mobile phone base stations exist worldwide today, and that number is significantly increasing with the emergence of new communication technologies. What about mobile phones? The International Telecommunication Union (ITU) claims that there are almost as many mobile phone subscriptions in the world as people. And that’s about 6 billion! The growth of wireless networks has enabled people to use personal devices anywhere at any time. Wireless communication nowadays has become a utility like water, electricity and gas. With the convenience and efficiency it brings, we also have to consider some issues with this technology, because it is so fundamental to our everyday lives. One critical and urgent issue is wireless availability. Like the laptops, mobile phones and tablets we know and love today, wireless devices communicate with each other using what are known as radio waves. Useful radio ...