Remote GSM Based Control Of Audio Hotspot Devices – A Boon Or Bane

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GSM-based remote-control technology is versatile, used in audio hotspot devices and air pollution monitors. However, its misuse, such as with malicious audio blasters, raises concerns. This discussion explores the capabilities and ethical dilemmas of audio hotspot technology.

Imagine a scenario where someone is peacefully sleeping in a hotel room or unfamiliar place, only to be repeatedly disturbed by loud noises throughout the night, like the sound of a lorry unloading or an ambulance horn. These disruptions leave them feeling confused and drowsy the next morning. Initially attributing the noises to nearby traffic, they discover they were the only one hearing them. In reality, they were exposed to an audio hotspot in the room, wirelessly transmitted via a GSM modem to the hotspot hardware.     

Remote control via a GSM modem is  a common student project nowadays. EFY has also published articles for home control remotely using a  SIM908 and Arduino board.

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There have been many useful applications so far installed with such modem control in several places. noteworthy is the air pollution monitor for sensing fine particles in the air in several crowded cities. This work has been done by Prof. Tripathi and Dr Mariselvam from IIT, Kanpur . They have set up hundreds of such remote GSM-based air pollution sensing in Chennai, Delhi and several other cities.

It is shown in Fig. 1 as installed in a college premises near a crowded main street in Chennai. It actually houses a power chargeable battery, a modem, a processor and the laser-based unit for particle sensing  Laser light is scattered by fine particles of 2.5 microns to 10 microns at varying angles to the ray and using photo sensors at two angles, it is able to sense the air around the laser ray and evaluate the PM2.5, PM10 values.  This data, along with temperature and humidity, are sent periodically all 24 hours, and the units have been working for more than a year now. The data is collected from a central monitoring computer 

Fig. 1 A useful remote GSM controlled Particle pollution monitor module installed in a building.

There have been publications on medical monitoring applications from remote medical counsel for patients.

While such useful applications are endless, there are some malefic applications that have slowly grown in recent years. Of these the audio blaster using the audio hotspot technique is one which has caused much concern and disturbance to the authors. Like the early date pure internet data, which has today become plagued with attacks from several virus programs,  every new useful development also is subject to such mischievous attempts with sadistic disturbing and stealthy ends.

What is an Audio Hotspot?

In the realm of audio technology, directional sound systems are quite literally making waves. Companies like Focusonics and Holosonics are pioneering this space, offering speakers that deliver audio in a precise, fixed direction, much like a beam of light. They can be used to send targeted audio messages to an individual in a reasonably crowded room. Imagine you are in a museum, engrossed in an artwork. A directional sound system could provide an immersive audio guide, narrating the history and significance of the piece, without disturbing other visitors.

The technology opens a world of possibilities for enhancing individual experiences in public spaces. However, every coin has two sides. These speakers are compact enough to be concealed in a mini closet or drawer. The same technology that can enrich experiences can also be weaponised for nefarious purposes. Consider a candidate in an interview room or a student in an examination hall. Targeted communication could be sent to them, compromising the integrity of the entire process.

Fig.2 Shows how they install audio hotspot in a room along with other remote control gadgets

The so-called decorative wood sheet projection on the wall houses the GSM modem and other electronic devices. A magnetometer brought near the sheet shows intense deflection, indicating the hidden electronics inside.

Audio hotspot modulates the audio signal over an ultrasound (US) frequency (of 25KHz to 40KHz). The signal is sent through an amplifier of high gain to a loudspeaker using piezoelectric elements, which can vibrate at US frequency. This outputs a sound pressure wavefront, which is narrow compared to the audio loudspeaker. Any person in the wavefront cannot ordinarily hear the US frequency because the human ear is insensitive to it. But as the beam traverses and strikes a target, the wave gets demodulated, and sound is heard. They use sounds of large intensity in these applications when they want to disturb a person

The nonlinear interaction mixes ultrasonic tones in the air to produce sum and difference frequencies. A DSB (double-sideband) amplitude-modulation scheme with an appropriately large baseband DC offset to produce the demodulating tone superimposed on the modulated audio spectrum is one way to generate the signal that encodes the desired baseband audio spectrum. This technique suffers from extremely heavy distortion as not only the demodulating tone interferes, but also all other frequencies present interfere with one another. The modulated spectrum is convolved with itself, doubling its bandwidth by the length property of the convolution. The baseband distortion in the bandwidth of the original audio spectrum is inversely proportional to the magnitude of the DC offset (demodulation tone) superimposed on the signal. A larger tone results in less distortion. F. Joseph Pompei of MIT developed technology he calls the ‘audio spotlight’ and made it commercially available in 2000 through his company Holosonics, which, according to their website, claims to have sold ‘thousands’ of their ‘audio spotlight’ systems. Disney was among the first major corporations to adopt it for use at the Epcot Center, and many other application examples are shown on the Holosonics website.]A sound signal can be aimed so that only a particular user-by, or somebody very close, can hear it. In commercial departmental stores, a transducer can be made to project a narrow beam of ultrasound modulated ultrasound that is powerful enough, at 100 to 110 dB SPL, to substantially change the speed of sound in the air that it passes through. The air within the beam behaves nonlinearly. It extracts the modulation signal from the ultrasound, resulting in sound that can be heard only along the path of the beam, or that appears to radiate from any surface that the beam strikes. (Fig3).

Fig.3. The two compartments in this department store have announcements regarding the items in these compartments through hotspot audio.

The trolley man can hear only the announcement particular to the compartment he is near while the other walker hears what pertains to the products, he finds near him.

Directional speakers in museums, exhibitions and art galleries can offer a huge upgrade to your visitor experience. In a quiet exhibition space, you don’t want to ruin the ambience of the environment. Visitors want to concentrate on the displays without distractions, like loudspeakers leaking sound from other exhibitions.

This technology allows a beam of sound to be projected over a long distance to be heard only in a small, well-defined area; for a listener outside the beam, the sound decreases substantially. This effect cannot be achieved with conventional loudspeakers, because sound at audible frequencies cannot be focused into such a narrow beam. In such applications, it can target sound to a single person without the peripheral sound and related noise of a loudspeaker. There are some limitations to this approach. Anything that interrupts the beam will prevent the ultrasound from propagating, like interrupting a spotlight’s beam. For this reason, most systems are mounted overhead, like lighting.

 Volume: 23, Issue: 15, 01 August 2023) IEEE Sensors journal describes the circuit improvements using a sensor array. (fig 4)  This enables a clearer sound than what most common devices provide. For the purpose of disturbance,  a single US transducer would suffice. They generate US frequency easily using transistor circuits with an oscillator coil and amplify the signal sufficiently amplify the signal sufficiently, and then drive a 300V  US transducer using a power amplifier IC. Are some people suffering as a result of increasing mass exposure of the public to ultrasound in air?

Fig. 4. The refined method of producing a narrow beam with high clarity is achieved using an array of piezo elements.

A typical circuit with a modem and an Arduino microcontroller board can use the voice recorder module APR33 to send up to sixteen loud sounds to a narrow beam. The Ultrasonic oscillator is usually a Colpitts coil-based sine wave circuit with high voltage transistors for the circuit so that the signal is at a high value of a hundred or more volts. The modulator is a diode bridge type, and it outputs the modulated signal through a high-frequency power amplifier to the piezo horn speaker

Fig. 5: A modem such as SIM 900 is used with this multiple sound blaster circuit (for disturbance or announcement or otherwise)

While there is a cacophony of discussions around the ethical implications of generative AI, technologies such as audio hotspot based directional sound and Wi-Vi (wireless vision, which lets you see through walls!) are not much discussed.

Flat builders of the huge multi-storeyed types might be hiding remote GSM-based hotspots to disturb the inmate at will for their own sadistic or other intents, even from as far away as a different country. Apparatus such as shown in the figure together with spying hardware are likely to be housed in the area shown in fig.2.

It is imperative to think about defensive measures against such technology, ensuring that one must not fear integrity compromises or the risk of continuously getting disturbed.  The jammer for GSM wireless, if only permitted to use at least inside, will be a boon. Health Protection Agency (HPA) published its report, which recommended an exposure limit for the public to airborne ultrasound sound pressure levels (SPL) of 100 dB (at 25 kHz and above).

Author: K.padmanabhan, P.swaminathan And S.ananthi

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