It turns out that humans can be made to smell by emitting ultrasound

Smells are processed in a region of the brain called the olfactory bulb. It has been found that stimulating this bulb with ultrasound can make humans perceive a variety of odors.

We Induced Smells With Ultrasound

Lev Chizhov and his colleagues from the Moscow Institute of Physics and Technology conducted an experiment in which they irradiated humans with ultrasound to create various sensations.

Chizhov and his colleagues focused on the sense of smell. To direct ultrasound waves toward the olfactory bulb, which processes odors, they developed a special head pad and conducted trial and error to accurately focus the ultrasound toward the olfactory bulb, located just below the forehead.

To ensure safety, we adjusted the output to an order of magnitude lower than the intensity of transcranial focused ultrasound (tFUS) used in the medical field, and set the irradiation angle narrow to avoid irradiating the optic nerve. As a result, we set the frequency to 300 kHz, which allows sufficient penetration through the skull, the focal depth to approximately 39 mm, where the ultrasound energy converges below the forehead, and the beam angle to 50-55°, which limits the focal area to the olfactory bulb, and performed five short, high-speed cycles at a frequency of 1200 Hz.

When the device was tested on two subjects, it was able to induce four distinct sensations: a sensation of fresh, oxygen-rich air, a smell like fruit peels that had been left out for days, a sensation of ozone, like being near an air ionizer, and a smell like a campfire with burning wood.

Chizhov and his colleagues distinguish between 'smell' and 'sensation' because they are subjectively different experiences. The 'smell' experienced by the subjects was strong and localized, almost like sniffing around to find its source. The 'sensation,' on the other hand, was more diffuse, weak, and slow-onset, often associated with other sensations, such as a slight tingling sensation in the face. Chizhov and his colleagues speculate that the tingling sensation was likely a placebo effect.

The subjects held the device to their foreheads and gently sniffed the surroundings. Both the smell and the sensation were strongest when they inhaled lightly; some smells were subtle and only lasted a few breaths, while others were sudden and overwhelming. When one subject smelled garbage, he or she opened their eyes, thinking it was the garbage truck. However, because the subject was indoors, it was impossible for them to smell the garbage truck.

The four different smells were related to the position of the ultrasound beam. By slightly shifting the beam angle, Chizhov and his colleagues were able to induce different smells in the subjects.

The length of an adult's olfactory bulb is about 6 to 14 mm, while the wavelength of ultrasound is 5 mm. It's remarkable that different scents could be evoked at such a small angle, Chizhov and his colleagues argue. Since there are 400 different receptors in the nose, adjusting the ultrasound irradiation may enable fine-tuning receptor activation and a variety of sensations.

'We were able to identify four scents within a few days,' Chizhov and his colleagues said. 'With further refinements to the technology, we should be able to significantly enhance the olfactory stimulation.'