The principles of audio feedback were first discovered by Danish scientist Søren Absalon Larsen . Microphones are not the only transducers subject to this effect. Record deck pickup cartridges can do the same, usually in the low frequency range below about 100 Hz, manifesting as a low rumble. Jimi Hendrix was an innovator in the intentional use of guitar feedback in his guitar solos to create unique sound effects. He helped to develop the controlled and musical use of audio feedback in electric guitar playing,  and later Brian May was a famous proponent of the technique. 
While reasonable accommodations for marijuana use remain a pipe dream for advocates, the Barrett case and others like it bear watching. Under the right circumstances, a court could rule that an employer has a duty to accommodate a disabled employee’s off-duty medical marijuana use. For example, some forms of medicinal marijuana (such as strains used to treat children with epilepsy), contain very little of the psychoactive chemical “THC” found in most marijuana plants. It would be difficult to argue that this type of off-duty use would negatively impact performance or safety. Complicating the matter further, a positive test for marijuana does not necessarily mean an employee is impaired. Urine tests—the most popular form of drug testing—measure not THC itself, but a non-psychoactive byproduct of THC created in the liver than can linger for weeks.