With Gus Machlup, Dr. David Littmann founded "Cardiosonics, Inc." to sell his stethoscopes. Stethoscopes and neckties are common fomites associated with health care providers. Less popular object groups focused on headwear, stethoscopes, wristwear, pacifiers, and diapers (diaper fetishism). These hearing aids are designed for medical practitioners with hearing loss who use stethoscopes. Electronic stethoscopes (also digital stethoscopes) convert acoustic sound waves into digital electrical signals. Electronic stethoscopes can be recording devices, and can provide noise reduction and signal enhancement. Since then, inventions such as X-rays and stethoscopes have progressed and revolutionized the medical field. Stethoscopes are a symbol of healthcare professionals. Currently, a number of companies offer electronic stethoscopes. One problem with acoustic stethoscopes was that the sound level was extremely low. Audio tours are free with entry and accessed via a set of stethoscopes.