5. Sound Instruments

Instruments in this category measure hearing ability, study vibrations of strings, or amplify and transmit sound.

Sound is a mechanical wave that requires a material medium (solid, liquid, or gas) to travel through, and cannot travel through a vacuum — unlike light, which can. This single fact explains several details tested in exams: sound travels fastest through solids, slower through liquids, and slowest through gases (because particles are packed most tightly, and therefore transmit vibrations most efficiently, in solids); and instruments that rely on sound, such as the stethoscope and the sonometer, work by efficiently conducting vibrations through a solid or an enclosed air column to the ear.

5.1 Audiometer

Audiometer. An instrument used to measure and test the acuity (sharpness) of a person's hearing, i.e., the ability of the ear to hear sounds of different frequencies and intensities. It generates pure tones of controlled, calibrated frequency and loudness through headphones, and the softest sound a person can hear at each frequency is plotted as an audiogram, used by audiologists to diagnose hearing loss.

5.2 Sonometer

Sonometer. An instrument, essentially a resonance box with a stretched string/wire, used to study the relationship between the frequency of a vibrating string and factors such as its length, tension, and mass per unit length (the laws of vibrating strings). It consists of a hollow wooden sounding box over which a metal wire is stretched between two bridges, with the tension adjustable using weights; commonly used in physics laboratories to verify the laws of transverse vibrations.

5.3 Stethoscope

Stethoscope. A medical acoustic instrument used by physicians to listen to (auscultate) internal sounds of the body, most commonly the heartbeat and breathing (lung) sounds, and also bowel sounds. It consists of a chest piece (with a diaphragm and/or bell) that picks up sound vibrations from the body, connected via flexible rubber/plastic tubing to earpieces; the enclosed air column in the tubing transmits and somewhat amplifies the sound to the listener's ears. It was invented by the French physician René Laennec in 1816.

5.4 Megaphone

Megaphone. A cone-shaped (or electronically amplified) device used to amplify and direct a person's voice or other sounds so they can be heard over a longer distance or above background noise, commonly used by public announcers, sports coaches, and in crowd-control situations. The simple horn/cone type works by directing sound waves in a specific direction and reducing their spreading, thereby increasing the perceived loudness in that direction; modern megaphones add electronic amplification with a microphone, amplifier, and loudspeaker.

It is worth noting that human hearing is generally sensitive to frequencies roughly between 20 hertz and 20,000 hertz; sound below this range is called infrasound and above it is called ultrasound. Instruments such as the fathometer, sonar, and many medical imaging devices (like the ultrasound scanner used in medical diagnostics, which is closely related in principle to sonar but used to image soft tissues inside the human body) work using ultrasound precisely because such high-frequency waves can be focused into narrow, directional beams that give more precise ranging and imaging than audible sound would allow.

Quick Revision Table — Sound Instruments