Acoustics is a branch of physics that deals with the study, production, transmission, and effects of mechanical waves in gases, liquids, and solids including topics such as vibration, sound, ultrasound, and infrasound. The ear itself is another biological instrument dedicated to receiving certain wave vibrations and interpreting them as sound.
The first studies on sound were performed by Pythagoras in the 6th century BC, but the hypothesis that sound was a consequence of pressure waves was supported by Chrysippus. Pythagoras and his students discovered the relationship between the length of a vibrating string and the pitch of the emitted sound, basing the musical scale on rigorous mathematical assumptions. The knowledge of the ancient Greeks was however somewhat refined, as evidenced by the famous theater of Epidaurus.
A few centuries later Claudius Ptolemy (100-170 AD), wrote The Harmonics, the first treatise on the musical theory that has come down to us. After criticizing the approaches of his predecessors, Ptolemy argued for basing musical intervals on mathematical ratios (in contrast to the followers of Aristoxenus and in agreement with the followers of Pythagoras), backed up by empirical observation (in contrast to the overly theoretical approach of the Pythagoreans). Ptolemy wrote about how musical notes could be translated into mathematical equations and vice versa in Harmonics. This is called Pythagorean tuning because it was first discovered by Pythagoras. However, Pythagoras believed that the mathematics of music should be based on the specific ratio of 3:2, whereas Ptolemy merely believed that it should just generally involve tetrachords and octaves. He presented his own divisions of the tetrachord and the octave, which he derived with the help of a monochord. His Harmonics never had the influence of his Almagest or Planetary Hypotheses, but a part of it (Book III) did encourage Kepler in his own musings on the harmony of the world (Kepler, Harmonice Mundi, Appendix to Book V). Ptolemy’s astronomical interests also appeared in a discussion of the “music of the spheres”.
The seventeenth century was an era of important discoveries in the field of acoustics. The Dutch physicist Christiaan Huygens (1629-1695), in his Tractatus de Lumine, claimed that the acoustic phenomena were generated by the vibrations produced by the particles of elastic bodies. The English physicist Isaac Newton discovered the formula for calculating the speed of sound by varying the elasticity and density of the medium (air, water, iron) through which it propagates. Mersenne is remembered among the protagonists, who made the first measurement of the speed of sound. The German physicist Otto von Guericke (1602-1686) showed that sound does not propagate in a vacuum.
However, the most notable developments occurred in the nineteenth century, thanks to John William Strutt Rayleigh (1842-1919), who, in his Treatise on sound, clearly exposed the fundamental physical principles of sound vibrations (waves), transmission and the refraction. Subsequently, the progress in the instruments that transmit and reproduce the sounds allowed an ever more in-depth analysis of sound vibrations. With electroacoustics, vibration frequencies have reached a million times higher than those perceivable by the human ear.
The acoustic defects that an environment may present are substantially due to a non-uniform sound intensity distribution or echo phenomena. A satisfactory uniformity of distribution, such that both focal concentrations of sound and sound ‘shadows’ are avoided, is achieved by resorting to various precautions: choosing appropriately or modifying the architectural forms, studying the distribution and size of any obstacles (such as pillars, columns, etc.) so that they do not create sound shadow zones, providing the environment of acoustic reflectors or appropriately positioned speakers.
Application sectors of acoustics
The study of sound waves also leads to physical principles that can be applied to the study of all waves. Applications of acoustic technology include music and the study of geologic, atmospheric, and underwater phenomena. Its origins began with the study of mechanical vibrations and the radiation of these vibrations through mechanical waves and still continues today. The research was done to look into the many aspects of the fundamental physical processes involved in waves and sound and into possible applications of these processes in modern life. From an application point of view, acoustics can be divided into numerous sectors:
- architectural acoustics: which deals with the acoustic quality of buildings, theaters, and other spaces that have a pleasing sound quality and safe sound levels; includes Architectural Acoustics, Engineering Acoustics, Physical Acoustics, Structural Acoustics, and Vibration;
- the acoustics of musical instruments: which deals with properties and characteristics of how music is made, travels and is heard; includes Musical Acoustics, Psychological and Physiological Acoustics, Noise;
- noise and environmental acoustics: which deals with problems related to outdoor noise (natural and man-made noise); include Noise, Structural Acoustics, and Vibration, Speech Communication;
- building acoustics: which aims to isolate rooms from disturbing noises;
- underwater acoustics: which deals with the propagation of waves and their perception in marine environments; includes Underwater Acoustics, Acoustical Oceanography, Animal Bioacoustics, Physical Acoustics;
- medical acoustics that deals with developing methods and tools in the therapeutic and diagnostic field based on the propagation of acoustic waves within the human body; includes Biomedical Acoustics, Engineering Acoustics, Speech Communication, Noise
- animal bioacoustics: study of how animals make, use and hear sounds; includes Acoustical Oceanography, Animal Bioacoustics, Underwater Acoustics;
- speech and hearing: study of how our ears sense sounds, what types of sounds can damage our ears and how speech is made, travels and is heard; includes Speech Communication, Physiological and Psychological Acoustics, Noise
- intensimetric diagnostics such as imaging acoustics are one of the latest application frontiers.
The perceptual and biological aspects of acoustics are then the subject of specific study areas such as psychoacoustics, which studies the psychology of sound perception in humans, and audiometry, which deals with the evaluation of the physiological characteristics of the ear and hearing ability measurement.
A scientist who works in the field of acoustics is an acoustician. The application of acoustics in technology is called acoustical engineering. There is often much overlap and interaction between the interests of acousticians and acoustical engineers. There are many different kinds of acousticians. Here are some examples:
- An acoustical engineer can design transducers such as microphones, earphones, and loudspeakers.
- A bio-acoustician might research bird populations to determine whether or not man-made noise changes their behavior.
- An audiologist can diagnose hearing impairments.
- A hearing scientist can conduct research about hearing loss prevention.
- An architectural acoustician could design an opera house so that people in the audience can enjoy the music to the fullest.
- A noise specialist could do work to reduce noise caused by airplanes, cars, and trains.
- An underwater acoustician might design sophisticated sonar hardware to explore the ocean floor.
- A biomedical acoustician could develop medical equipment to destroy kidney stones.
Psychoacoustics, the study of the physical effects of sound on biological systems, has been of interest since Pythagoras first heard the sounds of vibrating strings and of hammers hitting anvils in the 6th century BC, but the application of modern ultrasonic technology has only recently provided some of the most exciting developments in medicine.
- Ptolemy. Wikipedia. https://en.wikipedia.org/wiki/Ptolemy#Music