Beat (Physics)
In physics, "beat" refers to an auditory phenomenon that occurs when two or more sound waves of nearly identical frequencies are played together. This interaction results in oscillation, where the sound alternates between being amplified and diminished in volume, creating a unique auditory effect. The brain processes these oscillations, but it can struggle to interpret them, often finding oscillation unpleasant, especially at certain frequencies. When the beat frequency exceeds approximately 125 beats per second, the brain perceives it as a single new tone, known as a difference tone. Conversely, beat frequencies below 10 Hz can lead to a single perceived pitch with fluctuating volume, while frequencies between 10 Hz and 60 Hz produce a dissonance that is difficult for the brain to decipher. These auditory effects have practical applications, such as in police whistles and medical devices, where beats can create attention-grabbing sounds or help measure vital signs. Musical instruments can also intentionally produce beats for dramatic effects through similar note pairings. Understanding beats in this context highlights the complexities of sound perception and its implications in various fields.
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Beat (Physics)
In physics, the term beat refers to a specific auditory phenomenon. Beats are created when two or more tones of an identical frequency are played almost simultaneously. Together, these tones create an effect called oscillation. When oscillating, a sound is simultaneously rising and falling in volume. When oscillation occurs, the brain often has difficulty processing it. For this reason, oscillation is unpleasant to hear.
Background
The sensation of sound is caused by a variation in pressure. Sound travels as a longitudinal, mechanical wave. Sound waves consist of regions of increasing pressure and decreasing pressure. The parts of a sound wave that are increasing in pressure are called compressions. The parts of a sound wave that are decreasing in pressure are called rarefactions.
Sound waves are caused by several events. Sudden expansion or compression of air, which takes place in events such as explosions and implosions, can cause sound waves to form. Air flowing around obstacles can also cause sound waves to form. Sound waves most commonly form through the vibration of solid objects. This is how many musical instruments make noise.
Longitudinal sound waves travel at different speeds through different mediums. They travel at high speed through solid and liquid mediums and low speeds through gases. For this reason, sound travels faster underwater than through the air. Because sound waves need a medium through which to travel, they cannot pass through vacuums.
Sound waves are measured in amplitude and frequency. The amplitude of a sound measures its volume. The frequency of the sound measures its tone. High-frequency sounds are high-pitched, while low-frequency sounds are low-pitched.
Frequencies are measured in hertz (Hz) and kilohertz (kHz). Most humans can hear between 20 Hz and 20 kHz. Human speech produces sounds between 100 Hz and 1000 Hz. Many sounds fall outside the range of human hearing. Some of these sounds, primarily sounds caused by large objects in motion, are called infrasound. Infrasound is caused by tornadoes, aircraft turbulence, and massive ocean waves. Some animals, including whales and elephants, can hear infrasound. In humans, sound waves are captured by the ear. They travel from the outer ear and through the auditory canal before striking the eardrum. The compressions and rarefactions cause the eardrum to vibrate, moving various tiny bones in the inner ear and transmitting the vibrations to the brain.
Overview
The brain uses several mechanisms to turn sound waves into the sounds we hear. Many of these mechanisms are located in the superior temporal gyrus, often called the auditory cortex. The auditory cortex is responsible for processing all the sound waves interpreted by the ear. For example, the parts of the brain usually responsible for deciphering speech are located on the left side of the auditory cortex. Damaging this area of the brain has been shown to damage an individual's ability to recognize and understand speech.
The auditory cortex is also responsible for recognizing beats. Beats form when two sound waves with the same frequency intersect in an imperfect manner. If two waves with the same frequency interact in a perfect manner, the sound is amplified to a much greater volume. If two waves with the opposite frequency interact in a perfect manner, the sound is cancelled out. However, when two waves with the same frequency interact in an imperfect manner, the sound is alternately amplified and cancelled. This produces an effect called oscillation, in which two sounds quickly raise and lower their volumes in a controlled pattern. The brain often has trouble processing oscillation. If oscillation occurs within a certain range of frequencies—roughly 125 beats per second or faster—the brain is unable to perceive the difference between the two frequencies. Instead, it interprets the oscillating frequencies as a new, individual frequency that is not occurring outside the brain. The tone produced by this effect is called the difference tone.
Other oscillation frequencies produce different auditory effects. If the beat frequency is less than 10 Hz (or fewer than 10 beats per second), the auditory cortex is unable to distinguish between the two frequencies. The brain will interpret it as a single pitch, and its volume will amplify and fade with the oscillation. If the beat is between 10 Hz and 60 Hz, the brain will struggle to interpret the sound. The oscillation between the pitches of the sound waves is too slow for the brain to completely ignore but too fast for the brain to accurately interpret. The brain registers this dissonance but is not sure how to turn it into sound. For this reason, beats in this frequency are extremely unpleasant to hear. After the beat frequency surpasses 60 Hz, the brain is able to interpret the different frequencies as separate sounds. While they may oscillate, these sounds are significantly less unpleasant to hear.
Audio engineers have devised several uses for intentionally created beats. For example, the police whistle used by London police officers intentionally uses beats to create oscillation, resulting in an extremely shrill, unpleasant sound that quickly grabs the attention of anyone within earshot. Because London police whistles create a beat composed of three oscillating tones instead of two, their effect is particularly powerful. Calculating the oscillation of a beat is one of the mechanisms behind police radar, which allows police officers to easily detect the speed of moving vehicles. Machines used in hospitals also utilize beats. They compute the oscillations made by the human body to precisely measure a patient's pulse. Some musical instruments may be used to create beats for dramatic effect. By playing two extremely similar notes, these instruments have the potential to create a muddy, oscillating frequency.
Bibliography
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