Acoustics

Acoustics is the area of ​​physics that studies sound waves and their phenomena. Acoustics addresses wave phenomena , such as reflection, refraction, diffraction , absorption and the Doppler effect , as well as the characteristics of sound — a mechanical wave capable of propagating through solid, liquid and gaseous media.

acoustics and sound

Sound is a mechanical wave , so it needs a physical medium through which it can propagate. Furthermore, sound is a longitudinal wave , that is, the disturbance that produces a sound wave occurs in the same direction in which this wave propagates, as in the case of stretching a spring and releasing it.

Sound is a pressure wave that produces regions of compression and rarefaction in air or other media . When these vibrations reach our ear, they are transmitted to a set of ossicles that help transmit the information to our brain.

Not all sounds are audible, humans can hear sounds whose frequencies are between 20 Hz and 20,000 Hz , this range of frequencies is known as the audible spectrum . Sounds with a frequency lower than 20 Hz are known as infrasound , while sounds with a frequency higher than 20,000 Hz are called ultrasound .

sound characteristic

The frequency of the sound is related to one of its characteristics: the pitch. High-frequency sounds are called loud sounds , perceived as high- pitched tones . Low-frequency sounds, in turn, are bass sounds , called low sounds .

Sound intensity is another characteristic of sound that allows us to differentiate different sound sources. We say that the sound intensity is the amount of energy that the sound wave is able to transfer, every m², every second. The unit of measurement for sound intensity is W/m² .

For practical purposes, sound intensity is usually presented on the decibel scale , this scale compares the intensity of a sound wave to the lowest audible intensity, which is about 10 -12 W/m². This value is known as the threshold of hearing and corresponds to the noise of an ant gnawing a tree leaf, for example.

N – number of decibels

I – sound intensity (W/m²)

0 – audibility threshold (10 -12 W/m²)

The propagation speed of sound depends exclusively on the medium in which it propagates. In air, at room temperature (25 °C) and at a pressure of 1 atm, the propagation speed of sound is about 340 m/s. The speed of sound in a medium is related to factors such as the density of the medium and its elasticity , that is, the ability of molecules to conserve their mechanical energy during collisions between neighboring particles.

v – speed of sound (m/s)

λ – wavelength (m)

f – sound frequency (Hz)

In addition to pitch and intensity, timbre is the other feature that allows us to differentiate two identical musical notes, produced by different sources (violin and piano, for example). The timbre concerns the form of vibration of the wave and is a characteristic of each sound source.

acoustics in music

Acoustics is present in music, especially when we study the formation of standing waves . Standing waves are produced when some boundary condition restricts the propagation of sound, in which case an incident wave and a reflected wave produce mutual interference , thus, under certain conditions, harmonics are formed.

In string instruments, clamps are used to control the size of the string, in addition, the tension applied to the strings also affects the speed of propagation of sound waves ( Brook-Taylor law ). In this type of instrument, harmonics are formed when the length of the string is long enough for standing waves of length equal to an integer multiple of half a wavelength to form .

L – length of string

n – harmonic number (n = 1, 2, 3…)

Acoustics also studies the formation of harmonics in sound pipes , as in flutes. In this case, there are two types of sound tubes: open and closed. In these, one side is closed, in those, both ends are open.

Acoustics in Enem

Acoustics has been one of the most discussed topics at Enem in recent years. To prepare for the test, study the different phenomena suffered by sound waves and try to understand how each of them can be present in technologies such as radar and ultrasound and in other technological applications or in everyday life.

exercises on acoustics

Question 1) (Enem) When listening to a flute and a piano emitting the same musical note, it is possible to differentiate these instruments from each other.

This differentiation is mainly due to:

a) sound intensity of the sound of each musical instrument.

b) sound power of the sound emitted by the different musical instruments.

c) different speed of propagation of the sound emitted by each musical instrument.

d) timbre of the sound, which causes the waveforms of each instrument to be different.

e) pitch of sound, which has different frequencies for different musical instruments.

Template: Letter D

Resolution:

The sound quality that allows us to distinguish sounds of the same frequency and intensity is timbre, so the correct alternative is the letter D.

Question 2) (Enem) As a teenager, our afternoons, after classes, consisted of taking the guitar and Almir Chediak’s chord dictionary in hand and challenging our friend Hamilton to discover, just by listening to the chord, which notes were chosen. We always lost the bet, he has perfect pitch.

Absolute pitch is a perceptual characteristic of a few individuals capable of identifying isolated notes without other references, that is, without having to relate them to other notes of a melody.

LENT, R. My accordion teacher’s brain. Available at: http://cienciahoje.uol.com.br. Accessed on: 15 Aug. 2012 (adapted).

In the context presented, the physical property of waves that allows this distinction between notes is:

often.

b) intensity.

c) waveform.

d) wave amplitude.

e) propagation speed.

Template: Letter A

Resolution:

The sound quality that allows us to distinguish different musical notes is the sound frequency, so the correct alternative is the letter A.

Question 3) (Enem) In order to reduce noise pollution in a city, the City Council passed a law that imposes a maximum limit of 40 dB (decibels) for the noise level allowed after 10 pm.

By approving the aforementioned law, the councilors are limiting which characteristic of the wave?

a) The height of the sound wave

b) The amplitude of the sound wave

c) The frequency of the sound wave

d) The speed of the sound wave

e) The timbre of the sound wave

Template: letter B

Resolution:

The sound intensity of the wave is related to the amplitude of the wave, so the correct answer is the letter B

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