Acoustics: what is it, formula, exercises

Acoustics is an area of ​​study in Physics that studies all aspects related to mechanical waves, such as sound, ultrasound and vibrations that propagate in solid, liquid and gaseous media. It focuses on the study of phenomena such as propagation, reflection, absorption and interference between sound waves.

See too: Wave classification - types, propagation forms and characteristics

Sound

O soundit's a mechanical wave and, for this reason, it is only capable of propagating in material media, such as air, water or metals. THE sound propagation is three-dimensional, is sound wave it propagates in a circular fashion in a homogeneous medium, covering equal distances in all directions. Also, the sound is a transverse wave, that is, the sound waves travel in the same direction as the disturbance that originated them.

Sound is a mechanical, longitudinal wave.

You soundsaudible by humans fall within a range of frequencies called the audible spectrum. These frequencies are distributed, on average, between 20 Hz and 20,000 Hz

. Sounds of frequencies lower than 20 Hz are called infrasounds, while sounds with a frequency greater than 800 Hz are known as ultrasound.

The detection of ultrasound and infrasound it is used in a large number of technologies and the applications are many:

• detection of earthquakes;

• conducting exams;

• study of underground structures etc.

THE velocityinpropagation of the sound waves ismiddle feature where these waves travel. Such characteristic depends on factors such as density, temperature and elasticity of the medium. Generally, sound waves travel faster in solid media such as metals.

See too: 5 things you need to know about sound waves

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Acoustic formulas

At acoustic formulas they are used to calculate sound characteristics, such as propagation speed, frequency, wavelength, sound intensity, etc. Check out the main ones below!

v – speed of sound (m/s)

λ – wavelength (m)

f – frequency (Hz)

The following formula is used to determine the sound intensity level of the sound in decibels:

I₀ – threshold of human hearing (10^-12 W/m²)

I – sound intensity (W/m²)

The following formula is used to calculate the apparent frequency when a sound source moves relative to an observer:

f – apparent frequency (Hz)

f₀ – broadcast source frequency (Hz)

v_s – speed of sound (m/s)

v_F – sound source speed (m/s)

v_M – middle speed (m/s)

Acoustics and music

Acoustics has direct applications in music, and one of them is the study of harmonics in strings and in sound tubes, used in most musical instruments. Want to know more about the subject? Access our specific text: Physics in imusical instruments.

See too: Physiological characteristics of sound - intensity, timbre and pitch

Solved Exercises on Acoustics

Question 1 —The phenomenon that occurs due to the fact that sound waves undergo an apparent change in frequency due to the relative movement between the emitting source and the observer is called:

a) diffraction.

b) Doppler effect.

c) Tindall effect.

d) polarization.

Resolution:

The phenomenon that causes the apparent change in wave frequency is known as Doppler effect, so the correct alternative is the letter B.

Question 2 — The “volume” of the sound is actually the intensity of the sound waves, that is, the amount of energy transmitted by the sound every second, every square meter. Check the alternative that presents the unit of measure corresponding to the sound intensity.

a) m/s

b) m³

c) kg.m/s²

d) W/m²

Resolution:

As stated in the statement, the sound intensity is the power per square meter, so the correct alternative is the letter D.

By Rafael Hellerbrock
Physics teacher

Would you like to reference this text in a school or academic work? Look:

HELERBROCK, Rafael. "Acoustics"; Brazil School. Available in: https://brasilescola.uol.com.br/fisica/acustica.htm. Accessed on June 27, 2021.