Sound

Question 1

What is a mechanical wave?

Answer 1

• spreading disturbance (forced displacement of particles)
• in elastic medium.
• transfers energy but not mass.
• may take a form of an elastic deformation in solids or a variation of pressure in gases.

Question 2

What is sound ?

Answer 2

anyMW / mechanical vibration producing hearing perception.

Question 3

Spectrum of mechanical waves

Answer 3

Low frequency mechanical waves - INFRASOUND

Acoustic range (sound) – mechanical waves with freq. from 20 Hz to 20kHz ·

High freq. mechanical waves - ULTRASOUND

Question 4

Sound pressure (formula, explanation, diagram)

Answer 4

P(t)= Δ P sinφ
Where:ΔP is the maximal pressure increase/decrease (+ or - ), φ is the phase of the given mechanical wave showing the rate of alterations.

Question 5

Which two form of matter vibration are there concerning sound? (diagram)

Answer 5

Longitudinal sound waves: typical togasses and liquids
- displacement occurs in same direction of propagation

Transverse waves: - typical for solids
- particles displacement occurs perpendicularly to
wave propagation

Question 6

Sound intensity (formula and definition)

Answer 6

I= E/t.S

• the sound energy transmitted per sec. trough unit area,
• placed perpendicularly to the sound propagation.
• is measured in watt per square meter. [W/m2]

Question 7

Formula for relation between intensity and pressure

Answer 7

I = p2/2Za

Question 8

Sound frequency (definition)

Answer 8

number of vibrations (repetitions of motion of the particles) per sec.
measured in Hz – 1 Hz equals to 1 vibration per sec.

Question 9

Definition for sound period

Answer 9

the time duration of 1 vibration

Question 10

Sound velocity

Answer 10

depends on the properties of the medium through which sound propagates.
For example: –At 20º C SV in the air is about 340 m/s, but in water SV is about 1500 m/s.
The exact relationship between sound speed and matter properties threats elasticity as major factor.

Question 11

Sound wavelength (formula and definition)

Answer 11

λ = v.T=v/f

The distance between two consecutive sound fronts

Question 12

Acoustic impedance (formula, explanation and diagram)

Answer 12

Z = ρ.v
- depends on density of propagating medium (p) and the sound velocity (v)
- when sound wave encounters border between two media, part of wave reflects and other part passes due to different acoustic impedances (Z1 and Z2)
- the amount of reflection and transmission depends on difference between (Z1 and Z2)
reflection coeffiecient: a2=(Z2-Z1)2 / (Z2+Z1)2

Question 13

DEF: sound is called simple tone when

Answer 13

vibration has sine shape and certain frequency

Question 14

Def: complex sound

Answer 14

• consits of several tones

- can be drawn as a graph in which amplitudes and frequencies of each tone are combined

Question 15

Weber - Fechner law:

Answer 15

Perception ~ log(stimulus)

Perceptions increases logarithmically with the stimulus

Question 16

Sound Intensity level E

formula, explanation, additional infos

Answer 16

• is subjective perception of sound volume, depends on frequency and intensity
• only at 1000 Hz sound frequency, the sound volume is described as sound intensity level E

E=k. lg( I/I0)
I is intensity of the sound and I0 is minimal intensity (treshold)
(at 1000 Hz the threshold is approximately 10-12 W/m2- the lowest in power vibration audible for human ears)

Intensity level is measured in bel B/ decibel dB

Question 17

Loudness of the sound

Answer 17

L= k. lg( I/I0)

k is coefficient depending on frequency,
I is intensity of given sound,
I0 is threshold of audibility of this sound

L is measured in Phon

Question 18

Audibility area

Answer 18

• the closed area between threshold of audibility and threshold of pain
• contains all the sounds we can perceive.

(threshold of audibility = minimal intensity requiered for hearing, different for different frequencies)
(treshold of pain = minimum intensity needed to cause pain, normally 130-140db)

our ears are most sensitive to sounds from 1000 – 4000 Hz range.

Question 19

Pitch of sound

Answer 19

subjective quality, depends on frequency frequency
the higher frequency - the higher pitch
determined by the vibrating characteristic of the vocal chords

Question 20

Timbre

Answer 20

• characterizes complex sounds.

- allows different sound sources to be distinguished by their acoustic spectra

Question 21

Lithotripsy

Answer 21

• destruction of kidney/bladder stones by sound wave
• wave that passes through stone causes vibration and resonance
• – increases amplitude, which increases pressure - stone breaks

Question 22

Use of contact gel in Ultrasound

Answer 22

• in ultrasound imaging, US waves would fully reflected due to different acoustic impedances of air and skin (Z skin&raquo_space; Z air)
• solution: contact gel on transducer, which has same acoustic impedance as the tissues, - US penetrates body without losses

Question 23

Phonocardiograph

Answer 23

Records heart sounds and transfers them into audible sounds

Question 24

Audiometry

Answer 24

Examination of personal threshold of audibility,

Determines relation between objective sound intensity and subjective loudness