Introduction to Acoustics dr inż. Michał Bujacz [email protected] Visitng hours: tuesday 15:00-16:00 wednesday 10:00-11:00 „Lodex” 207.

Introduction to Acoustics

dr inż. Michał [email protected]

Visitng hours:

tuesday 15:00-16:00

wednesday 10:00-11:00

„Lodex” 207

Acoustics[gr. akoustikós ‘related to hearing’]

field of physics and engineering that deals with the study of all mechanical waves in gases, liquids, and solids including topics such as vibration, sound, ultrasound and infrasound

http://www.physics.byu.edu/2

Sound

„phys. mechanical disturbance in a compressive medium capable of inducing an auditory sensation, as well as the auditory sensation itself” (Encyklopedia PWN)

3

Longitudinal wave (wzdłużna)

Particles oscillate in the direction of wave propagation.

Areas of increased and decreased pressure.

http://www.physics.byu.edu/

What’s the range of movement of the air particles moved by sound?

0.008nm to 0.1 mm

4

Harmonic signal

http://hyperphysics.phy-astr.gsu.edu

T

dttxE0

2)(

T

dttxT

P0

2)(

1

5

Physics vs. perception

Physical definitions:

- Intensity (natężenie) or

amplitude (amplituda)

- Frequency (częstotliwość)

- Spectrum (widmo)

- Speed (prędkość)

Psychoacustic definitions:

- Loudness (głośność)

- Pitch/tone (wysokość/ton)

- Tembre (barwa)

6

Speed of soundc = 344 m/s

c = 331 + 0,6*T[oC]gestosc

sztywnoscCc

7

Intensity of soundIntensity = the amount of energy passing through an area in time

224 m

W

r

PI

http://hyperphysics.phy-astr.gsu.edu

How does amplitude change with distance?

22 ~~~ ApEP

rrIA

1~

1~~

28

Measures of sound level

Pressure (rms)

from 0.00002 N/m2 (threshold of hearing)

to 20 N/m (threshold of pain)

atmospheric pressure

100000 N/m2

9

Decibel scaleRatio of power:

Ratio of amplitude:

dB power ratio amplitude ratio

60 1 000 000 1 000

50 100 000 316.2

40 10 000 100

30 1 000 31.62

20 100 10

10 10 3.162

6 3.98 2

3 2 1.414

0 1 1

-3 0.5 0.7071

-6 0.251 0.5

-10 0.1 0.3162

-20 0.01 0.1

-30 0.001 0.03162

-40 0.000 1 0.01

0

110log10P

PLdB

20

21

10log10A

ALdB

0

110log20A

ALdB

10

Confusing „sound levels”Sound Power Level (SWL)

- energy emitted by sound source per second (cause)

Sound Pressure Level (SPL)

- pressure (amplitude) changes at receiver (effect)

Sound Intensity Level (SIL)

- energy delivered to receiver area per second (effect)

μPa200 p 212

0 mW10P

11

http://www.physics.byu.edu/

12

Loudness1Fon = 1dBSPL at 1kHz

13

Frequency

14

Audio frequency ranges

Tones Examples

Low bass: 20 – 80 Hz

Lowest two octaves. Explosions, storms, lowest church organ notes

Upper bass: 80 – 320 Hz

3th – 4th octavesDrums, bass, cellos, wind instruments

Lower mid: 320 – 2560 Hz

5th – 7th octavesMost instruments and human speech

Upper mid: 2560 – 5120 Hz

8th octaveHighest notes on most instruments, whistles, most sensitive perception range

High/treble: 5120 Hz – 20000 Hz

9th -10th octavePercussion (hi-hat, cymbal), higher harmonics of sounds, noise

15

Human perception

16

Musical tone (pitch)

Musical scales

– divided into octaves (intervals), e.g. C (dur)

A4 = 440Hz

Psychoacoustic scales

– Mel or Barkhttp://www.sfu.ca/

17

Freq. components

18

Fourier transform

19

Spectrum of a sound

20

Real spectrumA

mpl

itud

a (d

B)

Częstotliwość (Hz)

21

Cutting the spectrum into octaves

22

Tembre/qualitySet of features distinguishing sounds of the same base frequency on different instruments

Spectrum dependent (harmonics and noise) and transients (sudden changes)

Described semantically (np.brightness, warmth, coarsness, clarity)

23

Sound envelope

24

Anatomy of hearing

25

Cochlea

26

Future topics - Psychoacoustics- Spatial audio- Room acoustics (wave phenomena) - Spectrum analysis (Fourerier, DFT, FFT) - Noise and sound standards- Math of Music- Electroacoustics- Studio equipment- Sound synthesis - Speech signals- Speaker sets

27