Chapter 16 – Waves and Sound

16.1 – The Nature of Waves

A wave is a traveling disturbance that carries energy.

Transverse Wave – disturbance is ┴ to

wave direction (up and down); light, vibrating strings.

Longitudinal Wave – disturbance is // to wave direction (compression); sound.

16.2 – Periodic Waves

time required for 1 cycle (sec)

number of waves per unit time (Hertz or Hz)

length of the wave (m)

max. displacement during a cycle (m)

Period (T) -

Frequency (f) -

Wavelength (λ) -

Amplitude (A) –

The speed of a wave (v) is determined by dividing the wavelength by its period.

v fT

1fT

Remember that

16.3 – SKIP

16.4 – SKIP

16.5 – The Nature of Sound

Sound – L-wave created by a vibrating object; needs a medium (solid, liquid, gas) for the disturbance to travel.

Sound is NOT a mass movement of air, the air molecules are in SHM

16.6 – SKIP

16.7 – Sound Intensity

Sound waves carry energy that can do work.

POWER = Energy/Time [Joule/sec] = [Watt]

Sound Intensity (I) – Power/Area [Watt/m2]

Threshold of hearing = 10-12 W/m2

16.8 – Decibels (dB)

The ear responds to sound in a logarithmic way.

Intensity Level (β) – compares sound intensity to a reference level; logarithmic ratio.

0

(10 dB) logI

I

2

0

-12 Wm

Sound Intensity

= Sound Intensity of Reference Level

(often 10 )

I

I

A 1-dB change in intensity level is smallest change noticeable by humans.

When intensity level increases by 10 dB, the new sound seems ~ 2X louder.

Ex. 70 dB is twice as loud as 60 dB.

REVIEW of TERMS…

Power – [Watt], Intensity – [Watt/m2]

Intensity Level – [dB], Loudness - subjective

Table of sound levels L and correspondingsound pressure and sound intensity

ExamplesSound PressureLevel Lp dBSPL

Sound Pressure pN/m2 = Pa

Sound Intensity IW/m2

Jet aircraft, 50 m away 140 200 100

Threshold of pain 130 63.2 10

Threshold of discomfort 120 20 1

Chainsaw, 1m distance 110 6.3 0.1

Disco, 1 m from speaker 100 2 0.01

Diesel truck, 10 m away   90 0.63 0.001

Curbside of busy road, 5 m   80 0.2 0.0001

Vacuum cleaner, distance 1 m   70 0.063 0.00001

Conversational speech, 1m   60 0.02 0.000001

Average home   50 0.0063 0.0000001

Quiet library   40 0.002 0.00000001

Quiet bedroom at night   30 0.00063 0.000000001

Background in TV studio   20 0.0002 0.0000000001

Rustling leaf   10 0.000063 0.00000000001

Threshold of hearing     0 0.00002 0.000000000001

ASSIGN:

Chapter 16 # 8, 51, 63, 66;

Page 489

Due

Do your homework, kid

16.9 – The Doppler Effect

The observed frequency (pitch) of a sound increases as the sound comes closer; lowers as sound moves away.

16.9 – The Doppler Effect

16.10 – SKIP

16.12 – SKIP

The ear can respond to sounds within the 20 to 20 kHz range.

The ear is most sensitive to sounds b/w 1-5 kHz

Most alarms (and screams) are near the 1-5 kHz range.

Did human speech evolve to match the frequency band the ear is most sensitive to OR did the ear evolve to be sensitive to the frequency band humans mostly speak at – OR is it all

just a coincidence???

16.11 – The Sensitivity of the Human Ear

Fletcher-Munson Curve

dBA – weighted dB scale that approximates human sensitivity to different frequencies.