Sound

Sound Intensity

To remember: Power = Energy per unit time

P = E/t with units: Watts [W]

Electric power : light bulb ~ 100 W
Acoustic power : person talking ~ 10-6 W

Intensity = Power per unit area

I = P/A

units: W/m2

What range of sound intensity can we hear?
  • threshold of hearing: 10-12 W/m2
  • loudest (below pain): 1 W/m2

These are 12 orders of magnitude!

Inconvenient for drawing into a diagram.

Scales

There are 2 different scales:

(a) linear scale:
  • Go from one marking to the next: ADD 1
  • Successive equal increments correspond to the addition of a constant

(b) logarithmic scale:
  • Go from one marking to the next: Multiply by 10
  • Successive equal increments correspond to the multiplication by a constant.

Define new unit

Sound intensity level (SL) :

B = 10 log (I/I0)

with I0 = 10-12 W/m2

[dB] decibel

selected values for the sound intensity level:

I / I0 dB
1 0
2 3
4 6
10 10
20 13
40 16
100 20
1000 30
10,000 40
100,000 50

This scale is NON LINEAR.

On a logarithmic scale, the markings correspond to equal steps in powers.

10 = 101 - log 10= 1
100 = 102 - log 100= 2
1000 = 103 - log 1000= 3

The decibel scale of sound intensity levels with sounds from various sources is given in this table .

Attention:

Problems: Buzzers

Loudness

Loudness is a subjective measure and depends on the individuals perception and physical condition of the ear.

Less crowded summary diagram:

About 1% of the people in the United States can hear sound below the levels of the lower curve. Perhaps 50% can hear below the dashed curve. The shaded region corresponds to the levels of normal music.

Decibles [db] explained: 5 things you need to know

Ch. Elster
Aug 26 14:27:03 EDT 2019