Sound waves, Intensity

longitudinal wave

What kind of wave is a sound wave?

our perception of how loud a sound is, is related to the intensity of the sound wave

Remember: our perception of how loud a sound is, is related to the intensity of the sound wave

the source of the sound waves, emits waves equally in all directions

what is a point source?

a spherical wave

when waves are emitted by the point source equally and in all directions, what kind of wave is that?

the intensity of the wave decreases as you move further away from the source

Remember: the intensity of the wave decreases as you move further away from the source

the intensity of the wave decreases as distance from source increases because the power of the source becomes more spread out over a larger surface area

why does the intensity of the wave decrease as you get further from its source?

Intensity

• def: the power per unit of area

• SI units: W / m²

the power per unit of area

what is the definition of Intensity?

spherical wave

• power is divided by the area of the sphere

• Area of the sphere (A) = 4πr²

I = Psource / 4πr²

Intensity equation for a spherical wave:

Sound level / decible level (β)

• wide range detectible by the human ear: 10⁻¹² to 10² W/m²

• Eqn: β = 10 x log₁₀(I/I₀)

β = 10 x log₁₀(I/I₀)

what is the equation for sound level (β)?

I₀ = 1.0×10⁻¹² W/m²

what is the reference intensity (I₀) equal to?

Threshold of hearing

• Def: the softest sound a typical human can hear

• I = 1.0 × 10⁻¹² W/m²

I = 1.0 × 10⁻¹² W/m²

Threshold of hearing value?

Threshold of pain

• Def: a sound that is so loud that it hurts your ears

• I = 10 W/m²

I = 10 W/m²

Threshold of pain value?

Doubling the sound intensity does NOT double the sound level

Remember: Doubling the sound intensity does NOT double the sound level

the higher on the sound level graph the louder the sound

Remember: the higher on the sound level graph the louder the sound

High frequency (treble)

• to the right of the sound level graph

• has a high pitch

Low frequency ( Base)

• to the left of the sound level graph

• has a low pitch

Sonic frequency

• Def: sounds that humans can hear

• in the middle of the sound level graph