Sound and acoustics

Question 1

How is Acoustics defined?

Answer 1

The generation, propagation, and measurement of sound

Question 2

How is sound defined?

Answer 2

The propagation of density changes through an elastic medium

Question 3

What is the elastic limit?

Answer 3

The greatest stress that can be applied to a material without causing its permanent deformation

Question 4

What are the units of Intensity?

Answer 4

Watt/m^2

Question 5

What are the units of Pressure?

Answer 5

1 Pascal = 1N/m^2

Question 6

What is the formula for Intensity (Ix)

Answer 6

Ix = 10^(dBIL/10) x 10^-12

Question 7

What is the formula for Intensity Level (dBIL)

Answer 7

dBIL = 10log(Ix/10^-12)

Question 8

How to add intensity levels which are different
(or pressure levels, as dBSPL = dBIL)

Answer 8

1. Find the Ix for each source’s dBIL
2. Add the Ix for each source to determine the combined dBIL
3. Find the new dBIL using the combined Ix value

Question 9

What is the formula for Pressure (Px)?

Answer 9

Px = 10^(dBSPL/20) x 10^-12

Question 10

What is the formula for Sound Pressure Level (dBSPL)?

Answer 10

dBSPL = 20log(Px/10^-12)

Question 11

How to add intensity levels that are the same

Answer 11

dBIL N = dB IL original + 10logN

Question 12

What is Acoustic Impedance?

Answer 12

The opposition that sound waves encounter when they pass from one medium to another

Question 13

What is the Natural Frequency?

Answer 13

The frequency at which a system oscillates / vibrates freely and impedance is lowest

Question 14

How do mass and stiffness of a system impact the Natural Frequency?

Answer 14

Increased mass decreases the natural frequency, increased stiffness increases the natural frequency

Question 15

What is the formula for Acoustic Impedance (Z)?

Answer 15

Z = square root (R^2+X^2)

R = Resistance
X = Reactance

Question 16

What is Resistance?

Answer 16

• The opposition a medium present to the passage of sound
• Measured in ohms
• Independent of frequency

Question 17

What is Reactance?

Answer 17

The forces that oppose motion in a frequency selective way

Question 18

What is Mass Reactance (Xm)?

Answer 18

• A measure of how particles in a medium resist motion
• Directly proportional to frequency, gets larger at higher frequencies

Question 19

What is Stiffness Reactance (Xc)?

Answer 19

• A measure of the ability of a material to deform or compress in response to pressure variations in a sound wave
• Inversely proportional to frequency, so larger at low frequencies

Question 20

When is a system mass dominant?

Answer 20

In the high frequencies (>f-nat)

Question 21

When is a system stiffness dominant?

Answer 21

In the low frequencies (<f-nat)

Question 22

What is Admittance (Y)

Answer 22

The energy accepted (or admitted) into a system

Question 23

What is Acoustic Mass?

Answer 23

• A property in acoustics that represents the inertial characteristics of a fluid medium e.g. air
• The opposition of a fluid to acceleration when subjected to acoustic pressure
• E.g. a thinner tube is harder to blow air through than a wider tube

Question 24

How is frequency related to Acoustic Mass

Answer 24

• Low frequencies overcome inertia easier than high frequencies
• Small masses are easier to move than large masses

Question 25

How is Acoustic Mass related to the length and diameter of a column of air?

Answer 25

• Acoustic Mass increases with the length of the column of air
• Acoustic Mass increases with the inverse of the diameter squared (1/d^2), smaller diameter = increased Acoustic Mass

Question 26

What is the equation for Acoustic Mass (Ma)?

Answer 26

Ma = 1500(l/d^2)

l = length
d = diameter