Section 4

Question 1

What is a logarithmic/exponential scale?

Answer 1

A number expressed as an exponent (or power) that tells how often a number (the base) is multiplied by itself

Question 2

What number do we use as the base in acoustics?

Answer 2

10

Question 3

Why do we use logarithms in acoustics?

Answer 3

The intensity ratio of the loudest sound to the smallest sound is 100,000,000,000,000 to 1, which is too large to work with

Question 4

How do we represent the ratio of the loudest sound to the faintest sound in log? Hint: bels

Answer 4

10 with 14: 1

Using just the logarithm, we get 14 bels

Question 5

Why do we use decibels instead of bels?

Answer 5

Because the entire range of human hearing is only 14 bels.

Question 6

What is a decibel?

Answer 6

1/10th of a bel. Ex: 5 bels= 50db, 1 bel= 10db

Question 7

What does it mean to be a relative unit of measure?

Answer 7

Decibels must have a reference in order to be useful.

Question 8

What is intensity? (IL)

Answer 8

Intensity is a measure of energy flow per unit of area, per unit of time, which is also power, measured in watts/m squared

Question 9

Define power- what is it measured in?

Answer 9

The capacity to exert physical force or energy is expressed as the rate at which energy is expended (watts)

Question 10

Does the logarithmic scale contain a zero?

Answer 10

No, it is a scale of ratios, always comparing two values. If we want to compare two sound intensities or powers, then one must have a reference.

Question 11

What reference do we use for intensity level?

Answer 11

10 to the -12 watts/m squared or 10 to the -16 watts/cm squared

Question 12

What is the formula to convert from watts/m squared to decibels?

Answer 12

dB IL= 10 x log (Io/Ir)

Io= the output of the sound you are measuring
Ir= the reference, which is 10 to the -12 watts/m squared

Question 13

What happens if the intensity output and reference are equal?

Answer 13

Then the ratio is 1:1, and since the log of 1= 0, the answer is 0 dB IL. This does not mean there is no sound, but that it is the softest sound possible for humans to perceive.

Question 14

What is sound pressure level?

Answer 14

The common measurement and reference for acousticians to use for sound is in terms of sound pressure.

Question 15

What is force?

Answer 15

A push or pull. When vibrating bodies move, they exert force on air molecules, and the greater the force, the greater the displacement/amplitude.

Question 16

What is the Dyne?

Answer 16

A unit of measurement for quantifying small changes in force. 1 dyne is a force sufficient to accelerate a mass of 1 gram at 1 cm per second/squared

Question 17

What is a Newton?

Answer 17

A measurement of force that will accelerate 1 kg of mass per second squared.

Question 18

What is pressure?

Answer 18

Generated when a force is distributed over a surface area (P=F/A)

Question 19

What is the unit for pressure?

Answer 19

Dynes per cm squared or Pascals

Question 20

One Pascal is equal to…

Answer 20

1 Newton/square m

Question 21

What do we use to express sound pressure in the range of human hearing?

Answer 21

Micropascals- smallest pressure variation to produce just audible sound is 20 micropascals

Question 22

What is the formula for converting a sound in pressure?

Answer 22

dB SPL= 10 x log(Posquared/Prsquared) OR

dB SPL= 20 x log(Po/Pr)

Question 23

What is the reference for pressure?

Answer 23

The average ear can detect a sound at 20 micropascals, so this is the reference. If plugged into the formula, 20 micropascals is equal to 0 decibels SPL.

Question 24

What happens if the intensity of a wave is doubled?

Answer 24

You increase the decibels by 3 IL because the intensity output of the two signals are added together and then converted back to decibels.

Question 25

What is an interval scale?

Answer 25

A scale in which the intervals between successive values are equal or linear (2, 4, 6, 8…)