Measuring & Quantifying Sound

Question 1

What does a scale consist of?

Answer 1

• Reference point

- Method of dividing up the scale

Question 2

What is an Interval Scale?

Answer 2

• When a base is known, we can say that one object is a certain number of intervals more/less than another
• EX: Base of 1 –> 5 is 1 interval > 4
• EX: Base of 2 –> 6 is 1 interval < 8
• EX: Base of 10 –> 20 is 3 intervals < 50

Question 3

What is a Ratio/Logarithmic/Exponential Scale?

Answer 3

• One object is so many times greater/lesser than another
• Successive units formed by multiplying/dividing each number by the base
• Unit of measure covers a smaller amplitude change at the bottom than the top of the range
• Scale used for sound intensity (amplitude)

Question 4

Why is a logarithmic scale used for sound intensity?

Answer 4

• Response of the human ear is logarithmic: loudness is proportional to the logarithm of sound intensity
  - Weber’s Law: minimum detectable change is a constant proportion of the overall level (as sound intensity increases, the min. difference detectable increases)
  - Correlates more closely with the subjective perceptual experience, at both ends of the scale
• Addition/subtraction can be used instead of multiplication/division
• Convenient way of dealing with large range of acoustic magnitudes we deal with (20uPa - 20Pa)

Question 5

Describe Acoustic Scales.

Answer 5

• References vary a lot, but the increment is always dB
• -> Bel = log (I/Iref)
• -> dB = 10 * log (I/Iref)

Question 6

What is Power (P)?

Answer 6

• Rate at which energy is emitted
• Sound power of a source is the total power emitted by that source in all directions
• Reference of 10^-12 W

Question 7

What is Intensity (I)?

Answer 7

• Sound of energy passing per second through a unit area held perpendicular to the direction of propagation of sound waves
• Only positive

Question 8

What is Pressure (p)?

Answer 8

-A positive and negative quantity

Question 9

How are Power, Intensity, and Pressure related?

Answer 9

-P, I, and p^2 are all proportional

Question 10

Describe Decibel (dB) Scales.

Answer 10

• dB is a relative measure (not absolute)
• ->L2 - L1 = 10 * log (I1/I2)
• ->L2 - L1 = 20 * log (p1/p2)
• Not linear because it’s a logarithmic measure with an arbitrary reference point
  
  • 0 dB doesn’t mean no sound (it means equal to the reference)
  • Any positive dB means greater than the reference
  • Any negative dB means less than the reference
  • No sound can’t be represented because log(0) doesn’t exist

Question 11

Describe the dB SPL scale.

Answer 11

• Equal to dB IL or dB SIL
• Measure of relative intensity (amplitude)
• ->dB SPL = 20 * log (pressure in uPa / 20 uPa)
• Uses reference of threhold of hearing at 1 kHz tone:
  - P0 = 20 uPa
  - I0 = 10^-12 W

Question 12

Describe the dB SWL scale.

Answer 12

• SWL: Sound Power Level

- P0 = 10^-12 W

Question 13

Describe the dB HL scale.

Answer 13

• Equal to dB HTL (Hearing Threshold Level)
• Reference level varies with frequency according to ANSI RETSPLs
• May need to convert to dB SPL for calibration

Question 14

Describe the dB SL scale.

Answer 14

-Reference is individual’s own thresholds for a given frequency

Question 15

What is dB(A) weighting?

Answer 15

• Follows human perception based on a 40 phon curve
• Widely adopted for environmental noise measurement
• Originally designed for low level sounds
• Discrepancy above 10 kHz

Question 16

What is a phon?

Answer 16

• Unit on an Equal Loudness Contour

- Level in dB SPL of an equally loud 1000 Hz sine tone

Question 17

What is dB(B) weighting?

Answer 17

• Update to dB(A) weighting for more intense sounds

- Not often used

Question 18

What is dB(C) weighting?

Answer 18

• Flatter frequency response

- Used most often after dB(A)

Question 19

What is dB(D) weighting?

Answer 19

• High level aircraft noise

- Not often used

Question 20

What is dB(Z) weighting?

Answer 20

• Zero weighting
• Originally, manufacturers could choose end points
• Results in different peak level readings

Question 21

What is Interaural Level Difference (ILD)?

Answer 21

-Measure difference using other ear as reference

Question 22

Describe the measurement of Equal Sound Intensities.

Answer 22

• ->dBn = dBi + 10 * log(n)
• i: dB SPL or IL from one source
• n: # of sources being combined

Question 23

Describe the combination of sound properties.

Answer 23

• Level can’t be added linearly (L3 doesn’t equal L1 + L2)
• Intensity can be added linearly (I3 = I1 + I2)
• Pressure can’t be added linearly because of phase effects (p3 doesn’t equal p1 + p2)

Question 24

Describe speech calibration in the sound field.

Answer 24

• 2 steps:
  
  1. 1000 Hz tone is used to calibrate the level at the input to the audiometer
  2. Noise is used to calibrate the output from the loudspeakers in the sound field