Measuring & Quantifying Sound Flashcards
What does a scale consist of?
- Reference point
- Method of dividing up the scale
What is an Interval Scale?
- 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
What is a Ratio/Logarithmic/Exponential Scale?
- 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)
Why is a logarithmic scale used for sound intensity?
- 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)
Describe Acoustic Scales.
- References vary a lot, but the increment is always dB
- -> Bel = log (I/Iref)
- -> dB = 10 * log (I/Iref)
What is Power (P)?
- 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
What is Intensity (I)?
- Sound of energy passing per second through a unit area held perpendicular to the direction of propagation of sound waves
- Only positive
What is Pressure (p)?
-A positive and negative quantity
How are Power, Intensity, and Pressure related?
-P, I, and p^2 are all proportional
Describe Decibel (dB) Scales.
- 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
Describe the dB SPL scale.
- 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
Describe the dB SWL scale.
- SWL: Sound Power Level
- P0 = 10^-12 W
Describe the dB HL scale.
- 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
Describe the dB SL scale.
-Reference is individual’s own thresholds for a given frequency
What is dB(A) weighting?
- 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
What is a phon?
- Unit on an Equal Loudness Contour
- Level in dB SPL of an equally loud 1000 Hz sine tone
What is dB(B) weighting?
- Update to dB(A) weighting for more intense sounds
- Not often used
What is dB(C) weighting?
- Flatter frequency response
- Used most often after dB(A)
What is dB(D) weighting?
- High level aircraft noise
- Not often used
What is dB(Z) weighting?
- Zero weighting
- Originally, manufacturers could choose end points
- Results in different peak level readings
What is Interaural Level Difference (ILD)?
-Measure difference using other ear as reference
Describe the measurement of Equal Sound Intensities.
- ->dBn = dBi + 10 * log(n)
- i: dB SPL or IL from one source
- n: # of sources being combined
Describe the combination of sound properties.
- 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)
Describe speech calibration in the sound field.
- 2 steps:
- 1000 Hz tone is used to calibrate the level at the input to the audiometer
- Noise is used to calibrate the output from the loudspeakers in the sound field
