Lecture 7: Masking

Question 1

What is the definition of masking?

Answer 1

The process by which the detectability of one sound, the signal, is impaired by the presence of another sound, the masker

Question 2

What are the different types of masking?

Answer 2

• 18 different types of masking
  1) Monaural (ipsilateral): signal and masker presented to the same ear

2) Binaural: both signals and maskers presented to both ears
Diotic: SoNo: signals in phase, noise in phase
Dichotic: SpiNo: signals out-of-phase, noise in phase
(could also be SoNpi)

3) Central: signal to one ear, masker to contralateral ear
- Causes poorer threshold in test ear due to central interaction

Question 3

What is temporal masking?

Answer 3

• A type of monaural masking
• Masking for signals and masking noises at different points in time

Forward masking: threshold shift by 20 dB at 10 ms
Backward masking: threshold shift by 10 dB at 10 ms

Question 4

What is laboratory masking?

Answer 4

• Usually a signal and masker are presented to the same ear

“ipsilateral masking”

Question 5

What is clinical masking?

Answer 5

• Signal is presented to one ear but crosses to non-test ear
• Masker is presented to non-test ear
• Purpose: mask detectability of signal that has crossed over to non-test ear
• Masker for non-test ear creates threshold shift from Q to N conditions in that ear ( a form of ipsilateral masking)

“We present the masking in the non-test ear to shift the detectability of the signal that crosses over to the non-test ear”

Question 6

What is the white noise masker?

Answer 6

Spectrum:

• all frequency components: 1 Hz-100,000 Hz
• Each component is equal in amplitude

Effective spectrum: determined by frequency response of transducers

• Dense and flat across spectrum
• Most predictable of all maskers

Problem: white noise is very loud

Question 7

What are narrow bands of noise?

Answer 7

Assumptions
1) For a given tone, only the energy in the white noise (WN) masker around that pure tone’s frequency is effective in masking the pure tone

2) At the masked threshold, the power in the tone equals the power in the critical band
3) The bandwidth of the CB is determined by the pure tone you are masking

Question 8

How can you describe a band of noise?

Answer 8

• The width of the band of frequencies (f2-f1, in Hz)
• The relative intensity of the bandwidth re: one component in the band (in dB)
  
  • Use power formula: 10log(p1/p2)
  • 10log (bandwidth/1)

Question 9

What are the components of noise?

Answer 9

1) Overall intensity: SPL generated by all components in noise

2) LPC (level-per-cycle, or spectrum level):
- SPL generated by one component of the noise

3) Bandwidth (in dB or Hz): how wide is the band of noise ( in Hz) or the relative level of the band of noise compared to 1 cycle (in dB)

These are related by
OL = LPC + BW (in dB)

Question 10

How can you convert bandwidth (in Hz) to dB?

Answer 10

BW (in dB) = 10log BW (in Hz)

Question 11

How can you determine the LPC?

Answer 11

Need to know the overall level & BW (in dB or in Hz) and calculate LPC

Ex. OL = 100 dB SPL
BW = 5500 Hz

LPC = OL - BW
LPC = 100 dB SPL - 37.40 dB
LPC = 100 dB SPL - 37 dB
LPC = 63 dB SPL

Question 12

What are the direct measurements of critical band?

Answer 12

Method: measure masked threshold in noise with decreasing bandwidth

Findings: width of CB > CR by 2.5 times, or by + 4 dB, above 500 Hz

Width of NBN masker in audiometer: used to be based on critical ratios, then critical bands

Question 13

What does the narrow band noise sound like in the audiometer?

Answer 13

• ANSI 2010 requires bandwidths to be wider than CBs
• Bandwidths are now between a 1/3 octave band and a 1/2- octave band
• Need to check audiometer specs. and compare NBN BW to audiometer

Question 14

What is effective masking level (EM)?

Answer 14

Definition: SPL of a band of noise whose geometric center frequency coincides with that of a specific pure tone that masks the pure tone to 50% probability of detection

• EML is analogous to hearing level
• For normal hearing listeners, EML = #dB that a given band of noise shifts a pure tone threshold

Alternate definition: how much a masker shifts threshold, relative to threshold in quiet (0 dB HL), for normal-hearing listeners

Question 15

How can you complete the calibration of a masking unit in EML?

Answer 15

• Know the RETSPLs for pure tones at each frequency, for your earphones
• Know the bandwidth of the narrow-band noise for your audiometer
• Add correction factor for noise bandwidth to pure tone RETSPL
• Provides RETSPL for each noise band