BMLD

Question 1

What is the binaural masking level difference (BMLD)?

Answer 1

“a measure of the improvement in detectability of a signal that can occur under binaural listening conditions. It is the difference in threshold of the signal (in dB) for the case where the signal and masker have the same phase and level relationships at the two ears and the case where the interaural phase and/or level relationships of the signal and masker are different.”

Question 2

What is the cocktail party effect?

Answer 2

The detection of a signal in noise is improved when either the phase or level differences of the signal at the two ears are not the same as the masker. It is the ability to discriminate sounds in complex acoustic environments based on the sources’ spatial separation.

Question 3

What is spatial release from masking?

Answer 3

The improvement patients receive from the spatial separation of speech and noise.

Question 4

Why is the spatial release from masking important?

Answer 4

It makes it easier to detect the signal and is important for speech understanding in complex listening environments.

For example, it makes it easier to hear a particular speaker at a noisy party or to identify an instrument in an orchestra.

Question 5

What do experiments examining spatial release from masking involve?

Answer 5

They require changing the location of the signal and measuring the improvement in understanding the signal

Question 6

What are the benefits from spatial release from masking?

Answer 6

• localization

- better speech understanding when the target and masker are separated

Question 7

What is the masker that can be used in a BMLD experiment?

Answer 7

• A narrowband noise or a broadband masker can be used.

Question 8

True or False: The BMLD is not limited to a pure tone signal.

Answer 8

TRUE

It can be observed using complex tones, clicks, and speech signals.

Question 9

Who first described BMLD for pure tones?

Answer 9

Hirsch (1948)

Question 10

Who first described BMLD for speech signals?

Answer 10

Licklider (1948) first described the phenomenon for speech signals.

When speech signals are used, the difference in speech threshold for a dichotic versus diotic condition is referred to as the binaural intelligibility level difference (BILD).

Question 11

Is the MLD smaller or larger when speech signals are used vs. tones?

Answer 11

When speech signals are used, the MLD is smaller than when tones are used (Wilson, 1994).

AKA larger MLD with tones

Question 12

Does the MLD increase or decrease as the frequency of the tones increases?

Answer 12

the MLD decreases as the frequency of the tone increases (Koehnke et al., 1986).

Question 13

Define monotic

Answer 13

Indicates that the stimulus is presented to only one ear

Question 14

Define diotic

Answer 14

Indicates that there is an identical stimulus presented to both ears or there is no interaural difference for the signal and the masker presented to each ear

Question 15

Define Dichotic

Answer 15

A condition in which different stimuli are presented to the two ears

Question 16

Describe a BMLD experiment

Answer 16

An example experiment would be to use narrowband noise and a 500 Hz pure tone signal that is presented to normal hearing individuals over headphones. The patient is instructed to indicate when they hear the signal, in order to find their threshold for detecting the sound using an adaptive procedure. The threshold is obtained in 2 separate conditions. The first condition is when the masker and signal are presented binaurally with no interaural difference (N0S0), and the second condition is when the masker and signal are presented binaurally with no interaural difference and the signal is simultaneously presented 180 degrees out of phase with the signal in the other ear (N0Sπ). The BMLD is the difference between these two thresholds. Furthermore, this testing can be completed using pure tones at different frequencies.

Question 17

How do BMLD experiments differ from experiments measuring spatial release from masking?

Answer 17

BMLD experiments involve changing the phase of the signal and/or masker

Spatial release from masking experiments involve changing the location not the phase of the signal

Question 18

Describe the diagnostic value of BMLD for APD

Answer 18

BMLD has diagnostic value when evaluating for central auditory processing disorder (CAPD).

Central auditory processing disorder is a complex disorder that is characterized by difficulties in processing information by the central nervous system (ASHA, 1996).

Individuals with CAPD have difficulty in one or more areas of auditory function, which includes binaural interaction.

Thus it is important to include a test such as BMLD as part of the CAPD test battery to assess a patient’s binaural processing.

Question 19

Describe a study that described the use of BMLD for CAPD diagnosis.

Answer 19

Delb et al., 2013

One study completed in Germany examined whether the binaural interaction component (BIC) could be used to differentiate normal children from those at risk for CAPD and what correlation these measurements had with subjective audiometric tests.

The binaural interaction component (BIC) is the “difference between the sum of the monaurally evoked auditory brainstem responses and the responses after binaural stimulation” which is similar to BMLD.

What they found was that the BIC is more evident in children at risk for CAPD than normal children. Furthermore, the BIC is 76% sensitive and specific at identifying children with CAPD.

Question 20

Describe the age effects on BMLD, particularly for older adults

Answer 20

Grose et al., ,1994

• study examining the masking level difference using tones and speech in adults 63-80 years of age and older who had relatively normal/near normal thresholds compared to young (21-25 years old), normal hearing adults.
• A 500 Hz tone and a narrowband masker centered at 500 Hz were used as the stimuli and were presented in 2 conditions (N0S0 and N0Sπ) over headphones.
• In the N0S0 condition, the signal and masker were presented bilaterally in phase with each other.
• In the N0Sπ condition, the masker was presented in phase to both ears, while signal was presented to both ears 180 degrees out of phase.
• The participants indicated what they heard using a three-interval forced-choice. The level was adjusted using an adaptive procedure.
• The same participants then underwent testing using 36 spondaic words of the CID W-1 list that were filtered at 4000 Hz and a broadband noise masker with the same LTAS as the speech.
• To obtain the MLD, the stimuli were presented at a fixed SNR with at least 2 measures for each SNR for both conditions (the same ones used with the tones).
• What they found was that the older adults have reduced BMLDs as well as a reduced ability to separate the target from the masker (spatial release from masking).

Question 21

What is one way to rectify the complications of age effects in the diagnosis of CAPD?

Answer 21

Use other tests that measure the binaural response of the patient which includes electrophysiological testing.