Materials and Methods: It is for the first time in Iran that designing and construction of phase-shifter instrument for performing Masking Level Difference have been studied. For reducing limitations of MLD test in middle ear pathologies, new methods of performing MLD has been considered.
In this method noise is conducted by bone vibrator, located on forehead. The comparison between standard method and proposed method has been obtained.Forty 20-25 year-old men referring to the Faculty of Rehabilitation Science in Iran university of Medical Sciences have been tested in the period of time between 10.30.1998 to 12.21.1998.The MLD test was performed in both method at frequencies of 250, 500, 1000, 2000 & 4000 Hz.
Results: The mean of MLD results n the standard method was 11.175 dB ranging from 10 to 13 dB that is in accordance with normative data.
- There was no significant difference between the means obtained in the methods for performing MLD.
- In both methods the MLD means decreased with frequency increments.
Conclusion: It is preferable to use proposal method in middle ear disorders due to normal cochlear sensitivity and subsequently enough noise that is conducted by bone conduction (60 dB SPL).

Background and Aim: There are evidences that indicate a relationship between auditory processing disorders and stuttering, and any disorder in the central auditory function can be at least one of the underlying causes of stuttering. Even though, using the most state of the art radiographic technologies, i.e. MRI, no definitive answer has been given in relative to this question. In this research, using Mask-ing Level Difference (MLD) and Synthetic Sentence Identification (SSI) tests, the central auditory function of stutters and normal group was evaluated.
Materials and Methods: In this study was analytic cross-sectional, fifteen male patients with stutter-ing and 15 male normal cases with the age range from 16 to 40 years (average age 26.78 year) were evaluated. SSI-ICM, SSI-CCM and MLD tests were performed. The results were compared in both groups.
Results: Although stutterers mean MLD was less than that of normal group, the different was not significant between stutters and normal group in SSI test in right ear at negative MCRs. There was a significant difference in ICM state, but in CCM state, there was no significant difference between the average score of two groups in various MCRs.
Conclusion: The findings of this research is compatible with those of similar researches about the SSI test and the pattern of results, probably indicates a partial dysfunction of brainstem in some of the stutters.

Background and Aim: Multiple sclerosis (MS) is a neurological disorder that involves central nervous system. Studies have showed that multiple sclerosis affects behavioral central auditory tests, such as masking release or masking level difference (MLD). The purpose of this study is to compare the masking level difference between multiple sclerosis patients and normal subjects.
Methods: This cross sectional and non-interventional study was conducted on 32 multiple sclerosis patients aged between 20-50 years and 32 controls matched for age and gender in Faculty of Rehabilitation, Tehran University of Medical Sciences. masking level difference test was performed on each subject.
Results: The mean masking level difference in the two groups was significantly different (p<0.01) however, gender did not prove to play a role in this difference.
Conclusion: As part of the multiple sclerosis diagnosis panel, masking level difference test is an efficient modality for evaluation of hearing impairment and monitoring of rehabilitation progress.