IntroductionGENERAL INFORMATIONMasking experimentsPart 1: Masking patternObjectiveStimuliMethodData storagePart 2: Are all maskers equal?ObjectiveStimuliTargetMaskerMethodData storageDATA ANALYSISLAB REPORTFundamentals of Perceptual Audio Encoding“All maskers are equal, but some are more equal than others.”Michael K. [email protected]: 258-5880INTRODUCTION No aspect of auditory psychophysics is more relevant to the design of perceptual auditorycoders than masking, since the basic objective of perceptual audio coders is to use the masking properties of sounds to hide quantization noise. In the spirit of MIT’s motto, Mens et Manus or learn by doing, this lab allows you the opportunity to carry out some psychophysical measurements on yourselves and gain some “ear-on” experience with auditory masking. The experiments should be carried out in pairs, so you can take turns running the experiments. GENERAL INFORMATION The experiments are to take place in the sound booth in the middle of Room 36-744.The entire lab session will take approximately 2-3 hours. The lab session is divided into two parts. In part one, you will be measuring the masking pattern associated with a narrowband noise. In part two, you will be measuring the masking thresholds in the presence of various types of masker. The waveforms are created on a PC using Matlab. Sound is generated from those waveforms using a 24-bit digital-to-analog converter (DAC) in the PC. The electrical signal is then fed via a headphone buffer (TDT HB6) to the booth. In the booth, the stimuli are presented via Sennheiser HD580 headphones (located in the booth).Before you start the experiment, it is very important to make sure that the wiring and the attenuation settings are correct. Make sure the HB6 switch is set to 6 dB.Log onto the computer with the user name: HST723and with the password: (provided in class)Start up Matlab. On the command line, enter: cd M:\Experiments\Perceptual_encoding There should be a handheld voltage meter outside the booth. Use this to verify the voltage at the headphone amplifier left output. To check the voltage enter: calibrate(‘mid’,+6,’l’)This tells the system that you have +6 dB attenuation in the path (from the headphone amplifier) and that you are testing the left channel only (‘l’).On the screen will then appear the voltage you should expect to measure at the output to the headphone buffer. Check that the actual voltage does not differ from the predicted voltage by more than about 10%. MASKING EXPERIMENTS Masking refers to the process by which the presence of one sound (masker) elevates the threshold (changes the audibility or sensitivity) of another sound (target). The hearing threshold in the presence of a masking signal is called the masked threshold. Figure 1: Masked threshold and threshold in quietThe masked threshold is the threshold intensity of a target signal in the presence of a masking stimulus with a specific intensity (Fig. 1). When the masker intensity is set equalto zero, the masked threshold is just the probe intensity at the hearing threshold in quiet.Part 1: Masking patternObjectiveIn this part of the lab, you will be measuring your hearing threshold in quiet and the masked threshold associated with a narrowband noise center at 1 kHz. You have the option of measuring your left ear or your right ear, but it is important to stay consistent.StimuliThe target stimuli are tones from 200 Hz to 4 kHz, starting at 30dB SPL.The masker stimulus is a narrowband noise with a bandwidth from 990 to 1010 Hz and a spectrum level of 60 dB SPL. The experimental parameter is the level of the target tones. MethodThe method of adjustment is used in this part of the lab.This method is also known as the Bekesy method. As thesubject, your objective is to adjust the levels of the targettones to maintain them at the just detectable levels. To start the experiment, enter the following line in theMatlab command window:  cd Mask_pattern;bsy_main('Mask_pattern','XX_repX','mid','0','1');Note that all the arguments are in single quotes and are separated by commas. The first argument is the experiment name; the second is for your initials and the repetition number (e.g., ‘js_rep1’ for John Smith); the third is the booth name (‘mid’ in this case), the fourth is the amount of attenuation set on the TDT PA4, and the fifth is the condition under examination (‘1’ for the threshold in quiet in the left ear, ‘2’ for the threshold in quiet in the right ear, ‘3’ for the masked threshold in the left ear, and ‘4’ for the masked threshold in the right ear).After enter the above command, a dialog window will appear. You will be instructed to press the space bar to start the experiment session. If you are measuring your threshold in quiet, you will hear a high pitch tone at the beginning of the experiment session. The level of the target tone should begin at an easily detectable level. By pressing on the space bar you slowly lower the level of the target tone. When you release the space bar,the level of the target tone will slowly increase. As the experiment processes, the target tone will sweep slowly from high frequency to low frequency. As the subject, your only task is to maintain the target tone at the just detectable level. If you are measuring your masked threshold, at the beginning of the experiment session you will hear a high pitch tone accompanied by a mid frequency tonal sound. The high pitch tone is the target tone and the mid frequency tonal sound is the masker. Here, as in the case of measuring threshold in quiet, the target tone should also begin at an easily detectable level. By pressing on the space bar you slowly lower the level of the target tone. When you release the space bar, the level of the target tone will slowly increase. However, the masker will not be affected by your actions. As in the above case, your as the subject has the task of maintaining the target tone at the just detectable level. The procedure is not easy to master. I highly recommend going though procedures two orthree times. To indicate the multiple experimental runs, you can add a repetition number to the end of your initials.Data storageThe results from your experiment are stored in the “Result” directory, under the file named “mask_pattern_xx.dat” (where xx should be your initials). Part 2: Are all maskers equal?ObjectiveIn this part of the lab, you will be