Critical Bands by Masking Lab



  1. Purpose and Goals
    1. To determine if critical bands, as determined by masking, are the same for all frequencies.
    2. To learn to test for limits.
    3. To design your own experiment
  2. Critical Bands - Review from material in class
    1. Observations
      1. How effectively a noise masks a tone depends not so much on the loudness of the noise but the match between the frequencies of the noise and the tone.
      2. There is a region around the frequency that is trying to be detected where noise can act as a masker.
      3. Noise outside this band is ineffective as a masking stimulus.
    2. Question: Are these critical bands always the same size for every frequency?
  3. Finding Limits
    1. Below is our graph from the class critical bands experiment
      1. Note how for the 1200 Hz tone, the masking stays about the same until the frequency of the noise is less than 100 Hz.
      2. This suggests that we can place that critical band at between 30 and 100 Hz.
      3. This is a send of finding the limit or size of the critical band and an example of what is needed to answer the question posed for th experiment.
    2. In this experiment you will ask some question about critical bands.
      1. You will be asked to design your own experiments to conduct this test.
  4. Experimental Design
    1. Experiments involve:
      1. Independent Variable(s): the variable(s) the experimenter manipulates
      2. Dependent Variable: the variable the experimenter measures
      3. Control: Change only one thing at a time.
    2. In this experiment, the IV and DV will be decided upon by you.
      1. IVs: These will depend upon the question that you want to ask.
      2. DV: this depends upon what you need to observer and the method that you use.
    3. Your task is to develop this experiment so that you can learn something about critical bands.
    4. Your design can be adjusted as you go to make sure you get a good answer, though you need a well structured experiment to begin with.
      1. This freedom is often part of research in many areas of science.
      2. However, in many psychology experiments, all of the conditions have to be set up before the data collection begins because of the nature of testing groups of subjects.
      3. Since psychophysics tests individuals, and the few of you are easily recollected, you can make some adjustments on the fly.
      4. The inability to answer the question of this lab report will lead to a significant negative impact on the grade for this lab.  Remember you may need to add new conditions to test.  Give yourself and your partners time.

The Experiment:

  1. Equipment
    1. Back to the Experiencing Sensation and Perception Software
    2. Use the Masking and Critical Bands Activity.
    3. There are headphones available as well.  You will want to use them in your rooms if you have any.  In lab you many need them to keep the sounds straight.
  2. Common Design Elements:
    1. Here are the common settings on the Experimental Parameters Page that everyone will use:
      1. Type of Activity: Experiment
  3. Variable Design Elements:
    1. Any of the stimulus parameters on the stimulus setup page for the experiment.
      1. These will depend upon your experimental design.
      2. Chose them to try to see clearly what is going on in the data
      3. Adding new conditions is one way to clarify your findings
    2. The experimental method
      1. You can choose:
        • Method of Limits,
        • Method of Constant Stimuli, or
        • Method of Adjustment
      2. The settings for the method
      3. Choose the method that allows for clear results.
        • Too easy can allow for sloppy results
        • Too hard can cause fatigue which can mess up your results without taking breaks
      4. Do not under/over collect data.
    3. You will work in a group of 3-4 to design your experiment.  I will assign people.
  4. Procedure:
    1. Most will depend upon the nature of your experiment.
    2. There are some common element:
      1. The top graph shows the tone. 
      2. The bottom graph shows the noise.
      3. At the bottom of the screen are the response buttons: The response buttons will depend upon the method.  Instructions will be displayed there.
  5. Data
    1. Convert the gain measures you get out of the computer program to decibels
    2. Use this spreadsheet to do it
  6. Write-up: (Full report)
    1. For next week, as a group, write up your design to share with me.