Project 3 Spring 2009
A "Functional" Representation of Audition
With and Without Hearing Loss and Amplification

Download PROJ3TEMP2007.xls from Weekly Schedule

A.  Introduction

The general goal of the project is to apply the principles of systems analysis to analyze a real-world speech and hearing problem. Project 3 involves the development of a model of sound transmission through the auditory system.  Each major anatomical division of the auditory system will be treated as a separate subsystem represented by a characteristic transfer function.  Students will add hearing loss to the system, choosing from among several different types and degrees of hearing loss provided in the template.  Finally, gain will be added to "correct" for that hearing loss, similar to the gain provided by a hearing aid.  In each case, the output of the total system will be treated as the sound that is heard by the listener. 

B.  Background

The softest sound that can be heard by the listener is termed the absolute threshold for that sound.  This serves as a measure of the sensitivity of the auditory system.  The Excel template for Project 3 is PROJ3TEMP2007.xls and must be downloaded from our website, Assignment Details.  The worksheet "MAIN dB" estimates three transfer functions in dB. Those three functions are (1) the ear canal transfer function (Shaw, 1974); (2) the middle ear transfer function (Kurokawa and Goode, 1995); (3) the cochlear transfer function. The latter function is modeled as a constant loss across frequency (Rosowski, 1991).  The worksheet "HEARING LOSS" contains the gain values associated with the specific hearing losses listed in the table below. The worksheet "Prescriptive Fitting Rules" is for graduate-level work for determining the target gain of a hearing aid.

C.  Follow the steps below and the Guidelines for Writing Project Reports to complete Project 3. 

  1. Predict absolute threshold in dB SPL.

    1. Create an Excel chart for each of the transfer functions in the "MAIN dB" worksheet (remember to use a logarithmic scale on the abscissa and to label your charts appropriately). 

    2. Similar to PS 3.2, compute the composite function representing sound transmission through the auditory system. Create a chart of the model output, and place it to the right of your composite transfer function.

    3. Now, assume that the auditory system requires a constant level of stimulation at the output of the cochlea to achieve detection threshold. This constant level of stimulation is the same at all frequencies, and we will assume that it corresponds to 0 dB at the output of the cochlea. In other words, when a stimulus passes through the external ear, middle ear, and cochlea and still deliver the equivalent of 0 dB of sound pressure to the sensory cells, then we will assume that it is detected.
      • Compute the input amplitudes in dB SPL necessary to reach the output threshold of 0 dB at 250, 500, 1000, 2000, 4000, and 8000 Hz, given your composite transfer function obtained in (b).
      • Create a chart of these threshold values. This is the curve for absolute thresholds for normal hearing.

  2. Simulate a hearing loss.

    1. Choose a specific "site of lesion" or location of auditory pathology (outer ear, middle ear, or cochlea). Choose a specific configuration of hearing loss, listed in the table below that corresponds to the chosen site of lesion. A transfer function representing the gain characteristic of each hearing loss option is provided on worksheet "HEARING LOSSES" of the template. Note that these hearing loss transfer functions provide gain values in dB at each of the octave frequencies from 250 to 8000 Hz and values in between octaves. The values are negative in sign because the loss produces negative gain.

        Type/Site Degree/Configuration
      H1 COND-OE Mild, flat hearing loss
      H2 COND-ME Mild, flat hearing loss
      H3 COND-ME Mild, sloping hearing loss
      H4 COND-ME Mild, rising hearing loss
      H5 SNHL-C Mild to moderate, flat hearing loss
      H6 SNHL-C Normal to moderate, sloping hearing loss
      H7 SNHL-C Normal to severe, steeply sloping hearing loss
      H8 SNHL-C Moderate, 4000-Hz notched hearing loss

      COND = Conductive hearing loss (pathology in outer or middle ear or both)
      SNHL = Sensorineural hearing loss (pathology in the cochlea, brainstem, or central pathway)

    2. On the "Hearing Losses" worksheet, create a chart to represent the chosen hearing loss transfer function.

    3. Insert the transfer function representing the desired loss at the appropriate location to the right of the transfer function for the EE, ME or CO subsystem.

    4. Compute a new composite output with hearing loss included.

    5. Create a chart of the new composite output.

    6. Following the reasoning in step 1(c) above, plot the absolute threshold sensitivity in dB SPL, that is the input required to reach threshold in dB for the ear with the hearing loss.

  3. Simulate a hearing aid that uses amplification and filtering to "correct" hearing loss.

    For those who are interested:  A hearing aid is an electronic device that takes sound in the environment, amplifies and filters that sound, and then delivers the modified sound directly to the listener's ear.  The four most common body styles are behind-the-ear (BTE), in-the-ear (ITE), in-the-canal (ITC), completely-in-the-canal (CIC), and body aids.  In the most simple of cases, a hearing aid consists of a microphone, an amplifier, a power supply (battery), a volume control, and a speaker (sometimes called receiver). The user can adjust the volume control for a given sound environment and can turn the aid on and off as needed.  Modern hearing aids offer many more features, including miniature sizes, computer chips for signal processing, and remote control capability.

    1. Estimate the amount of amplification needed to compensate for the hearing loss above. This is computed as the inverse function of the hearing loss transfer function, similar to the equilization function from PS3.1.

    2. Chart the transfer function representing amplification and place below your Hearing Loss chart.

    3. Place the amplification transfer function before the EETF in order to simulate a hearing aid. Compute the composite transfer function with both amplification and hearing loss to verify that you have a perfect hearing aid. Create a chart for this composite.

  4. For the Results section of your report, explain in your own words what were the primary outcomes of steps 1, 2 and 3. Include print outs of only those charts that you referenced in your results. Upload both your .xls spreadsheet and your report to the Oncourse dropbox.

  5.  

References

Kurokawa, H., and Goode, R. L. (1995). Sound pressure gain produced by the human middle ear. Otolaryngology: Head and Neck Surgery, 113, 349 – 355.

Mueller, H. G., Hawkins, D. B., and Northern, J. L. (1992). Probe Microphone Measurements: Hearing Aid Selection and Assessment. Clifton Park, NY: Singular Publishing Group.

Rosowski, J. J. (1991). The effects of external- and middle-ear filtering on auditory threshold and noise induced hearing loss." Journal of the Acoustical Society of America, 90, 124 – 135.

Shaw, E. A. G. (1974). Transformation of sound pressure level from the free field to the eardrum in the horizontal plane. Journal of the Acoustical Society of America, 56, 1848 – 1861.