### In Class Notes for Project Assignment

```EE 221 Mid-Term “Hearing Aid” Project
Objective
The purpose of this project is to design, optimize and
construct a hearing aid to correct your client’s hearing
response to that of a normal hearing response.
Background
This project combines your knowledge of the frequency
response of AC circuits, transfer functions, passive and
active filters to enable the design of a filter system with a
complex transfer function.
Outcomes
Understanding a design process
Top-down design
Increased knowledge of more complex filter design and
Cascading, summing transfer functions for a more complex
frequency response
Team work
EE 221 S. Gedney, University of Kentucky
General Design Process - SIDDE
“S”top!
“I”dentify the problem
Identify the specifications and constraints
Formulate a clear and precise problem statement
“D”iscuss
Brain storm a set of potential solutions
Sketch a top level design for each proposed solution
Question if each design will meet the specifications and
constraints of the problem
“D”ecide
Choose the solution that best meets the constraints & specs
“E”xecute
Optimize the design, prototype, improve, optimize again,
validate
EE 221 S. Gedney, University of Kentucky
SIDDE
STOP
DISCUSS
DECIDE
No EXECUTE
Did not Meet
contraints & specs
Meets contraints &
specs
Yes
Prototype
EE 221 S. Gedney, University of Kentucky
Top-Down Design Process
Top-Level View
View of the problem as a whole
Break up the problem into high-level functions
Develop a block diagram of highest of the problem
Each block represents a high-level description of a system
function
Identify the purpose of each function block
Hierarchal Description
Under each function block lies another level of design
Set of function blocks describing each facet of the system
Continue to the lowest level
EE 221 S. Gedney, University of Kentucky
Top-Level Design of the Hearing Aid
EE 221 S. Gedney, University of Kentucky
Hearing Aid Transfer Function
Clients Ear
a ( jω )
C ( jω )
Hearing Aide
a ( jω )
Therefore
or
H ( jω )
o( jω ) = C ( jω )a ( jω )
Clients Ear
C ( jω )
o( jω ) = N ( jω )a ( jω )
N ( jω )
H ( jω ) =
a ( jω )
H=
( jω ) dB N ( jω ) dB − a ( jω ) dB
EE 221 S. Gedney, University of Kentucky
The Hearing Aid
Problem description
In this project you are asked to design a custom hearing aid that
amplifies sound non-uniformly over the hearing spectrum to
compensate for frequency dependent hearing deficiencies.
Objective is to design, optimize, and prototype a physical circuit of a
broad-band active filter that corrects a client’s hearing threshold to
a normal hearing threshold.
Constraints:
Circuit will correct the hearing over the frequency spectrum of 40 Hz to
10 kHz
The circuit will be designed to minimize the mean and variance errors of
the circuit transfer function relative to the ideal transfer function
measured from the client’s threshold of hearing
The circuit will be fed by a source with a 50 ohm impedance
The circuit will be terminated by a 2 Mega-Ohm audio amplifier load
The circuit cannot pass DC
The transfer function must be < -3 dB, and falling at least at -20
dB/decade at f = 20 kHz.
No more than 4 op-amps can be used in the circuit
EE 221 S. Gedney, University of Kentucky
Threshold of Hearing
Threshold of hearing is the power level at which you
can no longer hear a signal
Measure your client’s threshold of hearing
http://www.engr.uky.edu/~donohue/audio/fsear.html
Threshold of Hearing
60
Normal Threshold
Client Threshold
50
40
dB
30
20
10
0
-10
1
10
2
10
3
10
H
4
10
5
10
EE 221 S. Gedney, University of Kentucky
Transfer function of the Custom Hearing aid
Compute the transfer function
Recall: |  | = |()| − |  |
From the hearing threshold:
( jω ) dB Ct ( jω ) dB − N t ( jω ) dB
H
=
Threshold of Hearing
Ideal Transfer Function
60
10
Normal Threshold
Client Threshold
50
|Ct|dB - |Nt|dB
5
40
0
dB
dB
30
20
-5
10
-10
0
-10
1
10
2
10
3
10
H
4
10
5
10
-15
1
10
2
10
EE 221 S. Gedney, University of Kentucky
3
10
H
4
10
5
10
Project Overview
Measure and tabulate your client’s threshold of hearing at a set
of discrete frequencies.
Determine the desired transfer function for your filter at these
discrete frequencies.
Develop a top-level design for the filter by devising an
appropriate scheme to realize the filter response.
Optimize the ideal response using MATLAB or MathCad such
that the mean and variance errors are minimized based on the
discrete data. The continuous response should also represent a
smooth interpolation of the transfer function, and should not
contain sharp peaks or nulls between the discrete points.
program) using non-ideal op-amp models (741)
Optimize the B^2 SPICE response to realize as close as possible
the ideal response with off the shelf parts.
Build a physical circuit using the Digilent Explorer board. Test
and measure each sub-circuit, and measure the transfer function
of the full hearing aid circuit.
EE 221 S. Gedney, University of Kentucky
Reporting
The project will require three reports to be handed in.
Preliminary report (Due, Thursday, 4/3/2014)
Problem statement
Hearing threshhold
Top-Level Design
First cut of a transfer function
Phase-1 report, due Tuesday, 4/15/2014.
Optimized transfer function
Minimize mean and variance errors
First cut of the SPICE Circuit model
Final report due on Thursday, 4/24/2014.
Optimized SPICE model (based on off the shelf components)
Prototype the circuit using the Digilent Explorer board
Final Demonstration:
A final demonstration of your physical circuit must be completed
with your instructor by 4 pm on Friday, 4/25/2014.
EE 221 S. Gedney, University of Kentucky
```