Project Documentationteam: SLAC: Headphones

Project DocumentationTeam: SLAC: Headphones

Project Management Document

and

Design Document

Team: SLAC

Project: Earphone Limiter

Table of Contents

1. Revision History
2. Project Charter
3. Description of Community Partner
4. Stakeholders
5. Project Objectives
6. Project Goals
7. Deliverables
8. Project Timeline
9. Budget
10. Basic Information
11. Headphone Safety
12. Similar Marketed Products
13. Design Description
14. Headphone Structure
15. Limiting Circuit
16. Justification of Design Choice
17. Specifications
18. Headphones
19. Circuit
20. Testing of Prototype
21. Testing Procedures
22. Results of testing
23. Summary
24. Possible weaknesses/errors
25. Maintenance
26. Marketability
27. Contact Information

Revision History

Project Charter

Community Partner

The project partner is Dr. Robert Novak, Ph. D., CCC. He is a professor at Purdue University and the Department head of Speech, Language, and Hearing Sciences. According to the Purdue University Audiology Clinic website (2010), the goal of the audiology clinic is, “to improve communication through diagnostic evaluation and rehabilitative intervention.” They address the problem of hearing damage and loss in society.The Audiology Clinic at Purdue serves the general public with a focus of the Lafayette community, especially those with hearing damage or loss. Cognitive disabilities have an impact on the patients of the audiology clinic because it could impact their level of hearing damage to some extent.The clinic is a part of the Speech, Language and Hearing Sciences at Purdue University. The deliverable from this project will be a marketable product available to the public for purchase. People who use headphones will benefit from the project.

Stakeholders

The stakeholder is the project partner, who will probably market this device to prevent hearing damage in the public. They will be involved in the maintenance of the project in the respect that they will handle customer service for the product. The direct users of this product will be the general public. The product will be available for everyone to purchase at a cost comparable to regular headphones, but with the added safety benefit of sensing when the volume of audio is above the maximum safe level.

Project Objectives

The project objective is to modify and build a pair of iPod ear buds to measure the decibel level of sound coming from each ear bud and gradually cut off audio once this level has reached 85 decibels. This is a preventative measure for hearing loss, which will benefit the users of the project.

Project Goals

Project goals for the semester include the following: to design headphones that can determine the decibel level of audio in the ear and that is economically feasible, applicable to all audio devices, and durable, to construct the sensing headphones and connect it with the limiting circuit, to test the product and deliver results of sensing abilities.

Deliverables

The product of this project will be headphones that can determine the decibel level of audio in the ear and that is economically feasible, applicable to all audio devices, and durable. By the end of the semester, we expect to have a completed pair of headphones with a circuit.

Project Timeline

1/25/2011:

- Piezoelectric applications research; determine initial parts that are needed

- Look into wiring structure for compressor circuits; figure out what parts we need hopefully by week of 1/25/2011 or 2/1/2011 (Yu)

2/08/2011:

- Have parts that we know we need ordered.

2/15/2011 and 2/22/2011:

- Work on documentation and prepare designs for prototypes.

3/1/2011:

- Test piezoelectric response

- Basic circuit testing

- Response curve analysis

- Make changes to design based on design review

3/8/2011:

- Advance testing using calibrated equipment

If for some reason, initial testing shows that this idea, as a whole, is infeasible, then we will still have time to go back to the drawing board with the previous implementation and extend on their work.

3/15/2011:

-Continue testing and improving ideas. Revise design as needed.

3/22/2011:

-Work on documentation of testing and reflections.

3/29/2011:

-Re-test prototype, revise documentation as appropriate

4/5/2011:

-Begin work on final presentation

-Finish prototype and complete final testing

-Search for potential problems with prototype and correct

4/12/2011:

-Practice presentation; receive feedback

4/19/2011:

-Correct presentation based on feedback

-Complete reflection and documentation

4/26/2011:

-Present final design

Budget

Basic Information

Headphone Safety

The Occupational Safety and Health Administration states that people should not be exposed to volume levels greater than 85 decibels for more than eight hours in order to conserve hearing (OSHA, n.d). Therefore, the headphones will measure the decibel level in the ear and cut off after this level of volume has been reached.

Similar Marketed Products

Some products on the market currently include myPhones by Griffin, headphones from dB Logic, and some kid’s headphones from companies such as Maxell and Sony. The myPhones by Griffin are headphones that cover the ears that cut off at 85 dB (Griffin, 2011). Kid’s headphones by Maxell simply provide volume control for the user (Maxell, 2011). The sound pressure limiting technology in dB Logic’s headphones create waves to lower the pressure in the ear (dB Logic, 2011). Our design will be unique in that it uses small and extremely sensitive piezoelectric sensors in the ear buds to convert pressure measurements into dB and cut off the headphones when the level reaches 85 dB. Our headphones will also account for specific ear geometries, as this affects the pressure in the ear.

Design Description

Headphones (Piezo Sensor)

In order to detect the sound intensity and loudness as accurately as possible, while still

keeping the price as low as possible, we have decided to test using piezoelectric sensors.

The piezoelectric sensors will work for this application by detecting a variation in

pressure via the deformation of the material itself. This converts the deformation into a

voltage which is interpreted by the limiting circuit to reduce volumes. The mounting

for the piezoelectric sensor will be within the casing for the speaker itself, allowing it

to accurately measure the dB level in the ear. A volumetric reduction test will be

performed to figure out the minimum amount of piezoelectric material required for this

application, which should help reduce cost as well as allowing us to minimize the impact

on the output sound. Second phase testing for the sensors after the first prototype is

finished will mainly focus on optimizing sensor placement as well as fail-safe

applications (upper response ceiling, lower response floor). The final stage of testing will

mostly be focused on testing durability. The cyclic loading nature of the speaker on the

sensor may cause wear over relatively short periods if the sensor is improperly chosen or

mounted.

Limiting Circuit

In order to limit the volume audio signals, we looked into several types of circuits in particular: limiter and compressor circuits. A limiter circuit serves to prevent the signal level from exceeding a preset limit while a compressor circuit serves to reduce the dynamic range of the input signal, which makes all signals have a more constant (similar) volume. In this case, we don't want to limit the dynamic range of music or other sounds from the earphones; we only want to limit the top end volume. The schematic of the initial test circuit is shown below:

Justification of Design Choice

Limiters are a specialized form of audio compressor. Some designs are very simple, and others are very complex. Diode-based limiters are simple in design and have relatively accurate loudness response, but may distort in certain regions. Voltage controlled amplifier limiters have lower distortion but can have other extraneous unwanted effects stemming from poor gain control. In our research, we found several good resources with designs, explanations, and implementations of various different types of headphone limiters as well as their applications. They ranged from simple hard limiters to more complex split band limiters with low frequency distortion canceller.

One particularly promising spec showed an example of how to use a hard limiter with a headphone amplifier design. It seemed simple, cheap, relatively easy to implement, and had applications very similar to our project description. However, the basic hard limiter may produce a harsh clipping in some cases. Therefore, we decided to modify the designinto a soft limiter, which is only slightly more complex but will provide a better sound quality by effectively routing any sound above the cutoff to a compressor instead of completely chopping off all sound above the cutoff.

Specifications

Headphones

Circuit

Testing of Prototype

Testing Procedures

Results of Testing

Summary

Possible Weaknesses/Errors

Maintenance

Marketability

Contact Information

Works Cited

dB Logic, (2010). Sound Pressure Limiting 2 Technology. Retrieved from

Griffin (2011). myPhone. Retrieved from

Maxell (2011). Kids Safe Headphone (KHP-2). Retrieved from

Occupational Health and Safety Administration, (n.d.). Noise and Hearing Conservation(1910.95(a)). Retrieved from

Speech, Language and Hearing Sciences, Purdue University, (2010).Audiology Clinic. Retrieved from http://

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