Hearing Aid Isolation Chamber Design Documentation

Hearing Aid Isolation Chamber

Project Team

Project Background

The M.D. Steer Speech-language and Audiology Clinic is the community partner for the Hearing Aid Isolation Chamber project. Working specifically with Dr. Joshua Alexander, assistant professor of audiology/hearing science, the purpose of the project is to create a functioning test chamber in which to perform acoustic tests on hearing aids.

Though various hearing aid test units currently exist, Dr. Alexander seeks a unique design to serve his testing needs. Specifically, Dr. Alexander requires that the testing unit meet basic industry standard hearing aid test system specifications, but also be able to easily interface with his computer system. Interfacing with personal computers is not possible with current existing units, which are designed to perform and analyze tests from start to finish using proprietary hardware and software.

The main testing area needs to have a flat frequency response from 200Hz to 10KHz. This will require significant acoustic considerations. The microphones, couplers, and hearing aids used in the testing area will be provided by Dr. Alexander. The sound source will be accessible through external BNC connections to receive audio from external sources. The general size of the whole unit should be around 9” x 9” x 9” (within approximately 3”), and weigh under 25 lbs.

This project is expected to be delivered by the end of the 2010 spring semester. The successful delivery of this project will benefit Dr. Alexander and his research. His research, in turn, contributes to the innovation and improvement of hearing aid technology and ensures that those who suffer from hearing loss are able to receive more accurate and better functioning hearing aids.

Specifications

Acoustic performance

•Testing chamber must have a flat frequency response from 200Hz-10KHz

Speaker

•Must be able to test up to 110db without distortion

•Flat frequency response within ± 3dB from 200Hz-10KHz

Interface

•The speaker and microphone must be able to interface with a computer (microphone and equipment relating to the microphone will be provided by Dr. Alexander)

•Speaker will be accessible to external audio via a BNC connection

Size

•Unit should be 9” x 9” x 9” within ± 3”

Weight

•Unit must weight under 25 lbs total

Conceptual Design

In the conceptual design stage, my team primarily looked at existing units and discussed them with Dr. Alexander. The two primary units we considered were the FONIX 7000 Hearing Aid Analyzer and the AudioScan Verifit.

After some discussion, we decided the overall design of our chamber should be similar to the Fonix 7000. This was primarily due to practical concerns relating to construction. As the chamber construction itself is not the primary focus of our project, we quickly decided that a simple opening box design with a speaker at the bottom would allow for the most freedom and efficiency in meeting the rest of our specifications.

Detailed Design

Box Design:

*For further details, please refer to Box Considerations.

After considering various materials, we decided that, if at all possible, we would like to use a pre-made box. This would save us significant time in construction and possibly even be more cost efficient. It was agreed that this would be the best use of our resources.

The box we have decided to use is the Vaultz File Security Box. It measures at 13.5”x13”x10”, just nearly outside our original specification of 9”x9”x9” (± 3”). It weighs only 7 lbs, which will ensure it will be possible for us to meet the weight specification of less than 25 lbs. At $40, we have agreed it would be a wise use of our resources. Below is a picture of the item.

Acoustical Considerations:

*For complete details and analysis, please refer to our acoustic design documentation.

For a space to have a flat frequency response over a certain range, the space must be anechoic over that range – free from internal reflections and external sound. The major challenge in this area is that anechoic chambers are limited in their ability to isolate low frequencies by the size of the space (and specifically, the size of the absorbers used in the space). With our current specifications, it would be difficult to line the box with more than 4” of absorbent material on each side. This corresponds to a cutoff frequency of roughly 3 kHz. Thus, completely anechoic design is not possible. As such, we are using a variety of methods to further isolate the chamber and obtain a flatter frequency response.

First, we will be using wedge absorbers. The use of wedge-shaped absorbers has become a standard in anechoic design. The conceptis to disperse sound waves while simultaneously absorbing them. Ideally, the dispersion will allow for greater overall absorption. The use of acoustically treated foam will also lend greater absorption. One such product that exhibits these characteristics and that we are planning on using is Auralex’s Studiofoam® Acoustical Panels. We are planning on using 2” of this material to account for the high frequencies, and 2” of a more dense material behind it to account for the lower frequencies, such as the Sound Seal Quilted Fiberglass Absorbers. Using these two materials in this way, we were able to calculate a projected sound attenuation based off of their combined absorption coefficients (provided by the manufacturer), shown below.

Second, we will decouple the box from its surroundings. This greatly reduces issues caused by vibration of the surfaces the box will rest on. For this, we have specified the AuralexGramma Isolation Riser.

We have considered the effects of exterior noise to be negligible. The potential laboratory spaces Dr. Alexander will be using this device in are already acoustically treated. Beyond this, we will achieve significant acoustic isolation due to the material of the box and the fiberglass directly within the box.

Speaker Design:

*For further details, please refer to Electrical Component Considerations.

When it came to implementing the speaker, we started by simply seeing the range of products available. Like the box material, we were hoping we could find something pre-made simplify the construction. We were unable to find a readily available speaker that would meet our specifications and design limitations. As such, we are planning to build the speaker system primarily from scratch. The speaker itself was selected with regards to size (ability to fit in our specified box), frequency response (flat within ±3dB from 200Hz-20KHz), and price (original budget estimate was $75). The other components were selected with primary considerations of functionality with the speaker. The key components are:

•Speaker

• Cone size of 2.5”, flat frequency response within ±3dB from 200Hz to 10KHz
• We have selected the Vifa NE85W2.5" Full Range Speaker ($27.40)

•Power amp

• 15W, full-range, mono
• We have selected the FK607: Power Amplifier Kit BTL 15W ($9.95)

•Power supply

• 12VDC, 1.5 amps
• We have selected the ARM Electronics 12VDC 1.5 Amp DC Regulated Power Supply ($13.00)

•Speaker enclosure

• We will be building the enclosure for the speaker according to recommended design specified by the speaker manufacturers
• Enclosure will be made from 0.5” plywood and have dimensions 5” long x 3” wide x 3.75” deep (see Speaker Enclosure Considerations for more information)

Overall Timeline, Status, and Future Goals

The project began in Spring 2010.

At this point, we are in the detailed design phase. We have a design complete with detailed documentation available on SharePoint (SLAC > Project Documentation > HAIC > Design Documentation).Some of the materials and components have been purchased and are in the EPICS lab in the SLAC locker. Currently, detailed instructions for ordering additional materials, constructing the device, and methods for testing acoustic properties are being written so that it can be quickly and efficiently completed next semester.