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NOTE AN-11NOTE AN-11

ELECTRONICSELECTRONICS

application note

VEF Vibration Isolation receivers

Introduction

The VEF Vibration Isolation receiver

integrates a vibration-isolated EF receiver

into an outer magnetic shielding case (see

figure 1). In use, the VEF receiver can be

directly mounted into a hearing aid housing

and the required electrical connection made

with standard wire. This eliminates the

need for hearing instrument manufacturers

to source and assemble additional flexible

wiring, rubber boots, tubing, and external

magnetic shielding to the receiver.

Additionally, the VEF Vibration Isolation

receiver’s electro-acoustic and vibration

performance is verified by Knowles

Electronics prior to shipment, reducing the

risk of feedback and closure problems

during hearing aid assembly.

Vibration Isolation Design

Mechanical vibration isolation is important

because receivers generate undesired

mechanical forces that propagate through

the hearing aid. These forces, through a

variety of transmission mechanisms,

produce acoustic pressure at the

microphone input port and accelerate the

microphone case, creating a feedback loop.

VEF Vibration Isolation receivers

incorporate compliant rubber isolators to

substantially reduce the mechanical

vibration forces transmitted to the hearing

aid (see figures 2 and 3). In principle, the

VEF Vibration Isolation receiver’s isolation

system is an extension of traditional

approaches that use compliant rubber

Figure 1: VEF Vibration Isolation receiver. The yellow

sections are compliant rubber mounts that isolate the

internal EF receiver from the outer metal shield.

Figure 2: VEF Vibration Isolation receiver front cutaway.

Note the rubber front tube mount that acts as both an

acoustic outlet (sound port) and a vibration isolation

mount.

AN-11

“boots” and tubes. The receiver is

suspended between two points inside the

outer metal shell by a front tube mount and

a rear terminal mount. The front tube

mount serves as both an acoustic outlet and

an isolation mount. The rear mount also

provides appropriate clearance for the

flexible lead wires to run from the inner EF

receiver to the outside solder terminals.

The entire receiver and suspension

assembly was analyzed using finite element

techniques to optimize the VEF system for

the inner EF receiver’s specific vibration

response characteristics.

Vibration Isolation Measurement

There are a number of ways to measure

vibration isolation, including open loop gain,

laser velocity measurements, and force

measurements. The open loop technique is

often used as a definitive measure of the

isolation in a complete hearing aid. Hearing

aid isolation, however, is strongly dependent

on the system acoustical and mechanical

properties such as housing geometry, mass

distribution, transducer type and location,

acoustic sealing, etc. In order to focus on

the receiver vibration independent of the

hearing aid system, Knowles utilizes laser

velocity measurements and force

measurements. While laser measurements

can yield insights into receiver and isolator

performance that are useful for design

purposes, force measurement remains an

inexpensive, accurate, and rapid vibration

characterization method.

Figure 4 depicts a typical force

measurement. A piezoelectric force sensor

is fixed to a stationary base. The VEF

receiver is oriented such that the diaphragm

normal vector is along the force sensing

direction (vertical orientation in Figure 4) so

that the maximum force output is measured.

The receiver is attached to the sensor using

a thin, stiff layer of wax or cyanoacrylate to

reduce the effects of fixturing on the

measurement. A spectrum analyzer is used

to measure the frequency response

magnitude of the force cell output with

respect to the receiver input drive voltage.

Figure 3: VEF Vibration Isolation receiver rear cutaway.

Note the rear rubber isolation mount that isolates the

internal EF receiver from the external metal case.

Device Under Test

Massive Base

Dual Channel

Spectrum Analyzer

HP 3562A

SRC Ch1 CH2

Signal Conditioner

Force

Cell

Figure 4: Typical receiver force measurement system.