Earmolds

Acoustic Impedance

______ to the flow of acoustic energy through an element which results in a _______ of energy

Impedance

This happens in two ways:

energy is temporarily stored within an element and then returned to the source

(______ impedance)

energy is dissipated through conversion to heat (________ impedance)

Total Acoustic Impedance

Determined by the interaction between

Acoustic resistance (_____ across frequency)

Acoustic mass

Acoustic compliance

Acoustic reactance (_____ with frequency)

Acoustic Resistance

When air particles ______ with

        each other

        sides of a tube

        other obstacles

        (such as sintered pellets, lamb’s wool, mesh screens)

acoustic energy is dissipated as heat

Acoustic _____ (Inertance)

When a _______ of air is set into motion as a unit without undergoing significant compression

            (such as in tubing, vents, diaphragm or rec)

impedance of mass of air

            ~ length of tube

            ~ 1/diameter

acts as low pass device

Acoustic __________

When air particles enclosed in a cavity are _________ and __________ but do not move as a unit such as...

               the cavity in front of the earphone

                      diaphragm the cavity drilled for the snap ring

                      in the receiver mold

                  the cavity of air beyond the tip of the earmold

Impedance due to acoustic compliance~volume (imped ^ as vol ^)

Acts as high pas device (imped ^ as freq decreases)

Acoustic _________

Occurs when the ________ to the flow of energy is minimal

Types in Earmold
Coupling System

	________
	Helmholtz
	Combination of wavelength and Helmholtz

SUMMARY OF COUPLING EFFECTS

Component

Type of element

Ways it can Vary

Effect

Dampers (mesh screen, sinted pellets)

Resistive

Placement in tubing System

Smooths out the response

Degree of resistance (in ohms)

Reduces overall SLP (Fig. 9, Cox)

Cavity of air beyond earmold tip

Reactive compliance

As earmold canal portion lengthened,

As volume decreases, SPL increases &

cavity volume decreases

resonant peak moves down (Fig. 4 Cox)

Cavity in front of receiver in snap ring

Reactive compliance

May increase in volume

As vol increases get peaked response &

lower high freq cutoff (Fig. 4.13 Leavitt)

Tubing

Reactive Inertance

Diameter

As diameter decreases SPL decreases &

resonance peak R1 moves down

Fig. 3 Cox; Fig. 4.12, Leavitt

Length

As length decreases, resonant peak moves

up, greater high-freq cutoff, but reduced SPL

(Fig. 4 Cox; Fig. 4.11, Leavitt)

Vents

Reactive Inertance

Type-Sidebranch/Parallel

Sidebranch reduces high freq response

as well as low (Fig. 4.17, Leavitt; Fig 16 Cox)

Diameter/Length

As diameter increases & length decreases

more low frequency reduction (Table 4.2

Leavitt, Fig. 24/25 Cox)

Open molds

Reactive Inertance

Length of tubing

As length decreases more low frequency

reduction. Resonant peak moves up

(Fig. 28, Cox)

Diamter of tubing

As diameter decreases SPL & resonance

peak R1 moves down (Fig. 29, Cox)

Leavitt, Ron (1968). "Earmold: Acoustic and Structural Considerations" in Wm. Hodgson ed. Hearing Aid Assessment

and Use in Audiologic Habilitation, Baltimore: Williams & Wilkins.