Open Fittings and RITE

(vents in hearing aids)

Summercamp 2008

Lars Bramsløw, Ph.D.

Audiology, Oticon A/S

2

Take–home messages:

Physics is the law!

Openness is here to stay.

3

Program

Why: Occlusion

How: A little physics….

What: Different shapes

Side effects: Howl, ’thin’ sound, echo

Directionality / Noise reduction

Other RITE advantages

Wrap up

4

Program

Why: Occlusion

How: A little physics….

What: Different shapes

Side effects: Howl, ’thin’ sound, echo

Directionality / Noise reduction

Other RITE advantages

Wrap up

5

Why vent(ilation)?

Occlusion

Moisture / itch in the ear canal

Static pressure relief

6

Occlusion

TRY IT!!

”There is a hollow

sound in my head””eeeeeeeeeeeee”

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Body conducted sound

What causes occlusion?

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Occlusion

Frequency, Hz

-10

10

30

50

70

90

110

0

20

40

60

80

100

120

Heari

ng

Lev

el,

dB

HL

125 250 500 1000 2000 4000 8000

Risk

Less or none

Increased own voice sound pressure

/u/ with different vents. HA turned off

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Solutions

Reduce LF gain? - ’occlusion manager’

Occlusion can NOT be removed by means of amplification!

Deep fittings – To the bony part

For example Sebotek (BTE), CIC

Vent as large as possible

10

Body conducted sound

The vent ’leaks’ low frequency energy out

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Real-ear sound pressure level for /u/

12

Program

Why: Occlusion

How: A little physics….

What: Different shapes

Side effects: Howl, ’thin’ sound, echo

Directionality / Noise reduction

Other RITE advantages

Wrap up

13

Now for physics…

Mass of air

14

mg

The acoustic system

Compliance = Spring

Mass = Weight

mg

Another example:

15

Acoustic Mass

Acoustic mass =

Constant * (effective vent length / vent diameter2)

Typical dimensions:

BTE: Length19 mm, Diameter = 0 - 4mm

ITE: Length19 mm, Diameter = 0 – 3 mm

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Acoustic Mass

..depends on the opening area and the length

NOT the shape.

NOT the volume!!!

Larger mass needs lower tones to move

mg

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The vent is an acoustic low pass filter

The acoustic mass is too heavy for the high frequencies

Allows low frequencies to pass

The cut-off frequency depends on the acoustic mass

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Outgoing (lost) sound

-40

-30

-20

-10

0

10

20

100 1000 10000

dB

Hz

Outgoing Response re closed mold

Closed

1.0 mm

1.3 mm

2.0 mm

3.4 mm

5.0 mm

Open mold

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-40

-30

-20

-10

0

10

20

100 1000 10000

dB

Hz

Incoming response re no mold

1.0 mm

1.3 mm

2.0 mm

3.4 mm

5.0 mm

Open mold

Incoming sound

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Program

Why: Occlusion

How: A little physics….

What: Different shapes

Side effects: Howl, ’thin’ sound, echo

Directionality / Noise reduction

Other RITE advantages

Wrap up

21

How about short vents?

Shorter vent is more open:

Diameter (mm)

Standard BTE

vent

(19 mm)

0 1.4 2.4 4.0

Micro Mould

vent

(7 mm)

0

No

vent

0.8

S

1.4

M

2.4

L

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How about domes?

The vent length in Open Dome is VERY

short => small acoustic mass

=> very open vent

~ 9-10 mm BTE vent

For Plus Dome the vent is longer and the

area is smaller

=> Larger acoustics mass

=> Not as open

~ 3-4 mm BTE vent

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Program

Why: Occlusion

How: A little physics….

What: Different shapes

Side effects: Howl, ’thin’ sound, echo

Directionality / Noise reduction

Other RITE advantages

Wrap up

24

Side effects:

Just do it! ??

Howling – feedback

‘thin’ sound – not enough LF gain

Echo/timbre – there is a delay through the HA

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Howl

Larger vent = larger risk

Solution: Lower HF gain

Solution: Anti-feedback system to obtain target gain

Especially for open fittings!

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AFB system provides more HF

Thin sound

Lack of LF gain

Target not reached

Close the vent?

Increase HA LF gain?

How?

The literature: real-ear ->

coupler gain

transformations.

NAL-NL1 suggests 100%.

Simple physics suggests

this solution.

High demands on LF gain

and MPO!!!

-40

-30

-20

-10

0

10

20

100 1000 10000

dB

Hz

Outgoing Response re closed mold

Closed

1.0 mm

1.3 mm

2.0 mm

3.4 mm

5.0 mm

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Low frequency vent compensation

Our listening tests show that full LF compensation is to much

Boomy sound…

Pushes the HA LF output to the limits -> saturation

Oticon uses 50 % compensation

gain

Frequency

Target

Actual

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How about open fittings?

Impossible to compensate in LF

Problems with sound quality

Stay tuned…

Better to turn off LF bands in

the HA and use incoming sound

only (IG = 0)

For example Delta

-40

-30

-20

-10

0

10

20

100 1000 10000

dB

Hz

Incoming response re no mold

1.0 mm

1.3 mm

2.0 mm

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How about Streamer/DAI/FM?

No incoming sound!

Target: 100% compensation

Epoq: In the Streamer program

As much as possible…

-40

-30

-20

-10

0

10

20

100 1000 10000

dB

Hz

Outgoing Response re closed mold

Closed1.0 mm1.3 mm2.0 mm

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Delay in the hearing aid

Amplified path

Direct path

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The sound through a digital HA is delayed..

Delay < 10 ms can be perceived as timbre changes

(‘pipe’ sound…)

Delay > 10 ms can be perceived as echo / reverberance

Delay > 20 ms may cause lip-reading (sync) problems

Echo / timbre

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Is delay a problem?

May not be audible for HI

Recent results: 5 – 10 ms is equally bad

Sounds like a pipe / seashell

Trouble if the two sound contributions are about equal

So: more or less gain in the HA!!

Experiment with removing LF gain up to an increasing

frequency (high pass).

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Program

Why: Occlusion

How: A little physics….

What: Different shapes

Side effects: Howl, ’thin’ sound, echo

Directionality / Noise reduction

Other RITE advantages

Wrap up

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Directionality and noise reduction

…in conflict with incoming sound

Noise reduction:

Less LF damping

Problem? Must reduce vent size.

No LF DIR

Most critical speech information placed higher in frequency

(especially 1-3 kHz)

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Testing DIR and vents

N=30 ITE and ITC users (Oticon Syncro).

Average collection vent size: 2.4mm (range: 1 – 3mm)

Dantale2, speech from the front, noise from behind:

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Did we…

Throw the baby out with the bath water?

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Program

Why: Occlusion

How: A little physics….

What: Different shapes

Side effects: Howl, ’thin’ sound, echo

Directionality / Noise reduction

Other RITE advantages

Wrap up

RITE - Receiver In The Ear

Amplifier Unit

ReceiverSound Wire

(thin wire that connects the

receiver to the amplifier)

Why is RITE relevant?

Sound Quality

No Occlusion

Cosmetics

Delivery of sound straight to

the eardrum

No tube resonance

No calibrating corrections

Best possible platform for

delivery of the clearest

sound

Peaks in the

response due to:

A. Receiver Placement

B. Tone hook

C. Length and diameter

of tube

And also

1. Moisture build up in

tube

2. Stiffness of Tube Wall

Tube Resonance

Avoidance of Tube Resonance

44

Program

Why: Occlusion

How: A little physics….

What: Different shapes

Side effects: Howl, ’thin’ sound, echo

Directionality / Noise reduction

Other RITE advantages

Wrap up

45

How is the vent size prescribed?

An important task for fitting

software

The goal is to make the vent as

large as possible without howl

and with sufficient LF gain

Two criteria:

How much gain?

(Hearing loss & rationale/gain rule)

Is anti-feedback activated?

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Sound advice

Occlusion:

Larger vent

Howl:

1. DFC

2. Smaller vent

Thin sound

more LF

Echo/’pipe’ sound

Less LF

Try increasing high-pass cut-off

Directionality

OK…

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Take-home messages:

mg

Physics is the law!

Openness is here to stay.

Thank you!

????