Evaluation of Techniques for Measuring Distortion Products of Bone Conduction Vibrators and Bone Conduction Stimulated Otoacoustic Emissions ARO 2014

1. Teru Kamogashira, Shotaro Karino, and Tatsuya Yamasoba
Department of Otolaryngology,
Faculty of Medicine, University of Tokyo
Evaluation of Techniques for
Measuring Distortion Products of
Bone Conduction Vibrators and
Bone Conduction Stimulated
Otoacoustic Emissions

2. Introduction
 Bone conduction vibrators (BCV)
have poor distortion performance
particularly at low frequencies and
resonant frequencies.
 The specified distortion quality rating
is approximately 5% at maximum.
 The absolute distortion is not accurately
known.
From the B-70 series
bone conduction
transducers
specification

3. Introduction
 Techniques for calibrating BCV have
been developed.
( Margolis & Stiepan 2012, Ginter & Margolis 2013 )
– The specific harmonic distortion (HD)
is not evaluated.
 The signal processing techniques of
measuring impulse response and
distortion products have been
applied to the BCV and OAE measuring
in the recent studies.

4. BCV signal recording system
Artificial masoid
Bone conduction
vibrator
DA/AD converter
and computer
Power amplifier and
attenuator
Charge amplifier
Output
Input
Signal loopback for
synchronization

5. OAE signal recording system
OAE Ampilifier
Bone conduction
vibrator
DA/AD converter
and computer
Power amplifier and
attenuator
Output
Input
Ear
Microphone
Vibrator placement:
The forehead
(including dual placement)
The ipsilateral mastoid
The contralateral mastoid
Signal loopback for
synchronization

6. Stimulation signals
 Pulse Sequence
 Time Stretched pulse (TSP)
– Linear (Lin-TSP), dual Lin-TSP
– Exponential (Exp-TSP)
( Algorithm: Aoshima 1981, Farina 2000,
OAE: Todor 2011, Bebbett 2010, Chen 2013 )
 Maximum Length Sequence (MLS)
( Algorithm: Rife 1989; OAE: Thornton 1993, Hine, et. al. 1997 )
 Silence Sweep (SS)
( Algorithm: Farina 2009 )
Stimulation algorithms review:
( Stan 2002 )

7. Signal processing tools
 Signal processing tools for
stimulation signal production and
offline output signal processing
– R version 3.0.2
– Audacity version 2.0.0
– Freeverb3 signal processing library
– In-house signal processing software

8. Stimulation signals
 Sampling rate: 48kHz
 Input voltage scale: -40dBV ~ 10dBV
– (BCV RETFL@1kHz: -90dBV)
 Pulse Sequence
 Time stretched pulse (Lin-TSP, Exp-TSP)
– 5Hz ~ 24kHz for 20 seconds
 Maximum Length Sequence (MLS)
– 18th order (2^18-1 samples)
 Silence Sweep (SS)
– 17th order MLS, 5Hz ~ 24kHz sweep

9. Output Normalization
 Output signals were convoluted with
inverse filter and normalized with
the peak amplitude.
 The sine wave or pulse wave were
converted to dBV in the following
equations.
– Sine waves 0 dBV = 2*sqrt(2) Vpp
– Pulse waves 0 dBV = 2 Vpp
 The output voltages were converted to
the forces (N) with the artificial mastoid
coefficient.

10. BCV normalized output (MLS)
-40 -30 -20 -10 0 10 20
-60
-50
-40
-30
-20
-10
0
10
dBV
dB re. 1uN
The power amplifier
output
The artificial mastoid
output

11. BCV normalized output (MLS)
The power amplifier
output
The artificial mastoid
output

12. BCV output (Exp-TSP)
-50 -40 -30 -20 -10 0 10 20
-80
-70
-60
-50
-40
-30
-20
-10
0dBV
dB re. 1uN

13. Exp-TSP distortion products
BCVOutput
PowerAmplifier
Output
2
3
4
5
3
2
5
4

14. Exp-TSP distortion products
-120
-100
-80
-60
-40
-20
0
50 500 5000
-80
-70
-60
-50
-40
-30
-20
-10
0
100 500 2500
The 1st, 2nd and 3rd order
output from the BCV.
(-20dBV TSP stimulation)
dB re. 1uN
[Hz] [Hz]
dBc
The 2nd and 3rd order harmonic
distortion at each frequency.
(-20dBV TSP stimulation)
2nd order HD
1st order
3rd order HD
2nd order HD
3rd order HD

15. Exp-TSP harmonic distortion
2nd order HD 2nd order HD
3rd order HD 3rd order HD

16. Lin-TSP IMD products
BCVOutput
PowerAmplifier
Output

17. BCV output (SS and pulse)
Silence sweep Silence sweep
Pulse Pulse

18. Summary of BVC output
 Fundamental output
– The output forces increase linearly.
 Transient response
– Any methods showed similar waves.
– The two negative waves and
resonance wave follow after
the one main positive wave.
– The transient waveforms change
depending on the voltage in the
stimulation of the sweep, but not
in the MLS stimulation.

19. Summary of BVC output
 Distortion output
– The 2nd order harmonic distortion
is dominant at high voltage input.
– The 3rd order harmonic distortion
and intermodulation distortion remains
about 45dBc at any low voltage input.
– The transient waveforms of the
harmonic distortion change
depending on the voltage.

20. Ear microphone output
Lin-TSP Exp-TSP
MLS
BCV: the forehead
Voltage input:
-8dBV ~ -28dBV
EarMic: the right ear
Exp-TSP

21. BC Exp-TSP HD
2
3
4
5
BCV: the forehead
EarMic: the right ear

22. BC TSP HD
2nd order HD 3rd order HD
4th order HD 5th order HD
BCV: the forehead
Voltage input: -8dBV ~ -28dBV
EarMic: the right ear

23. BC OAE (MLS)
BCV: the forehead
Voltage input: -3dBV
EarMic: the right ear
The output from the normal
stimulation signal
The output from the inverted
stimulation signal

24. BC OAE (lin-TSP)
BCV: the forehead
Voltage input: -9dBV
EarMic: the right ear
The output from the normal
stimulation signal
The output from the inverted
stimulation signal

25. BC OAE pre-ringing waveform
Pre ringing
waveform
Reconstructed
OAE wave
The linear TSP reconstructed
Impulse response and
pre/post-ringing wave

26. BC OAE (dual lin-TSP)
BCV: two BCV were
used at the forehead
Voltage input: -5dBV
EarMic: the right ear
The output from the normal
stimulation signal
The output from the inverted
stimulation signal

27. Summary of OAE output
 The main waveforms of the ear
microphone were direct stimulation
waves from the bone conduction
vibrator.
 The distortion products of the output
of the ear microphone were similar to
that of the artificial mastoid.
 The direct extraction of OAE from the
output of the ear microphone was
difficult because the bone conduction
vibrator’s stimulation wave lasts over
few milliseconds.

28. Summary of OAE output
 The signal inverting techniques with
time stretched pulse (TSP), dual TSP
or maximum length sequence were
useful in extracting OAE products.
 The dual sweep stimulation techniques
were useful in stimulation and
extraction of the BC OAE.
 Further studies are needed.
– The input-ouput function.
– The frequency width of the sweep.

29. Summary of OAE output
 The pre-ringing waveforms were
extracted from the positive and negative
sweep stimulation, however they were
absent in the MLS stimulation.
 The origin of the pre-ringing waveforms
are not known.
– This may be related to SOAE or
tonotopic frequency range.

30. Conflict of Interests
 This research did not receive any
financial support.

34. Signal recording system
 Bone conduction vibrator
– RION BR-41
 Bone conduction amplifier
– Technics SE-C01
 Artificial mastoid
– B&K 4930
 Otoacoustic emission probe
– RION AG-04 OAE amplifier
 Digital sound player and recorder
– Edirol UA-25

35. Pulse Sequence
 The dirac delta
 The simple method to extract the
impulse response from the system.
 The pulse stimulation was used in
many types of physiological recordings.
– Neurological examinations:
ENoG, Nerve conduction velocity, …
– Otological examinations:
ABR, TEOAE, VEMP, …

36. Time Stretched pulse (TSP)
 The stimulation frequency changes
dynamically from low frequency to
high frequency.
 The impulse response from the system
can be calculated using convolution
technique and inverse filter.
 The TSP is used in many areas of
transfer function analysis.
 The application in OAE have been
developed recently.
(Todor 2011, Bebbett 2010, Chen 2013)

37. Time Stretched pulse (TSP)
 The distortion products of the system
can be extracted from the reconstructed
impulse response sequence.
 The harmonic distortion (HD) and
the intermodulation distortion (IMD)
products can be calculated from
specific stimulation sweep.
 Variant types of tsp
– Linear TSP (Lin-TSP)
– Exponential TSP (Exp-TSP)

38. Time Stretched pulse (TSP)
 Linear

39. Time Stretched pulse (TSP)
 Linear
Inverse filter Calculated response

40. Time Stretched pulse (TSP)
 Exponential

41. Time Stretched pulse (TSP)
 Exponential
Inverse filter Calculated response

42. HD and IMD of TSP
Dual sweep stimulation
HD and IMD of the impulse

43. Maximum Length Sequence
 One of pseudorandom binary sequences.
 The frequency response is flat.
 The impulse response can be
calculated using the hadamard
transform or reverse sequence
convolution.
 The distortion products act as noises.

44. Maximum Length Sequence

45. Maximum Length Sequence
Reverse sequence

46. Silence Sweep (SS)
 Time stretched MLS, a variant MLS.
– (Farina 2009)
 The two step convolutions are needed
to recover the impulse response.
 The non-distortion noise can be
extracted in some conditions.

47. Silence Sweep

48. Silence Sweep
Two reverse filters
are needed to
recover impulses.

49. BCV output (Exp-TSP)
The power amplifier
output
The artificial mastoid
output

50. BCV output (Lin-TSP)
-50 -40 -30 -20 -10 0 10 20
-80
-70
-60
-50
-40
-30
-20
-10
0dBV
dB re. 1uN

51. BCV output (Lin-TSP)