**1. Introduction**

258 Acoustic Waves – From Microdevices to Helioseismology

Sun K., R. M. Lec, Cairncross R. A., Shah P., Brinker C. J. 2006. Characterization of

Urbakh M. and Leonid Daikhin. 2007. Surface morphology and the quartz crystal

Voinova M., Jonson M., Kasemo B., 2002. Missing Mass effect in biosensor's QCM

Voros J. 2004. The Density and Refractive Index of Adsorbing Protein Layers. *Biophysical* 

Yang D., Huang C., Lin Y., Tsaid D., Kao L., Chi C. Lin C., 2003 Tracking of secretory

Young-Doo Kwon, Soon-Bum Kwon, Seung-Bo Jin and Jae-Yong Kim. 2003. Convergence

continuous optimization problems. *Computers and Structures*. 81. 1715-1725 Wegener, J, et al., 2000. Analysis of the composite response of shear wave resonators to the

Werner C. 2008. Interfacial Phenomena of Biomaterials. *Polymer Surfaces and Interfaces*.

Westphal S. and Bornmann A. 2002. Bimolecular detection by surface plasmon enhanced

Zhang Q, Desa J., Lec R., Yag G., and Pourrezaei K. 2005, Combination of TSM and AFM for

Zhuang H., Pin Lu, Siak Piang Lim, and Heow Pueh Lee. 2008. Effects of Interface Slip and

Investigating an Interfacial Interaction of Particles with Surfaces. *Joint IEEE International Frequency control Symposium (FCS) and Precise Tie and Time Interval* 

Viscoelasticity on the Dynamic Response of Droplet Quartz Crystal Microbalances.

Application. *Biosensors and Bioelectronics*. 17, 835-841

attachment of mammalian cells. *Biophys. J.* 78, 2821–2833.

Ellipsometry. *Sensors and Actuators B*. 84. 278-282

*(PTTI) Systems and Applications Meeting*. 4490454

Dresden, Germany. Springer-Verlag Berlin Heidelberg. pp. 299

*Cybernetics*. 5. 3336-3341

*Journal*. 87. 553-561

*Microscopy*, 209, 223-227

*Anal. Chem*. 80. 7347-7353

*Engineering Aspects*. 134. 75-84

Superhydrophobic Materials Using Multiresonance Acoustic Shear Wave Sensors. *IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control*. 53. 1400- 1403 Szabad, Z. Sangolola, B. and McAvoy, B. 2000. Genetic optimisation of manipulation

forces for co-operating robots. *IEEE International Conference on Systems, Man, and* 

microbalance response in liquids. *Colloids and Surfaces A. Physicochemical and* 

vesicles of PC12 cells by total internal reflection fluorescence microscopy. *Journal of* 

enhanced genetic algorithm with successive zooming method for solving

The analysis of hearing mechanisms and research on the influence of various internal (pathologies, ageing) and external (trauma, vibration, noise) factors on sound perception are usually done using acoustic waves induced in the external ear canal. Stimuli which have been used for this purpose are: clicks, tone bursts, half-sine-waves, single tones or pairs of tones. The Corti organ's responses to the external stimuli have either an electric or acoustic character. In the former case, these are cochlear microphonics (CMs) picked up from the surface or from the inside of the cochlea, which are usually used as an indicator of damage to the organ of Corti in animals. In the latter case, these are acoustic waves that appear in the external ear canal as a result of stimulation. The acoustic waves have an important clinical value. Taking into account the presence of nonlinear distortions in the cochlea, the waves that appear after stimulation with a pair of tones are called distortion product otoacoustic emissions (DPOAE).

In studies on CMs, the origin of stimulating waves is often a single earphone (controlled by a generator of defined, often periodical, electrical signals) placed in the external auditory canal. In studies on DPOAE, a probe with two miniature earphones and one microphone is placed in the external auditory canal. The earphones are controlled by two generators of frequencies *f*1 and *f2* and the microphone converts the returned DPOAE wave with a combination frequency, e.g. 3 12 *f ff* = − 2 , into an electrical signal.

The acoustic wave which induces CM signals is usually a periodic wave, while the waves inducing DPOAE signals consist of two pure tones. Thanks to the easy access to the output(s) of the generator(s) the phase-sensitive detection (PSD) technique can be used to measure both CM and DPOAE signals. Very weak (even below single microvolts) CM and DPOAE signals originating from the unimpaired cochlea can be measured in this way. Thanks to this technique signals obscured by other disturbing sources (even thousand times larger) can be measured accurately. This is possible because the phase-sensitive detector singles out the input signal with a specific reference frequency while signals with frequencies different from the reference are rejected. The fundamentals of this technique and its measuring potential are described in section 2.

In section 3, the authors' own experiments aimed at determining the effect various factors on the electrical function of the Corti organ are described. The factors include: vibration

Analysis of Biological Acoustic Waves by Means of the Phase–Sensitivity Technique 261

( ) 2 0 cos *VA t ref ref* = + ω

The two signals have different amplitudes, frequencies and initial phases. At the phasesensitive detector inputs there are signals with unchanged frequencies, but with different

> α

1 1 01 1 2 01 cos()() cos *VA t A t PSD sig* = +⋅ + ωα

1 1 ( ) 1 2 ( ) 01 01 ( ) 1 2 ( ) 01 01 0,5 cos cos *A A sig ref* =

At the output of the PSD there are two signals: a slow-changing signal with differential

are passed through a low pass filter, the fast AC signal will be removed. When the

slowly changing DC signal proportional to the signal amplitude and cos(α01-β01). When

The phase dependency of the output voltage of the lock-in amplifier with one PSD unit can be eliminated by adding a second PSD multiplying the same measured signal by a reference signal shifted by 900. A block diagram of the lock-in amplifier with a double PSD is shown

1 2 = , the filtered signal is exactly a DC signal. By adjusting the phase of the reference

 α β

1 2 − and a signal with overall frequency ( )

change over time and *Vout* of the lock-in amplifier will not be a DC signal.

 BANDPASS FILTER

 90º PHASE SHIFT

> PHASE SHIFTER

α β

The output of the second PSD, filtered by the low pass filter, is proportional to the signal

( ) ( ) ( ) ( ) 2 2 1 01 01 1 01 01 <sup>1</sup> *R A* = −+ − = *sig* cos

*Asig* sin

Fig. 2. Block diagram of lock-in amplifier with double PSD

ω

amplitudes and phases: *VA t sig sig* = + 1 1 01 cos( )

frequency ( ) ω ω

signal one can make (

true if both initial phases

*Signal input* INPUT

AMPLIFIER

 REFERENCE TRIGGER

> α β

When

amplitude and ( ) 01 01 sin .

*Y A*= − *sig*1 01 01 sin . ( ) α β

ω ω

in fig. 2.

*Reference input* 

of the PSD is simply the product of the two sine waves

frequencies of the two signals are approximately equal (

α0 and β

α01β ωω

 β

and 1 20 ( ) <sup>1</sup> cos . *VA t ref ref* = +

 β

 ω

> ωω

− +− + + ++ *<sup>t</sup> <sup>t</sup>* (3)

ω ω

ω ω

01) equal to zero, in which case only *Bsig* can be measured. This is

0 do not change over time, otherwise cos(

 PHASE SENSITIVE DETECTOR

 PHASE -SENSITIVE DETECTOR

− Now there are two outputs: *X A*= − *sig*1 01 01 cos( )

α β

*ref*

(2)

 β

1 2 + . If the PSD output signals

1 2 ≈ ), the filtered PSD output is a

 LOW PASS FILTER

 LOW PASS FILTER

> α

*Asig* (4)

β

 *R* and *θ* CALCULATOR

α01β01) will

> *R θ*

and

The output

ω

 α β

(3.3), ototoxic medicines (3.4) and laser beams used in ear microsurgery (3.6). The role of the signal phase in the measurements is given special attention.

In section 4, experiments involving acoustic waves being nonlinear products of the Corti organ are presented. The individual subsections describe the way in which the PSD technique is applied (4.1), compare the latter with the previously used methods (4.2) and discuss the authors' own experiments in which the phase-sensitive technique is employed to measure DPOAE signals (4.3) and to measure simultaneously DPOAE signals and CMDP (cochlear microphonic distortion product) signals (4.4). It is shown that the phase of DPOAE signals plays an essential role in otoacoustic emission studies.

All the experiments described in sections 3 and 4 were carried out on coloured guinea pigs, each weighing 500-650 g, being under general ketamine/xylazine (15 mg/kg and 10 mg/kg body weight, respectively) anaesthesia. A Homoth measuring probe was placed in the external auditory meatus of the animals. The probe contained two mini earphones and a standard microphone. Prior to the measurements the probe had been graduated in a Brüel&Kraej artificial ear 4144, using a measuring amplifier 2607 made by the same company. Permission to carry out the experiments had been given by the Bioethical Committee in Wrocław.

Section 5 presents the final conclusions and discusses the future of the phase sensitive detection technique in investigations into the function of the cochlea exposed to various hazards.

### **2. Phase-sensitive detection technique**

### **2.1 Fundamentals**

The measurement apparatus based on the phase-sensitive detection technique is called a *lock-in amplifier* or a *lock-in nanovoltmeter*. Lock-in measurements require a frequency reference which should be strictly connected with a fixed frequency of the function generator used in the experiment. The reference signal can be either a square wave or a sinusoid. A block diagram of a typical lock-in amplifier is shown in fig. 1.

Fig. 1. Block diagram of lock-in amplifier with single phase-sensitive detector

Let us assume that the input signal can be described as:

( ) 1 0 cos , *VA t sig sig* = + ω α(1)

and the reference signal as:

(3.3), ototoxic medicines (3.4) and laser beams used in ear microsurgery (3.6). The role of the

In section 4, experiments involving acoustic waves being nonlinear products of the Corti organ are presented. The individual subsections describe the way in which the PSD technique is applied (4.1), compare the latter with the previously used methods (4.2) and discuss the authors' own experiments in which the phase-sensitive technique is employed to measure DPOAE signals (4.3) and to measure simultaneously DPOAE signals and CMDP (cochlear microphonic distortion product) signals (4.4). It is shown that the phase of DPOAE

All the experiments described in sections 3 and 4 were carried out on coloured guinea pigs, each weighing 500-650 g, being under general ketamine/xylazine (15 mg/kg and 10 mg/kg body weight, respectively) anaesthesia. A Homoth measuring probe was placed in the external auditory meatus of the animals. The probe contained two mini earphones and a standard microphone. Prior to the measurements the probe had been graduated in a Brüel&Kraej artificial ear 4144, using a measuring amplifier 2607 made by the same company. Permission to carry out the experiments had been given by the Bioethical

Section 5 presents the final conclusions and discusses the future of the phase sensitive detection technique in investigations into the function of the cochlea exposed to various

The measurement apparatus based on the phase-sensitive detection technique is called a *lock-in amplifier* or a *lock-in nanovoltmeter*. Lock-in measurements require a frequency reference which should be strictly connected with a fixed frequency of the function generator used in the experiment. The reference signal can be either a square wave or a

> PHASE-SENSITIVE DETECTOR

 LOW PASS FILTER

 OUTPUT AMPLIFIER

(1)

*Output* 

sinusoid. A block diagram of a typical lock-in amplifier is shown in fig. 1.

 BANDPASS FILTER

 REGULATED PHASE SHIFTER

( ) 1 0 cos , *VA t sig sig* = + ω

 α

Fig. 1. Block diagram of lock-in amplifier with single phase-sensitive detector

signal phase in the measurements is given special attention.

signals plays an essential role in otoacoustic emission studies.

Committee in Wrocław.

**2.1 Fundamentals** 

**2. Phase-sensitive detection technique** 

 INPUT AMPLIFIER

 REFERENCE TRIGGER

and the reference signal as:

Let us assume that the input signal can be described as:

hazards.

*Signal input* 

*Reference input* 

$$V\_{ref} = A\_{ref} \cos(\omega\_2 t + \beta\_0) \tag{2}$$

The two signals have different amplitudes, frequencies and initial phases. At the phasesensitive detector inputs there are signals with unchanged frequencies, but with different amplitudes and phases: *VA t sig sig* = + 1 1 01 cos( ) ω α and 1 20 ( ) <sup>1</sup> cos . *VA t ref ref* = + ω β The output of the PSD is simply the product of the two sine waves

$$V\_{\rm PSD} = A\_{\rm s\rm g1} \cos(\alpha\_1 t + \alpha\_{\rm o1}) \cdot A\_{\rm n\prime 1} \cos(\alpha\_2 t + \beta\_{\rm o1})$$

$$= 0.5 A\_{\rm s\rm g1} A\_{\rm n\prime 1} \left[ \cos \left[ \left( \alpha\_1 - \alpha\_2 \right) t + \left( \alpha\_{\rm o1} - \beta\_{\rm o1} \right) \right] + \cos \left[ \left( \alpha\_1 + \alpha\_2 \right) t + \left( \alpha\_{\rm o1} + \beta\_{\rm o1} \right) \right] \right] \tag{3}$$

At the output of the PSD there are two signals: a slow-changing signal with differential frequency ( ) ω ω 1 2 − and a signal with overall frequency ( ) ω ω 1 2 + . If the PSD output signals are passed through a low pass filter, the fast AC signal will be removed. When the frequencies of the two signals are approximately equal (ω ω 1 2 ≈ ), the filtered PSD output is a slowly changing DC signal proportional to the signal amplitude and cos(α01-β01). When ω ω 1 2 = , the filtered signal is exactly a DC signal. By adjusting the phase of the reference signal one can make (α01β01) equal to zero, in which case only *Bsig* can be measured. This is true if both initial phases α0 and β0 do not change over time, otherwise cos(α01β01) will change over time and *Vout* of the lock-in amplifier will not be a DC signal.

The phase dependency of the output voltage of the lock-in amplifier with one PSD unit can be eliminated by adding a second PSD multiplying the same measured signal by a reference signal shifted by 900. A block diagram of the lock-in amplifier with a double PSD is shown in fig. 2.

Fig. 2. Block diagram of lock-in amplifier with double PSD

The output of the second PSD, filtered by the low pass filter, is proportional to the signal amplitude and ( ) 01 01 sin . α β − Now there are two outputs: *X A*= − *sig*1 01 01 cos( ) α β and *Y A*= − *sig*1 01 01 sin . ( ) α βWhen

$$R = \sqrt{\left(A\_{\text{s\\_1}} \cos\left(\alpha\_{01} - \beta\_{01}\right)\right)^2 + \left(A\_{\text{s\\_1}} \sin\left(\alpha\_{01} - \beta\_{01}\right)\right)^2} = A\_{\text{s\\_1}}\tag{4}$$

Analysis of Biological Acoustic Waves by Means of the Phase–Sensitivity Technique 263

MULTIPLIIER DIVIDER 3 2 1

Fig. 3. Basic experimental set-up with lock-in amplifier for measuring first harmonic (1),

The situation becomes more complicated when the examined object is nonlinear. The signal spectrum at the PSD input differs from the one at the generator output (the nonlinear object changes the input signal spectrum). This is true for all the signal waveforms, including the sinusoidal one. Then the lock-in amplifier measures the rms of both the first harmonic and the *n*-harmonic (*n*-subharmonic). When harmonic or subharmonic distortion is measured, the function generator supplies a pure sinusoidal signal without any harmonics. The basic experimental setup shown in fig. 3 was used to carry out experiments described in section 3.

The double phase-sensitive detection technique and modern digital technologies offered new possibilities of examining nonlinear objects. An example of the measuring systems which have been developed is shown below (fig. 4). The main component of the setup is a digital sinus generator of three signals with synchronous frequencies. Two of the signals are delivered to the examined object while the third one serves as a reference signal. The frequency of the third signal is a linear combination of the frequencies of the other two

Let us assume that the output-input function for the examined object can be described by

where *C* is a constant value. The input signal is the sum of signals with different amplitudes,

1 1 01 2 2 02 cos cos *V CA t A t out* = ++ + ωα

After trigonometric conversions it is possible to receive a signal frequency spectrum at the object's output. The frequencies, amplitudes and phases of the particular spectral components (assuming that the examined object does not change its phase relations, i.e. it is

3

( )( ) <sup>3</sup>

 ωα(7)

, *V CV out in* = ⋅ (6)

 MEASURED NONLINEAR OBJECT

*measured signal*

*amplitude* 

*phase* LOCK-IN

*switch* 

AMPLIFIER

 FUNCTION GENERATOR

signals.

the formula:

phases and frequencies and so:

TTL *output*

higher harmonic (2) and subharmonic (3)

**2.3 Nonlinear object testing with two synchronous signals** 

characterized by pure resistances) are shown in table 1.

the phase dependency is removed. Phase difference (α01β01) can be measured according to

$$
\alpha\_{01} - \beta\_{01} = \tan^{-1} \left( \mathbf{Y} / \mathbf{X} \right) \tag{5}
$$

The first lock-in amplifiers were based on analogue technology. The measured signal and the reference were analogue voltage signals and they were multiplied in an analogue PSD. The results of multiplication were filtered through a multistage RC filter. In such lock-ins the reference signal phase at the PSD input had to be manually adjusted to the phase of the measured signal so that ( ) 01 01 cos α β − = 1. It was technically difficult to perform measurements by means of such lock-ins and it was practically impossible to register the amplitude and phase changes of the measured signals. Digital technology made it possible to build lock-in amplifiers in which both signal and reference inputs were multiplied and filtered digitally. Dual phase-sensitive detection eliminated the need for manual phase adjustments and enabled the simultaneous measurement of signal amplitude and phase. Such simultaneous measurements can be performed in *real time*, practically without any delay to the inducing signal. It also became possible to register short (below 0.1s) and slow changes in amplitude and phase over time.

### **2.2 Application of double-phase detection**

The PSD technique offers greater measuring possibilities owing to the fact that:


The basic experimental setup is shown in fig.3.

The generator used in the setup has two synchronous outputs. One of them (sync. output) supplies a TTL signal. Depending on the frequency of the reference signal one can measure the first harmonic, higher harmonics and subharmonics. In switch position 1 (fig.3), the reference signal is taken directly from the generator's synchronous output whereby the first harmonic can be measured. In switch position 2, the synchronous signal is multiplied by integral number *n* whereby the *n*-harmonic can be measured. In order to measure the *n*subharmonic the generator's synchronic output must be divided by integral number *n* (the switch in position 3).

In the simplest case, the waveform of the signal directed to the examined object is sinusoidal. When the examined object is linear, using the PSD technique one can very precisely (with an accuracy of 1 nanovolt) measure the electrical response of the object. If the signal is a simple square wave or another periodical wave with frequency *f*, the examined linear object does not change the signal spectrum and the filtered PSD output is a DC signal proportional to the root mean square (rms) of the first component of the signal.

The first lock-in amplifiers were based on analogue technology. The measured signal and the reference were analogue voltage signals and they were multiplied in an analogue PSD. The results of multiplication were filtered through a multistage RC filter. In such lock-ins the reference signal phase at the PSD input had to be manually adjusted to the phase of the

measurements by means of such lock-ins and it was practically impossible to register the amplitude and phase changes of the measured signals. Digital technology made it possible to build lock-in amplifiers in which both signal and reference inputs were multiplied and filtered digitally. Dual phase-sensitive detection eliminated the need for manual phase adjustments and enabled the simultaneous measurement of signal amplitude and phase. Such simultaneous measurements can be performed in *real time*, practically without any delay to the inducing signal. It also became possible to register short (below 0.1s) and slow

3. the reference signal frequency can be equal to the frequency of the signal being delivered to the examined object, but it also can be an integral multiplicity (or

4. two coherent signals can be introduced to the examined (usually nonlinear) object; the reference signal can be used at a frequency that is a linear combination of the

The generator used in the setup has two synchronous outputs. One of them (sync. output) supplies a TTL signal. Depending on the frequency of the reference signal one can measure the first harmonic, higher harmonics and subharmonics. In switch position 1 (fig.3), the reference signal is taken directly from the generator's synchronous output whereby the first harmonic can be measured. In switch position 2, the synchronous signal is multiplied by integral number *n* whereby the *n*-harmonic can be measured. In order to measure the *n*subharmonic the generator's synchronic output must be divided by integral number *n* (the

In the simplest case, the waveform of the signal directed to the examined object is sinusoidal. When the examined object is linear, using the PSD technique one can very precisely (with an accuracy of 1 nanovolt) measure the electrical response of the object. If the signal is a simple square wave or another periodical wave with frequency *f*, the examined linear object does not change the signal spectrum and the filtered PSD output is a DC signal proportional to the root mean square (rms) of the first component of the

01 01 α β

The PSD technique offers greater measuring possibilities owing to the fact that: 1. the signal fed to the examined object may have various periodical waveforms,

α β α01β

tan *Y X* <sup>−</sup> − = (5)

− = 1. It was technically difficult to perform

( ) <sup>1</sup>

01) can be measured according

the phase dependency is removed. Phase difference (

measured signal so that ( ) 01 01 cos

changes in amplitude and phase over time.

**2.2 Application of double-phase detection** 

submultiplicity) of this frequency.

frequencies of the inducing signals. The basic experimental setup is shown in fig.3.

switch in position 3).

signal.

2. the examined object can be linear or nonlinear,

to

Fig. 3. Basic experimental set-up with lock-in amplifier for measuring first harmonic (1), higher harmonic (2) and subharmonic (3)

The situation becomes more complicated when the examined object is nonlinear. The signal spectrum at the PSD input differs from the one at the generator output (the nonlinear object changes the input signal spectrum). This is true for all the signal waveforms, including the sinusoidal one. Then the lock-in amplifier measures the rms of both the first harmonic and the *n*-harmonic (*n*-subharmonic). When harmonic or subharmonic distortion is measured, the function generator supplies a pure sinusoidal signal without any harmonics. The basic experimental setup shown in fig. 3 was used to carry out experiments described in section 3.

### **2.3 Nonlinear object testing with two synchronous signals**

The double phase-sensitive detection technique and modern digital technologies offered new possibilities of examining nonlinear objects. An example of the measuring systems which have been developed is shown below (fig. 4). The main component of the setup is a digital sinus generator of three signals with synchronous frequencies. Two of the signals are delivered to the examined object while the third one serves as a reference signal. The frequency of the third signal is a linear combination of the frequencies of the other two signals.

Let us assume that the output-input function for the examined object can be described by the formula:

$$\mathbf{V}\_{out} = \mathbf{C} \cdot \mathbf{V}\_{in}^3 \tag{6}$$

where *C* is a constant value. The input signal is the sum of signals with different amplitudes, phases and frequencies and so:

$$V\_{out} = C \left[ A\_1 \cos \left( a \mathbf{o}\_1 \mathbf{t} + a \mathbf{o}\_{01} \right) + A\_2 \cos \left( a \mathbf{o}\_2 \mathbf{t} + a \mathbf{o}\_{02} \right) \right]^3 \tag{7}$$

After trigonometric conversions it is possible to receive a signal frequency spectrum at the object's output. The frequencies, amplitudes and phases of the particular spectral components (assuming that the examined object does not change its phase relations, i.e. it is characterized by pure resistances) are shown in table 1.

Analysis of Biological Acoustic Waves by Means of the Phase–Sensitivity Technique 265

Corti. For the first time they were registered by Wever and Bray in 1930 (Wever &Bray, 1930). The discovery of the signals made it much easier to examine the function of the inner ear and made it possible to assess the impact of various external and internal factors on this function. It is widely believed that cochlear microphonics (CMs) are generated mainly by outer hair cells (OHCs). Therefore it seems reasonable to use CMs as an indication of the OHC function. On the basis of measurements performed over a long period (e.g. a few weeks or months) one can assess if given hearing damage is temporary or permanent. The CM signal originating from different places in the human ear (or the animal ear) can be recorded. In humans CMs are usually picked up from the round window during surgical procedures performed on patients with various hearing pathologies. There are much fewer reports describing the reception of CM signals from the promontory or the ear canal near the eardrum. The past and present studies of the mechano-electrical cochlear function (based on the reception of CMs) are conducted mainly on animals, using: *in vivo*  preparations of anaesthetized animals with positive Preyer's reflex, *in vitro* preparations of the cochlea or *in vitro* preparations of the hair cells. As regards the research into the impact of various external and internal factors on the hearing organs, the *in vivo* studies seem to be

In the 1930s and 1940s CMs were measured at the cochlea's round window (Wever & Bray, 1930). In most animals the round window is relatively easily accessible and so measuring electrodes were usually placed on it or in its direct proximity. In the first years after the discovery of inner ear potentials, CM signals were measured by a single active probe. Several years later the first mapping of CMs on the cochlear surface was described (Thurlow, 1943). It was probably the first attempt ever to place the probe so close to the source of cochlear microphonics. CM potentials are continued to be measured at the cochlea's round window today (Brown, 2009). This measuring technique was not abandoned after the introduction of very sensitive (but invasive) procedures (Tasaki et al., 1952). Tasaki monitored CMs using a pair of active intracochlear electrodes in the basal turn (one electrode in scala tympani, the other in scala vestibuli). The electrodes were connected to a balanced differential amplifier. The reference electrode was placed on the neck muscles. This enables the measurement of the potentials very close to the organ of Corti and eliminates the auditory nerve potentials. The largest drawback is the mixing of perilymph

A new recording technique has been described by Carricondo (Carricondo at al., 2001). In this technique, CM potentials are recorded by subcutaneous electrodes in animals or by surface electrodes in humans. Two active electrodes are placed on the mandibular muscles while the reference electrode is located on the head's vertex. All the three electrodes are connected to a differential amplifier. The signal coming from the amplifier's output is filtered and subsequently averaged through in-phase synchronization with the sound

In 1960 at the Wroclaw University of Technology an oscillograph was built and used as part of an experimental setup for registering cochlear microphonics (CMs). In the following years Jankowski and Giełdanowski started a series of experiments on animals – first on cats, later on guinea pigs (Jankowski et al., 1962). A Biopotentials Research Workshop was founded, where biopotentials were measured after damage to the inner ear or the skull, in acoustic

most clinically valuable.

stimuli.

and endolymph when the probe is introduced.

**3.2 History of CM studies in Wroclaw** 

Fig. 4. Block diagram of experimental setup for more complicated studies of nonlinear objects


Table 1. Exemplary spectrum at output of object with 3rd-order nonlinearity, tested by pair of pure tones

The amplitude of each of the spectral components can be measured using this technique if a proper reference signal frequency is selected. The interpretation of phase shifts between the particular spectral components is much more complicated and requires taking into account the phase shifts introduced by the examined object. Moreover, the phase shifts introduced by the examined object may be a function of frequency and so they may be different for each spectral component. The technique was used to examine nonlinear distortions during the stimulation of the cochlea by a pair of pure tones. The results of this research are presented in section 4.
