**5.2 Discussion of problems encountered during measuring process**

One of the major challenges faced during the tests was positioning of the listeners relatively to the microphones. In the first tryout the microphones were fixed in a way similar to medical stethoscope. Microphones were coupled with flexible wires; these were attached to ears in such a way that the microphones were suspended and their transducers were on the level of ear canals entrances. The head of the human subject was placed on a holder fixed to the extension of the armchair's back. The distance between the head and the head holder was adjusted using cushions of different sizes. By increasing or reducing the amount of cushions the head of the test participant was placed at varied distances from the holder. The position of the head was controlled trough electronic visual system. On the screen the researcher could see the lines matching the position of ear canals entrances and adjust the position of the head accordingly.

This method was verified negatively. The participants during the tests do move their heads slightly. Using a band to fasten the head to the holder did not bring any significant improvement. Those minimal head motions have an impact on the geometry of the measurement arrangement. In the case of high-resolution measurement performance the stability of geometrical configuration: the sounds source – the microphone, is crucial for the accuracy of the measurement.

The other method of attaching measurement microphones was then proposed and tested. The microphones were fixed on a nonflexible construction. The construction had the possibility of adjusting the position of the microphones, though. The microphones were placed on the level of ears' canals entrances like before. Applying the fixed construction resulted in the fact that the test participant felt the microphones support structure limitation. In this case it was easier for the test participant to control head motions: when they appeared, it was a simpler task to put the head back in right position. The other advantage was that the distance between the microphones and the head holder was preset. The researcher avoided long process of positioning the head in relation to the holder. The only thing to be done was locating the listener in a proper elevation according to the sound sources. This solution is presented in Figure 2.

System for High Speed Measurement of Head-Related Transfer Function 15

The impact of the research room is neutralized as much as possible by measuring the reference response. While measuring the reference responses the components with frequencies around 80 Hz were singled out (Figure 8). It could be said that it was the effect of the wave interference inside the room. Although the tests are conducted in anechoic chamber, it is a place designed basically to make measurements involving machines and there is a concrete platform in the middle of the chamber intended for placing machines. This can contribute to forming interference phenomenon. Repositioning the device inside the chamber reduced the presence of the interference occurrence. Nevertheless, the phenomenon was observed only for frequencies outside the operational range of the device.

Fig. 8. The impulse response containing a small frequency component.

The HRTF measurement system allows a very fast measurement of HRTF with high spatial and frequency resolution. The applied operational algorithms of the system guarantee repeatability of measurements and minimalization of the influence of many disadvantageous factors on measurements results. Compact structure and modularity of construction of the system allows an easy transport of the device. The encountered problems were discussed together with the eventual solutions to them. On the basis of conducted measurements and subjective tests it could be assumed that the device measures the HRTFs

0 2 4 6 8 10 12 14 16 18 20 22 24 <sup>15</sup>

Time [ms]

**6. Conclusions** 

10

5

0

HRI h1

R

5

10

15

The most important of all the advantages of this particular way of setting the microphones is the possibility of the precise microphones positioning while measuring the reference response and while conducting the tests with human participant, as well. It is very significant for the accuracy of measurements, particularly when the impact of the measuring set and that of the research room should be minimized.

Although conducting the measurement of reference response for each assessment spot excludes the impact of the measurement set, some acoustic phenomena cannot be reduced this way. During the tests it was observed that for the 90° elevation angle and for the angles close to this value, in the reference impulse response the sound reflection from the seat of the armchair was observed. (Figure 7, Time ≈ 7 ms). During the test involving the participant the reflection does not occur because the person is seated in the armchair and therefore covering the seat surface. The phenomena of reflection while measuring the reference response, after the sound reaches the seat of the armchair, could be eliminated by using additional sound diffusion device.

Summing up, in the case of sound reflection from elements covered by the test participant, the use of the reference response is not sufficient. Similar phenomena were observed for different angles but never to such extent as in the case of 90° elevation angle.

Fig. 7. The impulse response for vertical 90 ° angle.

The impact of the research room is neutralized as much as possible by measuring the reference response. While measuring the reference responses the components with frequencies around 80 Hz were singled out (Figure 8). It could be said that it was the effect of the wave interference inside the room. Although the tests are conducted in anechoic chamber, it is a place designed basically to make measurements involving machines and there is a concrete platform in the middle of the chamber intended for placing machines. This can contribute to forming interference phenomenon. Repositioning the device inside the chamber reduced the presence of the interference occurrence. Nevertheless, the phenomenon was observed only for frequencies outside the operational range of the device.

Fig. 8. The impulse response containing a small frequency component.

#### **6. Conclusions**

14 Advanced Topics in Measurements

The most important of all the advantages of this particular way of setting the microphones is the possibility of the precise microphones positioning while measuring the reference response and while conducting the tests with human participant, as well. It is very significant for the accuracy of measurements, particularly when the impact of the measuring

Although conducting the measurement of reference response for each assessment spot excludes the impact of the measurement set, some acoustic phenomena cannot be reduced this way. During the tests it was observed that for the 90° elevation angle and for the angles close to this value, in the reference impulse response the sound reflection from the seat of the armchair was observed. (Figure 7, Time ≈ 7 ms). During the test involving the participant the reflection does not occur because the person is seated in the armchair and therefore covering the seat surface. The phenomena of reflection while measuring the reference response, after the sound reaches the seat of the armchair, could be eliminated by

Summing up, in the case of sound reflection from elements covered by the test participant, the use of the reference response is not sufficient. Similar phenomena were observed for

<sup>0</sup> 5 10 15 20 25 30 <sup>4</sup>

Time [ms]

different angles but never to such extent as in the case of 90° elevation angle.

set and that of the research room should be minimized.

using additional sound diffusion device.

Fig. 7. The impulse response for vertical 90 ° angle.

2

0

2

4

h1 HRI

R

6

8

10

12

The HRTF measurement system allows a very fast measurement of HRTF with high spatial and frequency resolution. The applied operational algorithms of the system guarantee repeatability of measurements and minimalization of the influence of many disadvantageous factors on measurements results. Compact structure and modularity of construction of the system allows an easy transport of the device. The encountered problems were discussed together with the eventual solutions to them. On the basis of conducted measurements and subjective tests it could be assumed that the device measures the HRTFs

System for High Speed Measurement of Head-Related Transfer Function 17

Hen, P., Kin, M.J. & Plaskota, P. (2008). Conversion of stereo recording to 5.1 format using head-related transfer functions, *Archives of Acoustics*, 2008, Vol. 33, No 1, pp. 7-10 Minnaar, P., Plogsties, J., Olsen, S.K., Christensen, F. & Møller, H. (2000). The Interaural

Møller, H., Sørensen, M.F., Hammershøi, D. & Larsen, K.A. (1992). Head Related Transfer

Møller, H., Sørensen, M.F., Hammershøi, D. & Jensen, C.B. (1995). Head Related

Møller, H., Sørensen, M.F., Hammershøi, D. & Jensen, C.B., (1996). Binaural Technique:

Moore, B.C.J. (1997). *An Introduction to the Psychology of Hearing*, 4th Ed., (1997) Academic,

Plaskota, P. (2003). Evaluation of microphone parameters for HRTF measurement (in

Plaskota, P. (2006). Measurement of acoustical impedance of human skin, *In: The 6th* 

Plaskota, P. (2007). Acoustical model of the head for HRTF calculation, *In: LIV Open Seminar* 

Plaskota, P. & Dobrucki, A.B. (2004). Selected problems of HRTF measurement (in Polish),

Plaskota, P. & Dobrucki, A.B. (2005). Numerical head model for HRTF simulation, *In: The 118th AES Convention, Barcelona,* Spain, May 28-31, 2005, Preprint 6510 Plaskota, P. & Kin, M.J. (2002). The use of HRTF in sound recordings (in Polish*), In: IX New* 

Plaskota, P. & Pruchnicki, P. (2006). Practical aspects of using HRTF measuring device,

Plaskota, P., Wasilewski, M. & Kin, M.J., (2003) The use of Head-Related Transfer

Pralong, D. & Carlile, S. (1994). Measuring the human head-related transfer functions: A

Pruchnicki, P. & Plaskota, P. (2008). HRTF Automatic Measuring System, *Archives of* 

Weinrich, S.G. (1992). Improved Externalization and Frontal Perception of Headphone

Signals, *Presented at the 92 AES Convention, February 1992, Viena*

Functions in auralization of sound recordings (in Polish), *In: X International Symposium the Art of Sound Engineering. ISSET 2003.* Wrocław, Poland, 11-13

novel method for the construction and calibration of a miniature in-ear recording

*Society of Finland.* Tempere, Finland, 30 May - 1 June 2006

*In: Metrology Congress,* Wrocław, Poland, 6-9 September 2004

*Trends in Audio and Video.* Warsaw, Poland, 27-28 September 2002

*Archives of Acoustics*, 2006, vol. 31, no 4, suppl., pp. 439-444

system, *J. Acoust. Soc. Am*. 95, pp. 3435–3444. 1994

*Acoustics,* 2008, Vol. 33, No 1, pp. 19-25

*on Acoustics.* Rzeszów-Przemyśl, Poland, 10-14.09.2007

February 2000, Paris, Preprint 5133

Wrocław, Poland, 11-13 September 2003

*February 1992, Viena*

pp. 300-321

San Diego, 1997

September 2003

451-469

Time Diference in Binaural Sythesis, *Presented at the 108 AES Convention*, 19-22

Functions: Measurments on 24 Human Subjects, *Presented at the 92 AES Convention,* 

Transfer Functions of Human subjects, *J. Audio Eng. Soc*., Vol 43, No 5, May 1995,

Do We Need Individual Recordings?, *J. Audio Eng. Soc*., Vol 44, No 6, June 1996, pp.

Polish), *In: X International Symposium the Art of Sound Engineering. ISSET 2003*.

*European Conference on Noise Control EURONOISE 2006, Proceedings, Acoustical* 

accurately enough to recreate the position of the sound source in the space surrounding the listener. The scope for future tests is to verify if the proposed adjustments eliminate the impact of the research room by conducting tests in the reverberation room sufficiently. To eliminate the influence of physical movements of the participant it is recommended that the tests should be conducted using a dummy head.

#### **7. References**


accurately enough to recreate the position of the sound source in the space surrounding the listener. The scope for future tests is to verify if the proposed adjustments eliminate the impact of the research room by conducting tests in the reverberation room sufficiently. To eliminate the influence of physical movements of the participant it is recommended that the

Algazi, V.R., Avendano, C. & Thompson, D. (1999). Dependence of subject and

Algazi, V.R., Duda, R.O., Thompson, D.M. & Avendano, C. (2001). The CIPIC HRTF database, *Proceedings of IEEE WASPAA 2001*, New Paltz, NY, pp. 99–102 Batteau, D.W. (1967). The role of the pinna in human localization, *Proc. R. Soc. London, Ser. B* 

Bovbjerg, B.P., Christensen, F., Minnaar, P., Chen, X. (2000). Measuring the Head-Related

Bujacz, M., Strumiłło, P. (2006). *Stereophonic representation of virtual 3D scenes – a simulated mobility for the blind*, In: New Trends in Audio and Video, Vol. 1, Białystok 2006 Cheng, C.I., Wakefield, G.H. (2001). Introduction to head-related transfer functions (HRTFs):

Dobrucki, A.B. (2006). *Electroacoustic transducers* (in Polish), Wydawnictwa Naukowo-

Dobrucki, A.B., Plaskota, P. (2007). Computational modelling of head-related transfer

Dobrucki, A.B. Plaskota, P., Pruchnicki P., Pec, M., Bujacz, M., Strumiłło, P. (2010).

Grassi, E., Tulsi, J., & Shamma, S.A. (2003). Measurement of headrelated transfer functions

Hartmann, W.M. & Wittenberg, A. (1996). On the externalization of sound images, *J. Acoust.* 

Horbach U., Karamustafaouglu, A., Pellegrini, R., Mackensen, P. & Theile, G. (1999). Design

sighted individuals. *J. Audio Eng. Soc.,* 2010, vol. 58, No 9, pp. 724-738 Gardner, W.G. & Martin, K.D. (1995). HRTF measurements of a KEMAR, *J. Acoust. Soc. Am*.

function, *Archives of Acoustics*, 2007 Vol. 32, No 3, pp. 659-682

*Displays \_ICAD 2003\_*, Boston, MA, pp. 119–121

Hartmann, W.M. (1999). How we localize sound, *Phys. Today 1999*, pp. 24–29

*Presented at the 106th AES Convention*, 8-11 May 1999, Munich

measurement position in binaural signal acquisition, *J. Audio Eng. Soc.*, Vol. 47, No.

Transfer Functions of an Artificial Head with a High Directional Resolution, *Presented at the 109th AES Convention*, 22-25 September 2000, Los Angeles, Preprint

Representations of HRTFs in time, frequency, and space, *J. Audio Eng. Soc*., Vol. 49,

Measurement system for personalized head-related transfer functions and its verification by virtual source localization trials with visually impaired and

based on the empirical transfer function estimate*, Proc. 2003 Intl. Conf. on Auditory* 

and Applications of a Data-based Auralization System for Surround Sound,

tests should be conducted using a dummy head.

11, (November 1999), pp. 937–947

Blauert, J. (1997). *Spatial Hearing*, The MIT Press, Massachusetts, 1997

*168*, 1967, pp. 158–180

No 4, April 2001, pp. 231-249

Techniczne, Warsaw 2006

*Soc. Am*. 99, pp. 3678–3688

97, 3907–3908

**7. References** 

5264


**Precise Measurement System for** 

*1Himeji Dokkyo University* 

*4Okayama University of Science* 

*3Okayama University* 

*Japan* 

*2Kawasaki University of Medical Welfare* 

 **Knee Joint Motion During the Pendulum** 

 **Test Using Two Linear Accelerometers** 

Yoshitake Yamamoto1, Kazuaki Jikuya2, Toshimasa Kusuhara3, Takao Nakamura3, Hiroyuki Michinishi4 and Takuji Okamoto3

The pendulum test is a means to evaluate the knee joint reflex from the pendulum motion induced by letting the lower leg drop freely after it has been lifted up (Watenberg, 1951). Many researchers have attempted to quantify the spinal cord stretch reflex from this pendulum motion in order to diagnose spasticity (Fowler et al., 2000; Kaeser et al., 1998; Lin & Rymer, 1991; Nordmark & Andersson, 2002; Stillman & McMeeken, 1995; Vodovnik et al., 1984). However, even today, much remains unknown about the relationship between this pendulum motion and the mechanism that produces the stretch reflex. For this reason,

One method to advance the quantification of the stretch reflex may be to implement the

1. Analyze the unknown behaviors in the pendulum test from various view points by trial and error, using existing physiological, clinical, and control engineering knowledge and

2. Modify the existing pendulum test model (Jikuya et al., 1991) based on the results of 1. 3. Elucidate the detailed mechanism of the stretch reflex using the model in 2, and

We have already elucidated various phenomena following this procedure, but in this process, it has often been necessary to know angle, angular velocity, and angular acceleration values at arbitrary times during knee joint motion as initial and boundary conditions to solve nonlinear differential equations. Obtaining this kind of waveform with

In principle, various existing sensors can be used to detect knee joint motion. However, several such sensors are not practical because of the knee joint's unique structural

quantification studies on the stretch reflex have progressed slowly.

**1. Introduction** 

following items in order:

theory as appropriate.

investigate quantification methods.

existing simple methods is difficult, as described below.

Zahorik, P. (2000). Limitations in using Golay codes for head-related transfer function measurement, *J. Acoust. Soc. Am*., Vol. 107, No 3, March 2000, pp. 1793-1796 **2** 
