**5. Practical aspects of using the HRTF measuring device**

#### **5.1 Verification of the measurements results using dummy head**

It is not impossible to verify results of measurements given by presented device directly, but the correctness of measurement results can be verified in indirect process. The first method is a subjective test for a person who had been measured using this device. During the test

System for High Speed Measurement of Head-Related Transfer Function 13

the diagram. The particular letters represent the following cases: i – the measurement result, ii – the result of simulations without impedance boundary conditions (the rigid model), iii – the result of simulations with impedance boundary conditions in whole modeled area except for the pinnas, for which the same boundary condition as for the rigid model was

Measurements results are in good accordance with calculation results below 6 kHz. On the basis of comparison between the measurement and calculation results, it was found that measurements results are proper. There are some reasons of difference between measurement and calculation results above 6 kHz. At first, the microphone set has been not taken into consideration during the numerical calculation: microphone enclosures probably produce the wave phenomena in frequency range of 8-10 kHz. Secondly, the high cut-off

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

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

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

assumed.

accordingly.

accuracy of the measurement.

sources. This solution is presented in Figure 2.

frequency of a numerical model is about 7 kHz.

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

the signal convolved with a result of HRTF measurement is presented – this operation sets up a virtual sound source in specific point in space around a listener (Dobrucki et al., 2010). The consistence of point determined by convolution process and point indicated by listener is tested. If the consistence is correct, the result of measurement is also correct. Other method for verification of measurement result is a comparison of measurement results with the results of numerical calculation (Dobrucki & Plaskota, 2007).

The correctness of a measurement result was examined by measurement of dummy head (Neumann KU100). The result of measurement was compared to the results of numerical calculation. The dummy head had been placed in measurement device, next the whole measurement process was conducted. The use of a dummy head can eliminate some inconvenient occurring during measurement of a person, i.e. the head movement that provides to large measurement deviation.

The Boundary Elements Method (BEM) has been used to perform the numerical calculation of HRTF (Dobrucki & Plaskota, 2007). The numerical model is a representation of geometrical shape of dummy head, especially with emphasis on accordance of pinna model with geometry of real object. Differences between real object and numerical model are smaller than 0.1 mm (Plaskota, 2007; Plaskota & Dobrucki, 2005). The measurement of acoustical impedance of dummy head has been done (Plaskota, 2006) and the result was used as a boundary condition.

Figure 6 show HRTF measurement and numerical calculation results for azimuth 90°, elevation 0°, for ipsilateral ear (located closer to the sound source). There are three graphs in

Frequency [kHz]

Fig. 6. Measurement and simulation results: azimuth 90°, elevation 0°, ipsilateral ear. Detailed description of symbols in text.

the signal convolved with a result of HRTF measurement is presented – this operation sets up a virtual sound source in specific point in space around a listener (Dobrucki et al., 2010). The consistence of point determined by convolution process and point indicated by listener is tested. If the consistence is correct, the result of measurement is also correct. Other method for verification of measurement result is a comparison of measurement results with

The correctness of a measurement result was examined by measurement of dummy head (Neumann KU100). The result of measurement was compared to the results of numerical calculation. The dummy head had been placed in measurement device, next the whole measurement process was conducted. The use of a dummy head can eliminate some inconvenient occurring during measurement of a person, i.e. the head movement that

The Boundary Elements Method (BEM) has been used to perform the numerical calculation of HRTF (Dobrucki & Plaskota, 2007). The numerical model is a representation of geometrical shape of dummy head, especially with emphasis on accordance of pinna model with geometry of real object. Differences between real object and numerical model are smaller than 0.1 mm (Plaskota, 2007; Plaskota & Dobrucki, 2005). The measurement of acoustical impedance of dummy head has been done (Plaskota, 2006) and the result was

Figure 6 show HRTF measurement and numerical calculation results for azimuth 90°, elevation 0°, for ipsilateral ear (located closer to the sound source). There are three graphs in

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Frequency [kHz]

Fig. 6. Measurement and simulation results: azimuth 90°, elevation 0°, ipsilateral ear.

the results of numerical calculation (Dobrucki & Plaskota, 2007).

provides to large measurement deviation.

used as a boundary condition.

30

Detailed description of symbols in text.

20

i ii iii

10

HRTF [dB]

0

10

20

the diagram. The particular letters represent the following cases: i – the measurement result, ii – the result of simulations without impedance boundary conditions (the rigid model), iii – the result of simulations with impedance boundary conditions in whole modeled area except for the pinnas, for which the same boundary condition as for the rigid model was assumed.

Measurements results are in good accordance with calculation results below 6 kHz. On the basis of comparison between the measurement and calculation results, it was found that measurements results are proper. There are some reasons of difference between measurement and calculation results above 6 kHz. At first, the microphone set has been not taken into consideration during the numerical calculation: microphone enclosures probably produce the wave phenomena in frequency range of 8-10 kHz. Secondly, the high cut-off frequency of a numerical model is about 7 kHz.
