**4.3 Measurement procedure**

The measurement procedure comprises several phases. The first is the system activation and configuration. It involves determining the horizontal and vertical resolutions of measurements. The next step is the selection and fixing of active test loudspeakers position. At this stage the kind of measurement signal and the number of averages should be chosen as well as the calibration of sound level should be carried out.

In the second phase, participant of the test should be properly positioned in the chair, so that the 0° loudspeaker is placed on the ear canal entrance level and the microphones are located at ear canals entrance. The setup of the loudspeakers' arc in relation to the microphones can be monitored using the camera view.

After the test participant measurement is completed, the reference responses are measured. Once the preparation is finished, regular HRTF measurements are carried out according to earlier parameter setups.

In the last phase of the procedure, measurement results are saved in plain text files in the form of the HRIR. Such storing allows access to the test results from any other application at the same time, and is clear to the user.

#### **4.4 System control software**

In order to apply the measurement procedure, dedicated software was designed. The modularity of this software, which consists of two basic elements, is its special feature. Figure 3 presents the main window used to control measurements. Via this interface the operator can influence the measurement course and conditions as well as all configuration parameters. Additionally, there is also a test participant communication part.

A separate element of the software is an OCX control which exchanges data between the device and the user interface. Calling certain functions of the control it is possible to steer such parameters as the armchair rotation, the loudspeakers movement or switching.

The whole measurement procedure is comprised of two parts: the measurement of reference responses and the measurement of regular HRTFs. The measurement of reference responses is made for all measurement spots determined by the system operator. During this procedure microphones, loudspeakers and the whole system work exactly like during any regular measurement. The only difference is that there are no test participants. The HRTF measurement results obtained in the second part are related to reference responses obtained

Using a reference response for each measurement point in the space allows limiting many inconvenient effects which decline measurement accuracy (Plaskota & Pruchnicki, 2006). Especially the influence of frequency responses and directivity responses of loudspeakers and microphones is eliminated. The influence of a test room and the reflection from the

The final result of the measurement process are HRIRs (Head Related Impulse Response, that is HRTF's reverse Fourier transform) produced to allow their direct use in convolution

The measurement procedure comprises several phases. The first is the system activation and configuration. It involves determining the horizontal and vertical resolutions of measurements. The next step is the selection and fixing of active test loudspeakers position. At this stage the kind of measurement signal and the number of averages should be chosen

In the second phase, participant of the test should be properly positioned in the chair, so that the 0° loudspeaker is placed on the ear canal entrance level and the microphones are located at ear canals entrance. The setup of the loudspeakers' arc in relation to the

After the test participant measurement is completed, the reference responses are measured. Once the preparation is finished, regular HRTF measurements are carried out according to

In the last phase of the procedure, measurement results are saved in plain text files in the form of the HRIR. Such storing allows access to the test results from any other application at

In order to apply the measurement procedure, dedicated software was designed. The modularity of this software, which consists of two basic elements, is its special feature. Figure 3 presents the main window used to control measurements. Via this interface the operator can influence the measurement course and conditions as well as all configuration

A separate element of the software is an OCX control which exchanges data between the device and the user interface. Calling certain functions of the control it is possible to steer such parameters as the armchair rotation, the loudspeakers movement or switching.

parameters. Additionally, there is also a test participant communication part.

device elements on measurement results is partly reduced.

as well as the calibration of sound level should be carried out.

microphones can be monitored using the camera view.

before.

with real signals.

**4.3 Measurement procedure** 

earlier parameter setups.

the same time, and is clear to the user.

**4.4 System control software** 

Applying this solution allows to use the device for purposes not provided by the user interface of the system.

Fig. 3. The main window of the HRTF measurement control software.

#### **4.5 Parameters of the device**

The HRTF measuring device has 16 sound sources. The reason for using such number of loudspeakers is the need to conduct tests for many various spots in the listener's surrounding in the shortest time possible. The different positions are found in the following way: the participant in the test turns around his vertical axis while taking a step in defined direction. The distances between the steps define the spatial resolution of the measurement in horizontal dimension. The vertical dimension of spatial resolution is determined by the arrangement of loudspeakers placed on the arc including range of vertical angles between - 45° and +90°.

For the precision of the measurement it is important to use a point sound source. The source should produce test signals in the entire operational frequency range of the device. In order to fulfill these conditions two-way car loudspeakers were applied. According to producer data the loudspeakers should operate within a small box. Figure 4 presents an example of amplitude frequency response of the used loudspeakers. The loudspeakers' operational range of frequency is between 200Hz and 20kHz. It should be noted that the frequency responses are not equalized and differ slightly for each loudspeaker less than 4dB. The applied measurement of reference response in the device for each tested spot neutralizes the influence of measuring set on the results of the tests.

System for High Speed Measurement of Head-Related Transfer Function 11

a significant impact for 10 kHz frequency and above (Dobrucki, 2006). But that is transversal dimension of applied elements; the length of the microphones cover is more significant dimension in this case and can influence acoustic field within the operational range of the

Fig. 5. An example of the measurement microphones frequency response.

10 100 1

**5. Practical aspects of using the HRTF measuring device 5.1 Verification of the measurements results using dummy head** 

One of the methods to eliminate the impact of microphones' cover elements on the acoustic field around the head of the test participant is using microphones placed directly in the matter closing ears' canals (Møller et al.,1995). In this case the usage of cover elements could be avoided and the solution is more advantageous for the precision of the results. On the other hand, the use of plain microphone without a rigid support construction attached to the measurement device gives way to the uncontrolled head motions. The impact of this fact on the tests' results is described in section 5.2. It should be noted that the use of the microphones without rigid support increases the amount of time needed for exact positioning of the participant's head and also makes the measurement of the reference

f [Hz]

Frequency [Hz]

103 <sup>1</sup>

104

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

device.

response more difficult.

50

40

30

20

H1 [dB] Frequency response [dB]

10

0

Fig. 4. An example of frequency response of the loudspeaker used in the HRTF measuring device.

The measurement microphones used in the device are the same as those used in hearing aids. It should be underlined that the particular type of microphones has equal frequency response in its entire operational range of ca. 60Hz and 8kHz (Figure 5). It means that these microphones are not commonly used in the hearing problems treatment. The choice of microphones was determined by the importance of the quality of the device and therefore the similarity of frequency responses of each microphone was achieved. The other advantage of this particular type of microphones is their small size. That is indeed a significant feature since it allows reducing the size of the outer cover. This minimizes cover impact on the acoustic field around the head of the test participant.

The operational frequency range of the HRTF measuring device is limited by the lower cutoff frequency of the anechoic chamber in which the tests are conducted. The other factor influencing lower frequency is the operational frequency range of loudspeakers. The lower cut-off frequency within the operational range of the loudspeakers is higher than the value of the cut-off frequency of the anechoic chamber thus the operational frequency range for the entire device starts at around 200Hz.

The upper cut-off frequency limit of the device is determined by the frequency range of the microphones. Hence the upper cut-off frequency is about 8kHz. The other factor carrying impact on operational frequency range of the device is the influence of microphones' covers on the acoustic field around the head of the test participant. The microphones are placed in ca. 5-mm diameter tubes. The wave phenomena for this type of construction elements have

Fig. 4. An example of frequency response of the loudspeaker used in the HRTF measuring

<sup>10</sup> <sup>3</sup> <sup>1</sup>

f [Hz]

Frequency [Hz]

<sup>10</sup> <sup>4</sup>

10 100 1

The measurement microphones used in the device are the same as those used in hearing aids. It should be underlined that the particular type of microphones has equal frequency response in its entire operational range of ca. 60Hz and 8kHz (Figure 5). It means that these microphones are not commonly used in the hearing problems treatment. The choice of microphones was determined by the importance of the quality of the device and therefore the similarity of frequency responses of each microphone was achieved. The other advantage of this particular type of microphones is their small size. That is indeed a significant feature since it allows reducing the size of the outer cover. This minimizes cover

The operational frequency range of the HRTF measuring device is limited by the lower cutoff frequency of the anechoic chamber in which the tests are conducted. The other factor influencing lower frequency is the operational frequency range of loudspeakers. The lower cut-off frequency within the operational range of the loudspeakers is higher than the value of the cut-off frequency of the anechoic chamber thus the operational frequency range for

The upper cut-off frequency limit of the device is determined by the frequency range of the microphones. Hence the upper cut-off frequency is about 8kHz. The other factor carrying impact on operational frequency range of the device is the influence of microphones' covers on the acoustic field around the head of the test participant. The microphones are placed in ca. 5-mm diameter tubes. The wave phenomena for this type of construction elements have

impact on the acoustic field around the head of the test participant.

the entire device starts at around 200Hz.

device.

60

50

40

30

H1 [dB] Frequency response [dB]

20

10

0

a significant impact for 10 kHz frequency and above (Dobrucki, 2006). But that is transversal dimension of applied elements; the length of the microphones cover is more significant dimension in this case and can influence acoustic field within the operational range of the device.

Fig. 5. An example of the measurement microphones frequency response.

One of the methods to eliminate the impact of microphones' cover elements on the acoustic field around the head of the test participant is using microphones placed directly in the matter closing ears' canals (Møller et al.,1995). In this case the usage of cover elements could be avoided and the solution is more advantageous for the precision of the results. On the other hand, the use of plain microphone without a rigid support construction attached to the measurement device gives way to the uncontrolled head motions. The impact of this fact on the tests' results is described in section 5.2. It should be noted that the use of the microphones without rigid support increases the amount of time needed for exact positioning of the participant's head and also makes the measurement of the reference response more difficult.
