**4. Measuring system**

4 Advanced Topics in Measurements

for the frequency to 1 kHz, 5 dB for the frequency to 8 kHz and about 10 dB for the higher frequencies. The first reason is an obvious dependence on individual physical body differences. Other reason are the measurement errors which are hard to be calculated in the final results – e.g. the error resulting from the differences in positioning the head in relation to the sound source or the differences in placing the measurement microphone in the ear canal. The individual HRTF variable is lower for the measurements conducted with a closed

In general, the HRTF parameters are measured in anechoic chamber, e.g. Møller et al., 1995. During measurement it must be possible to place the sound source in a distance of minimum 1 m from the middle of the listener's head in each direction. Especially the direction above the listener's head is important because of chamber size. Taking into account the listener's height and minimal distance between the loudspeaker and the human head it can be assumed that the minimal height of the measurement room is ca. 3 m. The intermediate solution is to place the listener sitting on a chair, although in this case reflections from knees can be observed (Møller et al., 1996). The reflections from measurement device placed into the measurement room have more significant influence on the result of the measurement in comparison with the reflections from body parts (Møller et

The HRTF measurement can be provided in ordinary room, e.g. auditorium (Bovbjerg et al., 2000; Møller et al., 1996). Measurements in non-anechoic chamber are convenient because of availability of this kind of room. Usually, when measurements with people go on a few days, there is a necessity to leave measurement devices in a fixed setup for long time. To make measurements in an ordinary room a noise-gate must be used for eliminating the

In the measurement room it is necessary to place the video devices for controlling and eventually recording the head position and head movements. Head movements are a significant source of errors. Verifying a head position allows to increase the measurement

For measurements in many points in space around the listener it is needed to use many sound sources in fixed positions or use movable set of loudspeakers. Generally, it is possible to apply two methods of changing the position of the loudspeaker relatively to the listener's pinna. One of them is a movement of the sound source (one loudspeaker or set of loudspeakers) around the listener's head (Algazi et al, 2001; Bovbjerg et al., 2000; Grassi et al., 2003). The listener can improve measurement's accuracy by a visual control of head position. In the case of changing the listener's position relatively to the loudspeaker set (e.g. by chair rotation) it is needed to use an additional equipment for monitoring the head position (e.g. video camera) (Møller et al., 1995). A convenient situation is when the position of the listener and positions of the loudspeakers are fixed. In this situation very good control of measurement setup is obtained,

The next important parameter of the measurement system is a placement of measurement microphone in an ear canal. In publications four main positions are considered: a few

ear canal than for the measurements with an open ear canal.

**3. HRTF measurement requirements** 

al., 1995), so these last can be omitted.

reverberation signals (Plaskota & Dobrucki, 2004).

accuracy (Algazi et al., 1999; Gardner & Martin, 1995).

but the number of measurement points is limited (Møller et al., 1996).

#### **4.1 Conception of measuring system**

The HRTF measuring device is built for a special group of test participants. It is assumed that the measurement will be made for people with severe vision problems (Bujacz & Strumiłło, 2006; Dobrucki et al. 2010). Therefore, the device is designed to reach many demands such as the highest automation of measurements which assures a short measurement time (ca 10 minutes) and offers great ease of manipulation. The participant of the test should feel comfortable during the measurement process and should be given sufficient information on each part of the measurement. To reach these demands, the device is equipped with a bidirectional communication system allowing the participant to report the problem at any time. In addition to voice communication, a visual control of the room is provided. It is possible to monitor the test room using a camera mounted on an arc with loudspeakers.

To provide a short measurement time the HRTFs are measured for both ears simultaneously. The way sound sources are configured significantly shortens this time too. The loudspeakers are mounted on vertically positioned arc (see Fig. 1). It allows to measure the range of vertical angles from -45° to +90° in one chair position. In certain points in the space of the room the measurement is made by switching the measurement signals to subsequent loudspeakers by an electronic switch.

The number of measurement points for elevation angles is adjusted by changing the number and position of the loudspeakers. On the other hand, the number of measurement points for horizontal angles depends on the size of the rotation step of the chair. The rotation of the chair is controlled by a stepper motor which assures high horizontal resolution. Default vertical resolution is 9° in regular sound source positions. Assuming the same horizontal resolution the number of measurement points is 640. The measurement in 16 points for one horizontal angle and simultaneous measurements for both ears allows to conduct the whole

System for High Speed Measurement of Head-Related Transfer Function 7

The system is operated via portable IBM PC computer to control measurements and data acquisition (Pruchnicki & Plaskota, 2008). The device communicates with the computer through a USB interface. At the same time signals operating the device, measurements signals and camera pictures are transmitted via interface. A special feature of the device is its compact construction and modularity which makes it very easy to assemble or

The measurement of a single HRTF is accomplished using a transfer method, which is popular in digital measurements systems. A wide spectrum measurement signal is used for stimulation. The system uses the following signals: chirp, MLS, white noise, pink noise, Golay codes. The length of a generated signal can be changed within the range from 128 up to 8192 samples. Sampling frequency is 48 kHz but it is possible to decrease it. The stimulating signal is repeated several times in order to average the answer of the system in the time domain. This operation allows improving the S/N ratio of received responses. There is no need to apply longer measurement signals because, according to other researches, HRTFs may be presented even with such resolution as 100 Hz. On the other hand, responses determined in the system will be used for convolution with real signals and therefore they cannot be too long. Moreover, long measurement signals make the

Fig. 2. The scheme of setting the measurement microphones.

disassemble and convenient to transport.

**4.2 Measurement algorithm** 

assessment time longer.

measurement in less than 10 minutes. Obviously, the number of measurement points can be modified. Changing the resolution in a vertical plane means changing the position of the loudspeakers. In a horizontal plane, changing the resolution means changing the rotation step of the chair.

Fig. 1. Overview of the HRTF measurement equipment.

The HRTF measurement can be done within the range of frequencies from 200 Hz to 8 kHz. The lowest frequency depends on the test room parameters. The device works in an anechoic chamber, therefore the cut-off frequency of the chamber limits the operational range of the device. The high cut-off frequency of the device is on the one hand confined by the set of loudspeakers, and on the other – by the set of microphones. Miniature microphones used in hearing aids, but with an untypical flat frequency response, are used in the device (Fig. 2). Another factor limiting the high cut-off frequency are the dimensions of microphone fixing elements. For 5-mm tubes the wave phenomena are significant for the frequencies above 10 kHz.

measurement in less than 10 minutes. Obviously, the number of measurement points can be modified. Changing the resolution in a vertical plane means changing the position of the loudspeakers. In a horizontal plane, changing the resolution means changing the rotation

step of the chair.

Fig. 1. Overview of the HRTF measurement equipment.

frequencies above 10 kHz.

The HRTF measurement can be done within the range of frequencies from 200 Hz to 8 kHz. The lowest frequency depends on the test room parameters. The device works in an anechoic chamber, therefore the cut-off frequency of the chamber limits the operational range of the device. The high cut-off frequency of the device is on the one hand confined by the set of loudspeakers, and on the other – by the set of microphones. Miniature microphones used in hearing aids, but with an untypical flat frequency response, are used in the device (Fig. 2). Another factor limiting the high cut-off frequency are the dimensions of microphone fixing elements. For 5-mm tubes the wave phenomena are significant for the

Fig. 2. The scheme of setting the measurement microphones.

The system is operated via portable IBM PC computer to control measurements and data acquisition (Pruchnicki & Plaskota, 2008). The device communicates with the computer through a USB interface. At the same time signals operating the device, measurements signals and camera pictures are transmitted via interface. A special feature of the device is its compact construction and modularity which makes it very easy to assemble or disassemble and convenient to transport.

#### **4.2 Measurement algorithm**

The measurement of a single HRTF is accomplished using a transfer method, which is popular in digital measurements systems. A wide spectrum measurement signal is used for stimulation. The system uses the following signals: chirp, MLS, white noise, pink noise, Golay codes. The length of a generated signal can be changed within the range from 128 up to 8192 samples. Sampling frequency is 48 kHz but it is possible to decrease it. The stimulating signal is repeated several times in order to average the answer of the system in the time domain. This operation allows improving the S/N ratio of received responses. There is no need to apply longer measurement signals because, according to other researches, HRTFs may be presented even with such resolution as 100 Hz. On the other hand, responses determined in the system will be used for convolution with real signals and therefore they cannot be too long. Moreover, long measurement signals make the assessment time longer.

System for High Speed Measurement of Head-Related Transfer Function 9

Applying this solution allows to use the device for purposes not provided by the user

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

influence of measuring set on the results of the tests.

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 -

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

interface of the system.

**4.5 Parameters of the device** 

45° and +90°.

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 before.

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 device elements on measurement results is partly reduced.

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 with real signals.
