**3. HRTF measurement requirements**

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

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 reverberation signals (Plaskota & Dobrucki, 2004).

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

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

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 millimeters over an ear entrance, an ear entrance, a few millimeters under an ear entrance, directly over the tympanic membrane (Pralong & Carlile, 1994). Additionally, the ear entrance closing influence on the measurement result is considered. It was found out that a smaller individual variation is obtained in measurements with closed ear entrance (Møller et al., 1995). It was also determined that the ear canal transfer function is independent of sound source position in the space around the listener (Bovbjerg et al., 2000).

The parameters of electroacoustic transducers have a great influence on the measurement result, especially a frequency response. The frequency responses of microphones are more important than the frequency responses of loudspeakers (Plaskota, 2003). It is suggested to use loudspeakers with a frequency response without large deeps (Møller et al., 1995).

In the studies there are informations available about used signals during the HRTF measurements. One of the applied signals is the Maximum Length Sequence (MLS) (Møller et al., 1995). It is possible to use Golay codes (Algazi et al., 2001), but difficulties in results interpretation are known (Zahorik, 2000). In anechoic chamber, the use of chirp signal is adequate to measurement conditions. It can be supposed that in a non-anechoic chamber the impulse signal is applied. It comes from a necessity of providing good measurement conditions.
