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Spatial and Spectral Properties of the Dummy-Head During Measurements in the Head-Shadow Area based on HRTF Evaluation

Wersényi György

SZÉCHENYI ISTVÁN UNIVERSITY, Department of Telecommunications

AbstractIn accurate and repeatable measurements dummy-heads are often used to model the average human head and body. They are suited for standardized measurements and for investigating the human spatial hearing and localization performance. The monaural Head-Related Transfer Functions (HRTFs) of the dummy-head can be used for various investigations. This paper uses the HRTF-set of a Brüel & Kjaer head and torso simulator focusing on the so called monaural head-shadow area, where one of the ears is shadowed by the head itself. Based on long-term measurements using the bare torso as well as other accessories (glasses, clothing etc.) on it, the extent of the head-shadow area will be presented in frequency and space. The head-shadow area is investigated in connection with the overall SNR of the measurement and sensitivity domains of the ears.

Introduction• HRTF: Head-Related Transfer Function, transmission from free-field to the eardrum(s)• Basic localization cue for directional hearing• Measurement on real humans or on dummy-heads • Reproduction through (equalized) headphones• In listening tests are dummy-head HRTFs inferior to individual HRTFs

Measurement

• HRTFs of a Brüel Kjaer 4128 dummy-head• Anechoic room, white noise excitation• Resolution: 1 degree horizontal, 5 degrees vertical• Accuracy in the HRTFs: 1 dB• Goal:

– Investigating the role of hair, cap, glasses, clothing etc.?– Repeatable property of the dummy-head (left-ear only, horizontal plane) ?– Role of the head-shadow area ?

HRTFD• HRTFD: Differences in dB between two HRTFs from the same direction but under modified conditions:

• If condition1 = condition2, HRTFD gives the repeatability property (accuracy) of the measurement (including dummy-head and environment)

• Eliminates individual differences in re-measured results!

2

1

C

C

HRTF

HRTFHRTFD

Results

• Sensitivity and antenna-model of the pinna• Directional filtering, directional dependent sensitivity• Spatial domains where the overall signal level of the HRTFs are high is a „sensitive” region• Head-shadow area decreases the sensitivity (both spatial as well in frequency)

• Footprints: ±20º „stereo area” (binaural sensitivity area)• Highest signal HRTF-level: 45º axes (pinna placement)• This kind of monaural sensitivity can be recognized between elevations -10º to +30º• Worst sensitive area at the contralateral ear about 250º-260º horizontally (head-shadow area)

Frequency Limits• 1500-1600 Hz:

– well-known from the literature– separates localization: envelope vs. fine structure (ITD)– We did not observe any effects or deviations in the HRTFDs below 1600 Hz

• About 3000-3500 Hz:– Shadowing effect = random incidence and high frequency disturbances– Cut-off frequency depends on azimuth and elevation– Figure 5 shows the head-shadow area (averaged for all elevations)– Lowest value is about 3000 Hz, average about 3500 Hz

SummaryDummy-heads are well suited for long-term, accurate and standardized HRTF measurements. A precisely controlled

system is able to set and re-set the spatial directions, to re-measure and to analyze recorded transfer functions easily. We investigated the role of the HRTFs in human localization through accurate HRTF measurements.

By analyzing the monaural HRTFs of the dummy-head, we can determine the monaural sensitivity regions of the hearing system. We have found this to be symmetrical to the median plane (±20º) and the axe of 45 degrees as the most sensitive monaural direction. The extent of the monaural „antenna footprint” of the ear in the horizontal plane is from about -20 degrees to 90 degrees.

Shadowing effects can be seen as the worst effect: the HRTFs are influenced and they vary too rapidly to allow high-frequency recognition. Re-measured HRTFs in this region vary more than 10 dB above about 2 kHz and they make the incoming signal “noisy”. This is a natural phenomenon in the head-shadow area, especially near to the horizontal plane. Above 30 degrees of elevation it is becoming less important. This fact was also supported by the measurement with the baseball cap.

Finally, we tried to determine the spatial extent of this area depending on the environment near the head. The cut-off frequency of the head-shadow area varies from 3-6 kHz, although there seems to be a constant value of about 3500 Hz that separates the lateral-contralateral evaluation.

Fig.1. Effect of the pinna at frontal incidence. HRTFs of the Head and Torso Simulator with and without the pinna.

Fig.2. Footprints of the ears representing the monaural sensitivity regions based on the HRTF evaluation.

Fig.3. 2D spatial representation of the magnitude of HRTF data for a fixed elevation as function of azimuth and frequency. The polar histogram shows the deviations between HRTFs from repeated measurements in the horizontal plane for the right ear. The natural deviations of the HRTFs caused by the filtering and shadowing effects are shown as unsigned absolute values in dB. The circles correspond to frequency domains with 1 kHz bandwidth marked with the center frequency (linear scale). Note the head-shadow area (filled black).

Fig.4. HRTF data using a baseball cap on the head of the dummy-head from elevation 10 degrees (left) and 20 degrees (right). Compare with Fig.3, note the effect of the visor.

Fig.5. Frequency limit of the head-shadow area as function of azimuth. Result is averaged from all measurement for all elevations. The lowest cut-off freqency of about 3000 Hz is in the head-shadow area (minimum of the monaural sensitivity)