Demonstrations of 3D Acoustic Signal Synthesis Results

Chapter 4 Experiment Results

4.4 Demonstrations of 3D Acoustic Signal Synthesis Results

Results

In Fig. 4.30, we show the 3D acoustic signal synthesis flow. By dividing the separated signals into parts, we are able to build the 3D acoustic signal as the designed HRTF scenario.

It can be done by filtering each divided parts with its corresponding ATF and HRTF. The order of ATF filtering and HRTF filtering does not affect the output signal but the computational complexity since the HRTF filtering produce a two channel signal for each input signal.

Fig. 4.30 Flow Diagram of 3D Acoustic Signal Synthesis

For each sequence data, we provide three kinds of waveforms: the SLAB synthesis waveform, the HRTF+ATF waveform from the original source signals and the HRTF+ATF waveform from the separated signals.

The demonstrations show two kinds of HRTF scenarios. The first scenario which is shown as Fig. 4.31 has 25 frames and the frame interval is about 0.5 second. The second scenario which is shown as Fig. 4.32 has 27 frames and the frame interval is also about 0.5

second. The red point represents the source 1, the green point represents the source 2 and the blue and red parts of the headphone represent the left and right ear of HRTF respectively.

(a) (b) (c)

(d) (e) (f)

Fig. 4.31 HRTF Scenario 1, 25 Frames, Frame Interval0.5 sec,

Red: Source 1, Green: Source 2 (a) Frame 1 (b) Frame 5 (c) Frame 10

(d) Frame 15 (e) Frame 20 (f) Frame 25

(a) (b) (c)

(d) (e) (f)

(g)

Fig. 4.32 HRTF Scenario 2, 27 Frames, Frame Interval  0.5 sec,

Red: Source 1, Green: Source 2 (a) Frame 1 (b) Frame 5 (c) Frame 8 (d) Frame 13 (e) Frame 18 (f) Frame 21

(g) Frame 27

In order to amplify the noticeable effect of the ATF, we demonstrate the 3D acoustic signals for three different room sizes: large room with 20 x 20 x 20 (m), median room with 10 x 10 x 10 (m) and small room with 4 x 4 x 4 (m), which are shown in Fig. 4.33.

(a)

(b)

(a) Large Room (b) Medium Room (c) Small Room

From Fig. 4.34 to Fig. 4.41, we can observe the effects of ATF to the waveforms and the spectrograms. By the comparisons of the figures in (a) and the ones in (c), it can be identified that the ATFs change the waveforms of the separated signals; the difference is implicit without reflection (NR), but it is visible for perfect reflectors (PR) as the wall material in the three different room sizes (Small, Medium, Large). The effect of room sizes

to ATFs can be observed in (f). The longer the reverberation time is, the faster the changes in the adjacent frequencies are. The explanation comes from the sum of different time domain shifting of signals cause the frequency domain magnitude variation:

Therefore, for a larger room, there exists some larger value of t_k t_m which cause a faster oscillation of the spectrum. By comparing the spectrograms in (e) with those in (b), we are able to see some blue slices at the frequencies with lower spectrum magnitudes in (f). After the HRTF filtering, the interchannel level difference (ILD) is noticeable in (d), which is related to the HRTF azimuth angle. For the signals at 45^, the left channel amplitude is much larger than the right one; in the other hand, for those at 45^, the right channel amplitude is larger than the left one.

(a) (b)

(e) (f)

Fig. 4.34 “f01m01”, Separated Signal 1, NR, HRTF at 45^

(a) Separated Signal in Time Domain (b) Separated Signal in Time-Frequency Domain (c) After ATF in Time Domain (d) After HRTF in Time Domain