FORMALISMO Y CONFLICTIVIDAD

The effect of participants pressing down on the disc with their fingertip was assessed using the accelerometer underneath the disc. The change of acceleration due to pressing lightly compared to without any finger on the disc was 0.5dB on average across the range of test tones. It was deemed that this would not affect the measurement of detection thresholds during subjective tests.

3.2.2 Relative pitch discrimination and learning

Figure 3.6 shows a block diagram and details of the experimental set-up for which a range of training variables were considered for the development of learning through training, as suggested in [61, 125]. Apart from the shaker configuration, other design variables included the accessibility of the training device, user characteristics, type and amount of training, response formats and evaluation characteristics as discussed later regarding the subjective measurements that are presented in Chapter 5. 100 200 300 400 500 600 700 800 900 1000 -3 -2 -1 0 1 2 3 20l og 10 ( a / 2 a) (d 1 B ) Frequency (Hz)

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Figure 3.6 Diagram (left) and details (right) of portable experimental set-up with a participant being tested.

The items of equipment are listed below.

o Shaker, LDS, V200 Series – type 201/203

o Infrared thermometer, TENMA – type 72-6700

o Infrared thermometer, Fluke – type 61

o Notebook PC, HP – ProBook 6555b

o Sound card, Trust – 5.1 surround

o Power amplifier, Clever little box – four-channel 4 x 12W

o AC-DC power supply, Powerpax – type PTD-1250P

o MP3 player, SWEEX – Clipz 4 GB (used as white noise generator)

o Stereo Headphones, Yoga – CD-98

For calibration purposes, the below items of equipment were used.

o Dual channel real-time frequency analyser, B&K – type 2144

o Sound Level Calibrator, B&K – type 4231

o Calibration exciter, B&K – type 4294

o Accelerometer, B&K – type 4393

The masking noise provided through the headphones was measured using the equipment described in Section 3.2.1.1.C.

3.2.3 Learning relative and absolute vibrotactile pitch

This experiment used a two-octave electronic piano (ION Discover Keyboard USB) that was reconfigured so that each key press produced QWERTY code (i.e. standard computer keyboard output) instead of a musical signal. Essentially, the

37

original microcontroller from the piano was replaced by a microcontroller of a standard computer keyboard. This allowed for mapping QWERTY code from the piano to the graphical user interface used in this experiment which is explained in Chapter 6 (see Figure 3.7).

The use of a piano unit on a portable experimental set-up was in line with previous research using the Teletactor [38, 128] which was previously used to train children with deafness for the appreciation of music and poetry via their fingertips. In addition, the stimuli used in the present study were similar to those reported recently with regard to melodies played on piano that are restricted to five pitches only [130, 131].

The remaining items of equipment and the objective measurements for this experiment were similar to those from the experiment on relative pitch discrimination in Section 3.2.2.

Figure 3.7 Diagram (left) and details (right) of experimental set-up with electronic piano keyboard.

3.3 FEET

3.3.1 Establishing vibrotactile detection thresholds

For the vibrotactile experiment using the feet, in line with the above experimental design to measure on the fingertip, this section describes the measurements of reference levels of test tones, masking noise conditions and vibration on the participant’s seat. In addition, larger contactor discs were used to measure transfer functions, vibration uniformity over the contact area and the foot load that had to be maintained constant with larger volumes of skin [35].

Headphones

Piano Laptop

Disc Thermometer

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The semi-anechoic chamber that was used to measure detection thresholds on feet was adjacent to the audiometric booth that was used to measure detection thresholds on fingertips. As a result, some of the equipment to measure on fingertips was also used to measure on feet. The experimental set-up is shown in Figures 3.8 and 3.9. For brevity, only the new main items of equipment are listed below.

New equipment on experimenter’s bench:

o Analogue mixing desk, Mackie – Onyx 1620i Premium

o Power amplifier, Acoustical Mfg Co Ltd – Quad 50E (2 units) New equipment in semi-anechoic chamber:

o Loudspeaker (active nearfield monitor), Fostex – type PM1 MkII (2 units)

o Video camera, Panasonic – CCD 92B15468

o Accelerometer, B&K – type 4393 (2 units)

o Perspex contactor disc, diameter 12 cm, thickness 2.5 cm (used for forefeet)

o Perspex contactor disc, diameter 10 cm, thickness 2.5 cm (used for heel)

o Shaker, LDS – Type V406 M4-CE (2 units)

o Support trunnion (2 units) and auxiliary suspension (2 units) for shakers

The surface roughness for the Perspex contactor discs was estimated to have a centre line average of 1.6μm. As with the disc for the fingertip, this assessment was carried out according to the roughness inspection procedure described in the Standard ISO 4288 [148].

The auxiliary suspension consisted of a centralising and support system that adds stiffness to the standard shaker suspension. This way, the shaker can bear safely the heavy static load of the participants’ feet. Extra care was taken to ensure an appropriate separation distance between both shakers to avoid transfer of vibration between them during each test. This is indicated by the yellow arrows in Figure 3.9. For ergonomic purposes, there is an inclination for the horizontal of 10° for the bottom shaker (left-hand side of the photo) and 25° for the top shaker (right- hand side of the photo).

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Figure 3.8 Diagram of experimental set-up.

Figure 3.9 General view of semi-anechoic chamber is shown (top left) along with details of the shakers, contactor discs and a participant’s foot being tested. Distances are shown on the top right graphic.

TEST INSTRUMENTATION SEMI-ANECHOIC CHAMBER CONDITIONING AMPLIFIER AMPLIFIER C CAMERA STEREO AMPLIFIER SOUND CARD SIGLAB ANALYSER WHITE NOISE GENERATOR MIXING DESK AMPLIFIER PARTICIPANT EQUALISER LOUDSPEAKER LOUDSPEAKER RESPONSE BUTTON SHAKER SHAKER AUDIO-VISUAL RESPONSE INDICATOR MONITOR SHOWING FEEDBACK ACCELEROMETER ACCELEROMETER 9 cm 22 cm 3.5 cm 35 cm 38 c m Speaker Shakers Isolation Monitor Camera Speaker

40 3.3.1.1 Calibration and measurements

As with the experimental set-up for the fingertips, the objective measurements described below were performed before running the subjective testing of participants. A two-channel system was used on this occasion. Channel one was used for the forefoot and channel two for the heel.

A. Reference levels of test tones

The test tones were generated and prepared in a similar way as in the experimental set-up for detection thresholds on fingertips. The measurement procedure to obtain the reference levels for calibration routines was also similar. The chain of equipment levels was adjusted; some settings remained as in Section 3.2.1.1 with the new settings described in this section. For the two channels used on the mixing desk, the gain was set to 40dB; each fader level was set to unity gain and the common main mix to 5dB.

Both signal spectrum and linearity were checked as before. Spectral coherence was also measured by comparing the signal at the output of the sound card (i.e. system input) with the signal at the contactor discs (i.e. system output). The coherence function examines the relation between these two signals and was always equal to one for the entire set of test tones at the reference level 0dBV. This indicated that the system output was fully related to the system input, without noise affecting the measurements.

The procedure to measure the reference levels included the use of the calibrator and mounting the accelerometer beneath the contactor disc. Accelerometers were mounted approximately 1.3cm from the disc edge towards the top right of the disc as seen from the participants’ sitting position. As with the tests on fingertips, the measurements were checked before and after each subjective test session to ensure that they remained stable within approximately ±0.5dBVrms of the required levels. The reference levels are shown in Tables A.1 and A.2 of Appendix A.