# Design and Application of the BiVib Audio-Tactile Piano Sample Library
## 3 Application of the BiVib Library
### Sample Equalization and Limitations  
BiVib users should hear the same sounds as the binaural pressure signals measured by the dummy head microphones, and should receive vibrations at the fingers which are identical to the acceleration signals measured by the accelerometer. The audio reproduction condition can be satisfied by using headphones or earphones that bypass the outer ear and provide a frequency response that is as flat as possible. In this way, the performer would listen to stereo sounds that contain the binaural information created by the dummy head. Note that these sounds also contain the contribution of room reverberation, as the pianos could not be located inside anechoic rooms during the recording sessions: especially the laboratory hosting the grand piano was moderately reverberant, and no reverberation data could be collected for this room at the time of the acquisition of samples. Conversely, the upright piano (ZH) benefited of a silent studio room whose impulse responses have been measured and recently included in BiVib (folder 'IR'). More precisely, responses from two source points generated by a pair of Genelec 8040A loudspeakers were taken in correspondence of the two ear canal entrances of the Neumann KU 100 dummy head with both outer ears removed: As shown in Figure 5, the loudspeakers were placed symmetrically at both sides of the upright piano, 80 cm above the floor, pointing toward the dummy head (angle with the vertical plane parallel to the piano equal to 47\({}^{\circ}\)) which was positioned 55 cm far from the upright panel of the piano. Logarithmic sweeps were synthesized in the Audacity audio editor using the Aurora modules [35], then reproduced using the loudspeakers, and finally deconvolved again with Aurora to find four room transfer functions forming the source-ear transfer matrix. Such transfer functions were included in the dataset and can be used to remove the echoes of the recording room through inversion of the transfer matrix, using standard deconvolution techniques [36]. Also, because of the low energy of these echoes, binaural recordings were intentionally left unprocessed, thus allowing musicians and performers to use them as they are, while leaving any decision on their possible manipulation to advanced users. The vibration reproduction condition may be ideally satisfied by implementing a weighted MIDI keyboard (such as those found on digital pianos) where each key is provided with an actuator, similar to the prototype technology described in [37]. Even in the unlikely case that such actuators would offer a flat frequency response over the range of interest for tactile sensation [38], vibration reproduction would be altered by the shape, material and construction of the keyboard. That requires the implementation of a compensation procedure--similar to that described in the previous paragraph to remove room acoustic components--that would enable the accurate reproduction of the vibrations recorded by the accelerometer on each key. Unfortunately, on such a keyboard there would be 88 touch points requiring deconvolution. In alternative, two arrays of 88 transfer characteristics can be formed each by measuring vibrations on the same touch points, after exciting the digital piano body with a corresponding pair of tactile transducers. Figure 6 (left) [12] shows a detail of the haptic digital piano that the authors realized according to the latter solution, where two transducers were mounted below the keybed, one mid-way the keyboard length and another approximately at one quarter way the same length--see also Figure 4 (right). By playing an impulse simultaneously on both of them, advanced BiVib users should accurately measure on each key the corresponding impulse response, and then (with Aurora or similar tools) design an equalizer which on average flattens the \"coloured\" response affecting the vibrations due to their path from the audio interface to the keyboard.