UNIVERSITY OF DUBLIN TRINITY COLLEGE FACULTY OF ENGINEERING & SYSTEMS SCIENCES School of Engineering and SCHOOL OF MUSIC Postgraduate Diploma in Music and Media Technologies Hilary Term 31 st January 2005 Printing House 10.00-12.30 Psychoacoustics 1 Dr. D. J. Furlong Please answer Section A (40%) and any 3 questions from Section B (60%)
Section A (40%) Q.1 For each of the questions below, please answer True or False with a short accompanying note to clearly explain your answer. (4 marks per question) (vi) (vii) (viii) (ix) (x) Beats occur when two signals with similar frequencies are combined. However, the perception of beats can be altered by adjusting the relative phase of the two signals. Due to the insensitivity of the ear to relative phase, the time differences that might arise from different positioning of multiple microphones around a single instrument will not influence the timbre of a recording generated from a mix of these microphone signals. Fourier analysis provides details of frequency component amplitudes for periodic waveforms. However, application to aperiodic waveforms suffers from limitations in frequency resolution. Perceived loudness of a complex tone is influenced by the frequency of the fundamental. The scale of perceived loudness has a span of approximately 96dB. However, musical dynamics exploit only a fraction of this. The frequency range of musically identifiable pitch extends to approximately 20kHz in normal hearing, but further for some individuals. A tonal complex consisting of equal amplitude 220Hz, 330Hz, and 440Hz tones will be perceived to have identical pitch, but different timbre, to a single pure tone at 220Hz. However, the perceived pitch would be significantly affected by increasing each of the component frequency values by 20Hz. Because the ear is maximally sensitive in the 3-4kHz region, we are much more likely to detect a 5Hz change in frequency in this region as compared to lower or higher spectral regions. Time reversal of a note waveform results in an alteration to the spectrum which manifests as a timbre change. Variations over time in the amplitude characteristic of any individual harmonic component will allow it to be heard out in a tonal complex, and consequently alter the timbre of the tonal complex.
Section B (60%) Q.2 Explain, using diagrams, the meaning of Simple Harmonic Motion in the context of acoustical waveforms. How does Simple Harmonic Motion relate to sine waves? Using diagrams, explain clearly the meaning of waveform frequency, amplitude and phase. What is the difference between longitudinal and transverse waveforms? What is meant by waveform wavelength? A waveform has a periodic time of T = 0.00908 sec/cycle. What is its wavelength? A spectrogram is a joint time-frequency representation of a sampled signal waveform. Explain this statement. What is meant by window length in this context? How might window length influence time and/or frequency resolution in spectrogram presentation? Sketch Spectrograms for the following signals: a. Constant amplitude sine wave at 110Hz with duration of 1 sec. b. 1 sec. duration complex tone consisting of sine wave components at 55, 256, and 512Hz, with ramping amplitude increase. c. 1 sec. duration sine wave with fixed amplitude with frequency increase from 110Hz to 220Hz. (8 marks)
Q.3 Explain clearly the difference between sound intensity and sound pressure level? If the minimum sound pressure level detectable is given by P 0 = 2 * 10 5 N/m 2 what is the sound pressure level in dbs corresponding to a measured pressure of P 1 = 0.0025 N/m 2? Why does the Weber-Fechner Law imply that loudness perception will be logarithmic? Audio amplifiers often have a Loudness switch. By referring to the Fletcher-Munson curves, explain when such a switch should be used and why? Using diagrams, explain the fundamental principles of dynamic range compression as used in studio production. (6 marks)
Q.4 Explain the relationship between waveform frequency and perceived pitch with particular reference to differences between sine wave and complex waveform stimuli. What are the stimulus characteristics required for perception of well defined pitch? What is meant by virtual pitch? Under what circumstances does this phenomenon occur? In what way does virtual pitch relate to Rameau s theory of Fundamental Bass? (vi) Explain what is meant by Just Noticeable Differences (JNDs) for pitch perception. How might JND influence the determination of pitch scales for musical use? How does Pitch categorisation contribute to scale design? Explain the phenomenon of the Shepard-Risset illusion of continuously rising pitch.
Q.5 Explain what is meant by the modes of vibration of a physical system. In the case of a vibrating string, why does the position at which the a string is plucked influence the harmonic components that are produced? Why do the modes of vibration of both strings and musical instrument bodies play a significant role in the specification of both steady-state and transient characteristics of musical waveforms? Explain what is meant by modal resonance Q-factor. What is relationship between Q-factor, coupling, and damping in a source-filter modelled system? Give musically relevant examples of high and low Q-factor resonances. What is meant by a formant? Explain how formant characteristics help define the identifiable timbre of a musical instrument as it is played in different registers.
Q.6 The investigation of timbre in the late 20 th century has lead to a reevaluation of the physical parameters which contribute to timbre recognition. The understanding of timbre which has emerged from these investigations has contributed greatly to the efficient digital synthesis of musical timbres. Discuss this statement with reference to: a. The Classical conception of timbre; b. Problems relating to the Classical conception arising from observations of note transients and formant characteristics; c. Identification of significant dimensions of timbre using multidimensional scaling techniques; (6 marks) d. The concepts of timbre space and timbre morphing. e. Effective simplifications of timbre representation for resynthesis;