Fundamentals of Music Technology

Transcription

1 Fundamentals of Music Technology

2 Juan P. Bello Office: 409, 4th floor, 383 LaFayette Street (ext ) Office Hours: Wednesdays 2-5pm URL: Course-info: Tuesdays pm (Studio F) E : Fundamentals of Music Technology Course materials at:

3 Lectures tentative schedule Weeks 1-2 What is sound? Weeks 2-3 Hearing Week 4 Microphones Week 5 Loudspeakers Weeks 6-8 Mixers Week 7 Mid-term exam (30%) October 20 Weeks 8-9 Cabling and Interconnection Weeks Basics of digital systems Week 12 Communication\MIDI Week 13 MIDI code Week 14 MIDI sound control/synthesis Week 15 Final exam (30%) December 15

4 Demonstrations schedule Teaching Assistant: Langdon Crawford (demonstrations, assignments + teaching in week 8) Thursdays pm, Studio F Week 3: Wave propagation Week 5: Transducers Week 6: Mixers and signal flow Week 10: Building your own studio Week 12: Sampling and Quantization Week 14: MIDI and Synthesis 8 Assignments (weeks 1, 3, 5, 6, 8, 10, 11 and 12). Due a week later.

5 Evaluation and Resources Mid-term exam = 30% Final exam = 30% Assignments = 40% Attendance and class participation All relevant information is (or will be published) on the class website - Please read it carefully and keep checking for updates. Assignments will be announced with sufficient time and published online Penalties will apply to delays Book: Francis Rumsey and Tim McCormick (2002). Sound and Recording: An Introduction, Focal Press. Further reading will be recommended as the course progresses. USE THE OFFICE HOURS (Wednesdays 2-5pm)

6 What is sound? Juan P Bello

7 Sound Sound is produced by a vibrating source that causes the matter around it to move. No sound is produced in a vacuum - Matter (air, water, earth) must be present! Air particles The vibration of the source causes it to push/pull its neighboring particles, which in turn push/pull its neighbors and so on. Pushes increase the air pressure (compression) while pulls decrease the air pressure (rarefaction) The vibration sends a wave of pressure fluctuation through the air

8 Waves Waves can be longitudinal (the particles move in the same direction of the wave) or transversal (the particles movement is perpendicular to the wave s direction) Longitudinal Transversal

9 Sound waves (1) In sound wave motion air particles do not travel, they oscillate around a point in space. The rate of this oscillation is known as the frequency of the sound wave and is denoted in cycles per second (cps) or hertz (Hz). The amount of compression/rarefaction of the air is the amplitude of the sound wave. The distance between consecutive peaks of compression or rarefaction is the wavelength of the sound wave (denoted by λ) A fast traveling wave results on a greater λ

10 Sound waves (2) If the frequency of oscillation is fixed, then the sound wave is periodic (with period t, and frequency 1/t) The simplest periodic wave is a sinusoid t Compressions Rarefactions Because of the inverse relationship, the higher the frequency, the shorter the time between oscillations. Humans frequency range: 20-20kHz (20,000 Hz)

11 Sound waves (3) The speed of a wave (c), depends on the density and elasticity of the medium (and thus in its temperature). In air, at 70 F (21 C), c = 769 mph (344 meters/s). This is slow when compared to most solids. If the speed c and the oscillation frequency f are known, the wavelength can be calculated as: λ = c/f wavelength

12 Frequency and wavelength There is then an inverse relationship between wavelength and frequency E.g. for f = 20 Hz, λ = 56.4 ft, and for f = 20kHz, λ = 0.67 in Frequency range is behind size differences in, e.g. musical instruments and loudspeakers

13 Different types of sounds Sinusoids are only one possible type of sound They correspond to the simplest mode of vibration, producing energy at only one frequency They are often called pure tones and are extremely rare in real life (e.g. a recorder produces an almost pure tone) However most sounds are not so simple resulting in complex waveforms The more complex, the noisier the sound is - when the pattern of vibration is random, the sound is said to be noise Demo: ftm_demo1

14 Periodicity (1) If a waveform pattern is repeated at regular intervals, then the sound wave is periodic and has definitive pitch We can use Fourier Analysis to break down the waveform into a series of frequency components known as harmonics These components can be seen in an amplitude vs frequency graph of the sound known as frequency spectrum Consider a sinusoid: it has a simple pattern that repeats at its oscillating - or fundamental - frequency (f0) Jean Baptiste Joseph Fourier ( ): French mathematician and physicist Fourier Analysis f0

15 Periodicity (2) For more complex patterns, more complex configurations of spectral lines will appear (see ftm_demo1) The underlying assumption in Fourier Analysis is that any sound can be made out of the combination of (many) simple sinusoids with different amplitudes + Note that a sound wave is periodic (and pitched) no matter how complex the repeated pattern is Pitch perception occurs as long as the repetition rate is within the human audio frequency range (see ftm_demo2)

16 Spectral Analysis (1) What are these complex spectral configurations and what waveforms do they produce? Harmonics (or Overtones or Partials) are frequency components that occur at integer multiples of the fundamental frequency Their amplitude variations determine the timbre of the sound T = 1/f Amplitude time Amplitude f Frequency T = 1/f Amplitude time Amplitude Fundamental frequency (first harmonic) f 2f 3f 4f 5f 6f Harmonics Overtones Partials Frequency

17 Spectral Analysis (2) Example: Square wave - only odd harmonics (even are missing). Amplitude of the nth harmonic = 1/n

18 Harmonic modes Most sources are capable of vibrating in several harmonic modes at the same time Examples: a guitar string, this room 1st harmonic nodes antinodes 2nd harmonic 3rd harmonic 4th harmonic

19 Complex sounds Most pitched instrumental sounds also present overtones which are not integer multiples of the fundamental. These are known as inharmonic partials Harmonic Inharmonic

20 Non-periodic sounds Non-periodic sounds have no pitch and tend to have continuous spectra, e.g. a short pulse (narrow in time, wide in frequency) Amplitude time Amplitude Frequency The most complex sound is white noise (completely random) Amplitude Amplitude Frequency time

21 Phase (1) In phase: cycles coincide exactly (sum duplicates amplitude) 1 2 Amplitude time Out of phase: half cycles are exactly opposed (sum cancels them) 1 2 Amplitude time -2

22 Phase (2) There is a range of partial additions and cancellations in between those extremes (ftm_demo3) What causes phase difference? t 2 delay t 1 Amplitude 0 t 1 t 2 time The phase difference depends on the delay time and the wave s frequency

23 Phase (3) Phase is commonly measured in terms of degrees of the oscillating cycle of a periodic wave The frequency defines the number of cycles per second, thus the delay x frequency x 360 returns the (unwrapped) angular phase difference

24 Sound power and intensity A source (e.g. bell) vibrates when a force (e.g. striking hammer) is applied to it. The force applied and the resulting movement characterize the work performed by the source (W = F x Δs) Power (P = W/t) is the rate at which work is performed and is measured in watts. An omnidirectional sound source produces a 3-D longitudinal wave. The resulting wavefront is defined by the surface of a sphere (S = 4πr 2 ), where r is the distance from the source. r Sound source Wavefront The original power is distributed on the surface of the wavefront. As r increases, the power per unit area (intensity) decreases: I = P/S

25 Intensity and SPL The effect of sound power on its surroundings can be measured in sound pressure levels (SPL) - much as temperature in a room relates to the energy produced by a heater. Both intensity (Watts/area) and sound pressure (Newtons/area) are usually represented using decibels (db) db are based on the logarithm of the ratio between two powers, thus describing how they compare (db = 10log 10 (P 1 /P 2 )). This can be applied to other measures (intensity, SPL, voltage), as long as their relationship to power is taken into account. In the case of intensity and SPL, the denominator of the ratio is a reference value, defined according to the quietest sound perceivable by the average person. Thus by convention, 0 db corresponds to SPL = 2x10-5 N/m 2 or I = watt/m 2

26 Acoustic fields The previous model of sound energy distribution only applies to omnidirectional sources and free fields (acoustic areas with no reflections) Free fields are extremely rare as there are always reflections (from the ground and nearby surfaces) In rooms there is both direct and reflected sound Where reflected sound energy is predominant the field is said to be reverberant (or diffuse) The near field is close to the source, where direct energy is much higher than reflected The distance from the source at which reverberant energy becomes predominant depends on the room s volume and absorption.

27 Useful References Francis Rumsey and Tim McCormick (2002). Sound and Recording: An Introduction, Focal Press. Chapter 1: What is sound? Dave Benson (2002). Mathematics and Music. Chapter 1: Waves and harmonics