Timbre perception

Objective Timbre perception and the physical properties of the sound on which it depends Formal definition: ‘that attribute of auditory sensation in terms of which a listener can judge two sounds similarly presented and having the same loudness and pitch as being dissimilar.’

Timbre Related to musical instrument / object recognition To distinguish between the same note played on different instruments perceived quality of a sound Some sound quality descriptors: mellow, rich, harsh, shrill etc. Judgments of timbre – subjective

Timbre Pitch and loudness – related to the physical properties of frequency and amplitude – may be considered as one dimensional attributes of sound – can be ordered on a single scale – pitch (from low to high); loudness (from quiet to loud) Timbre is a multidimensional attribute of sound – no single scale along which we can order the timbre of sounds

Timbre Timbre depends on: Spectral envelope – patterning of energy as a function of frequency Temporal properties – fluctuations over time

Spectral envelope A major determinant of timbre Overall distribution of energy over frequency e.g. Strong lower harmonics – ‘dark’, ‘mellow’ e.g. Strong higher harmonics – ‘bright’, ‘shrill’ More specifically timbre is related to the relative level in each critical band rem: The critical bandwidth variation with frequency – this dscribes the spectral analysis carried out by the ear.

Spectral envelope In a complex sound the partials are described as either resolved (lower components) or unresolved (high components) by the auditory system. Resolved and unresolved - related to the critical bands Timbre depends on whether most of the energy lies in resolved or unresolved components.

Spectral envelope example The effect of spectrum on timbre – asa demo 28, trk 53 – recognise the musical instrument Listen and note the following: the point where you can identify each sound the point where recognition of the sound becomes unambiguous

Temporal properties Timbre recognition may also depend on: whether a sound is periodic; waveform changes over time; spectral changes over time; preceding and following sounds (e.g. masking). The frequency components in a sound change over time - described by the temporal envelope Attack (onset) – increase in amplitude Steady state Decay portion (offset) – amplitude decreases

Temporal envelope Diagram – indicates different attacks for a plucked vs. a bowed violin string, also speech sounds ‘ba’ ‘wa’ have different attacks Plucked strings – attack followed by decay the onsets (attacks) of sounds can be used to identify them – recognition depends strongly on onsets and temporal structure of sound envelope

Effect of temporal envelope Piano tone – rapid onset and gradual decay Effect of temporal envelope in timbre perception - asa demo 29 (trk 54-56) Chorale played on piano and recorded Chorale played from end to beginning and recorded The recording of the backward chorale is played out in reverse – causing each note to be reversed in time.

Effect of temporal envelope Reversed piano tone – completely different timbre – no longer sounds like a piano Spectral energy distribution is unchanged by this time reversal temporal envelope changed

Some instruments have noise like qualities that influence their perceived timbre At the start of a flute note Important for the synthesis of a convincing flute sound To create a realistic synthesis of a sound – spectral envelope and the variation in time of the components – some harmonics may have different time envelopes

Some timbre measures Spectral centroid – related to the subjective ‘brightness’ of a sound - measures the frequency at which the energy of the sound is centred.

Tristimulus a measure of tone colour, calculates the proportion of energy in the fundamental, mid and high frequency components Tristimulus1 – the amplitude of the first harmonic divided by the sum of all the amplitudes of all the harmonics Tristimulus2 – the sum of the amplitudes of the 2- 4 th harmonics divded by sum of all amplitudes Tristimulus3 – sum of amplitdues from the 5 th upwards divided by the sum of all the amplitudes

Irregularity variation in energy between the partials A value close of zero – indicates little variation between the amplitudes of adjacent frequency components A value close to 1 – large difference in amplitude between adjacent frequency components

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