sgn-14006 / a.k. sgn14006/pdf/l03- آ  2 ear physiology sgn-14006 / a.k. ! the human ear consists

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  • Hearing 1 SGN-14006 / A.K.

    Contents: 1.  Introduction 2.  Ear physiology 3.  Masking 4.  Sound pressure level 5.  Loudness 6.  Pitch 7.  Spatial hearing

    Hearing Sources: Rossing. (1990). ”The science of sound”. Chapters 5–7.

    Karjalainen. (1999). ”Kommunikaatioakustiikka”. Moore. (1997). ”An introduction to the psychology of hearing”.

    Hearing 2 SGN-14006 / A.K. 1 Introduction

    !  Auditory system can be divided in two parts –  Peripheral auditory system (outer, middle, and inner ear) –  Auditory nervous system (in the brain)

    !  Ear physiology studies the peripheral system !  Psychoacoustics studies the entire sensation:

    relationships between sound stimuli and the subjective sensation

    Hearing 3 SGN-14006 / A.K. 1.1 Auditory system

    !  Dynamic range of hearing is wide –  ratio of a very loud to a barely audible sound pressure level is

    1:105 (powers 1:1010, 100 dB)

    !  Frequency range of hearing varies a lot between individuals –  only few can hear from 20 Hz to 20 kHz –  sensitivity to low sounds (< 100Hz) is not very good –  sensitivity to high sounds (> 12 kHz) decreases along with age

    !  Selectivity of hearing –  listener can pick an instrument from among an orchestra –  listener can follow a speaker at a cocktail party –  One can sleep in background noise but still wake up to an

    abnormal sound

    Hearing 4 SGN-14006 / A.K. 1.2 Psychoacoustics

    !  Perception involves information processing in the brain –  Information about the brain is limited

    !  Psychoacoustics studies the relationships between sound stimuli and the resulting sensations –  Attempt to model the process of perception –  For example trying to predict the perceived loudness / pitch /

    timbre from the acoustic properties of the sound signal

    !  In a psychoacoustic listening test –  Test subject listens to sounds –  Questions are made or the subject is asked to describe her

    sensasions

  • Hearing 5 SGN-14006 / A.K. 2 Ear physiology

    !  The human ear consists of three main parts: (1) outer ear, (2) middle ear, (3) inner ear

    Hearing 6 SGN-14006 / A.K. 2.1 Outer ear

    !  Outer ear consists of: –  pinna – gathers sound; direction-dependent response –  auditory canal (ear canal) - conveys sound to middle ear

    Nerve signal to brain

    [Chittka05]

    Hearing 7 SGN-14006 / A.K. 2.2 Middle ear

    !  Middle ear contains –  Eardrum that transforms sound waves into mechanic vibration –  Tiny audtory bones: hammer (resting against the eardrum, see

    figure), anvil and stirrup

    !  The bones transmit eardrum vibrations to the oval window of the inner ear

    !  Acoustic reflex: when sound pressure level exceeds ~80 dB, eardrum tension increases and stirrup is removed from oval window –  Protects the inner ear

    from damage

    Hearing 8 SGN-14006 / A.K. 2.3 Inner ear, cochlea

    !  The inner ear contains the cochlea: a fluid-filled organ where vibrations are converted into nerve impulses to the brain.

    !  Cochlea = Greek: “snail shell”. !  Spiral tube: When stretched out, approximately 30 millimeters long. !  Vibrations on the cochlea’s oval window cause hydraulic pressure

    waves inside the cochlea !  Inside the cochlea there is

    the basilar membrane, !  On the basilar membrane

    there is the organ of Corti with nerve cells that are sensitive to vibration

    !  Nerve cells transform movement information into neural impulses in the auditory nerve

  • Hearing 9 SGN-14006 / A.K. 2.4 Basilar membrane

    !  Figure: cochlea stretched out for illustration purposes –  Basilar membrane divides the fluid of the cochlea into separate

    tunnels –  When hydraulic pressure waves travel along the cochlea, they

    move the basilar membrane

    Hearing 10 SGN-14006 / A.K. Basilar membrane

    Best freq (Hz)

    Travelling waves:

    !  Different frequencies produce highest amplitude at different sites !  Preliminary frequency analysis happens on the basilar membrane

    Hearing 11 SGN-14006 / A.K.

    !  Distributed along the basilar membrane are sensory hair cells that transform membrane movement into neural impulses

    !  When a hair cell bends, it generates neural impulses –  Impulse rate depends on vibrate amplitude and frequency

    !  Each nerve cell has a characteristic frequency to which it is most responsive to (Figure: tuning curves of 6 different cells)

    2.5 Sensory hair cells Hearing 12

    SGN-14006 / A.K. 3 Masking

    !  Masking describes the situation where a weaker but clearly audible signal (maskee, test tone) becomes inaudible in the presence of a louder signal (masker)

    !  Masking depends on both the spectral structure of the sounds and their variation over time

  • Hearing 13 SGN-14006 / A.K. 3.1 Masking in frequency domain

    !  Model of the frequency analysis in the auditory system –  subdivision of the frequency axis into critical bands –  frequency components within a same critical band mask each

    other easily –  Bark scale: frequency scale that is derived by mapping

    frequencies to critical band numbers !  Narrowband noise masks a tone (sinusoidal) easier than

    a tone masks noise !  Masked threshold refers to the raised threshold of

    audibility caused by the masker –  sounds with a level below the masked threshold are inaudible –  masked threshold in quiet = threshold of hearing in quiet

    Hearing 14 SGN-14006 / A.K. Masking in frequency domain

    !  Figure: masked thresholds [Herre95] –  masker: narrowband noise around 250 Hz, 1 kHz, 4 kHz –  spreading function: the effect of masking extends to the spectral

    vicinity of the masker (spreads more towards high freqencies)

    !  Additivity of masking: joint masked thresh is approximately (but slightly more than) sum of the components

    Hearing 15 SGN-14006 / A.K. 3.2 Masking in time domain

    !  Forward masking –  masking effect extends to times after the masker is switched off

    !  Backwards masking –  masking extends to times before the masker is been switched on

    !  Forward/backward masking does not extend far in time " simultaneous masking is more important phenomenon

    forward masking

    backward masking

    Hearing 16 SGN-14006 / A.K. Masking: Examples

    !  A single tone is played, followed by the same tone and a higher frequency tone. HF tone is reduced in intensity first by 12 dB, then by steps of 5 dB. Sequence repeats twice: second time the frequency separation between the tones is increased.

    !  Attempt to mask higher frequencies !  Attempt to mask lower frequencies (not masked as

    easily)

  • Hearing 17 SGN-14006 / A.K. Application to audio steganography

    !  Idea: hide a message in the audio data, keeping the message inaudible yet decodable

    !  Example –  Here robustness to environmental noise was important

    Hearing 18 SGN-14006 / A.K. 4 Sound pressure level

    !  Sound signal s1(t) at time t represents pressure deviation from normal atmospheric pressure

    !  Sound pressure is the (linear) RMS-level of the signal –  E{ } denotes expectation (RMS = root-mean-square level)

    !  Due to the wide dynamic range, decibel scale is convenient –  pdB = 20 log10 (pRMS / p0) = Lp

    where p0 is a reference pressure

    2{ ( ) }RMSp E s t=

    Hearing 19 SGN-14006 / A.K. 4.1 Threshold of hearing and dB scale

    !  Threshold of hearing –  Weakest audible sound pressure at 1 kHz frequency is 20 µPa,

    which has been chosen to be the reference level p0 of the dBscale –  Lp = 20 log10(p/p0) = 10 log10(p2/p02)

    !  Threshold of pain –  Loudest sound

    that the auditory system can meaningfully deal with

    –  130 dB @ 1 kHz

    Hearing 20 SGN-14006 / A.K. 4.2 Multiple sources

    !  Two sound sources: s(t) = s1(t) + s2(t) !  RMS pressure level of the summary signal:

    !  If the signals are uncorrelated and the above formula simplifies to If p1 = p2, the sound pressure level of the summary signal is 3 dB higher than that of p1 (why?)

    2 2 2 1 1 2 2{ ( ) } { ( ) 2 ( ) ( ) ( ) }RMSp E s t E s t s t s t s t= = + +

    1 2{ ( ) ( )} 0E s t s t =

    2 2 1 2RMSp p p= +

  • Hearing 21 SGN-14006 / A.K. Multiple sources

    !  Two sources with 80 dB sound pressure level –  Source signals uncorrelated: together produce 83 dB level –  Sources correlate perfectly (same sound): results in 86 dB level

    !  Doubling the sound amplitude increases the sound pressure level by 6 dB –  Because: Lp = 20 log10(2·p/p0) = 20 log10(p/p0) + 6 [dB] –  Equivalent to adding another identical source next to the first one

    !  Intuitively: if the two sources do not correlate, the components of the two audio signals may amplify or cancel out each other, depending on their relative phases, and hence the level will be only 83 dB

    Hearing 22 SGN-14006 / A.K. 5 Loudness

    !  Loudness describes the subjective level of sound –  Perception of loud

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