Presentation on theme: "The ear, the brain & psychoacoustics"— Presentation transcript:
1The ear, the brain & psychoacoustics Sound PerceptionThe ear, the brain & psychoacoustics
2Plan About sound... How does the hear work? Absolute thresholds of hearingAuditory maskingSound spatialisationSummary
3Some definitions, and reminders about the nature of sound AbOUT SOUND
4Sound"Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations” – Wikipedia.
5Sound (cont’d)Sound is a pattern of compression and depression of the airRecord it using microphonesPerceive it from our earsGenerate it by speaking or using speakersEnergy per m2 decreases with the square of distance...
6Sound is a waveform Sound is a waveform, Can be reflected when hitting a non-transmissive surfaceIf the surface is flat, reflected in cohesive wayOtherwise depends on frequency and surface textureSound proof studio wall, forabsorbing high frequencies
7Effect of weatherBecause sound is carried by air compression and decompression, sound travelling can be affected by temperatureEg. Air temperature near the ground is coolerEg. Wind
8How does the ear work. How do we perceive sound How does the ear work? How do we perceive sound? How does it relate to sound synthesis techniques?thE EAR
10Outer ear The outer ear is composed of: The pinna The pinna (visible part)Auditory canal (meatus)Tympanic membrane (eardrum)The pinnasignificantly modifies incoming sound (esp. High frequencies)is important for sound localizationpinnasound
11(Outer ear in bats)Bats rely heavily on sonar for localization, navigation and hunting.Generate high pitched ultrasounds and listen for echoes.Highly refined sound perception & localisation (accurate enough to catch a bug in flight!)
12The middle ear Sounds make the eardrum vibrate Vibrations are transmitted through middle ear by 3 bones:Malleus (hammer)Incus (anvil)Stapes (stirrup)Stapes is connected to a membrane called the oval windowTransmits sounds to the inner ear.Tensor tympaniMalleus(hammer)Incus (anvil)Inner earOuter earStapes (stirrup)Auditory canalTympanicmembraneEustachian tube
13The middle ear (function?) Efficient transfer of sound from the air to the fluid in the cochlea (inner ear)Otherwise, sound would mostly be reflected.The oval window’s resistance is higher than air... Also higher than the eardrum’s.... But surface is smaller! middle ear is an efficient transfer mechanism (like in a bicycle), esp. within 500Hz-4kHz
14The middle ear (function - cont’d) “Barany (1938) suggested that middle ear reduces transmission of bone-conducted sound to the cochlea” Moore (2003)Internally generated sounds: Chewing, flow of air, blood, creaking of joints...Would cause masking...Middle ear only transmits differential movements between ossicles and skull (when skull vibrates, spates vibrates in sync does not transmit)!Note: birds & reptiles usually swallow food whole...
15The middle ear Acoustic reflex Muscles in the middle ear (tensor tympani) can contract to pull the stapes away from the oval window, and therefore reduce drastically transmission. The ear reacts to loud sound by contracting the ossicles muscles and attenuating following sounds.Usually needs 70-90dB sounds to trigger.Protects the ear against loud sounds BUT slow, doesn’t help against sudden loud noises!
16The inner ear The inner ear is also called the labyrinth Vestibular system:balanceSpatial orientationHorizontal, posterior and superior canals...Cochlea is used for hearing...Posterior canalSuperior canalutriclecochleaHorizontal canalvestibulesaccule
17The CochleaShaped like a spiral (no functional reason – space economy?)Filled with uncompressible fluidsRigid, bony walls transmit sound pressure without loss!Divided across length by two membranesBasilar membraneReissner membrane
19Basilar membrane When the oval window moves Round window moves in opposite mannerBasilar membranes moves tooWaves propagates through the BM
20Basilar membrane (cont’d) Mechanical properties of BM varies across length:Narrow & stiff at baseWider & less stiff at apex position of the peak depends on frequency!High frequency: near the baseLow frequency: near the apex
22Basilar membrane (cont’d) BM acts as a (imperfect) Fourier analyser!Frequency that gives best response at a point of the BM is called Characteristic Frequency (CF) for this point.In response to a steady frequency, all points vibrate at the same frequency some point with greater amplitude.
24Organ of Corti & Hair cells Between BM & tectorial membrane hair cells which form the Organ of CortiInner hair cells (12,000 cells – 140 hairs each)Tunnel of CortiOuter hair cells (3,500 cells – 40 hairs each)(Hairs are called stereocilia)
26Stereocilia (cont’d)Transforms mechanical movements into neural activityStereocilia are joined by fine links (“tip links”)Deflection of the stereocilia apply tension to those links opens “transduction channels” flow of potassium ions, voltage alteration, etc.
27Inner hair cells Each inner hair cell is connected ~ 20 neurons Most (all?) information is transferred by inner hair cells.
28Outer hair cells What about the outer hair cells? Actively influence mechanics of the cochleaHigh sensitivitySharp tuningEvidenced by experiments with drugs that affect outer hair cells’ performance.Control from above? 1,800 efferent nerve fibers! hearing is not a passive phenomenon, even earlier stages are influenced by higher brain areas!
29Otoacoustic emissions Experiments by Kemp (1978): if a click is sounded next to the ear, it is possible to detect a sound coming out of the ear (using a microphone sealed into the ear canal)Reflexions?Not only! Sound can be heard with delays from 5 to 60ms (Kemp echoes).Relative level greater at low emissions (grows by 3dB for each 10dB of input)May be stronger than the actual input!Disappears even with moderate cochlear pathologies
30Neurons in the auditory nerve Approximately 30,000 neurons in each auditory nerves (left,right)Study this using fine tipped micro-electrodes to record voltage in single cellsMost neurons fire spontaneously (0-150Hz)Most neurons are tuned to specific frequencies.Phase locking: spikes occur at specific phase of the stimulating waveform temporal regularity.
32Some comments on the absolute limits of hearing Absolute thresholds
33Minimum Audible Pressure An absolute threshold is the minimum detectable level of a sound, in the absence of other external sounds.Depends on set-up: important to define precisely how the intensity is measuredProbe microphone (ideally close to the eardrum)Usually using headphones.Threshold is called the Minimum Audible Pressure (MAP)
34Minimum Audible FieldAlternatively, loudspeakers in large anechoic chamber (walls, floor, ceiling are highly sound absorbing)Measurement made after the subject is removed, at the point occupied by the center of the listener’s headMinimum Audible Field (MAF)
35MAF vs. MAP MAF is binaural, MAP is monaural MAF factors in the head, pinna & meatus effects (broad resonance)Thresholds increase rapidly at very high and very low frequencies transmission characteristics of the middle ear!
36Absolute ThresholdsDepends on people: individuals may vary by up to 20dB and have “normal” hearing.Highest audible frequency depends on age: kids up to 20kHz, adults about 15kHz
37Hearing lossMAP can be used for generating Audiograms and evaluating hearing loss.Two types of hearing loss:Conductive: middle ear problems reducing sound transmission to cochlea (eg, infection: otitis media, bone growth around stapes or oval window, wax in the ear canal) elevation in absolute thresholdHelp: hearing aid, surgerySensorineural: defects in cochlea or auditory nerve or higher centers in the brain.Extent of the loss increases with frequency (esp. Elderly)Makes it hard to understand speech, esp. In noisy environment.Generally, no surgery is possible.
38Conductive hearing loss middle ear problems reducing sound transmission to cochleaeg, infection (otitis media), bone growth around stapes or oval window, wax in the ear canal elevation in absolute thresholdHelp: hearing aid, surgery
39Sensorineural hearing loss Due to defects in:cochleaauditory nervehigher centers in the brain.Extent of the loss increases with frequency (esp. Elderly)Makes it hard to understand speech, esp. In noisy environment.Generally, no surgery is possible.UK data, using frequencies 0.5,1,2, and 4kHz:61-71: 51% with loss >20dB, 30% > 40dB71-80: 74% > 20dB and 30% > 40dBUsing 4,6, and 8kHz71-80: 98% > 20dB and 81% > 40dB
40Temporal effect in Absolute Threshold Absolute thresholds of sounds depend on duration (Exner, 1876)For sounds > 500ms, no effectFor sounds < 200ms, minimal sound intensity increases as duration decreasesThe ear appear to integrates a stimulus energy over timeIn practice: (I-IL).t = IL.tIL threshold intensity for a long sound (>500ms)t constant for the auditory system integration time
41Limits of the human hearing, how one sound can hide another... Auditory masking
42Auditory maskingThe human auditory system has a limited capacity to resolve sinusoidal components of complex soundsEg, if a we listen to two tuning forks, one tuned at C (262Hz) and the other at A (440Hz), we hear two separate tones, each with its own pitch.Yet, one sound can be obscured, or rendered inaudible by other sounds (music from a car radio may mask the car’s engine – or conversely!)
43Auditory Masking (definition) Definition: “1. The process by which the threshold of audibility for one sound is raised by the presence of another (masking) sound; 2. The amount by which the threshold of audibility of a sound is raised by the presence of another (masking) sound. The unit customarily used is the decibel.” American Standards Association
44Auditory maskingA sound is more easily masked by another having a similar frequency. Limitations of the Basilar membrane Limits of frequency selectivityMasking is very dependent on time:Simultaneous presentation of the soundsForward maskingBackward masking
45The Critical BandFletcher (1940) suggested the auditory system works as a bank of bandpass filters, with overlapping passbands, based on the BM.When detecting a sound in a noisy background, a listener is assumed to make use of the filter with the closest center frequency.Threshold is determined by the amount of noise passing through this filter “power spectrum” view on masking (Patterson & Moore, 1986)
46Critical Band (cont’d) Fletcher’s idea:Only a narrow band of frequencies surrounding the tone contribute to masking the toneWhen the noise just masks the tone, the power of the tone, divided by the power of the noise is a constant KAssuming rectangular bandpass filters (not true, but convenient!), we have:P/(W.N0) = KWhere W is the bandwidth, N0 is the noise power, and P is the tone power.
50Auditory masking curves Auditory masking curves show how much masking occurs at which frequenciesUseful for efficient compression: no need to encode a frequency if we can’t hear it!Eg, MP3
51Contralateral masking Another form of masking is when the signal is presented to one ear, and the noise is presented to the other.This is called contralateral maskingWhen both sound and noise are presented to the same ear, this is called ipsilateral masking
52Temporal MaskingThis occurs when the masking and masked signals are not simultaneousIf the masking sound precedes the masked sound, it is called forward masking.If the masking sound follows the masked sound, it is called backward maskingMasking effectiveness attenuates exponentially from the onset and offset of the maskerOnset attenuation ~ 20ms.Offset attenuation ~100ms.Note: different to the ear’s acoustic reflex (reduce ear’s sensitivity after loud sound)
53Perception of Loudness Psychoacoustic perception of loudness, versus soud pressure.Perception of Loudness
54Loudness Fletcher Munson (1933) Robinson & Dadson (1956) Subjects listen to pure tonesVarious frequenciesamplitude inc. per 10dBRobinson & Dadson (1956)more accurateBasis for standard ISO-226Perceived Loudness (Phons)1 Phon = 1dB 1kHzBritish Standard BS ISO 226 (2003) (source wikipedia)
55Sound localization in space and stereo hearing... Sound spatialization
56Sound SpatializationLocalization accuracy is 1 degree for sources in front of the listener and 15 degrees for sources to the sides.Humans can discern interaural time differences of 10 microseconds or less
57Cues for Localization Interaural time difference The sound will reach the ears at different time, depending on its location in spacePhase delay at low frequenciesGroup delay at high frequenciesInteraural level differencesThe sound will be louder in one hear compared to the other.Distance can be estimated fromspectrum: high frequencies are attenuated more quickly lower loudnessMovement: parallax depends on distance
58Cues for LocalizationIn addition, the pinnae modifies the spectra of incoming sounds in a way that depends on the angle of incidence of the sound to the headHead+pinna form a direction-dependent filterMeasured by comparing the spectrum of the sound source vs. the spectrum reaching the eardrum: Head Related Transfer Function (HRTF)High frequencies (>6kHz) interact especially strongly with the pinna.
60SynesthesiaSynesthesia: Perceiving one sense as another eg. Sound as colors.Prevalence: unknown, could be as high as 1 in 23 (Simner J, Mulvenna C, Sagiv N, et al. (2006))This is believed to have a neurological basis (FMRI evidence)Famous synesthetes include David Hockney, who perceives music as color, shape, and configuration, and who uses these perceptions when painting opera stage sets – but not while creating his other artworks
61Synesthesia Some facts: Synesthesia is involuntary and automatic Synesthetic perceptions are spatially extended (sense of location)Synesthetic percepts are consistent and genericSynesthesia is highly memorableSynesthesia is laden with affect Richard Cytowic (2002,2003,2009)
62Plan About sound... How does the hear work? Absolute thresholds of hearingAuditory maskingSound spatializationSummary
63Additional ReadingBrian C.J. Moore (2003) An Introduction to the Psychology of Hearing, Academic Press.