Lecture 1 (Summer) - Psychoacoustics Flashcards
What is Psychoacoustics?
Study of the psychology of human hearing.
It’s about PERCEPTION.
• Perception depends both on the stimulus and on our cognition.
• Physical stimulus:
change in waveform frequency
• Perceptual Response: pitch
How can we understand what we hear when we hear sound?
It’s a multidisciplinary approach between
1. Physics
2. Psychology
3. Physiology
These contribute to our PERCEPTION
Study of the interaction between these three domains and the experience of sounds = PSYCHOACOUSTICS.
What is sound localisation?
Human perception of the position of auditory events.
Differences in signals received by the ear are essential.
Two main cues:
- Level differences at the ears
- Time differences at the ears
What is Interaural Time Difference (ITD)?
If a sound source is to the side of the head, the wavefront will hit one ear first before the other.
We can use ITD cues to recognise the localisation of a sound source to a resolution of a few degrees.
• The maximum time delay between the ear is of about 0.65ms, but it depends on the size of the head!
• Time difference cues are easier to detect at the starts and ends of sounds and mostly based on the low-frequency content of signals.
What is Interaural Level Difference (ILD)?
The human ear is also sensitive to level differences.
The head is a barrier for sound at high frequencies but not at low frequencies.
• If the wavelength is small (high frequencies) in comparison to the head/torso sound will be reduced at the ear further away from the sound source as there is a shadowing effect. So, localisation can be achieved.
• If the wavelength (lower frequencies) is larger sound travels around the listener (diffraction), so localisation is more difficult.
• Hence, ITD cues work better for low frequency sounds, and ILD cues work better for high frequencies sounds.
Why do ITD cues work better for low frequency sounds? and ILD for high frequency sounds?
Because low frequency sounds have a larger wavelength, they are not sufficiently reduced by the head and thus it is more effective to localise the sound through ITD.
High frequency sounds have a smaller wavelength and will be reduced significantly by the time they reach the furthest ear, so ILD is an effective localisation cue.
What is a wavelength? What is the equation?
The distance travelled by a wave within a cycle.
λ=c/f
λ - wavelength
c - speed of sound (340m/s)
f - frequency
Calculate the wavelength for the following frequencies: 40Hz, 200Hz, 500Hz 1kHz
40Hz = 8,5m
200Hz = 1.7m
500Hz = 0.68m
1kHz =0.34m
What is the Duplex theory?
Together, ILD and ITD cues allow us to localise sound. This is known as Duplex Theory, first proposed by Lord Rayleigh around 1900.
What is the Cone of Confusion?
For any ITD and ILD pair, the source can exist in a ‘cone of confusion’
To the listener, audio sources A and B, and sources C and D have identical interaural time difference and interaural level difference.
(see slide image)
What is the third cue that allows us to distinguish between up/down and front/back?
The Pinna!
Pinnae spectrally shape the signals at the ears.
• This filtering action allows us to determine the elevation of the source and helps overcome front/back confusion.
Everyone has differently shaped pinna!
What are HRTFs?
Filtering effect that is dependent on the shape of the pinna, together with the diffracting and reflecting characteristics of the head and torso of a listener
Intensity Stereo - if we put the same signal to both loudspeakers where do we hear the sound?
Phantom centre - in between the two loudspeakers.
Intensity Stereo - if one loudspeaker is louder relative to the other, where do we hear the sound?
Towards the louder speaker. Full image shift when 1 loudspeaker is approximately 15dB higher.
Time-Based Stereo - If we put the same signal to both loudspeakers and at the same time, where do we hear the sound?
In the centre.
