POST MIDTERM-Audition

The Range of Sound Frequencies for Humans and Some Other Animals. What Does Range Depend On?

• Humans can perceive sound between 20-20000 hz. Whales, dolphins and dogs have a wider frequency range, while birds and frogs have a much narrower range. The range of frequencies a species can hear depends on the length of their basilar membrane

• Fish can hear very low frequencies and bats can hear very high frequencies.

How do Soundwaves Work?

• Sound waves are caused by vibrations that displace the air around it and create waves of dense and less dense air molecules. This is like a ripple in water with areas of dense and less dense water. This can be graphed as a sine wave.

  • These bands of more and less air molecules push the eardrum (tympanic membrane) in and out. Bands of more condensed air molecules push the eardrum in, while less condensed molecules push it out. 

How do Light and Sound Waves Differ?

• Light is a Transverse wave which does not require a medium to travel. Sound is a longitudinal wave and does require a medium to travel (air).

What are the Characterists of the Sine Wave of Sound and What do they Mean?

• Sine Wave of Sound

  • Wavelength = 1 cycle = pitch (Hz = cycles/second. The more cycles/second, the more the hz, the higher the pitch). Many frequencies exist outside our detectable range.

  • Amplitude = height = loudness. Loudness is measured using a log scale. This means that perceived loudness doubles for every 10 db increase.

  • Purity = # of frequencies = Timbre/complexity.

What Does the External Ear Do? What 3 Things Does it Consist Of?

• External Ear: Detects changes in air pressure

  • Pinna: Outer portion of ear; collects sound waves to send to ear canal

  • Ear Canal: Narrows as it moves towards the eardrum which amplifies the sound waves.

  •  Eardrum: Vibrates at the frequency of the incoming sound wave

What Does the Middle Ear Do and Consist of?

• Middle Ear: Begins on the other side of the eardrum. Amplifies these changes to set off changes in fluid pressure in the inner ear

  • Contains ossicles called hammer, anvil and stirrup. Ossicles work like a lever system to amplify the eardrums vibrations into the oval window even more

What Does the Inner Ear Do and How Does it Work?

• Inner Ear: Changes in fluid pressure convert to nerve signals. Other side of oval window

  • The vibrating oval window connects to the cochlea of the inner ear. The vibration of the oval window displaces cochlear fluid. The round window balances this pressure by bulging in and out appropriately.

  • Inside the cochlea, there is a basilar membrane. When the cochlea is pushed in, the basilar membrane is pushed down. When the oval window is pushed out, the basilar membrane is pushed up.

  • The basilar membrane widens towards the apex of the cochlea, while the cochlea narrows. Because the basilar membrane varies in width and frequency, different sound waves make different sections of the membrane vibrate. High frequency sounds vibrate the membrane near the base, while low frequencies vibrate it near the apex. This is because a low frequency sound has a large wavelength, so it’ll touch the end of the membrane more while a high frequency sound has smaller wavelengths so it will touch the base more.

  • The basilar membrane has hair cells. These hair cells move when the membrane moves, and convert the sound into a neural impulse. 

  • The cochlea is a curled up organ that contains nerve tissue to make the vibrations nerve impulses.

How Does the Trasmission of Neural Info in the Cochlea Occur?

• Transmission of neural info

  • The hair cells send their signal to the bipolar cells of the cochlear nerve through neurotransmitters. This neurotransmitter triggers EPSP’s in the cochlear nerve fibers. The cochlear nerve fibers then send the signal to the cochlear nucleus in the hindbrain.

How is the Auditory Pathwaty Similar to the Visual Pathway?

• The cochlear nucleus has dorsal and ventral streams that function similar to vision.

• The auditory pathway is also similar to the visual pathway in that it has the same spatial mapping (topographic organization). In the auditory pathway, this is called tonotopic organization, in which neighboring hair cells signals are neighbors all the way till the primary auditory cortex. 

Outer vs Inner Hair Cells

• Outer: More numerous (4:1), fewer connections per cell to brain, amplify sound, unmyelinated.

• Inner: Less numerous, more connections to brain per cell, detect pitch, myelinated. Take primary responsibility for transmitting signals to the brain. 

What Are the Three Intra Aural Cues

Difference in arrival time between two ears

Difference in intensity between two ears

Pinna Cues

Difference in arrival time between two ears

The difference can be measured in sub milliseconds, but neurons in the superior olivary complex respond to these subtle differences to localize sound. The medial superior olivary nuclei detects ITD’s

Difference in intensity between two ears

• A sound at your left ear would have to travel through or around your head to get to your right ear. Your head therefore casts a sound shadow which makes your right ear perceive the sound as less intense. This effect is diminished with low frequency sounds, because the wavelength might be longer than your head. The lateral superior olivary nuclei detects IID’s

Pinna Cues

Localize the elevation of a sound source. Because ear shapes are unique, everyone has their own pinna cues they learn.

Azimuth

• Whether your head is turned left or right

Binaural Cues:

• Require both ears

Inferior Colliculus

•  Integrates info about the spatial location of sound

Echolocation

• Process by which a receiver emits sound pulses and analyzes the returning echo to form a perceptual image of objects in the environment.

How Does Echolocation Detect Proximity?

• A distant objects echo will echo back later than a proximal object

How Does Echolocation Detect Motion?

• A moving object will have doppler shifted echoes that a still object will not

How Does Echolocation Detect Texture?

•  Textured objects will produce echoes that vary slightly in their return time

Coevolution

Evolution of one species directly influences the evolution of another.

Ex: Some prey of bats evolved hearing to hear bat calls