P4: Loudness & Pitch

The sound stimulus

“If a tree falls and there is no one to hear it, would there be a sound?”

What is sound? def.

• Perceptual definition = sound is the experience we have when we hear  

 

• Physical definition = sound is pressure changes in the air or other medium caused by the vibration of an object  

• A pure tone occurs when changes in air pressure form a perfect sinusoidal wave  

Characteristics of sound - pure tones

Amplitude 

 

• Size of the variation in air pressure (I.e difference between peak and trough  

• Related to perception of loudness 

 

Frequency 

• Number of cycles per second (1 hertz = 1 cycle/s)  

• Related to perception of pitch  

 

Characteristics of sound - complex sounds

• Most sounds encountered in the world are more complex than pure tones 

• All sound waves can be described as some combination of sine waves 

• Natural sounds often consist of a fundamental frequency superimposed by additional waveforms with higher frequencies (the harmonics) 

Overview of the ear

The human ear is divided into 3 sub-divisions: outer, middle and inner  

Outer ear

• Pinnae = visible external parts of the ear  

 

• Auditory canal = 3cm tube-like structure, protects middle ear  

 

• Tympanic membrane (eardrum) = Cone-shaped, separating the outer and middle ear  

• Sound waves induce a difference in pressure either side of tympanic membrane causing it to vibrate  

 

• Larger amplitude sounds  = larger vibrations  

• Higher frequency sounds = faster vibrations  

 

Middle ear

The middle ear = small cavity (approv. 2 cubic cm) that contains the ossicles 

 

• Malleus (hammer)  

• Incus (anvil)  

• Stapes (stirrup)  

 

The bones amplify the vibrations of the tympanic membrane and transmit them to the inner ear at the oval window  

Inner ear

• The main structure of the inner ear is the cochlea, a snail-like liquid filled organ  

 

• Vibration of the oval window displaces fluid in the cochlea, resulting in a change in pressure which propagates up and down the spiral structure  

 

• Cochlea consists of 3 parallel canals: vestibular, middle and tympanic  

 

• Auditory transduction is triggered by motion of the basilar membrane, which separates the middle and tympanic canals 

Auditory transduction

• Motion of the basilar membrane are translated into neural signals by structures in Organ of Corti, which extends along its surface  

 

• A voltage is generated when specialised hair cells contained within the organ of corti are bent  

 

• This produces impulses in auditory nerve cells which are sent to the brain  

 

• Hair cells are extremely sensitive  

 

• Overstimulation by loud sounds can damage hair cells and lead to hearing loss 

 

Loudness

• Our perception of loudness is related to the amplitude of sound waves  

The range of amplitude we encounter is extremely large  

Eg. The amplitude of a very loud sound at a rock concert might be up to 1,000,000 times that of a sound you could barely hear  

Loudness (2)

• In order to describe differences in amplitude, sound levels are measured on a logarithmic scale in units called decibels (dB)  

 

• A change of 20dB corresponds to a ten-fold increase in amplitude 

 

Loudness (3)

Rate code: Sound amplitude is coded in the firing rate of auditory nerve fibers  

 

• Responses increase with increasing sound intensity  

• Note, some fibers have high spontaneous rates and saturate rapidly, while others have low spontaneous rates and saturate slowly  

•  This enables us to discriminate loudness across a range of sound levels  

Loudness (4)

Loudness depends on amplitude (but the two are not directly proportional)  

 

• For a sound to be perceived as twice as loud, its amplitude needs to be increased by a factor of approx. 3.16 (10dB)  

 

Loudness (5)

 Loudness depends on the frequency:  

 

• Our auditory systems are not equally sensitive to all sound frequencies  

 

• The red curves here indicate the number of decibels required to create the same perception of loudness at different frequencies  

Pitch

• Humans are sensitive to a wide range of sound frequencies  

 

• The lowest frequency humans can hear is 20Hz (below that we can't hear the sound, but are able to feel it)  

• The highest frequency humans can hear is 20,000Hz (20kHz)  

Pitch - Place code

Sound frequencies cause vibrations in specific areas along the basilar membrane  

 

• Low frequencies – near apex  

• High frequencies – near base  

Pitch - Timing code

Frequency is not only signalled by which auditory nerve, but also WHEN they respond  

 

• Auditory nerve responses are synchronised to changes in pressure  

• This property is called phase-locking and occurs up to frequencies of about 4,000Hz