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Physics SUM Second Semester > Acoustics > Flashcards

Flashcards in Acoustics Deck (38):

Sound Intensity

  • defined as the sound of power per unit area
  • basic units are watts/m or watts/cm2. Many sound intensity measurements are made relative to a standard threshold of hearing intensity Io.


Threshold of hearing intensity Io

  • the maximum sound level of pure tone that an average ear with normal hearing can hear in a noiseless environment.
  • I0=10^-12(watt/m^2) for frequency of 1000 Hz
  • measurement of absolute hearing threshold provides some basic information about our auditory system
  • Pure tone audiometry (PTA) is the key hearing test used to identify hearing threshold levels of an individual, enabling determination of the degree, type and configuration of a hearing loss


Mechanical Wave

  • are a local oscillation of material 
  • only the energy propagates; the oscillating material does not move far from its initial equilibrium position 
  • transport energy and not material 


Sound level L

  • The decibel (dB) is commonly used in acoustics to quantify sound levels relative to some 0 dB reference.
  • The reference level is typically set at the threshold of perception of an average human and there are common comparisons used to illustrate different levels of sound pressure



  • is the vibration of matter, as perceived by the sense of hearing
  • physically, sound is vibrational mechanical energy that propagates through matter as a wave 


Sound waves are characterized by the generic properties of waves, which are: 

  1. frequency
  2. wavelength
  3. period
  4. amplitude
  5. intensity
  6. speed (propagation speed)



  • is a measure of the number of occurrences of a repeating event per unit time 
  • also referred to as Temporal Frequency  



  • is the duration of one cycle in a repeating event
  • is the reciprocal of the frequency 



< 16 Hz


Audible Sound 

  • 16 Hz- 20 kHz (20000 Hz)
  • for humans, hearing is limited to frequencies between about 16Hz- 20000 Hz, with the upper limit generally decreasing with age 



> 20 kHz



  • is the distance between repeating units of a propagating wave of a given frequency
  • is commonly designated by the Greek letter lambda: λ


Sound level formula

  • L= 10log I/Io(dB)
  • I - sound intensity
  • Io - sound threshold


Ear Diagram


3 parts of Ear

  • The sense organ that detects sound is EAR which has 3 parts
    • Outer ear
    • Middle ear
    • Inner ear




Outer ear

  • the outer ear is the most external portion of the ear 
  • the outer ear includes the pinna (also called auricle), the ear canal and the very most superficial layer of the ear drum(also called the tympanic membrane)



  • is the magnitude of change in the oscillating variable, with each oscillation within an oscillating system 
  • maximum value of intensity 


Speed of Sound

  • describes how much distance such a wave travels in a certain amount of time 
  • in dry air with a temp. of 21 ºC (70 ºF) the speed of sound is 344m/s (1230km/h, or 770mph, or 1130 ft/s)


Middle ear

  • The middle ear, an air filled cavity behind the ear drum(tympanic membrane), includes the three ear bones r ossciles: the malleus(or hammer), incus (or anvil) and stapes (or stirupp)
  • the opening of the Eustachian tube is also within the middle ear


Inner ear

  • The inner ear includes the organ of hearing (the cochlea)
  • its core component is the Organ of Corti, the sensory organ of hearing, which is distributed along the paritition seperating fluid chambers in the coiled tapered tube of the cochlea


Outer Ear: Function

Middle Ear: Function

  • The outer ear collects sound waves and channels them down to the ear canal, where they cause the ear drum to vibrate
  • This, in turn, causes the middle ear bones to move, increasing and amplifying the vibrations and transmitting them into the inner ear, (the cochlea)


Material Speed 

Most Important one from picture

Soft Tissue: 1540 m/s


Image of Cochlea


Audiogram indicating normal hearing


Sound scale diagram


Inner Ear: Function

  • The inner ear resembles a snail shell containing fluid. The vibrations cause the fluid to move, setting tiny hearing nerves, (hair cells), in motion.
  • Then, an electrical signal is sent along the auditory nerve which the brain translates into the sounds we hear.


Diagram for middle ear


Hearing Loss: Two Types

  • There are many causes of hearing loss. But it can be divided into two basic types:
    • Conductive Hearing Loss
    • Sensorineural Hearing Loss


Conductive Hearing Loss

  • Caused by anything that interferes with the transmission of sound from the outer to inner ear


Sensorineural Hearing Loss

  • Caused by damage to the pathway that sound impluses take from the hair cells of the inner ear to the auditory nerve and the brain


What is hearing loss measured in, and what is considered normal hearing?

What are the thresholds of the mild to severe hearing loss?

  • Hearing loss is measured in decibels, or dBHL. 
  • Normal hearing is a range of sound frequencies from 0-20dB for a person
  • Thresholds from mild to severe hearing loss:
    • Mild: 25-39dBHL
    • Moderate: 40-69dBHL
    • Severe: 70-94dBHL


Audiogram Diagram


Sounds used for Echolocation by animals:

  • Some Birds:
    • 4-7 kHz
  • Shrews:
    • 20-64 kHz
  • Bats:
    • 25-210 kHz
  • Dolphins:
    • up to 280 kHz


Production of Ultrasound:

Piezoelectric Crystal Image


Production of Ultrasound:

How does it work?

What is a transducer?

  • the ultrasound is produced through the conversion of electrical into mechanical energy and is detected by the reverse process by converting mechanical into electrical energy
  • a transducer is a device that is both a transmitter and reciever of the ultrasound signal and it serves a dual role in pulse echo imaging


Longitudinal Waveform

  • the ultrasound beam propogates as a longitudinal waveform from the transducer face into the medium
    • near field (for 1.5 cm diameter crystal element at 3.5 MHz the near field extends to 12.5 cm)
    • far field


Interaction of Ultrasound with Matter

  • reflection
  • refraction
  • attenuation