The natural resonance and efficiency of the outer and middle ears: Enhances sound transmission for 500 to 3000 Hz. Thus, the transformer ratio of the middle ear is approximately 22:1 (the combination of the area effect of the tympanic membrane and the lever action of the ossicles: 17 × 1.3 = 22) which translates to approximately 25 dB. Because the handle of the malleus is approximately 1.3 times longer than the incus long process, the force (pressure) received at the stapes footplate, through the use of leverage, is greater than that at the malleus by about 1.3:1. Lever action of the ossicular chain: As the eardrum vibrates, the ossicular chain is set into motion about an axis of rotation from the anterior process of the malleus through the short process of the incus. Thus, the ratio of the vibrating portion of the tympanic membrane to that of the stapes footplate results in a 17:1 increase in sound energy by concentrating it into a smaller area. The area effect of the tympanic membrane: Although the area of the adult tympanic membrane is between 85 and 90 mm 2, only about 55 mm 2 effectively vibrates (the lower two-thirds of the tympanic membrane) the stapes footplate is 3.2 mm 2. This impedance matching is accomplished by the following four factors: It is an impedance-matching system that ensures only minimal energy is lost, when structurally and functionally normal. The middle ear transforms acoustic energy from the medium of air to the medium of liquid. Thus, the ear has greatest sensitivity at those frequencies most important to understanding speech. The tympanic membrane and ossicular chain most efficiently transmit sound between 5 Hz. This difference facilitates the transmission of high frequencies. The malleus and incus weigh approximately the same, but the stapes is about one-fourth the mass of the other ossicles. The displacement of the ossicular chain varies as a function of the frequency and intensity of the sound. Sound waves from the tympanic membrane travel along the ossicular chain, which comprises three bones (the malleus, incus, and stapes), to the oval window.
The middle ear is an air-filled space approximately 5/8 inch high (15 mm), 1/8 to 3/16 inch wide (2-4 mm), 1/4 inch deep, and 1 to 2 cm 3 in volume in the average adult. Natural resonance of external auditory canal is approximately 3000 Hz of middle ear, 800 to 5000 Hz, mostly 1000 to 2000 Hz of tympanic membrane, 800 to 1600 Hz of ossicular chain, 500 to 2000 Hz. Resonant frequency: frequency at which a mass vibrates with the least amount of external force determined by elasticity, mass, and frictional characteristics of the medium. However, the term “noise” can also mean any unwanted sound. Types of noise frequently used in clinical audiology are white noise (containing all frequencies in the audible spectrum at average equal amplitudes), narrow band noise (white noise with frequencies above and below a center frequency filtered out or reduced), and speech noise (white noise with frequencies > 3000 and < 300 Hz reduced by a filter).
Pure tone: single-frequency sound rarely occurs in nature.Ĭomplex sound: sound comprising more than one frequency. Human ear capable of hearing from approximately 20 to 20,000 Hz. Hertz (Hz): in acoustics, unit to express frequency (formerly cycles per second or cps). Time for completion of one cycle is the period. One-tenth of a Bel (named for Alexander Graham Bell).įrequency: number of cycles (complete oscillations) of a vibrating medium per unit of time psychoacoustic correlate is pitch. Sound pressure: sound force (related to acceleration) over a surface per unit time.ĭecibel (dB): unit to express intensity of sound more specifically the logarithm of the ratio of two sound intensities. Intensity of sound: amount of sound energy through an area per time refers to sound strength or magnitude. In psycho acoustics, sounds with higher amplitudes are perceived as louder than sounds with lower amplitude.
Sound: energy waves of particle displacement, both compression (more dense) and rarefaction (less dense) within an elastic medium triggers sensation of hearing by vibrating the tympanic membrane.Īmplitude of sound: extent of vibratory movement from rest to farthest point from rest in compression and rarefaction phases of energy waves.