Otology Seminar
90-7-4
R3 葉春風
Acoustic Changes of Sugical Modified External Ear
I. Sound transmission in normal ear
Pinna flange
 For human, small in relation to head size, almost rudimentary acoustically, for
sound localization.
 3 dB sound pressure gain from 3000 to 6000 Hz
Concha
 In human as a cavity resonator, 10 dB sound pressure gain from 4000 to 5000
Hz. Depth1.3 cm, radius 0.89 cm, volume 4.5 cc.
 Resonant frequency= 4 times depth + 0.822 X radius
Ear canal
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Length 26 mm, diameter 7 mm, volume 1cc, size of opening vertical 9 mm,
horizontal 6.5 mm.
Sound pressure gain 12 dB at tympanic membrane with peak btw 3400 and
3900 Hz as compared to the canal opening.
Fundamental resonance: wave length = 4 times canal length
II. Acoustic effects produced by changes in external ear anatomy
 Produce acoustic change in higher speech frequency above 1500 Hz, rarely
exceeding 20 dB
 Bandwidth of 200 Hz or greater is necessary in 1500 to 3600 Hz to produce
measurable effect on speech discrimination
 At 4k Hz and above have little effect on speech reception and discrimination
Conchal concavity using KEMAR
 Occlusion of conchal cavity: 10 dB decrease in conchal resonance peak 4.5k
Hz, 7 dB decrease in canal resonance peak 3.4k Hz
 Larger deeper concha(1.9 cm depth, 0.92 cm radius): downward resonant peak
shift to 2.2k Hz, 9 dB increase of amplitude, decreased bandwidth
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Shaw 1974: concha has some role in producing acoustical gain in 3k Hz
Op Hint—conchal depth should be kept or increased, remove conchal cartilage
or concha flattening may produce detrimental effect.
Ear canal opening size
 Decrease of resonance peak with decreased canal opening size
 3 mm: 7 dB broad peaked increase at 1000 Hz.
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(Chandler1964), 3mm hearing loss 1 dB at 1000 Hz, 13 dB at 2000 Hz, 14 dB
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at 4000 Hz
2 mm 9 dB loss at 1000 Hz, 23 dB at 2000Hz, 21 dB at 4000 Hz
 1 mm: marked decrease at all key speech frequency
 Loss at 2k 3k begins at 6mm in diameter.
 Barely adequate lumen may become inadequate due to cerumen
 Op Hint—canal opening should be at least 6 mm in diameter or equivalent area
Ear canal length
 Add 2 cm to Kemar’s ear canal, 2.15 cm to 4.15 cm, resonance peak downward
shift to 2400 Hz without change of resonance peak
 Short canal shift the peak too high above speech frequency
 Op Hint—canal length should be as long as possible, ideally 3 cm, to make
resonant peak 3k Hz or below
Ear canal shape using fresh temporal bone, canal 1.7cm, peak at 5kHz
 Widened EEC to 1.2 1.4 cm diameter , not entering antrum: no acoustical
change
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Surgically connect canal and antrum: loss of normal resonant peak,
antiresonance appeared.
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(Fresh temporal bone)Wide opened mastoid via transcanal approach: 24 dB
sharp dip at at resonance peak. 15 dB antiresonant dip at 2500 dB. 12 dB
resonant point btw 1000-2000 dB
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Radical mastoidectemy patients, despite size and shape mostly had normal
resonant peak in 2k to 4 k with gain of 15 to 25 dB
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Difference: Patients mastoid were well healed. Temporal bone specimen had
many residual air cells as sound absorbers. Also with open aditus
Closure of aditus and antrum with clay: improvement at 2500 Hz
antiresonance (open aditus acted like a perforation)
Gardner Hawley1973: cone shape canal with apex facing insidepeak
ampiltue at higher frequency. Not demostrated in Goode 1977
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Tolley 1992: open mastoid surgery : decrease resonant frquency, not affecting
amplitude and Q factor. Cavity obliteration with silastic foam : no change in
canal resonance. Surgical technique to obliterate the mastoid cant restore the
resonant property.
Liu T, Lin K 1998: REUR(real-ear-unaided- response) post open mastoid
surgery ears—significant decrease of resonant frequency. No difference in peak
amplitude. Ear canal volume can not predict resonance frequency. There are
also negative responses at higher frequencies.
Op Hint --Shape is not important until it connect antrum.
Evans 1989—Open cavity mastoid surgery, effect on acoustics of ext ear canal.
5 normal vs 5 post op ear: not alter mean frequency of resonance peak,
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increase amplitude 9 dB. 12 Preop vs postop ears: small but significant
lowering of mean resonant peak frequency from 2.2 to 2.5k Hz, no change in
amplitude, 8 dB attenuation in 8 dB. 6 temporal bone pre vs post modified
radical mastoidectemy: lowered mean peak resonant frequency from 3.9 to
1.9k Hz and negative peak at 4.2k Hz, +14 dB at 1.9k Hz –22dB at 4.2k Hz.
Creating a tympanic membrane defect: peak shift to 1.8kHz. Radical
mastoiddectemy: peak shift to 1.85kHz
TM perforation
 In a temporal bone, arteficially TM perforation was made at post inf quad
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0.75 mm perforation: no change
1.5 & 2.0 mm perforation: 3-4 dB decrease from 300 to 3000 Hz, sharp 10 dB
antiresonance at 3200 Hz, 17 dB antiresonance at 5900 dB
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1.5 & 2.0 mm perforation: produce deep antiresonance at 2k 4k Hz
Kruger, Tonndorf 1977: 2 mm TM perforation in cat low frequency loss
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improved at a rate of 12dB per octave up to 1600 Hz
Harrison 1972: small perforation in human produce significant 19 db loss at 2k
Hz. Consistence with Goode 1977—high tone antiresonance
Op Hint—1. Post tympanoplasty small perforation is occasionally present, and
is desirable if poor E tube function. 2. Middle ear ventilation tube size
TM impedance raise raise amplitude and narrowing of resonant frequency
peak bandwidthincrease in Q factor
Reference
1.
Goode RL. Friedrichs R. Falk S. Effect on hearing thresholds of surgical modification of the external ear.
Annals of Otology, Rhinology & Laryngology. 86(4 Pt 1):441-50, 1977 Jul-Aug.
2.
Liu TC. Lin KN. Probe-tube microphone measures in patients with open-mastoid surgery:
real-ear-to-coupler differences and real-ear unaided responses. Audiology & Neuro-Otology. 5(2):59-63, 2000
Mar-Apr.
3.
Evans RA. Day GA. Browning GG. Open-cavity mastoid surgery: its effect on the acoustics of the external
ear canal. Clinical Otolaryngology & Allied Sciences. 14(4):317-21, 1989 Aug.
4.
Tolley NS. Ison K. Mirza A. Experimental studies on the acoustic properties of mastoid cavities. Journal of
Laryngology & Otology. 106(7):597-9, 1992 Jul.
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