The Journal of Laryngology and Otology July 1992. Vol. 106, pp. 597-599
Experimental studies on the acoustic properties of mastoid cavities N. S. TOLLEY, M.D., F.R.C.S., K. ISON, M.Sc, PH.D., A. MIRZA, M.SC. (London)
Abstract This study investigated the effects of open-mastoid surgery upon ear canal resonance. In particular an attempt was made to alter resonant properties by obliterating each cavity with silastic foam. The results from this study showed that open-mastoid surgery significantly decreased resonant frequency without producing an effect upon either the peak amplitude or the quality of the resonant peak as defined by the Q-factor. Restoring the natural resonance properties of an operated ear, if indicated, is likely to be a difficult objective to achieve in surgical terms.
Introduction The surgical treatment of cholesteatoma by open-mastoid surgery is commonplace in clinical practice. Despite this, little basic scientific research has been conducted into assessing the effects of such surgery upon acoustic resonance properties. This is particularly surprising since many of these patients require auditory rehabilitation in the form of a hearing aid. Patients may be difficult tofitwith hearing aids because of the undesirable resonant peaks in the external auditory meatus and cavity observable with real ear measurement systems. In addition, the problems that some patients experience with persistent otorrhoea will be all too familiar to the surgeon. Techniques for obliterating a mastoid cavity in order to achieve a dry ear are well known in otological practice. The acoustic benefits obtained as a consequence seems to be an aspect that has received little attention. The sound pressure distribution in the normal ear has
been well researched in classical studies by Wiener and Ross (1946) and, more recently, by Djupesland and Zwislocki (1972). Evans et al. (1989), studied the effect of open-mastoid surgery upon resonant properties of the external ear canal in both temporal bones and patients. They found that surgery significantly reduced the resonant frequency of the outer ear. This effect, however, was less pronounced in patients compared with that found in temporal bones. The effect on resonance properties of obliterating open-mastoid cavities has not been previously reported. The aim of work presented in this paper was to investigate the effect of experimental mastoid cavity obliteration on acoustic resonance properties. If beneficial changes in ear resonance could thereby be brought about, this might provide another indication for cavity obliteration, particuCENTREFREQUENCY
500
1000
2000 5000 FREQUENCY (HJ| LOGARITHMIC SCALE
FIG. A = Loudspeaker B = Subject C s Probe Microphone D = Rastronics Real Ear Measurement System E = Operator
2
Illustration of methods of measurement of amplitude, centre frequency and band width of resonant peak. The Q-factor is calculated by dividing the resonant frequency of the peak by its band-width, the latter being the frequency interval between the two points of intersection on the resonance curve, of a horizontal line drawn 3 dB SPL below the resonant peak.
Fig. 1 Experimental set-up for measurements of resonance.
From The Royal National Throat, Nose and Ear Hospital, Gray's Inn Road, London WC1X 8DA. Accepted for publication: 27 March 1992
597
598
N. S. TOLLEY. K. ISON. A. MIRZA
TABLE I RESONANT FREQUENCY, BANDWIDTH, Q-FACTOR, AND AMPLITUDE IN TEN HEALTHY CONTROL STUDIES
Resonant frequency (H,) 2.600
Bandwidth (H2)
Q-factor
Amplitude dbSPL 16.6
684
larly in those patients who are difficult to fit with hearing aids due to the existence of undesirable resonant peaks. Methods Ten subjects were used for the study, mean age 57 years, range 37-75 years. Each possessed a unilateral open mastoid cavity with a normal contralateral ear. Acoustic measurements were made by using a Rastronics CCI-10 Frequency Response Analyser equipped with a probe microphone. Measurements of canal resonance were performed in a class C acoustic room with each subject facing the sound source at a distance of 1 m, (Fig. 1). The sound source was a warble tone at 80 dB SPL with a frequency sweep from 0.125-8 kHz. First, a sound level calibration was performed with the subject in place but with the probe microphone placed at the level of the pinna. Recordings of the open canal resonance were then taken externally with the probe microphone tube located just lateral to the drum, placed under otoscopic control. After recording the resonance response from both ears, measurements were repeated with each mastoid cavity obliterated with silastic foam. Control measurements were also performed on ten healthy volunteers with normal ears. From each resonance curve, the amplitude and frequency of the resonant peak was measured. The sharpness of the peak was described by calculating its Q-factor. The Q-factor was determined by dividing the resonant frequency of the peak by its bandwidth, the latter being the frequency interval between the two points of intersection on the resonance curve, of a horizontal line drawn 3 dB SPL below the resonant peak. The methods of measurement are illustrated in Figure 2. Results Table I shows the mean resonant frequency, amplitude of resonance and Q-factor in the ten healthy controls. A series of t-test comparisons were performed on the results for different subject groups, as summarized in Table II. No significant differences were found between right and left ears in the ten healthy controls or, when comparing these, to the ten normal contralateral ears.
Open mastoid surgery significantly decreased the resonant frequency from 2.6 ± 0.11 kHz (mean ± sem) to 1.9 ± 0.17 kHz (mean ± sem), P
Experimental studies on the acoustic properties of mastoid cavities N. S. TOLLEY, M.D., F.R.C.S., K. ISON, M.Sc, PH.D., A. MIRZA, M.SC. (London)
Abstract This study investigated the effects of open-mastoid surgery upon ear canal resonance. In particular an attempt was made to alter resonant properties by obliterating each cavity with silastic foam. The results from this study showed that open-mastoid surgery significantly decreased resonant frequency without producing an effect upon either the peak amplitude or the quality of the resonant peak as defined by the Q-factor. Restoring the natural resonance properties of an operated ear, if indicated, is likely to be a difficult objective to achieve in surgical terms.
Introduction The surgical treatment of cholesteatoma by open-mastoid surgery is commonplace in clinical practice. Despite this, little basic scientific research has been conducted into assessing the effects of such surgery upon acoustic resonance properties. This is particularly surprising since many of these patients require auditory rehabilitation in the form of a hearing aid. Patients may be difficult tofitwith hearing aids because of the undesirable resonant peaks in the external auditory meatus and cavity observable with real ear measurement systems. In addition, the problems that some patients experience with persistent otorrhoea will be all too familiar to the surgeon. Techniques for obliterating a mastoid cavity in order to achieve a dry ear are well known in otological practice. The acoustic benefits obtained as a consequence seems to be an aspect that has received little attention. The sound pressure distribution in the normal ear has
been well researched in classical studies by Wiener and Ross (1946) and, more recently, by Djupesland and Zwislocki (1972). Evans et al. (1989), studied the effect of open-mastoid surgery upon resonant properties of the external ear canal in both temporal bones and patients. They found that surgery significantly reduced the resonant frequency of the outer ear. This effect, however, was less pronounced in patients compared with that found in temporal bones. The effect on resonance properties of obliterating open-mastoid cavities has not been previously reported. The aim of work presented in this paper was to investigate the effect of experimental mastoid cavity obliteration on acoustic resonance properties. If beneficial changes in ear resonance could thereby be brought about, this might provide another indication for cavity obliteration, particuCENTREFREQUENCY
500
1000
2000 5000 FREQUENCY (HJ| LOGARITHMIC SCALE
FIG. A = Loudspeaker B = Subject C s Probe Microphone D = Rastronics Real Ear Measurement System E = Operator
2
Illustration of methods of measurement of amplitude, centre frequency and band width of resonant peak. The Q-factor is calculated by dividing the resonant frequency of the peak by its band-width, the latter being the frequency interval between the two points of intersection on the resonance curve, of a horizontal line drawn 3 dB SPL below the resonant peak.
Fig. 1 Experimental set-up for measurements of resonance.
From The Royal National Throat, Nose and Ear Hospital, Gray's Inn Road, London WC1X 8DA. Accepted for publication: 27 March 1992
597
598
N. S. TOLLEY. K. ISON. A. MIRZA
TABLE I RESONANT FREQUENCY, BANDWIDTH, Q-FACTOR, AND AMPLITUDE IN TEN HEALTHY CONTROL STUDIES
Resonant frequency (H,) 2.600
Bandwidth (H2)
Q-factor
Amplitude dbSPL 16.6
684
larly in those patients who are difficult to fit with hearing aids due to the existence of undesirable resonant peaks. Methods Ten subjects were used for the study, mean age 57 years, range 37-75 years. Each possessed a unilateral open mastoid cavity with a normal contralateral ear. Acoustic measurements were made by using a Rastronics CCI-10 Frequency Response Analyser equipped with a probe microphone. Measurements of canal resonance were performed in a class C acoustic room with each subject facing the sound source at a distance of 1 m, (Fig. 1). The sound source was a warble tone at 80 dB SPL with a frequency sweep from 0.125-8 kHz. First, a sound level calibration was performed with the subject in place but with the probe microphone placed at the level of the pinna. Recordings of the open canal resonance were then taken externally with the probe microphone tube located just lateral to the drum, placed under otoscopic control. After recording the resonance response from both ears, measurements were repeated with each mastoid cavity obliterated with silastic foam. Control measurements were also performed on ten healthy volunteers with normal ears. From each resonance curve, the amplitude and frequency of the resonant peak was measured. The sharpness of the peak was described by calculating its Q-factor. The Q-factor was determined by dividing the resonant frequency of the peak by its bandwidth, the latter being the frequency interval between the two points of intersection on the resonance curve, of a horizontal line drawn 3 dB SPL below the resonant peak. The methods of measurement are illustrated in Figure 2. Results Table I shows the mean resonant frequency, amplitude of resonance and Q-factor in the ten healthy controls. A series of t-test comparisons were performed on the results for different subject groups, as summarized in Table II. No significant differences were found between right and left ears in the ten healthy controls or, when comparing these, to the ten normal contralateral ears.
Open mastoid surgery significantly decreased the resonant frequency from 2.6 ± 0.11 kHz (mean ± sem) to 1.9 ± 0.17 kHz (mean ± sem), P
