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Snoring is a prevalent disorder affecting 20–40% of the general population. The mechanism of snoring is vibration of anatomical structures in the pharyngeal airway. Flutter of the soft palate accounts for the harsh aspect of the snoring sound. Natural or drug-induced sleep is required for its appearance. Snoring is subject to many inﬂuences such as body position, sleep stage, route of breathing and the presence or absence of sleep-disordered breathing. Its presentation may be variable within or between nights. While snoring is generally perceived as a social nuisance, rating of its noisiness is subjective and, therefore, inconsistent. Objective assessment of snoring is important to evaluate the effect of treatment interventions. Moreover, snoring carries information relating to the site and degree of obstruction of the upper airway. If evidence for monolevel snoring at the site of the soft palate is provided, the patient may beneﬁt from palatal surgery. These considerations have inspired researchers to scrutinize the acoustic characteristics of snoring events. Similarly to speech, snoring is produced in the vocal tract. Because of this analogy, existing techniques for speech analysis have been applied to evaluate snoring sounds. It appears that the pitch of the snoring sound is in the low-frequency range (<500 Hz) and corresponds to a fundamental frequency with associated harmonics. The pitch of snoring is determined by vibration of the soft palate, while nonpalatal snoring is more ‘noise-like’, and has scattered energy content in the higher spectral sub-bands (>500 Hz). To evaluate acoustic properties of snoring, sleep nasendoscopy is often performed. Recent evidence suggests that the acoustic quality of snoring is markedly different in drug-induced sleep as compared with natural sleep. Most often, palatal surgery alters sound characteristics of snoring, but is no cure for this disorder. It is uncertain whether the perceived improvement after palatal surgery, as judged by the bed partner, is due to an altered sound spectrum. Whether some acoustic aspects of snoring, such as changes in pitch, have predictive value for the presence of obstructive sleep apnea is at present not sufﬁciently substantiated. Ó 2009 Elsevier Ltd. All rights reserved.
q This work was in part supported by the special research fund BOF 011/033/04, grant of the Ghent University. * Corresponding author. University of Ghent, Faculty of Medicine and Health Sciences, Department of Internal Medicine, 25 Sint-Pietersnieuwstraat, 9000 Ghent, Belgium. E-mail address: dirk.pevernagie@ugent.be (D. Pevernagie).
These novel directions in the investigation of snoring may disclose additional clues relevant to clinical practice. apneas and hypopneas.31 Habitual loud snoring may be socially unacceptable. as a consequence of which nadir intrathoracic pressures may double or triple. Resumption of breathing is associated with a sequence of snores. or both). Some recent publications on this subject will be reviewed in this paper. or a combination thereof).9 From this study it was concluded that to a large extent snoring is ‘‘in the ear of the beholder’’. Moreover.38. age. It can schematically . Corresponding respiratory events that last at least 10 s are called obstructive hypopneas.36 Other non-surgical therapeutic interventions such as the use of mandibular advancement devices have become available in recent years. However. snorers signiﬁcantly increase inspiratory muscle effort. Furthermore. as such. although the variability is extremely large. an association was demonstrated between sleepiness and snoring sound intensity.19 The positive likelihood ratio from the combination of these symptoms may be sufﬁciently high to obviate the need for polysomnography in this age group. and even aggression and homicide.34 Female snorers may feel embarrassed and stigmatised by their nocturnal behaviour as snoring is intuitively associated with the male gender. Bed partners may have a signiﬁcantly impaired sleep quality30 and suffer from secondary sleep disorders.12 naturally occurring vs induced sleep. may be a useful and an easily accessible marker to screen for obstructive SDB. and this association was independent from the AHI. and are increasingly used for treatment of snoring. Because the perception of snoring is highly subjective. Pevernagie et al. divorce.27 Excessive negative intrathoracic pressure increases cardiac afterload by increasing myocardial transmural pressure28 and may facilitate gastro-esophageal reﬂux. airﬂow is decreased but not abolished. Intense ﬂutter of the upper airway structures may cause vibratory trauma. different target populations.10 the predominant sites of upper airway narrowing (palatal segment. Snoring may have several other side effects.14 With partial collapse of the upper airway. resulting in early inﬂammation21 and permanent damage of the pharyngeal tissues22. the vibratory activity of the pharyngeal airway is referred to as ‘simple snoring’. / Sleep Medicine Reviews 14 (2010) 131–144 prevalence. and therefore.29 Perhaps most importantly. For the listener it is quite obvious to discern snoring from other breathing sounds such as stridor and wheezing.18 Snoring.20 These ﬁndings would indicate that snoring is independently associated with daytime somnolence in some particular groups.37 Furthermore. It is subject to many inﬂuences.e.11 sleep stage and body position.25 To overcome increased upper airway resistance.24. smoking.19 Excessive daytime sleepiness and daytime fatigue were related to habitual snoring independent of the AHI. The loud rattling noise may keep spouses and people in adjacent bedrooms from falling or staying asleep. The number of apneas plus hypopneas per hour of sleep is the apnea–hypopnea-index (AHI).8 its characteristics cannot easily be deﬁned. excessive daytime sleepiness.. The quality of the sound is determined by many factors such as the route of breathing (oral. there is preliminary evidence to suggest that chronic exposure to loud snoring may predispose bed partners to presbyacusis. there is evidence to show that a substantial proportion of the chronic snorers seek medical help for their discomforting malady.13 and presence or absence of sleep-disordered breathing. It is known to be an important clinical hallmark of OSA15 and. Whilst self-reported snoring has limited accuracy in predicting the presence of OSA. obesity. Evidence is now accumulating that snoring by itself may be linked to daytime symptoms. During apneas there is no breathing sound. the most accepted estimate for the prevalence of chronic snoring is 40% in adult men and 20% in adult women.000 people undergo surgery to alleviate snoring. its presentation may change in the course of the sleep period and it may vary from night to night.16 Snoring is commonly regarded as a laughable circumstance or a source of irritation to the observer. and learning problems were found highly speciﬁc for SDB in 6. However. Snoring persists during these events and may show a crescendo pattern of increasing loudness.7 Subjectivity. In apneics. and not merely a proxy for sleep apnea. Combining results from all surveys that have been done so far.39 The aim of this article is to review the present state of scientiﬁc knowledge regarding acoustic assessment of snoring and to deal with the following topics:  Physical characteristics of snoring sound generation in the upper airway  Principles of acoustic measurement of sound  Advanced analysis and modelling of snoring sounds `  Acoustics of snoring assessed vis-a-vis clinical outcomes  Unresolved questions that are suitable for further research  Practice points Physical characteristics of sound generation in the upper airway Speech and singing is produced by the vibratory excitation of a hollow anatomic system called the vocal tract. supraglottic space.26. Early work has shown a poor correlation between measured loudness of snoring and subjective appreciation by different observers. snoring may bring to the surface the underlying lack of harmony. marital disharmony. The sound quality of these consecutive interapneic snores may vary markedly. i.23 and adjacent vessels. and sleep parameters in a populationbased sample of women. there is a need to objectively measure snoring if one wants to do accurate patient assessment and evaluate treatment effects. nasal. as annoyance has a broader psychoacoustic scope than the mere assessment of noise intensity and loudness. Snore-related phenomena may comprise a sound spectrum ranging from modest audible breathing to loud vibratory sounds that are readily perceived as ‘snores’ by human observers. ranging from 5 to 86% in the male and from 2 to 59% in the female population.7 Whilst snoring is a ubiquitous phenomenon that is also known to occur in animals.17 acoustical analysis of snoring seems to carry a better potential to discriminate between ‘simple snorers’ and patients suffering from OSA syndrome.15 When no apneas and hypopneas occur during sleep and the individual has no daytime complaints. Snoring is the audible sign of increased upper airway resistance. to establish a working deﬁnition that would provide a paradigm for the objective identiﬁcation and quantiﬁcation of snoring-related events has proven elusive so far.32 If it does not disrupt a harmonious relationship. the questionnaires used and whether or not the bed partner was asked to answer the questions may account for the inconsistency of these results.to 11-year old children.33 Moreover. snoring is a social nuisance. Whilst no information is available on prescription or consumption rates for the latter type of treatment. its intrinsic clinical relevance has become increasingly obvious in recent years. An apnea is deﬁned as a complete cessation of breathing of at least 10 s. snoring is not a homogeneous acoustic phenomenon.132 D.14 Sleepdisordered breathing (SDB) is characterized by the frequent occurrence of pathological respiratory events. about which little can be done but to awaken the unwitting culprit. The combination of an AHI ! 5 per hour and excessive daytime sleepiness is referred to as the obstructive sleep apnea (OSA) syndrome. and may constitute a reason for sleeping apart. tools to quantify the snoring-induced annoyance have to be developed. tongue base. this contention would imply that the phenomenon ‘snoring’ is to be acquitted from its connotation ‘simple’.35 In the UK each year more than 11.
face and external ﬁeld. 1. the energy is concentrated high up in the frequency band. Finally. Resonance peaks R1 and R2 add gain to speciﬁc frequencies of the harmonic spectrum.42 In speech and singing. Figure reproduced by courtesy of Joe Wolfe..41 The laryngeal sound source generates longitudinal compression waves in the vocal tract. 1) that deﬁnes the pitch of the voice. School of Physics. BSc Qld. The thoracoabdominal muscle-apparatus operates an air pressure system that drives the primary sound generator located within the pharynx. which ﬁlters the ‘buzzy vocal sound’ into a person’s particular voice. the vocal cords. The tract behaves like a variable ﬁlter (B). Each formant corresponds to a resonance level in the vocal tract. Pevernagie et al. / Sleep Medicine Reviews 14 (2010) 131–144 133 be presented as an acoustic box which is terminated at one end by the lips and nares and at the other end by the glottis. together with the radiation properties of the mouth. harmonic series.D.k. because of the physiological similarities and the availability of common methods for digital Fig. which is called a ‘formant’. The input signal and the vocal tract. Australia. Fig.a.43 The last two sound components are not emitted by vocal cord vibration. PhD ANU. The vibration of the vocal folds produces a varying airﬂow which may be treated as a periodic signal (A) that produces a spectrum of equally-spaced frequency peaks or harmonics. and /k/ are impulsive sounds which occur with the sudden release of air by using the tongue and lips. /t/. the quality of the voice is deﬁned by the articulatory system that comprises a set of movable structures. The sound produced by the vocal folds has a character of vibrating strings. jaw etc. Vibration of the vocal folds is the result of air passing through the narrowed glottis and is referred to as ‘phonation’. tongue. starting with a fundamental frequency (F0). as it consists of a fundamental frequency and harmonics or overtones (a. 1). the voice is characterized by a speciﬁc set of formants (F1 through F3). Therefore. i. The resonances R1 and R2 can be determined approximately from the peaks in the envelope of the sound spectrum. Narrowing of the glottic aperture is caused by adduction of the vocal cords and their surrounding folds. The result is a concentration of acoustic energy around a particular frequency. Speech is the generic name given to sounds that carry language content.e. BA UNSW.40. and is quite disorganized or ‘noise-like’ in its appearance. The University of New South Wales. They are produced in the laryngeal and supra-laryngeal part of the vocal tract and are referred to as ‘unvoiced sounds’. These peaks are called the formants (F1 and F2). Plosives. deﬁning its unique aspect and explaining the audible characteristics that typify different individuals (Fig. such as /p/.e.43 In recent years. different formants are produced simultaneously. many authors have approached snoring from the perspective of speech analysis. oral and nasal cavities) serve as a resonating system. produce a sound output (C). . The supralaryngeal upper airways (i. Speech has three basic components: voiced sounds. This source signal is input to the vocal tract. soft palate and jaw are the so-called ‘articulators’ that change the shape of the resonator in accordance with the subtle anatomical changes that are required for intelligible speech. The lips. pharynx. In unvoiced fricative sounds. The vocal folds emit the so-called ‘voiced sound’. Sydney 2052. Its response is different for different frequencies and the frequency response may be further adjusted by changing the position of the tongue. fricative sounds and plosive sounds. This ﬁltering is based on both attenuation and ampliﬁcation of the original laryngeal sound waves..
11 Snoring occurs in sleep. whilst for appropriate analysis of snoring sounds the use of air-coupled microphones is indispensable. A direct-current (DC) voltage. Capacitor microphones are high-end products.55 From this study it was concluded that contact microphones might be used in screening devices. may exhibit vibration or ﬂuttering (snoring). An ambient microphone should be placed in this free ﬁeld. Moreover. equally-spaced peaks of power (comblike spectrum). Capacitor (or condenser) microphones have a membrane or ﬂexible plate that moves with the air pressure variations. depending on mechanical properties of the pharynx. consisting of a movable wall in a channel segment that connects to the airway opening via a conduit with a resistance. The capacity changes that result from the movements of the membrane are converted into voltage signals. not by the vocal cords. whereas in the direct ﬁeld (or free ﬁeld) the relative amount of reﬂected sound is negligible. and correlated these properties with mechanical events relevant to the mechanisms of snoring. In the frequency domain. it is characterized by multiple.48 Gavriely and Jensen proposed a theoretical model of the upper airways. which is at the boundary between the direct and indirect ﬁeld. equallyspaced. In an early study. the principles of recording. have introduced a model implying two basic biomechanical concepts. Therefore. or may show repetitive closure.49 Liu et al. characterized the acoustic properties of snoring sounds in the time and frequency domains. The unit of sound pressure is pascal [Pa].44 Acoustical analysis of snoring and vocal sounds demonstrates that fundamental frequencies and harmonics can be observed in both phenomena.52 Principles of acoustic measurement of sound Sound is the vibration in a physical medium. The complexwaveform snores may result from colliding of the airway walls and represent actual brief airway closure. Snoring is caused by vibratory activity of pharyngeal structures. There is no articulation of the sound. the driving pressure is directed interiorly. furniture. They operate similarly to a condenser microphone but require no external polarizing voltage. SPL ¼ 20 log10 ðp=p0 Þ (1) Hence. applied the concept of structural intensity to a three-dimensional ﬁnite element model of a human head. 2). Pevernagie et al. The vibration causes a propagation of pressure waves that can be measured. Sound recording Sound waves that propagate in air are recorded with microphones. The capacitor consists of this membrane and a second plate which is ﬁxed. The analogy between snoring and speech lies in the fact that both are generated in the vocal tract. which is expressed in decibel (dB).45 They observed two dominant snoring patterns: the ‘simple-waveform’ and the ‘complex-waveform’. Their power spectrum contains only 1–3 peaks. Piezoelectric or ceramic microphones – the third class – have a membrane that is connected to a piezoelectric element. and ‘static divergence’ to give insight into the pharyngeal mechanisms of snoring.46 However.54 In a recent study it was shown that the recorded sound frequency range may depend on the placement of the microphone. as snoring is mainly associated with inspiration. / Sleep Medicine Reviews 14 (2010) 131–144 processing and analysis. which may be combined with noise generation by vibration of other structures. . tongue base and epiglottis. starting with a large deﬂection followed by a decaying amplitude wave. it is important that the received signal is for the major part the sound going directly from the nose and mouth to the microphone. is applied between the plates. as yet. there are several dissimilarities as well. Electret microphones are small-sized and low budget. Alternatively. The model predicts that. such as tonsils. The recording of sounds reﬂected by objects. during which the upper airway is in a passive state. p0 is equal to 20 mPa. namely ‘ﬂutter’ to describe palatal snoring. or in the ambient air.50 Such models may be further adapted to study various snoring mechanisms for different groups of patients. gas density.47–50 Huang et al. much attention should be given to the position of the microphone. and almost no secondary internal oscillations.49 The effects of different variables. To predict the snoring noise level as a function of a preset airﬂow loading. no recommendations as to standardization have been made. which is the average threshold of human hearing at 1 kHz. These impulses result from the repetitive opening and closure of the nasopharyngeal airway. ceiling. and airway dimensions on the pressure–ﬂow relationships were studied. known as the polarizing voltage. The reason to use a logarithm measure is that it ﬁts better to the human perception of loudness than a linear measure. and are the most commonly used type in acoustic laboratories. a three-dimensional boundary element cavity model of the upper airway was constructed. When the piezoelectric element moves a voltage is generated. of which the ﬁrst is the most prominent. Several theoretical models have been developed to deﬁne the relationship between the characteristics of snoring and the functional anatomy of the human upper airway. the walls of the upper airway may be stable (normal breathing). Commonly. These physiological differences should be kept in mind when applying speech techniques to the acoustic analysis of snoring. A sound signal can be captured with electronic equipment. Simple-waveform snores are of higher frequency and probably result from oscillation around a neutral position without actual closure of the lumen. the simple-waveform with tongue base snoring. Endoscopic appraisal of the pharyngeal structures during snoring has revealed that the complex-waveform is associated with palatal snoring.47.53 The positioning of the microphone to record snoring is highly variable in published literature and. Endoscopic evaluation of upper airway structures during snoring has revealed ﬂutter of the soft palate. The logarithm of this ratio is the Sound Pressure Level (SPL). With respect to measuring snoring. in the nasal cannula. By convention. The complex-waveform snore is characterized by a repetitive. which corresponds to the release of packets of sound energy. the microphone may be placed on the forehead. a given pressure level p is related to a reference pressure p0. and walls should be minimized.51 The harsh property of snoring is related to the ‘explosive’ feature of the sound production. processing and analyzing sound will be brieﬂy discussed (Fig. Possible locations for placement of the microphone include the skin regions of the larynx or trachea. Prepolarized or electret microphones are a second category. Simple-waveform snores have a quasi-sinusoidal waveform. Beck et al. with a range of variants. higher frequencies being lost with microphones that have contact with the skin. These devices convert air pressure variations into an electrical signal (Volt) and are grouped in three principal classes.50 They observed that pressure loads in the range of 20–60 Hz yield tissue vibrations mainly in the areas of the soft palate. The space where reverberation is relevant is called the indirect ﬁeld. such as airway wall compliance. train of sound structures. In the section below.134 D. like the bed. the tongue and the nasal cavity.45 The same holds true for formant features. an increase in the sound pressure of a factor 10 will result in a SPL increase of 20 dB. the range to the mouth being shorter than the critical or reverberation distance.
preferably 2.56 For a reference. and to remove high frequencies (by a low-pass ﬁlter). which may subsequently be used for several purposes. It is possible to ﬁlter or weight the microphone signal to reduce the inﬂuence of certain frequencies in the measured signal. or it can be applied in the subsequent off-line processing of the signal. They may be Sound analysis: physical sound strength measures The digitized sound record must be further processed to obtain a format that is useful for various purposes. An important aim of recording snoring sound is to obtain measures of snoring intensity and quality. Pevernagie et al. C. other measures have to be used. the sampling frequency must be equal to at least two times. etc. wav-ﬁle). anatomical site of snoring. is called the signal to noise ratio. The true snoring events are then futher classiﬁed into different categories that are relevant for speciﬁc clinical outcomes. The amount of noise ‘N’. respectively.g. and to decrease external sounds.14. Sound processing from source to clinical outcomes. The reason for using weighting is to mimic the perception of loudness. received by the microphone.g. The converted signal can be stored as a sound record on the PC (e. e.1 kHz comply with the musical CD standard and are widely available on PC-sound cards. A high-pass frequency of 20 Hz is largely sufﬁcient for acquiring relevant information of snoring sounds at the lower spectral end. like airconditioning noise and other non-snoring related sounds. The ﬁltered sound signals are subsequently digitized with an analog to digital converter (ADC).57 For a low-pass value of 3 kHz. like snoring. It samples the analog microphone signal at a frequency rate that is determined by the low-pass ﬁlter. time and spectral analysis. graphical plotting of the signal. a WAV-ﬁle). whereas a low-pass frequency of 3 kHz or higher is usually suitable for recording the higher frequencies.5 kHz.38 Loud breathing measured at a 1 m distance from the mouth can reach sound levels up to . but may produce quite large amounts of data when applied in all night recordings. or Dweightings. such as sound replay. it is preampliﬁed and ﬁltered. / Sleep Medicine Reviews 14 (2010) 131–144 135 Snoring: from sound to outcome micro phone preamplification A/ D Conversion (ADC) record replay process plot snoring events classification clinical outcomes analysis & model other events Fig. The digitized record is stored as a computer ﬁle (e. sound intensity assessment. one has to choose a sampling frequency of at least 7. Methods to improve the SNR are mounting the microphone close to the mouth. identiﬁcation and classiﬁcation of events. B. The sound is captured with a suitable microphone. the frequency range of an ordinary telephone connection has a limited bandwidth from 300 to 3400 Hz. For recording of snoring sounds a sampling frequency of 12 kHz is largely sufﬁcient. These two ﬁlters together are referred to as a band-pass ﬁlter. Filtering and conversion to the digital domain The signal picked up by the microphone is pre-ampliﬁed and submitted to electronic ﬁlters in order to remove very low frequencies (by a high-pass ﬁlter). The sound may be replayed. A is most often used as it reﬂects the human perception of sound pressures across the sound frequency range (Fig. (2) used instead of lower frequency samplers. Weighted sound intensity measurement. which should be sufﬁciently high. should be small with respect to direct snoring sound ‘S’. 2. The ratio SNR ¼ 20 log10 ðS=NÞ.. and prediction models for efﬁcacy of treatment. Converters sampling at 44. Before the analog signal is converted to a digital format. loudness and annoyance.5 times the low-pass frequency of the signal. The ultimate goal is to identify individual snoring events and to discriminate them from non-snoring sounds. For more complex signals. apneic versus non-apneic snoring. graphically plotted or further analyzed and classiﬁed using different kinds of mathematical models. 3). Their settings depend on the sound frequencies of interest. The ﬁlter may be included in the microphone ampliﬁer. using directional microphones which have a narrow sound recording angle. Calm breathing at a 10 cm distance from the mouth is barely audible and produces sound pressure levels of 25 and 17 dBA for inhalation and exhalation.D. Analog-to-digital conversion (ADC) means that the signal is sampled at a sufﬁciently high rate to enable adequate sound reproduction in the further process. This implies transformation of the signal into physical and mathematical data that can be interpreted. which are determined by the experimental objectives and modalities. To avoid spectral aliasing. Very common are the so-called A.58 The Aweighted SPL is denoted as dBA or dB(A). Loudness is a perceptive measure. In the following paragraphs some basic measurement methods will be discussed. preferably exceeding 20 dB. in contrast with technical and physical measures like the volt or dB discussed above.g. A simple signal like a sinusoid is fully characterized by its amplitude and frequency.
5]. L5 and L10 are the sound levels that are reached or exceeded 1%. a rating level (LR) can be computed that adds penalty points to LEq. Four plots in the time domain. The equivalent noise level or LEq. and certain sources and situations.2 s segments of a snore. percentile levels (LPer) are computed. the rms value is equal to amplitude A.. 3. Application of crest factor analysis is suitable to recognize palatal from nonpalatal snoring sounds (Fig. for example by the A-weighting. Pevernagie et al. In other words. . averaged over a time T.M.T is equal to the squared ratio of the signal and a reference signal p0. with time (sec) in the abscissa and sound amplitude (wV) in the ordinate. is deﬁned as the square root of the time average value of the square of a signal. / Sleep Medicine Reviews 14 (2010) 131–144 For a constant signal with amplitude A.59 RMS value and crest factor. which has the same level as a signal with a ﬁxed SPL. tone and information content. for factors that are known to increase annoyance. right). [From Aarts R. Vrms vﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ u n u1 X ¼ t V2 n i¼1 i (3) Fig.. the corresponding pﬃﬃﬃ rms value is equal to A/ 2. Very common are the so-called A. right) than in nonpalatal snoring (2. indicating the sound intensity level that is reached or exceeded Per% of the time.2. For a sinusoid with amplitude A. B.6.36 Equivalent noise level. top panel. or it can be applied in the further processing of the signal. For a constant signal with amplitude A. 5% and 10% of the time. If the signal is A-weighted. A common feature to these curves is that the weighting is equal to 0 dB. e. 40 dBA. They indicate the noise peaks in a sound recording. which value is proposed by some authors as the threshold for the transition between breathing and snoring. a microphone voltage.g. respectively. it is the average level of a timevarying signal. the crest factorﬃﬃﬃ equal to 1. sound impulses. this is expressed in dBA. The crest factor is the ratio between the highest absolute value of a signal divided by its rms value. The ﬁlter may be included in the microphone ampliﬁer. C.g. left). It is customary to compute the Fig. Snores associated with ﬂutter of the soft palate contain a series of impulses with high energy content.58 Fig. In order to quantify this aspect. Filtering or weighting the microphone signal reduces the inﬂuence of certain frequencies in the measured signal. bottom panel. A high crest factor may identify a ‘peaky-signal’. This A-weighted SPL is denoted as dBA or dB(A). e. For more complex signals this value may drop further. [Reproduced with permission from36]. 4). a ratio of peak sound amplitude (99th centile measurement – gray bars) to effective average sound amplitude (rms – hatched bars) measurement was calculated during 0.60 Spectrum and spectrogram. at 1 kHz. A is the most used one as it reﬂects the human perception of sound pressure across the sound frequency range.61 To get some insight into the statistical distribution of the signal’s intensity. Regular explosive peaks of sound at very low frequency can be seen during palatal snoring (top panel.. The root-mean-square (rms) value of a signal V. Moreover.60 If the signal is weighted.136 D. and then converted to a logarithmic value expressed in dB. This ratio is higher in palatal (4. The signal may contain several frequencies at the same time. left) but not during tongue base snoring (bottom panel. time of day. it is expressed in dBA.37. for a sinusoid p is with amplitude A the crest factor is 2. L1. 4. or unity gain. or Dweightings. For instance.
5 s. which can be plotted with the magnitude of the signal (dB) in the vertical axis and the frequency (Hz) in the horizontal axis (Fig. Annoyance may have particular relevance for snoring sounds. The peak frequency describes the harmonic with greatest magnitude. the tone color. This reference sound consists of a sinusoidal plane progressive wave of frequency 1 kHz that is administered directly in front of observers. The spectrogram can give valuable information of the spectrum in case of a time dependent spectrum. . This results in a spectrogram. 5C). C. psychoacoustic annoyance depends on the loudness. These techniques allow discrimination between true snoring events and other sounds. The center frequency describes the average sound frequency for the range of frequencies over which sound occurs.57 These p coefﬁcients are mathematically processed in a way that the model gives an optimal Fig. advanced methods for analysis and modelling of snoring sounds are addressed. Advanced analysis and modelling of snoring sounds In the following section. and the temporal structure of sounds.63 Basically. The event in A is shown after Fourier analysis. For instance. Amplitude of the sound in the time domain. 5A) to get the frequency spectrum of that signal. Connecting the peaks yields the spectral envelope. The vertical frequency axis is on a log scale. 5. The loudness level is expressed in phon. Linear predictive coding (LPC) is widely used in advanced speech analysis. The magnitude is coded as a color or gray value (Fig. where p is the order of the model. For time-varying signals. but also to the psychological state of the observer. as it is not only related to the characteristics of the noise. The gray scale is a measure of the intensity of the signal. which is not the case in plot A and B. Outside episodes of snoring there is silence represented by white columns. music. in addition to the physical and psychophysical measures.63 In this paragraph some of the important psychoacoustic measures will be addressed. A. and snoring. Pevernagie et al. Spectrogram. one can compute the frequency spectrum for successive parts of the signal. Loudness is expressed in sone. It is based on a mathematical model containing p coefﬁcients. it is the point at which the area under the graph is equal on both sides. The loudness level of a given sound is the equivalent SPL of a reference sound. as well as differentiation between sounds qualiﬁed as ‘simple snoring’ vs events belonging to the spectrum of obstructive sleep apnea. To highlight the differences between physical and psychophysical measures an overview is given in Table 1. Within each snoring sound epoch the fundamental frequency is equal to about 40 Hz. Sound analysis: psychophysical measures of noise Some aspects of assessment of strength and quality of sound involve the human ear and brain. It is adjusted up to a level where these observers judge that the reference sound becomes equally loud as the given sound. Three consecutive snores are shown occurring every 3. The discipline that studies the relationships between acoustic stimuli and hearing sensations is called psychoacoustics. a third level. Frequency (Hz) in the abscissa is plotted against power (SPL) in the ordinate. Frequency spectrum of the sound. The ﬁrst peak at about 40 Hz is called the fundamental frequency.D. Finally. where the dB reference level is arbitrary. subjective parameters are not routinely taken into account in the objective assessment of annoyance. Loudness is an attribute of the auditory sensation by which sounds may be ordered on a scale extending from soft to loud. 5B). the annoyance of a sound can be assessed. and can therefore not be expressed in pure physical measures. indicated by the reference bar ranging from a high (black 40 dB) to a low (white À60 dB) intensity. where the time axis is plotted horizontally and the frequency vertically. annoyance depends on psychophysiological aspects such as the mood of the listener and his or her relation to the snoring partner. like speech. The bar denotes the SPL in dB. The power ratio compares the relative amount of sound emanating below and above a set frequency. who are otologically normal persons. / Sleep Medicine Reviews 14 (2010) 131–144 137 Fourier-transform or the Power Spectral Density (PSD) of a time signal (Fig. Within a spectral plot different markers can be observed. Psychoacoustic annoyance (PA) can quantitatively describe annoyance ratings obtained in psychoacoustic experiments.62. The most apparent feature in C is that the three snores are clearly visible. Whilst the mental state of the listener is of signiﬁcant importance. The ﬁrst snore of a sequence of three. Frequency analysis of a simulated mechanical palatal snoring model. the peaks at higher frequencies are called harmonics. B.
i.). Finally. known as polyspectra. silence. Examples for the detection of sleep apnea from snoring sounds have recently been published. a need for objective assessment is obvious.64 Ng et al. duvet noise. As such. The accuracy of this method for snore episode detection was found to be 97.65 Spectral analysis performed by Fiz et al.2 (7.52. using the Fourier-transform. Polysomnography is the gold standard for diagnosis of this disorder. but without clearly identiﬁed harmonics. who found signiﬁcant differences between simple snorers and OSA patients.71 Snoring episodes show a regular pattern on the spectrogram. Recent investigations have demonstrated that with the application of suitable acoustic analysis techniques it is possible to differentiate palatal from nonpalatal snoring. To this goal. Physical Frequency [Hz] Sound pressure [Pa] Sound pressure level (SPL) dB Psychophysical Pitch [mel] Loudness [sone] Loudness level [phon] description of the input signal for a certain time segment or window. other noise (including car noises. In order to analyze speech. It is to some extent similar to the spectrogram as discussed above. the pitch will change as well. much interest is directed at identifying the primary sound-generating site in the upper airway. They found that 82–89% of snores could be identiﬁed correctly. or the lack thereof.71 Pitch is an important characteristic of speech.73 Recent studies have focused on the features of changing pitch in the characterization of snoring events.8% in OSA patients. pitch tracking is interesting because pitch is – like in the voice – an important attribute of snoring sounds. which can be ‘snoring’ or ‘no snoring’. Measuring snoring loudness and annoyance Hoffstein et al. The second pattern was characterized by a low-frequency peak with the sound energy scattered on a narrower band of frequencies.33 Systematic measurement in the sleep laboratory environment of snoring sound intensity in 1139 subjects revealed a mean (SD) LEq of 46. these models should be able to yield additional information on the nature of the snoring-related events and thus offer predictive value with respect to clinical outcomes. techniques for measurement of snoring loudness and annoyance have been employed. higher order statistics (HOS) were introduced with many applications in diverse ﬁelds including biomedicine. As such. revealed that there are two different patterns. If useful information on SDB can be revealed by analysis of snoring events. 1 phon is equal to 1 dB sound pressure level (SPL) at a frequency of 1 kHz.69 In recent years.72 In analogy with speech. The challenge is to determine the hidden parameters from the observable data. In acoustics.77 ` Acoustics of snoring vis-a-vis clinical outcomes Acoustic measurements of snoring have been applied in clinical medicine for various purposes.e. in particular for speech recognition. pitch is the time domain counterpart of the fundamental frequency. Because speech changes over time. and that it has a multi-resolution analysis and synthesis ability. known as cumulants. also tried to discriminate apneic from simple snorers using LPC techniques. one can transform the microphone signal directly to the frequency domain. Because the snorer is most often referred by the bed partner whose appreciation of the snoring noise is inherently subjective. / Sleep Medicine Reviews 14 (2010) 131–144 Table 1 Physical and psychophysical measures. The rest of the OSA patients showed the second pattern. a way to easy and affordable screening for OSA may be disclosed. Pevernagie et al. These ﬁndings surpass by far acceptable standards deﬁned by the WHO (World Health Organization) for interior sound pressure levels at night. It is believed that if the soft palate is the principal source of snoring. It is associated with the vibration frequency of the vocal cords and is an acoustic correlate to tone and intonation of speech. The mel scale. loudness is the subjective perception of sound pressure.74 During the last two decades.66 The ﬁrst pattern was characterized by the presence of a fundamental frequency and several harmonics. as ‘‘snoring is in the ear of the beholder’’. or labeling of the input signal.65 It is a statistical paradigm in which the system is modeled as a number of states with unknown parameters.75 These statistics. is a perceptual scale of pitches judged by listeners to be equal in distance from one another. breathing.138 D. The method of Hidden Markov Models (HMM) is an analysis technique that is widely used in classiﬁcation problems. By deﬁnition. or other classes. The sone is the unit of perceived loudness. A particularly fast and efﬁcient method is the fast Fourier-transform or FFT. The phon is the unit of perceived loudness level for pure tones. From the input signal features are derived and combined by a statistical framework that eventually leads to a decision. etc. A third method is wavelet analysis. certain tools can be employed for the analysis for snoring. and their associated Fourier transforms. segmentation and assessment of snoring sounds. The simple snorers and two of the 10 OSA patients showed the ﬁrst pattern. or between observers. It was concluded that perception of snoring is highly subjective and that validated methods to measure snoring had to be further developed. but is cumbersome and expensive to perform. Wavelet analysis has been carried out on snoring signals to explore clinical usefulness in detecting obstructive sleep apnea. and can easily be distinguished from other sound events. applied HMM to automatic detection. In particular.76. other sounds must be discarded. it is common to track the pitch and to display this over time.49). In Ear-Nose-Throat (ENT) medicine. Furthermore.68. reveal amplitude information about a process as well as phase information. were the ﬁrst to test subjective perception of snoring and to compare it with objective measurements in 25 snoring patients. Only the real snores should be selected in the process. The spectral envelope method has been used by Sola-Soler et al.78 While no data on the partner’s .3% of the patients. barking dogs.46 Instead of using LPC to acquire spectral information. Duckitt et al. The agreement between both observers in judging snoring severity was moderate (weighted Cohen’s kappa ¼ 0.59 L10 levels of 55 dBA were exceeded by 12. different models have been designed that allow distinction between snoring-related and other events. based on their characteristic sub-band energy distribution.91) dBA. objective assessment of snoring before and after therapeutic interventions should be carried out to evidence efﬁcacy of treatment. it is a method to estimate the signal’s PSD or its spectral envelope. acoustic researchers have tried to identify and quantify those characteristics of snoring sounds that point towards the coexistence of OSA.70 They recorded various sounds at the bedside and manually screened the recordings in the following classes: snoring.3% in simple snorers and 86.33 Comparison of the number of snores scored on polysomnography vs the count of the snores perceived at the replay of an audiotape by two technicians demonstrated that an agreement within 25% occurred in only 11 of 25 subjects. surgical therapy will be more successful.. Another way to make out snoring from nonsnoring events is to analyze the 500 Hz sub-band energy distributions in sound segments recorded during sleep.67 The advantage of wavelet analysis over the Fourier analysis is that it is better suited to representing functions that have discontinuities and sharp peaks. The average levels of snoring sound intensity were signiﬁcantly higher for men than for women. Furthermore. In the other patients there was a disagreement between either perceived vs objective snore counts. LEq ¼ 30 dBA and LMax ¼ 45 dBA.
028). respectively: 44.81 They demonstrated that both groups had a large low-frequency peak in SPL at around 80 Hz. The ﬁrst hint for acoustic differences between these two phenomena was provided by Perez Padilla et al. the OSA group displayed a substantially larger high frequency sound component. which were termed ‘intra-snore-pitch-jumps’ (ISPJ). additional investigation is required to assess the general clinical applicability of this particular score. The snoring events were modeled using a LPC technique. Free-ﬁeld snore sounds were acquired in 19 habitual snorers.. snoring periods are characterized by discontinuities in pitch. Thirtyone patients with a documented absent history of stopped breathing during sleep had signiﬁcantly lower average LEq values than 290 subjects with a known history of SDB.80 The ﬁrst attempt at objectively quantifying annoyance due to snoring noise was carried out by Cafﬁer and coworkers. It was found that the ratio of power above 800 Hz to power below 800 Hz could be used to separate snorers from patients with OSA. peak sound intensity was determined from the power spectrum in sixty male patients with suspected SDB and reported snoring.83 It appears that snoring loudness and SDB are correlated. Accordingly.78 The mainly affected parameters were LEq. All but one OSA patient and only one nonapneic snorer showed a peak frequency below 150 Hz. Another frequency pattern was characterized by a low-frequency peak with the sound energy scattered on a narrower band of frequencies.46. The scores that were observed in the 19 subjects substantially exceeded the prescribed limits deﬁned by WHO noise guidelines. maximum level (LMax). proposed a paradigm to solve the issue of deﬁning a snore.6) dBA (p ¼ 0. Apneic snores exhibited higher values than benign snores. McCombe et al. A very signiﬁcant difference in sound intensity was noted between apneic and nonapneic snoring patients.7–48. whose maximum was exceeded by up to 32 dBA. as well as the sleep of the bed partner. Finally. which shows little variation and little or no interruptions. It was concluded that the listeners were not stable in their own ratings and that the ratings also differed between listeners. and was associated with a signiﬁcantly lower peak frequency of snoring. a distinction is made between steady snoring. other sound and voice analysis techniques have shown differences between apneic and nonapneic snorers as well. and 236 normal and 429 postapneic snores from 8 OSAS patients.66 They observed the presence of a fundamental frequency and several harmonics in the simple snorers.2–47. It was concluded that acoustic signatures in snore signals carry information for the diagnosis of OSA. non-harmonic snoring noise pattern in OSA patients. and the irregular snoring that characterizes the resumption of breathing in between obstructive apneas.79. Whilst the LEq and the peak values for L1 and L5 were more than 5 dBA louder for apneic snoring patients with an RDI of !10 than they were for nonapneic snoring patients with an RDI of <10. 46.7 (95% CI. which are responsible for creating the vibratory sound particular to snores and which deﬁne the ‘pitch’ of snoring.39 Snoring as a marker of the coexistence of OSA By convention. Patients with OSA had residual energy at 1000 Hz. but also by psychological intricacies and the mental state of the observers. investigated snoring sounds of 30 apneic snorers (24 males and 6 females) and 10 benign snorers (6 males and 4 females). This pattern was present in the majority of OSA patients. the hypothesis has been tested that acoustic analysis of snoring may provide clues to the diagnosis of OSA syndrome. 41. Pevernagie et al. Moreover. In a recent study.82 In this study. and a threshold value of F1 ¼ 470 Hz that best differentiated apneic snorers from benign snorers. Patients with primary snoring revealed peak intensities between 100 and 300 Hz. Out of different sound characteristics and levels. Yet another investigation in which FFT analysis was applied conﬁrmed the presence of a high frequency. Patients with apnea showed a sequence of snores with spectral characteristics that varied markedly through an apnea-respiration cycle.14 They analyzed snoring noise from 10 nonapneic heavy snorers and 9 OSA patients. even when nonpostapneic snores were considered. In accordance with the previous study.52 They ﬁgured out that sounds perceived as ‘snores’ by humans are characterized by repetitively released packets of energy. it was found that the inter. In contrast with previous studies. which they named the ‘Berlin snore score’. OSA patients showed peak intensities above 1000 Hz. the listeners’ noise sensitivity seems at least equally relevant for the assessment of snoring annoyance as the snoring sound itself. especially with respect to F1. and the peak power was usually below 500 Hz. 1. / Sleep Medicine Reviews 14 (2010) 131–144 139 perception of nuisance were collected in this study.64 In a study by Sola-Soler et al.59 In recent years. mean and minimum SpO2) as well as body mass index correlated with peak intensity of the power spectrum. no cut-off point was proposed that would discriminate both groups from each other. two percentile levels for frequent maxima (L5 and L1) and snoring time.59 Patients with LEq values ! 38 dBA were 3.44 (95% CI.4) vs 47. no residual power in the higher frequency bands was observed in the OSA group.D. Quantitative differences were demonstrated between apneic and benign snores in the extracted formant frequencies F1. studied 10 OSA patients and 7 simple snorers. Polysomnographical data (AHI.46 Besides investigation of spectral properties of snoring events.39 Taking into account the changing properties of snoring events throughout the recording period. Methodological issues could have accounted for this discrepancy.3 (95% CI.44. whereas the nonapneic snorers did not. and included the following compounds: rating level (LR). However. The study yielded a sensitivity of 88%. The ﬁrst postapneic snore consisted mainly of broad-band white noise with relatively more power at higher frequencies. annoyance is not only determined by the noise itself. even after controlling for demographic and clinical factors. Abeyratne et al. several investigators have embraced the notion that snoring carries acoustic information on the presence of SDB. Their ﬁnal score graded objective acoustic annoyance on a scale from 0 to 100.99–5.95) times more likely to have an RDI ! 10 after controlling for different confounders. it was contended that the noise generated by snoring was sufﬁciently loud to disturb or disrupt a snorer’s sleep. they retrieved those parameters that were most relevant for annoyance. Data from formant analysis also point towards the presence of higher frequencies in patients suffering from SDB. employed thirdoctave sound analysis and calculated dB(A)/dB(SPL) for LMax in 9 OSA patients and 18 subjects with simple snoring. Fiz et al. it was shown that the mean annoyance caused by snoring differs from one kind of event to another.38 They adapted methods developed for environmental medicine and used psychoacoustic measures to establish a new scale.and intra-rater reproducibility of the annoyance scores was only moderate. LR and the standard values of brief noise peaks. but have thus far not been validated against relevant acoustic indices. Therefore.59 Questionnaires to evaluate and report the severity of snoring have been elaborated. analyzing 447 snores from 8 simple snorers. In a cohort of 1139 patients undergoing polysomnography. the levels of snoring sound intensity were signiﬁcantly associated with the respiratory disturbance index (RDI). or at least to the identiﬁcation of subjects who are at risk for having the disease. a speciﬁcity of 82%. Most of the power of snoring noise was below 2000 Hz. F2 and F3. The Berlin snore score was proposed for interindividual comparison and to evaluate effects of therapy.64 Ng et al. but without clearly identiﬁed harmonics. some snoring sounds being more ‘acceptable’ than others. signiﬁcant differences were found in formant frequencies variability between simple snorers and OSAS patients. ‘ISPJ .
87 In one study it was noticed that the palatal ﬂutter type of snoring occurred with the oral airway closed. deﬁned by an arbitrary cut-off value for AHI.0 Æ 5.96 In another study.2 Æ 25. Snores per sleep minute. Reported center frequencies are highly divergent (mean ﬁgures for palatal vs nonpalatal snoring): 420 vs 650.51 Whilst the acoustic spectrum of palatal vs nonpalatal snoring is clearly different. acoustic analysis techniques alone are insufﬁcient if certainty about monolevel snoring is required before initiating surgical treatment. This method thus carries the potential for the development of a suitable screening tool for OSA. that patients whose snoring is not associated with ﬂutter of the soft palate should be excluded from surgery. endorsed in contemporary ENT-literature. were unable to ﬁnd consistent results using spectral analysis methods or observed that snoring is not steady state and thus changes in the course of sleep.87 Therefore. The different trials that have compared anatomical locations of snoring with acoustic ﬁndings are summarized in Table 2. In one study. This technique was introduced by Croft and Pringle. they have a more ‘palatal’ character. To date.87 and 69 vs 117. Sound analysis requires neither sedation nor nasopharyngeal instrumentation.90 center frequencies.52 Other models have used variability of snore parameters. whilst energy ratios for low-frequency bands decreased signiﬁcantly as sedation levels increased. CPAP effectively controls snoring at therapeutic pressure levels. Comparative research of visual and acoustic assessment techniques has been incited by the belief that sound analysis by itself could disclose the different mechanisms of snoring.13.91 peak frequencies and power ratios. the cut-off value being 2.97 Each patient underwent a home sleep test at baseline and following 3 weeks using MAD.13 At each sedation level snoring sound was recorded. whereas the mouth was open in the tongue base type of snoring. the use of MAD resulted in a signiﬁcant decrease of the ‘spouse dissatisfaction scale’ but no signiﬁcant difference in the objectively assessed snoring index.51 The crest factor of these snores is higher than their tongue base counterpart. as in sleep nasendoscopy.2% (p < 0. as is explained below.01) and 26.87. they should be further developed in a way that severity of SDB can be predicted. When applied to a clinical database.91 Differences in acoustic analysis techniques or in the signal acquisition method may have accounted for these discrepancies.98 It is obvious that further randomized controlled studies with sufﬁcient power are needed to gauge the effect of MAD on snoring events.89 The authors inferred that the snoring mechanism may change in some individuals during the night and concluded that a single recording.95 However. overnight snore recordings and subsequent sleep nasendoscopic examination were performed using incremental steady-state sedation levels of propofol. respectively.51 391 vs 1094. in a recent prospective case series of 15 individuals with conﬁrmed simple tongue base snoring. showing a moderate effect of protriptyline on snoring in 14 nonapneic snorers. may not be representative.91 Therefore.86 and is widely employed in ENT medicine to evaluate whether the source of the snoring sound is palatal or not.51. and can therefore be applied during normal night time sleep. This has been shown consistently using different markers of spectral analysis.51.70. There is recent evidence to accept that induced sleep nasendoscopy is ﬂawed.140 D.0 Æ 25. with frequencies ranging from 20 to 100 Hz. and not merely the presence or absence of OSA. Furthermore.90.94 There are currently two frequently applied treatment modalities for snoring without signiﬁcant SDB: the use of mandibular advancement devices (MAD) and palatal surgery. corrected for time in apnea. 60 patients with a chief complaint of snoring with or without apnea were enrolled. including fundamental frequencies.93 Continuous positive airway pressure (CPAP) is the treatment of choice for OSA patients. As a conclusion. Literature on the effect of drugs is limited to one randomized controlled crossover trial. Anatomical site of snoring Palatal surgery is a treatment option for snoring. screening methods based on the analysis of snoring sounds must be validated in large target populations before they can be introduced for everyday clinical practice. High satisfaction rates have been reported in patients who continue treatment with MAD for a prolonged time.88 The crest factor in naturally occurring snoring changed signiﬁcantly in an overnight study in three out of ﬁve habitual snorers. ISPJ yielded OSA detection sensitivity of 86– 100% whilst holding speciﬁcity at 50–80%. A statistically signiﬁcant improvement was found in the number of snores per hour.0 (p < 0. Snoring loudness increased signiﬁcantly.97 In contrast. These drugs induce artiﬁcial sleep which may signiﬁcantly alter the physiological characteristics of natural snoring. / Sleep Medicine Reviews 14 (2010) 131–144 probability’ was introduced as a model to assess pitch jumps and seemed to correlate with the presence of OSA. randomized controlled clinical trials relating to snoring surgery. It was concluded that the subjective beneﬁt could have been due to a placebo effect. A signiﬁcant difference between natural snoring and snoring induced at the lowest sedation level was shown. Palatal surgery is frequently carried out for socially disturbing snoring.85 Such models can be adjusted to obtain maximum speciﬁcity with a sufﬁcient corresponding sensitivity to identify good candidates for subsequent polysomnography. however.87 In an elegant study in which 21 patients were enrolled. it appears that multisegmental snoring can hardly be distinguished from monolevel palatal snoring.0% to 9. 39 of whom had an AHI ! 10. the ﬁgures on the respective frequencies vary considerably in the literature. It is a common principle.13 Snoring sound analysis and outcomes of treatment for snoring Different medical and surgical options are available for the treatment of snoring. the effects of MAD on snoring and OSA were evaluated after a few months of treatment in 57 subjects with habitual loud snoring.87. maximum and average snoring loudness and the percentage of palatal snoring.36. Of . However. most of the evaluations of therapeutic efﬁcacy were based on questionnaires.36.0 Æ 6.8 and 42.87–92 The key observation of these studies is that the sound energy spectrum of palatal snoring lies in the lower frequency sub-bands.36 Palatal snoring is characterized by a series of high energy impulses. there are only few rigorously performed. and sound intensity of snores (% snores ! 50 dB) decreased with MAD from 11. Pevernagie et al. whereas tongue base snoring has an energy content in the higher frequency regions.13. Naturally occurring snores have a higher energy content in the low-frequency sub-bands than snores induced by either midazolam or propofol. Moreover. Only few trials involved actual measurement of snoring noise. It is likely that sedating agents add a tongue base component to snoring during induced sleep.96 Snores were scored where inspiratory noise was greater than 5 dB above background. Some authors. The commonly used term ‘sleep nasendoscopy’ is misleading because either midazolam or propofol are administered during these procedures.84 and logistic regression fed by several parameters from the time and frequency domains.51 It is nowadays regular practice to assess the primary snoring site using nasendoscopic examination of the pharynx under intravenous sedation. MAD increase oropharyngeal and hypopharyngeal dimensions and may improve snoring and mild forms of SDB.01). doubt was casted on the pretence that the technique of sleep nasendoscopy is a suitable predictor for the outcome of snoring surgery.
MPR: mean power ratios.D.8 Hz Combined: 115. tonsils.7 Æ 144.4) Nasendosopy using midazolam and incremental doses of propofol in comparison with acoustic analysis to distinguish palatal.8 Æ 34. meso-. and hypopharynx and esophagus Acoustic analysis of snoring sound is useful as a supportive screening method to diagnose the site and grade of upper airway obstruction during sleep Intraluminal pressure measurement and dynamic image recordings are most accurate to assess the level of UA obstruction 1999 Hill88 11 adults Sleep nasendoscopy using midazolam with direct visual conﬁrmation of snoring site. . This method is possibly useful as a noninvasive diagnostic technique 2000 Hill89 3 male and 2 Naturally occurring snores (15) were recorded at In some recordings crest factor The snoring mechanism may change in female different times in the same night showed good reproducibility. but no differences in PF Induced snores showed a higher frequency component then natural snores Data on CF showed values <90 Hz for Acoustic analysis will unlikely replace sleep palatal and >90 Hz for tongue base nasendoscopy snoring. / Sleep Medicine Reviews 14 (2010) 131–144 141 Table 2 Acoustic ﬁndings in snoring produced at different anatomical sites. Publication First date author 1990 Schafer92 Subjects 5 children with SDB 1 adult with chronic snoring Methods and outcomes A comparison between time series of spectral density and mean power spectra Findings Simple snorer: frequency spectrum with a low-frequency component and a lot of harmonics Children with apnea: a lack of lowfrequency components and harmonics Discussion ‘Simple snoring’ in the adult is caused only by vibrations of the soft palate ‘Apneic’ snoring in the children has a pathomechanism of enlarged adenoids. with or subjects others marked changes were without a change of the snore site Crest factor was calculated observed an hour apart A single recording. CF did not identify multisegmental snoring Blind assessment of waveforms of individual snores gave poor accuracy and poor interobserver agreement Loudness (dBA) increased signiﬁcantly with increasing sedation levels Energy ratios for different lowfrequency sub-bands decreased when sedation level increased A signiﬁcant difference between natural snoring and snoring induced at the lowest sedation level was shown No signiﬁcant effects were found regarding snore duration and periodicity (%) There is a signiﬁcant difference in acoustic characteristics between sedation-induced snores and natural snores Further research is needed to determine if sleep nasendocopy is a valid predictor of outcome of snoring surgery 2002 Agrawal87 16 subjects 2004 Saunders91 27 male and 8 female subjects (mean AHI 10.70) The nonpalatal group consisted of epiglottic (1). but in some individuals during the night. hypopharyngeal (2) and tongue base snoring (2 subjects) 1998 Miyazaki90 65 male and 10 female subjects Daytime polysomnography after administration of zopiccone and simultaneous recordings of intraluminal pressures of the UA Comparison of snoring sound with the amplitude of the respiratory pressure swings at the level of the epi-. UA: upper airway. steady-state sedation levels of propofol SDB: sleep-disordered breathing. causing an impeded movement of the soft palate Craniofacial anomalies are characterized by special spectral patterns Distinct difference of the waveform and frequency patterns between palatal ﬂutter snoring and tongue base snoring Differentiation between different snoring sites seems possible 1996 Quinn51 10 subjects with nonapneic snoring Sleep nasendoscopy (sedative used not mentioned) and simultaneous snore sound recording Analysis of waveform and frequency to differentiate the snore sites 6 subjects: snoring at the level of the soft palate 3 subjects: snoring at the tongue base level 1 subject: snoring at soft palate and tongue base levels Snoring type strata resulting from intraluminal pressure measurement: Soft palate: 28 subjects Tongue base: 14 subjects Combined: 27 subjects Larynx: 6 subjects Fundamental frequencies of the snores: Soft palate: 102. as in sleep nasendoscopy may not be representative for showing the sites of snoring overnight Nasendoscopy using either midazolam or propofol to observe the site of snoring in comparison with recording of natural and sleep induced snores to compare their frequency spectra Signiﬁcant differences in MPR and CF Nasendoscopy may not accurately reﬂect between induced and natural snores natural snoring for 12 palatal snorers. Pevernagie et al.7 Æ 58. tongue base and multisegmental snoring 2006 Jones13 20 male and 1 female subjects (AHI < 15) Comparison of the acoustical analysis between the inspiratory sound of natural snoring versus sedation-induced snoring (propofol) The nasendoscopy was performed at sequentially increasing.9 Hz Tonsils/tongue: 331. combined with crest factor calculation to measure the degree of modulation of the palatal snoring loudness The palatal and nonpalatal snorers can be distinguished by their crest factor. CF: center frequency: PF: peak frequency.9 Hz Larynx: Æ250 Hz Crest factor was signiﬁcantly higher in the palatal group of 6 subjects (cut-off value: 2.
99. Moreover. comprising the inspiratory sound of the ﬁrst 100 snores whilst sleeping in a supine position. Loud snoring in the absence of SDB may produce upper airway inﬂammation. 2. . 0 to 250-Hz energy ratio measurements maintained a statistically signiﬁcant improvement at the time of the late post-operative recording. Perceptual issues and relational attitudes may be relevant. Whilst no patient was cured from snoring. and Roy Raymann from Philips Research. 3.  That snoring without apneas and hypopneas has no medical relevance is an oversimpliﬁcation. despite an obvious drift back to pre-operative levels. Okke Ouweltjes.and post-operatively for audio recording of snoring sound and video recording of sleeping position. It is recommended to perform some kind of acoustical evaluation before and after treatment with MAD or palatal surgery.101. one should take into account the psychological factors that to some extent make up the degree of annoyance. sufﬁcient reliability about the site of snoring can be assumed.  It is widely accepted that subjects who show palatal snoring are the best candidates for palatal surgery.103 They were admitted pre. Daytime sleepiness in regular heavy snorers. The authors concluded that following palatal surgery changes in the acoustic parameters of snoring sound are demonstrable but short-lived. an operational deﬁnition of ‘snoring’ should be provided. Results from druginduced sleep nasendoscopy should be corroborated with adequate nocturnal recordings of naturally occurring snores..  There may be a role for acoustic interventions in the management of socially disturbing snoring.  Concealed acoustic information in snoring events that points to the presence of SDB is an ongoing line of research. Jones et al. Ghent. Research agenda  In the literature.142 D. J Appl Physiol 1985. selection of ‘palatal types’ of snoring patients for surgery may be superﬂuous.  If acoustic assessment techniques show only mild to moderate snoring.and early post-operative recordings for snore periodicity and energy ratios in the low-frequency ranges. The Netherlands. which would be in contrast with serious complaints by the bed partner. Only if an agreement between these two evaluation methods is found. periodicity (%) and energy ratios for low-frequency bands.99. 2000. Measuring sound pressure levels and showing snoring events in the time domain can be easily performed with commercial devices. loudness (dBA). data on sound quality of different types of snores are inconsistent. were analyzed for snore duration (s). tongue base and multilevel types of snoring are lacking. is still in a stage of pioneering. investigated the effectiveness of palatal surgery for nonapneic snoring in 35 patients. Standardization of hard.99(1):40–8. Moreover. Airway resistance and respiratory muscle function in snorers during NREM sleep. only few have utilised acoustic changes of snoring sound as objective outcome measurements. Skatrud JB. This contention is still an unproven hypothesis. and. the acoustic information in snoring that occurs during obstructive hypopneas (e.  Most publications on the effects of palatal surgery disclose large differences between subjective (questionnaires) and objective (sound measurement) assessments of residual snoring. Belgium for reading the manuscript and providing pertinent suggestions. An international Task Force of physicians and engineers should elaborate guidelines on recording and analysis of snoring sounds.59(2):328–35. though expensive and time-consuming. Counting the number of snores on polysomnography is feasible with modern equipment. A trial with CPAP-treatment may be warranted to prove this cause-effect relationship. but the degree of SDB. References 1. The subjective and objective results correlated poorly. ‘‘Is there a correlation between snoring events suspect for SDB and the AHI?’’ could be a relevant research question. Stoohs R. Dempsey JA. some techniques can by now be applied in clinical practice.104. Active noise reduction is a recently developed technology that is employed to diminish ambient noise (e. especially in children and women. is not the presence or absence. Appropriate methods for unequivocal classiﬁcation of snoring vs other events are to be elaborated. in aviation).and software as well as type and placement of microphones is necessary to allow reproducibility of data recording and comparison between groups.102 In a carefully designed study.  Induced sleep is not natural sleep. This possibility deserves further investigation. Sound ﬁles. Pevernagie et al. Duncan S. and Johan Rijckaert from Artevelde Hogeschool. 4th ed. Only the Practice points  While the acoustics of snoring. John Lamb. acoustic analysis methods have so far proven unreliable regarding prediction of treatment response to palatal surgery. as a medical science. even though subjective improvement was observed by some patients and their bed partners. Boston: Houghton Mifﬂin Company. This may be explained as a placebo effect or perceptual adjustment of the observer. Whilst beneﬁcial effects on objectively measured snoring were described in two publications. may be a cause of excessive daytime somnolence. Psychoacoustic analysis of snoring sounds. Guilleminault C. The American HeritageÒ dictionary of the English language. The question. the crescendo pattern of the sound during consecutive inspiratory phases) is still a virgin territory. could be used as a more precise way to describe the symptom of snoring. Comparative studies on the effects of palatoplasty in palatal. Eindhoven.g. Psychological counselling may be indicated in some selected cases.105 Acknowledgements The authors whish to thank Werner de Bruijn. A considerable lack of uniformity in methodological approach may account for this observation. however. / Sleep Medicine Reviews 14 (2010) 131–144 those that do exist.. and that may ﬁnd an application in masking the snoring volume of the bed partner. Until clear differences in surgical outcomes between these groups are demonstrated. Chest 1991. Finally. Snoring (I).g. statistically signiﬁcant changes were found between pre.100 most studies reported no signiﬁcant reduction of snoring sound intensity. * The most important references are denoted by an asterisk. An alternative account could be the eventuality that subtle changes in quality of the noise may signiﬁcantly reduce the annoying effect of snoring.
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