Proceedings Fonetik 2009 - Institutionen för lingvistik

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Proceedings, FONETIK 2009, Dept. of Linguistics, Stockholm UniversityEffects of vocal loading on the phonation and collisionthreshold pressuresLaura Enflo 1 , Johan Sundberg 1 and Friedemann Pabst 21 Department of Speech, Music & Hearing, Royal Institute of Technology, KTH, Stockholm, Sweden2 Hospital Dresden Friedrichstadt, Dresden, GermanyAbstractPhonation threshold pressures (PTP) havebeen commonly used for obtaining a quantitativemeasure of vocal fold motility. However, asthese measures are quite low, it is typically difficultto obtain reliable data. As the amplitudeof an electroglottograph (EGG) signal decreasessubstantially at the loss of vocal foldcontact, it is mostly easy to determine the collisionthreshold pressure (CTP) from an EGGsignal. In an earlier investigation (Enflo &Sundberg, forthcoming) we measured CTP andcompared it with PTP in singer subjects. Resultsshowed that in these subjects CTP was onaverage about 4 cm H 2 O higher than PTP. ThePTP has been found to increase during vocalfatigue. In the present study we compare PTPand CTP before and after vocal loading insinger and non-singer voices, applying a loadingprocedure previously used by co-authorFP. Seven subjects repeated the vowel sequence/a,e,i,o,u/ at an SPL of at least 80 dB @0.3 m for 20 min. Before and after the loadingthe subjects’ voices were recorded while theyproduced a diminuendo repeating the syllable/pa/. Oral pressure during the /p/ occlusionwas used as a measure of subglottal pressure.Both CTP and PTP increased significantly afterthe vocal loading.IntroductionSubglottal pressure, henceforth P sub , is one ofthe basic parameters for control of phonation. Ittypically varies with fundamental frequency ofphonation F0 (Ladefoged & McKinney, 1963& Cleveland & Sundberg, 1985). Titze (1992)derived an equation describing how the minimalP sub required for producing vocal fold oscillation,the phonation threshold pressure(PTP) varied with F0. He approximated thisvariation as:PTP = a + b*(F0 / MF0 ) 2 (1)where PTP is measured in cm H 2 O and MF0 isthe mean F0 for conversational speech (190 Hzfor females and 120 Hz for males). The constanta = 0.14 and the factor b = 0.06.Titze’s equation has been used in severalstudies. These studies have confirmed that vocalfold stiffness is a factor of relevance to PTP.Hence, it is not surprising that PTP tends to riseduring vocal fatigue (Solomon & DiMattia,2000 & Milbrath & Solomon, 2003 & Chang &Karnell, 2004). A lowered PTP should reflectgreater vocal fold stiffness, which is a clinicallyrelevant property; high motility must be associatedwith a need for less phonatory effort for agiven degree of vocal loudness.Determining PTP is often complicated. Onereason is the difficulty of accurately measuringlow values. Another complication is that severalindividuals find it difficult to produce theirvery softest possible sound. As a consequence,the analysis is mostly time-consuming and thedata are often quite scattered (Verdolini-Marston et al., 1990).At very low subglottal pressures, i.e. in verysoft phonation, the vocal folds vibrate, but withan amplitude so small that the folds never collide.If subglottal pressure is increased, however,vocal fold collision normally occurs. LikePTP, the minimal pressure required to initiatevocal fold collision, henceforth the collisionthreshold pressure (CTP), can be assumed toreflect vocal fold motility.CTP should be easy to identify by means ofan electroglottograph (EGG). During vocal foldcontact, the EGG signal can pass across theglottis, resulting in a high EGG amplitude.Conversely, the amplitude is low when the vocalfolds fail to make contact. In a previousstudy we measured PTP and CTP in a group ofsingers before and after vocal warm-up. Theresults showed that both PTP and CTP tendedto drop after the warm-up, particularly for themale voices (Enflo & Sundberg, forthcoming).The purpose of the present study was to explore24
Proceedings, FONETIK 2009, Dept. of Linguistics, Stockholm Universitythe potential of the CTP measure in female andmale subjects before and after vocal loading.MethodExperimentSeven subjects, two female (F) and five male(M), were recruited as subjects. One female andone male were amateur singers, one of themales had some vocal training while the remainingsubjects all lacked vocal training.Their task was to repeat the syllable [pa:] withgradually decreasing vocal loudness and continuinguntil voicing had ceased, avoiding emphasisof the consonant /p/. The oral pressureduring the occlusion for the consonant /p/ wasaccepted as an approximation of Psub. The subjectsrepeated this task three to six times on allpitches of an F major triad that fitted into theirpitch range. The subjects were recorded in sittingposition in a sound treated booth.Two recording sessions were made, one beforeand one after vocal loading. This loadingconsisted of phonating the vowel sequence/a,e,i,o,u/ at an SPL of at least 80 dB @ 0.3 mduring 20 min. All subjects except the twosingers reported clear symptoms of vocal fatigueafter the vocal loading.Audio, oral pressure and EGG signals wererecorded, see Figure 1. The audio was pickedup at 30 cm distance by a condenser microphone(B&K 4003), with a power supply (B&K2812), set to 0 dB and amplified by a mixer,DSP Audio Interface Box from (Nyvalla DSP).Oral pressure was recorded by means of a pressuretransducer (Gaeltec Ltd, 7b) which thesubject held in the corner of the mouth. TheEGG was recorded with a two-channel electroglottograph(Glottal Enterprises EG 2), usingthe vocal fold contact area output and a lowfrequency limit of 40 Hz. This signal wasmonitored on an oscilloscope. Contact gel wasapplied to improve the skin contact. Each ofthese three signals was recorded on a separatetrack of a computer by means of the SoundswellSignal Workstation TM software (Core 4.0,Hitech Development AB, Sweden).Figure 1: Experimental setup used in the recordings.The audio signal was calibrated by recording asynthesized vowel sound, the sound pressurelevel (SPL) of which was determined by meansof a sound level recorder (OnoSokki) held nextto the recording microphone. The pressure signalwas calibrated by recording it while thetransducer was (1) held in free air and (2) immersedat a carefully measured depth in a glasscylinder filled with water.AnalysisThe analysis was performed using the SoundswellSignal Workstation. As the oral pressuretransducer picked up some of the oral sound,this signal was LP filtered at 50 Hz.After a 90 Hz HP filtering the EGG signalwas full-wave rectified, thus facilitating amplitudecomparisons. Figure 2 shows an exampleof the signals obtained.Figure 2: Example of the recordings analyzed showingthe audio, the HP filtered and rectified EGG andthe oral pressure signals (top, middle and bottomcurves). The loss of vocal fold contact, reflected asa sudden drop in the EGG signal amplitude, ismarked by the frame in the EGG and pressure signals.25
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