Voice restoration with voice prosthesis after total laryngectomy. Assessment of survival time of 378 Provox-1, Provox-2 and Blom-Singer voice prosthesis]

Voice Restoration with Voice Prosthesis after Total Laryngectomy. Assessment of Survival Time of 378 Provox-1, Provox-2 and Blom-Singer Voice Prosthesis. BACKGROUND: Indwelling voice prosthesis are used in Trier for voice restoration after total laryngectomy since 1991. OBJECTIVE AND METHOD: To assess the voice prosthesis survival times the patients of the years 1993-1999 are assessed retrospectively. PARTICIPANTS: 58 patients provided with indwelling voice prosthesis were seen regularly at follow-up. 378 prosthesis were changed. Provox 1, Provox 2 and Blom-Singer-Prostheses were used. RESULTS: The average survival lifetime of the prosthesis was 224 days for Provox-1, 96 days for Provox-2 and 107 days for Blom-Singer respectively. There is no significant difference found between Provox-2 and Blom-Singer Prosthesis. The survival times of the Provox-1 Prosthesis is significant longer. CONCLUSION: Using indwelling voice prosthesis for voice restoration after total laryngectomy an average survival time of the prosthesis of three months can be expected. There are relevant individual differences. Provox-1 Prosthesis have a significantly longer survival time, but as they are more difficult to handle they are not suitable for routine use. The indication for the choice between Blom-Singer or Provox-2 Prosthesis should be influenced by the surgeons' experience.     

A new problem-solving indwelling voice prosthesis, eliminating the need for frequent Candida- and "underpressure"-related replacements: Provox ActiValve

OBJECTIVES: To develop and clinically assess a new prosthesis for voice rehabilitation after total laryngectomy that solves the problem of frequent Candida- and "underpressure"-related replacements. MATERIAL AND METHODS: We designed a voice prosthesis with a new valve mechanism, applying Candida-resistant fluoroplastic (Teflon-like) material for the valve and valve seat and magnets to generate an active closing force, preventing inadvertent opening of the valve during swallowing or deep inhalation. Several prototypes were tested in 13 laryngectomized patients and, subsequently, the final design was assessed in a prospective clinical trial in a cohort of 18 patients with a short device lifetime of their standard indwelling voice prosthesis (mean 30 days). RESULTS: Prototype testing and the long-term clinical trial confirmed that the new valve material remained free of Candida growth and that the use of magnets can prevent inadvertent opening of the valve during swallowing and/or deep inhalation. This resulted in a highly significant increase in device lifetime in the 18 laryngectomized patients in the prospective trial (14-fold increase on average, range 3-39-fold; p < 0.001). Lubrication with special medical-grade fluoridated silicone oil is favorable in patients who experience possible adhesion of the valve to the valve seat. CONCLUSIONS: This new voice prosthesis, the Provox ActiValve, represents a solution for patients who have the problem of requiring very frequent voice prosthesis replacements due to excessive Candida growth and/or inadvertent opening of the valve by swallowing and inhalation-related underpressure in the esophagus.     

Are modern voice prostheses better? A lifetime comparison of 749 voice prostheses

The aim of the study was to compare device life of more recent indwelling voice prostheses Provox Vega and Blom-Singer Dual Valve to device life of well-known standard devices (Provox 2, Blom-Singer Classic). In a prospective, non-randomised study, device life of Blom-Singer Classic, Blom-Singer Dual Valve, Provox2, Provox Vega and Provox ActiValve voice prostheses was recorded in a group of 102 laryngectomised patients. In total 749 voice prosthesis were included. Average overall life time was 108 days, median 74 days. The prosthesis with the longest dwell time was the Provox ActiValve (median 291 days). Provox Vega had longer device life compared with Provox2 (median 92 days vs 66 days; p = 0.006) and compared with Blom-Singer Classic (median 92 days vs 69 days; p = 0.004). In conclusion, device lifetimes of Provox Vega and ActiValve were better than those of Provox2 and the Blom-Singer Classic. New voice prostheses, with a defined valve opening pressure (Provox Vega, Provox ActiValve, Blom-Singer Dual Valve) had longer lifetimes than prostheses without a defined opening pressure (Blom-Singer Classic and Provox 2).     

A New Problem-solving Indwelling Voice Prosthesis,Eliminating the Need for Frequent Candida- and “Underpressure”-related Replacements: Provox ActiValve

Objectives—To develop and clinically assess a new prosthesis for voice rehabilitation after total laryngectomy that solves the problem of frequent Candida- and “underpressure”-related replacements. Material and Methods—We designed a voice prosthesis with a new valve mechanism, applying Candida-resistant fluoroplastic (Teflon-like) material for the valve and valve seat and magnets to generate an active closing force, preventing inadvertent opening of the valve during swallowing or deep inhalation. Several prototypes were tested in 13 laryngectomized patients and, subsequently, the final design was assessed in a prospective clinical trial in a cohort of 18 patients with a short device lifetime of their standard indwelling voice prosthesis (mean 30 days). Results—Prototype testing and the long-term clinical trial confirmed that the new valve material remained free of Candida growth and that the use of magnets can prevent inadvertent opening of the valve during swallowing and/or deep inhalation. This resulted in a highly significant increase in device lifetime in the 18 laryngectomized patients in the prospective trial (14-fold increase on average, range 3–39-fold; p<0.001). Lubrication with special medical-grade fluoridated silicone oil is favorable in patients who experience possible adhesion of the valve to the valve seat. Conclusions—This new voice prosthesis, the Provox ActiValve, represents a solution for patients who have the problem of requiring very frequent voice prosthesis replacements due to excessive Candida growth and/or inadvertent opening of the valve by swallowing and inhalation-related underpressure in the esophagus.     

The long-term indwelling tracheoesophageal prosthesis for alaryngeal voice rehabilitation

OBJECTIVE: To analyze the initial experience at Oregon Health Sciences University, Portland, with the use of long-term indwelling tracheoesophageal voice prostheses. DESIGN: Retrospective case series. SETTING: Tertiary referral academic medical center. PATIENTS: Thirty patients undergoing speech rehabilitation after laryngectomy during a period of 18 months. INTERVENTION: Insertion of a long-term indwelling tracheoesophageal voice prosthesis. MAIN OUTCOME MEASURES: Duration of use, complications. RESULTS: The mean duration of placement for a single prosthesis was 4.9 months (148 days), with a range of 14 to 330 days. Sixteen of the 30 patients encountered problems with leakage because of fungal colonization, the majority of which (15 of 16 cases) were solved with either oral or topical application of nystatin. Size matching in terms of prosthesis length and tract length was critical, and problems of this nature were encountered in 11 of 30 patients. The incorporation of a second system of prostheses that offered an increased number of size options solved these problems in all of these patients. Ultimately, 27 of 30 patients were able to successfully wear these prostheses. CONCLUSIONS: The indwelling tracheoesophageal voice prosthesis offers patients all the advantages of tracheoesophageal speech rehabilitation after laryngectomy without the inconvenience of frequent prosthesis changes. With careful attention to the details of fitting and care, it can be worn by the majority of patients successfully.     

Long-term results of Provox ActiValve, solving the problem of frequent candida- and "underpressure"-related voice prosthesis replacements

OBJECTIVES: To assess the long-term results of the Provox ActiValve, a prosthesis for voice rehabilitation after total laryngectomy that was designed to lessen the need for frequent replacements caused by Candida and "underpressure." PATIENTS AND METHODS: Retrospective assessment of device lifetime, indications for replacement, voice quality, and maintenance issues, measured by a structured trial specific questionnaire, in a cohort of 42 laryngectomized patients, experiencing a short Provox2 device lifetime (median, 21 days). RESULTS: The median device lifetime of Provox ActiValve, replaced for leakage through the device and those still in situ at the date of data collection (N = 32), was 337 days (mean 376 days): a statistically significant 16-fold increase compared to the Provox2 prosthesis (P < .001). In 10 patients, replacement was fistula-related (median after 86 days): esophageal pouch (N = 4), fistula granulation (N = 3), extrusion of the device (N = 2), and periprosthetic leakage (N = 1). Eighty-six percent of the patients used a special lubricant to diminish "stickiness" of the valve. Provox ActiValve was preferred by 90% of the patients who completed the trial-specific questionnaire. CONCLUSIONS: For patients requiring frequent device-related replacements, Provox ActiValve, also long-term, provides a true solution and thereby is a valuable addition to prosthetic voice rehabilitation.     

A decade of postlaryngectomy vocal rehabilitation in 318 patients: a single Institution's experience with consistent application of provox indwelling voice prostheses

OBJECTIVE: To assess long-term results with consistent use of indwelling voice prostheses (Provox; Atos Medical AB, H?rby, Sweden) for vocal rehabilitation after total laryngectomy. DESIGN: Retrospective clinical analysis. SETTING: Comprehensive national cancer center. PATIENTS: Three hundred eighteen patients (261 men and 57 women; mean age, 62 years) from November 1988, through May 1999. INTERVENTION: Standard wide-field total laryngectomy (287 patients) or total laryngectomy with circumferential pharyngeal resection (31 patients), and 2700 prosthesis replacements. Prostheses remained in situ during 364,339 days (1000 patient-years). MAIN OUTCOME MEASURES: Device lifetime, indications for replacement (device or fistula related), adverse events, and voice quality. RESULTS: Median patient-device follow-up was 67 months. Mean actuarial device lifetime for all indications for replacement was 163 days (median, 89 days). Main indications for replacement were device-related, ie, leakage through the prosthesis (73%) and obstruction (4%), or fistula-related, ie, leakage around the prosthesis (13%), and hypertrophy and/or infection of the fistula (7%). Adverse events occurred in 11% of all replacements in one third of the patients, mostly solvable by a shrinkage period, or adequate sizing and/or antibiotic treatment. Definitive closure of the tracheoesophageal fistula tract occurred in 5% of the patients. Significant clinical factors for increased device lifetime were no radiotherapy (P =.03), and age older than 70 years (P<.02). Success rate with respect to voice quality (ie, fair to excellent rating) was 88%, which was significantly influenced by the extent of surgery (P<.001). CONCLUSION: The consistent use of indwelling voice prostheses shows a high success rate of prosthetic vocal rehabilitation, in terms of the percentage of long-term users (95%), and of a fair-to-excellent voice quality (88% of patients).     

Prospective randomized comparative study of tracheoesophageal voice prosthesis: Blom-singer versus provox

Objectives: Compare the most commonly used types of tracheoesophageal voice prostheses, Blom Singer and Provox. Study Design: Prospective study of 113 prostheses placed in 52 patients randomly selected to receive Blom-Singer and Provox. Methods: Postoperative voice was recorded at 1 and 4 months after valve placement. Survival time of the prosthesis and four objective voice parameters were analyzed. Eight judges rated all recordings in a random order for six subjective voice parameters. Subgroup analysis for primary versus secondary placements and type of procedure was performed. In addition, patients were asked for their subjective assessment. Results: Overall, Blom-Singer and Provox prostheses give very similar voice quality, lifetime, and patient satisfaction. Cleaning management is somewhat better for Provox, but there is a trend toward better overall voice quality for the Blom-Singer prosthesis. In subgroup analysis secondarily placed prostheses score somewhat better than primary placements, and patients with total laryngectomy have better voices than patients with extended laryngectomy combined with partial pharyngectomy. Conclusions: Given the equal and good results in terms of voice quality, other factors (e.g., costs, surgery-related factors, maintenance, patient preference) should be taken into account when deciding which type of tracheoesophageal voice prosthesis to use. Laryngoscope, 108:1561–1665, 1998     

Speech rehabilitation using Provox voice prosthesis

The first voice prosthesis was described in 1972 by Mozolewski. Eight years later Blom and Singer constructed the first commercial prosthesis. In 1988 another prosthesis was presented as Provox system prosthesis. The aim of the study was to describe the technique of tracheoesophageal puncture (TEP) and to present two years results of the technique performed in 21 patients. Primary TEP with immediate implantation of the Provox 2 prosthesis was applied in 16 (76%) patients. In five patients (24%) secondary TEP was performed. All the patient with primary TEP had cricopharyngeal myotomy. In 7 patients the vocal prosthesis was exchanged. In five because of leakage through the valve and in two patients the vocal prosthesis was extruded. Leakage around the prosthesis occurred in two patients with secondary TEP. The mean device-related lifetime was 216 days and ranged from 30 to 540 days. In non-radiated patients the lifetime of the prosthesis was 255 days and in patients after radiotherapy the lifetime was 150 days. In all the patients the prosthetic voice was more similar to normal voice than in patients with esophageal speech. The implantation of the voice prosthesis is a simple method of restoring of a good quality voice enabling communication.     

Perceptual characteristics of tracheoesophageal voice produced using four prosthetic/occlusion combinations

This study examined and compared perceptual characteristics of tracheoesophageal (TE) voice produced using four different prosthetic/occlusion combinations in 15 tracheoesophageal speakers: 1. using the Blom-Singer duckbill prosthesis and digital occlusion of the tracheostoma; 2. using the Blom-Singer duckbill prosthesis and valve occlusion of the tracheostoma; 3. using the Blom-Singer low-pressure prosthesis and digital occlusion of the tracheostoma; and 4. using the Blom-Singer low-pressure prosthesis and valve occlusion of the tracheostoma. Characteristics of speech produced in each condition were also compared to those of 19 normal speakers. Differences in perceptual ratings made by judges who varied in knowledge about laryngectomees were examined. Results revealed a number of speaking parameters that discriminated among normals and the four TE speaking conditions, and that differed according to the judge's expertise level. Clinical implications are emphasized.     

Temporal and perceptual characteristics of tracheoesophageal voice

This study compared the temporal and perceptual characteristics of tracheoesophageal voice produced in four different prosthetic/occlusion conditions: 1. using the Blom-Singer duckbill prosthesis and digital occlusion of the tracheostoma; 2. using the Blom-Singer duckbill prosthesis and valve occlusion of the tracheostoma; 3. using the Blom-Singer low-pressure prosthesis and digital occlusion of the tracheostoma; and 4. using the Blom-Singer low-pressure prosthesis and valve occlusion of the tracheostoma. Characteristics of speech produced in each condition were also compared to those of normal speech. Results revealed significant differences between normal speech and tracheoesophageal speech (in all prosthetic/occlusion conditions) for several temporal and perceptual parameters. In addition, important trends were noted among tracheoesophageal prosthetic/occlusion conditions.     

喉全切除后气管造口及发音管的应用研究

目的 探讨喉全切除术后建造不狭窄的敢管造口同时防止造口处“碗状”塌陷形成的术式。方法 回顾分析１９９６年以来我科应用Ｂｌｏｍ－Ｓｉｎｇｅｒ技术行喉全切除后重建气管食管音患者２５例,采用五种方法气管造口术,其中６例在一期手术中采用颈前“Ｕ”形切口＋切断双侧胸锁乳突肌胸骨端＋Ｇｒｉｆｆｉｔｈ气管造口扩大术（综合法）,并安装常植式发音管和呼吸瓣膜器。结果 ２５例中２３例成功发音,成功率９２％,气管造口方     

In Situ Lifetime,Causes for Replacement,and Complications of the ProvoxTM Voice Prosthesis

One hundred Provox^TM voice prostheses consecutively inserted in 37 patients and replaced only when phonation was impaired or complications noted were analyzed for in situ lifetime, causes for replacement, and complications. The mean in situ lifetime was 311 days. Actuarial analysis(Kaplan-Meier method) of the in situ lifetime revealed that 65.9%, 23.7%, and 16.4% of the voice prosthesis were still in situ by the 6th, 12th, and 18th postinsertion month. Cause for replacement was salivary leakage through the prosthesis valve, salivary leakage around the prosthesis, deterioration of the prosthesis, and excessive crusting resulting in increased airflow resistance in 33%, 27%, 24%, and 16% of cases, respectively. Complications included granulation tissue formation, cervical cellulitis, necrosis of the tracheoesophageal puncture, tracheostomal stenosis, and swallowing impairment in six, two, one, three, and one patient, respectively.     

Long-term performance of indwelling tracheoesophageal speaking valves in Chinese patients undergoing laryngectomy

OBJECTIVES: To assess the use of indwelling tracheoesophageal speaking valves in Chinese patients undergoing laryngectomy, to identify the clinicopathologic factors for favorable outcome, and to evaluate the factors that determine prosthesis lifetime. DESIGN: Retrospective review. SETTING: Tertiary care institution. PATIENTS: Sixty consecutive patients with total laryngectomy were included. All had indwelling tracheoesophageal speaking valves for voice restoration. INTERVENTION: Anterograde replacement of voice prosthesis in case of failure of functional speech production or leakage of saliva. MAIN OUTCOME MEASURES: Incidence of successful tracheoesophageal speech rehabilitation for daily communication, influence of different clinicopathologic factors on the outcome, and prosthesis lifetime and its relationship to different clinicopathologic factors. RESULTS: The success rate of speech rehabilitation was 78%. Age younger than 60 years was the only clinicopathologic factor associated with successful speech rehabilitation (P = .04, Fisher exact test). The median device lifetime was 8.2 months. Both age of 60 years or older and the use of subsequent prosthetic valves were identified as risk factors for valve failure on univariate analysis (n = 192; log-rank test; P = .02 and P = .03, respectively), with age of 60 years or older as the only risk factor that reduced the device lifetime in the Cox proportional hazards model (P = .03; relative hazard ratio, 1.5; 95% confidence interval, 1.1-2.4). CONCLUSIONS: Our success rate in using indwelling tracheoesophageal speaking valves was comparable to that reported in the Western literature. The median device lifetime of 8.2 months was satisfactory. Patient age was found to be a significant predictor of successful tracheoesophageal speech rehabilitation, with age of 60 years or more adversely affecting device lifetime.     

气管食管裂隙状瘘发音重建术的声学分析

目的 客观评价气管食管裂隙状瘘发音重建术后的发音效果，并以此指导临床工作。方法 对喉全切除术后I期与Ⅱ期气管食管裂隙状瘘发音患者、食管发音患者、安装Blom—Singer发音钮患者以及健康人分别进行语音测试，分析7项客观声学参数，比较发音效果。结果 经统计学t检验，气管食管裂隙状瘘语音最长发音时间短于健康人，明显长于食管音，但和Blom—Singer发音钮语音差异无显著性；其声音的强度和健康人及Blom—Singer发音钮差异无显著性，明显高于食管音；其基频明显低于健康人；其频率微扰和振幅微扰明显高于健康人，而明显低于食管发音，与Blom—Singer发音钮语音差异无显著性；其共振峰频率和能量仅在F1共振峰能量上明显高于食管音，其余各频率上差异均无显著性。虽然喉全切除I期气管食管裂隙状瘘发音重建术中制作帽状气室，而Ⅱ期发音重建术中不制作帽状气室，但两者语音声学分析各参数间差异均无显著性。结论气管食管瘘语音比食管音接近健康人语音，能满足日常生活需要。气管食管裂隙状瘘发音重建术中不制作帽状气室不影响术后的发音。     

The custom-fit voice prosthesis, for treatment of periprothetic leakage after tracheoesophageal voice restoration

BACKGROUND: Leakage around an indwelling voice prosthesis is detected during 13% up to 27% of all replacement procedures of voice prosthesis and causes serious complications in further voice restoration of the laryngectomee. Lots of therapeutic options to stop periprothetic leakage have been described (Injection of Bioplastique, autologous fat or collagen, suture techniques, spacer therapy) without convincing success rates. METHODS: Custom-fit voice prostheses are ordinary indwelling voice prostheses (Blom-Singer low pressure Indwelling 20 fr) with enlarged flanges and reduced shaft length that are individually sized for the shunt of the laryngectomee. Especially enlarging the esophageal flange provides a tight sealing of leakage around the prosthesis. PATIENTS: In a one year lasting clinical trial 692 voice prostheses were changed. In 77 cases periprothetic leakage was detected and fistulas were fitted with individually sized voice prostheses. RESULTS: In 76 cases total control of leakage was achieved without any specific complications taking place. Moreover two types of leaking tracheoesophageal fistulas were distinguished, a dilated-atrophic and an infected-necrotic type. 57% of the fistulas were dilated-atrophic type and 43% of the fistulas were classified infected-necrotic type. Infected-necrotic fistulas needed enlarged flanges tracheal an esophageal for tight sealing in 91% of the cases whereas dilated-atrophic fistulas needed double flanges only in 45%. 70% of infected-necrotic type fistulas needed only one singular history of a custom-fit prosthesis and could be changed back to ordinary indwelling prostheses after healing had taken place. CONCLUSIONS: As the insertion of a modified prosthesis is only slightly more effort than an ordinary voice prosthesis insertion, the success rate is high und complications are rare we recommend the custom-fit voice prosthesis for treatment of periprothetic leakage.     

Viscoaugmentation as a treatment for leakage around the Provox 2 voice rehabilitation system

Tracheo-oesophageal puncture for voice restoration is a well-established technique post-laryngectomy. A number of complications can occur with the creation of a tracheo-oesophageal fistula (TOF) and in the subsequent management of the patient with an indwelling voice rehabilitation system. This article is the first to report the use of Hylaform, a colourless viscoelastic gel, to treat an intractable case of leakage around a Provox 2 voice prosthesis. The procedure which required no anaesthesia resulted in no further leak around the valve to the present day, now more than four weeks post-viscoaugmentation.     

Incidence of tracheo-oesophageal fistulas after primary voice rehabilitation with the Provox or the Eska-Herrmann voice prosthesis

Salivary leakage can be a major symptom of valve failure or incorrect positioning of indwelling voice rehabilitation valves in a tracheo-oesophageal fistula. Usually, removal of the valve for a short time leads to shrinking of the fistula or a simple valve replacement procedure resolves the problem. If the fistula, however, does not close spontaneously, symptoms persist and the fistula may have to be closed surgically. In a retrospective study, data of 103 patients who underwent laryngectomy and primary voice rehabilitation between 1989 and 1998 with either the Provox or the Eska-Herrmann prosthesis were compared with regard to surgical fistula closure requirement. A total of 55 patients underwent laryngectomy and primary voice rehabilitation with the Eska-Herrmann and 48 with the Provox prosthesis. Initial tumour treatment also included post-operative radiotherapy for all patients in the study. In total, surgical fistula closure had to be performed in three patients, all of whom had been treated with the Provox prosthesis. The time span between initial voice rehabilitation and surgical closure of the fistula was 5 months, 21 months and 24 months in all three patients respectively. None of the fistulas developed in relation to recurring tumour disease. The Provox prosthesis seem to have a higher risk of developing fistulas necessitating surgical intervention, even years after initial tumour therapy, than the Eska-Herrman prosthesis. These complications may be due to the larger tracheo-oesophageal fistula necessary to fit the larger diameter of the Provox prosthesis. Received: 19 December 2000 / Accepted: 10 April 2001     

咽食管括约肌切开术在喉全切除术后发音重建中的作用

目的：探讨咽食管括约肌切开术对减少咽食管括约肌失弛缓对喉全切除术后安装Blom-Singer发音管发音重建的影响。方法：喉全切除术后，咽食管括约肌收缩或是痉挛都会不同程度地影响食管的气流并阻碍发音。咽食管括约肌的解剖位置是环咽肌以及其上的下咽缩肌的一部分和颈段食管上段的一部分。手术操作是在喉全切除术中、术后于气管造瘘口外上方切除长5cm，宽1cm的咽食管括约肌。结果：33例喉全切除术的患者进行咽食管括约肌切开术后有32例发音成功，发音重建的成功率是97％。其中包括12例喉全切除术中I期进行环咽肌切开术，21例是Ⅱ期进行咽食管括约肌切开术的，后者又有9例是安装Blom-Singer发音管后发音不能再行咽食管括约肌切开术的。患者3年存活25例，5年存活18例。同期行喉全切除术和咽食管括约肌切开术的12例患者，Kaplan-Meier法统计3年生存率81．82％，5年生存率42．86％。结论：咽食管括约肌切开术能提高Blom-Singer发音管发音重建的成功率。     

The artificial throat: a new method for standardization of in vitro experiments with tracheo-oesophageal voice prostheses.

After total laryngectomy, the voice can be restored successfully with a silicone tracheo-oesophageal voice prosthesis. Biofilm formation and subsequent deterioration of the silicone material of the prosthesis often limit the mean life of the device to an average of 3-5 months. Although device replacement can be considered an easy outpatient procedure, frequent replacements are inconvenient for the patient and may lead to malfunction of the tracheo-oesophageal fistula. Further understanding of the process of development and inhibition of the colonization of these polymer surfaces requires several comprehensive clinical studies. However, in vivo research of the biomaterials of the voice prostheses is difficult and time consuming. In order to simulate the natural process of biofilm development under dynamic nutrient conditions, an artificial throat was developed. Biofilm developed on Groningen button voice prostheses in vitro could not be distinguished from that formed over several months in vivo. This method can be used as a standardized approach for studying functional and structural aspects of all commercially available indwelling and non-indwelling voice prostheses, including the Groningen button, Provox, Voice Master, Blom-Singer and others under various laboratory conditions.