POLISH SENTENCE TEST FOR SPEECH INTELLIGIBILITY MEASUREMENTS IN MASKING CONDITIONS

The article reports on development of a Polish Sentence Test for accurate and reliable speech intelligibility measurements against noise. The Polish material is composed of 481 different sentences divided into 37 lists of 13 sentences each. All the lists are phonemically and statistically balanced, i.e. the respective lists reveal comparable phonemic content and similar psychometric functions. Moreover, the phoneme distribution for each list reflects mean phoneme content for Polish language. The test is optimised for speech reception threshold (SRT), measurements in speech-like maskers, i.e. babble noise or speech-shaped noise. For otologically-normal subjects SRT= 6.1 dB. The mean steepness of the psychometric functions (S50test) at the SRT point, is equal to 25.6%/dB, i.e. is slightly higher than that of comparable sentence tests developed previously for other languages. INTRODUCTION Different methods have been proposed for measuring speech intelligibility in quiet and in noise [2,3,5,7,9,16,18,19,23,24]. They differ in the following aspects: the structure of the speech material, details of the test procedure, presentation level, range of the signal-to-noise ratio (SNR), type of interfering noise and a presentation mode. As far as the structure of the speech material is concerned, one can distinguish three basic types of tests: word intelligibility tests [1,8,12-14], digit intelligibility tests [12,13,16] and sentence intelligibility tests. The sentence tests can be divided into those using meaningful, everyday utterances [7,9,11,17,18] and those using semantically unpredictable (nonsense) sentences [3,19-22]. The main disadvantage of word intelligibility tests, as used in a traditional speech audiometry, is the difficulty to determine a precise SRT value (defined as the signal-to-noise ratio, that yields 50% probability of correct response), in a time-efficient manner. Sentence intelligibility tests, on the other hand, have been shown to be much more accurate [3,7,9,10,18,24]. They have some advantages over the word tests in testing the quality of hearing aids with different signal processing algorithms since they provide a reliable distinction between unaided and aided patient performance. Furthermore, unlike words and digits, sentence materials reflect natural communication process. Many sentence tests have been developed for accurate measurement of the speech reception threshold (SRT) in the noise [5,7,9,10,18]. Since there had not been Polish sentence materials for speech intelligibility assessments in noise, the present study deals with the preparation and evaluation of the Polish Sentence Test to be used for accurate and reliable measurement of the SRT in noise. The test is structurally similar to the previously developed meaningful sentence tests in: Dutch [10], American [9] and German [7]. The speech test developed is designed for evaluation of speech intelligibility in clinical conditions as well as for laboratory measurements. DEVELOPING THE SENTENCE MATERIALS The process of the developing the sentence test was as follows: 1) initial selection of the sentences (in the written form); 2) recordings; 3) measurements of the sentence intelligibility in the noise and derivations of psychometric functions; 4) selection of the optimal sentences; 5) composition of statistically and phonemically balanced sentence lists; 6) verification measurements.