The measurement of sound transmission in human lungs has shown promise to reveal, by noninvasive methods, information about the structure of peripheral airways and lung tissue. The paper gives a detailed explanation of the instrumentation and testing methods developed to measure sound transmission through human lungs and thoracic structures in the 5-20 kHz frequency range and describes in detail experiments comparing the acoustic lung transmission patterns of four different subject groups. The experimental results are compared with those predicted by an acoustical model of sound transmission through lung parenchyma.

T h i s paper p r e s e n t s t h e r e s u l t s of a s t u d y designed t o i n v e s t i g a t e t h e e f f e c t s o f s u b t r a c t i v e -t y p e n o i s e r e d u c t i o n a l g o r i t h m s on LPC based s p e c t r a l paramet e r e s t i m a t i o n as r e l a t e d t o t h e p e r f o rmance of speech p r o c e s s o r s o p e r a t i n g w i t h i n p u t SNRs o f 1 5 d8. and b e l o w . S u b t r a ct i v e n o i s e p r e -p r o c e s s i n g g r e a t l y i mp r o v e s t h e S N R b u t system performance improvement i s n o t commensurate. LPC s p e c t r a l e s t i m a t i o n i s a f f e c t e d by t h e c h a r a c t e r o f t h e r e s i d u a l n o i s e which e x h i b i t s g r e a t e r v a r i a n c e and s p e c t r a l g r a n u l a r i t y t h a n t h e o r i g i n a l broadband n o i s e . The s t u d y shows t h a t removing l e s s t h a n t h e f u l l amount o f n o i s e and w h i t e ni n g i t improve s p e c t r a l e s t i m a t i o n and speech d e v i c e performance. Techniques and Derformance r e s u l t s a r e p r e s e n t e d . 1. I n t r o d u c t i o n The performance o f most speech r e c o g n i t i o n and p a r a m e t r i c speech c o d i n g systems b e g i n s t o s u f f e r a t s i g n a l -t on o i s e r a t i o s o f 1 5 de. and below. N o i s e i n t e r f e r e s w i t h t h e a c c u r a t e e x t r a c t i o n and m o d e l i n g o f speech s p e c t r a l and t e m p o r a l f e a t u r e s n e c e s s a r y f o r a c c u r a t e r e c o g n i t i o n and c o d i n g . P a r t i c u l a r l y a f f e c t e d a r e p i t c h e x t r a c t i o n , spectrum and f o r m a n t e s t i m a t i o n , and word boundary d e t e c t i o n a l g o r i t h m s . P a t t e r n m a t c h i n g systems s u c h as speech r e c o g n i z e r s and VQ based c o d e r s t e n d t o be a d v e r s e l y a f f e c -. t e d because t h e i n p u t speech f e a t u r e v e c t o r p a t t e r n s a r e supposed t o c o r r e spond w i t h t h e s t o r e d t e m p l a t e s g e n e r a t e d under non-noisy c o n d i t i o n s . Performance g e n e r a l l y i s h i g h l y c o r r e l a t e d w i t h i n p u t SNR a t r a t i o s o f 1 5 d8. and below and u s u a l l y degrades p r e c i p i t o u s l y . I t s t a n d s t o reason t h a t i m p r o v i n g t h e SNR o f t h e i n p u t speech t o p a r a m e t r i c sDeech p r o c e s s o r s by means o f some n o i s e r e d u c t i o n p r e p r o c e s s o r s h o u l d improve d e v i c e performance. V a r i o u s c l a s s e s o f n o i s e r e d u c t i o n a l g o r i t h m s have been developed and implemented o v e r t h e y e a r s w i t h some success. One such c l a s s i s t h e s o e c t r a l / c e p s t r a l s u b t r a c t i o n t e c h n i q u e s and t h e i r v a r i a n t s . I t has been demons t r a t e d [ 1 . 2 , 3 . 4 ] t h a t s u b t r a c t i v e -t y p e broadband n o i s e r e d u c t i o n t e c h n i q u e s can s u b s t a n t i a l l y improve t h e SNR of n o i s y speech and t...

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The purpose of this series is to publish work of high quality by authors who are experts in their respective areas of electrical and computer engineering. Each volume contains the state-of-the-art coverage of a particular area, with emphasis throughout on practical applications. Sufficient introductory materials will ensure that a graduate and a professional engineer with some basic knowledge can benefit from it.

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