الفهرس 
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Abstract
Auditory development starts from 23 to 24 weeks of gestational age (GA). At this time fetal auditory threshold is approximately 65 decibel (dB).Auditory system development continues during intrauterine life, and the threshold is gradually diminished to that of an adult level at 5 or 6 years of age(Almadhoob & Ohlsson, 2015).
Some parts of neonatal auditory system develop after birth shortly. Persistence of appropriate sensory stimulants is important for normal growth and development (Duranet al.,2012).
Preterm newborns are susceptible to negative effects of environmental stimuli due to immature central nervous system, and decreased autonomic and self-regularity abilities of dealing with stress. They are unable to coordinate autonomic responses to environmental and tactile stimulation until 32 to 34 weeks’ postmenstrual age (PMA) because of excessive stimuli, neurons may develop alternative pathways between the cerebral cortex and the brainstem (Abdeyazdan et al.,2014).
Preterm infants born before 28 weeks’ PMA have an immature auditory system. Myelination of the auditory pathway continues after birth.Microscopic analyses of the cochlea of neonates have shown that hair cells are still in a process of differentiation (Bott et al.,2015).
Many studies have documented that infants are exposed to continuous noise without interval periods of quiet in the neonatal intensive care units (NICUs)(Santos et al.,2015).
The sound levels in NICUs range from 7 dB to 120 dB, which exceed the maximum acceptable level of 45 dB which recommended by the American Academy of Pediatrics (AAP)(Pinheiro et al.,2011).
Exposure to noise in the NICU may lead to cochlear damage,Hearing impairment is diagnosed in 2% to 10% of preterm newborns versus 0.1% of the general pediatric population. There is evidence of synergy between noise, hypoxia, apnea and hyperbilirubinaemia in development of hearing impairment(Brandon et al.,2007).
Noise may cause apnea, hypoxemia, alternation in oxygen saturation, and increased oxygen consumption secondary to elevated heart and respiratory rates so decrease the amount of calories available for growth(Almadhoob & Ohlsson, 2015).
Increases in intracranial pressure ,electromyographic and behavioral changes are noted in newborns in response to sudden noise. Excessive sound may affect the neuroendocrine system and may have indirect effect on immunity (Santos et al.,2015).
Stress from noise causing activation of the hypothalamic-pituitary-adrenal axis, and causes growth-inhibition through the action of corticosteroids possibly (Almadhoob & Ohlsson, 2015).
Neonatologist should monitor the sound in the NICU, and in incubators. A noise level 45 dB is recommended by the US Environmental Protection Agency (EpA). NICU personnel should do simple instruction to reducenoise in the nicu (no tapping no writing on theincubatorstops and hoods, closing the incubator doors carefully, wearing soft shoes)lowering sound level can be done also by earmuffs or earplugs. These earmuffs, which are laid on infants’ external ears, diminish the sound intensity to at least 7 dB and the sound pressure level by over 50%, so that they do not block infants’ necessary hearing stimulation for nervous system development (Bott et al.,2015).