Osteoporosis is a common disease affecting a large proportion of the human population worldwide. It is described as a bone density and structure deterioration leading to a bone fracture risk. The World Health Organisation (WHO) recognises osteoporosis as one of the most health issue since it is asymptomatic. Globally, two third of women and one third of men over 50 years is diagnosed with osteoporosis.One million in Australia alone have osteoporosis and over six million is considered with low bone mass which cost the budget healthcare sector about 2.75 $ billion dollars a year. The expectation rate of population affected by osteoporosis is assumed to significantly increase with continuous increase of aging population. Therefore, an urgent powerful and precise technique for early detection of bone loss is required.There are several techniques in the market available but the Dual Energy Absorptiometry (DXA) is the only one authorised for this regard by the WHO.However, the controversy is that DXA can only measure the bone density but not yet the bone microarchitecture which is an important factor for the assessment of bone.Quantitative ultrasound (QUS), in terms of broadband ultrasound attenuation (BUA) and speed of sound (SOS), is an alternative method that has been investigated by numerous studies confirming that it is a reliable technique for the assessment of bone and predicting fracture risk. Yet, sufficient knowledge about the propagation of ultrasound waves through complex structure such as cancellous bone is not attained which affects the ability of accurate determination of bone mass and structure.A new proposed hypothesis is that phase interference is the leading attenuation mechanism in complex composite such as cancellous bone due to inhomogeneity transit time of the sonic rays. It also proposed that the propagation of ultrasound wave can be approximated as parallel sonic rays; each has a transit time determined by the proportion of bone (solid) and marrow (liquid) it travels through. Thus, a transit time spectrum (TTS) can be defined describing the proportion of sonic rays of the same transit times. Therefore, this thesis will illustrate a development and validation of an ultrasound transit time model of complex structure such as cancellous bone and identify its relation with phase interference. This study will further explore possible ULTRASOUND TRANSIT TIME SPECTROSCOPY IN COMPLEX MEDIA -APPLICATIONS AND RELATIONSHIP WITH PHASE INTERFERENCE IV applications of the TTS to enhance the effectiveness of ultrasound as a reliable assessment tool for bone.In particular, an examination of the phase interference, created by the variation in transit time, being the main attenuation factor in complex structure among other factors for instance absorption and structure-scattering was performed. Amore fundamental concept of phase interference was investigated showing that it has both temporal and spatial phase interference. The temporal phase interference depends on the pulse length and the transit time di...