if a listener were on the other side of a thick brick wall, they might not hear you at all.
if you and your listener were both underwater, they could hear you at a great distance.
if you were in a canyon or a cathedral, you could hear your own echo, but you might have a difficult time telling where the sound was coming from.
Penetration: the ability of the sound wave to go through an object.
Attenuation: how much energy does the sound lose over a distance (in water, it loses very little so you can hear noises from far away when you are under water).
Reflection: when the sound hits an object, it will bounce back to the sender.
as sound penetrates all tissues, the signal becomes attenuated.
the further the sound penetrates, the greater the attenuation (objects far away are harder to see).
reflection is required for the ultrasound probe to receive a signal to analyze ... so no reflection = no signal = you cannot see the object on the screen.
different tissues have different degrees of penetration, attenuation and reflection and therefore produce different appearances on the ultrasound screen.
bone: ultrasound does not go through bone but, reflection is close to 100% so bones give a strong signal. However, penetration is effectively 0%. Because penetration is close to zero, you can never see anything behind bone except for a black shadow.
soft tissues, fat and organs: penetration, attenuation and reflection are all "medium" levels. They give the appearance of different shades of grey.
liquids (blood, urine, amniotic fluid, cysts): penetration is high and attenuation is low. Therefore, it is very easy to see structures behind liquid. However, reflection of liquid is very low and therefore, liquid looks black on the ultrasound screen.
scatter is caused by air or gas and it produces a useless snow storm on the ultrasound screen.
any time bowel loops or the lung interfere with the ultrasound signal, the image on the screen is completely obscured.