Apparatus for measuring physiological parameters in a target blood vessel comprising: at least one acoustic sensor positionable on a part of a subject's body and configured to receive an acoustic signal from the target blood vessel, comprising at least: a first microphone, a second microphone and a processor for receiving signals from the first and second microphones, wherein the first microphone is configured to measure an acoustic signal from the target blood vessel and the second microphone is configured to determine background noise and the processor is configured to remove the background noise determined by the second microphone from the acoustic signal measured by the first microphone; an optical sensor comprising a light source, and a photosensor configured to measure one or more properties of the light source through absorption, refraction or reflection of the light source from the target blood vessel; a means for attachment of the acoustic sensor on the part of the subject's body; the apparatus further comprising a motion sensor for determining whether the apparatus is being worn by the subject, and; wherein the processor is configured to activate the first and second microphones when the photosensor determines light absorption, refraction or reflection representative of the apparatus being located in close proximity to the target blood vessel. at least one acoustic sensor positionable on a part of a subject's body and configured to receive an acoustic signal from the target blood vessel, comprising at least: a first microphone, a second microphone and a processor for receiving signals from the first and second microphones, wherein the first microphone is configured to measure an acoustic signal from the target blood vessel and the second microphone is configured to determine background noise and the processor is configured to remove the background noise determined by the second microphone from the acoustic signal measured by the first microphone; an optical sensor comprising a light source, and a photosensor configured to measure one or more properties of the light source through absorption, refraction or reflection of the light source from the target blood vessel; a means for attachment of the acoustic sensor on the part of the subject's body; the apparatus further comprising a motion sensor for determining whether the apparatus is being worn by the subject, and; a first microphone, a second microphone and a processor for receiving signals from the first and second microphones, wherein the first microphone is configured to measure an acoustic signal from the target blood vessel and the second microphone is configured to determine background noise and the processor is configured to remove the background noise determined by the second microphone from the acoustic signal measured by the first microphone; an optical sensor comprising a light source, and a photosensor configured to measure one or more properties of the light source through absorption, refraction or reflection of the light source from the target blood vessel; a means for attachment of the acoustic sensor on the part of the subject's body; the apparatus further comprising a motion sensor for determining whether the apparatus is being worn by the subject, and; wherein the processor is configured to activate the first and second microphones when the photosensor determines light absorption, refraction or reflection representative of the apparatus being located in close proximity to the target blood vessel. <EOS>