A system comprising: an earphone (100;200) comprising: a loudspeaker; an accelerometer (114;214); a housing supporting the loudspeaker and accelerometer; and an ear tip surrounding the housing and configured to acoustically couple the loudspeaker to an ear canal of a user; a second earphone comprising: a second loudspeaker; a second accelerometer; a second housing supporting the second loudspeaker and second accelerometer; and a second ear tip surrounding the second housing and configured to acoustically couple the second loudspeaker to a second ear canal of the user; and a processor configured to: receive input signals from the accelerometer in response to motion of the earphone; detect first peaks in the input signals; based on the first detected peaks, compute an instantaneous heart rate; measure a frequency of an oscillation within the instantaneous heart rate; based on the frequency of the oscillation, compute a rate of respiration, wherein the input signals comprise acceleration measurements for each of three axes, and the processor is configured to detect the first peaks by combining the three acceleration measurements into a norm of the three measurements; wherein the processor is further configured to: receive second input signals from the second accelerometer comprising second acceleration measurements for each of three axes of the second accelerometer; combine the second three acceleration measurements into a norm of the second three measurements; and detect second peaks by combining the norm of the three measurements from the first accelerometer with the norm of the three measurements from the second accelerometer and detecting the second peaks within the result of the combination. an earphone (100;200) comprising: a loudspeaker; an accelerometer (114;214); a housing supporting the loudspeaker and accelerometer; and an ear tip surrounding the housing and configured to acoustically couple the loudspeaker to an ear canal of a user; a second earphone comprising: a second loudspeaker; a second accelerometer; a second housing supporting the second loudspeaker and second accelerometer; and a second ear tip surrounding the second housing and configured to acoustically couple the second loudspeaker to a second ear canal of the user; and a processor configured to: receive input signals from the accelerometer in response to motion of the earphone; detect first peaks in the input signals; based on the first detected peaks, compute an instantaneous heart rate; measure a frequency of an oscillation within the instantaneous heart rate; based on the frequency of the oscillation, compute a rate of respiration, wherein the input signals comprise acceleration measurements for each of three axes, and the processor is configured to detect the first peaks by combining the three acceleration measurements into a norm of the three measurements; wherein the processor is further configured to: receive second input signals from the second accelerometer comprising second acceleration measurements for each of three axes of the second accelerometer; combine the second three acceleration measurements into a norm of the second three measurements; and detect second peaks by combining the norm of the three measurements from the first accelerometer with the norm of the three measurements from the second accelerometer and detecting the second peaks within the result of the combination. receive input signals from the accelerometer in response to motion of the earphone; detect first peaks in the input signals; based on the first detected peaks, compute an instantaneous heart rate; measure a frequency of an oscillation within the instantaneous heart rate; based on the frequency of the oscillation, compute a rate of respiration, wherein the input signals comprise acceleration measurements for each of three axes, and the processor is configured to detect the first peaks by combining the three acceleration measurements into a norm of the three measurements; wherein the processor is further configured to: receive second input signals from the second accelerometer comprising second acceleration measurements for each of three axes of the second accelerometer; combine the second three acceleration measurements into a norm of the second three measurements; and detect second peaks by combining the norm of the three measurements from the first accelerometer with the norm of the three measurements from the second accelerometer and detecting the second peaks within the result of the combination. receive second input signals from the second accelerometer comprising second acceleration measurements for each of three axes of the second accelerometer; combine the second three acceleration measurements into a norm of the second three measurements; and detect second peaks by combining the norm of the three measurements from the first accelerometer with the norm of the three measurements from the second accelerometer and detecting the second peaks within the result of the combination.