MVS Spectra Demystifier

A. Sound, what is it?

Sound, mechanically speaking, is alternating spaces of high and low air pressure which move, you guessed it, at the speed of sound (344 meters per second in air, 1570 meters per second through blood because it is more dense). These waves have one or more superimposed frequencies-- times per second that the pressure goes from low to high (causing one's eardrum to be sucked out and pushed in, respectively).

A spectrum, loosely speaking, is a representation of the relative strengths of the frequencies which make up a given sound. The pictures included in the McGill Virtual Stethoscope show the strength of a given frequency, say 100Hz (Hz is frequency in cycles per second--think ear drum movements/second), over time. Frequency is the Y axis, time is the X axis, and the color represents the strength of the frequency component (black/purple = little or none of that frequency and red/yellow is a lot of that frequency). Hence a heart beat has two 'red-yellow' (strong) zones on the 100Hz line at the time of S1 and S2. Inbetween S1 and S2 there are (usually) no sounds, so there is a purple-black area indicating very little of the 100Hz frequency component.

Below is a Shockwave movie of a midsystolic murmur. There are three complete beats, each with its own S1, murmur, and S2 sequentially. The upper, white squiggly line is a 'sonogram' which shows the amplitude of the sound. The bottom, colorful part is the spectrum. You will note that S1 and S2 are 'lower' frequency than the midsystolic murmur because the murmur's spectrum extends higher up towards the sonogram.