Intensity, or loudness, of a murmur is graded from one to six. A loud murmur can produce palpable vibrations called a thrill. Such a murmur is graded as four or more and cannot be considered innocent.
This separation between grades one to three and four to six is the most distinctive component of murmur intensity classification.
A grade one murmur is heard with difficulty, not immediately upon listening and requires a quiet environment. Grade two murmurs are readily heard, but are not very loud. Grade three murmurs seem loud, but no thrill can be felt. The distinction between grades two and three, therefore, is clearly both arbitrary and subjective.
A grade four murmur still requires contact of the stethoscope with the chest wall. Grade five murmurs can be heard with the stethoscope tilted on edge against the chest wall, and one can hear a grade six murmur with the stethoscope one centimeter above the chest wall. There is little significance to distinguishing between grades four to six.