Try this article from the Littmann website.
Short vs Long Tubing
Quite often health care workers raise questions regarding tubing length based on early publications claiming shorter tubing length provides better acoustic response. Some instructors have recommended their students buy the shortest tubing possible. In an attempt to clarify the information surrounding tubing length, Littmann stethoscopes has tested their product line to offer the following information about tubing length.
To explain our test results, it will be helpful to compare the tubing of the stethoscope to a garden hose. For example, an increase in the length of a garden hose will decrease the pressure at the end of the hose as a result of frictional and other internal forces. The same effect occurs when the tubing length of a stethoscope is increased. However, in the case of stethoscope tubing, change in length is relatively small; this decrease in acoustic pressure is not detectable by the human ear.
Additionally, as tubing length increases, resonant frequency decreases. Considering this fact, an increase in tubing length provides a better response to the lower frequency sounds (an advantage in auscultation). Many heart sounds fall below 150 Hz and are considered low frequency. Because it has been shown that the human ear is least sensitive to low frequency sounds, improved low frequency response is an advantage.
Taking these two factors into account, there is no detectable difference in acoustical performance between Littmann stethoscopes with shorter tubing vs. those with longer tubing. In fact, there may be some enhancements to low frequency sounds.
When purchasing a stethoscope, the health care practitioner needs to consider their own needs and practices. Longer tubing might be more appropriate for people wearing the stethoscope around their neck as it drapes better. The practitioner's height and arm length should also be a factor to determine optimal tubing length. Many practitioners would like a little more distance from sicker patients while auscultating. Longer tubing also reduces the amount needed to bend over the supine patient which can stress the practitioner's lower back.