In-ear drum reference point measurements are rarely performed due to invasiveness and unreliability.

Evidence

In the previous section, headphones were measured at ear drum reference point (DRP) where the microphone is located. The equivalent measurement for a human requires a small probe microphone to be inserted down their ear canal until it just barely touches the ear drum. The procedure is time-consuming and uncomfortable for the subject with the risk of causing damage to the ear drum. These measurements are rarely performed except in clinical research settings. Another challenge is the measurements are notoriously unreliable at high frequencies where small changes in microphone position can produce 10–20 dB differences. — Toole, Floyd E, Sound reproduction- the acoustics and…, 1736