Vd represents the maximum volume of air a driver can move within its linear operating range, calculated as:
Where:
- Sd = Effective diaphragm area (m²)
- Xmax = Maximum linear excursion (m)
A higher Vd means the driver can displace more air, which is essential for deep bass reproduction. This makes Vd particularly useful for comparing subwoofers, as it directly correlates with low-frequency output capability.
Since this formula is based on Xmax, it only accounts for the driver’s linear range, meaning the excursion where distortion remains minimal. Xvar (maximum excursion before noticeable distortion) and Xlim (absolute physical limit before damage) may provide additional insights into the driver’s real-world performance beyond its ideal operating conditions.