Other than the narrowing vertical coverage, the length of the array also plays a role in what wavelengths will be affected by this narrowing of dispersion. The longer the array, the lower frequency the pattern will control. At frequencies below 100 Hz (wavelength of 11.3 ft) the line array which is less than approximately 3 meter long will start to become omnidirectional, so the system will not conform to line array theory across all frequencies. Above about 400 Hz the driver cones themselves become directional, again violating the theory’s assumptions, and at high frequencies, many practical systems use directional waveguides whose behavior cannot be described using classical line array theory. In short, the geometry of real-world audio line arrays as used in public address systems can only be modeled approximately by line array theory, and only in the 100–400 Hz range.