Cavity pressure fluctuations induced by flow grazing over a long, narrow, open cavity were investigated. The apparatus stimulated a typical road vehicle door gap/door cavity/primary bulb seal assembly, tested using a wind tunnel. The cavity pressure distribution was measured over a range of flow velocities. Pressure spectra were decomposed into a source spectral distribution function and an acoustic frequency response function, using a system identification method based on dimensional analysis and a linear source-filter model. The results indicated a weak coupling between the cavity response and the flow excitation. The excitation mechanism was dominated by flow-edge interactions, i.e., the ``edge tone.'' The characterization method was validated by varying the acoustic properties of the cavity and the excitation strength. The source function was independent of sensor location. The system response was shown to be associated with transverse standing waves. Tapering the orifice trailing edge simultaneously reduced the excitation strength, the excitation frequency, and the cavity response. The presence of flow aspiration through the seal raised the broadband noise floor of the system response function. This anomaly underlined one limitation of the spectral decomposition method.