The phenomenon of squeal in disc brakes has been, and still is, a problem for the automotive industry. Extensive research has been done in an attempt to understand the mechanisms that cause it and in developing design procedures to reduce it to make vehicles more comfortable. Part of the research programme at Liverpool University has been the verification of finite element models of disc brakes using an experimental brake rig. Tests have been carried out on a Rover solid disc, a Mercedes solid disc and a Mercedes vented disc. The test rig houses the front knuckle in a rigid support, and the drive is to the wheel studs on the disc. There is no wheel in place. The brake calliper is mounted in the normal fashion to the knuckle. A 42 kW motor drives the disc against the action of the brake. A fixed array of 12 non-contact capacitance displacement transducers is arranged in an arc near the outer radius of the disc. These measure the axial displacement of the disc while it is rotating. During a squeal event, the measurement system records these displacements along with values of disc temperature, rotation speed, shaft torque and hydraulic pressure at a logging rate of 56 kHz. From this high definition time domain data, small time samples can be analysed using a least squares technique to determine the nature of the vibration mode at a given squeal frequency. The analysis shows that the vibration of the brake disc can take several forms. With different conditions, even on the same disc, the vibration mode can be a forward travelling wave, a backward travelling wave or a standing wave (relative to the calliper). This paper is in two parts. The first part describes the test rig and the testing procedures; the second part explains how the analysis is performed and shows some of the results from typical squeal tests.