from the DUT, both analog and digital, and convert it to appropriate signals (analog or digital) for analysis A signal analyzer that filters the response and calculates results of measurements, commonly a connected or embedded PC in modern solutions A form of output to the user (display, report, etc.) In a closed-loop test, the analysis engine controls the audio generator while simultaneously measuring the output of the DUT, as shown below: The signal analyzer can provide control to both the audio generator and the audio input stages, assuring that test conditions are met. This also permits precise time relationships between the stimulus and response of a DUT to be determined. In an open-loop test, the signal analyzer has no control over the audio source driving the DUT, and thus the user must take care to ensure that the source is providing a signal of appropriate characteristics. Open loop tests are useful for measuring DUTs that have no direct signal input, such as a CD or MP3 player. Electro-acoustic Devices Electro-acoustic devices such as loudspeakers and microphones present special problems for analysis, as they must receive or transmit signals through air. In these cases, the DUT in the model shown above must be replaced with the complete electro-mechanical system, e.g., a power amplifier to drive a loudspeaker, a loudspeaker, a measurement microphone and microphone pre-amplifier. The actual device under test can be measured only when the other devices in this system are fully characterized, so that the contributions from these devices may be subtracted from the response. Many modern audio analyzers contain measurement sequences that automate this procedure, and the focus of recent developments has been on quasi-anechoic measurements. These techniques allow loudspeakers to be characterised in a non-ideal (noisy) environment, without the need for an anechoic chamber, which makes them ideally suited for use in high volume production line manufacturing. Most quasi-anechoic measurements are based around an impulse response created from a sine wave whose frequency is swept on a logarithmic scale, with a window function applied to remove any acoustic reflections. The log swept sine method increases signal-to-noise ratio and also allows measurement of individual distortion harmonics up to the Nyquist frequency, something which previously impossible with older analysis techniques such as MLS (Maximum Length Sequence). Audio Generator An audio generator suitable for use in test and measurement must meet several criteria that apply to both analog and digital