Created on 2013-02-01 00:12:00
The impulse response of the DUT is derived from the APx500 log-swept continuous sweep measurement. This gives the same result (although with better signal to noise ratio) as would be produced by stimulating the DUT with an impulse and observing the output in the time domain. This view is useful to study artifacts that move or spread the stimulus in the time domain, such as delay, reverberation, echo or reflection, etc.
Impulse response is a property of the device or system under test, and for same-domain measurements (analog to analog, for example) the impulse response is unitless, which we represent in APx500 as “x/y”. For cross-domain measurements, the impulse response is expressed as the relationship between output and input. A DAC, for example, has an impulse response expressed in units of V/D. See Units of Measurement…
An ideal impulse response does not change with stimulus level.
Why do some instruments display impulse response with absolute units?
Other instruments may use absolute units (such as V or Pascals) for the impulse response. This is not actually the inherent DUT impulse response (which, as stated above, is unitless for DUTs which do not cross domains), but rather the response of the DUT to an impulse whose level is the generator level when the measurement was made. It has the same shape as the true impulse response, but is scaled according to the generator level, and will change as the generator level is varied.
A linear DUT’s impulse response is constant with generator level. The APx impulse response view reflects this. “Impulse response” views from other instruments may need to be scaled inversely to the generator level to recover the true impulse response.
Impulse Response in the APx Acoustic Response implementation
Historically, actual acoustic impulses such as gunshots have been used as stimulus signals for impulse response measurements of acoustic spaces and devices.
Other stimulus signals such as MLS and swept sine bursts can be mathematically transformed into impulse responses, with improved signal-to-noise ratio and less odor of gunpowder.
The APx500 series uses our continuous sweep signal (log-swept sine chirp) as the stimulus, then recovers the system’s impulse response from the acquired data.
For Acoustic Response measurements, the Impulse Response and the Energy Time Curve are primarily used to identify reflections in the acoustic space, as an aid to optimal adjustment of the Time Window.