The Exponential Sine Sweep (ESS) technique, popularized by Angelo Farina, has become a cornerstone of modern electroacoustic measurement due to its unique capability to simultaneously extract a system’s linear impulse response ; its individual harmonic distortion components. Standard implementation of this method almost exclusively utilizes a low-to-high (upward) exponential sine sweep. However, during a technical Q&A session at the AES Europe 2025 Convention in Warsaw, a question was raised: what are the practical consequences of reversing the sweep direction? This inquiry is particularly relevant given that several industry-standard measurement platforms often employ high-to-low (downward) sweeps to optimize the mechanical ; thermal stability of the device under test (DUT) while performing stepped or swept sinusoidal analysis. This paper provides an investigation into the temporal behavior of nonlinearities when the frequency gradient of an exponential sweep is inverted. Through formal mathematical derivation; numerical simulations the study proves that while the spacing between distortion orders remains identical in magnitude, the polarity; time distribution of these impulses is reversed. Specifically, we demonstrate that in a downward sweep, the distortion products shift from the "pre-causal" negative time region to the "post-causal" positive time region. This shift causes harmonic distortion pulses to emerge within the reverberant tail of the impulse response, leading to significant contamination of decay measurements; energy-time curves. By contrasting the "tracking filter" paradigm with "time-domain deconvolution," this work clarifies why sweep direction is a critical parameter that must be aligned with the specific goals of the measurement protocol.
