(a) Dots: measured magnitude |T| (ω) and phase advance δ (ω) of the transmission coefficient for fixed lB = 195 mm (VB = 163.8 cm3). Solid red curves: theoretical fits. (b) Time-domain measurements of the incident (larger, blue pulses) and transmitted (smaller, red pulses) acoustic pressure amplitude for the same VB. The Gaussian wavepackets have central frequencies 506 and 512 Hz, corresponding to maximum-negative and maximum-positive group shifts. The operating frequencies are marked as the green solid circles p and q in (a). (c) Measured space-time diagrams of the normalized pressure amplitude for positive and negative group delay. The volume VB was fixed to 170.5 cm3 and a single temporally-Gaussian pulse of spectral width 3 Hz was generated and recorded by a microphone scanned along the tube.