Full solar disk observations of radial velocity, obtained with the IRIS network of resonant scattering spectrometers, are used to analyze the acoustic p-mode spectrum around the acoustic cut-off frequency. Three different methods are used to determine it; two use the power spectrum, while the third uses the phase spectrum. The three values measured converge to a common value of 5.55 +/- 0.1 mHz, which is higher than any theoretical prediction. Beyond this frequency, the power spectra still decreases down to 10 mHz at which point it becomes flat, showing the presence of some power due to the so-called pseudomodes which we show are the result of interference between traveling waves. The acoustic power density measured beyond the cut-off frequency makes it possible to estimate the energy deposited into the chromosphere about 10 exp 7 erg/sq cm per sec, which is high enough to compensate the losses that take place in this layer.