Concerning the Structure of Turbojet Sound Field Free

The sound field of several turbojets was studied from recordings of sound levels, before analysis (40–10 000 cps) and by frequency octaves. These recordings obtained in many azimuths and at several distances have been plotted as isobaric curves. These curves show that the spreading of sound source zones is variable according to the considered frequency octave. For a constant gas velocity, the azimuth of the lobes is about 70° from the jet axis for high frequencies, and 20° for low frequencies. For increasing velocities, a rotation of the lobes towards the jet axis is observed. This displacement seems concomitant to the variations of the exponent n, of the velocity V in the expression: where W_a≃ρ²S^mVⁿ where W_a is the acoustic power, ρ the gas density, and S the cross‐sectional area of the nozzle exit. In this expression, n reaches a maximum value for mean frequencies. Its random value for the whole spectrum is around 5. The value of m is greatest at low frequencies and differs little from 1 for the whole spectrum. The sound source factor η, defined as η = W_a/W_i where W_i is the jet kinetic energy, seems to vary according to 4.5 power of the Mach number for M > 1.