Generate a sinusoid at 150hz and a whistle repeat once which is inverted. Modification of the envelope to create a fade in and fade out. Between the 2 whistle, modification to have no jump. Then generate a tone and placed it like the gradation with the whistle.
This is some room tone i recorded in a thankfully quiet time in my studio. It's an oktava 012 microphone in our whisperroom with the door wide open. Signal path is a neve 1272 and an apogee mini-me. Recorded in 24-bit at 48khz.
A sound stimuli to demonstrate combination tone from speakers. You should hear additional frequency of 500 hz. The sound itself is summation of1000 hz and 1500 hz.
To create this sound, i created a sinusoid of 500 hz and an amplitude of 0. 8. I then multiplied the speed of 0. 750. To get this final sound is to duplicate the sinusoid several times.