Example of FFT in R (because I always forget)

fft.R

# Sampling frequency and audio length
freq <- 400
duration <- 5

# radian constant for sin/cos
tau <- 2*pi

# Generate a composite signal (4, 6, 9 hz)
x <- seq(0, duration, length.out = freq*duration) 
y <- 
  1.5 * sin(4 * tau * x) + 
  2.5 * sin(6 * tau * x) + 
  1.2 * cos(9 * tau * x)

plot(x,y, type = 'l')

# Perform FFT and drop 2nd half 
fx <- fft(y)
fx <- head(fx, length(y)/2)

# List the strongest signals (yields back 4, 6, 9hz with amplitude)
df <- tibble::tibble(
  freq = (seq_along(fx)-1)/duration,
  energy = Mod(fx) / length(fx),
)
dplyr::arrange(df, desc(energy))