Morse code base in Haskell

morse.hs

mport Sound.Tomato.Speakers

sound freq duration = withSpeakers sampleRate 128 $ \s -> playSamples s sound
    where
    sampleRate  = 22050
    dt          = 1 / sampleRate -- time in seconds of a single sample
    sound       = take (ceiling $ duration / dt)
                $ map (0.3*) $ sine freq
    
    sine freq = [sin (2*pi*freq*dt*fromIntegral t) | t <- [0..]]

beep duration = sound 1000 duration

short = beep 0.2
long = beep 0.3

letter 'S' = [short, short, short]
letter 'O' = [long, long, long]

sentence  = sequence_ . concatMap letter

main = sentence "SOS"