Letters that make background signal
s, a, o, b, d, g, k, l, n, c, r, i, u are letters that make 'background signal'
Words within background signal
ground, sign, BA, si, UN, Ba, AC, Ro, SI, kg, round, signal, back, background, AL, ND, Ac, Al, Nd, Si, Na, Ig are words within 'background signal'