Abstract
<jats:p>We examined whether the spectrum differs before and after the spatial addition of low-frequency sound to indoor background noise in a quiet, real room. Because the possible carriers for such an addition are unlimited, three stationary noises with known spectral shapes (white, pink, and brown) were used as test signals. Background noise and a 40-Hz tone were reproduced simultaneously in the same space, and recordings with and without the addition were compared with the microphone fixed in place. In the low band, the spectrum, low-band ACI, and autocorrelation showed predictable changes that scaled with the amount added. These followed almost trivially from linear superposition and normalization and did not depend on the added frequency. A spectral difference appeared at 40 Hz and its harmonics (80 and 120 Hz). A high-band difference (1–8 kHz) was also observed; it exceeded the high-band variability of recordings without addition and did not arise from numerical linear addition to the same waveform. The difference was reproduced in a different room using different equipment and independent recordings (white and pink) and varied according to the carrier (Table 1). This study reports these differences as experimental observations and does not address their generation mechanisms.</jats:p>