Babble noise was added to the speech signals at levels of0, -3, -6 and -9 dB. This range was chosen after first informaltests showed that masking on audio only stimuli started at 0dB, whereas stimuli were completely masked at -12 dB.上面这段话里面，分贝为什么是负的？
这段话的意思是说在正常的声音信号（speech signal）上添加噪音信号（babble noise）。添加的噪音信号的强度是相对于正常声音信号的，因此0dB指噪音信号与正常信号强度一样，-2dB指噪音信号比正常信号少2个分贝，以此类推。
豆豆乖乖
喜洋洋和美羊羊
纳斯她爸
