Achieving "money notes" poses a particular obstacle for classical musicians, as compared to pop musicians, because opera singers do not usually sing with a microphone. While both pop and classical musicians aim at producing well-sung notes with a good tone (with the respective stylistic norms appropriate to each idiom), the pop musician does not have to sing loudly at high sound-pressure levels to be heard, because the amplification of the PA system speakers does this for them. In an opera hall, though, a classical singer has to project their voice over the accompaniment of a symphony orchestra without a microphone or PA system. In trying to project the voice with at such high volumes, a classical singer may find it harder to maintain an attractive tone quality and correct intonation than a mic-wielding pop singer. To ease the pressure on classical vocalists' voices, major opera halls of the 2000s are increasingly using electronic acoustic enhancement, a subtle type of sound reinforcement system.