A question of power: what is the sound of one amp clipping? (Audio Update)

For many serious music listeners, the advent of digital reproduction raised a troubling question: Can their present low- or medium-powered amplifiers cope with the 90-dB dynamic range inherent in CD technology?

Although my real-time analyzer confirms that well-recorded CDs register peaks 10 dB or so higher than the same music on an analog tape or disc, the average sound level in the listening room--which is determined by the volume control setting--is essentially unchanged. Furthermore, the dynamic range potential of a CD is not likely to be realized on most discs and, in any case, is usually manifested as a reduction in noise at low levels rather than as an increase in sound at high levels. And so, for the vast majority of listening, their CD players will not push amplifiers above and beyond the call of conventional duty.

Power adequacy

Whether your system has adequate amplifier power depends on: (1) speaker efficiency, (2) the acoustics and size of the listening room, (3) the kinds of music you listen to, and (4) how loudly you play it. Let's discuss the four factors in order.

(1) Speaker efficiency is usually given as a sensitivity rating written as 86 dB/W/m. This translates into the speaker producing a sound pressure level of 86 decibels when fed 1 watt of test signal and measured at an on-axis distance of 1 meter. A high-sensitivity (very efficient) speaker might have a rating around 94 dB, medium sensitivity is about 87 dB, and low sensitivity is about 81 dB.

For a practical perspective on these figures, it should be appreciated that a 3-dB increase in speaker sensitivity means that for a given volume level, 50 percent (!) less amplifier power will be required. In other words, a 30-watt amplifier feeding a speaker with an 84-dB sensitivity will sound as loud as a 60-watt amplifier feeding an 81-dB speaker. However, a 3-dB increase in output level is barely discernible; It takes a 5- or 6-dB increase (a tripling or quadrupling) of applied power to be audibly significant.

(2) The size and acoustic properties of a listening room can significantly influence power requirements. Reducing room size by half will cut the amplifier power requirement by about one third. Room furnishings can have an even greater effect, changing the power requirements over a range of about three to one.