Loudspeaker Sensitivity and Amplifier Power Requirement
By Chuck Hawks
All loudspeakers have a maximum power handling capability. In the case of home Hi-Fi speakers, this is usually somewhere between 50 and 250 watts RMS of program material (music).
For example, the PSB Synchrony Two-B bookshelf speaker, a small two-way bookshelf design using a 5.5" low/mid frequency driver and a dome tweeter, has a power handling specification of 150 watts maximum (program). The large, two-way, Klipsch RF-7 II floor standing tower speaker (two 10" woofers and a 1200 HZ horn mid/tweeter) has a maximum power handling specification of 250 watts RMS and 1000 watts peak. The very large, three-way, floor standing, Klipschorn corner-horn loudspeaker (15" woofer/450 Hz horn/4500 Hz horn) has a maximum power handling specification of 100 watts RMS and 400 watts peak.
The required power column in the tables below do not take into account the limitation imposed by the loudspeaker's power handling capability, but the user must. Apply excessive power attempting to drive your loudspeakers to a high sound pressure level (SPL) and you will burn them out!
As you can see from the charts below, the Klipschorn, because it is more efficient, will play much louder than the PSB Synchrony Two-B and do it with much less amplifier power. You would exceed the Synchrony Two-B's power handling limit long before it could approach the sound pressure level attainable by the Klipschorn. A loudspeaker's efficiency is much more important than its power handling capability in determining its maximum SPL, although both ultimately come into play.
From the amplifier perspective, it is amazing how much power it takes to drive inefficient loudspeakers to high listening levels (90db and higher). The efficient Klipsch RF-7 II (101db sensitivity) requires 53 watts RMS to achieve a average 90db SPL with 15db of headroom. To achieve the same output level, an inefficient speaker (83db sensitivity) would require 3354 watts RMS! While many, probably most, home stereo amplifiers can manage 50+ watts/channel output, I can't offhand think of any with 3300+ watt/channel capability. Nor do I know of any high-fidelity loudspeakers that can tolerate anywhere near that kind of input power.
Speaker Sensitivity: Usually quoted by the loudspeaker manufacturer and measured at one meter with one watt input. According to the amplifier manufacturer Crown, typical or average home Hi-Fi speakers have a sensitivity of about 85db. The PSB Synchrony Two-B bookshelf speaker, for example, has a sensitivity of 86db. The large and highly efficient Klipsch RF-7 II floor standing tower loudspeaker has a 101db sensitivity, while the very large and fully horn loaded Klipschorn speaker has a 105db sensitivity.
Listener Distance: The distance from the loudspeakers to the listening position; given in feet and meters. In the case of the charts below, the first is for a listening position 15 feet from the speakers (chosen to represent a medium size listening room) and the second is for a listening position eight feet from the speakers (representing a smallish listening room).
Desired SPL: The average sound pressure level (loudness) required at the listening position, measured with an SPL meter set at "C" weighting and SLOW response. For the 15 foot listening distance I chose 90db SPL, which is loud and might sound realistic for listening to most pop, country, jazz and folk-rock type music. For the eight foot listening distance I chose an 80db SPL, which is still reasonably loud and might be representative of classical chamber music, folk, new age and the softer types of jazz. Realistically reproducing hard rock and symphonic classical music require higher SPL's in the 95-110db range, which are usually difficult or impossible to achieve with most home stereo systems.
Amplifier Headroom: The reserve amplifier capability required for music's momentary higher volume excursions. Music typically has peaks between 6 and 25 decibels higher than the average level and the power amplifier must handle those peaks without distorting (clipping). I chose 15db as a reasonable amount of headroom, although the amplifier might clip (usually very briefly and probably inaudibly) on the highest decibel peaks.
Required Power: The RMS watts per channel required to produce music at the desired SPL at the listening position.
How loud will speakers play music, driven with their maximum permissible program power? Using the same three examples mentioned above, the PSB Synchrony Two-B bookshelf speaker, Klipsch RF-7 II floor standing tower and Klipschorn floor standing corner horn at an eight foot (2.46M) listening distance and with the same 15db headroom specified in the tables above, here are the results to the nearest decibel at the listening position without exceeding the speaker's maximum power handling capability.
PSB Synchrony Two-B: Sensitivity 86db, Max. power handling 150 watts RMS = 84db SPL at 8' with 121 watts. (85db SPL requires 152 watts.)
Remember, these SPL numbers indicate the average program loudness; peak excursions can be as much as 15db higher. In my experience, it requires high efficiency loudspeakers (about 98db sensitivity or higher) to achieve anything like realistic live music levels. Even with high efficiency speakers in the 98-99db sensitivity area, it is desirable to have an amplifier capable of delivering a minimum of 100+ watts RMS/channel, both channels driven, 20-20,000 Hz. Of course, the speakers must be able to handle that much power. Most people have no idea how loud live music really is. Attend a few concerts of various types, including rock, country, jazz, pop and classical to see for yourself.
Copyright 2011, 2012 by Chuck Hawks. All rights reserved.