Q. How much amplifier power do I need?
A. When selecting speakers, the second most commonly asked question by prospective buyers is "how much power do I need?" This question does not have the simple answer most people expect because it is determined by more than just the loudspeaker's efficiency.
The first thing to keep in mind is that sound quality is usually much more important than sound quantity. There can be large differences in the sonic performance of two amplifiers of equal power, and a smaller powered amplifier can sound superior to a larger powered amplifier. Almost everyone will be happier with a 50 watt amplifier of high sonic quality than with a 200 watt amplifier of mediocre sonic quality. For this reason, we feel there is no substitute for listening to make your amplifier decision.
It is important to have enough power to play at the volume level you desire without distortion. If you play the speakers more loudly than the volume the amplifier can cleanly produce, the amplifier will produce overload (clipping) distortion. The sound will become compressed, strained, and in extreme cases, obviously distorted. This distortion is actually non-musical additional energy and since it is concentrated in the high frequency region where the speaker is least able to handle it, the tweeter's voice coil can be burned in extreme cases. Driver damage in general is usually the result of having inadequate power rather than having too much. If your system does not play as loudly as you would like without becoming distorted, you need a more powerful amplifier.
There are three almost equally important factors that determine how much power is needed for a given situation. In order of importance they are:
1. The volume level desired
2. The speaker's efficiency
3. The size of the room
Each of these factors can make a 10-to-1 difference in the power required.
In the real world, if all 3 factors are average, about 50 watts/channel is required. Each factor can raise or lower this amount by a factor of about 3. Average values for each are:
- a volume level of 88dB SPL
- speaker efficiency of 87dB/W-m
- a room size of 3000 cu ft
The first factor is the most difficult to analyze or objectively determine. There are different ideas about what is "loud." Some people do not want to play music above a sound pressure level of 88dB and some do not want to play music below 88dB. Usually, people who like to listen only at low levels can decrease their power by a factor of 2. People who like music at very loud levels, even if only occasionally, should increase their power by 2 times or more.
A speaker with a low 84dB/W-m efficiency will require twice the power of an 87dB speaker. and one with a high rating of 90dB/W-m will require only half the power of an average speaker. Usually, high efficiency can be obtained only by trading off sonic quality ---there are very few speakers that provide a very high level of both. THIEL speakers are of average efficiency and therefore require an average amount of power.
A small room will need less power for a given loudness level than a large room. A very small room of 1000 cu ft (11 ft x 11 ft with an 8 ft ceiling) will usually require only half the power of an average room. A large room of 6000 cu ft (20 ft x 30 ft with a 10 ft ceiling) will usually require twice the average power. If the listening room is connected to another room by a large open area, the required power will increase, but not by the amount of the combined room volume. If the room has a "dropped" ceiling with lightweight panels, the ceiling will be almost transparent acoustically. In this case the space above the ceiling should be added. If the panels are heavy they will act more like a solid ceiling.
With all this in mind, a person who likes to play music only at lower levels, has a small room, and who has fairly efficient speakers can get the performance desired with only 20 watts; whereas a person who sometimes likes to play loudly, has low efficiency speakers, and a large room may need 300 watts or more.
Also, remember that how loud your system plays has nothing to do with the position of your volume knob. Source components such as CD players and tape decks, as well as recordings themselves, can all have different output levels. This means that to achieve the same volume level for each source, your preamp's volume control may need to be set at a different position for each source. For example, when some high output CD players are used with lower power amplifiers, maximum power can be reached with a volume knob setting as low as 4 on a scale of 10.