Jeff - this may be more than you wanted to know, but here are the primary variables involved; I've found over the years that having a good appreciation for the relationship between power, loudness and speaker capabilities can really help when thinking about putting together your system or making changes to it:
Speaker sensitivity - Different speakers have different sensitivity ratings. Sensitivity ratings are standardized in the form of decibels (dB) produced with 2.83 volts of power, measured at one meter from the speaker. 2.83V = 1 watt with an 8 ohm speaker, and 2 watts with a 4 ohm speaker. The lower the sensitivity, the higher the power necessary to achieve a given loudness.
Desired loudness (what's the loudest you will ever want to listen) - This is a personal choice, but some things to keep in mind include:
- Distance from speakers - each doubling of distance drops speaker output by about 6dB. So, an 8 ohm speaker with a sensitivity of 90dB will deliver "only" 84dB at 2 meters, and 78dB at 4 meters.
- Room size - other variables being equal, the smaller the space, the more reinforcement. Stated another way, it takes less power to produce a given loudness in a small space vs a large one.
- Average vs peak - in a well-engineered music recording, the difference between average loudness and short team peaks in the music will be 9-12dB (and quite a bit higher with some recordings). Each 3dB change = a doubling (or halving) of power. So, if it takes 1 watt of continuous (aka average, aka RMS) power from the amplifier to produce the desired loudness, the peaks will require 8-16 watts to keep distortion to a minimum.
Note: 3dB is generally considered to be the smallest change in loudness to be easily noticeable to someone with average hearing. Many folks can "hear" a 2dB change, and some can detect even a 1dB change.
- All speakers have limits to the amount of power they can handle. Unfortunately, there are not standards for defining these limits. Generally, if the manufacturer states a power limit, it will be in the form of continuous power. It is fairly safe to assume that if a speaker is capable of handling, for example, 75 watts on a continuous basis, it can handle 3-4 times that power (or more) during short term peaks in the music.
Amplifier power - The standard for rating the output capabilities of a stereo amp is in the form of its continuous (RMS) power capability. While just about all amps can deliver more than this for very short durations (tens or hundreds of milliseconds), it is generally good practice to leave this "buffer" out of of your considerations. With this in mind, you should just assume that an amplifier will clip any portion of the signal requiring more than its rated RMS power capability.
Combining this assumption with the need to handle 9-12dB peaks in the music, an amplifier needs to have an RMS power rating at least 8-16 times higher than the power required to achieve a given, continuous dB level of loudness. For example, if it takes 2 watts of continuous power to achieve your desired loudness (measured as an average level) with a particular pair of speakers, your amp should have an RMS rating of at least 16-32 watts to avoid clipping the signal, or at least keep it to a minimum.
To further illustrate, let's assume speakers with a sensitivity of 87dB, a listening distance of 4 meters, and the desire, at least once in a while, to achieve a loudness of 90dB (average). First, adding the second speaker (assuming stereo) pushes output by about 3dB, taking us to 90dB with one watt. But that's at 1 meter. At 12 meters, output will be down by about 12dB, putting us at 78dB at our listening spot. Getting back to 90dB will take 16x times the power, or 16 watts. But wait...we still need accommodate transient peaks in the music. That's another 9-12dB to avoid or at least minimize clipping..which requires an amp with an RMS rating 128-256 watts!
In the real world, you would probably be ok at half that (64-128 watts), but hopefully you are getting a sense of the exponential nature of power vs loudness.
In addition to adding distortion, clipping - when excessive - can result in more power than normal going to the tweeters. For example, let's assume that the tweeters, because of the crossover network in the speakers and the nature of energy distribution at various frequencies, normally get 10% of the power going to the speakers. And, let's assume the speakers are correctly rated to handle 75 watts. The tweeters in such speakers would be getting 7.5 watts at the amp's rated power, and might be capable of handling 10 watts. 75 watts of unclipped power from the amp to the speakers will therefore be no problem. But turn up the amp some more, and the lower frequencies run out of power and start clipping. However, there is still power available for the higher frequencies (which usually have lower peak amplitudes). So, the tweeters start getting more than the intended 7.5 watts. Push the amp harder, and it may well deliver more than the 10 watts required to destroy the tweeters.
To summarize, if your amp can produce the loudness you desire, without clipping (or, more realistically, with only minimal clipping), and your speakers can handle the power required to do so, you are good. In your case, your amp will be clipping well before you start pushing the speakers to their maximum output. So, your plan to keep volume to a moderate level is a good one. In your room, "moderate" can still be
quite loud. Just don't push the speakers to the point where the quality of the sound begins to deteriorate.
My personal point of view is having an amp capable of more power is just about always a good thing. It provides the ability to go louder (when you want to do so), and even more importantly, reduces the potential for clipping because of the power "in reserve" (headroom). I believe that clipping is far more common than most people appreciate.
Sure, you can crank a 25 watt amp up and get it LOUD with speakers of average sensitivity, but I guarantee it will be clipping fairly significantly when doing so...reducing sound quality and creating the potential for damage to both your speakers and your amp (some amps will start oscillating with heavy clipping). The only downside to a high-powered amp is the potential for over-driving your speakers and damaging them, but this is easily avoided with judicious use of the volume control (and not using it when drinking, when having uninhibited guests over, etc).
Unfortunately, higher powered amps of good quality will cost more than many folks are willing to spend. An alternative is to seek out speakers with very high sensitivity ratings (95dB or higher), thereby dramatically reducing the power required to achieve a given loudness. This a common strategy with tube amps, which by and large have quite a bit lower output capabilities than their solid state counterparts.
EDIT: As mentioned by a subsequent poster,
speaker impedance is also an important variable. Other than mentioning the relationship between impedance and power draw, I did not want to further complicate my dissertation by including this topic. Suffice to say that rated impedance does not equal actual impedance, and is often spec-ed higher than it should be.
A speaker's actual impedance, which varies by frequency and therefore is constantly changing, is not supposed to drop below 80% of rated impedance anywhere on its performance curve, but in many cases it does. The lower the impedance, the more current the amp must produce. Amps with the ability to handle low impedance loads (say, below 4 ohms) require stouter power supplies, output transistors and heat sinking...all of which add significantly to the cost of manufacturing.
The lower the impedance, the more stress on the amp. Depending upon the duration of impedance dips and how hard the amp is being pushed, amps can shut down or drop their power output (if they have corresponding protection circuitry), or be damaged.
Simply put, if you are going to run speakers with low impedance characteristics, you need an amp capable of handling such loads (for example, one rated to handle 2 ohm loads). These are generally quite a bit more expensive than amps rated for 4-8 ohm loads, although Class D amps are changing that paradigm.
Note that some amps/receivers are designed to handle only 8 ohms, or sometimes only 6. Also, note that running more than one speaker per channel results in significantly reduced impedance.
Read me right, yours IS NOT stupid.
