You have not provided enough information. If you are driving an high efficiency ear-phone, a couple of milliwatts might get you about 80dB. The power requirement will go up with the distance from the source to the measurement point. Most stereo speakers are rated for efficiency at 1 meter and 1 watt input (driven with pink noise). Very inefficient speakers are about 80dB at 1m, 1W. Very efficient speakers run over 108dB at 1m and 1W. (28dB is a huge difference.) Look for spl (sound pressure level) to drop approximately with the square of the distance from the source (depending on surroundings). Also expect to use about four times the watts to double the spl - but remember the doubling the spl adds 3 dB: the scale is exponential (or logarithmic, depending on how you set up the equation). So, 13dB is twice the spl of 10dB which is twice 7dB, etc. You have to answer: what kind of drivers (effiency)? how large a space, where do you measure? how "alive" is the environment? (standard measurements are done in a "dead" anechoic chamber, the inside of a concrete box will be much louder) how many channels/drivers are you driving?

Rule of thumb: Peak_level = Sensitivity + Power - Distance_factor (dBW) Speaker sensitivity is specified in terms of dB, measured 1 metre from the speaker. The distance factor is 0 if you are sitting one metre from the speaker. You subtract about 5 dB for each metre you move further away from the source. So, if you are 3 metres (about 10 feet) from the speaker, the distance factor is -10 dB. Amplifier power, in Watts, is converted to dBW using: 10 log(Power_Watts). If you have a peak level of 100 dB, a fairly sensitive speaker (e.g., 90 dB), and you are located 3m from the speaker, then: 100 = 90 + Power - 10 ==> Power = 20 dBW. Values expressed in dBW can be converted to Watts using: Watts = 10^(Power_dBW / 10). In this case: 10^(20/10) = 10^2 = 100 Watts. Note that a small change in speaker sensitivity has a significant impact, since a 3 dB increase requires double and a 10 dB increase requires ten times the power. A 3 dB change is audible; a sound that is twice as loud requires ten times the power. Since most amplifiers tick over somewhere under 1 W during normal operation, a 100 W amp may seem powerful enough. However, music is full of transients. The 1 W signal would need to jump momemtarily to 1,000 W to handle a 30dB peak - hardly an uncommon demand. This also explains why there is no effective difference between an amplifier rated, for example, at 85 Watts and one rated at 90.