impdsens

When considering speakers to buy, two specs I check for are Impedance and Sensitivity Ratings. Upon reading this article, you should have a better understanding about wattage and why 70 watts per channel isn't that bad!

Impedance refers to the amount of resistance amps face when powering a speaker.* Most home speakers are rated at 8 ohms, but there are also home speakers rated at 6 and 4 ohms. If one were to buy 4-ohm rated speakers, only a very good receiver or separate amps can do them justice because it puts more pressure on the amp to correlate the sound. Additionally, the amp will deliver more current to the speakers with lower impedance because as the resistance decreases, the current increases. For example, an Onkyo TX-DS575x is rated at 70 watts per channel into 8 ohms and 90 watts per channel into 6 ohms. Also, the TOTAL HARMONIC DISTORTION increases as the impedance decreases.

A speaker's Sensitivity Rating refers to the loudness factor and efficiency. In other words, a speaker rated at 89dB (measured with 1 watt, 1 meter away) won't play as loud as a speaker rated at 92dB (1w/1m).

The first set of figures is the 89dB rated speaker, the second set is the 92dB speaker. Measured from one meter away....

89dB/W/m
1 watt....89dB
2 watts....92dB
4 watts....95dB
8 watts....98dB
16 watts....101dB
32 watts....104dB
64 watts....107dB
128 watts....110dB
256 watts....113dB

Doubling the power indicates a 3dB increase in sound pressure level in theory. Klipsch speakers, which typically have high sensitivity ratings (~98-105dB/1w/1m), are popular because it doesn't take much to power them. 32 watts to a Klipsch, for example, could easily produce 116dB whereas it would take 1,024 watts to get 116dB out of a speaker rated at 86dB.

The most important point to illustrate is that during regular quiet passages, you might only need about 1-10 watts, and then when there is an explosion, SPL might increase by 20dB...that's when you'd need as many watts as you can handle. For example, if your speakers are rated at 92dB/W/m and regular dialogue requires 1 watt, an explosion or loud music that peaks at 113dB means you'd need about 128 watts. This range of SPL (dB) is important for the realism of what you hear, and that's why you need the juice.

*Note: Speakers often dip below and rise above their nominally rated impedance.

5.26.01, edited 5.31.01, again on 9.1.01, updated 1.24.02