Powerful Questions
The same questions come to almost every audio
shopper’s mind. The first is usually "How much money do I need to spend?"
The second is often "How much power do I need?"
Amplifier power can affect how loudly your speakers will be
able to play. But if you don’t overspend on power, you might be able to save yourself
some money. On the other hand, if you’re the kind of person who needs more juice, you
sure don’t want to underpower your system.
Power-amplifier output is always expressed in watts,
usually in watts per channel and into an impedance of 8 ohms, which is considered a normal
loudspeaker load. When looking at the specifications of stereo power or integrated
amplifiers, you’ll likely see something like "100Wpc into 8 ohms." This
means that each of the amplifier's channels will deliver at least 100 watts into an 8-ohm
load. If the same amplifier is presented with another load -- say, 6 or 4 ohms -- then
that rating would be different: the impedance the amplifier is presented with affects how
much power it can deliver to that speaker.
But if the power rating listed is for only one
channel, it’s likely that the power output will be lower when two or more channels
are driven simultaneously: It’s harder for an amplifier to drive multiple channels
than to drive just one. That’s why it’s important to have standards, so that
you’re always comparing apples with apples, and to look for that in the specs. Most
companies comply with these standards, but some try to fudge their numbers. This is why
it’s important to check for third-party measurements, such as the ones we publish on
the SoundStage! Network (see www.amplifiermeasurements.com).
However, when trying to determine how loudly a speaker will
play, it’s not enough to look only at an amplifier’s power output. Another
important specification is of a loudspeaker’s sensitivity. It tells you how
loudly a speaker will play with a given input signal, though it has no bearing on sound
quality. It’s simply a measure of how loudly something will play, not how well.
Still, it can give you an idea of how powerful your amplifier needs to be.
As with power-amplifier output ratings, there are some
general guidelines about measuring a speaker’s sensitivity so that consumers can
again compare apples with apples. The most common way is to place a microphone 1m in front
of a loudspeaker and measure the sound-pressure level (SPL) in decibels with an input
signal of 2.83 volts. This voltage is used because 2.83V into an 8-ohm load translates
into 1W of power. Quite often you’ll see a sensitivity specification that looks like
this: 87dB @ 1m/2.83V, or 87dB/m/2.83V. This means that the speaker will deliver an SPL of
87dB at 1m in front of the speaker with just 2.83V input. If the SPL in dB is less than
that -- say, 84dB -- it means the speaker is less sensitive, and will need more power to
get it up to the same volume level as a more sensitive speaker. We at the SoundStage!
Network measure speakers, too, and provide sensitivity measurements. You can find them at www.speakermeasurements.com.
As I said, though, a high or a low sensitivity is not a measure of a speaker’s sound
quality; it indicates only how loudly a speaker will play with a given input signal.
Now, let’s tie amplifier watts to speaker sensitivity
to show much amplifier power you might need.
An SPL of 87dB is quite loud. The average conversation is
about 60dB, a lawnmower is about 90dB, and anything over 90dB is really getting up there
-- a chainsaw is about 100dB, and a rock concert about 110dB. Not surprisingly, most
people who listen at a normal volume setting are listening at an average level somewhere
in the range of 80-90dB. Actually, 1W can get you quite far -- many speakers on the market
have a sensitivity of about 87dB (based on our measurements, that seems to be an average,
plus or minus 1dB). In my opinion, that’s why something like NAD’s C325BEE, the
50Wpc stereo integrated amplifier reviewed this month by Thom Moon, provides enough power
for most people. That’s also why so many people can derive so much pleasure from
lower-powered tube amps. (Jeff Stockton’s review of the Antique Sound Lab AQ1003 DT,
a tubed, 30Wpc integrated amp, appears next month.) Compared to amps rated to deliver
hundreds of watts per channel, those amps aren’t very powerful at all. If you
don’t overspend on power, you can save quite a bit of money.
But, that’s not to say that amps of lower power will
please everyone. Depending on your requirements, you might really need more power, and
thus really need to spend more.
While many speakers will play at respectable volume levels
on very little power, to get them to play louder and louder can take a lot of juice. In
fact, increasing a speaker’s output by just 3dB requires a doubling of
amplifier power. So if it takes 1W to deliver 87dB, it takes 2W to reach 90dB, 4W to reach
93dB . . . and 64W to reach 105dB. That’s more than the little NAD or ASL amp can
deliver. And if you want to play your music extraordinarily loud -- e.g., at
rock-concert level -- it can take hundreds and hundreds of watts, depending on your
speakers’ sensitivity.
Furthermore, that 87dB SPL that I keep referring to is at a
listening position quite close to the speaker -- only 1m away. Output measured at the
listening position decreases as the distance from the source increases. In other words, if
you move back another meter, to 2m, the SPL decreases. Move back 3m and it goes down even
more. So if you’re sitting far away from your speakers, that will strongly affect how
much power you need. So will the size of your room -- a speaker in a large room
must be played louder to sound as if it’s playing at the same level as in a smaller
room.
The answer to the question "How much power do I
need?" will be different for everyone because it will depend on many things. But
understanding power output and speaker sensitivity will get you some of the way there, and
taking into consideration listening distance, room size, and preferred listening levels
will get you the rest of the way. For example, although my room is very large, I
don’t need an amplifier more powerful than 100Wpc because I listen at relatively
modest levels compared to, say, home-theater enthusiasts -- I rarely go much above 90dB.
And in a much smaller room, maybe something measuring 14’ x 16’, I can’t
see needing more than the 50Wpc delivered by the NAD C325BEE.
But that’s me. Your needs will undoubtedly be
different. Now that you’re armed with the necessary knowledge, you can ask some more powerful
questions to get a much clearer answer and make much more informed buying decisions.
…Doug Schneider
E-mail comments to the editor@goodsound.com.
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