Hi,
I have been reading the technical articles on this site, and I'm an
Axiom owner. I have an A/V technical question.
Question one concerns a speaker's sentivity measured at 1 watt/per meter,
which equals a specific decibel rating. Example: 90 decibels output at 1
watt
sensitivity. My understanding is that to produce 93 decibels this speaker
would need 2 watts, or 4 watts to produce 96 decibels, or 8 watts to produce
99 decibels. What I am wondering is how can all the different speakers
on the market produce a 3-decibel increase with double the power? Is
this correct or is it a "rough" guideline.
Thanks,
Stephen
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Hi Stephen,
Thanks for your question. To put decibels in context, a sound
level of 90 decibels is very loud, and 99 decibels is about as loud as most
live acoustic music (except rock concerts) like a full symphony orchestra
ever gets on peaks. It does get louder if you are sitting really close, in
the first few rows. The other night I was at a concert of the New York
Philharmonic and sat in the 6th row for a Mahler 7th symphony that used two
sets of percussion, two bass drums, orchestral bells, gongs, etc., and I
measured
peaks of 106 dB SPL at the end of the symphony during the final crescendo.
Your statement about the
power increases in watts required to produce slight but noticeable increases
in loudness in dB is correct. Each 3-dB increase requires twice as much
power in watts, and if you went from 90 dB to 100 dB (a 10 dB increase in
loudness is perceived by most as sounding "twice as loud"), you would need
ten times as much power, or 10 watts.
So you are correct, but consider that most listening to music is between 75
and 85 dB SPL (C weighting), and most
speakers except really insensitive ones will normally require from about 1
watt to perhaps 10 watts to recreate those levels---BUT, music is very
dynamic, so some peaks may be 10 dB louder than 85 dB, in which case the
amplifier
would need to supply ten times as much power. If the speaker was using 10
watts to reproduce an 85-dB sound level (quite loud) and a 95-dB peak came
along, then 100 watts would be required to handle the peaks. These figures
are "rough" guidelines, as the distance you are from the speaker will affect
the loudness and power requirements as well as the "liveness" of the room,
the size of
a room, and so on. But it is a logarithmic relationship, so it applies to
all speakers. Also, those measurements for speaker output are done in an
anechoic chamber which has no reflections from any wall surfaces, and they
are for one speaker only. If you use two speakers in stereo, then that adds
3 dB to the sound output level, and walls in real rooms help, so the power
requirements will be reduced. However, most of us sit at least two or three
meters away from the speakers, so then the power requirements will rise
significantly. I tend to advise customers to buy as much power as they can
afford so you never have to worry about overtaxing your amplifiers and
reaching "clipping" which is gross distortion that can damage speakers.
Moreover, even before an amplifier reaches gross distortion from hard
clipping, distortion levels rise quite dramatically as the amp nears the
clipping point. This can add a hard, edgy strident quality to the sound
before gross distortion becomes audible.
Hope
that answers your questions.
Kind regards,
Alan
