Manual-4
UNDERSTANDING HEADPHONE POWER REQUIREMENTS
HEADPHONE SENSITIVITY
Headphone manufacturers specify a “sensitivity” rating
for their products that is very similar to loudspeaker sensitiv-
ity ratings. For loudspeakers, the standard is to apply 1 watt
and then measure the sound pressure level (SPL) at a distance
of 1 meter. For headphones, the standard is to apply 1
milliwatt (1 mW = 1/1000 of a watt) and then measure the
sound pressure level at the earpiece (using a dummy head
with built-in microphones). Sensitivity is then stated as the
number of dB of actual sound level (SPL) produced by the
headphones with 1 mW of input; headphone specifications
commonly refer to this by the misleading term “dB/mW.”
What they really mean is dB SPL for 1 mW input.
Think about these sensitivity definitions a moment:
headphone sensitivity is rated using 1/1000 of a watt; loud-
speaker sensitivity is rated using 1 watt. So a quick rule-of-
thumb is that you are going to need about 1/1000 as much
power to drive your headphones as to drive your loudspeakers
since both of their sensitivity ratings are similar (around 90-
110 dB-SPL). For example, if your hi-fi amp is rated at 65
watts, then you would need only 65 mW to drive comparable
headphones. (Actually you need less than 65 mW since most
people don’t listen to their loudspeakers at 1 meter.) And this
is exactly what you find in hi-fi receivers—their headphone
jacks typically provide only 10-20 mW of output Power.
Take another moment and think about all those portable
tape players. Ever hear one? They sound great, and loud. Why
you can even hear the headphones ten feet away as the
teenage skateboarder that ran over your foot escapes.
Power output? About 12 mW.
As you can see from the chart below, headphones near 75
ohms impedance produce the highest power levels from the
MH 4. However, heaphone sensitivities vary widely, and are
not merely a function of power.
MH 4 power with all Channels driven simultaneously or with any combination
THE LIST
As an aid in finding out how much power is available
from the MH 4 Headphone Console we have compiled a
listing of popular headphones. To the right is a column giving
the maximum SPL obtainable using the MH 4 and any
particular headphone—ultimately, it all gets down to actual
SPL. The power rating really doesn’t matter at all—either it’s
loud enough or it isn’t (of course it has to be clean power, not
clipped and distorted). The SPL numbers shown are for
maximum
continuous
SPL; for momentary peak SPL add 3
dB.
Note that the maximum achievable SPL varies widely for
different models and manufacturers, ranging from a low of
107 dB to a harmful 133 dB! The table also shows there is
very little relationship between headphone impedance and
sensitivity, and that power output
alone
means nothing, since
in one case 89 mW produces a maximum SPL of 107 dB, yet
in another case the same 89 mW yields an SPL of 117 dB!
Sensitivity dB is measured sound pressure level with
1 mW of power. The MH 4 Max Power mW column is typical
continuous average power, 20 Hz-20 kHz, with THD less than
0.1%.
If headphones are not yet owned, or replacements are
desired, use this listing as a guide for selecting head-phones
with sufficient sensitivity for the maximum desired SPL.
Note: headphones with an impedance of less than 32 ohms
are not recommended for use with the MH 4.
MH 4 POWER
0.089
0.146
0.207
0.239
0.233
0.207
0.146
0.089
0.000
0.050
0.100
0.150
0.200
0.250
600
300
150
75
50
32
16
8
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