Recording - July 2015 - 59

Earphones
I addressed earphone sensitivity in the July 2014 issue of
Recording, so I won't repeat that treatise here other than with
regard to published specs. Since we attach earphones directly
to our ears, the SPL is measured as close as possible to the
diaphragm rather than 1 meter away. Also, because earphones
are low-power devices, their input specification is in milliwatts
rather than watts.
When people complained that their older earphones weren't
loud enough when driven by their low voltage phones and
tablets, lower impedance earphones started coming to market,
with sensitivity as the SPL at 1 volt rather than the power necessary to achieve a specified SPL. A spec for classic earphones
will read something like: "Sensitivity: 94 dB @ 50 mW,
Impedance: 60 ohms", whereas for a newfangled lower impedance model you might see "Sensitivity: 106 dB @ 1V".
What about audio interfaces?
The spec sheet for one of these devices should include sensitivity, but it rarely does. When the input is a mic, instrument, or
line level analog source and the output is USB, Thunderbolt,
Ethernet, S/PDIF, or AES3, a conversion between signal formats
is involved. The output of an A/D converter is a 16- or 24-bit
digital word, with all bits on (e.g. 1111111111111111) representing the largest number it can handle. This is called "full

scale" and is the reference for digital level in dB. 0 dBFS (dB referenced to full scale) is the highest level possible-all levels
below full scale, when expressed as dBFS, are negative numbers. A D/A converter works in reverse. A 0 dBFS digital audio
signal produces the maximum analog output level.
So, how many volts in does it take to reach 0 dBFS? And conversely, how many volts do you get out when playing 0 dBFS
recording? Here are three important things to understand about
digital sensitivity:
1. There is no headroom above 0 dBFS in a digital system.
2. There is no standard for the relationship between volts and bits.
3. Input sensitivity is fixed on some devices and adjustable on
others.
The meter on early digital recorders (DATs, etc.), usually had
a mark at -20, -18, -16, or even -12 dBFS. Setting the eyeball
average level to that mark allowed its respective number of dB
of headroom, but you could make as much or as little headroom
as you wanted.
Today's computer audio interfaces are built around A/D
and/or D/A converter chips having fixed sensitivity, with the
manufacturer adding gain or attenuation to make a useful product. Mic inputs always have a variable gain preamp ahead of
the A/D converter, so the preamp gain control doubles as the
sensitivity adjustment. Sometimes you'll find a specification like:

"Mic Input (maximum gain): -56 dBu = 0 dBFS", though often
the mic preamp gain will be specified.
Fixed-gain line inputs are usually specified relative to a nominal operating level: "+4 dBu = -16 dBFS" or are referenced to
full scale: "+20 dBu = 0 dBFS" (these are equivalent). This is the
genesis of the vague input specification of "-16 dBFS".
Interfaces nearly always have an output level control, with the maximum output level at 0 dBFS ("0 dBFS = +22 dBu") sometimes specified. This can be helpful when setting up your monitoring system.
Putting it all together
Let's look at a typical signal chain to see if we can get from
here to there with the help of a spec sheet. In this example, our
job is to make a spoken word recording using a Shure SM57
mic connected to a computer through a typical USB interface.
Our first task is to figure out how many volts will come out of
the mic when our narrator is talking. Using my best narrator
voice, I became the "calibrated source" by speaking about 6
inches from my SPL meter. The eyeball average reading was 76
dB SPL. According to the spec sheet, the SM57 has a sensitivity
of -56 dBV/Pa (94 dB SPL). Since my measured SPL is 18 dB
(94 - 76) below the reference level, we can expect the mic's output to be 18 dB below what it would be at the reference SPL, or
-74 dBV (-56 - 18). This converts to -72 dBu (see Part 1,
February 2015, for how).

The interface manufacturer specifies the mic preamp's gain to
be 60 dB. An astute reviewer (that's me) wrote that at maximum
gain, the sensitivity from mic input to digital output is -56 dBu =
0 dBFS. (Thanks, Mike.) Since the mic's output is 16 dB (72 -
56) below the 0 dBFS reference, the record level will be around
-16 dBFS.
In real life, the DAW's meter will probably show peaks in
the -10 to -4 dBFS range, and here's why. The mic spec is based
on the RMS voltage of a sine wave, and this doesn't represent
human speech very well. The DAW meter responds to peak
rather than RMS levels, which will be higher than this theoretical
exercise predicts. It's fair to say that we have 16 dB of headroom, and the DAW meter shows that we're occasionally using
most of that.
In this case, it took all of the preamp's gain to reach a decent
record level. Replace the soft-voiced narrator with a powerful
rock singer and the preamp gain will need to come down by 20
dB or more in order to avoid overloading. Replace the SM57
with a hot condenser mic, and you might have to switch in a 20
dB pad. And that, folks, is how it works in the real world.
Mike Rivers (rivers@recordingmag.com) is a studio and live
sound engineer, gear design guru, and educator. Learn more
(lots more!) at mikeriversaudio.wordpress.com.
RECORDING July 2015

59


http://mikeriversaudio.wordpress.com

Table of Contents for the Digital Edition of Recording - July 2015

Contents
Recording - July 2015 - Cover1
Recording - July 2015 - Cover2
Recording - July 2015 - 1
Recording - July 2015 - 2
Recording - July 2015 - 3
Recording - July 2015 - 4
Recording - July 2015 - 5
Recording - July 2015 - Contents
Recording - July 2015 - 7
Recording - July 2015 - 8
Recording - July 2015 - 9
Recording - July 2015 - 10
Recording - July 2015 - 11
Recording - July 2015 - 12
Recording - July 2015 - 13
Recording - July 2015 - 14
Recording - July 2015 - 15
Recording - July 2015 - 16
Recording - July 2015 - 17
Recording - July 2015 - 18
Recording - July 2015 - 19
Recording - July 2015 - 20
Recording - July 2015 - 21
Recording - July 2015 - 22
Recording - July 2015 - 23
Recording - July 2015 - 24
Recording - July 2015 - 25
Recording - July 2015 - 26
Recording - July 2015 - 27
Recording - July 2015 - 28
Recording - July 2015 - 29
Recording - July 2015 - 30
Recording - July 2015 - 31
Recording - July 2015 - 32
Recording - July 2015 - 33
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Recording - July 2015 - 35
Recording - July 2015 - 36
Recording - July 2015 - 37
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Recording - July 2015 - 40
Recording - July 2015 - 41
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Recording - July 2015 - 45
Recording - July 2015 - 46
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Recording - July 2015 - 49
Recording - July 2015 - 50
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Recording - July 2015 - Cover3
Recording - July 2015 - Cover4
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