Download Adjusting the Receive Audio Chain

CAN I HEAR YOU NOW?
ADJUSTING THE
Introduction
Far too many contesters struggle unnecessarily to copy signals
through self-inflected interference and audio impairments. And
far too many of us finish the contest with temporary hearing
damage that, over the years, accrues to permanent hearing loss.
This article outlines improvements for received signal clarity and
hearing protection at contest stations.
RECEIVE AUDIO CHAIN
Eric L. Scace K3NA
[email protected]
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ERIC SCACE K3NA
CAN I HEAR YOU NOW? ADJUSTING THE RECEIVE AUDIO CHAIN
1 Dynamic range of receiver audio
Modern contest-grade receivers provide excellent operation over
a wide range of incoming signals. Let’s begin by examining how a
modern receiver transforms that range of incoming radio signals
to audio, taking the Orion transceiver as an example.
In a crowded contest band, several strong interfering signals
may be present nearby while the operator tries to copy a weak
one. Figure 1 graphs the audio output of a receiver in this situation, with audio frequency on the X axis and signal strength (in dB
above the average band noise level) on the Y axis. A dotted line
shows the receiver’s instantaneous internal noise level as a function of frequency. In this example, the operator correctly adjusted the receiver so that the band noise from the antenna (solid
line) hovers a few dB above the receiver internal noise floor. The
operator tuned an interesting weak signal to 450 Hz. Two other
strong signals, annoying multi-op stations calling CQ TEST, squat
nearby at 1100 and 1550 Hz. In the figure the receiver created
two artificial signals from strong signal mixing products, one of
which sits underneath the interesting weak signal at 450 Hz. (I
assume other artificial signals, such as IMD and reciprocal mixing
noise, to be negligible and these are not included in the figure.)
Fortunately for the operator, the two mixing products remain
weaker than the band noise and so go unnoticed.
Dynamic range measurements help describe how a receiver
performs in such situations. Sherwood Engineering measured the
Orion I receiver’s close-in dynamic range at 93 dB, and the Orion
II at 95 dB.1 “Close-in dynamic range” here means signals separated by 2 kHz but falling within the roofing filter passband used
in the test. Dynamic range degrades to 85 dB in Sherwood’s tests
when the signals all fall within the roofing filter passband. The
Orion’s close-in dynamic range currently stands as the best on
the market for commercial transceivers. Radio design continues
to advance and we can expect even better close-in dynamic
range in the future.
To put this range into perspective, the scenario of Figure 1
could describe a quiet 20 m band with antenna noise near the
receiver noise floor, a weak signal that doesn’t move the S-meter,
and those loud signals running S9+30 dB. As DXers and contesters, we wish to fully exploit the dynamic range of the receiver
and easily copy that weak interesting signal.
As we will see in this article, safely exploiting the full dynamic
range of the receiver requires proper adjustment of the receiver
audio chain: the connection between the receiver headphone jack
and the operator’s inner ear.
Figure 2 illustrates a simple receive audio chains. The operator
connected an ordinary headset directly to the receiver headphone
jack. This configuration exemplifies a single-op one-radio or
multi-op station. Even this simple case contains six factors influencing the quality of signals conveyed from the receiver to the
brain. The operator must understand and control each factor to
exploit fully the dynamic range of the receiver.
2 Receiver audio output
I made some simple measurements of headphone voltages using
a Heil Pro II headset, an Orion radio, and an oscilloscope. The
first measurements examined the receiver’s apparent noise floor
by disconnecting the antennas. I set the audio gain control so
that, while wearing the headset, I could hear a the receiver’s
noise floor just above the ambient noise of a quiet room. The
scope measured this noise signal at about 1 µVP-P.
Next I attached an antenna and tuned in a strong broadcast
carrier near the 40 m band. This very loud audio signal measured
40 mVP-P, +92 dB over the just-audible hiss.
Increasing the audio gain demonstrated the Orion could produce as much as 2 VP-P before distorting – over 120 dB above the
receiver noise floor level and stunningly loud. I couldn’t wear the
headphones at this level.
1 See www.sherweng.com/table.html.
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ERIC SCACE K3NA
CAN I HEAR YOU NOW? ADJUSTING THE RECEIVE AUDIO CHAIN
strong
signals
100
90
80
70
60
dB
50
40
30
20
10
weak
signal
band
noise
floor
mixing
product
mixing
product
0
-10
100 Hz
receiver
noise floor
300 Hz
500 Hz
1 kHz
2.5 kHz
5 kHz
Figure 1: Receiver audio output example. Vertical scale in dB relative to average band noise.
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ERIC SCACE K3NA
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other
operators
equipment
noise
2y
receiver
1
Ñ: 3
2
4 headphones
A/D
AF gain
5
middle
ear
brain
6
interest. This might typify ideal CW reception in a perfectly quiet
environment. In absolute terms, at this setting the band noise is
just louder than the sound of a mouse running across a wood
floor, or a mosquito buzzing 3 feet away!
The inner ear also has maximum limits, above which pain and
damage occur. Figure 3 also shows the threshold of pain. The two
loud signals fit below this threshold – but, at +98 dBA, represent
a danger discussed more fully below.
inner
ear
Figure 2: Simple receive audio chain.
This little exercise demonstrated that the Orion can easily deliver its 93 dB receiver dynamic range to the headset with plenty
of additional headroom. Safely maintaining these large dynamic
ranges between receiver and brain requires special attention to
safety.
3 Ear
The ear exhibits the widest range of sensitivity of the five human
senses. Point 6 in the Figure 2, the inner ear, functions as a biological analog-to-digital converter. This portion of the ear transforms analog sound energy (represented as pressure waves in
the cochlear fluid) into nerve pulses. Like any converter, the inner
ear has a minimum threshold below which it cannot detect
weaker sounds. The average threshold of hearing varies by frequency. The most sensitive point (“acute threshold”) lies between
3–5 kHz, a result of resonance in the outer ear canal. For measurement convenience, acute threshold as a function of frequency
has been defined as “A-weighting” and each point is defined as 0
dBA for that frequency.
In Figure 3 the operator adjusted the audio gain so that band
noise hovers just above the acute threshold of hearing in the frequency range beginning at 300 Hz. A little further up the audio
spectrum at 1 kHz, band noise stands about 15 dBA. “dBA” represents dB above the acute threshold of hearing at the frequency of
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120
strong
signals
110
100
threshold
of pain
90
threshold of
attenuation reflex
70
60
50
40
30
acute
threshold
of hearing
98 dBA
receiver audio: dB above average band noise
80
threshold
of hearing
after attenuation reflex
weak
signal
20
10
mixing
product
mixing
product
0
-10
-20
band
noise
floor
receiver
noise floor
-30
100 Hz
300 Hz
500 Hz
1 kHz
2.5 kHz
5 kHz
Figure 3: Positioning received signals within the dynamic range of average human hearing. Strong signals above the threshold of the attenuation reflex raise the threshold of hearing, masking the weak signal at 450 Hz.
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ERIC SCACE K3NA
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4 Attenuation reflex
To reduce risk of damage, the middle ear contains two muscles
that function together as an attenuator. As signal strength increases, these muscles tighten the eardrum and shift parts of the
middle ear’s bone structure to reduce the strength of signals
reaching the cochlea.2 This protective attenuation reflex kicks in
when sound levels reach 75–90 dBA. One medical reference cites
80 dBA as a typical triggering threshold for the attenuation reflex
for frequencies between 200–4000 Hz. Figure 3 includes a dotted
line showing the threshold of the attenuation reflex.
The two strong signals have crossed the threshold for the attenuation reflex. The reflex reduces signal strength to the inner
ear at a rate of –0.6 dB per dB. With these signals about 18 dB
above the threshold of attenuation, those muscles will attenuate
the operator’s hearing by –12 dB. A line in the figure graphs the
new threshold of hearing after the attenuation reflex kicks in.
Note that the interesting weak signal has disappeared below the
threshold!
The operator can restore the weak signal back above the
threshold of hearing by one of the following steps:
• Increase the receiver AF gain. A +10 dB increase will bring the
weak signal back above the threshold, but also brings the loud
signals closer to the threshold of pain. For reasons explored later,
this is a dangerous approach.
• Reduce the receiver bandwidth to weaken those strong signals.
Figure 4 shows the audio spectrum, including the effects of an
Inrad 400 Hz bandpass roofing filter centered at 500 Hz. (I assumed an ultimate rejection of –90 dB in the stopband.) The filter
pushed the strong signals well below the threshold for the attenuation reflex. The operator now hears band noise over a small
range of 350–800 Hz, and again hears the weak signal.
• Similarly, a notch filter wide enough to weaken both offending
signals by at least –15 dB could push those signals below the attenuation reflex threshold.
When optimizing the receive audio chain, the operator should:
a) Place the band noise very close to the acute threshold of
hearing; and,
b) Avoid triggering the attenuation reflex by keeping the strongest signals within 80 dB above the threshold of hearing.
c) When transmitting, reduce the CW sidetone and voice monitor
signals to the lowest practical level and well below the attenuation reflex threshold. In a 48-hour contest you will
make over 10,000 transmissions, totaling one-third to onehalf your operating time. You don’t need a loud sidetone to
send CW accurately or to know when the memory keyer approaches the end of its message. Give your ears some hours
of rest!
5 Hearing damage
While examining these charts, one might fairly ask why we cannot exploit the 25 dB of headroom between the attenuation reflex
threshold and the threshold of pain. So what if the attenuation
reflex reduces signals by ten or twenty dB? Just turn up the receiver gain to compensate! And, unfortunately, this is exactly
what most of us do. At the end of a 48 hour contest, the operator
removes his headphones to find the rest of the world sounds a bit
muffled. Maybe he even has a bit of ringing or white noise in the
ears. Damage to the cochlea has occurred, and some of that
damage is irreversible. This damage occurs from both long- and
short-term events during the contest.
2 Stevens, S. S., & Warshofsky, Fred,eds. Sound and Hearing, Time-Life
Books, NY, 1965.
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ERIC SCACE K3NA
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120
0
110
-10
100
-20
threshold
of pain
90
threshold of
attenuation reflex
-40
70
-50
60
-60
50
-70
40
-80
30
acute
threshold
of hearing
400 Hz filter response – dB
receiver audio: dB above average band noise
80
-30
Inrad 400 Hz roofing filter
response curve, centered
on 500 Hz
-90
weak
signal
20
10
strong
signals
-100
-110
mixing
product
0
-120
-10
-130
-20
band
noise
floor
-140
receiver
noise floor
-30
100 Hz
-150
300 Hz
500 Hz
1 kHz
2.5 kHz
5 kHz
Figure 4: Using a 400 Hz filter restores the threshold of hearing to the acute level, unmasking the weak signal. The filter’s response curve is
plotted against the vertical scale on the right.
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Whenever sounds above the threshold for the attenuation reflex are present, a risk exists for temporary or permanent hearing
loss. The USA National Institute for Occupational Safety and
Health established these limits for safe exposure to noise in the
workplace:
– 80 dBA: 25.4 hours. 3
– 85 dBA: 8 hours.
– 90 dBA: 2.5 hours.
– 100 dBA: 15 minutes.
– 110 dBA: 90 seconds.
Contesters interested in preserving their hearing for a lengthy
contesting career should observe these limits. That means keeping receiver gain at the lowest practical settings. If you typically
operate with the band noise 30 dB above the acute threshold of
hearing on a quiet band, and tune across an S9+20 signal, you’re
hitting that 100 dBA level and chewing up safe exposure time
rapidly.
Over the course of a single contest, lengthy exposure to
sounds above 80 dBA degrade the cochlea’s threshold of hearing,
reducing further the ear’s dynamic range. If, over the course of
the contest, you accumulate eight hours exposure to 100 dBA, for
example, you will lose about 40 dB of dynamic range from
threshold shift. Twelve to 48 hours after the end of the contest
most of this shift disappears, but a small amount remains as a
permanent loss in hearing. Over time, repeated exposure accumulates these permanent loses.
The protective attenuation reflex also has limits to its effectiveness:
• Maximum attenuation runs as much –20 dB for children and
teenagers (which partially explains why they tolerate louder music). As we age, the attenuation reflex degrades gradually toward
–10 dB, providing less protection.
3
• The attenuation reflex does not act instantaneously. When new
signals just over the threshold appear, 150 milliseconds elapse
before attenuation develops. If a very loud sound suddenly begins, the reflex still requires 25–35 ms to activate. Gunshots, a
dropped wrench on concrete, or a big signal suddenly firing up on
frequency will slam into the inner ear at full power.4
I hope you have been convinced about the importance of keeping audio signals largely within the 80 dB range between the
acute threshold of hearing and the threshold of the attenuation
reflex.
6 Ambient noise
One of the biggest challenges in maintaining audio signals within
the safe range comes from our radio room. Points 2 and 3 of Figure 2 identify two typical troublemakers: other operators and
equipment noise.
Normal speech runs about 60 dBA. An excited SSB operator sitting next to you can yell at 90 dBA, already triggering the attenuation reflex! If you were not convinced by last month’s discussion of the transmit audio chain to speak quietly into the microphone, these numbers should get your attention.
Equipment cooling fans tend to be as much as 20 dB louder at
lower frequencies (300 Hz and below) compared to mid-range
frequencies around 5 kHz. A single quiet muffin fan, mounted in a
cabinet and moving a modest amount of air, averages 25–35 dBA
in the frequency range most commonly used for copying CW.
Equipment noise in your shack likely stands substantially above
that number, depending on the type of equipment, location, and
orientation. In comparison, the ambient noise level in a library
4 Just before speaking, the attenuation reflex automatically kicks in to
protect the cochlea from one’s own voice. One can exploit this fact to
protect oneself when anticipating a loud noise (start of a power tool,
canon fire, etc): simply start humming a few seconds in advance! The act
of humming triggers the attenuation reflex, preparing the ear for the following loud noise.
Time weighted average. See www.cdc.gov/niosh/98-126a.html
www.aearo.com/pdf/hearingcons/earlog11.pdf for more details.
and
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CAN I HEAR YOU NOW? ADJUSTING THE RECEIVE AUDIO CHAIN
runs around 40 dBA. Let’s pick 40 dBA figure as illustrative for
this discussion, recognizing that it represents very quiet radio
room.
Absent any controls on ambient noise, the operator will set the
receiver gain until the band noise becomes audible over the ambient noise levels in the room. At a station with a library-like ambient noise environment, the operator may set the receiver gain
so that the band noise level runs about 45 dBA to listen on a
speaker or headphones with no isolation from room noise. This
gives him about 35 dB of audio range to play with before signals
start triggering the attenuation reflex. That’s not very much! If
the band noise sits around S1 on the meter, any signals over S8
trigger the attenuation reflex, perhaps covering up weaker signals of interest. More importantly, loud signals consume the
available budget of safe listening time before temporary or permanent hearing damage occurs… and, as an experienced contester, think of how much of the time you listen to signals at S8
and above.
You can do the arithmetic for SSB contest at a multi-op station:
the situation is grim. No wonder we finish the contest with muffled hearing and tinnitus (ringing in the ear).
We can do a lot to improve matters. Our goal: to reduce ambient noises down to 0 dBA.
7 Reducing ambient noise
The first step focuses on the radio room. As pointed out in a sister article in the preceding issue of NCJ5, a noisy radio room clutters up your SSB transmitted signal, reducing intelligibility. And
now we see a noisy radio room reduces your audio dynamic
range on reception.
Relocate or re-orient equipment that generates noise. An amplifier with a noisy blower might benefit from dense foam blocks
5 Scace, Eric K3NA, “Can You Hear Me Now? Adjusting the Transmit Audio
Chain”, National Contest Journal, 2006 May/June, p. 23 ff; ARRL, Newington CT.
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underneath to reduce sound transmission to the shelf. Wayne
Hillenbrand N2FB relocated his amplifier cooling fans behind a partition, using a duct to bring air to the amplifier chassis. If the operating position sits in the cellar near a noisy furnace, consider a
dividing partition containing fiberglass insulation batts or other
sound-absorbing material.
In a multi-op environment, separate the operators as much as
possible. On-duty operators should adjust their equipment so
they can speak quietly into the microphone, and should use voice
memory keyers as much as possible. Off-duty operators should
take their conversations into a different room.
8 Headphone isolation
Well-designed headphones provide substantial passive isolation
between ambient noises in the radio room and the ears. Unfortunately many headphone manufacturers provide little or no measurements of isolation. Nonetheless some data exists which can
point the way towards better isolation.
Figure 5 shows several popular headsets with significantly different isolation characteristics. Figure 6 shows isolation as a function of frequency for typical earmuffs and earplugs discussed in
the next few sections. Note that operators wearing eyeglasses
experience a 3 to 8 dB reduction in isolation caused by sound
leaking around the eyeglass stems. Use wire stems to minimize
sound leaks. Wire stem glasses also transmit much less force
from the earmuff to the tip of the stem behind the ear, reducing
spot irritation during long-term wear.
The Heil BM-10 typifies headsets that simply rest on the outside of the pinnae, the external flaps of cartilage and skin we
commonly call the ear. Such a design offers no isolation. Over the
duration of a contest, any headset that rests on the pinnae
gradually irritates these sensitive structures, to the annoyance of
the operator.
Similarly, the Heil Pro Set (not shown) features small earmuffs
that also rest on the pinnae. The earmuff provides limited isolation from ambient sounds, perhaps –5 to –10 dB.
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ERIC SCACE K3NA
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Heil BM-10
no isolation
$100
Heil Proset Plus
–15 dB est
$230
Dav id Clark H10-56
–24 dB
$300
K3NA kit bash
–20 dB
$130
Direct Sound
Extreme Isolation
–20 dB
$130
complete headsets
Elvex HB-650
–27 dB
$17
earmuffs
headphones only
Westone UM1+UM56
–20 dB est.
$210
including custom mold
Elvex TP-401
–27 dB
$1 in quantity
flanged earplugs
passive ear protection
Westone ES2
–25 dB est.
$700
including custom mold
in-ear monitors
Figure 5: Examples of headsets and components that one may combine to for high isolation from ambient noise. Figures in dB are mean isolation between 200–3000 Hz.
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Heil’s Pro Set Plus is a partial step in the right direction. With
the cloth cover removed, the larger earmuffs do not rest on, but
fully surround, the pinnae. The earmuffs can be too shallow for
many people whose pinnae stick out a bit from the head, causing
the interior of the earmuff to touch or press against the pinnae.
Again, no published specifications exist. Comparing this headset
against other better-specified models, I estimate the isolation
somewhere in the –10 to –15 dB range.
Higher isolation headsets exist in other industries and may be
adapted to amateur radio use. The David Clark H10-56 headset,
designed for communications in helicopters, offers a measured
average –24 dB of isolation over the 200–3000 Hz audio range we
use in contesting, and a peak isolation at 4 kHz of –39 dB.6 Each
ear sits inside a large hollow shell. The shell contains soundabsorbing foam and small speaker(s); the shell and foam form a
passive noise reduction system. The shell depth is sufficient to
keep the interior foam and all other materials off the pinnae,
eliminating that source of irritation. To use aviation headsets in
amateur radio service, you must replace the aeronautical standard cable plugs with suitable connectors.
A less expensive approach is Direct Sound’s Extreme Isolation
headphones for drummers, but one must add a boom mic.
By the way, some operators find the ear gets hot or sweaty in
the dead air trapped inside the earmuff. George Baltz N3GB produced an excellent solution. A few seconds with an alcohol swab
or baby-wipe gives the pinnae a refreshing and cleansing break.
9 Kit bashing a better headset
Frustrated by existing headsets that irritated my pinnae and
lacked adequate isolation, I began a process of kit-bashing:
modifying some headsets in combination with parts from others.
Figure 5 shows an early prototype. An inexpensive set of earmuff protectors, purchased from the local hardware store, form
the basis of this project.
6 www.davidclark.com
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I disassembled a Heil BM-10, yielding two small speakers and
the boom mic assembly. After pulling out the foam from the ear
protector shells, I drilled a small hole for the speaker cable. On
the left shell a second hole and bolt form the boom mic mount.
The speakers sit behind the foam. A dab of silicone sealed the
holes. The earmuff headband didn’t have any padding. An easy
solution employs dense foam pipe insulation, available in six-foot
lengths in the plumbing section of the hardware store for a few
dollars. A short piece wraps around the headband and seals to
itself, forming a comfortable pad for the top of the skull.
Some earmuff protectors do not provide much clearance inside
the muff between the pinnae and the interior foam. With care,
one can pry the muff’s circular soft padding from the muff shell;
a small amount of glue or double-sided tape typically secures the
padding to the shell. Mass-loaded vinyl (MLV), used in the soundproofing industry, provides excellent isolation – even better than
the muff itself. Many inexpensive sources of MLV exist on the
Internet. One can cut MLV relatively easily with a sharp utility
knife. One or more layers of MLV, cut in the shape of a zero or
donut, and placed between the muff shell and the soft padding,
will increase clearance between the pinnae and the other interior
materials of the muff. A 1X3 ft sheet of ¼-inch thick MLV provides
plenty of material to create as much as 1 in additional depth in
each ear. Use a sharp, new utility knife to cut the MLV in the approximate shape, glue the layers together with Krazy® glue or an
equivalent, and shape the package to the exact exterior and interior dimensions with a half-round file.
This project created a communications headset with the high
isolation of industrial earmuffs for a new cost of about $130. The
homebrew headset represented a significant improvement. In a
library-like quiet radio room the headset reduced ambient room
noise to about 10 dBA. If I set the receiver gain so that the band
noise stood a few dB above the attenuated room noise, I now had
70 dB of dynamic range before triggering the attenuation reflex. I
could copy weak signals in the pileups significantly better because I was making better use of the receiver’s dynamic range.
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ERIC SCACE K3NA
CAN I HEAR YOU NOW? ADJUSTING THE RECEIVE AUDIO CHAIN
-10
David Clark H10-56 headset
Elvex HB-650 earmuffs
Elvex EP-401 earplug
bone conduction limit
-20
typical custom earmold monitor
custom earmold + muff
dB isolation
-30
-40
-50
-60
100 Hz
250 Hz
500 Hz
1 kHz
2 kHz
4 kHz
6 kHz
8 kHz
Figure 6: Isolation response vs. frequency for high-isolation earmuffs and earplugs. The bottom curve shows the limit to isolation from bone
conduction paths in the skull. A combination of earplug/mold and earmuff provides greatest isolation.7
7 Sources: manufacturer published data. Bone conduction curve and combined earmuff+earmold curve from Berger, Elliot; “Attenuation of Earplugs Worn in
Combination with Earmuffs”, Earlog, issue #13, 1984, Aearo Company, Indianapolis IN USA.
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SSB contests at multi-ops, however, remained a problem. Even
with –35 dB of isolation, an adjacent operator’s normal speaking
voice runs around 25 dBA. That left 55 dB of dynamic range to
work with, meaning signals above S9+10 dB on a quiet band still
triggered the attenuation reflex.
10 Combining muffs and plugs
Using earplugs together with earmuffs could achieve better isolation from ambient noise, as shown in Figure 6. Several types of
earplugs exist with different isolation characteristics:
• Polyurethane (PU) earplugs: These compressible foam cylinders
come in a variety of densities with differing isolation levels. The
user rolls up the cylinder between his fingers and inserts it deep
into the ear canal. The cylinder gradually expands to seal the canal, attenuating outside sounds. A denser foam plug inserted
deeper into the canal provides greater attenuation. The inexpensive disposable PU earplug available in drug stores, when properly
inserted, could average more than –20 dB isolation.
• Thermoplastic elastomer (TPE) reusable plugs: At $1 a pair, the
Elves TP-401 model typifies this style. The best plugs provide
about –27 dB isolation, and some models have a flatter response
curve; i.e., the isolation varies less with frequency. The flat curve
versions sometimes are called “musicians’ earplugs”.
By wearing earplugs together with my high-isolation earmuffstyle headset, I could achieve greater isolation from the ambient
noise.
One might think that –20 dB earplugs together with –35 dB
earmuffs provide –55 dB of passive isolation from the outside
world. Measurements show this is not quite the case. Ambient
sounds carry through the bones of the skull directly to the middle
and inner ear. Figure 6 graphs the bone conduction limit as well
as a typical curve for a combination of earmuffs and earplugs.
I found three drawbacks to the combination of drugstore PU
earplugs and earmuffs/headset:
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• After 24–30 hours, my ear canals tired of the pressure from PU
earplugs and felt irritated. I would have to remove them for the
remainder of the contest, losing their isolation benefits.
• The frequency response curve for PU earplugs is not very flat. The
increased attenuation at higher frequencies detracted from the
intelligibility of SSB signals. For receivers with an equalization control (such as the Orion), a boost of higher frequencies in the receiver can compensate.
• To compensate for the signal loss in the earplug, the headset
volume must increase. This 20 dB increase in headset volume
was OK for many signals, but the loudest signals would drive my
headset speakers (recycled from that disassembled Heil BM-10)
into distortion. The need to reduce volume to avoid distortion
partially defeats the benefits of added isolation.
After a few years experience with this solution, I began to explore two avenues for improvement. Custom earplugs, manufactured by Westone and others from a variety of hard or semi-soft
materials, offer more isolation and fit precisely into purchaser’s
own ear canal shape. For about $40, a doctor or nurse at a hearing health center makes a pair of molds from purchaser’s ear canals. The center or purchaser mails the molds to the manufacturer, who then creates the plug. A typical pair of plugs costs
$75. Custom earplugs are more comfortable for long-term wear,
especially the newer heat-sensitive models with elastomer that
softens slightly from body heat.
However, adding further isolation in the ear canal demands
more volume from the headset speakers, with a greater chance
of distortion or speaker damage. A different approach avoids this
problem.
11 In-ear monitors
In-ear monitors such as those shown in Figure 5 contain very
tiny, low-power, high fidelity speakers that sit in the ear canal as
part of the earplug. By using in-ear monitors together with inexpensive earmuffs (that have no speakers), the operator achieves
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maximum isolation from ambient noises in the radio room with
high fidelity, full dynamic range sound from the receiver.
In-ear monitors typically contain one or two drivers (speakers). The two-driver models, although more expensive, provide
flatter response below 100 Hz, useful for low-band CW operators
who tend to favor CW notes at 250 Hz or even lower.
In-ear monitors fall into two broad families:
• Detachable units, shown on the left in Figure 5: The monitor, a
separate unit, fits into a specialized earplug. The plug contains
one or two tunnels to carry the sound from the monitor speakers
into the ear canal. Both “universal” earplugs (similar to the PU
foam cylinder or flanged TPE insert) and customized molded earplugs (with better isolation and comfort but at extra cost) are
available.
• Integrated units, shown on the right in Figure 5: Here the manufacturer casts the drivers into customized earplugs. Isolation levels increase by 5 dB compared to the detachable units with custom earplugs.
Because musicians make extensive use of in-ear monitors, the
earplug design provides a flatter frequency response than a standard industrial or drug store plug. Musicians may plug their in-ear
monitor into a belt-mounted wireless unit, so cord lengths are
short – around four feet. One may require an extension cable to
reach the radio headphone jack at a contest station, as well as a
⅛ to ¼ inch stereo plug adapter.
In ear monitors have much greater sensitivity than traditional
amateur radio headsets. When I first connected one to an Orion
receiver, I heard a broadband hiss that remained unchanged with
audio gain setting, and even the lowest gain setting delivered far
too much audio. I added about 40 dB attenuation by constructing
two back-to-back T-pads in each of the left and right channel; see
Figure 7. Exact resistor values are not critical, but use very similar resistances for the left and right channels to maintain balance
between left and right audio level. I used two 10 Ω resistors in
parallel for the 5 Ω legs. The 100 Ω resistor value is not critical.
Source and termination impedance for these pads are 27 Ω,
V1.3
— 06 AUG 5 SAT 12:02
which matched the Orion headphone jack and the in-ear monitor.
If you need different impedance or attenuation, the Internet offers convenient T-pad calculators.8
I now use these in-ear monitors with a high-isolation, deep
shell industrial earmuff.9 The combination delivers the maximum
achievable isolation from ambient sounds in the radio room. A
Heil boom mic (scrounged from that old BM-10 and retrofitted
with an HC-5 mic element) bolts onto the left earmuff. The effect
of inserting the plugs and donning the muffs is striking: the outside world drops away and the sound of one’s own breathing
largely defines the noise floor.
With receiver gain set low so the band noise becomes just perceptible, the full dynamic range of hearing becomes safely available for radio reception. With no or very minimal receiver AGC
action, the clarity of signals is stunning. Loud signals are loud,
but weak signals remain unobscured. The effect resembles a walk
in a mature forest: tall trees (big signals) surround you, but
mushrooms and flowers (weaker signals) are easily spotted. Tuning across a signal with bad key clicks or splatter feels like wading through a patch of thorny bushes! But best of all, when the
contest ends, the ears feel neither exhausted nor deafened by
temporary threshold shift.
Initially I felt some reluctance to risk the significant amount of
money needed for good in-ear monitors with customized earplugs. No other contester I knew used this approach. I eventually
took the plunge, in the name of research (?!) for this article but,
more importantly, to protect my hearing. My ears are the only
part of my station that cannot be repaired or replaced. The results were outstanding… and the in-ear monitors work great with
my iPod!
8 www.rfcafe.com/references/calculators/attenuator_calc.htm
9 Elvex Ultra-Sonic HB-650, available for $17 from www.elvex.com.
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ERIC SCACE K3NA
CAN I HEAR YOU NOW? ADJUSTING THE RECEIVE AUDIO CHAIN
22 Ω
100 Ω
5Ω
22 Ω
• Many models do not use fully encompassing earmuffs. The headphones rest on the pinnae, adding irritation over a long contest.
Wireless headphones, used by some operators to eliminate the
headphone cord, share many of the drawbacks of noise-reduction
headphones. These systems have significantly smaller dynamic
range (some below 80 dB) than the signals delivered to the headphone jack of the amateur radio receiver (shown earlier to be as
much as 120 dB); as a result the wireless headset user simply
throws away signal quality.
22 Ω
5Ω
100 Ω
5Ω
5Ω
12 Noise-reduction and wireless headphones
As prices declined, some operators have tried so-called “noisecanceling” headphones. One should consider these factors:
• Despite the marketing name, noise cancellation reduces broadband noise by just –10 to –15 dB and only at lower frequencies.
• Noise cancellation does nothing to reduce ambient interference
that does not resemble broadband noise; e.g., the adjacent SSB
operators at a multi-op station.
• Some noise canceling headphones add a small background hiss,
adding to the workload of the operator trying to concentrate on
real signals.
• Noise cancellation models based on digital technology add an
asynchronous co-decoding to the audio path. A later section explains the disadvantages of these co-decodings.
— 06 AUG 5 SAT 12:02
SO2R
SO2R
Figure 7: Attenuators for in-ear monitors.
V1.3
audio switching
headphone audio switching may also introduce impairments
into the receive audio chain.
Computer-controlled analog switching systems such as WriteLog’s MK1100 multi-keyer may introduce ground loops between
the switching system, the computer, and the radios. Ground loops
add weak noise or tones to the operator audio. If your receiver
sounds clean with the headphones plugged directly into the
headphone jack, but you hear additional noises when listening
through the SO2R switchbox, then ground loops are the likely culprit. Last issue’s article discussed galvanic isolation techniques to
eliminate similar problems in the transmit audio chain; these
techniques apply equally to the receive audio chain. 5
Analog switching systems with manual switching may also introduce ground loop signals into the audio. The switchbox ties
together the signal grounds of both radios. Again, galvanic isolation will remove the impairments from the audio delivered to the
headphones.
N1MM logging software introduced SO2R headphone audio
switching using a computer soundcard. Users of this digital system must:
• Employ galvanic isolation;
• Set the radio’s audio output level and the computer soundcard’s
line input volume control to appropriate settings for the soundcard’s A/D converters.
13
22 Ω
— PAGE 15 OF 17 —
ERIC SCACE K3NA
CAN I HEAR YOU NOW? ADJUSTING THE RECEIVE AUDIO CHAIN
• Set the soundcard’s line output volume control appropriately for
the operator headphones.
Last issue’s article discussed all the techniques needed to
measure signal voltages, measure A/D encoder range use, and
configure and set soundcard levels. 5
An additional small impairment comes from the added conversion from analog to digital and back again. Recall that modern
receivers digitize the analog RF signal (usually at an IF stage) for
some signal processing, and then convert the result back to analog (usually at audio frequencies). In an ideal implementation,
only one such “co-decoding” would occur before delivering the
audio to the operator’s ears. Each additional asynchronous codecoding, such as that done by the soundcard, adds small errors
to the resultant audio.10 These errors accumulate as more asynchronous co-decodings occur in the audio path; e.g., receiver,
followed by soundcard switching, followed by an external DSP audio processor, followed by a wireless headset or digital noisereducing headset. Reduced signal quality results.
When using N1MM-style SO2R soundcard audio switching, configure the soundcard without any kind of data compression, and
use a high sampling rate (e.g., 44.1 ksamples/s) and high sample
bit size (e.g., 32 bits).
A test at www.phys.unsw.edu.au/~jw/hearing.html will give
you an idea of the collective sensitivity of your ears plus headphones and soundcard at frequencies between 30 Hz and 16 kHz.
If the test results are not flat between 200 Hz and at least 3 kHz,
a problem needs attention.
10 An “asynchronous” co-decoding means the encoding algorithms for the
adjacent D/A and A/D converters are not identical, or the sampling rates
and timing are not identical, or both.
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14 Summary
You can protect your hearing while improving your ability to hear
weak signals amongst the strong during the contest by combining
these techniques:
• Minimize the ambient noise in the radio room; strive for a librarylike atmosphere.
• For multi-op stations, adjust transmitters so that operators can
speak quietly. Use voice memory keyers as much as possible.
Keep conversations between off-duty operators outside the radio
room.
• Use a high-isolation headset with deep-shell earmuffs that fully
surround the ear. The earmuffs should not touch the pinnae at
any point. If you wear glasses, use flexible wire stems to reduce
pressure at the stem tip behind the ear and to minimize ambient
noise leakage around the earmuff.
• Use donut-shaped spacers made from layers of mass-loaded vinyl
to increase distance between the outer ear and the interior of the
headset cup.
• To improve isolation further, use earplugs or in-ear monitors in
combination with high-isolation earmuffs.
• Do not use wireless headphones.
• Noise-reduction headphones may add impairments to the receiver audio, and do not isolate the operator from many types of
ambient sounds.
• SO2R operators may need galvanic isolation between the radio
headphone jacks and the SO2R audio switching system.
• SO2R operators using the N1MM second soundcard solution for
audio switching must set soundcard configuration, encoding, and
levels correctly to accommodate the full dynamic range of the
headphone audio.
• Using the receiver’s RF gain and front-end attenuator controls, set
the antenna noise just above the receiver noise floor.
• Using the AF gain control, set the antenna noise just above your
threshold of hearing.
• Use the receiver’s notch and bandpass filters to weaken adjacent
QRM below the threshold of the attenuation reflex.
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ERIC SCACE K3NA
CAN I HEAR YOU NOW? ADJUSTING THE RECEIVE AUDIO CHAIN
• In a quiet radio room, using high isolation earmuffs and plugs,
and setting the controls as described above, you will need no or
very minimal AGC action in the receiver.
• Set the transmitter’s CW sidetone and SSB/RTTY monitor levels to
the lowest practical volume.
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ERIC SCACE K3NA