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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] V1.3 — 06 AUG 5 SAT 12:02 — PAGE 1 OF 17 — 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. V1.3 — 06 AUG 5 SAT 12:02 — PAGE 2 OF 17 — 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. V1.3 — 06 AUG 5 SAT 12:02 — PAGE 3 OF 17 — ERIC SCACE K3NA CAN I HEAR YOU NOW? ADJUSTING THE RECEIVE AUDIO CHAIN 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 V1.3 — 06 AUG 5 SAT 12:02 — PAGE 4 OF 17 — ERIC SCACE K3NA CAN I HEAR YOU NOW? ADJUSTING THE RECEIVE AUDIO CHAIN 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. V1.3 — 06 AUG 5 SAT 12:02 — PAGE 5 OF 17 — ERIC SCACE K3NA CAN I HEAR YOU NOW? ADJUSTING THE RECEIVE AUDIO CHAIN 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. V1.3 — 06 AUG 5 SAT 12:02 — PAGE 6 OF 17 — ERIC SCACE K3NA CAN I HEAR YOU NOW? ADJUSTING THE RECEIVE AUDIO CHAIN 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. V1.3 — 06 AUG 5 SAT 12:02 — PAGE 7 OF 17 — ERIC SCACE K3NA CAN I HEAR YOU NOW? ADJUSTING THE RECEIVE AUDIO CHAIN 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 V1.3 — PAGE 8 OF 17 — — 06 AUG 5 SAT 12:02 ERIC SCACE K3NA 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. V1.3 — 06 AUG 5 SAT 12:02 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. — PAGE 9 OF 17 — ERIC SCACE K3NA CAN I HEAR YOU NOW? ADJUSTING THE RECEIVE AUDIO CHAIN 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. V1.3 — 06 AUG 5 SAT 12:02 — PAGE 10 OF 17 — ERIC SCACE K3NA CAN I HEAR YOU NOW? ADJUSTING THE RECEIVE AUDIO CHAIN 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 V1.3 — 06 AUG 5 SAT 12:02 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. — PAGE 11 OF 17 — 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. V1.3 — 06 AUG 5 SAT 12:02 — PAGE 12 OF 17 — ERIC SCACE K3NA CAN I HEAR YOU NOW? ADJUSTING THE RECEIVE AUDIO CHAIN 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: V1.3 — 06 AUG 5 SAT 12:02 • 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 — PAGE 13 OF 17 — ERIC SCACE K3NA CAN I HEAR YOU NOW? ADJUSTING THE RECEIVE AUDIO CHAIN 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. — PAGE 14 OF 17 — 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. V1.3 — 06 AUG 5 SAT 12:02 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. — PAGE 16 OF 17 — 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. — END — V1.3 — 06 AUG 5 SAT 12:02 — PAGE 17 OF 17 — ERIC SCACE K3NA