You can’t work ’em if you can’t hear ’em

You can have a modern transceiver with a good sensitive receiver that won’t turn turtle under the pressure of strong signals connected to a good antenna, but the signal still has to make it from the rig to your ears (unless you’re running some digital mode).


Some of today’s transceivers throw in a small downward (or upward) firing speaker.  These mostly sound rather tinny with some buzzing (how bad depends on where you’ve set the radio down on your operating position).  The built in speaker is better than nothing, and will probably even be just fine for a rag chew with a strong local on a quiet band.  However you won’t want to use it for serious DXing, or in a contest.

Yeasu, Kenwood, Icom, and Elecraft offer matching speakers for their transceivers, which solves the problem of having a front facing speaker.  Hopefully, these speakers are designed acoustically correct to avoid unwanted internal resonances, and to fully cover the frequency range of the human voice necessary for communication (about 300hz to 3000hz).  Expect to pay between $100-$300 for such a speaker.

You can, of course, get a very good communications quality speaker for a lot less by buying something originally intended for another purpose.  I have a small Optimus (Ratshack) speaker that nearly matches my IC-746, and it does a good job.  The shack made a few different small two way book shelf hifi speakers that make decent communication speakers.

You can also build your own.   Let me tell you more than what you probably wanted to hear (unless you are into building speakers).   What you need to find is a suitable driver (raw speaker), and then build a correct sized box to house it in.  Since the frequency response required is one octave from 300hz to 3000hz, a mid-woofer or mid range driver of about 3.5″ to 5.5″ in diameter will suit our needs perfectly.  It has to be a fairly efficient driver because most transceiver will have an audio output power range somewhere around 1.5 to 3 watts of power.  This isn’t as limiting as it sounds, since we aren’t going to fill an auditorium or even a living room with sound, just a semi-circle of space, about an arm’s length in diameter.  Several sources of such drivers come to mind, ranging from the auto sound after market, and drivers made for musical instrument practice amps.

One thing to look out for in selecting a driver, is the cone surround.  This is the flexible band that connects the edge of the cone to the driver frame.  Foam, rubber, treated cloth, and pleated paper are the most common materials used here.
Avoid foam based surrounds like the plague.  They have a half-life of about two years or so before they start to fall apart in a pile of dust.  Your mileage will vary on this, but if you live in a warm-humid climate you are in prime foam rot land.  (I threw out two different sets of expensive tower speakers that had their woofers and mid-range drivers develop this cancer).
Rubber surrounds are vastly superior to foam, and will probably last forever.  I have a pair of home made speakers in my media room that are over 25 years old, their rubber surround drivers are still going strong.
Treated cloth surrounds were once very popular in wide range speakers (woofers with a ‘wizzer’ cone for mid-tweeter range).  They will also last forever.  Finally pleated paper.  This is just the edge of the cone.  During manufacture, the outer inch of so of the cone is soaked in water (maybe with some other solvent) and is compressed between two halves of a mold that form the pleats.  Some heat is applied and when the paper dries the pleating is permanent.  Drivers for PA systems and musical instrument amps are built this way.

In order to know how large a box you need to build for a speaker, you need to know the driver’s Thiele-Small parameters.  These are the acoustic specifications for the driver.  We need be concerned with only three of these.   Rs (or Fs) is the drivers free air resonant frequency.  This must be lower than the lowest note we must reproduce.  So for our case, it must be below 300hz.   Qts is the total Q of the driver.  It will determine the shape of the high frequency cutoff of the driver.  Different types of speaker enclosures require different ranges of driver Qts.  Finally there is Vas.  This parameter gives us the effective volume of the driver cone.  The speaker enclosure must have a larger internal volume than the Vas, how much larger depends on the enclosure type, and desired response.

The common types of speaker boxes are sealed, ported, and infinite.   The sealed box enclosure is the easiest to design and build.  I won’t discuss the math here, there are plenty of websites that have on line calculators for working out suitable designs.  Drivers for sealed box enclosures usually have a Qts from 0.33 to 0.55.  The box itself is usually designed for a Q of 0.707 (Butterworth  LP response).
Ported boxes have an opening in either the front or the back via a tube (PVC pipe) inside the box.  The box is ‘tuned’ by adjusting the length and diameter of this pipe.  Ported boxes do not have rational cutoff response curve (I’m referring to the math here), but there are several design equation sets that are commonly used.  At the low end the speaker impedance drops suddenly like a rock, and a high pass filter is usually employed to protect the driver from excessive movement.  Drivers with lower Qts values are suitable for ported enclosures.
At the high end of driver Qts ranges (0.707 and higher) we have the infinite baffle design.  The ideal infinite baffle is an infinitely sized board with the driver mounted in the middle.   Open back cabinets, car doors and rear decks approximate what is required here.  Those old time, tube era transceivers had such open backed matching speakers.


Even the best speaker isn’t going to cut the mustard during weak signal conditions in the middle of a DX pileup during a contest.  You know, when the guy you REALLY need to hear is being stepped on by the QROO crowd.  What you need are a pair of CANS!

Communications headphones used to be rather uncomfortable things.  That’s probably where the radio slang term ‘cans’ came from, they felt like a pair of tin cans clamped to your ears.   High impedance, magnetic headphones go back to the days of Marconi.  They have a thin flexible iron or steel diaphragm that is pulled by a magnet.  A coil wound with many turns of very fine wire surrounds the permanent magnet.  The weak radio signals drive this coil, which adds to, or subtracts from the magnetic field, causing the diaphragm to flex and move the air next to the ear, thus producing sound.  There is a variation in the construction of this type of headphone where a lever arm is attached to a smaller diaphragm inside of the earpiece.  This lever arm transmits the movements to a thin mica diaphragm that generates to sound.  This type of headphone (Baldwin) is based on the structure of the human ear.  Magnetic headsets were made with impedance values (with both earpieces wired in series) somewhere between 2000 to 5000 ohms, with 2000 and 3000 ohm values being common.   Early magnetic headphones used individual ‘tip’ plugs and jacks for connection to the circuit, but even the first available commercial communications receivers were set up for the now common phone plug and jack system.
During WWII, most communications headphones were magnetic types, but these had to be ruggedized.  Extreme sensitivity was no longer a requirement as the radio equipment had plenty of internal audio gain.  These headsets usually were built to a 500 ohm impedance, a standard that carries over today in the aviation industry.

Amateur radio equipment from the pre-war era, usually had receivers set up to drive medium to high impedance headphones.  By the 1960’s, during the AM to SSB switch over, lower impedance headsets became common.  I’ve looked back at the specs on old Drake, Collins, and Heathkit rigs.  Only Heathkit continued to require the 2000 ohm headset impedance on their SB line receivers and transceivers, the others had switched to speaker level impedance headphones.  Perhaps the adoption of hifi headsets in the shack was the thing driving the lower impedance of today’s headphone jacks.  BUT, they are still mostly wired as monaural jacks.  BTW, hifi headsets are not magnetic drivers, they are dynamic drivers, having actual pint-sized speakers in each earpiece.

If there is one place where a communication quality headphone has to keep outside noise from interfering with the operator, it is in the cockpit of an airplane.  Technology changes slowly in aviation, and bits of the past are ingrained in newer tech.  Many standards have been inherited from the military, the 3/16″ diameter tip/ring microphone plug for example.  (The only maker of amateur radio gear to use that style of microphone connector was Collins, probably due to their involvement in avionics.  BTW the sleeve is ground, the tip is PTT, and the ring is the microphone).

Back in the late 70’s, I started taking flying lessons in Cessna 150’s.  Rather than use the flying schools crappy headsets with the plane’s hand held microphone, I bought myself a  David Clark H10-30 headset.  This time proven unit is still available today, though with an improved microphone.  It’s a medium impedance headset (around 300 ohm, with a series volume control).  While I haven’t flown in years now, I’ve kept the headset for use with my ham gear.  Even though there is quite the impedance miss-match, most transceivers will still drive it, and I can always use an output transformer ripped from an old transistor radio wired ‘backwards’ to step up the impedance if necessary.  The microphone is an amplifier dynamic type that requires power supplied by the radio to operate.  Like most aviation microphones, it’s designed to work in a carbon microphone circuit.  These noise canceling microphones have a frequency response that matches the human voice range of 300hz to 3000hz.  They won’t sound hifi, but they will cut through the crap.
Most transceivers today provide a 5 to 12 volt power pin on their microphone jacks to power electret units.  I made an inline adapter to power the microphone through a 470 ohm resistor from this power source,   Audio is then coupled to the microphone input via a 10uf capacitor back to the rig with a 10k pot in between.

There is nothing like a pair of well made (for the purpose) headphones for bringing the weak signals right to your ears.  Your radio may have the latest in DSP audio processing (perhaps done at the IF frequency), but the best DSP filter for separating a weak voice out from a crowd of others is still that lump of gray matter between your ears.  But you have to provide your brain with the entire input, as directly as possible.

David Clark makes a whole line of aviation headsets over a wide price range.  They still sell parts for anything they ever made.  I recently ordered new padding and filters for mine that had been turning to dust over time.
There are other brand names in the market today, but I’m happy that this “made in the USA” supplier is still going strong.


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