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Microphones

While talking to Junior about recording sounds, Grandad had mentioned the use of microphones. Junior thought that these sounded interesting.
"How does a microphone work? Can it hear things like we do?" he asked.
Grandad had to think about how to answer.

"Microphones work in the same way as our ears," he replied, "So I suppose that they do hear things in the same sort of way. Both have a device which vibrates at the frequency of the sound waves in the air, then converts these vibrations into something more useful. Ears use lots of tiny hairs and nerve sensors to make signals that our brains interpret as sound. Microphones use several ways to convert the vibrations into an electrical signal that we can use in our stuff."

Grandad showed Junior two types of carbon microphone.
Carbon hand "lollipop" microphoneGEC Home Broadcaster"These are a very simple type of microphone. They are based on a very old concept, where a carbon rod just touching another rod vibrates with the sound waves, changing the resistance of the contact. By connecting a battery and headphones in the circuit, the changing resistance altered the current flowing, to produce an alternating current that made sound in the headphones.
"This was even used as a form of amplifier, because the output from the circuit was large even for small vibrations.
"Both of these microphones use a telephone type of cartridge. Telephones used carbon microphones because of their high sensitivity, but also because they ideally suited the way that the telephone network operated and the need for speech quality only. These cartridges were cheap, so often became the heart of cheap microphones.
"They have a diaphragm made of steel, or of mica coated with carbon, which vibrates with the sound. As it moves it changes the force on some carbon granules, changing their resistance - the tighter they are packed, the lower the resistance.
Carbon microphone insert"One terminal is the body; the other is a button at the back, insulated from the body, against which the carbon is in contact. The rest of the space is packed with cotton wool, to hold the carbon granules in their place and prevent the diaphragm resonating. The picture is a telephone cartridge (centre) with a mica/carbon diaphragm which has broken. The tin that it was in (left) contains a bit of diaphragm and most of the carbon granules. The lid of the tin is on the right"
Junior asked what resistance the carbon microphone had, so they took three inserts from their tins and measured their resistances using a modern multimeter. After being shaken (they rattled slightly as the granules moved) the resistances were between 80 and 150 Ohms.
"If I press the diaphragm in gently, the resistance drops to three or four Ohms," said Junior, "And when I release it, the resistance goes high, off the scale."
"Normally the diaphragm only moves a tiny bit," said Grandad, "But the resistance still changes a lot! Sometimes the microphone can become noisy or insensitive while being used, as the granules change their position, just like after you released the diaphragm. When this happened, we would shake the 'phone or bang it on something to move the granules around and make it work properly again."
Junior wondered if that was the origin of the 'good thump' that people used on the radio or television when it didn't seem to be working properly.

Carbon cartridge"What is that microphone that looks like a lollipop?" asked Junior.
 "That one has a carbon cartridge in it, just like the one above, and not much else. The holder looks like a modified telephone microphone casing, all mounted on an aluminium rod with an ebonite handle. I can't make up my mind if this is home-made or a commercial microphone - it looks too good for home-made."

"You didn't make it yourself, then, Grandad?" asked Junior.
"No, not this one," replied Grandad.
 
"The second of the carbon microphones is definitely commercial. You already saw it, sitting on the box it came in. I never used it. It is called 'GEC Home Broadcaster BC. 1901'.
Under the Home Broadcaster"This also uses a standard telephone type of insert, with the wires soldered to tags on it. Under the stand it has clips for a battery, a rheostat for crude volume control, and a transformer. There's no cover at the bottom.
"The rheostat is just a variable resistance in the circuit, to increase or reduce the current in the carbon capsule. It can go right off the end of the resistance track, to turn off the microhone. If left on, the battery would soon go flat!"

"What does the transformer do?" asked Junior.
"It gives a path for the battery current to flow through the carbon insert, so that it works, then isolates that current from the small changes due to speech. The speech alternating current in the secondary of the transformer can be joined using two wires to the 'Gramophone' input of a wireless, to give people their own 'Home Broadcasting' setup, as it lavishly says on the box."

"Doesn't the lollipop one also have a battery and transformer?" asked Junior.
"Well, it needs them if it is going to function," replied Grandad, "But this one is much older and expects that whoever is using it will provide these things in whatever it is used with."

"How old are they, then?" asked Junior.
"The lollipop one, as you call it, is from the twenties or early thirties," grinned Grandad.
"The GEC Home Broadcaster BC. 1901 is from the forties or fifties."


"If these were only used for voices, what sort of microphone was used for music and stuff?" asked Junior.
"There are lots of better types of microphone," replied Grandad, "And probably all the other sorts are better than carbon!"

Film Industries ribbon micReslo ribbon mic"Here are two ribbon microphones. These can give very good performance. The only moving part is a very light, thin aluminium ribbon, quite small, that is wrinkled by being run between two special gear wheels. This makes the ribbon able to deflect more easily, so that it can vibrate with the sound waves reaching it. The wrinkles make it have a very low mechnical resonance itself, so that it doesn't upset the signals. Each side of it are powerful magnets. As the ribbon waggles with the sound waves, it moves in the magnetic field and produces electricity, just like a generator does."
"Or the gramophone pickup?" asked Junior.
"The gramophone pickup is usually more like the next type of microphone, but some did use ribbons," replied Grandad.
"Is the ribbon microphone easy to use?" asked Junior.
"Quite easy," answered Grandad. "The main disadvantage is that it has a very low impedance and low output, because it is just a bit of foil waggling in a magnetic field. You can't put lots of turns of wire on it, like with moving coil microphones, so a transformer is used. This can be inside the microphone or at the other end of the cable - both ways have some advantages and disadvantages.

Film Industries ribbon mic

This microphone was made by Film Industries. It needs an external transformer. I broke the ribbon by speaking too close to it. The ribbon is so delicate that just a slight breath into it can break it. I replaced the original ribbon with a bit of chocolate wrapping foil in 1960, but it never worked that well afterwards. I don't think the foil was thin enough.


Reslo ribbon mic"The other microphone is a Reslo RV. This one works just fine. It was made in 1950 or thereabouts. I used this with a Ferrograph tape recorder, using the Ferrograph matching transformer.

"Ribbon microphones respond to sounds that waggle the ribbon most. That means, sounds coming straight onto the flat part of the ribbon, from perpendicular to it. If the sound comes onto the edge of the ribbon, it not only sees the very thin ribbon edge, but also the ribbon just doesn't move that way and can't produce any output. This means that the response is like a figure 8, sensitive from front and back but not from the sides. It's even called a 'figure 8' response."

"I mentioned moving coil microphones. These are sometimes called dynamic microphones, for no good reason that I can see. They have a small diaphragm made of metal or plastic to catch the sound vibrations in the air; this is joined to a coil of wire that is mounted in a powerful magnetic field. As the diaphragm vibrates, it moves the coil in the magnetic field and generates electricity, much in the same way that the ribbon does. The big advantage is that many turns of fine wire can be put into the coil, so the impedance and voltage output are much higher than the ribbon. Even so, a transformer is often needed in order to get enough signal for the amplifier, especially if it is a valve amplifier."

"Grandad, you said about plastic diaphragms," said Junior. "Are metal ones better?"
"Well, no," replied Grandad. "Plastic is best, because it can be made very thin and shaped and moulded to give great strength in the centre, but wrinkled at the edges so that it can move freely, like a piston, for driving the coil. This gives a very good sound quality, even for cheap microphones."

Junior was amazed that Grandad was extolling the virtues of plastic! He went quiet for a while.

"Here are three moving coil microphones. This one is a really cheap one, made by BRC, whoever they are. I'll take it apart so that we can see the diaphragm."

BRC moving coil unitGrandad unscrewed the top of the microphone and took out the capsule that did the work.
"You can see the thin plastic diaphragm, with wrinkles around the edge to allow it to move easily, and a bit of a dome on the top to make it rigid where it 'catches' the sound. That dark ring is the coil, wound on a paper cylinder called the 'former'. The metal around and inside the coil is the powerful magnet.
"This microphone is called 'omni-directional', because it picks up sound from all around. It is a bit more sensitive to sounds from the front, though."


AKG cardioid mic"Now, this is one an expensive microphone, made in Austria by AKG. This one has slots along the body, which feed sounds from the back of the microphone to the back of the diaphragm. The slots and the cotton padding inside delay the sound a little, so that when it arrives at the back of the diaphragm it cancels the sound at the front of the diaphragm. What this does is to make the microphone insensitive to sounds from behind, but still sensitive from the front. This gives a response that we call cardioid, heart-shaped. It's good for people speaking in, say, a hall, when the microphone needs to pick up the person speaking but ignore the sounds in the hall. If it picked up the sounds in the hall, these would get amplified and fed back into the hall, then picked up again and re-amplified. This makes a howling noise often called howl-round that is very unpleasant, due to acoustic feedback. A cardioid microphone helps to avoid this."
"Is a ribbon microphone good for this, too?" asked Junior.
"No," replied Grandad. "The 'figure 8' response of a ribbon makes it just as sensitive to sounds from both sides, so it howls like a good 'un. But having said that, some ribbon microphone also contained an omni-directional moving coil cartridge, wired up so that the phase of the outputs cancelled signals from behind, giving an overall cardioid response. They could also have shields and pads to stop sound coming from one side reaching the ribbon, to give a similar cardioid result."

Grampian DP6 on stand"This next microphone is British. It is a Grampian DP6 on a desk stand. This is omni-directional, like the cheap BRC one, but this one was used in a factory for making announcements, so the receptionist spoke very close to it and produced a lot of signal compared with any other sounds.
"These Grampian microphones are actually quite good, and were very popular in the 'fifties and 'sixties. They didn't cost as much as the AKG microphone, but were nice and robust and worked well."

Grampian GC2X box"Here is another Grampian dynamic microphone. I never een used this one - it is still in the box! Although this one looks different to the DP6, it works in the same way. This one has a cardiod response, unlike the omnidirectional DP6. Do you remember how a cardiod microphone behaves?"
Junior was ready for this sort of question, and, just to prove to Grandad that he had been taking notice, he replied "Yes, Grandad, you only just explained to me that it means 'heart-shaped' and it doesn't hear any sounds from behind."
In the Grampian box"It does hear some of the sound from behind," corrected Grandad, "But not as much as if it was turned around so that the front was pointing backwards instead of forwards."
Junior was so delighted with Grandad's explanation that he didn't say anything for a few seconds.
Unboxed Grampian GC2XThey took the inner holder out of the box. Junior looked at the long lead that was spooled around the holder, and thought how useful it was that everything was in one place. He held the microphone, then said "Shall we unwind the cable and see how long it is, Grandad?"
So they unwound the lead, which reached across the room. "Not long enough for everything," said Grandad, "But it's there if you need it, and you can always use a longer one. Now let's put it all back in the box."  So they did.


"Grandad," said Junior, "The way that you have described these moving-coil microphones sounds quite like a loudspeaker. Can a loudspeaker be used as a microphone?"
"Well done!" said Grandad. "Yes, a loudspeaker makes quite a good microphone! Not good enough for Hi-Fi, but far better than a carbon microphone. Many intercom units use the loudspeaker both as a loudspeaker and a microphone, but of course only for one thing at a time."
Spectrum Computer showing 'loudspeaker'"In fact," said Grandad, "Sometimes a microphone insert was used as a loudspeaker! It was not very good, since it's small size did not move much air, so it only works for higher frequencies. When you look inside my Sinclair Spectrum computer you will see one of these. It's at the lower right-hand corner, with the number '2' on it. Cheap and cheerful."

"Are there any other types of microphone?" asked Junior, feeling pleased.
"Yes, there are two other major types," replied Grandad.
"Crystal microphones use a piece of rochelle salt or similar that make electricity when it is deformed. The diaphragm, like the moving coil diaphragm, connects to the rochelle salt and vibrates it with the sound, to generate an electric signal. This type produces a lot of signal, and is very cheap to make. It also has a very high impedance, unlike a moving coil microphone, so it is easy to feed to a valve amplifier and doesn't need a transformer. But the quality is not that good. Still better than carbon, though! It works like the crystal pickup that I showed you on the gramophone."

"The last type is the condenser microphone. For some reason these are still usually called condenser rather than capacitor microphones, I don't know why.
"These are very simple, but can give really high quality. It's easy to make a simple one yourself. They have two metal plates, one is fixed and quite thick; the other is very thin and can vibrate with the sound. Usually the thin plate is made of plastic film which is metallised on the outside with a very thin deposit of metal. The plastic film stops the two plates short-circuiting together if they touch.
"That's it! By feeding a voltage called the polarising voltage between the plates through a very high resistance, the capacitor made by the two plates charges up. When the plates move together, the capacitance increases and therefore the voltage between them reduces. When they move apart, the capacitance reduces and the voltage increases. Because of the high resistance in circuit, current can't flow rapidly into the changing capacitor, so the voltage changes that occur can be fed to an amplifier. A fixed capacitor is needed, to stop the battery voltage from getting into the amplifier but allow the signal from the moving plate, which is, of course, the diaphragm.
"Sometimes the diaphragm is made large, to get better sensitivity. I once had a Grundig microphone that was about three inches diameter, so it was quite sensitive. But for better quality, the diaphragm is made smaller so that it has few resonances."

Electret data"A type of condenser microphone is called electret. I've lost the microphone, but here is the information on it. Just 10mm diameter and 12.5mm long! This is actually quite a large one - modern electrets can be extremely small. I'll show you a tiny one in a minute or two.
"Electrets works in the same way as the normal condenser mike, but don't need a polarising voltage because charge is permanently stored within the plates when they are made, like a perfect capacitor. These microphones are often very small, and usually incorporate a pre-amplifier using a field-effect transistor (FET) so that the final impedance is low enough to drive the wire that connects them to the amplifier. Because the pre-amplifier needs power, it sometimes seems as though the microphone needs a polarising voltage, just like the normal condenser mike. Actually, I suppose that the voltage could be used this way. I wonder why it isn't?
"Without the pre-amplifier, the output voltage is reduced because of the capacitance of the cable. If the microphone itself has a very small capacitance, maybe only 20pF, then the capacitance of the cable, perhaps 2000pF, will reduce the signal dramatically. But the frequency response stays the same, of course.
"This is the reason why condenser microphones used to be made with large plates - the higher capacitance allowed a longer cable to be used. We didn't have FETs then, because they hadn't been invented, and a valve in the mike would have been rather daunting, although it would have worked in the same way. I expect someone probably used a valve in one..." He looked thoughtful and rather vacant for a minute or two.

Electret on Thumb"Look at this one, from a modern pocket computer! I'll put it on my thumb..."
The microphone was only about 4mm diameter and very thin.
Junior seemed surprised at how small a microphone could be.

"Why doesn't the frequency response change when a long cable is used with a condenser microphone?" asked Junior, still thinking about something Grandad had said earlier. "I expected that it would."
"Well, you can work it out yourself!" said Grandad.
So Junior did.
"But it doesn't work if the microphone has a built-in pre-amplifier, like that electret one, does it?" he asked.
"No, it doesn't, that's perfectly right," replied Grandad. "It only happens if the plates of the microphone connect to the cable."



The next time Junior visited, he held his hand behind his back and said
"Grandad, I brought something of mine for you to see."
"What have you brought?" asked Grandad, his eyes shining.
Junior's Condenser Microphone"It's the microphone from my karaoke machine," replied Junior. "I wondered what sort it is?"
Grandad took a look at it, then carefully unscrewed the top and bottom sections, which was quite easy. Then he removed the 1.5v cell from the holder near the cable end, just in case.
"Just in case of what?" asked Junior.
"Just in case something short-circuits and blows it all up," said Grandad. Junior thought that being blown up sounded quite dangerous, but Grandad said that in electronics it usually just meant that something stops working.
The dismantled microphone"With a battery in the handle, it is most likely a condenser microphone," he said.  "One and a half volts is what electret microphones usually use for their little amplifier, so I think that you have an electret microphone here.  If we look at the business end, it seems to have a fairly large insert. The holes at the side, covered with gauze, which connect to the outside world through those slots in the top cover, are probably there to modify the polar pattern and make the microphone unidirectional, so that it doesn't howl round so easily."
Junior remembered the AKG microphone, which had slots down the body to make it unidirectional.
"Is this microphone worth as much as one of you AKG microphones?" he asked, hopefully.
"Sorry, but this is a modern mass-produced microphone worth a few pounds only," said Grandad. "Nevertheless it is possibly quite a good one."
The 'business end'Grandad looked at the business end of the microphone with a funny glazed expression. Finally he said:
"Shall we take it all apart and see what's inside that top bit?"
The tables were turned - Junior looked horrifed and said "No, Grandad, you might break it."
"Yes, O.K." said Grandad, putting it all back together and handing it back to Junior.

Then, one day, Roger Jelbert had a look at the photos and recognised the microphone - he had had one just like it. Roger said that he thought that it is not an electret microphone, but dynamic.
Grandad thought out loud.  "Well," he said, "It seems that having a 1.5v cell in the body is exactly right for an electret microphone, to power the FET buffer that drives the cable. Why would you want a cell in a dynamic microphone, which already has a low output impedance?"

So Grandad asked Junior to bring his microphone with him on his next visit.
"O.K. Grandad," said Junior, "But no taking it apart, remember?"
Grandad promised not to take it apart, and soon he had his hands on it again. He unscrewed it just as he did before.
"What are you going to do?" asked Junior. "Don't break it open!"
WW1 Compass set upGrandad put his hand in his pocket and brought out an old magnetic compass in a leather case, that looked as if it had survived since the first World War! On the back was stamped the date, 1917. Grandad said "That's my old First World War compass. Now, let's see if the microphone is magnetic." He carefully laid it down and let the pointer stabilise.
"Now what will you do?" asked Junior.
"Let's put the microphone top bit near the compass and see if the pointer moves," said Grandad. "If it's a dynamic microphone it'll have a powerful magnet in it."

Pointer moves! It's dynamic.The pointer moved, quite a long way.
"It's definitely a dynamic microphone," said Grandad. "Roger was perfectly right. Well, I wonder what the cell is there for?".
They though about it over a cup of tea and Grandad gave his opinion. "You know, it must be just to light the little LED indicator that shows when the microphone switch is 'on'," he said. "There's no way it does anything else!"   He muttered something that sounded like "Well, I'll be bothered!"
Grandma looked up from darning the socks and put her finger to her lips.

The dilemma was resolved some time later, when Roger obtained a second-hand microphone. This is a Panasound PS-928 Microphone, which had not only a wired connection but also a radio interface.

Roger's radio microhoneRoger sent some photographs of the microphone to Junior, showing all the parts both in and out of the box in which it came. These clearly show the 'rubber duck' aerial that plugs into the microphone in place of the cable, and the receiver box with a bit of wire as an aerial that plugged into the jack socket on the amplifier. Roger has kindly allowed the use his photos so that we can all see what it looks like. The microphone body is a bit different to Junior's.

Roger's radio microphone
Junior was well impressed and immediately asked if he could have one like it so that he didn't keep tripping over the cable on his wired microphone. Grandad listened patiently, then said "Do you still use your karaoke machine then?" 

Junior didn't quite know how to answer this. He had somewhat grown out of that type of toy and hadn't used it for ages. He looked around to see if Daddy was going to say anything, then replied "I don't use it much now, Grandad, but if I had a radio microphone like Roger's I would want to use it a lot."