Dynamic mics - why so difficult to reproduce?

[MicMan]

soapfoot said "The M380 is a bidirectional, single-pattern moving coil with (I believe?) a headphone driver as mic element".

The UK outfit 'Aston' sold a cheap mic with a so-called 'revolutionary' mic element inside - the mic was called 'Element' - made of (patented, no less! ..we-ell, the name was apparently patented) 'Ridyon capsule technology'. Inside mine was half a headphone.

So it's recently been done, but the quality was gruesome.

 

[engineerkyle]

soapfoot said "The M380 is a bidirectional, single-pattern moving coil with (I believe?) a headphone driver as mic element".

The UK outfit 'Aston' sold a cheap mic with a so-called 'revolutionary' mic element inside - the mic was called 'Element' - made of (patented, no less! ..we-ell, the name was apparently patented) 'Ridyon capsule technology'. Inside mine was half a headphone.

So it's recently been done, but the quality was gruesome.

The Beyer M380 you mention is literally “half a headphone” wired to the xlr jack. No transformer, no electronics.

I’ve purchased a clone made from NOS beyer parts as well as made 2 of my own from (very) used old stock drivers and they all sound very similar. The Alien8 is another headphone driver mic that sounds fantastic, though less flexible than the M380, imho. But on kick drum with both capsules on…

I haven’t used that Aston, but I wouldn’t knock anyone for using headphone drivers.

I suspect that a company like Beyer (read - self serious and giant) would not retool their machining to make a mic that they’ll sell 1000 of. They stopped making that driver/capsule in the 80s when they changed the DT 770/880/990 construction apparently. For all we know the mic could have been a happy accident out of already in-use parts.

 

[hop.sing]

The Beyer M380 you mention is literally “half a headphone” wired to the xlr jack. No transformer, no electronics.

I’ve purchased a clone made from NOS beyer parts as well as made 2 of my own from (very) used old stock drivers and they all sound very similar. The Alien8 is another headphone driver mic that sounds fantastic, though less flexible than the M380, imho. But on kick drum with both capsules on…

I haven’t used that Aston, but I wouldn’t knock anyone for using headphone drivers.

I suspect that a company like Beyer (read - self serious and giant) would not retool their machining to make a mic that they’ll sell 1000 of. They stopped making that driver/capsule in the 80s when they changed the DT 770/880/990 construction apparently. For all we know the mic could have been a happy accident out of already in-use parts.

I have a sennheiser hd414 sl with one broken side. This headphone has 600 ohms impedance, like the capsule in the Beyerdynamic. So just for the fun of it I soldered a XLR plug to the working "capsule" and tried it in front of kick drum pretty close to the inner head of the drum.
The hd414sl sounds huge and very attractive there, the same in front of a bass cab. I have no experience with the m380 but I guess the same principle (figure of 8 response with a huge proximity effect) that makes the m380 a good microphone for certain tasks apply here also.
I tried various cheap 32 ohm headphone capsules from old Walkman phones but they lacked and measured and sounded crappy.
Here is a comparative measurement with a line audio cm3 ca. 30cm (actually, I do not remember the actual distance, but that is my usual one), in front of a speaker:

 

[engineerkyle]

I have a sennheiser hd414 sl with one broken side. This headphone has 600 ohms impedance, like the capsule in the Beyerdynamic. So just for the fun of it I soldered a XLR plug to the working "capsule" and tried it in front of kick drum pretty close to the inner head of the drum.
The hd414sl sounded huge and very attractive there, the same in front of a bass cab. I have no experience with the m380 but I guess the same principle (figure of 8 response with a huge proximity effect) that makes the m380 a good microphone for certain tasks apply here also.
I tried various cheap 32 ohm headphone capsules from old Walkman phones but they lacked and measured and sounded crappy.
Here is a comparative measurement with a line audio cm3 ca. 40cm in front of a speaker:

Same principle! I’ve tried probably a dozen headphone drivers now, inspired by learning about the M380. I’ve found the vast majority follow two rules

1) the pickup quality (as a mic) corresponds to playback quality (as headphones).

2) sometimes bad sounding very low impedance (32 ohms and below) drivers sound significantly better with the even a cheap transformer to step up the impedance to something like 300-600ohms.

I’ve found one driver that actually beats out my m380, atm25 and 421 on crunchy tube amp bass guitar. No transformer needed and costs less than 20 bucks for the driver and I mounted in a shock mount with the clips removed. These headphone drivers mics are a really fun and cheap project.

 

[hop.sing]

Same principle! I’ve tried probably a dozen headphone drivers now, inspired by learning about the M380. I’ve found the vast majority follow two rules

1) the pickup quality (as a mic) corresponds to playback quality (as headphones).

2) sometimes bad sounding very low impedance (32 ohms and below) drivers sound significantly better with the even a cheap transformer to step up the impedance to something like 300-600ohms.

I’ve found one driver that actually beats out my m380, atm25 and 421 on crunchy tube amp bass guitar. No transformer needed and costs less than 20 bucks for the driver and I mounted in a shock mount with the clips removed. These headphone drivers mics are a really fun and cheap project.

That is a very cool way to mount the capsule, I think I am inspired, thank you!

 

[ricardo]

Is it really that much harder to recreate an accurate moving-coil capsule than it is to recreate a K67?

Yes. Anyone with a machine shop, know how and loadsa patience can DIY a good working copy of K67. You can even get suitable diaphragm material easily.

Making a MC capsule is like making a MC speaker unit ... just LOADS more finicky.

How many DIYers make their own MC speaker units?

 

[engineerkyle]

Yes. Anyone with a machine shop, know how and loadsa patience can DIY a good working copy of K67. You can even get suitable diaphragm material easily.

Making a MC capsule is like making a MC speaker unit ... just LOADS more finicky.

How many DIYers make their own MC speaker units?

The original question was about manufacturers cloning old mics. Your point is well taken though. Very hard for a DIY person to make a moving coil, but the hundreds (thousands?) of moving coil manufacturers in business today would surely be up to the task.

I’m a believer that there are enough people out there desperate to get rich that if the customer market really was there it would be happening already. But it seems like Warm wants to take a swing at it, so we may see how consumers react to it.

I’m with the guy that said D12 reissue. That could be a crowd pleaser, which would make other manufacturers consider doing the same. Of course it would have to sound good too…

 

[Rud]

I've only really repaired or assembled dynamic mics because designing and building a dynamic capsule is intimidating. Tuning dynamic mics electronically is different from condensers as it's more like a speaker where it's all subtractive.

 

[ricardo]

The original question was about manufacturers cloning old mics. ... but the hundreds (thousands?) of moving coil manufacturers in business today would surely be up to the task.

If you don't have the original tooling, (ie you aren't the original maker) just cloning the diaphragm and other important bits so it sounds the same is not easy.

Even cloning the AKG CK12 capsule is trivial compared to cloning a dynamic mike. There's the cost of tooling for plastic parts which may or (probably) not do the same job as the original.

I don't think there are many people today competent to design a condensor capsule from scratch. That's why there are so many clones. You just have to get certain dimensions right.

But to clone a dynamic mike/capsule, you need to be competent to design one ... and these folk are even rarer.

 

[OneRoomStudio]

A lot of good points here. I still think there are a ton of manufacturers currently making speakers and headphone drivers...so making a decent dynamic mic shouldn't be all that difficult.

Let's stick with the Beyer M380 design for a moment, since in many ways, it's the simplest form of a moving coil mic.

I know the original drivers were "left overs" from their headphones. Is that why they were 600 Ohms? Why 600? Why not 200? How does that impact the sound? Is the output "sensitivity" of the driver (i.e., 109dB/W) directly related to its sensitivity as a microphone (i.e., voltage output relative to dB SPL input)? How do other parameters of a driver affect its use as a microphone?

There are tons of 40mm and 50mm drivers out there to choose from, and a mount/body would be easy to 3D print....

 

[abbey road d enfer]

I know the original drivers were "left overs" from their headphones. Is that why they were 600 Ohms? Why 600? Why not 200?

Probably because 600 ohm headphones are not very sensitive, i.e. they require a lot of voltage to produce their nominal level.
Most hp's are comfortable with about 1mW of power. For a 600 ohms driver, it means 0.77Vrms or about 2.2V p2p. For a 60 ohms driver, it's 0.25V or 0.7V p2p. Since equipment has constantly evolved towards lower and lower supply voltage, the choice is obvious. That's the reason why most hp's are 32 ohms.
It is very likely the reason why Beyer were left with extra inventory of 600 oms drivers.

How does that impact the sound? Is the output "sensitivity" of the driver (i.e., 109dB/W) directly related to its sensitivity as a microphone (i.e., voltage output relative to dB SPL input)?

Headphones and mics are transducers.
If a transducer requires a high voltage to work, it is true in reverse, moving it will produce a higher level. So a 600 ohm driver used as a microphone will produce about 3 times more output than a 60 ohm.

How do other parameters of a driver affect its use as a microphone?

Size is very important; directivity narrows with diameter. For a 50mm front-radiating diaphragm, the diffraction-induced peak is at about 7kHz 4.5kHz, followed by a dip at 14kHz 9kHz.
For a symmetrical diaphragm (radiating in both directions), cancellation happens at 2.25kHz.
Obviously, it limits the use to low frequency sources.
Considering that almost everybody assesses a microphone by speaking into it, it's understandable manufacturers are not very enthousisatic.

 

[OneRoomStudio]

Probably because 600 ohm headphones are not very sensitive, i.e. they require a lot of voltage to produce their nominal level.
Most hp's are comfortable with about 1mW of power. For a 600 ohms driver, it means 0.77Vrms or about 2.2V p2p. For a 60 ohms driver, it's 0.25V or 0.7V p2p. Since equipment has constantly evolved towards lower and lower supply voltage, the choice is obvious. That's the reason why most hp's are 32 ohms.
It is very likely the reason why Beyer were left with extra inventory of 600 oms drivers.


Headphones and mics are transducers.
If a transducer requires a high voltage to work, it is true in reverse, moving it will produce a higher level. So a 600 ohm driver used as a microphone will produce about 3 times more output than a 60 ohm.

Size is very important; directivity narrows with diameter. For a 50mm front-radiating diaphragm, the diffraction-induced peak is at about 7kHz, followed by a dip at 14kHz.
For a symmetrical diaphragm (radiating in both directions), cancellation happens at 2.25kHz.
Obviously, it limits the use to low frequency sources.
Considering that almost everybody assesses a microphone by speaking into it, it's understandable manufacturers are not very enthousisatic.

This is really helpful information, thank you!

I'm especially interested in the last part. How do you calculate the diffraction-induced peak/dip based on the diaphragm size?

 

[JohnRoberts]

Headphones and mics are transducers.
If a transducer requires a high voltage to work, it is true in reverse, moving it will produce a higher level.

I am not a microphone guy like you all but if the "high voltage to work" criteria is because of poor efficiency I wouldn't automatically assume it will make high output if used backwards.

There is/was a popular kick drum mic that is just a loudspeaker used backwards.

Did I mention I am not the mic guy?

JR

 

[KrIVIUM2323]

I'm especially interested in the last part. How do you calculate the diffraction-induced peak/dip based on the diaphragm size?

Diffraction is a two sided coin with 'edge diffraction' and 'low frequency loss' ( this is the inverse in fact, a high frequency increase because there is a transition from 4pi to 2pi as freq increase- iow the driver become more directional as freq increase):

https://trueaudio.com/st_diff1.htm

https://www.tonestack.net/software/speaker-directivity-simulators.html

This will help to define directivity.

For diffraction ( low end loss) use driver diameter (in meter) and from there apply 115/x.
Eg: For a 50mm driver 115/0.05= 2300hz

Not far away from what Abbeyroad wrote.

For edge diffraction, subject is more complex, you can find info about it there:
https://www.linkwitzlab.com/diffraction.htm
https://www.speakerdesign.net/understand.html

 

[OneRoomStudio]

I am not a microphone guy like you all but if the "high voltage to work" criteria is because of poor efficiency I wouldn't automatically assume it will make high output if used backwards.

There is/was a popular kick drum mic that is just a loudspeaker used backwards.

Did I mention I am not the mic guy?

JR

I wonder if the speaker excursion might play into that. A lot of high-power subwoofers have large excursion (x-max), which helps them create large air movement, and therefore low frequencies at high volume. It seems like most acoustic events that would be recorded (short of a slamming door, a gun shot, etc.) wouldn't generate enough acoustic pressure to move those types of speakers to generate the voltage level that they might be capable of generating.

Just a guess though...

 

[abbey road d enfer]

I am not a microphone guy like you all but if the "high voltage to work" criteria is because of poor efficiency I wouldn't automatically assume it will make high output if used backwards.

Of course, but relatively, with a similar electromechanical structure, a 600 ohm coil will deliver more than a 60 ohm.

 

[abbey road d enfer]

I wonder if the speaker excursion might play into that. A lot of high-power subwoofers have large excursion (x-max), which helps them create large air movement, and therefore low frequencies at high volume. It seems like most acoustic events that would be recorded (short of a slamming door, a gun shot, etc.) wouldn't generate enough acoustic pressure to move those types of speakers to generate the voltage level that they might be capable of generating.

That's a very common subject in transducers. Efficiency requires low mass. A loudspeaker used in reverse has a high mass, but it is compensated by the large area.

 

[KrIVIUM2323]

I wonder if the speaker excursion might play into that. A lot of high-power subwoofers have large excursion (x-max), which helps them create large air movement, and therefore low frequencies at high volume. It seems like most acoustic events that would be recorded (short of a slamming door, a gun shot, etc.) wouldn't generate enough acoustic pressure to move those types of speakers to generate the voltage level that they might be capable of generating.

Just a guess though...

Abit off topic but related:
Often sub dedicated loudspeakers are low efficiency ( think about car audio subwoofers, they rarely have more than low 80 dbspl/1w/1m).

There is Hoffman law at play: low end extension, spl and cabinet volume needed are the three parameters. From there you can pick and optimise two of them, the last one will be compromised.

You can't have more than two at a time: if you want low and loud, then cabinet volume will be large. In the example of car audio requirements are going low in freq and in small cabinet volume so they are inefficients...

Spl outcome is defined by membrane area ( sd) x Xmax (excursion). That's why large diameter drivers are prefered for (real) sub duties ( 15", 18", 21",...).