LTspice@groups.io | Building Microphone model (2024)

Hello All,

I'm new to this group and to LT Spice. Could anyone point me in the right direction regarding building models for Electret and True Capacitor Microphone capsules? Thank you all in advance for your time and assistance. I'm starting to build some mic's for a gospel group I am the sound man for.

Sincerely

Bill Weinstein

In a message dated 7/22/2010 11:07:58 PM Central Daylight Time,
billssupport@... writes:

I'm new to this group and to LT Spice. Could anyone point me in the right
direction regarding building models for Electret and True Capacitor
Microphone capsules? Thank you all in advance for your time and assistance. I'm
starting to build some mic's for a gospel group I am the sound man for.

Both Electret and True Capacitor (usually referred to as Condenser)
microphones have amplifiers build into the microphone itself. The principle upon
which these devices work is vibrations in the air causes minute changes in
displacement between two plates (or membranes) which have an electric field
between them or "charge". Charge is the product of voltage and capacitance.
When the distance between the planes change due to vibrations in the air,
the capacitance changes, and since charge must be conserved (one of the basic
laws of the universe), the voltage between the two membranes changes
proportional to displacement. This principle allows for very thin low mass
membranes which are well-coupled to the surrounding air even at very high
frequencies. A condenser microphone is known for being extremely accurate and wide
range.

The basic element of a condenser microphone must operate into an extremely
high impedance. The capacitance of even the shortest length of cable would
swamp the minute changes in charge from sound. So in the microphone itself
are electronics to supply the required charge as well as provide a buffer
from the element to the connecting cable. In older condenser microphones, a
common mode DC voltage was supplied via the balanced low impedance microphone
cable. This was called "phantom" power as there was no seperate power
cable. The original condenser microphones used vacuum tube charge amplifiers
which took considerable power. Eventually, the electronics went to solid
state but the element still operated at a rather large voltage. Electret
elements worked at much smaller voltages. So small, a single pinlight or other
battery could supply the necessary power for a FET charge amplifier allowing
the microphone to be completely self-powered and have a single ended cable
for low cost applications just like a dynamic microphone.

Professional microphones run the output of the charge amplifier through a
transformer to get a balanced low impedance output. Most designed to work
into 150 to 200 ohms.

The exact electrical model would depend on the design of the particular
microphone, but a reasonably accurate model for a professional microphone would
be a floating ideal AC voltage source of around 1mVpk amplitude with 75
ohms in each leg. LTSpice does not like floating sources, so if your circuit
does not provide a path ground, such as using an input transformer, you will
need to add a 100K or so from each side of the balanced input to ground. If
your input transformer has a center-tap on the primary, you can just ground
the center-tap and forget the resistors.

Your circuit should be able to accommodate an input as high as perhaps
50mVpk before overloading.

Mike

John Woodgate

In message <i2ar25+pg97@...>, dated Fri, 23 Jul 2010, captainvideo_007 <billssupport@...> writes:

I'm new to this group and to LT Spice. Could anyone point me in the right direction regarding building models for Electret and True Capacitor Microphone capsules? Thank you all in advance for your time and assistance. I'm starting to build some mic's for a gospel group I am the sound man for.

I'm going to tell you about the capsules, as you have already received an explanation of head amplifiers etc.

Both types of capsule behave electrically as small capacitors e.g. 10 pF. However, if you want to model the mechanical properties of the diaphragm and the acoustic environment (one or two sound entrances) you are in a different world. Not only are the basic consideration less than simple, actually finding the values of the relevant parameters is extremely difficult. It isn't a subject that can be dealt with in this forum, which is about using LTSpice; you need a textbook, a very good one, on microphone design.

You might consider joining the Audio Engineering Society (optional) and its microphone standards group (open to all) SC-04-04. That would put you in touch with leading designers of microphones, some of whom may well try to help.

Mind you, it's very unusual for people to make their own microphones, since quite good ones are inexpensive. The difference between a $30 mic and a $300 mic of the same type is likely to be very subtle.
--
OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
I should be disillusioned, but it's not worth the effort.
But I support unbloated email http://www.asciiribbon.org/

Tony Casey

--- In LTspice@..., "captainvideo_007" <billssupport@...> wrote:

Hello All,

I'm new to this group and to LT Spice. Could anyone point me in the right direction regarding building models for Electret and True Capacitor Microphone capsules? Thank you all in advance for your time and assistance. I'm starting to build some mic's for a gospel group I am the sound man for.

Sincerely

Bill Weinstein

Hello Bill,

It's possible you're in the wrong group. You could try here for actually building microphones:
http://tech.groups.yahoo.com/group/micbuilders/
This is seemingly an active group of thousands of members suggesting that lots of people build them. In fact, I'm often surprised at what people will do for hobbies. There's even a Yahoo group for building PCBs by baking them in souped-up laser printers and using noxious chemicals to etch the copper. I'm afraid that's a job I'm happy to offload to Eurocircuits for £30.

Anyhow... look here for a spice model that other folk have come up with in this group:
http://tech.groups.yahoo.com/group/LTspice/files/%20Lib/ELECTRET%20MICROPHONE/ - you might need the Babelfish translator to hand to help understand the model, though, as the subcircuit is commented in Spanish.

Lot's of other interesting things can also be found here:
http://tech.groups.yahoo.com/group/LTspice/files/%20%20Tables%20of%20Contents/all_files.htm - use your browser's search facility, usually <Ctrl>-F. Seek and ye shall find.

If you've not used spice before, electret modelling is probably not the best place to start, as has been observed.

Regards,
Tony

My assumption was our friend Bill, being a sound man for a gospel group,
was looking for a microphone model to simulate an input to a preamp/mixer and
wasn't planning on building microphones. As has been mentioned, that that
would take alot of technical moxy not to mention a machine shop and access to
materials one isn't likely to find at Home Depot.

Also having been mentioned, it would be far from a cost saving proposal.
There are good sounding mics on the market for very reasonable prices. There
is also eBay. Being a gospel group, if money is short, he might want to
check around at local churches that periodically upgrade their sound
reinforcement systems and may have residual components.

As a side note, it is nice to see some agreement on an audio issue. I'm
surprised no one found fault with the model I suggested.

Mike

One needs to ask the following: What will you do with these
microphone models in LTspice?

If you want them to tell you how the microphone will sound, even what
its frequency response will be, that is unlikely. A microphone is a
complex electro-mechanical system, and the response of many mics is
dictated by mechanical resonances of the diaphragm, the case, etc..

It might be possible to create a SPICE model that represents
everything starting with the sound pressure wave, but I think it take
a lot of work to do that, and even then it would tell you nothing
about the polar response of the mic, for example.

So I am not sure SPICE is the way to go if you are interested in mics,
unless you have already worked extensively on mic design and know
quite a lot about them and want to take it a step further; but be
prepared to make it a full-time profession to do that.

I can see using a SPICE model of the microphone element's output to
investigate things like interaction with long cables or with certain
mic preamps where the input impedance is complex, but other than that,
I wouldn't think it would be useful.

Just my opinion, mind you.

Andy

--- In LTspice@..., Bill Weinstein "captainvideo_007" wrote:

I'm new to this group and to LTspice. Could anyone point me in the
right direction regarding building models for Electret and True
Capacitor Microphone capsules? Thank you all in advance for your
time and assistance. I'm starting to build some mic's for a gospel
group I am the sound man for.

It isn't entirely clear from your post, but I am guessing that you
only wish to put together finished microphone assemblies, rather
than construct the actual transducer elements from raw materials.
If so, another member of the forum has already given you some good
information about what to expect in the way of electrical transducer
levels for the basic signal source in your model. For some good
examples of practical microphone powered assemblies see:

http://www.epanorama.net/circuits/microphone_powering.html

Regards -- analogspiceman

John Woodgate

In message <f5a85.7463ce53.397b379f@...>, dated Fri, 23 Jul 2010, mjgrant@... writes:

Being a gospel group, if money is short, he might want to check around at local churches that periodically upgrade their sound reinforcement systems and may have residual components.

In US it's practicable, but in UK churches upgrade about once a century. (;-)

As a side note, it is nice to see some agreement on an audio issue. I'm surprised no one found fault with the model I suggested.

See my .sig tag. (;-)
--
OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
I should be disillusioned, but it's not worth the effort.
But I support unbloated email http://www.asciiribbon.org/

Tony,

Thanks for the quick reply, I'm already a member of the n mic builder group and posted there also. My thought was, as this group is dedicated to LT Spice, I'd try here as well. I will check out the links you referred. Again thank you.

Bill

Andy,

Your last paragraph is pretty much where I'm going. The ides is to emulate the behaviour of the preamp, I don't think substituting the passive equivalent components will really yeild much. Thanks for the input, any ideas about creating a model? Thanks again for your time and assistance.

Bill

mjgrant,

Your close, I do intend to build the finished assembly not the actual capsule. I realize that there are commercially available poducts, but All manufacturers design around a very broad set of parameters, I'm building for specific vocalists and instruments,as well as acoustical environments (Churches vary tremendously in configuration and acoustics). The practice of "tight" micing will only get you so far when noise is high or echo (acoustic) is too long. I've been in the sound arena for 40 years not only in the Gospel area but Rock, Blues, Country, etc. So the availbility of this tool to aid in the design AND implamentation of microphones is of emormous value to me. I appreciate all input in this matter, and thought I'd share my intent so as to clarify any confusion or misconception. Thank you ALL for your time and assistance,

Bill

Well, I'll say "very cool!" and let us know how you get on with the model. LTSpice is a very powerful tool, and if you can establish the appropriate analogues to successfully model a mic and its environs, I'll be very ininteristed in seeing how you did it and what you discovered.

I've had occasion to mic a vocal group, and am no audio expert, but it is a challenge. I'd feel better if there were a mechanism for modeling vocals capture. I want to use shotgun mics rather than close-micing with cardiods, but WTHDIK. (And our church has no money for such frivolities.)

Thanks,
Donald.
--
"Plain Text" email -- it's an accessibility issue
() no proprietary attachments; no html mail
/&#92; ascii ribbon campaign - <www.asciiribbon.org>

I do not have alot of experience in this area other than to say that
mic-ing seems to be both a science and an art. Mic-ing in different for recording
than sound reinforcement.

Doing sound reinforcement for difficult venues can be challenging.

Ideally, every mechanical parameter has an electrical analog, at least that
is what I was told in analog computation back when dinosaurs grazed outside
the classrooom window when I was engineering school. You should be able to
do a LTSpice simulation on the total electromechanical system.

The course was taught under the ME rather EE department. The crowning
achievement of course instructor's career was the simulation he did of a commode
for a bathroom fixture manufacturer which produced the optimal flow choke
for the water closet resulting in the fastest flush without overflowing the
bowl.

I bet I could do the "bouncing ball" simulation in LTSpice if I can
remember my physics and how to take deriatives.

Mike

Mike

In a message dated 7/26/2010 9:16:04 PM Central Daylight Time,
billssupport@... writes:

The ides is to emulate the behaviour of the preamp, I don't think
substituting the passive equivalent components will really yeild much.

Most likely I am missing something, but isn't a preamp basically just an
amplifier? As for circuit peculiar idiosyncrases, then you would have to
simulate the preamp you intent to use down to the component level.

Mike

John Woodgate

In message <i2ianm+lokp@...>, dated Sun, 25 Jul 2010, captainvideo_007 <billssupport@...> writes:

Thanks for the input, any ideas about creating a model?

What is your difficulty in creating a model of the preamplifier? While there are subtle techniques that have claimed advantages, the basics are quite simple.

What is in an electret capsule as a 'head amplifier' is a FET with typically a 1 Gohm resistor between gate and source. The mic element itself is connected between gate and source as well. The drain is the output, and is connected via a resistor to the power supply, typically 1.5 V to 12 V. The resistor may be 1 kohm to at least 4.7 kohm.

This is OK for feeding a few metres of cable to an input designed for electret mic powering, but for more up-market applications, you would embed the capsule and head amplifier in a pre-amplifier using an op-amp or four, that produces a balanced line-level or mic level if you wish) output. The output source resistance of this preamp should be 50 ohms to 100 ohms according to current practice, and be designed to feed a lowest load resistance of 1 kohm.
--
OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
I should be disillusioned, but it's not worth the effort.
But I support unbloated email http://www.asciiribbon.org/