simplest way to measure a mics frequency response?

Archived from groups: rec.audio.pro (More info?)

Lacking the necessary funding for brand new condenser mics, I am going to be
constructing one of my own soon. I understand the basics of construction
(circuit, housing, grounding, etc) but I am not sure how to measure the
frequency response once completed. What is the simplest way to do it? I
don't really have much experience doing something like this by ear,
unfortunately. Also, regarding the housing of the mic, I understand that if
done improperly, it can effect the lower response of the mic so any
suggestions in this area would be helpful also.

Thank you,
Chase

Archived from groups: rec.audio.pro (More info?)

On Fri, 29 Oct 2004 07:30:51 GMT, in rec.audio.pro you wrote:

>On Fri, 29 Oct 2004 07:23:00 GMT, "Chase Sunderlin"
><zxyt6f6yu@wervf.met> wrote:
>
>>Lacking the necessary funding for brand new condenser mics, I am going to be
>>constructing one of my own soon. I understand the basics of construction
>>(circuit, housing, grounding, etc) but I am not sure how to measure the
>>frequency response once completed. What is the simplest way to do it? I
>>don't really have much experience doing something like this by ear,
>>unfortunately. Also, regarding the housing of the mic, I understand that if
>>done improperly, it can effect the lower response of the mic so any
>>suggestions in this area would be helpful also.
>>
>>Thank you,
>>Chase
>>
>
>A good reliable way is to set up an external electrode fairly close to
>the diaphragm. Put a high voltage on this electrode - this will
>distort the diaphragm slightly away from its rest position. Now remove
>the voltage cleanly and sharply.
>
>Read the resulting pulse from the microphone into a PC through a sound
>card, and perform an FFT on it. Compare this to an FFT of the edge of
>the voltage pulse itself, and you have the frequency response of the
>capsule.
>
>Most DAW programmes have an FFT (frequency analysis in Audition) that
>will do this for you.
>
>d
>Pearce Consulting
>http://www.pearce.uk.com
Hi Don
How about the acoustic pulse from an piezoelectric gas lighter
thingummy?


martin

Serious error.
All shortcuts have disappeared.
Screen. Mind. Both are blank.

Archived from groups: rec.audio.pro (More info?)

On Fri, 29 Oct 2004 11:25:23 GMT, in rec.audio.pro you wrote:

>On Fri, 29 Oct 2004 13:18:47 +0200, martin griffith
><martingriffith@yahoo.co.uk> wrote:
>
>>On Fri, 29 Oct 2004 07:30:51 GMT, in rec.audio.pro you wrote:
>>
>>>On Fri, 29 Oct 2004 07:23:00 GMT, "Chase Sunderlin"
>>><zxyt6f6yu@wervf.met> wrote:
>>>
>snip
>>Hi Don
>>How about the acoustic pulse from an piezoelectric gas lighter
>>thingummy?
>>
>>
>>martin
>
>The purpose of the technique is to remove the uncertainty of the
>acoustic path and just test the microphone. You know exactly what you
>are doing with the diaphragm when you pull it electrically. Of course
>when you let go, you are recovering the way the microphone diaphragm
>moves exactly.
>
>If you fancy trying an impulse response from an acoustic source, an
>electric spark is a good way to do it, but it needs to be a much
>louder splat than you get from a gas lighter.
>
>d
>Pearce Consulting
>http://www.pearce.uk.com

Ok
Sloppy thinking by me. Should have thought out the basic principles
before posting!




martin

Serious error.
All shortcuts have disappeared.
Screen. Mind. Both are blank.

Archived from groups: rec.audio.pro (More info?)

>Lacking the necessary funding for brand new condenser mics, I am going to
>be
>constructing one of my own soon.

Huh? Mics are so cheap today I don't understand how making one yourself could
possibly ne cheaper.
Phil Brown

Archived from groups: rec.audio.pro (More info?)

"Don Pearce" <donald@pearce.uk.com> wrote in message
news:4190f0cf.88477375@news.plus.net...
> On Fri, 29 Oct 2004 07:23:00 GMT, "Chase Sunderlin"
> <zxyt6f6yu@wervf.met> wrote:
>
> >Lacking the necessary funding for brand new condenser mics, I am going to
be
> >constructing one of my own soon. I understand the basics of construction
> >(circuit, housing, grounding, etc) but I am not sure how to measure the
> >frequency response once completed. What is the simplest way to do it? I
> >don't really have much experience doing something like this by ear,
> >unfortunately. Also, regarding the housing of the mic, I understand that
if
> >done improperly, it can effect the lower response of the mic so any
> >suggestions in this area would be helpful also.
> >
> >Thank you,
> >Chase
> >
>
> A good reliable way is to set up an external electrode fairly close to
> the diaphragm. Put a high voltage on this electrode - this will
> distort the diaphragm slightly away from its rest position. Now remove
> the voltage cleanly and sharply.

What sort of high voltage source are you talking about here?

>
> Read the resulting pulse from the microphone into a PC through a sound
> card, and perform an FFT on it. Compare this to an FFT of the edge of
> the voltage pulse itself, and you have the frequency response of the
> capsule.
>
> Most DAW programmes have an FFT (frequency analysis in Audition) that
> will do this for you.
>
> d
> Pearce Consulting
> http://www.pearce.uk.com

Archived from groups: rec.audio.pro (More info?)

donald@pearce.uk.com (Don Pearce) wrote in message news:<4190f0cf.88477375@news.plus.net>...
> On Fri, 29 Oct 2004 07:23:00 GMT, "Chase Sunderlin"
> <zxyt6f6yu@wervf.met> wrote:
>
> >Lacking the necessary funding for brand new condenser mics, I am going to be
> >constructing one of my own soon. I understand the basics of construction
> >(circuit, housing, grounding, etc) but I am not sure how to measure the
> >frequency response once completed. What is the simplest way to do it? I
> >don't really have much experience doing something like this by ear,
> >unfortunately. Also, regarding the housing of the mic, I understand that if
> >done improperly, it can effect the lower response of the mic so any
> >suggestions in this area would be helpful also.
> >
> >Thank you,
> >Chase
> >
>
> A good reliable way is to set up an external electrode fairly close to
> the diaphragm. Put a high voltage on this electrode - this will
> distort the diaphragm slightly away from its rest position. Now remove
> the voltage cleanly and sharply.
>
> Read the resulting pulse from the microphone into a PC through a sound
> card, and perform an FFT on it. Compare this to an FFT of the edge of
> the voltage pulse itself, and you have the frequency response of the
> capsule.
>
> Most DAW programmes have an FFT (frequency analysis in Audition) that
> will do this for you.
>
> d
> Pearce Consulting
> http://www.pearce.uk.com

================
Interesting, you use the high voltage step function to apply an
acoustical step function to the mic's diaphragm.

Question...

How do you keep the mic from electrically responding directly to the
electrical step function. I would think it would be very difficult to
keep the high voltage step from being picked up directly (electrically
not acoustically) by the mic's preamp etc. If the mic' responds to
both the electrical and acoustical stimulation, it would be hard to
separate the two effects in the results.

Mark

Archived from groups: rec.audio.pro (More info?)

On Fri, 29 Oct 2004 15:22:37 GMT, "Chase Sunderlin"
<zxyt6f6yu@wervf.met> wrote:

>
>"Phil Brown" <philcycles@aol.communged> wrote in message
>news:20041029095748.11054.00002711@mb-m14.aol.com...
>> >Lacking the necessary funding for brand new condenser mics, I am going to
>> >be
>> >constructing one of my own soon.
>>
>> Huh? Mics are so cheap today I don't understand how making one yourself
>could
>> possibly ne cheaper.
>
>Define "cheap"? Sorry, I don't think $200 or more for a mic is inexpensive.
>Electret condenser elements, if carefully selected, can be had for about $5,
>placed into a capsule and wired up. The reason I was asking about testing
>was because I would then need to determine where eq might need to be
>applied.

Most here will suggest you construct the mic for flat response (or
for whatever response you want it to have) rather than using EQ. If
you want to test its response, you can probably compare it with a
'standard' - there's been a $40 Behringer model that's been described
as having a flat enough response for many testing purposes.
I recall the late Stephen Paul discussing mic acoustics on a
webforum a few years back, regrettably he closed the forum and removed
the content due to some controversy/misunderstanding, and more
recently, I saw mic acoustics discussed on a webforum moderated by
Harvey Gerst.
There's probably one or more books' worth of info for "comparing"
two mics (frequency response sweep from high quality loudspeaker,
acoustically dead room (or the outdoors doesn't reflect much), bla bla
bla), but this is at least a start. Perhaps that free
frequency-response measuring program Arny has mentioned (Rightmark?)
would be usable here.

Someone suggested using a gas lighter to generate an acoustic
impulse and then do FFT to get the response, and someone else gave
some reasons it wouldn't work (IIRC the gas lighter's spark isn't loud
enough). I would think the biggest problem would be the mechanical
hammer that hits the piezoelectric element probably makes more sound
than the spark itself.

>Simply doing this by ear would not have been the way to do it for
>me.

You may want to look at the "Mitey Mike":
http://www.audioxpress.com/bksprods/kits/kd-4.htm
I saw the original Mitey Mike construction article in Audio Amateur
or perhaps Speaker Builder (both of which are incorporated into the
current AudioXpress magazine) many years ago. You might want to order
the issue(s) describing it and read up on it. I recall that the mic
element is set into the end of a fairly long (about 1 foot) tube
that's just slightly larger in inside diameter than the element, to
get a flat reponse and prevent the frequency-response effects of
nearby objects (such as the preamp box at the other end of the tube).
Also there's a DIY mic mailing list on yahoogroups.com. I recall
that it's mostly for the "stealth tapers" crowd, but it may have some
useful ideas.

>Chase
>> Phil Brown

-----
http://mindspring.com/~benbradley

Archived from groups: rec.audio.pro (More info?)

>A good reliable way is to set up an external electrode fairly close to
>the diaphragm. Put a high voltage on this electrode - this will
>distort the diaphragm slightly away from its rest position. Now remove the
voltage cleanly and sharply.

>Read the resulting pulse from the microphone into a PC through a sound card,
and perform an FFT on it. Compare this to an FFT of the edge of the voltage
pulse itself, and you have the frequency response of the capsule.

>Most DAW programmes have an FFT (frequency analysis in Audition) that will do
this for you.

That's just what I was going to say. Really.

Archived from groups: rec.audio.pro (More info?)

Ben Bradley wrote:


> Someone suggested using a gas lighter to generate an acoustic
> impulse and then do FFT to get the response, and someone else gave
> some reasons it wouldn't work (IIRC the gas lighter's spark isn't loud
> enough). I would think the biggest problem would be the mechanical
> hammer that hits the piezoelectric element probably makes more sound
> than the spark itself.

There is a low frequency rolloff from a spark of about 12 db
per octave and the corner frequency is in inverse proportion
to the gap length. Even at a half inch, I've measured that
corner to be a couple of kHz. There isn't enough low
frequency energy in a spark gap to get anywhere near ambient
noise much less above it.

An exploding wire might be a good idea. I built a high
voltage, high energy supply for this purpose from a kit a
couple of years ago but when I came to realize just how
lethal the thing is, I decided to let it gather dust rather
than turn it on.

Impulsive methods are problematic for other reasons too, the
main one being signal to noise ratio. Their advantage is
mainly for measuring halls and spaces because they are
omnidirectional. When all the energy is in a pulse, the
total isn't very much at all. Swept sin methods are much
better because you are spreading your energy out over time
at relatively high power at all frequencies and can get
_much_ better SNR in your measurement.

To make this work, you need to calibrate your sound source
with a calibration mic and do impulse response division of
all measurements by the one measured by the calibration mic.
The Beringer mic is supposed to be flat enough but since
it contains a Panasonic WM-61A capsules I just don't believe
that. From the factory they have a 12 kHz presence peak
that is plus or minus about 2 kHz and plus or minus about 6
dB by my measurement of about 50 of them. I really don't
believe Behringer selects for matched characteristincs, the
standard deviation is just too broad, and they just don't
come flat.

A much better choice for a calibration mic that is useful
also for recording is the Earthworks TC30K. The actually do
the capsule qualification and selection and ask you to pay
for it in the price. With a matched pair you get the
calibration curves and they _are_ flat. But then thats
clearly outside your budget. It's gonna be hard to do what
you want on the cheap. (Maybe I ought to sell measurement
mics that don't pretend to be flat but are cheap like the
Behringer and come with their calibrated impulse response on
a floppy. :-)

If you want to take this on, Chase, I've done a lot of work
with all this (and still am) and would be happy to advise
via email.


Bob
--

"Things should be described as simply as possible, but no
simpler."

A. Einstein

Archived from groups: rec.audio.pro (More info?)

Ben Bradley wrote:


> Aha, that explains why a step response would be a better idea. I
> saw the description in this thread of electrostatic excitation of a
> step response, but I can't think of a way to do that acoustically.

Yes it is very difficult to make atmospheric pressure
suddenly jump to a new value and stay there. :-)

I'm not sure what the value of electrostatic excitation is.
Might tell you information about diaphragm tension and
port accuracy if you have something standard to compare it
with but it sure isn't going to tell you much about how it
transduces sound.

>>you want on the cheap. (Maybe I ought to sell measurement
>>mics that don't pretend to be flat but are cheap like the
>>Behringer and come with their calibrated impulse response on
>>a floppy. :-)
>
> CD-R's are cheaper, not that the price of the media matters much,
> but more importantly, newer computers don't have floppy drives
> anymore. Also, you can put more data on a CD-R.

My age is showing. :-)

> But that does sound like an interesting business idea. Are there
> services that will make such a measurement on a mic the customer sends
> in? You could do that as well.

I have thought about it. With the general availaibility of
convolution, having the impulse responses and the inverses
at several orientations could be useful. Not sure how many
people would know how to use the results though and I doubt
they'd pay just for a graph.


Bob
--

"Things should be described as simply as possible, but no
simpler."

A. Einstein