Basic question about phantom and sensitivity

[[email protected]]

I saw some specs from Sennheiser about my lav mic.
It said the maximum output voltage of the mic would
be 1.8V with P48 powering, and 0.6V max with P12 powering.
Does this mean the lav gets more sensitive as the
phantom powering increases?

[gunnar]

wrote:
> Does this mean the lav gets more sensitive as the
> phantom powering increases?

Main effect is that max sound level decreases.

G.

[[email protected]]

What I'm trying to find out is, for the same volume level
from the talent, will using P48 instead of P12 require
less preamp gain?
I have the proper gain figured out for using P12 but
I will be switching to P48. Will I need to dial down the
gain?

[Don Pearce]

On 22 Aug 2006 00:52:37 -0700, wrote:

>What I'm trying to find out is, for the same volume level
>from the talent, will using P48 instead of P12 require
>less preamp gain?
>I have the proper gain figured out for using P12 but
>I will be switching to P48. Will I need to dial down the
>gain?

Changing the phantom voltage does not change the sensitivity of the
microphone. For a given volume from the talent, you will get the same
signal from the microphone. What does change is the maximum level the
mic can handle before it starts to overload. You are unlikely to run
up against this.

d

--
Pearce Consulting
http://www.pearce.uk.com

[Laurence Payne]

On 21 Aug 2006 22:15:34 -0700, wrote:

>I saw some specs from Sennheiser about my lav mic.
>It said the maximum output voltage of the mic would
>be 1.8V with P48 powering, and 0.6V max with P12 powering.
>Does this mean the lav gets more sensitive as the
>phantom powering increases?

No. It means it will accurately reproduce a higher level.

[Henk]

schreef:

> What I'm trying to find out is, for the same volume level
> from the talent, will using P48 instead of P12 require
> less preamp gain?
> I have the proper gain figured out for using P12 but
> I will be switching to P48. Will I need to dial down the
> gain?

A microphone sensitivity specification tells you how much electrical
output (millivolts) a microphone produces for a certain sound pressure
input (the loudness of the source in dB SPL). If two microphones are
subject to the same sound pressure level and one puts out a stronger
signal (higher voltage), that microphone is said to have higher
sensitivity. So when the maximum o/p level doubles when going from 12
to 48V phantompower the microphone will get more sensitive. This means
that for a similar SPL it produces a higher o/p voltage so indeed you
will have to reduce the i/p gain of the preamp in order to have similar
post i/p gain level in your micpreamp as you had when using 12V phantom
power.
There is a max dB SPL a microphone will be able to handle that is
related to the amount of (usually 1%) total harmonic distortion the
cirquit causes but this is pretty inaudible. In many cases it is almost
always the diaphram clipping (reaching its absolute displacement limit)
or the preamplifier that cannot handle the current that really is the
problem. Things are tricky though: In some microphones phantom power is
only used to power the mic's cirquit and in others it also supplies the
voltage used for polarizing the microphone's transducer element.
Depending on this constructions it varies what happens when a
microphone is given less phantom power than it is designed for. A
microphone will NOT be able to handle higher SPL levels before
diaphragmclipping occurs when the capsule is differently polarized (for
this is a mechanical thing). But the internal preamp will. Because you
say you ave a lavelier mic it is safe to assume your mic is an electret
which means the backplate is prepolarized and so its output level is
not affected by the amount of phantom power that is applied. The
internal cirquit of the microphone however is capable of better
performance when using 48V in stead of 12V (especially the internal
preamp) so maybe 12V phantom power will cause distortion at high SPL
levels because of the preamp incapability of properly handeling its i/p
level (since this remains constant). In that case applying 48V of
phantom power will just provide the preamp with more gainpotential and
thus causing a higher o/p level and thus more apparent sensitivity.
Normally though, when you compare two DIFFERENT microphones, higher
sensitivity compromises the clipping level.

Henk

[Scott Dorsey]

> wrote:
>I saw some specs from Sennheiser about my lav mic.
>It said the maximum output voltage of the mic would
>be 1.8V with P48 powering, and 0.6V max with P12 powering.

That makes sense.

>Does this mean the lav gets more sensitive as the
>phantom powering increases?

It might actually do so, but that's not what the spec means. The spec
means that the _overload point_ at which the mike clips is higher with
the higher operating voltage.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[David Satz]

Henk,

> A microphone sensitivity specification tells you how much electrical
> output (millivolts) a microphone produces for a certain sound pressure
> input (the loudness of the source in dB SPL). If two microphones are
> subject to the same sound pressure level and one puts out a stronger
> signal (higher voltage), that microphone is said to have higher sensitivity.

That's completely correct, and admirably precise.


> So when the maximum o/p level doubles when going from 12 to 48V
> phantompower the microphone will get more sensitive. This means that
> for a similar SPL it produces a higher o/p voltage so indeed you will have
> to reduce the i/p gain of the preamp in order to have similar post i/p gain
> level in your micpreamp as you had when using 12V phantom power.

Unfortunately there is some misunderstanding here. The maximum output
voltage of a condenser microphone can increase when its circuitry is
given more operating current, with no change in sensitivity. The
circuit maintains the same gain, but is able to deliver more power when
needed. This generally increases the maximum SPL capability of the
complete microphone, since it is nearly always the amplifier which sets
this limit. If the input impedance of the preamp is on the low side or
the cables are long, the difference can be heard even at everyday SPLs.

The capsules of typical studio condenser microphones can handle sound
pressure levels of 150 dB quite well, but modern transformerless
microphone circuitry usually reaches its limit around 130 - 140 dB SPL,
and condenser microphones with output transformers typically overload
at about 115 - 125 dB SPL.


> There is a max dB SPL a microphone will be able to handle that is related
> to the amount of (usually 1%) total harmonic distortion the cirquit causes
> but this is pretty inaudible. In many cases it is almost always the diaphram
> clipping (reaching its absolute displacement limit) [ ... ]

This is a fairly common misconception. In fact the diaphragm
displacement of a condenser microphone is miniscule when propelled by
the energy of ordinary sound; it is astonishingly small even at quite
high SPLs. In practice only an explosion or a direct blast of wind or
breath would ever cause a diaphragm to touch its backplate.

--best regards

[Henk]

David Satz schreef:


> Unfortunately there is some misunderstanding here. The maximum output
> voltage of a condenser microphone can increase when its circuitry is
> given more operating current, with no change in sensitivity. The
> circuit maintains the same gain, but is able to deliver more power when
> needed. This generally increases the maximum SPL capability of the
> complete microphone, since it is nearly always the amplifier which sets
> this limit. If the input impedance of the preamp is on the low side or
> the cables are long, the difference can be heard even at everyday SPLs.

Do you mean to say that although the max o/p level doubles when
applying 48V in stead of 12V the max SPL handling is also very likely
to increase by 6dB because of the additional powerhandling of the
preamp? And since a doubling of Voltage is equal to a 6dB difference
this "cancels" each other out when we speak about its sensitivity? You
could very well be right! It must be exactly 6dB though, otherwise the
sensitivity does change. Mmmm, thanks, I'll think about that


> > There is a max dB SPL a microphone will be able to handle that is related
> > to the amount of (usually 1%) total harmonic distortion the cirquit causes
> > but this is pretty inaudible. In many cases it is almost always the diaphram
> > clipping (reaching its absolute displacement limit) [ ... ]
>
> This is a fairly common misconception. In fact the diaphragm
> displacement of a condenser microphone is miniscule when propelled by
> the energy of ordinary sound; it is astonishingly small even at quite
> high SPLs. In practice only an explosion or a direct blast of wind or
> breath would ever cause a diaphragm to touch its backplate.

Just where you put the brackets in my quote I wrote: "or the
preamplifier".... I have been using schoeps microphones quite a bit and
although I love them on drums I have experienced some nasty clipping
when I first started using them. I've always assumed that this was the
diaphragm for in the recorded waveform you could see a very tipical and
strange curve that I could not otherwise explain. It's true though that
I never experienced this with other mics so maybe the preamp is indeed
more often the 'weakest link' in that regard. Thanks again for making
me rethink this.

regards, Henk

[David Satz]

Henk wrote:

> Do you mean to say that although the max o/p level doubles when
> applying 48V in stead of 12V the max SPL handling is also very likely
> to increase by 6dB because of the additional powerhandling of the
> preamp? And since a doubling of Voltage is equal to a 6dB difference
> this "cancels" each other out when we speak about its sensitivity? You
> could very well be right! It must be exactly 6dB though, otherwise the
> sensitivity does change. Mmmm, thanks, I'll think about that

Hmm--no, actually, it's simpler. A condenser microphone's sensitivity
is determined by physical characteristics of its capsule, by the
voltage at which the capsule is polarized, and by the voltage gain of
the microphone's circuitry. With electrets (such as the lavalier
microphone which the original poster asked about) the polarization
voltage isn't a function of external powering, while with microphones
such as your Schoeps, the polarization voltage is kept constant by an
internal DC converter. Thus the sensitivity of the microphone is simply
not affected by the changeover from 12 V to 48 V.


> [ ... ] I have been using schoeps microphones quite a bit and although I
> love them on drums I have experienced some nasty clipping when I first
> started using them. I've always assumed that this was the diaphragm for
> in the recorded waveform you could see a very tipical and strange curve
> that I could not otherwise explain. It's true though that I never experienced
> this with other mics so maybe the preamp is indeed more often the
> 'weakest link' in that regard. Thanks again for making me rethink this.

Well, with Schoeps microphones, the capsules can handle far higher
sound pressure levels than the circuitry--but the circuitry can handle
levels of 130+ dB SPL (unless you have the blue dot version of the
amplifier, which has 5 dB greater sensitivity but a correspondingly
lower maximum SPL limit). So if you really are overloading the
capsule--a relatively rare occurrence--I would suspect that this may be
due to air motion rather than sound energy per se.

You might try a simple foam windscreen to see whether it makes the
distortion decrease or go away. If it remains, then I'd suggest trying
a resistive pad at the input of your preamp or mixer. A Schoeps
microphone can put out nearly a Volt without distorting, and some
preamp inputs can't handle such high signal levels.

Capacitive pads (Schoeps DZC 10 or DZC 20) can also be placed between
the capsule and amplifier of the microphone if the capsule's output is
overloading the amplifier, but they are normally needed only for sound
pressure levels so high that your hearing would be damaged by continued
exposure to them.

--best regards

[Henk]

> Hmm--no, actually, it's simpler. A condenser microphone's sensitivity
> is determined by physical characteristics of its capsule, by the
> voltage at which the capsule is polarized, and by the voltage gain of
> the microphone's circuitry. With electrets (such as the lavalier
> microphone which the original poster asked about) the polarization
> voltage isn't a function of external powering, while with microphones
> such as your Schoeps, the polarization voltage is kept constant by an
> internal DC converter.

Yes, I get all this, but it does not answer my question

>Thus the sensitivity of the microphone is simply
> not affected by the changeover from 12 V to 48 V.

I don't see this folowing from the above in any way. Follow this:

A microphone sensitivity specification tells you how much electrical
output (millivolts) a microphone produces for a certain sound pressure
input (the loudness of the source in dB SPL). So, when the maximum o/p
changes that means that the whole mapping of i/p SPL against o/p
Voltage changes; except for when the maximum i/p handling changes
accordingly. Now we all know that a doubling of Voltage corresponds to
a change of 6dB's. So, ONLY when his mic is also capable of handling a
6dB higher SPL level when applying 48V the microphone will be able to
put out the exact same voltage for any i/p SPL as with 12V. In any
other case, the mics sensitivity has changed.

Henk

[Mike Rivers]

Henk wrote:

> A microphone sensitivity specification tells you how much electrical
> output (millivolts) a microphone produces for a certain sound pressure
> input (the loudness of the source in dB SPL).

Right.

> So, when the maximum o/p
> changes that means that the whole mapping of i/p SPL against o/p
> Voltage changes

Wrong. Raising the maximum output level doesn't change the gain or the
sensitivity. For example, if, for a given supply voltage, the maximum
output level is 1V, if you increase the SPL up to the level where you
get 1V peaks out, that's all you can get. Increase the SPL beyond that
point and you'll still only get 1V out, but it will be distorted
because peaks will be clipped at 1V.

If you increase the supply voltage to whatever it takes so that you can
get 2V out of the mic, that higher SPL won't be distorted since the
output stage will allow a greater swing that represents the higher SPL.


When dealing with phantom powering, there isn't necessarily a linear
relation between the DC input voltage and headroom of the output stage,
just that, in general, for many (but not necessarily all) mics, it will
be higher with a higher supply voltage - just like a device that uses a
+/- 15V power supply doesn't have the maximum output capability of one
that uses a +/-24V supply.

Now there are capsule designs that can adjust their sensitivity by
adjusting the polarizing voltage on the capacitor formed by the
diaphragm and backplate, but in general, the polarizing voltage is
regulated for a specified phantom supply voltage range so changing the
phantom voltage doesn't change the sensitivity, at least not by enough
to worry about.

[Bob Cain]

Henk wrote:
>> Hmm--no, actually, it's simpler. A condenser microphone's sensitivity
>> is determined by physical characteristics of its capsule, by the
>> voltage at which the capsule is polarized, and by the voltage gain of
>> the microphone's circuitry. With electrets (such as the lavalier
>> microphone which the original poster asked about) the polarization
>> voltage isn't a function of external powering, while with microphones
>> such as your Schoeps, the polarization voltage is kept constant by an
>> internal DC converter.
>
> Yes, I get all this, but it does not answer my question
>
>> Thus the sensitivity of the microphone is simply
>> not affected by the changeover from 12 V to 48 V.
>
> I don't see this folowing from the above in any way. Follow this:
>
> A microphone sensitivity specification tells you how much electrical
> output (millivolts) a microphone produces for a certain sound pressure
> input (the loudness of the source in dB SPL). So, when the maximum o/p
> changes that means that the whole mapping of i/p SPL against o/p
> Voltage changes;

There isn't a remapping with phantom voltage variation; that would be
a change in sensitivity. With the higher voltage the amplifier's
voltage rail is increased which allows the mic to reach higher SPL
before clipping at that higher rail. It's just a matter of more
output signal swing before clipping occurs.


Bob
--

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

A. Einstein

[[email protected]]

There must be a minimum phantom voltage that a mic
needs to operate. For example, I have an ME2 lav
and the specs say that it can run on P12 to P48, but
I measured the voltage applied by the Senn G2
system's transmitter and it is 9 volts. The ME2
even seems to operate on the 4 volts supplied
by the plug-in power from my minidisc.
What is the phantom voltage actually powering?
If the mic is electret that means the capsule
is pre-polarized, so that it doesn't require any
external voltage? You say that usually the
voltage is regulated, so that applying more
phantom will not increase sensitivity, but what is
the minimum phantom that a mic needs? Is there
any way to determine the minimum?

[Scott Dorsey]

> wrote:
>There must be a minimum phantom voltage that a mic
>needs to operate. For example, I have an ME2 lav
>and the specs say that it can run on P12 to P48, but
>I measured the voltage applied by the Senn G2
>system's transmitter and it is 9 volts. The ME2
>even seems to operate on the 4 volts supplied
>by the plug-in power from my minidisc.
>What is the phantom voltage actually powering?
>If the mic is electret that means the capsule
>is pre-polarized, so that it doesn't require any
>external voltage? You say that usually the
>voltage is regulated, so that applying more
>phantom will not increase sensitivity, but what is
>the minimum phantom that a mic needs? Is there
>any way to determine the minimum?

Sure. Rig up a bench supply. Set it to 48V. Put a signal source into
the mike at the maximum level you care about. Turn down the supply until
it sounds like crap. The minimum operating voltage is slightly higher
than that.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Mike Rivers]

wrote:
> There must be a minimum phantom voltage that a mic
> needs to operate. For example, I have an ME2 lav
> and the specs say that it can run on P12 to P48, but
> I measured the voltage applied by the Senn G2
> system's transmitter and it is 9 volts. The ME2
> even seems to operate on the 4 volts supplied
> by the plug-in power from my minidisc.


Well, specs are specs. When they say it needs at least 12V, that means
that it will meet its specs (whatever they are) at that voltage. It
doesn't mean that it won't work at all at a lower voltage.
Incidentally, some mics (Shoepes?) sense the phantom voltage and
reconfigure themselves depending on whether it's 12V or 48V. And I
recall reading about a mic that uses the detected phantom voltage to
adjust the directivity pattern. The official power supply (which is
optonal) has a control to adjust the pattern and what it does is
changes the actual voltage applied. If you run it at 48V +/- the
standard official tolerance (4V I think) it's a cardioid.

As far as your Minidisk powering the mic goes, I don't know what's
happening there. Since MD mic jacks are unbalanced and applies the mic
powering voltage between the tip and ring, you really can't phantom
power with that scheme.

> What is the phantom voltage actually powering?

Typically, it powes a DC-DC converter that makes 60V or so to polarize
the capsule if it's extnerally polarized (an electret is "permanently"
polarized and doesn't require the high voltage) and in addition it
makes whatever voltage the impedance converter circuit requires. That's
generally pretty low since the maximum output voltage of a mic is
rarely greater than 1V or so.

> If the mic is electret that means the capsule
> is pre-polarized, so that it doesn't require any
> external voltage?

Yup.

> You say that usually the
> voltage is regulated, so that applying more
> phantom will not increase sensitivity, but what is
> the minimum phantom that a mic needs? Is there
> any way to determine the minimum?

The easiest thing is to ask the manufacturer he uses as the minimum
powering voltage when he tests the mic. You could put the mic in front
of a sound source, look at the output, and, with the full 48V applied,
increase the SPL until you start to see clipping. (Don't do this with
your bare ears or friend around). Then drop the SPL back to where it
jsut doesn't clip. start reducing the phantom voltage, and see when it
starts to clip again. But if you can work with a mic that clips at a
lower SPL than its rated maximum, just expose it to the loudest SPL you
expect, and start dropping the voltage until it starts clipping. Then
make sure that, with the SPL just below clipping, it still sounds OK.

By the way, testing microphones isn't simple.