Live Sound & Production

[dboomer]

Some people believe that some microphones are better (or worse) at picking up sound as the distance increases.  In general terms this is not the case for conventional live sound microphones.  Remember ... microphones do not go out and search for sound, the sound comes to them. To do otherwise would require that some kind of amplification that could kick in for a distant sound and that the mic could tell the difference between distance and SPL.  It is the sound itself that falls under the inverse square law.  IOW it's your voice that is dropping off in feeding the mic.

The example given was pulling a mic away a foot and having it go dead.  So here's how that breaks down.

Just to keep it simple we'll stay in tens.  If you start singing at .1 foot away and that produces 100 dB spl (wherever you are measuring it) then moving away from your mic to 1 foot the signal would drop by 20 dB to a new spl of 80 dB.  This is the normal loss due to inverse square from the source over distance.  Microphones are simple but dumb devices and cannot distinguish distance from simple lower level. If this were the case then the mic would become unusable as you sang softer. They just convert, in a reasonably linear fashion (at least at these distances) the pressure on their elements to voltage.  It makes no difference whether the element is dynamic or condensor.

Now if you factor in proximity effect you will see a bit of an exception to this ... but only to the frequencies below about 200 Hz.  I have made the measurements at 18" compared to 1" and can tell you at that distance an SM58 will exhibit a bump of about +10 dB at aprox 120 Hz.  An OM-5 will exhibit less bump ... about +7 dB at about 180 Hz. (the bump being at the closer distance of course)

So back to the claim that the Audix picks up less at distance ... Compared to an SM 58, they both pick up the same at distance for the normal voice range of 300-3kHz but the Audix picks up about 3 dB less than the 58 at 18" but only below 200 Hz..  But that is 3 dB compared to 20 dB and not very significant.  Likely another dB at 1 foot instaed of 18".  It's not the exact level of the bump that is important, just the delta between the two.

So what about "shotguns" picking up at great distances?  They don't pick up on axis any better than anything else.  What they can do is pick up less off axis spill so the intended to un-intended ratio is better, but the signal itself is no stronger.  If you are trying to pick up birds on the wire across the street with a shotgun compared to a 58 you'll get the same level of birds in both mics, but you'll get less traffic noise mixed in with it with a shotgun.

[agedhorse]

This is IMO good, accurate information. When the pattern control is factored in, this is exactly what happens. What the user hears is the relative on axis acceptance to off axis rejection ratio.

[OneEng]

---

dboomer wrote:

Some people believe that some microphones are better (or worse) at picking up sound as the distance increases.  In general terms this is not the case for conventional live sound microphones.  Remember ... microphones do not go out and search for sound, the sound comes to them. To do otherwise would require that some kind of amplification that could kick in for a distant sound and that the mic could tell the difference between distance and SPL.  It is the sound itself that falls under the inverse square law.  IOW it's your voice that is dropping off in feeding the mic.

The example given was pulling a mic away a foot and having it go dead.  So here's how that breaks down.

Just to keep it simple we'll stay in tens.  If you start singing at .1 foot away and that produces 100 dB spl (wherever you are measuring it) then moving away from your mic to 1 foot the signal would drop by 20 dB to a new spl of 80 dB.  This is the normal loss due to inverse square from the source over distance.  Microphones are simple but dumb devices and cannot distinguish distance from simple lower level. If this were the case then the mic would become unusable as you sang softer. They just convert, in a reasonably linear fashion (at least at these distances) the pressure on their elements to voltage.  It makes no difference whether the element is dynamic or condensor.

Now if you factor in proximity effect you will see a bit of an exception to this ... but only to the frequencies below about 200 Hz.  I have made the measurements at 18" compared to 1" and can tell you at that distance an SM58 will exhibit a bump of about +10 dB at aprox 120 Hz.  An OM-5 will exhibit less bump ... about +7 dB at about 180 Hz. (the bump being at the closer distance of course)

So back to the claim that the Audix picks up less at distance ... Compared to an SM 58, they both pick up the same at distance for the normal voice range of 300-3kHz but the Audix picks up about 3 dB less than the 58 at 18" but only below 200 Hz..  But that is 3 dB compared to 20 dB and not very significant.  Likely another dB at 1 foot instaed of 18".  It's not the exact level of the bump that is important, just the delta between the two.

So what about "shotguns" picking up at great distances?  They don't pick up on axis any better than anything else.  What they can do is pick up less off axis spill so the intended to un-intended ratio is better, but the signal itself is no stronger.  If you are trying to pick up birds on the wire across the street with a shotgun compared to a 58 you'll get the same level of birds in both mics, but you'll get less traffic noise mixed in with it with a shotgun.

---

Don,

I don't think that you are incorrect as far as you go in your assessment.  I simply think that you don't cover everything.

First, the microphone pickup element is not a point, but rather a disk.  When you are really close to the disk, your mouth doesn't look like a far-field source, it looks like a near-field source as wide as your mouth.  The near field effect can be enhanced or diminished by placing the disk in a tube or placing it outside of the tube.

If you test an OM7 against an SM58, the results are even more pronounced.

If you design a microphone to be very dependent on a near-field source, you can reject lots of outside noise since you can essentially lower the over-all gain of the microphone and rely on the amplification of the near field source to make-up the gain.  This also has the effect of reducing feedback.

[wesg]

Can somebody explain the near field effect?

[abzurd]

---

OneEng wrote:

---

dboomer wrote:

Some people believe that some microphones are better (or worse) at picking up sound as the distance increases.  In general terms this is not the case for conventional live sound microphones.  Remember ... microphones do not go out and search for sound, the sound comes to them. To do otherwise would require that some kind of amplification that could kick in for a distant sound and that the mic could tell the difference between distance and SPL.  It is the sound itself that falls under the inverse square law.  IOW it's your voice that is dropping off in feeding the mic.

The example given was pulling a mic away a foot and having it go dead.  So here's how that breaks down.

Just to keep it simple we'll stay in tens.  If you start singing at .1 foot away and that produces 100 dB spl (wherever you are measuring it) then moving away from your mic to 1 foot the signal would drop by 20 dB to a new spl of 80 dB.  This is the normal loss due to inverse square from the source over distance.  Microphones are simple but dumb devices and cannot distinguish distance from simple lower level. If this were the case then the mic would become unusable as you sang softer. They just convert, in a reasonably linear fashion (at least at these distances) the pressure on their elements to voltage.  It makes no difference whether the element is dynamic or condensor.

Now if you factor in proximity effect you will see a bit of an exception to this ... but only to the frequencies below about 200 Hz.  I have made the measurements at 18" compared to 1" and can tell you at that distance an SM58 will exhibit a bump of about +10 dB at aprox 120 Hz.  An OM-5 will exhibit less bump ... about +7 dB at about 180 Hz. (the bump being at the closer distance of course)

So back to the claim that the Audix picks up less at distance ... Compared to an SM 58, they both pick up the same at distance for the normal voice range of 300-3kHz but the Audix picks up about 3 dB less than the 58 at 18" but only below 200 Hz..  But that is 3 dB compared to 20 dB and not very significant.  Likely another dB at 1 foot instaed of 18".  It's not the exact level of the bump that is important, just the delta between the two.

So what about "shotguns" picking up at great distances?  They don't pick up on axis any better than anything else.  What they can do is pick up less off axis spill so the intended to un-intended ratio is better, but the signal itself is no stronger.  If you are trying to pick up birds on the wire across the street with a shotgun compared to a 58 you'll get the same level of birds in both mics, but you'll get less traffic noise mixed in with it with a shotgun.

---

Don,

I don't think that you are incorrect as far as you go in your assessment.  I simply think that you don't cover everything.

First, the microphone pickup element is not a point, but rather a disk.  When you are really close to the disk, your mouth doesn't look like a far-field source, it looks like a near-field source as wide as your mouth.  The near field effect can be enhanced or diminished by placing the disk in a tube or placing it outside of the tube.

If you test an OM7 against an SM58, the results are even more pronounced.

If you design a microphone to be very dependent on a near-field source, you can reject lots of outside noise since you can essentially lower the over-all gain of the microphone and rely on the amplification of the near field source to make-up the gain.  This also has the effect of reducing feedback.

---

My thoughts as well. Don, aren't you only explaning part of the physics involved? The size of the on axis "sweet spot" is the practical difference between microphones. An OM7 has a smaller on axis area and more of the signal falls outside of it as you back away versus a microphone with a larger on axis area. So at X inches, if only 1/2 the signal is hitting the on axis area of the microphone, it's only picking up 1/2 the signal and therefore the signal will be less loud.

[Mogwix]

---

abzurd wrote:
So at X inches, if only 1/2 the signal is hitting the on axis area of the microphone, it's only picking up 1/2 the signal and therefore the signal will be less loud.

So microphones have an output that's directly proportional to the size of it's diaphragm?

Or rather, microphones with smaller diaphragms are more directional?

[abzurd]

---

Mogwix wrote:

---

abzurd wrote:
So at X inches, if only 1/2 the signal is hitting the on axis area of the microphone, it's only picking up 1/2 the signal and therefore the signal will be less loud.

So microphones have an output that's directly proportional to the size of it's diaphragm?

Or rather, microphones with smaller diaphragms are more directional?

---

I would think it's based on the design of the microphone. I don't know what makes one microphone have a wider pick up pattern than another, only that they do.

It seems kind of like how putting a horn flare on a compression driver would work (actually the opposite I guess). The sound source is the same, but the flare design changes the characteristics. A conical 100 degree flare will have the high end falling off faster than a 30 degree conical design. Stand on axis 50 feet back in the room and, you'll be able to tell the difference, no? The tighter pattern will project further than the wider pattern, but at the expense of coverage.

[dboomer]

Hey One
When speaking of "free field" mics (as live sound mics are) as opposed to "random incident field" mics (omnis) or "pressure field" mics ( instrumentation) I believe you are correct, but only at wavelengths similar to the diaphragm size. Otherwise they just behave like the mic wasn't there. I'll have to build a rig someday to test your theory.

Absurd ...
Your point would speak to initial mic sensitivity. But once you adjust your trim so that both mics are the same it would have no bearing to pickup vs distance

[dboomer]

Diaphragm size is part of the sensitivity equation, but just like speakers, all 12" speakers aren't the same sensitivity.

Directionality depends mainly on how the intended sound reaches the front of the diaphragm vs the back so it's more related to the size of the mounting than the size of the diaphragm ... Except that a smaller diaphragm could result in less directionality it you purposely wanted that.

None of this is related to horn patterns however

[agedhorse]

Diaphragm size also relates to polar differences with respect to frequency if the design exploits this. A smaller diaphragm will generally be easier to design with at higher frequencies because of relative size compared with wavelength.

[Mutha Goose]

---

wesg wrote:
Can somebody explain the near field effect?

---

Near field is typically defined as being within 1/2 wavelength from the source. Within the near field, pressure is constant and approximately equal to the source in magnitude. Outside of the near field, pressure will vary by location, and magnitude will be less than at the source. (quick note: there is a transistion period between near and far feilds, but I am glossing over that).

But as you can imagine with full bandwidth complex audio (i.e. the sum of many sinewaves of various frequencies) you can have a receiver (in this discussion, a mic element) being in the near-field for certain frequencies (low frequencies) and yet far-field for others (high frequencies). This creates a non-linear response across the spectrum (source through receiver) resulting in an emphasis on the low end.

[agedhorse]

Note that near field is also relative to the size of the diaphragm, the acoustic assembly versus the wavelength and distance. Near field may be larger for low frequencies than high frequencies under this example for example.

[Mutha Goose]

If the receiver is completely within the near-field, it's size is imaterial as pressure is also completely constant within this area.

[agedhorse]

---

Mutha Goose wrote:
If the receiver is completely within the near-field, it's size is imaterial as pressure is also completely constant within this area.

---

I have to think about this. I am concerned that artificial bounderies (and proximity effect from such) created by the unidirectional design alters this. For omni mics, I can see this reflected in the polars.

[dboomer]

I'm concerned that we don't get bogged down in minutia.  There are  bunch of small factors but the big picture is still inverse square.

The claim was that moving a foot away from the Audix mic and it's essentially off.  At a foot away you are still in the near field below 500 Hz.

[agedhorse]

---

dboomer wrote:

I'm concerned that we don't get bogged down in minutia.  There are  bunch of small factors but the big picture is still inverse square.

The claim was that moving a foot away from the Audix mic and it's essentially off.  At a foot away you are still in the near field below 500 Hz.

---

Agreed, though it's gotten me thinking about something that I otherwise woudln't have thought about...

[OneEng]

I was thinking of the wave theory when I introduced the term "near field".

Think of it like this.

If I were attempting to figure out the electric field at some point in space near a long antenna, I would have to take into account the length of the antenna and my position relative to that antenna ........

..... but ONLY as long as I was close enough to the antenna that it looked like a line.

The moment I start getting some distance from the antenna, it starts looking like a point in space radiating a uniform spherical field ..... this is the far-field equation for the electromagnetic field.

When I am close to the antenna, it doesn't look like a point source at all .... and the field it creates looks nothing like a sphere.  This is the near-field effect and the equation looks nothing at all like a sphere

I know that some of you might be thinking that this is just for EM waves .... but I assure you that waves are waves as far as physics and math are concerned.

In the example, I asked you to use your eyes as the sensor to "see" a field shape being created by an antenna.

In the situation we are talking about, the sensor is a microphone "disk" or diaphragm.  When you are close to the disk, it looks like a disk.  When you get further away ..... it looks like a single dimensional point in space.

The same is true of your mouth which is generating the sound.  When you are close to the microphone, your mouth does not look like a single point.... but when you get further away, it does.

That is my take on this.

[dboomer]

---

OneEng wrote:

The same is true of your mouth which is generating the sound.  When you are close to the microphone, your mouth does not look like a single point.... but when you get further away, it does.

---

  So what you are saying is that there is a little "curve" in amplitude response in the front of linear inverse square law.  I'll grant you that ... but how does that change my statement that all mics fall off the same with distance?  Once you calibrate the mics to be the same in the beginning, at distance they are still the same (but now lower).

There are no magic mics that either "reach" nor "stop reaching"

[Pro Sound Guy]

So after reading this your saying a Shure SM58 has more Proximity effect then an Audix OM XX.

Also, a bump at different freqs.  120/Shure     180/Audix

In using small diaphragm condensors in live recordings there seems to be less phase issues between mics.

The best live recordings I have are all with small diaphragm condensers.

I have used large diaphragms,.and small diaphragms, and dynamics.

Which none of this might mean Sh$% to what you are trying to make a point of.

[wasRbts]

I only ever had one year of Physics in High School - and one year in College - and of course I forget most of it - but one thing that I HAVE carried with me is that the concept of "inverse to the square of the distance" is one of the most debilitating forces in nature.  ("Things" "fall off" quickly - even if they move on <somewhat> infinitly.)  (Never really having noticed logs having rhythm.)