MS mics placement

[abbey road d enfer]

https://gearspace.com/board/geekzon...-pressure-velocity-sensitive-microphones.html

I remembered that I have subscribed there a long time ago, so I posted my answer there.
https://gearspace.com/board/showpost.php?p=16667549&postcount=16

 

[Biula]

It is exactly the other way around. When I checked my own built ribbon prototypes against condensers to define the correct phasing I found this differene of 90 deg. even at moderate frequencies!
A pressure sensitive transducer provides maximum amplitude at the peak of sine soundwave --> maximum pressure. A veloctity transducer provides maximum amplitude at the stepest part of the sine wave --> maximum velocity. These are 90 deg. apart.

I Agree 100%: physics is physics. Condenser MS microphones are all pressure/ gradient-pressure (omni, cardioid or eight: the latter is in fact a couple of back-to-back cardioids...), the only velocity MS example is the original Blumlein Array!

 

[RoadrunnerOZ]

It is exactly the other way around. When I checked my own built ribbon prototypes against condensers to define the correct phasing I found this differene of 90 deg. even at moderate frequencies!
A pressure sensitive transducer provides maximum amplitude at the peak of sine soundwave --> maximum pressure. A veloctity transducer provides maximum amplitude at the stepest part of the sine wave --> maximum velocity. These are 90 deg. apart.

What about the phase lag of current in the primary coil of the ribbon transformer - doesn’t that shift the whole perspective as there is a lag in the time for the ribbon transformer to produce an output on its secondary?

 

[Barry Hufker]

is there a phase lag in a condenser with a transformer? Is there no lag in an "active" ribbon microphone?

 

[RoadrunnerOZ]

is there a phase lag in a condenser with a transformer? Is there no lag in an "active" ribbon microphone?

There is lag in the ribbon element by itself. A dynamic mic with a transformer will have lag - SM57 and 58. A dynamic mic will have lag in the voice coil anyway without a transformer. Literally there are phase differences between mics differing electronically of any type.
That’s possibly why I found the best results for M/S using two identical mics with the correct available two pattern choices.

In the long run, when recording a drum kit for example, if I’m getting cancellation problems - lots of mics, different types, different positions - I just go into the software and look at the relative wave positions of sidestick hits (I get the drummer to do a count-in in time with the intro click) on all the close mic tracks at the beginning of a take and slide each tracks’ audio into place to match that of the snare - room mic I leave - overheads I play with until it becomes cohesive. The same can apply to M/S recording.

 

[Barry Hufker]

That's very interesting. I do quite a bit of time alignment myself when necessary. What you're saying technically is what I have felt intuitively but never had an explanation for. Thank you.

 

[RoadrunnerOZ]

That's very interesting. I do quite a bit of time alignment myself when necessary. What you're saying technically is what I have felt intuitively but never had an explanation for. Thank you.

No problem. After spending countless (sometimes fruitless) hours on phase flipping and EQ’ing tracks on tape mixes without the luxury of being able to slide audio tracks in a DAW, it was a simple progression to get time cohesive drums and other instruments with multi-mics.
Added to mic variances and placement position errors there is also the delay added when using inline hardware such as boutique mic pre’s, some with A/D & D/A internal processing for EQ & compression for example. Can end up quite a phase nightmare. Sliding audio tracks is also good for tuning room size with room mics - there’s usually a sweet spot.

 

[k brown]

I Agree 100%: physics is physics. Condenser MS microphones are all pressure/ gradient-pressure (omni, cardioid or eight: the latter is in fact a couple of back-to-back cardioids...), the only velocity MS example is the original Blumlein Array!

Blumlein and MS are completely different things; Blumlein is stereo right out of the mics, MS requires dematrixing, and has quite different advantages and disadvantages. All they have in common is they are both coincident stereo arrays.

 

[abbey road d enfer]

I Agree 100%: physics is physics.

Of course.

Condenser MS microphones are all pressure/ gradient-pressure (omni, cardioid or eight: the latter is in fact a couple of back-to-back cardioids...),

Not always, check Sennheiser or Schoeps fig-8 condensers, they are true velocity sensors.

the only velocity MS example is the original Blumlein Array!

The origial Blumlein array used ribbon mics, because they were the only practical choice for fig-8, but it has been proved times and times that fig-8 condenser mics, wheteher they are natively so, or by combining cardioids, give excellent results, thus are not "the only velocity MS example".

 

[abbey road d enfer]

There is lag in the ribbon element by itself. A dynamic mic with a transformer will have lag - SM57 and 58. A dynamic mic will have lag in the voice coil anyway without a transformer. Literally there are phase differences between mics differing electronically of any type.

The only significant phase difference is that resulting from sensing velocity and/or pressure. The rest is phase differnces that are collateral to frequency response.

That’s possibly why I found the best results for M/S using two identical mics with the correct available two pattern choices.

Very likely because the frequency response matches, not because of an inherent phase difference.

In the long run, when recording a drum kit for example, if I’m getting cancellation problems - lots of mics, different types, different positions - I just go into the software and look at the relative wave positions of sidestick hits (I get the drummer to do a count-in in time with the intro click) on all the close mic tracks at the beginning of a take and slide each tracks’ audio into place to match that of the snare - room mic I leave - overheads I play with until it becomes cohesive. The same can apply to M/S recording.

Multi-micing is a different subject. Even using all condenser mics or all dynamic mics in a multi-micing situation leads to cancellation issues that are due to the physical distance and acoustic travel differences.
Here we are talking of phase response of mics of different technologies.

 

[GabrieleP]

This is a quite extraordinary claim. Actually, ALL figure-8 mics are velocity transducers, whatever the technology. Omni are strictly pressure transducers. Cardioids of all variations are pressure-gradient transducers.

I'd like a demonsration of this ASSertion.

Demonstration of how a wrong analysis leads to a wrong conclusion.

abbey, why you call it a "wrong analysis"?

maybe what bert37 means is that when SPL is @ max
- a capacitor transducer's output is max
- a moving coil/ribbon transducer's output is zero

here's my personal contribution to the matter:

http://www.tunedmiking.net/oh-such-a-perfect-phase-part-1-of-3/
I totally support what bert37 said friday: having a condenser as M and a ribbon as S theoretically speaking is not a good idea. Of course designers and manufacturers can manipulate the output phase.

The only place in the world where I could find confirmation of this (after weeks of research) is the "Handbook for Sound Engineers" (a reference and a link can be found in my blog, within the "Being Social" post).

I came to the conclusion that the "pressure vs. velocity" approach can be confusing.

Best, G.

P.S. Sorry, reading your capitals was quite disappointing

 

[MicUlli]

I see a lot of misconception here.
A plane wave (almost true also for waves excited by point sources only far enough away) is characterised by it's sound pressure p (scalar) and its sound velocity v (vector). p and v are linked together by Z (sound impedance). Because Z = rho x c (density times speed of sound) sound pressure p and sound velocity are ALWAYS in phase in a sound field obeying the plane wave rule.
If there is a standing wave (addition of two waves travelling in opposite direction) you might get LOCAL minima and maxima for sound pressure and sound velocity. Probably this is the cause of confusing interpretations seen in this thread..
Best regards
MicUlli

 

[GabrieleP]

I see a lot of misconception here.
A plane wave (almost true also for waves excited by point sources only far enough away) is characterised by it's sound pressure p (scalar) and its sound velocity v (vector). p and v are linked together by Z (sound impedance). Because Z = rho x c (density times speed of sound) sound pressure p and sound velocity are ALWAYS in phase in a sound field obeying the plane wave rule.
If there is a standing wave (addition of two waves travelling in opposite direction) you might get LOCAL minima and maxima for sound pressure and sound velocity. Probably this is the cause of confusing interpretations seen in this thread..
Best regards
MicUlli

thank you MicUlli... although I don't have a solid background to fully understand your technical and scientific explanation, nevertheless it is something I grasped in the past. I guess we can fairly assume a plane wave is not something a microphone transducer is always picking up, is it? Definitely not in close-miking arrangements...

I'm not talking (nor is bert37 I reckon) about sound waves, I am talking about different microphones' transducing principles.

 

[MicUlli]

In close miking applications the proximity effect comes into play, but only for pressure gradient microphones, not for pressure mics. At very low frequencies you would indeed see a phase shift of up to 90 degrees with a 6 dB/oct amplitude rise to lower frequencies for the gradient mic. If you eqalise the frequency response of your gradient mic the phase shift disapears..

 

[RoadrunnerOZ]

The only significant phase difference is that resulting from sensing velocity and/or pressure. The rest is phase differnces that are collateral to frequency response.

So there is no current lag in your mind for a velocity mic when an internal transformer passes a signal from a dynamic capsule or a ribbon to the XLR socket on the mic? I was taught that current lags voltage by 90deg in an inductor. The voltage can’t get through a transformer before the current flows so how can there be a perfect 90deg phase lead for a velocity sensing mic over a pressure or pressure gradient mic which senses voltage max at diaphragm excursion peak, when there is no current flow yet, so therefore no mag field, no coupling to transformer secondary, no output voltage yet - or am I missing something?
There are a lot of condenser mics that are transformerless.

There is also lag in any velocity sensing mic or pressure gradient mic due to the sprung element mechanical resistance of voice coil, ribbon or diaphragm - they don’t move instantaneously with air movement and because they are sprung it takes a pressure buildup to overcome the force of the spring resistance - there is back pressure also from the tuning chamber of a cardioid condenser or dynamic mic eg 57 & 58 use the same voice coil, different chambering, different mechanical resistance and of course frequency response - the chambers provide a time delayed and tuned phase cancelling pressure to the back of the diaphragm to reduce rear information. No two mics will have the same mechanical response time to overcome inertia.
It takes a buildup of pressure to move the diaphragm or ribbon, the higher SPL mics tend to have a stiffer diaphragm, so the more sensitive mic will lead the stiffer less sensitive mic timewise for the same SPL signal.

 

[molke]

I see a lot of misconception here.
A plane wave (almost true also for waves excited by point sources only far enough away) is characterised by it's sound pressure p (scalar) and its sound velocity v (vector). p and v are linked together by Z (sound impedance). Because Z = rho x c (density times speed of sound) sound pressure p and sound velocity are ALWAYS in phase in a sound field obeying the plane wave rule.

Probably, I still have several misconceptions. But I maintain that there are no velocity transducers in studio microphones—only pressure gradients. Your statement above is true for the velocity, but apparently not for the pressure gradient. Unfortunately, this document is in German, but the formulas should be understandable without the explanation.

As to why a combination of pressure gradient and pressure transducer (e.g. in the context of MS) is not a problem, the following seems to be a very plausible explanation:

If the pressure gradient is always 90° ahead to the sound pressure, why is MS stereophony with an omni-directional pressure microphone and a pressure gradient figure-of-eight microphone no problem?

Answer:
The frequency response of the pressure gradient increases with frequency at 6 dB/octave. To compensate for this, these microphones are "tuned damped". In the electrical analogy,this represents an RC pad above its transition frequency causes a drop of 6 dB/octave, and shifts the phase constant at −90°. In the whole area in which the "pressure gradient figure-of-eight microphone" has a constant frequency response, thus, the phase is equal to the "pressure omni-directional microphone".

 

[MicUlli]

That's it!
The naming velocity microphone is mainly related to ribbons and is based on the fact that the output voltage is proportional to the ribbon velocity. In fact the ribbon mic is of type pressure gradient with fig8 pattern. The pressure gradient rises with 6dB/oct, the ribbon velocity falls with 6dB/oct (because of the low tuning frequency). Both together causes a considerably flat response.
BR MicUlli

 

[molke]

That's it!
The naming velocity microphone is mainly related to ribbons and is based on the fact that the output voltage is proportional to the ribbon velocity. In fact the ribbon mic is of type pressure gradient with fig8 pattern. The pressure gradient rises with 6dB/oct, the ribbon velocity falls with 6dB/oct (because of the low tuning frequency). Both together causes a considerably flat response.
BR MicUlli

Maybe, that’s a language problem, but all the references (mostly in German) I have here consider also ribbon microphones to be pressure gradient transducers. Velocity microphones are certainly possible and apparently have no membrane.

 

[GabrieleP]

Any microphone's diaphragm is displaced by variations in atmospheric pressure, not velocity.

 

[abbey road d enfer]

abbey, why you call it a "wrong analysis"?

First because you think taht combining two signals thar 90° apart results in a "wrong" signal. Then for mixing acoustic phase issues, due to different acoustic paths with phase-shift related to transducers.

maybe what bert37 means is that when SPL is @ max
- a capacitor transducer's output is max

Not true. If the diaphragm was left with no damping, that may be partially true, but in order to achiebve a flat-ish response, it must be damped, which results in mechanical phase-shift that compensates whatever phase-shift due to the transduction process.

P.S. Sorry, reading your capitals was quite disappointing

Care to explain?