Harvey Gerst
New member
DonF has it 100% correct. A pressure mic is like a barometer that operates in the audio range. It responds to pressure changes in the room.
Dumb question
- Thread starter krs
- Start date
DonF
Call Me "Grandpa"
If the vibrating surface is open to the atmosphere on only one side, it will respond to pressure changes, such as sound waves, regardless of which direction they're coming from. If the vibrating surface is open to the atmosphere on both sides, then the velocity (speed and direction) of the wavefront comes into play. But I think we're jumping ahead of wherever Harvey was going with his diagrams. Especially if we start talking about voltages.
BTW, I'm resisting the use of the word "diaphragm" in favor of the more general term, "vibrating surface". Ribbon mics are a very important type of velocity microphone, and they don't use a diaphragm.
BTW, I'm resisting the use of the word "diaphragm" in favor of the more general term, "vibrating surface". Ribbon mics are a very important type of velocity microphone, and they don't use a diaphragm.
DonF
Call Me "Grandpa"
OK, I found an article with plenty of diagrams that I think will answer the original question:
Here's a link to the article, which originally appeared in Recording magazine in 1999: http://classes.berklee.edu/mpe/pdf_files/articles_pdf/nuts_bolts/05_mic_air_pressure_velocity.pdf
On the last page, the article says
The answer, apparently, is no; a hypercardioid pattern must have a rear lobe.
Thanks, krs, for asking a "dumb" question and making me learn this.
Don
krs said:
Here's a link to the article, which originally appeared in Recording magazine in 1999: http://classes.berklee.edu/mpe/pdf_files/articles_pdf/nuts_bolts/05_mic_air_pressure_velocity.pdf
On the last page, the article says
The answer, apparently, is no; a hypercardioid pattern must have a rear lobe.
Thanks, krs, for asking a "dumb" question and making me learn this.
Don
krs said:Is this difference in pressure/velocity acheived by different voltages across the diaphragms? Or? I feel like I don't understand something very basic about how one creates a diaphragm to be one way or the other.
Sorry to be nosy
Harvey, I hope you don't mind me jumping in, I've followed your contributions for years.
krs,
Maybe a little info on what sound is will help. Sound waves aren't really waves. They are areas of more and less pressure. When you see a sine wave picture of a sound wave, it is just a numerical/graphical representation of higher and lower air pressure passing a point (like a microphone) These areas of high and low pressure move from a source (like your mouth) through the surrounding air. So sound is said to have both pressure and velocity. How many times per second the pressure goes up, down, and returns to the zero point is the frequency (one full sine wave, in all those fancy pictures). How high and low the pressure goes is the SPL, sound pressure level, what we usually call loudness or volume.
The difference between pressure mics and velocity mics is in the capsule design, not the diaphragm itself. A mic capsule is the diaphragm, and an airspace behind it.
A pressure mic is like a filled baloon, literally. It is a sealed airspace, with a flexible boundary (the diaphragm). Behind the diaphragm, the capsule is sealed, no air gets in or out.
Put the baloon in a room. Hook a tube to an air pump and feed it into the room. If you pump air into the room, the baloon shrinks. If you suck air out, the baloon expands. It reacts to the pressure changes. When you speak, you change the pressure in the room just like the air pump. The pressure mic reacts the same way as the baloon.
A key thing is that the direction of the source of pressure change doesn't matter. Your air hose could be in the ceiling, floor, or wall. The baloon will react the same. In the same way, it doesn't matter where the sound comes from when using a pressure mic. The pressure mic reacts to the overall pressure changes in the room.
A velocity mic is an open airspace. There are holes in the capsule. It's like a tom-tom with no bottom head. So if you just pump air into the room, the diaphragm won't move, because the pressure on both sides of the diaphragm stays the same. A velocity capsule and diaphragm react like a piece of paper in front of your face. When you speak at it, you create sound waves, areas of high and low pressure with a velocity (they move, in other words). Hold the paper flat side towards your mouth, and it flutters back and forth. The higher and lower pressure on the front of the paper (relative to the ambient room air pressure on the back of the paper) as the sound waves hit the paper cause the paper to flutter back and forth. Hold the paper edge-on, and when you speak the paper does not move. There is equal pressure on both sides of the paper at once at all times as the sound waves hit it.
Pressure mics are omnis, directional mics are velocity designs.
Both types of mics rely on pressure in the end, but velocity mics absolutely depend on fairly rapid pressure changes from the right direction to be effective. If there is time for the pressure to equalize on both sides of a velocity mic diaphragm, it will not react. This is the basis behind various directional patterns. Sound waves coming towards the back of a cardioid mic, for example, are delayed so they reach the diaphragm at the same time as they hit the front, so the pressure is balanced, and you get no sound. The holes in a cardioid capsule aren't just holes, they are special, designed to delay sound waves just the right amount so sound from the back of the mic hits the back of the diaphragm at the same time it hits the front.
The terms velocity and pressure are a bit arbitrary, but they work.
Thought experiment:
Say you could make a sound wave stand still. That is, you could move a mic along the wave, instead of moving the sound wave past the mic. With a pressure mic, you could move it as slow as you wanted and get accurate results, even stop in the wave, as it reacts to absolute pressure, due to it's sealed capsule.
A velocity mic needs to keep moving. It's open capsule will allow the pressure to equalize on both sides of the diaphragm if it stops or moves too slow.
Theoretically, a pressure mic could measure any frequency, no matter how low. A velocity mic will be limited in low-frequency response by nature, once the frequency gets low enough, as the pressure changes happen too slowly.
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Marik
Pro Microphone Design
krs said:Is this difference in pressure/velocity acheived by different voltages across the diaphragms? Or? I feel like I don't understand something very basic about how one creates a diaphragm to be one way or the other.
Sorry to be nosy
It is a little tricky to explain... but here it is.
Think about the (double) diaphragm as two cardioid mics in one body. Bias voltages can be arranged in many different ways. Usually, in tube microphones front diaphragm is at 0V bias, back diaphragm voltage is variable, and center electrode (backplate) is at + of bias (lets say 60V) and used as a reference. What happens here is that in reference to backplate, the front diaphragm is actually at -60V.
If we put on back diaphragm 0V, the signal here will be at the same polarity as the front diaphragm (remember? our backplate is a reference, so front diaphragm although has a 0V bias, but in reference to the backplate is actually -60V). We get two cardioids. The voltages from both diaphragms get summed and two cardioids give us an OMNI pattern.
If we put on back diaphragm +60V, then in reference to backplate (+60V) it will be at the same potential and will be off, so we will get a CARDIOID pattern.
Now, if we put on the back diaphragm +120V, (in reference to backplate it is +60V), it will be at the same voltage, as the front one, but with opposite polarity. So back cardioid will be out of phase, and then combined with the front in-phase one, their signals will cancell each other, resulting in a FIG 8 pattern.
Any in-between voltages on the back diaphragm will result in in-between patterns.
Hopefully it is clear.
Marik
Pro Microphone Design
krs said:That's starting to get complicated
Huh, I did not really want to complicate the matter.
Simply speaking, there are two ways of getting different patterns.
1) Acoustically, by carefull combination of pure velocity and pressure components, or
2) Electrically, by summing or substructing the certain amount of signal, with the certain phase. That's what polarization voltage does.
There are two types of capsule design:
I. single-diaphragm, and
II. dual-diaphragm.
Single diaphragm includes "pure" velocity and pressure types of capsules, and with special acoustical arrangements also may be designed as a combination of both, for acoustically formed patterns I mentioned in 1) above.
Dual-diaphragm called Braunmuhl-Weber design. Each side is essentially an acoustically formed cardioid pattern, meaning that they have an equal amount of velocity and pressure components. When the back-diaphragm gets bias voltage equal to voltage/phase of the front-one, the VELOCITY component is electrically substructed, and mathematically we get an OMNI pattern.
Likewise, if the back-diaphragm gets bias voltage equal to the front one, but of opposite polarity, then the PRESSURE component gets substructed, and we get a FIG8 pattern.
Needless to say, because of construction, these kinds of patterns work and behave somewhat different than the "true" pressure or velocity types.
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OneRoomStudios
New member
That was the best dumb question ever
Harvey Gerst
New member
Great job, Marik and Easychair!!!
Does everybody understand how this pattern stuff works now?
That's exactly why ALL continuously variable, multi-pattern mics start with omni at one end of the pot and figure 8 at the other end, with cardioid in the middle. And it's why a hyper-cardioid pattern has to have a rear lobe.
If some of you are still confused about it, I might be able to give you a simpler explanation (with some more pictures), but if you all get it, then we can move on.
Does everybody understand how this pattern stuff works now?
That's exactly why ALL continuously variable, multi-pattern mics start with omni at one end of the pot and figure 8 at the other end, with cardioid in the middle. And it's why a hyper-cardioid pattern has to have a rear lobe.
If some of you are still confused about it, I might be able to give you a simpler explanation (with some more pictures), but if you all get it, then we can move on.
AcoosticZoo
New member
Microphone Polar Patterns
Old thread, but I was really surprised yesterday when I read this, cos changing polar patterns really affects the quality of a recording in terms of recording ambient room reflections with the source.
I've only been really keen on using cardiod and omni for so many years and maybe some figure 8 for MS recordings.
I started experimenting on my Neumann 149 and shure KSM 44 and I found that fig 8 has the most focused field and captures more of the source when compared to other polar patterns. That's probably why a ribbon mics are so fun to use cos it's only got fig 8. It delivers a focused recording when you point it dead on the source.
Old thread, but I was really surprised yesterday when I read this, cos changing polar patterns really affects the quality of a recording in terms of recording ambient room reflections with the source.
I've only been really keen on using cardiod and omni for so many years and maybe some figure 8 for MS recordings.
I started experimenting on my Neumann 149 and shure KSM 44 and I found that fig 8 has the most focused field and captures more of the source when compared to other polar patterns. That's probably why a ribbon mics are so fun to use cos it's only got fig 8. It delivers a focused recording when you point it dead on the source.