LD acoustic delay chamber size

[Jan]

A microphone question for the tech heads:

How does it come that large diaphragm cardioid microphones are relatively slim compared to small diaphragm mics?

(Considering the need for an acoustic delay chamber in order to work as a cardioid)

Thanks,

Jan

 

[littledog]

might as well take your question right to the source:

go to Stephen Paul's forum at: www.recording.org

 

[Harvey Gerst]

A cardioid pattern is a 50/50 mixture of pure pressure pattern (omni), combined with a pure pressure gradient pattern (figure 8, also called pure velocity, or bi-directional).

In a large diaphragm condenser mic, there are small capillaries (holes which go partway thru the backplate, creating the pressure mic pattern), while other capillaries go all the way thru the back plate to create the pressure gradient (velocity sensitive) pattern. These two patterns combine to create the cardioid pattern. It's not done using time delay, but thru the combination of patterns, using both pressure and velocity sensitve components.

The negative portion of the velocity pattern combines with the positive portion of the omni pattern to cancel out sounds coming from the back side of the mic. As you get in close to the front of the mic, the velocity portion of the pattern becomes more prominent, and you get the familiar proximity buildup, as with any pressure gradient mic.

The above explanation is a bit simplistic, but hopefully, it'll get ya closer to the answer you want. Littledog is right; Stephen Paul can explain it far better.

 

[Harvey Gerst]

Probably a little of both. It requires a pretty good knowledge of math, involving sines and cosines and math junk like that.

I believe the first condenser mics were omnidirectional. You'd hafta find the original Western Electric patent to check that out. Some of the early mics used ducts and vanes that would cover or expose the back side of the diaphragm to mechanically create the different patterns.

 

[audiofreak]

Harvey,

You brought out an interesting point about proximity effect. May I ask a few questions?

1. It seems that most mic manfacturers do not produce a mic which is "flat" (or conforms to the specification) without the proximity effect. Most manufacturers acutally factor in the proximity effect when balancing the response of the mics, so that the mic is "flat" WITH (at least a bit of) proximity effect; but loses the bottom end and become thin without the proximity effect. The question is: does it mean that a mic should not be used too far (e.g. a few feet) from the source, even in a dead room?

2. If the answer is "it depends on whether its pattern is wide cardioid, cardioid, hypercardioid, etc.", then which pattern can be used farther from the source without losing the "flatness" (assuming a dead room)?

3. If your answer is "it depends on individual mics", then how can I know which mic is designed for which distance? I seldom, if ever, come across a mic with a frequency response graph that specifies the source-to-mic distance on which the graph is based. I also seldom see a manufacturer's recommendation for an optimum distance. I suppose the users won't have to figure that out by trial-and-error.

Remarks:
For conceptual simplicity, the questions above assume a dead room with a single source, where room reverb and instrument bleeding can be conveniently ignored. True, we seldom prefer/have a dead room in real life, but I need to simplify the questions to a conceptual, anechoic level to get my head clear about source-to-mic distance and frequency response.

For simplicity, I also assume that I want a flat response, although in practice most people don't.

Thanks.

P.S. I have read your big thread again before posting questions.

 

[mixsit]

That's an interesting point. It would seem there are at least two extremes in directional condensors; voiced 'live' where proximity is combined with low rolloff for a balanced sound up close, and studio mics that are flatter at a distance?
wayne

 

[Harvey Gerst]

Harvey,

You brought out an interesting point about proximity effect. May I ask a few questions?

1. It seems that most mic manfacturers do not produce a mic which is "flat" (or conforms to the specification) without the proximity effect. Most manufacturers acutally factor in the proximity effect when balancing the response of the mics, so that the mic is "flat" WITH (at least a bit of) proximity effect; but loses the bottom end and become thin without the proximity effect. The question is: does it mean that a mic should not be used too far (e.g. a few feet) from the source, even in a dead room?

2. If the answer is "it depends on whether its pattern is wide cardioid, cardioid, hypercardioid, etc.", then which pattern can be used farther from the source without losing the "flatness" (assuming a dead room)?

3. If your answer is "it depends on individual mics", then how can I know which mic is designed for which distance? I seldom, if ever, come across a mic with a frequency response graph that specifies the source-to-mic distance on which the graph is based. I also seldom see a manufacturer's recommendation for an optimum distance. I suppose the users won't have to figure that out by trial-and-error.

Remarks:
For conceptual simplicity, the questions above assume a dead room with a single source, where room reverb and instrument bleeding can be conveniently ignored. True, we seldom prefer/have a dead room in real life, but I need to simplify the questions to a conceptual, anechoic level to get my head clear about source-to-mic distance and frequency response.

For simplicity, I also assume that I want a flat response, although in practice most people don't.

Thanks.

P.S. I have read your big thread again before posting questions.

Wow, some great points. First let's clarify what "proximity effect" is, and how it occurs. "Proximity effect" is a byproduct of pressure gradient patterns - all pressure gradient patterns. In simple terms, it's a velocity buildup, caused by getting so close to one side of the diaphragm, the other side doesn't have a chance to do its canceling thing.

If you imagine a figure 8 pattern, you have a completely velocity driven mic, where the direction of the source determines the output. As the pattern is changed to hypercardioid, to cardioid, to wide cardioid, to finally omni, the proximity effect gradually decreases.

Most of the proximity effect will be noticed within about 3 feet of the mic, increasing as you get closer. Some manufacturers count on the proximity effect to produce a flat response in close, but beyond 3 feet, the bass drops off rapidly. Others use the bass buildup of the effect to add bottom as you get closer, but the mic is flat in the far field.

Others compensate for the proximity effect by using a complementary low frequency rolloff switch for up close work to achieve flat response, but the switch is turned off for flat response in the far field.

To answer your specific questions, you'd hafta look at the manufacturers published response curves to see if the mic is precompensated for proximity effect, which means it's specifically designed for in close recording.

If the mic is NOT precompensated for proximity effect, it will show a rising bass curve as you move in closer, and it will be fairly flat from about 3' to 10' away, in the case of various pressure gradient patterns. Omnis will of course remain flat, since they only respond to pressure, not velocity.

 

[Harvey Gerst]

That's an interesting point. It would seem there are at least two extremes in directional condensors; voiced 'live' where proximity is combined with low rolloff for a balanced sound up close, and studio mics that are flatter at a distance?
wayne

Wayne,

Hopefully, my answer to audio freak's question also answered your question.

 

[mixsit]

Yes! Thank you Harvey.
If I may, one other? (This might be in the 'Big Thread' but I haven't seen it.)
When looking for the most total off axsis rejection (all rear angles included) does cardioid have the most? It looks like others (hyper,ect.) are tighter in front at the expence of lobes in the back. My application in this case is minimizing room bleed on my drum overheads.
Thanks again.
Wayne

 

[Harvey Gerst]

Yes! Thank you Harvey.
If I may, one other? (This might be in the 'Big Thread' but I haven't seen it.)
When looking for the most total off axsis rejection (all rear angles included) does cardioid have the most? It looks like others (hyper,ect.) are tighter in front at the expence of lobes in the back. My application in this case is minimizing room bleed on my drum overheads.
Thanks again.
Wayne

Yes, cardioid would offer the most rear rejection. Remember that most patterns are simply varying combinations of figure 8 and omni combined. Cardioid is a 50/50 mixture.

If you put omni at one end of the pattern scale and figure 8 on the other end, all the other patterns are simple ratios of these two patterns, with cardioid always in the middle of the pattern range.

75% figure 8 + 25% omni gives you a hypercardioid pattern, while 25% figure 8 + 75% omni gives you a wide cardioid.

 

[Harvey Gerst]

If, you sing slightly off axis, the proximity effect remains about the same. Singing directly into the side of the mic produces no sound at all, since the velocity is equal into both sides of the mic. canceling out.

Most singers sing directly into the front of the mic.

 

[Jan]

Thanks for al your replies. I actually subscribed myself to the recording.org forum but still didn’t post the question, mainly because I’m usually reasonably busy with work and family, but that’s a lousy excuse of course. I actually spend many hours reading posts on the homerecording forum and even printed our the Harvey big thread (made available in .doc form by someone here)

I have some observations after reading Harvey’s brilliant (as always) explanation on the special constructions applied in LD cardioid design;

LD microphones usually are of the “stand up” type. This way of constructing microphones has become a must-do fashion for manufacturers that want to create something presentable as professional.

Historically this way of constructing microphones had big advantages because the diaphragm had “free space” all around, and since all historic microphones were either: omni, figure 8 or multi pattern, this panoramic position on top of the microphone was the most logical place to be.

Not knowing anything particular about LD microphone history, I’m now going to say something stupid by suggesting that LD cardioid microphones were probably a necessity that evolved in later days when the need for lower budged “vocal” (cardiode) LD mics became eminent.

Being already used to the “professional” stand-up type LD microphones it became hard to propose huge U87 lying down, in which singers had to sing the same way that they do with their low budged SM58

So unless there is some very specific reason why a LD cardioid simply can’t be an enlarged SD cardioid with a huge acoustic delay chamber, I would say that it’s a matter of microphone fashion. Otherwise SD cardioid’s could have the same “capillary chamber” construction as their bigger LD brothers (or sisters).

Anyhow I learned a lot from the postings on this forum, and very mutch from the contributions of specific members like Harvey etc. Actually I know now that since my Rode NTK has little proximity effect and picks up lot of room noise, it simply means that it’s more omni then cardioid. My Neumann TLM 103 has lots of proximity effect and picks up little room noise, just as my AKG C2000B’s (the most underestimated mic ever), so I know that their pattern must be very cardioid.

Please keep the postings going.

Thanks,

Jan

 

[Harvey Gerst]

I feel like an idiot but I've got to be honest here.........

When I have seen diagrahms of M/S recording they always seem to show the Figure 8 mic turned 90-degrees to the cardioid. I guess that is just a diagramatical explanation of the concept.

I hate it when I look back and see footprints in the rice-paper.........

Yup, the figure 8 mic is turned 90° to the main source so that it's only picking up sound from the sides, not straight ahead. Aiming the cardioid straight ahead takes care of sounds from the front and (in combination with the figure 8 mic) controls the width of the stereo image.

One of the other advantages to figure 8 mics is that they have the best off axis frequency response, compared to any of the other pressure gradient polar patterns.