Mic tests for VO

It is obviously easier to tailor a flat high frequency response using a diaphragm smaller than 18 mm as the peak frequency is in the order of 1/2 wavelength. For SD this comes to the order of 15 kHz while for a LD it is closer to 10 kHz. Although the behaviour beyond 15 kHz may be rather a matter of a few fetishists, it is obvious that a rapid fall occurs above the peak. This explains why many (even rather cheap) SD microphones exhibit a reasonably flat response up to 20 kHz while even highly reputed types such as a U87 have a steep fall off above 15 kHz.

Fine, except that it's not true...

The problem isn't with the diameter of the mic at all when you measure the on-axis response; a plane or spherical wave arriving on-axis isn't affected by the diameter in the slightest. Where the diameter makes a difference is in the rate of fall-off of HF as you move off-axis, and this effect is same, whatever directional pattern you think you are using. If you think about it for a moment, it's obvious; the worst case is a signal arriving at 90 degrees to the diaphragm - if a whole wavelength is present across the diaphragm it will cancel itself out completely. This effect grows the further away from directly on-axis you get; no effect at first, and the whole shooting match at 90 degrees (but see later...).

The on-axis response of my C414s, as plotted by AKG is pretty much flat up to 20kHz. It may well be flat above that as well, but their measuring system stops at that point.

As for the extinction frequencies, you haven't quite got them correct, I'm afraid. The relationship is f=c/λ, where c is the speed of sound in air. I do this in metric because I can remember the speed of sound in m/s, but the answers are the same if you use imperial measurements: So, c=344m/s, which means that for a diameter of 25mm (1") you end up with 344/0.025 = 13.76kHz, and for a 12.5mm (1/2") diameter, obviously the figure doubles to 27.52kHz. But as I indicated, this only affects the off-axis response, and because the case and mountings of the microphone will provide diffusion at the sorts of frequencies concerned, the cutoff is never complete anyway, and the dip often occurs at a higher frequency than you think it might - a glance at the C414 graphs suggests that this occurs at around 16kHz.

The second is: I admit that this often high overrated with respect to the sound quality but the frequency characteristics of SD types in general look better than those of LD types. This leaves open the question why both so many manufacturers as users stick to the LD types ?

For one thing, they are a damn sight more sensitive. What this means is that you can capture a lot more low-level detail from them without having to attempt to extract it from the capsule noise. Typically, the self-noise figure for 1/2" mic capsules is is around the 15-17dB mark compared to an equivalent 1" self-noise of about 7dB at best. So, in a typical vocal situation the overall sound tends to be more 'intimate' (for want of a better descrption) than the sound from a similarly placed SD would be, and this is pretty noticeable in a direct comparison. As for the frequency response - well, for mics intended for vocal use, manufacturers often allow a 'presence' peak to remain at around 7-8kHz, which adds a general sense of 'air' to the sound - but quite frankly you can do that anyway with EQ if you want, because the effect is pretty much the same.

So, it's not just myth/hype about LD mics sounding better for vocal performances - they really do, but it's a by-product of the sensitivity, not the FR.

Thank you again Steve. Happy those who have a studio as silent as 7 dB acoustical noise. For my live recordings I am far away from such favorable conditions !

Happy those who have a studio as silent as 7 dB acoustical noise. For my live recordings I am far away from such favorable conditions !

I quite regularly record in situations where the mic self-noise is significant - anywhere where you use a distant mic pair, and the acoustic levels are lower, this can become a significant issue - simply because you have to crank the gain up a bit. With a capacitor mic, it's almost invariably the mic capsule/impedance converter self-noise that you hear, whereas with a dynamic mic (much lower impedance of the generating element), it's the self-noise of the first preamp stage in your mixer/preamp that dominates at lower levels. That's the (noise) price you pay for using a very high impedance generator...

LD mics appear to perform better with close-up vocals on-axis because they intercept and collect more of the acoustical wavefront than SD ones do. And being on a slightly larger scale, it's relatively easier to engineer the capsule response to suit the human voice. But, and this is really the important point, the manufacturer can also engineer them to have a pretty much flat response on-axis in just the same way, if they desire. But since most users don't appear to want to use them this way, they don't do it very often, and those mics attract something of a premium, because...

One of the ways in which SD mics generally score quite a bit better is in the response control as the pickup moves progressively away from 0 degrees. It's quite possible to engineer this response to be good in a LD mic too, but to make it worthwhile as an exercise, it involves a lot of care with the capsule housing as well. As a result, it's quite easy to engineer a SD mic with relatively good axis control for a reasonable sum (as a lot of manufacturers have demonstrated), but to do this with a LD and have it as good is invariably going to cost a lot more. So, the cheaper LD mics are often fine for vocals, but not so good at a distance, or as stereo pairs. As a result, there are far fewer LD mics that are rated for that sort of use, but when the design is good, the basic constructional layout remains in production for a long time. My AKG C414s have been in production with essentially the same acoustic layout for the last 37 years, are in use in virtually every major studio on the planet, and are showing no sign of running out of steam as a design at all. And the major difference between them and a cheap LD is how well they work at a distance. And that's because the design is holistic - it's more than just capsule design involved.

One other advantage of mics designed like this is that they are a lot easier to position at close range too - that controlled off-axis response means that you don't have to search anywhere near as hard for, for instance, the 'sweet spot' where a mic's response, the room response and an instrument's case radiation all sound good - generally, it's just down to the room and the instrument. And to be absolutely fair, this is also easier with the majority of SD mics. Which is why I tend still to use these on individual smaller instruments.

Thanks for the info on the mics. I also did some mic shopping recently and found that the AT 4040 "Printed" my vocals very well. I'm very pleased with this mic.

A smart purchase indeed.