Microphone engineering from Michael Joly

LDC Mic Head Baskets - A Necessary Evil

$0.00

The necessary and primary job of the head basket is to shield the sensitive, high impedance condenser capsule from electromagnetic interference.

Sources of interference can be AC line hum, AC power transformers or higher frequency EMI from light dimmers or radio transmissions.

A balance between effective EMI shielding and acoustical transparency must be struck. Total shielding of the capsule (by a solid metal cage for example) would effectively block EMI interference but would also prevent sound waves from striking the diaphragm. On the other hand, an unshielded capsule would be very open acoustically, but the mic would not function due to the "swamping" effect of AC line frequency hum which would be louder than the acoustic signal being recorded.

Neumann U 47
Three layer head basket design

Question: What's a Head Basket Evil?
Answer: Audible artifacts.

A wire mesh cage surrounding a condenser capsule is actually a small chamber with a complex acoustic signature - the wire mesh surfaces create acoustical reflections and standing waves within the basket. The more layers of wire mesh (or more strands of wire per millimeter) the more the head basket resembles a small reflective room. Now recording in the WC, toilet or bathroom (your choice) may be fine sometime, for an effect. But we don't want all our microphones to have reflection coloration all of the time.

Here's an experiment - cup your hands around your mouth in a megaphone shape, say something, take your hands away, say something. What you hear is the different level of early refections that color the sound of your voice. The same principle is at work in a head basket. In short, more layers of wire mesh equal more internal head basket reflections - and thus more coloration of the source sound.

Classic Mic Coloration Artifacts -

During the history of the condenser microphone various head basket designs have been used. These head basket designs contribute to the individuality of each mic's "sound".

Neumann Head Basket Examples -

The Neumann U 47 is one of the most famous, and revered, mics in the world - and part of the sound of this mic can be attributed to the early reflections created within its cylindrical / domed top / three layer wire mesh basket.

The effect of the U 47's head basket reflections can be heard as both time and frequency domain artifacts. In the time domain, reflected sound is heard as a lengthening and smearing of transient response. Sharp impulse sounds acquire a thickening - source transients are joined by refected transients arriving at the diaphragm some time later that the initial, direct transients. In the frequency domain, the summation of first arrival and early reflections creates very narrow band peaks and dips in the U 47's frequency response curve.

Call it "boom and honk" or whatever you like, but the U 47 head basket has a sound. Neumann tacitly acknowledged this fact - their subsequently introduced M 49 microphone had a much more open wire mesh weave and newly introduced angled shape basket.

Both of these features serve to reduce standing wave coloration in the M 49 relative to the U 47. Because of the M 49's relatively neutral signature compared to the U 47 it was, and is, a favorite of female vocalists (like Barbara Streisand for example) who love the warmth and presence of the Neumann voicing but want to avoid the "boom and honk" of the U 47 (which is much admired by male vocalists on the other hand).

Drawing on their success with the M 49, Neumann continued to use an angled head basket design in the classic U 67 and U 87 mics to reduce parallel wall standing wave coloration and achieve a more neutral head basket sound relative to the U 47.

The AKG Difference -

Let's consider another mic for a moment - the classic AKG C12, another of the worlds' most revered mics. Unlike Neumann with their multiple layer head baskets, the AKG C12 used a very open, cylindrical, single layer wire mesh basket - and delivers a more open and neutral sound as a result.

AKG C12
Single layer head basket design

Compared to Neumann's head baskets, the AKG C12 single layer wire mesh basket offers far greater open area percentage and thus much less internal reflection coloration. Btw - the Manley Reference mics also employ single layer mesh baskets to reduce internal head basket coloration. The use of single layer head baskets in the AKG C12 and Manley Reverence mics contribute to clear-and-open reputation these mic have.

Single Layer Head Baskets and Capsule EMI Shielding -

As I noted above, the necessary and primary job of the head basket is to shield the capsule from EMI interference. And yes, three layers of wire mesh are a more effective EMI shield than one or two layers of wire mesh. But in actual practice, a single layer head basket with proper mesh sizing provide more than adequate EMI shielding of the capsule.

As evidence of this, both AKG in their C12 mic and Manley in their Reference mics determined the sonic advantages (less coloration, more accurate transient response) of a single layer head basket outweigh the potential for AC / EMI pickup.

In fact, one could argue it is the recordist's job to locate the mic where there is a minimum of hum pick up in the studio (and this includes rotating both moving coil dynamic mics and ribbon mics to determine the best hum null orientation).

Of course, if a single layer head basket mic is operated in close proximity to strong EMI fields such as those emanating from AC power lines, AC power transformers, light dimmers etc. hum and noise may be heard. The solution is simple - operate the mic further away from offending EMI sources. The clear and open sound heard with a single layer basket outweighs the minor inconvenience of choosing the mic position to minimize the potential of EMI pick up.

Head Basket as Pop Filter -

Unlike the early days of pop music recording, external pop filters are now ubiquitous. Today we don't have to rely on the head basket mesh to provide protection from breath plosives. A pop filter used in front of a single layer head basket mic provides excellent plosive filtering and imparts minimal reflection coloration. The pop filter does not surround the mic and does not create the multiple back and forth reflections that are found within a multiple layer head basket.

The Michael Joly Engineering Approach -

MJE mics are known for their clarity, detail, reach, resolution and open / natural sound capture - all while providing a mid-focused, "Neumann-esque" presence. How is this achieved? The answer - a combination of head basket and capsule design.

Michael Joly Engineering v250 and Hulk 990 MJE-v250 and Hulk 990 LDC Mics Single Layer Head Basket / K47 Capsule Type Designs

During my training with David Blackmer (dbx Inc. and Earthworks Microphones founder) I was taught to listen for, and become exceptionally sensitive to the coloration effects of early arrival time reflections in the vicinity of transducers (both loudspeakers and microphones).

My subsequent design goals have been to minimize, to the greatest extent possible, coloration-causing reflections. This means using single layer mesh (with a high percentage of open area) formed into angled and / or angled-curve shapes to distribute the remaining internal reflections and avoid, as much as possible, the coloration effects of standing waves. One could say I'm an angled, single-layer basket case ;)

When you listen to an MJE large diaphragm condenser mic you'll hear great clarity, reach, resolution and openness due to my signature single layer / angled head baskets (my electronics topologies and parts choices contribute as well, but these are the subject of another email). And inside the head basket, a classic mid-focused presence is delivered by the iconic K47 capsule design (MJE-K47 capsules feature either 6 or 3 micron diaphragms for either a traditional or more modern sound, more on my capsules in a future email).

And here's something you may not have known ...

... when you listen to MJE large diaphragm condenser mics you're listening to the only angled / single layer head basket / K47 capsule mics available - anywhere.

That's an MJE difference - hear it for yourself.

Best, Michael

Ps - You're welcome to quote me exactly (with quotation marks) or forward this post. But please don't paraphrase as this can lead to misunderstandings ;)