Isolating Directional Microphones From Wind and Mechanical Noise Interference
an article by Leslie Drever
This article concerns itself with examining the causes and cures of wind, mic boom and camcorder generated noise as it affects the production directional condenser microphone.
Fabric Mesh Windscreens
Wind Noise: You know it when you hear it... your audience does too. They also know that the audio is unprofessional. Wind noise is typically dealt with by enclosing the microphone in a
windscreen, as just the slightest velocity of air movement will badly disturb the directional production microphone. The screen must act as a physical wind barrier, yet be of sufficient porosity to allow the sound pressure wave to transfer on through the particular material, without
degrading its frequencies, or bouncing around inside. If the material is too dense, the light high frequencies will be overly attenuated. If the material is too limp, the low frequencies will attenuate. As we'll see further on, a regulated amount of high and low end attenuation can be an
It used to be that silk was the standard windscreen material, though modern, advanced multi-dissimilar layer fabrics that are of greater thickness and greater porosity have relegated silk and similar synthetics to pop filter work, where air velocity is low and distance to
the microphone can be greater.
Sound disturbance of the microphone from wind interacting with the windscreen comes about in two distinct ways and windscreen design must address each as a whole.
- If the windscreen material porosity is too open, though allowing good acoustics, the sound track is overly degraded by the diaphragm disturbance.
- If the windscreen surface is uneven, such as with raised grid netting, noise (whistle or sing) will transfer through the air to the microphone diaphragm. An amount can travel through the physical screen and suspension mount to the diaphragm.
The ideal windscreen would therefore possess the following qualities: it would have a smooth, soft and highly contoured surface, have a sufficient porosity of sufficient depth to slow wind velocity to a crawl so that the microphone is physically surrounded with nonmoving "dead"
air. The screen should be neither too limp nor too rigid. As a lightweight shell or cage, the screen ideally should approach the more rigid factor to help the weaker high frequencies punch through, though must not to go too far, otherwise the response spectrum balance may become unbalanced and there is a danger of causing filibration (sympathetic resonance) harmonics in the screen itself. The windscreen as a shell should exhibit an acoustical neutrality or transparency, with sufficient wind resistance to cause the air to seek a lesser resistance, which is by compressing and passing around the body of the screen. Air that enters the screen under compression will displace the same volume, therefore as a two-way valve the acoustical material may allow an amount of variable velocity air to pass freely. The acoustical material, therefore, needs to be as open as possible, to slow the air to no morethan about one or two m.p.h. The inside of the acoustical fabric should be soft and absorbent. In the enclosed space, the ideal inner fabric surface will absorb and dissipate reverberation echo or sound bounce like the interior walls of an acoustical sound chamber. Hard plastic frame construction should be minimal in the frontal areas as this is the frontal lobe of the incoming sound pattern.
Although a screen's mating suspension or shock mount is designed to hold and isolate the microphone from unwanted incoming mechanical noise, the energies that travel to the isolator will spread out through the entire mass of the windscreen - another reason for the screen to not filibrate. Should this happen, it can often be negated by dampening with the addition of a screen cover, though the screen rigidity should stay safely back from this parameter. A well-designed screen will, however, "live" close to this edge for the higher performance it delivers.
The physical coupling of screen sections or screen cap and body should be secure with no ability to rattle, 'squeak or creak'. The microphone mount must couple to the windscreen and likewise be absolutely secure. The incoming mic cable should also be secure and not have an air gap. The best quick test of any screen/mount system is to place the base of the mount flat against the ear while "tweaking" the unit. One should hear nothing. The slightest sound will be magnified enormously by the microphone.
Just as one puts on more clothing to protect oneself from ever greater cold, so too does it become necessary to build up layers of acoustical fabric to protect the microphone diaphragm from increased wind velocity. Being able to have a minimal or greater level of fabric protection,
enables the microphone to always have the maximum sound transparency available with the minimum amount of added layers. In high velocity wind conditions, desirably, the wind ought to be able to howl in its given environment without any diaphragm breakup. Typically, lightweight acoustical knits, weaves or combinations of these, with good non wrinkled fit make up the first barrier of added defense, while in extreme wind conditions, heavyweight fur of the softest possible surface
is used. Good fabrics will not degrade performance in wet conditions through a change in outer softness or a swelling of fibers. Low absorption synthetics preferably, or treated natural material give the best results in this regard. In multi layer acoustical material design, a thin central foam layer will act as an excellent drain for heavy water saturation. This will keep the microphone dry.
Camcorder Microphone Windscreens - some myths and truths.
A high quality slip-on windscreen will allow surprisingly high quality sound transmission, yet give excellent protection to wind noise. Typically, a foam windscreen is near useless outdoors and will allow diaphragm breakup with just several m.p.h. of wind. Rub your finger
across the surface of the foam - it has a surface that wind loves to kick up noise on. Then what does that noise do? Why, it radiates directly through its own material down to the mic of course. It's not the foam substance that is soft, per se, but the air chambers - this also allows the wind to rush through, kicking up a fuss at each and every ragged twist and turn. Fur-covered foam is not much better - some protective dead still air volume remains supreme - this is why placing a foam sleeve over a mic before inserting it into a hollow fabric mesh screen is a mistake, and a hefty rise in noise is what you get. One presumes that all the ads showing high quality picture and sound camcorders are only for use indoors! Millions of professional through consumer level camcorder operators are handicapped by belief in the 'character who really isn't wearing any clothes!' Foam is OK for dynamic microphones, which are not prone to wind noise, but not for condenser mics - unless they're on a podium inside.
The best slip on windscreens should approximate a quality full sized type as much as possible - a dead air space inside, with a soft, smooth interior wall lining. It should be soft and/or smooth and unresponsive to wind contact on the outside, with a minimum mechanical sound
transmissibility. A good slip-on screen will be within about 5 dB's of wind noise reduction from its full sized counterpart and still be able to address high levels of wind.
The smaller size reduces the distance from the surface to the microphone. Therefore, the surface barrier needs to compensate by being more dense for a given wind speed. This causes a greater roll-off in the high and low frequencies. If the screen is well designed, the high and
low end will be harmonically balanced and a lopsided audio spectrum will not result. This is true of the full sized screen too, though not as delicate a balancing act. The roll-offs should be smooth in their descent for the sound to remain natural. Well-designed attenuation in the low end tends to counteract the boominess that is peculiar to the directional microphone. High end attenuation, though not desirable when listening to the piccolos, will help in reducing sibilance and scratch.
Finally, a wide, even midrange with minimal variation will give a luxuriant presence to dialogue and effects. Place clear, delineated sound on a track that is firm and free of mud (low through medium level noise) and you will have captured the seeming elusive. We see with the
mind, but we hear with the heart. Clear sound is a joy.
Isolation Suspension Mounts
Suspension mounts, otherwise known as shock-mounts both support the microphone and windscreen plus provide isolation from the microphone boom, camcorder or other manner of support. A quality mount will be physically adaptable, but most important, be able to prevent handling noise, cable rattle, etc. from reaching the microphone. A good mount/screen combination will radiate unwanted energy out from the entire structure, and transpose frequencies from about 15 Hz on up, into
frequencies below 15 Hz, where the microphone cannot 'hear' them.
This transposing process can be likened to the suspension of a road vehicle where the disturbing bumps and jolts are turned into a long soft floating rise and fall. The deeper the give, the smoother and quieter the ride. Typically, however, it is not shock that is the enemy. What is shock but the reaction to sudden dynamic intrusion - such as banging the back end of a mic boom against the wall behind you! Well, so much for that take. Shock is rare. Vibration from handling rub and cable rub is the usual culprit. No amount of floating spring will block the spreading vibrations. The noise waves will travel through the material itself!
In olden times when ribbon mics were the norm, springs were necessary - to protect the sensitive ribbon from physical damage. This is not so with the modern, taught, Mylar diaphragms. As the unwanted sound energy is radiating in all directions on its way to dissipating into the air, it is following, (like all things in nature), the line of least resistance. The steel spring that was so good at protecting the ribbon from inertia displacing damage, never-the-less, carried vibrations happily up around the steel coils - like sound traveling along the railroad line.
Now, many modern isolator materials are 'noisy' for the same reason. Although their springiness 'protects' mostly nonexistent shock, vibration is passing right through the isolator material itself! Try this. Press the tip of your finger against your ear. Now rub your finger with the other hand. You are now simulating mic boom 'handling' noise. Now tap your finger. This causes a large wave of noise and this too easily passes through your finger to your ear. Try the same thing with a high (or low) gain microphone and you may jump out of your headset. Of course if you like to drum a tune on your mic boom while recording, then you need a shock mount.
However if you want a dead quiet foundation upon which to lay your recorded sound, you'll require an isolation material that simply is unresponsive, making the unwanted energy concentrate its expanding radiation into the air. Therefore, the finest isolator materials possess an inherent ability to resist transference of energy through their composition. Energy will bog down as it travels and seek an easier route. Good isolation material works on a molecular level
whereby solids 'sink' into surrounding 'mush'. This is not so for rubber bands or any rubber, nor silicone, nor butyrate, nor viton (O-rings); such materials are merely slightly improved steel springs and much too dense.
To stop vibrational energy requires having millions, or billions of tiny springs all of which are dampened like a fluid filled ball and socket. Shock will be taken care of too, as a strong soft material will have the required displacement. The physical strength of the material is
determined by the cohesiveness factor inherent. Ultra soft, though strong synthetic compounds that are engineered for the above principal requirements are what is to be sought in a high quality isolator for the modern microphone of the real world. Very few microphone isolation
suspension systems available today work reasonably well - and the degree to which they do is mostly by accident and guesswork.
The microphone should be free to move in as many directions as the design will allow with equal increasing resistance. Distance of movement should be minimal to reduce echo effects inside a screen. Obviously, stereo microphones should not rotate around their longitudinal axis, but
otherwise move as freely. Gradually stepped resistance of movement is desired as this makes the unwanted energy progressively exhaust itself in a non abrupt manner.
Camcorder Noise - Lens Mechanism Noise
Camcorder microphones present an interesting challenge for noise isolation. Even the top professional camcorders produce unwanted frequencies. They travel through the air and through the solid mechanisms. The worst offenders in order are:
- Zoom motor noise: Bearing whine, rumble or drone and tapping gears.
- Focus and iris motor noise: Similar to zoom motor noise, though not as pervasive.
- Tape transport motor and mechanism noise.
- Handling noise.
From the perspective of the microphone, noise is noise. The microphone in a clamp holder is going to listen to all of it and you are too! What to do about it? Find a good suspension mount and use a good microphone. Many of the camcorder supplied microphones are marginal in their ability to ignore physical disturbances. You can often attenuate the low end in the microphone or camcorder; this may help, but it is an artificial way to go and often only gives marginal relief Electronic attenuation is only good for lower frequencies and misses the higher noises. Also, by
attenuating, you effectively cripple your rich sound foundation of lower response.
Fix it in post you might say? You know that an ounce of prevention is worth a pound of cure -and like doctors, you're only doctoring the symptoms, not curing or healing the sound. Prevention is the only real way to go. A little equalization in post should be all that is occasionally required, though the best audio people never need it. It is better to equalize at the source if necessary. For handling noise - a good isolator will greatly reduce or eliminate vibrations and handling noise.
With camcorders, there is another, less obvious, though equally important means by which noise will manage to reach the microphone. Lens noise; again, mostly zoom, travels through the air to the microphone. Note that when a screen is in place, (increasing the microphone surface), the microphone is gathering a greater square area of sound noise. The proximity of the microphone to the lens needs to be looked at. If the microphone has high gain, it will hear more of the lens
noise. Operate the lens while moving the mic around to find the best place it wants to be in. Directional microphones have a large lobe of receptivity to the front and highly reduced receptivity to the side. The side receptivity falls off rapidly as one moves into the rear quadrant area. This can be checked by snapping the fingers in a circle around the length of the mic. Note where the weak side area is and try to position the mic so that this area is toward the zoom motor area of the lens. Even the MS stereo mics will show you sweet spots to utilize to advantage. Do the above test with a screen in place.
Depending on these parameters, sometimes it is necessary or desirable to affix the microphone further from the lens - often they are closer than they should be and as sound intensity dissipates rapidly by the square of the distance, sometimes an inch or so can make a real difference.
Noise and precision mechanics: Sometime further on, it will be time to purchase a new camcorder. While tape is running, check the noise levels by placing the camcorder solidly against your ear. Operate the lens controls. Do this with any lens that you may consider purchasing. Not
only will you instantly know the noisy equipment from the less noisy equipment, you'll also realize the quality of mechanical precision, or lack thereof, in the item.
"The heart invented the ear that we may learn to be wise."
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