High sensitivity microphones in the field need protection from both shocks and wind. After building my own shock mounts, I wanted better wind protection. A blimp and dead cat delivers good signal levels, fidelity and wind resistance.
Building a blimp
A microphone blimp (or zeppelin) looks like an airship. When covered with fur, it looks like a dead cat. The blimp creates a volume of still air around the microphone. Fur provides a better wind barrier than cloth because the fibres move and absorb wind energy.
I made my blimp from 83 mm diameter PVC plastic pipe and push-on PVC vented caps. These can be purchased from hardware and plumbing stores. Larger diameters and dead air space will better attenuate wind noise but larger, heavier blimps are increasingly difficult to handle. I have also used 55 mm diameter PVC for a smaller microphone, but installation of the mic into that narrow tube was more difficult.
I cut the PVC to length and drilled it nearly full of holes with a 16 mm diameter hole saw. Round hole saws are necessary, because large drill bits make a mess in thin plastic. I made a hole guide out of a thin steel sheet.
I used another hole saw to drill out the ends of the caps. I also sawed off the base of the caps, leaving about 10 mm to grip the tube. Cutting away more plastic makes the blimp lighter and the tube acoustically transparent. If you put the “unholy” tube to your ear, it sounds like a seashell. As more holes are drilled you will no longer hear any effect.
I left some plastic at the bottom of the tube for the grip. My pistol grip is secured with a wing nut (the bolt is threaded into the tube). I contact-glued some rubber (recycled bicyle inner tube) to the base to prevent slipping and squeaking. See my earlier shock mount post for a simpler, fixed pistol grip handle.
The microphone suspension is made from rubber bands. The bands pass through the tube and are secured with “H” pieces at both ends. I used a permanent marker to mark the tube where the rubber bands pass, for whenever they have to be replaced. The “H” pieces were cut from PVC tube. I contact-glued some rubber (recycled bicycle inner tube) to the inside of the “H” pieces to prevent slipping and squeaking. The microphone is inserted through the gaps between each rubber band, then the “H” pieces are lifted and rotated to twist the rubber bands and secure the microphone.
The microphone cable passes outside the rear cap. It is secured with two rubber bands to stop transmission of shocks through the cable. I have also slotted the pistol grip handle, so that the cable can be tucked away. Underneath the rear of the tube I have made a large hole where I can access the switch on my microphone.
The dead cat skin is made from fake fur that I purchased in a fabric shop. It has a single pile and a stretchy, open-weave backing material. Beware of denser furs, that can strongly attenuate the acoustic signal. It is easy to hand sew the dead cat by turning the fur inside out. Use a thread that contrasts with the fur. The fur will hide bad sewing.
My home-made blimp and dead cat is lightweight and cost about AUD$30 in materials. Commercial dead cats are heavier and often cost more than the microphone!
|A comparison of blimps and dead cats that will fit a Sennheiser ME66 microphone. My home made blimp is the lightest and cheapest. The best price was selected from three sites: ebay, BHphotovideo USA, Video and VideoGuys Australia. Shipping costs are not included.|
The (optional) cloth is made from leftover material from a light diffuser project. The material was light, stretchy and difficult to sew (this was the first time I’ve done machine sewing). Perhaps there are other acoustically transparent cloths that are easier to work with.
For all tests, the naked mic is the reference response. A good wind protection system will have a frequency response and signal level close to the naked mic and much lower wind noise levels.
To compare frequency responses, I created broad spectrum noise by crumpling dry sheets of newspaper. The microphone was indoors (no wind), the distance to the source was fixed at about two metres (I didn’t write down the exact distance) and all recordings were made with a fixed manual gain. I made three recordings for each set-up and compared median spectra and third-quartile (Q3) frequencies. Q3 frequency is the frequency at 75% of the power spectrum.
The first graph compares frequency responses. Wind protection set-ups ordered from widest (best) to narrowest (worst) bandwidth are:
- Foam windscreen and microphone in shock mount. Q3 frequency = -258 Hz relative to naked mic.
- Cloth-covered blimp. Q3 frequency = -345 Hz.
- Fur-covered blimp (dead cat). Q3 frequency = -904 Hz.
- Fur over foam windscreen and microphone in shock mount. Q3 frequency = -1637 Hz.
To compare signal levels I played back white noise through a small speaker. I made the white noise signal using Audacity 2.0.6. The frequency response spectra were rough, but signal levels were more repeatable than for the newspaper sheets. I compared mean levels averaged over about 60 seconds of playback.
The second graph compares signal levels. A difference of -3 dB corresponds to half the power. Wind protection set-ups ordered from strongest to weakest signal levels are:
- Foam windscreen and microphone in shock mount = -1 dB relative to naked mic.
- Cloth-covered blimp = -2 dB.
- Fur-covered blimp (dead cat) = -3 dB.
- Fur over foam windscreen and microphone in shock mount = -4 dB.
To compare wind noise levels, I simulated wind using a pedestal fan. I compared mean levels averaged over about 60 seconds of wind noise.
The third graph compares wind noise levels. Wind protection set-ups ordered from weakest wind noise level (best) to strongest (worst) are:
- Fur-covered blimp (dead cat) = -25 dB relative to naked mic.
- Cloth-covered blimp = -20 dB.
- Fur over foam windscreen and microphone in shock mount = -20 dB.
- Foam windscreen and microphone in shock mount = -6 dB.
The final graph compares signal-to-noise amplitude ratios from the above two tests. Wind protection set-ups ordered from highest (best) to lowest (worst) are:
- Fur-covered blimp (dead cat).
- Cloth-covered blimp.
- Fur over foam windscreen and microphone in shock mount.
- Foam windscreen and microphone in shock mount.
Blimp is best
These tests illustrate the trade-off between signal strength and fidelity versus wind noise reduction. The light foam windshield is best for indoors, but hopeless for wind. The cloth-covered blimp is best for “light breezes”. The fur-covered blimp is best for “strong wind”. For high fidelity recording, I suggest to have all three wind protection systems in the field bag and to select the one appropriate to the conditions.
I always see professionals using the dead cat outdoors. For news recording and like, there is no second chance. For nature recording, we choose to avoid windy conditions and I would recommend lighter wind protection (i.e. cloth-covered blimp) for higher signal levels and wider bandwidth.
I will not use the fur over foam system because the signal level and bandwidth were lower than for other set-ups. Some manufacturers claim that their expensive fur is acoustically transparent. I invite them to send me a sample product (to fit a Sennheiser ME66) so that I can test it myself.
Disadvantages for the blimp/dead cat are the bulkier size and slower microphone installation. Installing the microphone actually only takes a couple of minutes because I leave the rubber bands on the blimp.