DIY microphone blimp and dead cat

January 5, 2015

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.

Naked and partly assembled blimp with my Sennheiser ME66 microphone inside.

Naked and partly assembled blimp with my Sennheiser ME66 microphone inside.

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.

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Hole guide. The large holes are aligned with a row of holes on the PVC tube. The smaller holes guide pilot holes for the next row.

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.

Blimp with fur covering. The forward half is slipped over the blimp and the rear half has a zipper.

Blimp with fur covering. The forward half is slipped over the blimp and the rear half has a zipper.

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!

Model Length (cm) Diam. (mm) Mass (g) Cost Remarks
My dead cat 44 83 370 AUD 20
Rode blimp 49 125 550 AUD 225 Mass excludes fur.
Rycote Super-Shield 40 100 694 USD 359 Mass includes cable.
Sennheiser SEBS2 USD 926 No detailed specs.
Proaim BMP40R 40 USD 132 No detailed specs.
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.

Blimp with cloth covering. It is secured with velcro (hook-and-loop) after stretching the material tight. This is a prototype and the sewing is a mess! An easier solution is to wrap the cloth around the blimp and secure it with rubber bands.

Blimp with cloth covering. It is secured with velcro (hook-and-loop) after stretching the material tight. This is a prototype and the sewing is a mess! An easier solution is to wrap the cloth around the blimp and secure it with rubber bands.

Blimp testing

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:

  1. Foam windscreen and microphone in shock mount. Q3 frequency = -258 Hz relative to naked mic.
  2. Cloth-covered blimp. Q3 frequency = -345 Hz.
  3. Fur-covered blimp (dead cat). Q3 frequency = -904 Hz.
  4. Fur over foam windscreen and microphone in shock mount. Q3 frequency = -1637 Hz.
Frequency response for different wind protection setups and dry newspaper sheets source. The spectra are aligned at 1–2 kHz to account for different signal levels. This comparison shows that fur coverings more strongly absorb higher frequencies (up to -1637 Hz reduced bandwidth). Measurements were made using Raven Pro 1.3 from 16 bit, 44.1 kHz recordings. The graph is zoomed to 0–11 kHz.

Frequency responses for different wind protection setups and dry newspaper sheets source. The spectra are aligned at 1–2 kHz to account for different signal levels. This comparison shows that fur coverings more strongly absorb higher frequencies. Measurements were made using Raven Pro 1.3 from 16 bit, 44.1 kHz recordings. The graph is zoomed to 0–11 kHz.

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:

  1. Foam windscreen and microphone in shock mount = -1 dB relative to naked mic.
  2. Cloth-covered blimp = -2 dB.
  3. Fur-covered blimp (dead cat) = -3 dB.
  4. Fur over foam windscreen and microphone in shock mount = -4 dB.
Signal levels for white noise source and different wind protection setups. The fur over foam windscreen gave a substantially weaker signal. Measurements were made using Raven Pro 1.3 from 16 bit, 44.1 kHz recordings.

Signal levels for white noise source and different wind protection setups. The fur over foam windscreen gave a substantially weaker signal. Measurements were made using Raven Pro 1.3 from 16 bit, 44.1 kHz recordings.

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:

  1. Fur-covered blimp (dead cat) = -25 dB relative to naked mic.
  2. Cloth-covered blimp = -20 dB.
  3. Fur over foam windscreen and microphone in shock mount = -20 dB.
  4. Foam windscreen and microphone in shock mount = -6 dB.
Wind noise waveforms for fan source and different wind protection set-ups. This comparison  shows very poor wind protection from the foam windshield. Waveforms from Audactiy 2.0.6.

Wind noise waveforms for fan source and different wind protection set-ups. This comparison shows very poor wind protection from this foam windshield. Waveforms from Audactiy 2.0.6.

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:

  1. Fur-covered blimp (dead cat).
  2. Cloth-covered blimp.
  3. Fur over foam windscreen and microphone in shock mount.
  4. Foam windscreen and microphone in shock mount.
Signal-to-noise ratios for different wind protection set set-ups and the experimental conditions in the preceding two graphs. The fur over foam system performed surprisingly well, however the Q3 frequency was substantially lower than for the cloth-covered blimp (see the first graph).

Signal-to-noise ratios for different wind protection set set-ups and the conditions in the preceding two graphs. The fur over foam system performed surprisingly well, however the Q3 frequency was substantially lower than for the cloth-covered blimp (see the frequency response spectra).

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.


A better microphone shock mount

April 29, 2011

Last year I drowned my Audiotechnica AT835b mic, ruined it. Then I upgraded to a Sennheiser ME67 long shotgun. The ME67 is a beast and required a better shock mount. Here is my new design. Now I can record low frequencies with my ME67 at maximum gain.

A better microphone shock mount for my Sennheiser ME67. I haven't needed to make a pistol grip yet.

Diagnosing the problem

The Sennheiser ME67 is a very sensitive mic (50 mV/Pa). It is long and doesn’t balance well in a pistol grip. My old home-made shock mount was not good enough. I experienced extreme low frequency handling noise with the bass roll-off switch off and high to maximum gain (Sony PCM-M10 recorder).

At first, the source of the noise was not clear since my old shock mount had performed wonderfully with the AT835b. People on the naturerecordists group suggested wind noise, structural noise (thin mic chassis), loose components, cable noise, wrong cable or mounting problems. I suspended my mic with elastic bands to find out the truth. There was no noise in my mic or cable and I concluded that handling noise was the mic slipping in my home-made shock mount.

Ultimate microphone suspension. Testing my ME67 for noise. The middle elastic band is for shaking the mic without bumping it.

Building a better shock mount

The ME67 needed a tighter grip to stop it slipping. I wrapped elastic bands firmly around the mic and used a larger diameter PVC pipe.

A better microphone shock mount.

A large diameter pipe avoids the mic bottoming out (banging the pipe wall) and permits finger access to pull on the elastic bands and also to operate the on/off switch. Four thick elastic bands are required: two each at the front and rear. To secure elastic bands around the mic body repeatedly loop, twist 180 degrees, loop, twist 180 degrees, until there’s no slack. Then pull on opposite sides of each band, and secure the ends to the pipe (I’ve cut tabs in the pipe).


DIY wind protection for the Sony PCM-M10 recorder

April 25, 2011

Wind noise is frequently an issue when recording outdoors. Furry windshields for the Sony PCM-M10 recorder cost between $22 and $80 on the internet. No thanks, I decided to make my own. This idea can be applied for different handheld recorders – be imaginative.

Windshield mounted on my Sony PCM-M10. The green and red peak level indicating LEDs (-12 dB/OVER) are covered. This shouldn’t be an issue because you can monitor recording levels on the LCD display.

Overview

What I’ve simply done is to cut a piece of fur to wrap around the top of the recorder and sewn it up like a sock (more details at the bottom of this post). If you’re stuck in the field without any kind of windshield, a thick woolen sock would be better than nothing!

Here are some recordings with my homemade windshield. WordPress won’t let me upload mp3s, you can listen to them on Stephen’s Archive.

Windy recording with naked Sony PCM-M10, Manual Gain=5. This was about the maximum gain without clipping. The mic was fixed on a tripod and all recordings here have not been filtered or edited.

Windy recording with Sony PCM-M10 and homemade windshield, Gain = 5. The wind noise has been reduced by about 19 dB.

Windy recording with naked Sony PCM-M10, Manual Gain=10. Recording was impossible at maximum gain without a windshield.

Windy recording with Sony PCM-M10 and homemade windshield, Gain = 10. The wind noise has been reduced but is still apparent with high gain.

Making it

To make this you need faux fur, elastic, a sewing machine and a sewing machine operator (my mum sews for me). I think that faux fur can be bought from craft shops.

Homemade windshield turned inside out. The vertical seam should be positioned roughly symmetrically between the left and right mics.

Windshield performance may be better with longer fur. I just used what I could scavenge. Another improvement would be to sew a tab at the base of the “sock” to help pull it over the recorder.

Here’s a sketch of how I constructed the wind shield. Exact measurements are not required. I used “guesstimation” to get a good fit. You don’t need a close-fit over the top. A dead-air cavity should not harm your recordings (zeppelins use dead-air cavities).

DIY windshield instructions.


Essential microphone pistol grip and shock mount project

January 14, 2011

I’m a keen nature recordist and use a shotgun microphone. I have done a lot of recording just holding the mic in my hand but one needs to be careful of handling noise and the bass roll-off switch should be on. With a shock mount, handling noise is much reduced and lower frequencies can be recorded. A pistol-style grip helps for aiming. This article describes a quick and easy pistol-grip and shock mount for a shotgun mic. It has improved my recordings.

Update April 2011: I built a better shock mount for my Sennheiser ME67 mic.

My home-made microphone grip and shock mount.

This mic grip takes about 30 minutes to make and should cost next to nothing. Here is a materials list:

  • Thin-walled PVC pipe. Hardware stores may not sell short lengths. Our local rubbish tip sells odd pieces.
  • Thick rubber bands.
  • Timber for the handle. I found a broken pool cue.
  • One wood screw.
  • Scrap aluminium for a washer.
  • RTV Silicon. All good projects use RTV!

To prevent sagging, cut the pipe long and try to balance the mic. The wind shield should not touch the pipe. Leave access at the back for connecting the mic cable. Next, cut slots for the rubber bands. I cut tabs, broke them off and filed the slots smooth. Position the handle so that the mic is balanced. To mount it, drill holes through both sides of the pipe (the top one is for screwdriver access). The timber handle is filed to match the circumference of the pipe. I fashioned a large, concave washer from aluminium for the inside, to reduce stress around the screw head. I spotted RTV silicon between the handle and pipe before final assembly, so it all fits together solidly.

Detail of microphone grip.

This shock mount works great and is well balanced. My mic is a little front heavy and if the rubber bands are slack, I shift them up a notch (see top picture). I always record with the bass roll-off switch off now. There’s no handling noise, unless I bump the mic cable (!). Search the web and you’ll find similar projects for shock mounts, wind screens, etc. Happy constructing.