Audio Production, DIY, Tech & Gear, Videography

DIY Boom Pole With Acoustic Considerations

16 Nov , 2015   Matt  

Warning: really long post. If you’re in a rush, see the pictures at the end.

I’ve recently started freelancing for local low-to-no budget film groups, including a sketch team organized by the Highwire Comedy Co. I’ve also learned that just having dedicated audio gear can make you “the audio guy” without any formal training. Whatever gets you on set, kids.

When first assigned audio duty, I realized I had no boom poll. An old monopod got me through a shoot, but I almost ruined my back before SIEGE. I needed a better solution, but the sketch team had no funds. Of course, a web search for “DIY Boom Poll” turned up dozens of guides, so that was that.

Wait, did I just tell you to read someone else’s guide? Yes, because I read someone else’s guide. Specifically, the Film Riot guide. Go watch that guide, but then come back! Why? Because I have some additional steps to improve the poll’s acoustics. Here is a basic list of equipment:

Never work without good safety glasses.

Disposable dust masks for PVC particles and glue fumes. Feeling woozy? Go out side and take the mask off for a while (in that order).

Common bolt sizes in audio/video DIY work (and a guide to reading them):

  • 1/4″ – 20: virtually all cameras in the DSLR range use this for mounting purposes.
  • 3/8″ – 16: the European standard bolt for audio equipment.
  • 5/8″ – 27: the American standard for audio equipment (insert size joke here).
  • 3/4″ – 5 Acme thread: the bolt on most broom handles which took forever to identify!

You won’t need to use all four at once, but will probably adapt between two or more depending on the mic and mount you use. The Acme bolt is not a “plumbing thread,” so don’t expect standard PVC adapters to fit it. Affix with glue or line the Acme thread grooves with strips of gripping material.

The Poll Itself:

  • Any telescoping broom handle between 10′ and 15′ will work. Something too long may require structural support. Something too short can’t be held properly during operation.

PVC Components:

  • Bushings (link) of the appropriate sizes to “step up” from your broom handle to the shock mount assembly. More than one may be required.
  • Tee joint (link) for housing your microphone and the shock-absorbing rubber bands. I recommend something between 1 1/2″ and 2 1/2″.

Additional items:

Thoughts on assembling DIY projects:

Power tools are your friends, but are not absolutely essential. You need a thin-bladed saw like a hack saw or coping saw. I also strongly recommend a small drill, even a hand-powered model, along with a heavy vice clamp.

A brief science experiment:

Once you have your materials, find a quiet place and listen to the ambient noise. Cup your hand to your ear in a half-circle. The “feel” of the room probably hasn’t changed, but some sounds are clearer and some are muted.

Now put one end of the PVC Tee joint’s long tube to your ear. You should notice that the sound of the room is echoing and distorted. “So what? I’m using a directional shotgun mic for this. Who cares what happens behind it?”

Actually, most “directional” or “cardioid” mics pick up regions of varying shapes behind and around themselves which the audio engineer and boom operator must be aware of. It is also important to note that the actual microphone in a shotgun is seated at the rear of the “cage”, or long perforated tube we think of as the front piece. This will usually be right around the front-facing ring of your shock mount.

In other words, if you build your boom exactly the way Film-Riot suggests, you’ll have mounted your mic to a rear-facing echo chamber!

Don’t think I’m criticizing Film-Riot. They are excellent film-makers and I did use their guide after all. It didn’t even occur to me until I recorded major distortion from an actress on a fluffy, non-reflective blanket. I went online, looked at a range of “professional” shock mounts, and realized none of them were simple tubes. Most of them were two thick rings connected by as few struts as possible.

As soon as I saw it, I modified the Film-Riot mount as follows:

  1. Cut out and remove the top central portion of your PVC Tee joint down to just under half-way through the pipe, leaving rings at either end between 3/8″ and 5/8″
  2. Cut strips of grip liner to fit around the outside of each PVC ring. Secure with a small piece of tape. Do not glue in place!
  3. If you use bailing wire for hooks, fashion 8 clips accordingly, measuring approximately 1″. Flare the non-hook ends for stability (see image below)
  4. Place your hooks in an X pattern around one ring. Secure the hooks all at once with a zip tie. This may require a friend’s help.
  5. Repeat for the opposite ring.
  6. For added stability, place a second zip tie on each ring (see image below)
  7. Finally, for extra sound dampening, cut a rectangle of grip cloth to fit the interior of your PVC Tee pipe. Test it by placing it inside the pipe, ensuring all edges are flush and that one end of the material does not overlap the other!
  8. Apply PVC adhesive to the entire interior portion of the PVC Tee pipe.
  9. Immediately line the interior carefully with your pre-cut grip liner.
  10. Allow several hours to dry, place rubber bands on your hooks, and enjoy your new acoustically treated shock-mount!

Final thoughts
“Wait! What about all that excess lining material?” I will admit I was originally going to cut that off. Once I had the rest of the shock mount assembled and the glue had dried, a few more acoustic facts occurred to me.

  • Decibels (link), one of the most commonly used measurements of audio signals, do not measure “volume” as pop-culture understands the term. It measures sound pressure. Pressure builds easily in confined spaces, and dissipates in perforated enclosures.
  • Audio waves bounce readily off of rigid, polished surfaces, but very poorly off of flimsy absorbent material.
  • Audio waves are also often combated with acoustic foam (link), a light weight material sometimes arranged in two intersecting wave forms, or a checkered pattern (link).

While the grip material was designed for other purposes, it meets many of the basic criteria for dampening and absorbing sound. Since the checker-patterned variety (linked above) is so thoroughly perforated, the pressure buildup versus a simple open-air section should be negligible. In other words, it should not only allow unwanted echos to easily escape from the shock-mount, but also impede ambient noise trying to get in.

I will say this makes your shock mount significantly more fragile, but I’m quite happy with the results. Thanks for reading, and please let me know if you have thoughts, opinions, or previous experiences with this sort of work by finding my Facebook page or Twitter feed.

Reference Pictures

The initial build with no modifications. Note the alternate rubber band arrangement.

The final modified product. Note the grip liner has not been cut away from the middle of the PVC Tee joint. Notice also that the padding on the rings will combat noise caused by loose metal hooks:

Probably don’t do this, though…

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