A shotgun mic is designed to ignore any sound arriving at the sides. This is how it focusses totally on the sound arriving from the front. It works great…

…unless side and front sounds are the same!

Read on to find out why…

Outdoor environments

are wide-open spaces. That seems so obvious right? But bear with me. Usually there are no reflective surfaces nearby. Nothing for the sound to bounce off. The main problem for sound outdoors is the ambient noise. On the other hand, indoor spaces are enclosed. You are surrounded by reflective surfaces such as walls, floors and ceilings. For us it is cosy, warm and sheltered :). But it can be a big problem for sound.

Let’s think about sound outdoors first.

You get in close with the mic using the boom arm. Great, we have nice clear dialogue. Now we will break the sound down into its component parts. The sound arriving at the sides of the mic (ambient) is completely different to the sound arriving at the front (dialogue). Put very simply, the shotgun mic has slots along the sides to break up the sound. Sound coming right in at the front goes straight down the tube. So it ends up being louder than any sound at the sides (known as”off-axis” sound). Incidentally, the longer the tube is, the better this works!

Now let’s think about indoors.

We have reflected sound made of the dialogue bouncing off all those surfaces. But indoors the sound arriving at the front of the mic is also dialogue! So it ends up canceling with (a delayed version of) itself. This creates what is known technically as phasing. What is that? It’s that metallic, harsh, whispery, un-natural quality that makes dialogue hard to take.

The solution

is to use a cardioid (or hyper or super cardioid) indoors. It will sound much more natural and engaging for your audience compared to a shotgun mic.

For more detailed information

and an explanation of exactly what’s happening with the sound waves, read these:
Rode Microphones Blog
How shotgun mics work- Hugh Robjohns, Sound On Sound Technical Editor