I've noticed that several have said Shotgun mics should preferably not be used inside (best outside), rather, that a hyper-cardioid should be used inside instead.

1. I thought a shotgun WAS a hyper-cardioid.
2. What is the issue with shotguns inside?

This is more of a guideline than a rule. It's *almost* guaranteed that you'll wish for a hypercardoid if you try to shotgun from above in a typical home.

You're right - a shotgun is a hypercardoid with a longer interference tube. For that matter, a hyper is a cardoid with a longer... An omni has no interference tube.

The issue is that while the longer interference tube reduces off-axis sound, it doesn't do so equally across frequencies, and, worse, develops a lobe of sensitivity at the back which responds very non-linearly.

Use it near ceilings or walls, get a reflection of non-linear response. This can be heard very readily if you know what you're listening for. If you don't know what's going on, you may notice that the sound is more "hollow" than expected.

In summary - all mics are going to catch some room sound. Smaller and live-er the room, more room sound. The longer the interference tube (generally), the more colored the reflected sound will be.

Close study of the polar patterns of your favorite mics will help to tell the story. Below, the polar pattern of the Rode NTG2, a medium/short shotgun. Notice that the off-axis response is different at different frequencies.

This is definitely a case where your mileage varies. Every room is different for reflectivity, mikes are slightly different, since the manufacturers are always working to reduce these effects.

What's the rule of thumb when not to use shotgun for inside?

Carl,

I'd say you generally don't want to use a shotgun inside where there are lots of reflective surfaces. So in a room that has hard wood floors, or a kitchen with tile or linoleum, for instance. Anyplace where you get a very "echo-y" sound. A hyper will be better in those situations. Now if there's too much echo, a hyper isn't going to be great all by itself either. You may have to lay down sound blankets, etc. to get rid of some of the echo. In a carpeted living room or bedroom, though, I think a shotgun like the Sennheiser 416 can work quite well.

Have fun!

Rob

What's the rule of thumb when not to use shotgun for inside?

Pretty much any space that has not been acoustically treated such as a soundstage might be is going to be too reflective for a shotgun. For that matter, 'guns can encounter the same problem outdoors, on the tarmack at an airport or on a paved tennis court for example. The problem comes about due to the way sound entering the front of the mic interacts inside the tube with its reflections coming in through the side ports of the interference tube. Other types of directional mics like a hypercardioid do not use the interference principle and so don't have the same drawbacks.

Standing next to stone walls can do it too. Especially if your in a corner with two walls and the stone pavement.

Also, keep in mind that some shotguns are actually bi-directional and have poor rejection from the rear.

This issue can get murky. At SOME distances in SOME interiors, the shotgun will win, in others the hypercardiod. If you've got a very bright space and you can't get close enough it's more of a question of which will sound worse, as they're both going to sound like crap. Some people believe that shotguns actually will attenuate echo better than hypercardiods in very large reverberant rooms, but I'm not committed to that.

Yep, it's all a continuum. The really nasty effects occur when the reflected sound arrives so close to the direct sound that you get comb filtering effects inside the tube. As the time lag of the reflection gets longer it transitions into reverb and then again into echo and the effect changes for each. It's a very arcane art.

Why not go through all of the nice sound samples I recorded both indoors and outdoors with hypercardioids and shotguns and decide for yourself? Much more accurate than discussing it As I Hear It - Choosing the Right Microphone (http://www.kenstone.net/fcp_homepage/right_mic_brockett.html)

Dan

Also, keep in mind that some shotguns are actually bi-directional and have poor rejection from the rear.

All shotguns pick up from the rear - it's the law of physics.

A gun mic. is normally a super-cardioid capsule wirh an interference tube that makes it more directional at higher frequencies.

The rear lobe is normally about 10dB down in level from the front at low frequencies. The published polar-patterns should show you what;s happening.