Film sound theory 201

When you set up the audio recorder before a shoot, you might choose settings for the sample rate and bit depth. Understanding what you're choosing will help you know when you need to change these basic guideline settings.

Sample Rate Explained

Ever wondered why we normally record at a sample rate of 48kHz in film and what that even means? This video explains what sample rates are.

Q. Can you figure out the sample rate you would need to record ultrasonic sounds?

If you had some specialist microphones and equipment and needed to record the sounds of bats echolocating (the highest frequency of bat echolocation is 200kHz which is well above what humans can hear).

What sample rate would you need to use to record all the frequencies of that sound?

Bit Depth Explained

Ever wondered why we record in 24bit?

Some audio recorders are offering 32 bit recording. 32bit isn't essential for dialogue/voice recording (most professional audio recorders used on feature films are still recording in 24 bits) but it does provide an even higher dynamic range and we all love shiny new tech right? The Mixpre6 can record in 32bit.

Limiters

In Sound Theory 101, we learnt that if you have your gain set too high, the recording will clip/peak and can distort and sound terrible.

But recording sound on set is often unpredictable and actors and subjects can sometimes get a little OVEREXCITED.

There is a backup tool that can save you.

You might have noticed that your audio recorder has a limiter on every channel. Limiters are there to protect against clipping. They 'catch' audio right before it clips and turn the level down slightly for a brief moment of time. Brilliant right?!?

Be warned though. Limiters only go so far. If your audio limiters are being used heavily, it sounds very similar to distortion. You only want to use a limiter to catch a small clip, not a whole word or sentence.

Unfortunately, limiters can't save you from a sudden scream. 😱I really wish they could. But they can't. It's too much for them.

https://edition.cnn.com/2015/07/16/health/scream-scare/index.html

What would you do?

If an actor unexpectedly screamed in a take and the recording distorted, would you be quiet and hope no-one notices in post, or would you ask to redo the take?

It happens to the best sound recordists in the world. There's no shame in redo-ing the take. It's better than having an awkward conversation with post or not being re-hired because you stuffed the audio.

Note: If you were warned the scream was coming and still distorted the take, actors will be annoyed at having to redo the scream because it hurts their voices. It's hard to get the gain right in these situations. Just do your best and be very apologetic and polite. This one is on you...

Microphones and diaphragms

Microphones are important when it comes to sound recording.

But what is a diaphragm, a shotgun, or a cardioid?

"Tympanoplasty" from https://www.aboutkidshealth.ca 2019

Ear Drums

In Sound Theory 101, we learnt that sound is caused when something vibrates. That vibration makes air molecules move, and as that something vibrates back-and-forth, it changes the air pressure, creating a high-pressure, low-pressure, high-pressure, low-pressure wave that travels through the air to your ear drum. Your ear and brain interprets this wave/vibration as sound.

Your ear drum is a thin circle of tissue that vibrates as the air pressure hits it.

"Mic Selection: What mic is right" at https://cras.edu/what-mic-is-right/ 2019

Microphone Diaphragms

A microphone works in a similar way. Microphones have a 'diaphragm' that is kind of like your ear drum. This diaphragm vibrates and the vibrations are converted into electrical signals which travel down your cable to your recorder.

Microphone Pickup Patterns

Ever seen these symbols on your microphones?

Pickup patterns, or polar patterns, refer to what direction the microphone picks up sound from.

"Take a look at each pattern and imagine each circle is the birds-eye view of a round room. The small circles in the centre of each pattern represent the microphone in the room, and the lighter shapes represent the areas the microphone is ‘hearing' sound from."

https://www.thepodcasthost.com/equipment/microphone-polar-patterns/

Three of these types of microphones are in your kit. An omnidirectional, a shotgun and a cardioid.

Omni Directional

Supercardioid Rode NTG3 or Sennheiser 416

Cardioid NT5

If you put your headphones on and point your shotgun mic in different directions, you'll notice how whatever is in front of the mic gets heaps louder. This doesn't happen with the lav mics (lavalier/lav/radio/bug) - they just pick up everything (you can even mount them on someone upside down). The NT5 is somewhere in the middle - not as directional as a shotgun but way more directional than the lav mic.

WHICH MIC WHEN?

It might seem like a shotgun would be perfect for every situation because you can get rid of noise right? And shotguns are fabulous - I use them almost every job. They sound better than radio mics and you don't have to worry about pesky rustling of clothing or radio interference from your local TV station/mobile phone/the wireless monitor/everything.

But different pickup patterns are good for different things.

I once had a job that I thought was going to be a piece of cake - interviews in a studio. I could put a boom mic on a stand, sit down, and have a cup of tea. It's the dream right? Except when I turned up, that studio was a photography studio with zero sound treatment. It was basically an echo chamber. The horror. Oh the horror. We quickly set about building a pillow fort to absorb the echo. But it wasn't enough...

My studio pillow fort

The answer: If this happens to you, build a pillow fort, and then use your radio mic and your NT5 cardioid REALLY close to your talent. Shotgun mics do weird things to echo/reverb and they accentuate echo. Omni-directional mics mounted close to the subject's mouth sound much better in an echoey space. The NT5 is good indoors in a slightly echoey space - think an empty classroom.

A combination of choosing the right pickup pattern and treating the room with doonas/everything soft, will get you out of trouble.

Remember to always use your ears👂

Sound theory can give you principles but sometimes weird stuff happens. Mics break down from humidity or sweat, echo comes from weird places, radio mics get interference, the transmitter gain is too high and the dialogue clips before it even gets to your recorder, or cables pick up radio waves from radio stations.... Use your ears and if it sounds good, it is good. If it sounds bad, it is bad.

Congrats! You've completed Sound Theory 201.

Did you know that Pawnix noise cancelling headphones for dogs are a real thing?

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