Film Sound Theory 101

Imagine your favourite movie without the sound... No-one wants to watch that...

While sound theory won't teach you how to create mind-blowingly good sound for film, basic sound theory can help you record dialogue well on set.

What is sound?

Sound is a longitudinal wave. We humans normally hear sound that travels through the air.

When something (anything) vibrates, it makes air molecules move. 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.

Sound can travel through more than air (it can travel through liquids, solids and gases) which is why you can hear underwater. You can't hear anything in space because space is a vacuum - there aren't any molecules at all.

In 'The Martian', Matt Damon would have barely been able to hear the giant storm (or anything) because the atmosphere on Mars is so thin that there aren't many air molecules to move around!

Q? Do you think sound travels faster or slower underwater?

Watch the next video to find out the answer.

What is a frequency? Amplitude???

Feel free to watch this next video at 2x speed.

The video explains it beautifully but he talks very slowly.

Also, when he gets to the part where he's showing what 50Hz sounds like, make sure you're wearing a good set of headphones - cheap headphones can't reproduce 50Hz.

Q? Can you now explain the Doppler effect?

Knowing what you know about frequencies and wavelength, why does the pitch of a siren/car/fast-moving vehicle get lower once it goes past you?

Frequency Identification Quiz

Are you ready for a challenge?

Listen to this section with decent headphones that can produce bass.

If you're wondering why you needed to know all that to be able to record sound, the next section of practical sound recording info will bring it all together.

SETTING GAIN

Apart from plugging everything in and hitting the record button, setting your gain (or levels of each microphone/channel) correctly is the most important part of sound recording. Getting this step wrong means your audio can be unusable.

Think of gain for audio like you think of exposure for cameras. If you set it too loud/hot, your audio will be 'over-exposed' and will be distorted. Too low and it's too noisy (literally - your recording will be hissy).

You have to set the gain for every channel of audio you have: eg. the boom and each radio microphone.

Clipping and Distortion

If you set your gain too high it will clip (aka. it will be distorted). Clipping is bad because it sounds bad.

Listen to the sweet sweet sound of my voice clipping (distorted)... (kidding, it sounds bad).

Noise Floors

Noise floor isn't referring to your upstairs neighbours (I used to have an upstairs neighbour who, every night, dragged something around on their floor that kind of sounded like it was the size of a human body). Noise floor refers to the inherent noise of microphones and recorder pre-amps - your equipment generates a certain amount of noise in every recording. If your microphone level (gain) is set too low, when you boost the volume in post, you will also boost the volume of the noise floor, making everything noisy/hissy, and basically, bad.

Here's an example.

An example of an audio recording where the gain was set too low. There is a lot of hissy noise floor once the recording is boosted to a normal level.

Professional microphones and recorders have very quiet noise floors - if you set your gain correctly during recording, you won't hear the hiss.

This is why setting your gain correctly is important.

In class, you will learn where and how to set your gain for different recording situations.

ZOOM F8: As a general rule, aim to have your levels bouncing around in between -10 and 0db on the Zoom F8.

Mixpre6: Aim to have your levels bouncing around -20db.

WIND NOISE and high-pass filters

If you have ever recorded outside, you know that wind can be a massive pain-in-the-butt for audio recordings. There are some practical things that help but there is also a bit of sound theory that will be your best friend from now on.

1. Use 'Deadcats'

Student: "It was windy so our audio isn't great". (By 'isn't great', they mean 'completely unusable')

Me: "Did you use a dead-cat?"

Student: "We didn't have one with us".

Me: ..............................

Deadcats are essential in windy conditions. Do not try to record in the wind without them. Don't go out on a job without them.

2. Use High-pass filters

You may have heard of high-pass filters. You may have wondered what they do and why they're useful. From a location sound perspective, high-pass filters are a very handy tool. (THEY DO NOT REPLACE DEADCATS)

High-pass filters let high frequencies 'pass' through, and 'cut' out low frequencies. See the video below.

Wind noise is mostly composed of low frequencies, so a high-pass filter can cut out those pesky low windy rumbles.

Only watch the first minute.

The rest isn't helpful.

He talks about a bunch of filters that usually get used in post-production or music production (the zoom F8 doesn't even have the extra filters because you don't need them).

Your location sound recorder/mixer (the Zoom F8 or Mixpre6) has in-built high-pass filters for every channel. You just have to turn them on.

A good basic setting is to set your HPF (High-pass filter) to 80Hz for every microphone recording dialogue. This will cut out frequencies below 80Hz.

Wind noise, handling noise and rumble BE-GONE!!

Congratulations! You have completed sound theory 101.

A photo of Schmidt just for cuteness.

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