This article is meant to inform and educate anyone in a deciding role in the filmmaking process. Whether you're a director, producer, UPM, line producer, DP or sound mixer, you should know these things.
1. Why boom mics are superior to lavs
2. Locations and acoustics principles
3. What you lose with ADR
Here's why boom mics are better than lavs
This topic is very necessary to understand when deciding how to cover a scene. The powers that be have to know the risks when choosing wide and tight or cross coverage where booms may not be usable.
A boom mic sounds more natural and clear compared to a lav mic for a number of reasons.
1. The first is that the boom isn't hidden under clothes that can muffle it. Even without getting into the cloth noise factor, you have to consider that the lav mic is always underneath a layer of cloth which will reduce the high frequency response.
2. If the lav is exposed and clipped to the outside of clothing then that's the best way to get a clean sounding lav mic, but you're still dealing with the acoustical block that comes from having a microphone up against a person. With lav mics you are only picking up 180 degrees of sound. The talents' body is blocking half of the sound from behind. While this might block out some unwanted noise from behind the talent, it ends up sounding less natural compared to the boom. Think of how we hear. Essentially, one ear picks up 180 degrees of sound and the other ear picks up the other half. Combined, you can basically hear 360 degrees of sound. Through your ears (and through a boom) you don't just hear the subjects voice. You hear footsteps, natural cloth rustle, foley created by your hands, the atmosphere of the room and the sounds that are reacting to the room as well.
3. Perspective plays another key role in sound. A lav sounds best placed in the center of the subjects' chest about 8 inches below the chin. Any higher would sound too "throaty", too much low-end. A lower placement would sound too hollow, not enough low-end. But, even if the costume allows the perfect placement without cloth noise, the subjects head is still bound to tilt and turn changing this perspective throughout the take.
Key Factors
Wavelength
(Warning, science ahead) The physical properties of sound are pretty simple. Lower frequencies (pitch) have longer wavelengths. A tuba is a much bigger instrument than a piccolo because to produce the longer wavelength you need a longer instrument. The piccolo is 13 inches long therefore it can only play as low as the note C6 at 1,047 Hz because that wavelength is 13 inches long. On the other hand, a tuba can play as low as 62 Hz at note B2 because its 18 feet long coiled up which is the same length of that wave. This applies to microphones as well. The larger the diaphragm of the microphone, the lower frequencies it can accurately pick up. So when looking at a lav mic versus the boom, the boom is bigger and can therefore capture lower frequencies more accurately. Check out this Wavelength calculator for fun.
Proximity Effect
Why is it sometimes more difficult to pick up low frequencies than high frequencies? The answer has to do with proximity effect. This is what happens as you start to pull a microphone away from the sound source and the low frequencies fade away quicker than high frequencies. This is because higher pitched frequencies travel faster than lower frequencies and are reacting to the room around them. The boom mic changes positions while shooting between close-ups and wides. So for wide shots the boom is farther away, but you want to still be able to capture clear, close-sounding dialogue. That's where acoustics come in.
Acoustics
Better acoustics can allow the actors voice to travel farther uninterrupted. If you're shooting in a kitchen or bathroom with hard surfaces the proximity effect is more harsh. You'll need to be right on top of the actor to get clear dialogue with the proper low frequency response from the mic. But if you're able to put a rug down and hang sound blankets off-screen, you will get the same results from farther away with the boom. This is important for every set location! The more treated stages with soft surfaces you shoot in the better your sound will be and you are more likely to have usable sound from the wide shots.
Phase Cancellation
(Warning, even more sciency) The technical reasons for acoustics' major affect on sound has to do with phase cancellation. Sound waves bounce off of hard things easier than soft things, just like a ball. Sound also travels at a speed of 767 mph. So while the subject is sounding out one word, the sound is hitting the wall 20 feet away and coming right back to interact with the same word before the first syllable is finished. That reduces the dialogues clarity.
Looking at the chart below you can see how waves interact with one another. The most noticeable wavelengths to change or cancel out are the big (long, low-pitched) waves since they cover the most area. The easiest way to describe this phenomenon is that the low-end of someones voice will become less present. Phasing like this won't affect pitch, but can drastically reduce the low-end in someones voice. This is all just from the sound of a voice hitting a wall, reflecting back, and interacting with the other sounds that voice is producing milliseconds later. Understanding the proximity effect can help combat this. In a reverberant room your microphone has to be closer to the source to capture low-end frequencies before they bounce back and phase cancellation occurs.
The good side to this is that higher pitched voices are less affected by acoustics and proximity affect than deep, low-end voices. So technically a microphone can be farther away when micing woman and children.
Acoustics and Locations
Frequently in film, the set will be built in a location that's entirely different than what it's shooting as. If the scene takes place outside, but art brings grass, leaves, and trees into an empty warehouse, you have a problem.
A lot of "The Strangers: Prey at Night" took place in trailers in a trailer park. The entire park had to be built from scratch because we needed to control every part of the world in front of the camera. They built this trailer park right next to the CVG airport. Turns out UPS and DHL do a lot of flying at night. This location didn't pose as much of a problem as we thought since I was able to track every flight with the "Flight Radar 24" app and tell them when a break in flights was coming up so we could shoot. The real problem was when they rebuilt the trailers inside a warehouse for the interiors so they could shoot in any weather and time of day.
Major locations problems in this warehouse included a constant transformer hum, the factory next to us was operational, when it rained it sounded more like a cacophony of rain hitting a huge tin roof rather than a small trailer roof. The worst was the acoustics. We could wrap the transformer in sound blankets and ask the factory for breaks of silence, but the acoustics were unfixable. Anytime an actor raised their voice it would reverberate up to 2 seconds (which would not happen in the outdoor trailer park). We couldn't put blankets inside the trailer because there wasn't room and the roof was a thin muslin fabric with lights above it so there wasn't room for anything above the lights. All we could do was get wilds outside and talk to post about reverb reduction.
Locations like this can kill the sound. Rooms and stages that are padded keep sounds from bouncing around. Padded rooms sound more dry and plain which makes it easy for post to conform the sound to the type of room they desire rather than be stuck with an echoic room that is harder to fix. ADR is sometimes the only solution to a bad location but it's far from ideal.
"So what, we'll ADR it"
ADR exists. Essentially everything sound-related can be fixed in post. So what's the problem? Narrative productions tend to rely on the "fix it in post" method a lot with sound. The trouble with that is:
1. ADR is very hard to do well and is therefore expensive
2. If you replace the production audio with ADR, you not only have to edit dialogue to match the room with reverb and EQ, but also have to replace everything else like foley and BGs.
3. You have to reproduce the best performances from your very talented actors who are now in a vocal booth without the visceral elements of costumes, set dec, supporting actors, etc. to act with.
ADR is automated dialogue replacement. There isn't a whole lot that's automated though. It's difficult to get right. For example, imagine a dialogue scene in a shower. Actors can't be miced if they're naked and wet, the shower is loud, there might not be room for a boom, etc. So you ADR this scene that has hard surfaces everywhere, but where do most people record ADR? A studio with soft surfaces talking right up against the mic. The recording location is entirely opposite and the mics used are usually different from production to post. This takes a very talented re-recording mixer to make the sound believable with EQ, reverb and other filters. It's not cheap for the post crew and of course the cast is paid for their time in ADR as well.
The second reason ADR is a killer is because it takes away from the actors' performance. For the most part, on higher budget films of certain dramatic genres, there are steps that can be taken to avoid ADR in any situation. If you're working with Leonardo DiCaprio and Meryl Streep in a dramatic scene, the actors shouldn't have to be worried about the quality of sound. The sound team should be prepared for anything. The prep work should have been done to ensure the location would be quiet in advance. They should have been in communication with appropriate departments about effects fans, picture car engines, camera fans, playback, HVAC, acoustics, costumes, background actions, camera movements, camera coverage, special lighting for shadows, etc. If ADR is needed, the actors are going to be upset about having to recreate this difficult scene and the director is going to be worried about getting that special romantic performance he captured on the Eiffel Tower but now has to get in a vocal booth.
Quentin Tarantino makes a very strong point of not ever needing to replace any dialogue after production. He wants to capture every natural element that happens in production. If there's wind coming from the open car windows then Quentin wants to hear that. He doesn't just want clean dialogue he wants to be immersed in what's happening in the scene. Quentins production sound mixer, Mark Ulano, has not had a line of ADR in the last 25 years. The reason for that is because the directors Mark works with care about the performance and they have the resources to prioritize production sound.
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