If you’re into physics, you understand the similarity sound waves have with other electromagnetic concepts like gamma waves or even certain states of light. But if you’re like most of us, how sound works is not something you’re intimately familiar with, and lack of sound theory can harm your recordings and mixes. Let’s get it handled right here.
A: Sound has different penetration based on frequency
Ever heard those “boom trucks” going by? You know the ones where your whole house shakes? You wonder how the heck they could
take the volumes it must be in the cab of the truck? Interestingly, the bass frequencies in the cab are slightly more than 1/4 what they are by the time they hit you in your home, and although the “why” is a deep discussion for another article, it does illustrate the fact that bass frequencies have incredible penetration power while high frequencies do not. While in your home you have no idea what the hi frequencies sound like from the truck. In fact, 90% of frequencies above 75 Hz aren’t leaving the cab at all.
Also, as we’ll see in “B,” the lack of high frequency penetration creates a near equal capability of reflection. A 10,000 Hz tone will hardly penetrate cardboard, but it will certainly carom off of it and keep going. Bass frequencies, on the other hand, have the same ratio but in the opposite way. Those boom-trucks’ low frequencies have virtually no reflective potential, but they will charge right through that cardboard while laughing.
What does this mean for we filmmakers? A few things:
- If you’re going to be mixing in a condominium or an apartment be sure you’re not running volumes above 75 dB – especially if you’re mixing in surround with a subwoofer – or else your neighbors will be quite unhappy.
- Tweet: Blocking street noise and rumble is extremely difficult and requires extra insulation #cinemasound, bass traps (which can be found at our friends at Auralex), and if you’re higher than the ground floor, insulation from the floor.
- If you’ve got reflective elements (like a smooth table top) in your listening environment, it’s likely you’re getting a bad representation of your mix due to primary reflection. We go into how to fix this a great deal in the MZed Pro Member Education.
- Just to recap: Low Frequencies have high penetration and low reflectivity. High Frequencies have low penetration and high reflectivity.
B: Sound has different dispersion based on frequency
Ever wondered why tweeters usually have funny fangled bits in front of them, but woofers don’t? It’s because in addition to penetration differences, Tweet: sound’s ability to be “coherent” or focused changes with frequency #cinemasound. The lower the frequency, the more the sound “spreads out,” and it is a function of its incredible penetration potential. Those weird things on tweeters help spread out the tweeter’s sound because high frequencies have a very focused dispersal or lack the ability to spread out. Tweeter horns and other devices use what’s called “diffraction” to assist in creating a wider field for the speaker to reach. Even with these devices you’ll notice how much more “high frequencies” there are when listening to a speaker directly in front of it instead of at an angle.
What does this mean for we filmmakers? A couple of things:
- Not only do your speakers need to be angle toward the listener in order to hear accurate high frequencies, the tweeter/high
frequency driver needs to be at the correct height as the listener’s ears. There’s no sense in have a $1,500 speaker angled toward you, but because you’re sitting and the speaker is on a stand, the tweeter is pointed 6″ over your head. You’ll need to turn the speaker upside down in order to have the tweeter closer to your transducers: your ears. Otherwise, your mixes will have far too much high frequencies.
- In the same fashion, if you have your speakers in a position where neither right-side-up or upside down will get you the right position, then sideways is your next bet. Unfortunately, sideways causes additional problems which we demonstrate in the Pro Member Education.
- First, by rotating the speakers 90º you’ve changed the distance between the speakers. How? Since we always measure speaker distance by the space between two tweeters – not the physical box regardless of size (thanks to “A” above) – rotating speakers so tweeters are on the outside creates a wider stereo listening field while tweeters on the inside create a narrower field.For example: if I’ve created the Magic Triangle (discussed in this article) at 48″ between speakers and listener, and I rotate my 18″ speaker 90º so tweeters are inward, I’ve shaved nearly a foot off of the distance between speakers…but not toward the listener. This will create a significantly diminished stereo field and will have you mixing sounds wider than they should be to counteract this. The easy solution here is to simply compensate by moving both speakers farther apart. But if you don’t, your mixes will never translate.
- Second – and this is worse – to have speakers sideways means that you have either your tweeter or woofers closer to the
listener in order for them to be angled appropriately in a triangle. This is unacceptable, because now you’ve introduced a massive phase issue between the tweeter and the woofer. In the picture here, notice that if you are not in the perfect distance from these speakers, either the woofer or the tweeter’s sound will hit you first. If you’re too close, the woofer’s sound will hit you first. If too far, the tweeter will hit you first. This creates a horrific phase issue and there’s little chance your mixes will translate to other systems to so say nothing about what this does to the stereo field. Unless you can afix your person in a position which is an inch in square for hours of work, Tweet: never position your speakers sideways! #cinemasound
C: Sound has different speed based on medium density
Most people thing that the speed of sound is a result of elevation. While elevation does change the speed of sound, it’s not the elevation itself that does it: it’s the density of the medium. As we go higher, air becomes colder and less plentiful: less dense. As a result, sound travels slower. But when sound travels through a medium like water or granite, sound speeds up by warp speed. What I like to tell folks is this: Tweet: sound travels at 1.1308 ms/ft at sea level at 72ºf/20ºC in your studio #cinemasound
What does this mean for we filmmakers? Two things:
- If sound travels so slowly in air, then it would really matter for us on set in two ways:
- Distance between lav and shotgun. If you’re running a lav on two actors and a boom op is panning a shotgun between them, you immediately know there will be a substantial delay between the lavs and the shotgun. It’s why when you try to mix your dialog with both the lavs and the shotgun, it sounds so terrible. Why? Because with that kind of 5-15 ms delay, the worst kind of comb filtering/phase cancellation issues occur, and it just sounds awful. It’s worse when actors approach each other, the distance to the shotgun gets closer (creating flanging) and both actors begin bleeding into each other’s lavs. Pig slop. It’s why I’m always saying: “Record with many microphones. Use only one in the mix.”
- Distance from reference mic to lav. If your on camera reference mic (already a bad idea) is 36 feet from the actors, your on camera reference is now one frame late. Move forward or backward from here, and you’re in fractions of frames which most NLEs won’t solve. You won’t even notice. You’ll run Pluraleyes or some in-NLE sync system and figure you’re in sync. You won’t be. Worse, when you line up the waveforms it will look like you’re in sync, but you still won’t be. Your camera reference is late. You’re FUBAR.
- Tweet: Speaker placement matters to within a 1/4 inch. #cinemasound Because if one speaker is a centimeter closer, it will occur as louder and we will unconsciously compensate in our mixes – which will then never translate to a system whose speakers are in the correct position. It’s also one of the reasons we like to keep our studios at a constant temperature. Believe it or not, changing that temperature even a few degrees changes how our brains perceive sound mainly in the panoramic field. Ever been underwater and tried to figure out where a sound was coming from? No way, right? That’s because sound is traveling so fast underwater, your brain can’t determine the delay between the sound hitting one ear to the next. If the speed of sound changes, our ability to perceive audio panorama is altered. It’s not a good idea to do that in the middle of a mix.
D: Sound hates going through multiple mediums
Tweet: The more mediums sound must penetrate, the more the amplitude of the sound is attenuated. #cinemasound This is dependent on the frequency, as we talked about in “A” but figure if you have a dry wall barrier between you and a mid-range driver, another dry wall barrier with a 1″ space between the two isn’t twice as good a sound barrier, it’s more than four times. Why? Because you’ve added three mediums for the sound to pass through to get to you: Solid, Gas, Solid – and then Gas again. In fact, the more mediums you can force sound to go through the better – if your goal is isolation. As we talk about in the Pro Member Education Work Space Volume 7, certain kinds of solids absorb sound amplitudes better than others.
There is also the matter: diversion. Waves tend to lose power when diverted. Ever noticed how well-made double paned glass is designed? One piece is parallel to the frame, but the other is slightly angled? This adds another layer of attenuation when the sound must not only pass through multiple mediums but also be diverted.
What does this mean for we filmmakers? This: when designing isolation for your work space, don’t simply add some foam to your walls and call it a day. Add some foam on top of something else. Nearly anything else. Have the resulting foam NOT be parallel with the wall or ceiling it’s on. Anything to force sound from outside coming in (or the opposite to keep your neighbors happy) to change state and direction.
E: Sound reflections are degraded by A, B, C and the texture of the reflecting medium. Boo.
The biggest issue we filmmakers face when trying to have our mixes translate to the widest theaters and playback scenarios possible is having good speakers. After that, it’s the sound of those great speakers reflecting off our smooth desks and smacking us in the ear a few milliseconds later. Why is this bad? Because as we’ve noted, the higher the frequency the more reflection. This means that you’ll be hearing crazy phasing and comb filtering in high frequencies but not in lows. If your speakers are in a vertical position, the woofers will reflect to your face before the tweeters do – which will make them seem louder – but only a fraction will reflect. This will cause additional phase issues.
Worse YET, the sound which is reflected has been colored and delayed by the texture of the reflecting medium! If the desk is super
smooth, this doesn’t matter, but if there is any texture or ripples or 3D design to the desk, that will radically affect the reflected sound. The result? You’ll tend to turn down 1.5-3.5kHz and turn up 100 Hz and below. This will cause all kinds of problems in music vs. dialog mixing, and it’s the main reason why I hear amateur mixer’s ask me,
“Why did it sound so bad in the theater/car/iphone/my buddy’s place?”
Simple. Reflective desk? 75% of the problem. We demonstrate how to fix primary “desk” reflections in the Pro Member Education in a jiff, but there are also secondary reflections off walls and ceilings. These should be solved with professional applications like those found at Auralex.com. But if budget is really an issue, you can use any number of absorbent materials to keep those pesky high frequencies from ruining your ability to perceive the direct sound of your speakers. You do have nice speakers don’t you?
Whether you’re on set or in studio, understanding how sound works is critical. Have another experience around this? Let us know or Tweet about it!