Science of Sound
I want to talk about a little bit about the science of sound um and you know, some of you might I understand all the stuff fluently and other others of you might not really understand much of it also apologies if this is kind of rudimentary stuff but I feel like it's it's very crucial to my king anything but particularly a drum set where it's you know, you might have very many mike's being used to capture the drums have you know the drum set? You know, some some people look at it as one instrument. Other people look at it as a as a collection of instruments but more than likely going to using more than one mike on the drums said so there's going to be a phase relationship between those those two or three year fifteen mike's so and the more you can optimize your phase relationship between the microphones, the mohr pure and punchy the drum tone will be so I'll make sure everybody understands what what phases and the difference between phase and polarity is I think that those two terms ge...
t thrown around and used interchangeably a lot when reality they're not the same. So um, sound is a is a pressure wave and so as sound is approaching you it's uh uh what is that sound is I think I wrote this down here right down yeah. So sound speed of sound at sea level and relatively low humidity is about one thousand one hundred twenty six feet per second so we're in seattle way probably have higher humidity so maybe it's a little bit slower than that but you know, sound sound as compared to say light or electricity moves quite quite slowly and what sound is is it's a pressure wave so you know as as I speak towards you you're hearing you're hearing my voice because you're getting these, you know, oscillating contractions and expansions of of air air pressure so you know when that air pressure goes to your ear and then causes your ear to vibrate and then your brain interprets this fight durations as sound so the simplest type of way to represent a sound wave would be just too just sketch out a sine wave I'm gonna do that in this white war over here. So um strong little access we've got our x y axes here and x is going to be time and why is amplitude or ah volume so the simplest sound wave that we can we can draw a sine wave so one period of a sine wave looks something like this um so um sort of a neutral sine wave would say be a one kilohertz sign away that means one thousand cycles per second, so that means you get a thousand of these per second and I have a little, um audio example of that back here in pro tools a good approach tools for the first time and just play what a soundly this are so excuse me a sine wave at one kilohertz way go that's a sine wave so um let's talk for second about um miking just a simple acoustical membrane like a single sided drum head so let's say you have like a remo roto tom or tim bali or something like that's a single sided drum head I'll just draw it sort of two dimensional say this is the that's the drone the shell of the drum and this is the drum head so you're looking at the drumhead from the side okay and then let's mic up this drum head with say you know standard standard microphone there you strike the drum head with a drumstick and started started resonating and to the initial strike of the drum set pushes the head down away for the microphone so the first I think that the microphone sees is the drum had moving away from it which is a you know and negative amplitude so this is this is a positive direction and this is the negative direction now let's say we want to double mike the drama wanted like the top and the bottom of the drum we're going to use the exact same brand and model of microphone and we're going to use the exact same mike preempt so everything is the set to the same game has the same frequency response okay, now we've got another mic on the bottom of the drum set now when this um stick strikes the head again, its initial amplitude is being pushed towards the bottom like so electrically since electricity actually moves near light speed according to your recorder it doesn't care that there's uh there's there's no there's no matter where you position the two mikes it won't perceive a difference in time due to the speed of sound it will are due to due to the speed of electricity you can use like a five hundred foot mic cable here in a five foot my cable there electrically they're essentially the same I mean obviously there's cable capacities and all that stuff but in terms of the speed in which electricity travels through the my cables essentially the same you don't have to worry about the difference. So what you're getting with this bottom microphone at the initial strike of the drumstick is a membrane moving towards this microphone so it's moving away from the top mike towards the bottom like so the bottom mike c's a positive amplitude okay, so this is okay so this is what the no backwards here but this is what the top make it's seeing we're sorry. This with the bottom, like a seeing and in the top, mike actually drawn this backwards. The top mike seeing that so let's, uh, we show you sort of the difference in tonality between this sine wave that starts with a positive sample tube versus a sign way that starts with a negative amplitude. And I could do that back in pro tools just by creating a duplicate of this track. And, um, seems goes solo, that guy and then we're going to use audio. Sweet invert. And this is just a quick audio sweet plugging it will invert the polarity of that sign, ladies let's blow these tracks a little bit just so we can see them a little bit better. And, uh, so now you can see the duplicate is, uh, in exactly the same in the x axis, but one hundred degrees reversed in the y axis. So just here's. How the regular sign way sounds right. And here's how the inverted subway sounds exactly the same. You can't perceive a difference, but if my homeless soul of otis wrong here, um, do this hello, latch if I plays together. There's no sound they perfectly they perfectly cancel each other because they're perfectly out of phase so if our little drum example over there if it's a ah perfect universe it's in an echo chamber and the membrane sounds exactly the same on both sides and the microphones are a hundred percent identical and entire single paths identical those two mikes would essentially cancel one hundred percent reality obviously is a lot more complicated than that but this is sort of an illustration of what of what polarity is so just the physical inversion of a signal is is polarity however so phase is actually a time discrepancy let me go back to the white board for a second here now um let's just forget about this bottom mike for now ok and you know I'm going to redraw this so he's here so uh here's my drum again this is thie membrane here and this is just sort of cross section of the edge of the shell and here's my microphone microphone cable it's you know it's pretty close to the shell and then let's say now I have um and overhead microphone so positioned up here is my overhead okay um so what happens now we have to remember that like I stated earlier electrically your recorder whether it's a tape machine or a dollar whatever is seeing these two microphones at the same time but because of the speed of sound the sound from this drum head is not arriving at two microphones at the same time so I did a little math on my own and determined that um let's say this let's say that this drum head is putting out in exactly a perfect one killer hurt sine wave takes that sine wave it um after we got to refer to my notes here um uh okay, so one kilohertz sine wave has a period of one millisecond because it's one thousand cycles per second so one period is one second so uh for one period that sine wave will move um thirteen point five inches so half a period so let's say let's say the overhead mike is only you know, six and three quarter inches from the snare of obviously it's a very low position but you're at the point and I can illustrate this if we go back to pro tools I can I can sort of illustrate this like zoom in a little bit more um so let's let me actually undo that inversions and now I have just a duplicate of the sign way which I wish if I play it is just louder than one happening at the same time because now they're the opposite of canceling their they're adding so the amplitude is doubled so basically what we're saying is that in thirteen point five inches this pressure wave will have moved through one period so one period of the sine wave is that okay? So in thirteen point five inches it will move that far which, um let's see uh going to samples here and see how far that is in terms of samples yes it's about forty four samples so let's say let's say that we, uh we have this microphone six and three quarter inches away from the membrane the perfect membrane, the unworldly perfect memory and that's putting out a one casein wave six six point three six point seven five inches away from that will give us half a period so now we've moved like we've moved in this case, I guess twenty two samples and um might I might not have moved at exactly twenty to samples, but it's uh, we should have now due to the timing offset we have something that well should mostly yeah it's not perfect. So you're still here in a little bit of audio, but not much. We're basically fully canceling because of not because of a polarity inversion but because of a time offset so we use of the polarity button on our microphone pre amp to compensate for this phase discrepancy because of the position of the microphones now in the real world, this phase discrepancy is nowhere near this perfect and there's a lot of subjectivity to it and it's real world audio signals are not nearly as perfect as the sine waves so it's comprised of many wavelengths stacked on top of each other and we can't perfectly, you know, phase, cancel our phase or like volume, cancel our value, multiply by doing this, but we can use the polarity switch on our consul or mike print to compensate for this phase discrepancy. So science in the case of a real drum set, we have in many probably many microphones. I mean, I typically record, you know, twelve to fifteen microphones on the drum said sometimes less sometimes sometimes mohr, but each one of those microphones is because there's not sonic isolation between each drum you know each each one of those microphones has a phase relationship to every other microphone, and we use a number of tools teo combat those phase discrepancies in the first one. The most important ways is mike positioning and you know, so you'll see me walk around my studio with a tape measure or with a string or my cable to measure distances between things to try to optimize certain microphones to get the best phase relationship between them sometimes after the fact I'll scoot, scoots and tracks around digitally to improve their phase relationship. Other times it's just a matter of flipping the clarity but on a switch that all being said, I should also say that, um sometimes perfect phase relationships are not what you want. Sometimes you get a greater sense ofthe space when you have in perfect phase relationships and equalizers actually work on the principle of phase discrepancies. So hee queuing. When you're skewing the sound, you're actually imparting a certain degree of phase shift in that tone, so sometimes you can use that to your advantage. You hear that actually just recently washed a cool video that george massenburg did where he's, talking about making a bass drum or double making a bass drum? And he'll actually use on the inside my candid outside mike and take advantage of the subtle phase discrepancy between the inside mike any outside mike in order to kind of scoop out the loam ids to get himself that whopping some base feel that he likes in his recordings on, and you could do that. So don't feel so. You have to get everything in face. But just be aware of what phases and how it effects the tones of what you're recording.