Hi everyone and welcome to Kristal Cross Music Production
This is Part 2 of the First Chapter of the Audio Engineering Class and I called it "Acoustic Principles"
We're going to cover a lot of important themes, also very underrated by a lot of people who deal with music production
Why do I know that? Because I was one of them. I had beed underrating the things I'm gonna tell you for years
I had to learn them afterwards and I had first-end experience of not knowing this stuff correctly
One of the consequences is that the studio I'm in (sooner or later I'll make a video-tour), had been built with wrong principles which led to disappointing results
Unfortunately I did that before I knew enough, so I want you to spare this mistakes to those who want to build a proper studio or just a home-recording one investing some money and time, to improve the room's acoustic
Good, let's begin!
When a sound is generated by a source, independently from the context and other factors, it'll follow this pattern:
The sounds starts from the source and reaches your ears, but only a part of it really reaches directly your ears
For the most part will bounce across the room on every reflecting surface it encounters and then will come to your ears, mixed with the other that reaches you directly
Same thing goes for a microphone, instead of your ears
There are various kinds of reflections and happen wherever there's any kind of reflecting surface
Inside the room, they can last for a fair amount of seconds
In most cases, dealing with rooms the size of an apartment, generally this refractions last less than a second
But there are cases in which they can last much longer. Think about an auditorium, for instance. In that case the reverberation time will be much longer. We're talking about many seconds
On the other side, we have an electric signal. When it's emitted (with some exceptions, there are many variations)
Theoretically when an electric signal is emitted, runs through a predefined medium that in most cases is a cable
From the source to the end point, it crosses a well predefined medium. In just one direction, inside a very narrow space without any variation and refraction
A sound the comes to an end point, emitted from a source, without external intervention... Almost
It's obviously a simplification, but you get the idea
In acoustics, sound moves three-dimensionally and as I said earlier will bounce on every reflecting surface that finds in its way and it'll bounce for a theoretically infinite number of times
Until it fades, clearly
Now pay attention, this is important: even with today's knowledge and technology, it's impossible to calculate mathematically and empirically every single sound reflection in a room
This means that once you "scientifically" calculate a room's response to a sound source, it'll have to be adjusted "by ears" by a human being, because the result will be "unpredictable"
The calculation can be done in a very professional and precise way, but there will always be a certain degree of unpredictability
A room's acoustic is determined by three factors: time of reflection, relative strength and the frequency balance
Look around you in this very moment, imagine you have a microphone in front of you: you're talking and the mic captures your voice
As I said earlier, a part of the sound emitted by your voice will directly reach the mic. Another big part of it will spread across the room, will bounce of the surfaces and will reach the mic with a certain delay
And this is called naturally "Delay"
Some surfaces sound encounters are not reflective but absorbent and those cause the sound to lose intensity as it hits them
But remember what we said about the wavelength in the previous lesson
Lower frequencies have a very wide wavelength and that means that if the absorbent surface is not big enough, it won't be able to hold those frequencies
That means if sound encounters let's say a curtain, or some pillows, only the lower frequencies will bounce back and the higher ones will be very attenuated
Or less attenuated, depending on the absorbing material but the point is that the wave will start with high and low frequencies and come back loaded with low frequencies
To tell you the truth, there would be a fourth factor that is "Movement". This interests us relatively, because we're dealing with a fixed source that emits sound and a fixed capture device that captures it
But it's ought to be mentioned, and to explain it better we can bring the so called "Doppler Effect" into play
And to explain the Doppler Effect, we can make the most classic example made in these cases: the Ambulance's Siren
Or a Police Car's Siren
As an ambulance approaches you, it seems that the siren's pitch get higher as it moves closer to you. As the ambulance moves further the pitch will seem lower
This is due to the fact that sound has its own speed of propagation through air, which is 340m/s
If the sources travels at a higher speed sound cannot compensate and the final effect will be a pitch variation
And this causes the Doppler Effect: as a source draws nearer fast, the pitch will seem to become higher. As the source passes by and travels further, it'll seem lower
But I repeat, it doesn't concern what we're studying now, because in a studio, sound sources are still
As we mentioned earlier, acoustics are a science. And this science is in the service of music, in the service of sound and sound we perceive with our human ears
And that means that the scientific conclusion of acoustics, don't guarantee the results we hope for
What you make to achieve good acoustics, cannot be done "universally" and won't work in all circumstances. Depending on the human ear, every room shall be treated differently based on what you're going to record
For example, if you have a room in which you want to record dialogues, it must have specific characteristics
It must be very silent, very intelligible and a very low reverberation
But if the room you're working on, will be a concert hall it shall be treated differently because it'll have other necessities
In some ways, opposite to the ones you'll find in a room to record dialogues
So keep in mind that this science, must always be applied to human perception. So there ins't an unambiguous and universal answer to the request "make this room sound great"
Because the room will sound great, depending on the applications, depending on the use you'll make of it
Now let's cover one of the most important topics: one of the most common problems that you can find in the average-sized room
Rooms you can find in flats, or in places we usually live in
The problem of "Standing Waves"
What are those "standing waves"? As we said in the previous episode, the human average hearing range goes from 20Hz to 20KHz
That means the wavelength of the frequencies that cover the human hearing range goes from 17mm to 17m
Now suppose that the distance between two parallel walls is 4m
4m is the exact half of the wavelength of a 42.5Hz sound wave
Which is more or less the lowest note of an electric bass guitar
Between these two parallel walls, a phenomena will take place: as we said, this 4m wavelength is the half of a 42.5Hz wave
While this wave bounces back and forth, the sequence of high pressure and low pressure will be stable. This means that we'll have high pressure on the surfaces and low pressure in the middle
And this will become a loop, given the distance is the exact half of the wave
The wave doesn't change , we will always have the same wave-form
This will result in the room resonating to that frequency. Every note of that frequency will have an emphasized high pressure and a hollow low pressure
Not in every single case, but for the most part this will surely happen and we'll have a longer reverberation time for that very frequency
This will happen also for the integral multiples of that frequency
This causes various problems to hearing, because the sound we're going to listen to won't be a good sound "representation" inside this closed environment
Why do small rooms sound worse? Because the standing waves that will create between parallel walls, will be placed "in the middle" of the human hearing range
If we have a huge room, we won't have particular standing waves issues. Because the frequencies interested, won't be in our hearing range
If the room's small, this frequencies will be in the middle of our hearing range. In the middle-low section, for the most part
Standing waves don't happen only between parallel surfaces, they happen every time the distance between reflection turns into a loop
Ironically it can happen even between not parallel walls, if a refraction goes in a loop diagonally and not perpendicularly
Remember that the floor and the roof are reflecting surfaces and commonly the most parallel of all
Where there is a standing wave, a phenomena known as "Flutter Echo" takes place
To experiment it, run a test: try to clap your hands in an average-sized room, without much furniture in it, and you'll hear an odd sound. A very peculiar vibrating one
It's a very annoying effect due to the fast refractions that move back and forth of a specific frequency
This obviously doesn't help hearing. And what's the solution to this standing waves phenomena? Simply, you shouldn't have parallel surfaces
Even those who don't have direct experience, should have seen pictures of well-known studios and should have noticed that their rooms are shaped oddly
They are not regular and have been built specifically for that matter. To minimize the problem of standing waves
This should make you realize that cubic rooms, are the worst possible scenario and should be completely avoided
Because in that kind of room, all surfaces are parallel and are all equidistant so you'll have a huge problem of standing waves which cannot be easily solved
A parallelepiped room is by far the most common and doesn't surely have the same issues as a cubic one, but the standing wave problem would be present in this one too
So always prefer irregular-shaped rooms. But pay attention:
Having an irregular-shaped room, with no parallel walls, doesn't guarantee you not to have standing waves. Because they're not created just by waves bouncing perpendicularly
They happen when waves go into a loop and bounce within the same distance
The could happen in scenarios you have never thought about, or maybe because of things you placed inside the room afterwards
So, in the end, the room's planning and design is a very important thing (for those who can afford it). But even if you have the money to do so, a good design is almost never enough and a good acoustic-treatment is needed in every circumstance
Its function is to reduce the reverberation time and minimize the standing waves problem
According to the principle where if surfaces can be made more absorbent, the refractions' intensity will be lower
You have different materials to do so: soft things like carpets, curtains, sponges, mineral wools, specific chemical sponges like pyramid-shaped polyurethane
They all have a function within this matter, but beware: listen carefully because no one applies this correctly
You must know that porous materials absorb low frequencies only if their thickness is at least half the wavelength of that specific frequency
That means that unless you have a stable porous surface that measures 17m in thickness, which is practically impossible, those materials will absorb only medium-high frequencies. Only the high ones in some cases
If the only acoustic treatment is composed of porous materials of any kind, placed on the walls without the predisposition of any other treatment, it'll absorb only the highest frequency
Leaving the lowest ones free to create standing waves "as they please" and you'll have a room that sounds muffled, dead and with the same low-frequencies problems
Believe me, I know that because I made this mistake first-hand
Porous materials are not a good solution to the standing waves issue. They are useful in other situations
To solve that problem, you have other means: like the membrane panels
They are composed by a flexible-wooden panel 18mm thick, placed in a sealed gap 300mm thick, that will resonate at low frequencies absorbing their energy
Depending on how you build this panel, it'll be quite specific for a certain frequency, but we'll see it in a minute
If you place in the cavity some mineral wool (that is the most common in this case), you'll have a wider absorption of low frequency, not specific for a narrower range, the so called "bass trap/broadband"
And if place some holes in the external casing, exposing the absorbing material, this panel will also absorb some high frequencies too
And we'll have a panel that covers, more or less, the whole human hearing range
We were talking about the panel that covers a very specific range and I'll tell you this: those panels can be "tuned up"
To respond specifically to a frequency. This is very useful if you have a standing wave problem that is precisely localized on a frequency and its multiples
You can build a panel that has a great effect on that frequency and even if it's less effective on the other ones
Now, let's stop talking about the building methods of self-made tools for acoustic adjustment, because I wouldn't push my self too far
There is a lot of information that can be found online, there are a lot more things to say, a lot more kinds of panels that can be built... Every kind of solution available
I don't know this matter that much, and know that there are people who made a job out of this topic. People that design and make acoustic treatments for recording studios and such
It's a very complex matter to go into and I made just some examples but I don't want to go deeper because this is not my field of expertise
I told you just what you need to now to grasp the importance of it
Now, there is another method to deal with refractions and stationary waves, and with other parts of acoustics that interest us
This method is called "Sound Diffusion" (Diffusion)
Standing waves happen between parallel surfaces, the more the waves repeat, the stronger the standing wave
The diffuser breaks the surface's regularity, according to a well-studied and precise pattern
Here you can see an example. Things like that are extremely useful to break the incoming high-intensity wave in many low-intensity ones
Let's stop here. There are many kinds of diffusers, almost in a greater number that the absorbing panels
Based on the discussions I read online and how some people speak about it, I can say that there are as many kinds of diffusers as many absorbing panels
Let's cover another very important topic, since many people are confused about it: acoustic treatment is completely different from "Acoustic Isolation"
Acoustic Isolation has not the same goals as Acoustic Treatment, because acoustic treatment deals with the room's insides: how it responds to ear's perception
Acoustic Isolation aims to stop the sound from going from the inside to the outside or vice versa
Generally, what's used and made for acoustic treatment doesn't work for Isolation
If you stuffed your room with pyramids or egg cartons hoping to stop the sound from propagating outside... Well, you're wrong
You'll lower the number of high-frequency refractions, but this won't work for low-frequency that will roam freely to the outside
Many materials used for acoustic isolation, can be the same for acoustic treatment but they're not used to build the same things
The purpose and the way of using them, will be much different
An effective Isolation comes from materials that reflect the sound energy. Concrete, bricks, plasterboard, dense wood, metal, glass, or other specific materials for that purpose
Have a high mass and are not porous
The three conditions for isolation are: mass, structural continuity and "no opening"
For mass is simple: for every doubling of the mass, you'll attenuate the sound by 6dBs
Structural continuity means not only not-porous, so the material shouldn't let the sound to pass through, but also means that every single bit of the room should be isolated
Every room's surface must be treated: walls, floor, ceiling, doors, windows, EVERYTHING
And "no opening" means that this work must be meticulously done. A single mistake could nullify a great deal of work and money invested
Now, there are different methods for a good isolation. We'll see some of them, just so you know
The construction techniques to isolate change based on the surface:
For walls: independently from the material we want to use, it's better to have two thin walls instead of a thick-single one
We must remember that isolation is achieved through sound reflection and sound reflection happens between cavities and gaps too
That means that having more reflecting surfaces, will help to minimize the sound going outside
The more the reflections, the better. And the surfaces should have as much mass as possible
Often a double plasterboard wall proves to be a good solution, and it's one of the most common ones
And it's even more effective if you uncouple the surfaces. And the image you see here, should give you a hint on how to build it
Now the ceiling: the hard part on isolating it, is to put as much mass as possible on it. Unlike the walls or the floor, this "thing" we're gonna build shall hang above our heads
The problem will be to hang it safely, without any danger. In this case, pay attention: you can isolate the walls, you can isolate the floor, but if you're not completely sure of it, don't isolate the ceiling by yourselves
The risk of it falling down on your head is to high, so ask a professional to do it
The procedure is very similar to the one for the walls: the double-layer of plasterboard applied on the ceiling, generally works great
Regarding the floor, the procedure is a bit different: one of the most common techniques is the "floating floor"
The ideal would be pouring concrete on a metal support, laid on isolating disks or springs
But this is not always doable because of the costs, or the weight of the structure. For example, if you live in a flat on the second floor, you couldn't do it unless you want your home to fall down
The alternatives would be wood, plasterboard and mineral wood work just fine according to similar principles like for walls and ceiling
About windows and doors: this is really complicated and I decided not to show you because it's virtually undoable for beginners
Isolating doors and windows is quite hard and quite expensive. I noticed how many suggest just to buy them pre-made
It could seem expensive, but considering buying materials and hours of work, it's more convenient to buy them instead of making them
To conclude, this second part has been very long but despite its duration, this is just a smattering
The info I gave you is rather partial, but also very important to understand. If you want to go further, if you want to design something properly this isn't just enough
You can look it up deeper on the internet, books or with professionals who work in this field that can make you achieve better results
What you just heard is only a tiny part
But know you know some concepts, and many don't know anything about this topics and the dangers that are related to
And I wanted to do something about it
So guys, don't underestimate acoustic correction. It's really one of the most important things to consider
And with the right knowledge and the right materials, it can be done effectively and chap
Well, that's all for now. As usual you'll find down here in the description the link to download the pamphlet regarding this part of the class
If you want to have a written landmark, that will surely come in handy
Subscribe to the channel, if you haven't already. Leave a like if you want to support me and this audio-engineering class I'm trying to bring on YouTube as good as I can
Alright, I'll see you in the next video. Good luck with your work! Bye!
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