Basics of Acoustic Engineering (Full Article)

acoustics
Basics of Acoustic Engineering (Full Article)
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#63

Also no good with exit ramps, turtles, sexual consent, flammable materials, anal warts, microwavable chicken, rubber lips, wax tits, balloon animals, pointy things, decorations, deer heads, animal stuffing, dead corpses, aspirin, ibuprofen, good decision making, career planning, getting a quality lay, starting a family, wives, cooking grease, kitchen fires, your liver, erotic toys, edibles, and yaks.


#64

Interesting thread. I take issue with 2 points in the OP:

The truth, if you want purest sound, climb on top of the tallest tower in the largest field out there where there is miles of nothing. Record there… thats the best place provided you can fight off the dragon.
Yep large open fields with miles of landscape, no obstacles, no parallel structures, no reflections, just plain perfect sound.

I think the point you are making is “no reflections”. I’d argue if that is your goal an anechoic chamber is better than being outdoors; no wind :wink:

The goal is to create and represent accurate sound.

Again it seems like you mean “no reflections” - i.e capturing only what is produced by the source.

My problem with this is that is absolutely not the goal. The goal is to create pleasing sound. Instruments are designed (or we are used to?) to live and interact in real spaces.

Being in a ‘perfect’ outside environment (whatever that is) is not then, the ideal recording environment. The ideal recording environment is a room in which the natural ambience sounds good for whatever you are trying to do.
“Good” will mean a lot of different things to different people (which is why there are 1000 different reverb plugins and presets available), but there are at least a few generally accepted ‘unpleasing’ effects which you would want to tackle with acoustic treatment such as build up of bass (or loss of bass?) or that nasty shrill reverb from lots of hard flat surfaces in a smallish room and so on.

But anyway, my main point is we are not seeking to completely deaden the room at all (can you imagine trying to perform in such a room? Urgh), but to dampen the unpleasing aspects of its’ sound.


#65

Also ice skating, sword fighting, skateboarding, gymnastics…hmm…basically any sport except for golf.

???


#66

I think he meant “golf with turtles”. While under the influence, turtles might seem like a fun amusement, especially if you find one in the rough where you thought your stray ball ended up. Why not “play through” and see how far a turtle-ball will travel? And you might even get creative and rename a “birdie” a “reptiley”. :laughing: A great adventure in aeronautics and acoustics as well. Turtles are “the bomb”! (especially if it lands on another players head)


#67

I think he is talking about pleasing accurate sound. Sound can be pleasing but if its not accurate it doesn’t serve the purpose in a mixing room. In recording room There are times when I want to record character of a particular space like a church hall or a warehouse, most of the times I prefer just plain dry sound. I d much rather record a flute outside, but not possible all the time.


#68

goal of any professional studio is “accurate sound” with or without an ambient envelop. Nothing else.
You can get close to an accurate frequency curve even with an ambient envelop . No one wants a totally dead room. If a room is sounding totally dead, that usually means too much high freq absorption. If treatment is good, room will sound lively and close to accurate. Perfect frequency response curve is hard to reach even with a lot of money in it, the next best thing is strive for consistency of response.

Regarding my outside conundrum, that was to introduce the idea in a basic way. Aside from being non-reflective and distortion free, outside still sounds good and lively because there is not much high frequency absorption going on and frequency responses (if there is no noise, wind or birds) are close to accurate. But we cannot record or mix outside ofcourse…

It is not just about what is pleasing to the ear as that is subjective - To a trained professional ear, a pleasing sound and accurate sound is one and the same. To an untrained ear, they can incorrectly judge room to be pleasing if it is adding a kind of sonic character they prefer.

There are times when room is designed just for recording and adding a particular character but you wouldn’t want to mix in those rooms. Purpose of the room drives most of its treatment.


#69

Turtles don’t like alcohol @Jonathan. And if you get liquored up and kill him it’s bad.


#70

To add to this, the hardest part about getting a dead room is that it’s near impossible to absorb all the low frequencies completely, so usually if a room is dead, it’s still actually ringing in the low end, which gives a boomy sound.

It would probably be better to get everything to decay evenly rather than try to kill it all completely, mostly because the latter is almost impossible to do.


#71

you can never have too much low end absorption - spot on.


#72

They did at Woodstock … but then they were all on “brown acid”. :sunglasses: Eddie Kramer mixed the live recording to tape from a semi trailer parked behind the stage IIRC, under horrible conditions. I might suggest that sometimes the “vibe” can be as, or more, important than accurate sound (depending on your goal).


#73

ofcourse :slight_smile: if you are shooting for a particular character in sound.
Kramer had replaceable equipment if bird pooped on it but many dont lol. However, if he was mixing “inside” a trailer that’s technically indoors. A lot of pros do record sound in different places to get their sonic attitudes and then revert to their trusted mixing rooms to mix them.

Kramer could mix while taking a dump in a truckstop men’s room, his ears could isolate sound like a BAMF…


#74

Again, I’m going on memory, but I think he was working on headphones … as any type of monitor situation might have been impossible. It wasn’t a trailer specifically and acoustically designed for him to mix in, it was what was available when he got there. And yeah, probably ‘indoors’ mainly due to rain, wind, lighting, bird poop, etc. It may have cut down on some of the noise around him - from the stage, the audience (hundreds of thousands), crew around the trailer, wind/rain, and generators. Perhaps all he had to go on were his ears in the headphones.


#75

added more to the original topic above (duplicating it here)

Resonance and Resonant Frequencies
Before we dive into extremely wide topic of room modes, lets take a moment to understand resonance.
Resonance happens when a force interacts with another medium that is either already vibrating or can vibrate. (not the vibrators you are thinking off, get your mind out of the gutter) . This is the kind I am talking about when you get slapped on the ear so hard that your ear drums start ringing. The result is an increased intensity (Greater amplitude).

When this driving force is periodic, and matches the frequency of a medium that is vibrating, the amplitudes will keep getting bigger and bigger and bigger indefinitely - unless there is some kind of dampening going on.

Swing conundrum:

Acoustic resonance like mechanical resonance is dependent on dynamics of simple harmonic motion. Lets take a swing for example. When you push a kid on a swing at the same rhythm as that of the swing, the kid keeps going higher and higher till the kid defies gravity and gets launched into outer space, never to be seen again. That result right there is resonance and the frequency of the swing is called the resonant frequency

Now if you break the rhythm and start pushing the kid at random times or from opposite end, the swing will eventually not go as high and eventually lose the battle with friction and come to a stop.

In the case of a swing, the resonant frequency is also its fundamental frequency (lowest resonant frequency).

No matter how hard you push, you cannot make a swing, swing at any faster rate without changing its chain length. A swing will always have the same frequency (its called fundamental frequency). A swing is a simple case, as it has only 1 resonant frequency. Vibrating mediums can exhibit many resonant frequencies (overtones)

A note here, resonant frequencies are not the same as harmonics. Things that vibrate produce both harmonics and can have resonant frequencies of their own. That is a whole another level of discussion for another time.

Same principle applies to acoustics of instruments and yes rooms. Rooms can have resonant frequencies of their own (room modes) caused by room reflections when you turn your speakers on. Will add more on these soon.

Next up, more on reflections, resonance, room modes and importance of geometry of the room
Anyway, that is all for now folks… will add more to it as time permits.


#76

Do you have a source? - Oh wait…I found one!

Can you imagine being the guy with the boom mic in this video??? That would be one recording gig I never forget!!!

Holy hell, I’ve recorded a lot of dumbass rappers, dumbass preachers, dumbass politicians, and dumbass comedians. I don’t even know what you’d call this guy…an extra dumbass I guess lol. Imagine being handed this video reel to add the music and sound effects. I’d be like Shiiiiiiiiittttt!


#77

more added to o.t.

Wine glass shattering connundrum
You must have all heard about wine glass shattering from high pitch sounds. This is due to resonance. When external frequency matches the natural resonant frequency of an object, resonance occurs and amplitudes get higher and higher causing distortions strong enough to break the glass.

Every system has a resonant frequency and they can cause a lot of issues. Good and bad.

When dealing with jet engines, frequencies from jet engine systems can cause harm to structures If proper sound treatment is not present.

Why do objects in nature have resonant frequencies?
Common question and a good one too. Short answer, a lot of research is going on in this area, but some of what we do know is that objects in nature are subject to many forces, some explained and some unexplained that cause the atomic structure lattice to stretch and contract. When we drill down to the basics of resonance, it is a cycle of energy shifts back and forth- potential to kinetic, kinetic to potential, electromagnetic to heat, photoelectric to magnetic etc.

Any kind of energy shift that is cyclic in nature will cause the system or object to have a resonant frequency.
Objects in the universe are vibrating and that is what gives them their resonant frequency.


#78

There are a lot of implications for that in quantum physics, as well as metaphysics and philosophy. Many have probably seen videos like this before, but it’s a really interesting demonstration of how ‘ordered’ and beautiful the universe is.
.


#79

more added to o.t

Reflections and Room Modes

Room modes are the core of the acoustics and acoustic treatment of confined spaces. Think of a room like a music box, or a sound box of a guitar or a piano. A room is hollow and filled with air. Perfect environment for resonant behavior. It is true that each room is like a musical instrument with hidden resonant frequencies that live within.

I tried to keep it as simple as possible to explain this phenomenon without going into too much nerdy details like nodes and anti-nodes, trying a basic mathematical approach to explain.
(however, requires a bit of knowledge of basic geometry)

It is no secret that sound can bounce of walls, ceilings and structures. These are called reflections. Although they are not exactly reflections like light rays off a mirror but When we are dealing with sounds in the human
hearing range inside a room where wavelengths are somewhat around or within the boundaries of the room- we can treat their behavior like light rays (this is a whole another topic)

If we treat them like rays, we can then use geometry to analyze the reflections in basic cases. When a sound wave is reflected back from a wall or combination of walls - at a certain wavelength of sound they will exactly combine (superimpose) on top of each other to deliver a standing wave. Its called a standing wave because when 2 waves traveling in opposite directions meet, they come to a halt as in appear to stand still but still transfer energy back and forth.

These standing waves are static frequencies and their multiples that resonate inside a room are called “Room Modes”

Room modes are internal resonance of a room that exist when you turn on your loudspeakers. These are, static frequency hums inside that get stronger when a particular frequency or multiple of that frequency is played.

Example:

Lets say sound reflected back and forth between two parallel walls (axial mode)

If the room is 10 feet long, the round trip of sound back and forth is 20ft.
A sound with 20 feet wavelength will fit snug like a condom within that length, holding a nice load of standing wave at that frequency.

Now what sound has a wavelength of 20 feet? lets find out

1130/20 = 57 Hz (reminder: 1130 is the speed of sound)

this means when you excite the room with sound around 57hz, the room will start moaning and groaning around that frequency and multiples of that frequency (harmonics). Room modes can exist from 20 hz upto 300hz in complex cases. After that they still exist but they are somewhat of a less problem.

There are 3 types of room modes


Axial Mode:
The one we discussed above is axial between 2 parallel walls, it is the simplest of the cases and a 6th grader should be able to calculate those.

tangential mode:
In tangent mode, sound bounces around the 4 walls instead of 2. These are less intense than those with 2 walls back and forth. Normally around half as strong as axial modes of the same room. (how? is a discussion for another time)

In lets say a room, if sound bounces off 4 walls it would generate a standing wave with a wavelength equal to the length of the diagonal of the room in its basic case. (Thank you Pythagoras)

In a basic case, in a 9 by 12 room, sound will bank tangentially, forming a standing wave of wavelength 15 ft (Note: 9,12 and 15 are Pythagorean triples). So the resonant frequency of tangential mode would be around a wavelength of a wave that fits snug in 15 feet is 1130/15 = 75 Hz (approximately)
75 hz and its multiples is one of the resonant frequencies for a Tangential room mode of a 9x12 room.

Oblique mode:
This time sound bounces around 6 walls (4 walls and ceiling and floor)
This is the weakest of the 3 modes and less problematic. This case of a reflection is a bit more complex to compute geometrically because pressure at more oblique angles is a lot less than the other 2 cases, but in one of the basic cases sound will approximately form a standing wave at a fraction (around inverse of square root of 2, roughly about two thirds) of the distance of the “large diagonal” (for example, the distance between top left and bottom right corners of the room in its basic case)

  • Mr Pythagoras visits again <-

If your room is 10 feet by 12 feet by 8 feet tall, the large diagonal will be about 17 feet.
Sound will form one of the standing waves at around roughly two thirds of it as wavelength, so about 11 feet . The frequency at 11 feet is 1130/11 = 102Hz (approximately)

102 hz and its multiples will become the resonant frequency of the room in oblique mode.

Note:
Ofcourse as we think of other ways sound can reflect, there are many ways it can bounce of the 6 walls. Geometry gets extremely complicated. while, using geometry we can approximate for basic scenarios but they get complicated as we progress further and a more pressure driven approach is needed.

Using calculus ( partial differential equations) we can use a pressure or molecular displacement driven wave equation to calculate resonant frequencies in confined spaces easily. After years of caffeine driven calculus you will arrive at an equation we can all easily use. Thankfully some crazy dude Rayleigh did it for us so we never have to.

Rayleigh Room mode equation:

raleighroommode

The above equation is a bit more accurate than the traditional geometry methods, while both methods will yield results around the same frequencies.
Lx, Ly, Lz are room dimensions -
Lx = Length
Ly = Breadth! @FluteCafe
Lz=Height
l, m and n are integers normally within (0 to 4) that help calculate different room modes
l, m and n correspond to axes x, y and z respectively.

For basic cases:
axial use [l = 1 , m=0, n=0 ] or [ l=0, m=1, n=0 ] or [l =0, m=0,n=1 ]
tangential use [l = 1 , m=1, n=0 ] or [ l=0, m=1, n=1 ] or [l =1, m=0,n=1 ]
oblique use [l = 1 , m=1, n=1 ]
v is speed of sound 1130 ft/sec , result will be in Hz.

If someone is interested in how this formula is derived, I can shed more lights on the calculus and wave equations involved if you pay for coffee .

Thats all for now, will shed more light on importance of geometry of a room.
Next up, more on reflections, diffusion, materials and importance of geometry of the room

Anyway, that is all for now folks… will add more to it as time permits.


#80

You got off to a rough start, but its coming along! This is good stuff.

Cool suggestion here…if you’re wanting to eventually evolve this to a formal article and publish it here, I think it’d help some of the more visual readers if you could include some diagrams…man, even if they’re only hand drawn scribbled on a piece of notebook paper, if its something that can help communicate the content, its worth the space on Bryans server! :smiley:

…and funny but not over-the-top comments like this add a lot imo…again - cool stuff!


#81

aye, good suggestions. I will find a fine motivated day and draw out some stuff lol - no Idea why I started writing, I am terrible at it, usually keep stuff in my head - holster is an inspirational guy!


#82

If x is the width of the room, what is Lx?

Also, if you put stuff into monospace font (surrounded by `), it’s easier to distinguish letters out of context.

l=1, m=0, n=0
or
l=0, m=1, n=0
or
l-0, m=0, n=1