Basics of Acoustic Engineering (Full Article)

acoustics
Basics of Acoustic Engineering (Full Article)
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#1
                **Introduction to Acoustic Engineering**

Greetings, my name is Andrew and I will be your guide as we cruise through the basics of Acoustic Engineering. My approach in explaining the topics will be in an informal question and answer format. My knowledge in Acoustics stems from my background in Heavy Electrical Engineering. Which means, aside from designing your quiet air conditioners, I can also design rocket engines, jet engines and power plants. Acoustics is a significant part of jet engine design, the noise of a jet engine is at hazard levels and if sound is not properly absorbed and diffused, it can self-resonate in jet cavities leading to catastrophic results similar to the infamous “Galloping Gertie” from Tacoma, Washington.

In electrical aeronautical engineering, we deal with sound dynamics of jet cavities and pressurized cabins. In civil engineering, acoustic engineering is most commonly used in design of concert halls, cinema halls, bridges, music studios and home theaters. For the sake of simplicity, I will approach the subject from a musical perspective instead of an engineering perspective.

To many, acoustics feel like some kind of magic that makes a studio sound good. It is that invisible technology that works for you night and day. Whether you are an engineer, audio enthusiast or a musician, understanding of basic principles of acoustics can make a difference at many levels. Please be advised, this is not a primer of what the best acoustic treatment is, instead it covers the basics of acoustics without going into cumbersome details of a myriad acoustic engineering products.

Interested in reading more? Download the full article here:
Basics of Acoustic Engineering IRD.pdf (497.7 KB)


Relevant animated illustrations (since PDFs cannot play gifs …)


Room Mode Animated Illustrations (links to these are also in the PDF)

Oblique Room Modes (with calculation)

Tangential Room Modes

Axial Room Modes

Speaker Boundary Interference Illustration


Basics of Acoustics (Part 3) Best Reverb Times!
Basics of Acoustics (Part 4 - Final Chapter) Quadratic Diffusion Explained & Simplified!
#2

Hey, that’s pretty damn good. Thanks for the write up, it is a behemoth of a post that makes me curl up in an insecure audio ball.


#3

its more of a statistical study than a scientific fact, I would quote the source but damn me… bottomline was in a blind test of accurate sound 4 out of 5 trained ears were able to distinguish accurate from altered. Truth is ears can go into a deeper dive of accurate sound, but its just very rare.


#4

I’m not sure I buy this explanation. Why would a sound wave being able to complete its cycle before reflecting off a wall be of any importance to the listener? If this were really the case, headphones wouldn’t work below 18kHz.

Can you give an explanation as to why “room to breathe” actually matters?


#5

I am starting to mix on phones more now .Im using Akg 701 phones with morhit then into goodhurtz can opener .Ive also just started using sounways too.Then when im nearly finished I check on monitors .I feel my mixes are definitely translating better to laptop and ear buds etc now


#6

“Room to breathe” is a generic term that relates to room size and room ratios with respect to certain wavelengths. Its a non-technical analogy. On the aspect of studios sizes, bigger is better yes but bigger is not always the solution as too big can still generate further complications.

(misunderstanding with boz on resonant boundary distortions vs room modes cleared up. We were talking about different things ) <<<<< topic cleanup

Standing waves and in-harmonics generated as a result of resonant boundary distortions alter the sonic character of the room. Resonant boundary collisions are not as predictable as non-resonant hard surface reflections (leading to room modes) but they can be understood to a certain degree with principles of in-harmonic or partial harmonic distortions. In reality it is hard to predict exactly what kind of distortion would be generated, that depends on type, material, physical properties and shape of the surfaces involved (a big topic for discussion in itself)

In headphones the levels are so low that internal reflections are less of an issue (for the materials). Most headphones already have absorbent materials and are treated to handle this issue.

(to add to this, I would like to clarify its not the levels as felt by the eardrum, this is relevant to absorbent material inside the headphones - live sound vs headphone sound - if you take a giant speaker the absorbing material inside wont be able to do much)


#7

Can you point to a reliable source that shows this? The acoustic chamber in headphones definitely plays a role in how they sound, but not as much at low frequencies as it does in the resonant frequencies.

What? No. Aside from the fact that acoustic reflections are largely linear with respect to SPL, the SPL inside of headphones is just as high as the SPL in a room.

The amplitude is a measure of sound pressure (or velocity, depending on whether you are measuring kinetic or potential energy), so of course higher pressure waves will reflect with higher reflections, but it’s still linear at sane listening levels.

Room acoustics are linear up until you get to the point where it’s either rattling the walls or nearing maximum possible SPL levels. I’m sure that rattling cases and distorted cones are very much a real thing in headphones when you get higher levels, but those are not acoustical issues.

It’s very trivial to confirm the fact that room acoustics are linear with respect to volume by running measurements at low and high levels. Aside from the higher noise at the low level measurements, the the readings are identical in every test I’ve ever done.

But you can’t design a room where all low frequencies meet a boundary at a velocity null. Whether it’s 1/4 wavelength or 1 1/4 wavelength to reach the boundary, some frequencies will hit the boundary at a null, and others wont.

It’s the idea of “breathing” that I don’t think has any relevance. I’ve heard people say it, but I’ve never heard anyone give any evidence that it has any effect.

Can you explain this some more? Of course a louder signal will create more heat when absorbed. It has more energy.

I’m not saying small rooms aren’t a lot harder to treat than large rooms, I just don’t think there’s any basis for blaming it on lack of breathing room


#8

FYI, I’ve changed a lot of what I said in the post above, so sorry for making a moving target.


#9

it is :slight_smile: that is the underlying basis of all acoustic treatments. Let me try explaining a bit more mathematically, you write plugins, youre mathematical and might be able to relate better. Then, I wont really need to quote the sources, these are well proven mechanics.

Intensity of sound = Power / Area of sound dispersion

which means intensity of sound drops the further away from the source you are. (which is why we yell when we are far)

anyhow, lets say there are 50000 watt speakers and sound is radial and depends on the area, so when your radius is lets say R the area of sound at that point is area of the sphere or the cone at that point. Let us say that it is a perfectly dispersed spherical sound, the area of the sphere at radius R would be 4 pi R^2 (four pi R squared)

Intensity1 = Power / 4piR^2

now lets say you have more room to breathe and moved to double the radius (2R)
Intensity2 = Power / 4pi(2R)^2 = Power / 4piR^2*4

=> Intensity2 = 1/4 (intensity1)
meaning when you move further the intensity drops significantly (by a quarter). This also affects reflections. The higher the intensity at the impact point the stronger the reflection. Which is why room to breathe is important in acoustic treatments. Not saying smaller rooms cannot be treated, they just have to treated differently.


#10

But this is a separate issue. Everything you are talking about here is frequency independent. I understand how energy dissipation works and how a larger room will have an effect on that. Initially you said that the room to breathe idea was specifically to help low frequencies.


#11

Curious to see what @Ethan_Winer’s take is on this.


#12

Just to be clear, the only part of your initial post that I don’t agree with is this:

I don’t think there’s any reality to that sentence. I think the ability for a wave to make a full cycle before hitting a boundary is not of any significance.

Yes, smaller rooms are harder to treat, especially in the low end. Low frequencies have a lot more energy, and are therefore a lot harder to absorb. You need lots of treatment to absorb low frequencies, and it’s hard to fit that much absorption into a small room. But it doesn’t have anything to do with breathing room.


#13

“Room to breathe” is a generic term that relates to room size and room ratios with respect to certain wavelengths. Its a non-technical analogy. On the aspect of studios sizes, bigger is better yes but bigger is not always the solution as too big can still generate further complications.

(misunderstanding with boz on resonant boundary distortions vs room modes cleared up. We were talking about different things ) <<<<< topic cleanup

Standing waves and in-harmonics generated as a result of resonant boundary distortions alter the sonic character of the room. Resonant boundary collisions are not as predictable as non-resonant hard surface reflections (leading to room modes) but they can be understood to a certain degree with principles of in-harmonic or partial harmonic distortions. In reality it is hard to predict exactly what kind of distortion would be generated, that depends on type, material, physical properties and shape of the surfaces involved (a big topic for discussion in itself)


#14

The reflections at a 6 foot wall would be very problematic, because it will cause the 100Hz wave to resonate. Far worse than the 9 foot wall. Same with the 12 foot wall. Those will both resonate and cause massive issues. Same with an 18 foot wall.

This is where I think your explanation is off. A 100Hz tone will resonate worse in the 12 foot room than the 9 foot room, and it’s not because the wave is hitting the wall with high impact. It’s because the initial wave and the reflecting wave create a nice velocity null at the boundary at all times.


#15

(misunderstanding with boz on resonant boundary distortions vs room modes cleared up. We were talking about different things ) <<<<< topic cleanup


#16

(misunderstanding with boz on resonant boundary distortions vs room modes cleared up. We were talking about different things ) <<<<< topic cleanup


#17

?? I have a pretty good understanding of wave mechanics. Far more than 2 days worth of lectures. But I’d be happy if you pointed me in the direction of any source that shows that a fully formed cycle causes any sort of beneficial properties when it comes to hearing and absorbing them.

We’ve already agreed that the perfectly fully formed wave is a worst case scenario for resonance. A 100Hz wave will resonate in a 12 foot room. So that fully formed wave isn’t benefiting any. Same with the room that is 1.5x the wavelength. Again, fully formed, terribly hard to deal with. So what exactly is the benefit of the fully formed wave cycle?


#18

intensity of a wave increases with amplitude and frequency

when a wave collides with resonant boundaries, higher frequency harmonics (odd harmonics or partial harmonics) could be generated that pool up together and create higher intensity standing waves and also create unpleasant sonic character in the room.

here is a diagram where waves collides a resonant boundary at maximum amplitude temporarily generating partial harmonics, these do even out over time however they can have a sharp effect for a few milliseconds or seconds.

Resonant boundary collisions are not as predictable as non-resonant hard surface reflections but they can be understood to a certain degree with principles of inharmonic distortions where tones been generated could be partial tones as a result of partial harmonics and odd harmonic. In reality it is hard to predict exactly what kind of distortion would be generated, that depends on type and shape of the surfaces involved.


#20

fuck
Here’s a two pack, have at it.


#21

oh my. I don’t even know what to say to this. Nothing in this entire post is correct. I guess the first sentence is half correct?

There is no distortion being caused by a wave reflecting off of a wall. The wave isn’t being cut off, it’s being reflected. Unless the wall is rattling, there are no overtones being created at all. You’re taking the concept of clipping and applying it to reflections, and the two have nothing to do with each other.

And those images you showed here do not show what room modes look like anyway. You are showing an opened end and a closed end. Rooms are two closed ends. In any room with standing waves, the velocity at the wall is 0, and the pressure is changing. So you have a pressure node at each end, and velocity antinodes at each end. Closed ends resonate at half wavelengths, not 1/4 wavelengths. That’s why an open ended flute is an octave lower than a closed ended flute.