Basics of Acoustics Engineering (contd.)

                **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.

                **Chapter 1: Basics of Acoustic Treatment**

The field of music resonates with a common question:

What is the most ideal environment to listen and record sounds and music?
The truth, if you want purest sound, climb on top of the tallest tower in the largest field out there where there are miles of nothing. That is the best place to listen and record music, provided you can fight off the dragon. Yes, large open fields with miles of landscape, no obstacles, no parallel structures, no reflections, no distortions, no standing waves, no high frequency absorption; just plain pleasant and lively sound with a consistent frequency response.

So why do we not record or mix outside?
You can; however, rain, wind and flying birds above you might have other ideas. If you are lucky enough to live in a quiet deserted area with miles of nothing, you just might get away with it.

Why is acoustic treatment needed?
If you cannot listen to music or record music out in the open, you are stuck listening to it inside halls, theaters, opera houses, your favorite home theater or a basement. Acoustic treatment is mandatory for professional auditorium buildings, without it, the reverb of big spaces and resonant characteristics of rooms will make it difficult to decipher a single word of speech or a single musical note.

In hospital buildings, acoustic treatment matters a lot due to health risks caused by sound pressures that could easily reach over 30 Decibels and cause anxiety and heart flutters. Our bodies are made of water, and water resonates with the sounds around it. You can test it by playing music next to a cup full of water. You will start seeing ripples in the water. Poor acoustic treatment in hospitals can cause nervous breakdowns due to elevated anxiety levels caused by resonance, not to mention a hazard to patients with sensitive ear conditions.

In homes, I have seen people invest thousands in sound systems for their home theaters but not many bother to invest in proper acoustic treatment. A $5000 sound system will sound like a $100 sound system in an untreated room with poor acoustics. At that point, investing thousands in a sound system is a sheer waste. Most homes are not architecturally designed for listening to loud music. I have seen expensive sound systems in small rooms and large rooms alike. In small rooms, the room resonances from reflections off parallel walls kick in after awhile and the whole sound just becomes one large pool of mush. High power sound systems In larger rooms are tainted with reverb and flutter echoes, phasing issues, boundary interferences and imaging issues. The end result is a sheer atrocity on the ears.

So what is next best thing?
To find what is the best form of indoor studio environment, we must first understand what happens to sounds inside rooms. Sound bounces off walls, ceiling and floors and create what we call “reflections”. Some reflections are good, some are bad. The result of bad ones are called “Standing waves”. Standing waves happen when reflected waves collide and converge to create a wave that is relatively standing still . Standing waves from bass and low-end frequencies are especially notorious from an acoustic point of view . Other problems are natural low pass filters like carpet, thick curtains and clutter. Combination of all this is what creates that boomy, inaccurate, nasty closet-like sound that we all dread as audio engineers.

To fix these, we need to absorb the extra sound energy created and diffuse the rest to minimize the effect of standing waves and other sonic distortions. That is where acoustic treatment comes in.

The goal is to create and represent accurate sound.

Low-end absorption is a hard beast to tame, the lower the frequencies, the denser, more finely porous, thicker, and more expensive material you need. That is where most of the money is spent. As your room is treated properly, it gets closer and closer to accurate and lively sound.

How do we test when our room is accurate?
Most professionals train their ears to detect accurate sound, however, even the sharpest of ears can only detect with about 80% of accuracy. To go further, the best way to detect is to place real time microphones in areas of the room. Play broadband static with equal energy per octave (pink noise) and analyze the sound curve at the listening end. If your curve matches, voila, you have accurate sound, if not, find out which areas have unnatural boosts or dips. Once you find the problem frequency ranges, you can treat (absorb or disperse) them.

Size of the studio
It is often contemplated, how big is too big and how small is too small of a room to listen and record sound? Short answer, there is indeed a good compromised middle ground, where room size and room ratios meet to create a decent overall balanced acoustic environment. The long answer is that there is a lot of math involved!
Anyway, let’s start with a little high school fun!

speed of sound = (frequency of sound) x (wavelength of sound)

In order to be an effective studio, we must be able to handle low frequencies; let us take an example of sound at a frequency of 30 hz. By handle, I mean give “room to breathe” or to flow unrestricted where the dimensions of the room are larger than the wavelength of most frequencies in the human audible range. When room is large enough, it weakens the resident resonant frequencies of the room (more on this later).
Speed of sound is 1,130 ft per second, frequency is 30 hz, let’s do the math, yes cold hard math!

Wavelength = 1130/30 about 37 ft

In an ideal world, a bit more than 37 ft of room (from the sound source), is important to let the sound flow unrestricted inside a room for low frequencies around 30 hz. Which means an effective studio could, in an ideal case, have about 37 ft long unobstructed path from the direction of the sound source. Remember, this is not some magic number that is going to turn your studio into the Boston Symphony Hall; several other factors - not just room size are considered as well. This is to give you an idea that room size is important.

A larger room will have axial resonances (room modes, more on that later) that are lower in the spectrum and less problematic, they will also have lower intensity resonances that build up over time among other modes compared to smaller rooms. Larger rooms may, however, have some issues with reverberation.
Not everyone has the money to get a larger studio, most studios are 20 ft on average, which means sound waves of low frequencies will not have enough “room” to travel, they will double back, resonate at higher intensities and create unpleasant sound. Which is why smaller studios, need a lot more low- end absorption than larger studios.

For a 20 ft studio, low-end sound waves will be restricted by about 17 ft (that doesn’t mean add 17 ft long or deep panels; no). To make up for the missing distance, acoustic engineers have designed many things from attack walls to air gap acoustic panels to porous polymers each of which handle it in their own way. Which one is the best? Only your budget can decide as you can never have too much low-end absorption.

More on spaciousness
The need for proper room size in good acoustics and inverse square law:
Intensity of sound = Power / Area of sound dispersion
which means intensity of sound drops the further away from the source you are.
Let’s say there are 50,000 watt speakers, dispersion of sound depends on the area so when your radius is R, the area of sound at that point is the area of the sphere at that point. Suppose that it is a perfectly dispersed spherical sound, the area of the sphere at radius R would be:
4piR^2 (four pi R squared)
Intensity 1 = Power / 4piR^2
Now let’s say you have more room to breathe and want to double the radius (2R):
Intensity 2 = Power / 4pi(2R)^2 = ¼ (Power / 4piR^2)
=> Intensity 2 = 1/4 (intensity 1)

This means when you move twice as far, the intensity drops significantly (a fourth) - this is the inverse square law. This also affects reflections. The higher the intensity at the impact point the stronger the reflection. Which is even more reason why having enough room is important in acoustic environments.

Not saying smaller rooms cannot be treated, they just have to treated differently. As we develop further, importance of room ratios and geometry takes priority over sheer brute force sizing of space. There is a sweet spot in the room size and ratios that can serve a good enough purpose to be a good middle ground or starting point.

Download the full article here:
Basics of Acoustic Engineering IRD.pdf (497.7 KB)

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