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Introduction to Acoustics

 

Rives Audio
Unregistered guest
"The room is the first thing we start with, and the last thing we think about." - unknown

Welcome to the first in our series of articles on acoustics. We strongly believe that room acoustics are the fundamental of any good sound system. It is really astounding how a good an average playback system can sound in a stellar room, and conversely how badly a state of the art, mega dollar system can sound in a room with no positive acoustic values.

Throughout this series we hope to educate and illustrate some of the fundamental points of room acoustics. This is by no means meant to be an intensive course on acoustics, which by the way, for those that are interested, we recommend and have a list of educational and training opportunities on our website. However, this will give the reader some fundamentals and resources that may help in solving their own acoustical needs and problems. In this introductory article we would like to accomplish a few goals: First will be to discuss many of the facts and myths that exist regarding acoustics. The second will be to give a very brief definition and description of some of the acoustical terms you may have heard. Lastly, we will discuss our direction in this series of articles.

FACTS and MYTHS:

The world of audiophiles is full of snake oil and witch doctors mystical powers. I have heard some strange and extreme claims, yet I must admit, there are things that I have heard that I can not physically explain. However, being a physicist and being able to hear it and mathematically prove it is far more comforting. Having said this, it is important to note, that I do not dispel things that I have heard, yet do not fully understand. I still look for the root or source of why something works (or doesn't work, for that matter). I have learned that when you can explain a complex concept to a six year old, that is when you truly understand it yourself. With that, let's examine some facts and myths in acoustics.

1. "It sounds bright-it could be my transport."

Okay this could be fact at least 1% of the time. In truth, it could be anywhere in the signal path of the components and/or speakers and it illustrates the point that we can often hear the nature of the sonic degradation in our system but have absolutely no idea what the true root of the problem might be. With today's advanced state of technology I doubt that in most cases the CD transport itself would be the main cause for a bright room, yet this is a comment that I actually encountered and after discussing the situation, the owner was still convinced that most of all he needed to upgrade his transport.

The truth is that most equipment today is capable of performing nearly flat from 20khz down to whatever the low frequency roll off of the speakers might be. If someone hears a bright, boomy, bloated, or collapsed sound stage with few exceptions, the listening environment (the room) is to blame. However, the myth is, it must be the speakers, the amplifier, or some other piece of hardware - No, I know - it's the cables.

2. "Room acoustics are ugly"

Well, this certainly can be true. And honestly, I can't blame people from steering away from room acoustics the way some of the devices on the market look. At least the pre-amp and other components can be reasonably well hidden and not make the living room look like a studio. I think this is the biggest barrier to dealing objectively with acoustics, and if I had the perception that my living room had to look like foam central, well quite frankly I wouldn't be interested either.

This statement is a MYTH. Room acoustics can look like anything you want them to look like-particularly if new construction is involved. In fact, with new construction, the acoustical treatment can be designed and integrated into the room as part of the construction, thereby saving money and creating an aesthetically pleasing atmosphere. Properly designed rooms can incorporate absorption, diffusion, traps, and many other acoustical devices into the walls, ceilings, and even floors using conventional finish materials such as glass and mirror, fabrics, carpet and wood. Looking at the room, the invited guest would have no idea what went into it, except that it sound far better than anything they've ever experienced.

3. "This one acoustical device will bring your room into sonic perfection"

I actually received an e-mail stating this very thing. This is a myth. The fact is there are no magic bullets. After replying to the e-mail that this really didn't make sense and that, although there could be an improvement (maybe even a very significant one for an untreated room), "achieving sonic perfection" was not really possible. The response I received was "open your ears." which I guess implied "not your mind." As I said, I don't dispel those items which I can not explain, but this is a little beyond even that.

Room acoustics require a balance. Every room is different and every room needs different acoustical treatment to optimize it.

4. "Room acoustics are complex"

This is fact. They are extremely complex, but not beyond understanding. This is the other area that I believe creates a barrier to people entering into the jungle of acoustics. The worst thing anyone can do is ignore the issue. For a start, take some very basic measurements, listen, and try to correlate the two. You might be amazed at how it accelerates your path to good, or at the very least reasonable acoustics.

So many believe "it's the transport" or "the cables" and they go buy new ones, only to discover a new issue. If only the basics were right in the room-we're not talking about a studio engineered listening room-these changes would be so much more meaningful and in a proper context. It would be like insuring your spelling was correct, all the while the grammar is so bad no one is going to read it anyway.

There are two routes with room acoustics. You can take some time to learn (there's an incredible amount of information on the web and in a few really well written books), make modest changes along the way, and achieve the acoustical goals, or you can hire a professional to do the design and engineering for you. The first will take longer, but may be more rewarding for many. I've always believed that "audio nirvana" is a road, not a destination. I have enjoyed changes in my own system a little at a time. Not everyone is like that, or perhaps they want to enjoy the changes solely in equipment. In that case hiring an engineering group may make more sense.

Definitions:

Following are several basic definitions of things that we will likely discuss in more detail throughout this series of articles. Our articles will advance into more complex concepts, so we want to start off slowly and build a good foundation for understanding.

Ambient Sound: See 'indirect sound'

Diffuse Sound: This is non-correlated sound. Unlike reflected sound, diffuse sound no longer carries the coherency of a reflected sound as different wavelengths of sound exit with different angles of reflection. Thus the energy maintained in diffuse sound may be as much as reflected sound (not absorbed) but no longer carries the coherency of reflected sound.

Direct Sound: This is the sound that arrives at the listener directly from the speakers. As opposed to indirect, reflected, or ambient sound.

Frequency Response: In our context we will refer to the frequency response as the normal response for human hearing, namely 20 Hz to 20 kHz.

Indirect Sound: Indirect sound, ambient sound, and diffuse sound for our purposes have the same meaning. This is the sound that arrives at the listener by some path other than directly from the speakers. It can be reflected, diffused, or partially absorbed sound. In an anechoic chamber there is no indirect sound as it is designed to eliminate all reflections of sound.

Reflected Sound: This is sound that has struck at least one other surface and is for the most part neither absorbed or diffused.

Reverberant Sound: This is another form of indirect sound. Reverberant sound has struck more than one reflection point and maintains some energy and for the most part coherency, though at lower levels this may be difficult to detect.

Reverberation Time: This is the time it takes for a sound in a room to decay to inaudibility. Most commonly people use the term RT-60, meaning the time it takes for a direct sound to decay by 60 db. RT-30 and RT-90 are also used depending on the size of venue and purpose. The issues of RT measurements in small environments will be discussed in a later article.

Total Sound: This is the sum of both direct sound and indirect (reflected) sound.

Octave: An octave is used in music. It is a doubling of frequency. For example, from 20 Hz to 40 Hz is one octave. From 40 Hz to 80 Hz is on octave.

Pink Noise: Pink noise has equivalent energy per 1/3 octave bands. Thus on a linear frequency scale it appears there is less energy in the higher frequencies than the lower frequencies. Pink noise is commonly used for most test measurement situations.

White Noise: White noise has equivalent energy per frequency. Thus on a linear frequency scale it appears linear. White noise is not commonly used in acoustical measurements.

By: Richard Rives Bird and Christopher Huston of Rives Audio.

Join us in about 1 month for our next article on Frequency Response and RT-60 and how they affect our listening room and our listening. We would like to know what your questions are. We have a basic outline of what will transpire through this series, but we will likely change our topics to include questions and comments from our readers. Please post any topics you would like to see discussed.
 

Unregistered guest
You mentioned "a few really well written books." I was wondering if you might suggest some of them and provide their reference information.

I think that it is an excellent idea to approach the idea of room acoustics as you have done. Bravo!
 

New member
Username: Rives

Post Number: 1
Registered: 01-2004
These are all on the Rives Audio website. If you go to the links section and click on Books there is a list, and it will likely be growing in the near future.

http://www.rivesaudio.com/Links/Linkframes.html
 

Anonymous
 
I'm thinking about placing my floor standing speakers (mains), center channel and Sub inside a large entertainment unit that takes up most of my family room wall. They will each have their own separate compartment with the opening covered using speaker grill cloth. It will also enclose my 65" widescreen. Should I line the inside of each speaker compartment with foam or sound absorbing material? My room is fairly reflective now. Not much soft material to absorb sound.

Thanks!
 

Silver Member
Username: 420pimp2

Baltimore Atlantic Ci..., MD, NJ

Post Number: 967
Registered: Jan-06
bump
 

Bronze Member
Username: Nency

Post Number: 89
Registered: May-09
Acoustics are fundamentally important to learning environments. Learning is intrinsically linked with communication, and aural (sound) communication is acoustics. Similarly, learning is about concentration, and external noise is a major distracting factor in education. This article is about typical classroom environments, up to about 1,000 square feet. Large specialized rooms like auditoriums, gyms, and cafeterias needs careful acoustical engineering and should not be designed using the rules of thumb described below.

The importance of acoustics is not limited to classrooms. Noise in corridors and public spaces can soar if they are too reverberant (too much echo), with voices raised louder and louder to overcome the background echo, just like shouting conversations at a noisy cocktail party or restaurant. In addition, sound is an important navigational tool for people who are blind or low vision, and either end of the reverberation scale (too "live" or reverberant, or too "dead" or absorptive) can prevent them from finding their way.
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