FLINT - Original HT Acoustics Article from 2004

[Flint]

Introduction

I have created and contributed to many online forum threads discussing acoustic treatments in rooms. Very often others will ask, “Where do I place my absorbers in this room?” or, “If you only put up one absorber, where would it be placed?” The answers to those questions are not always easy to answer. Most of my advice comes from decades of experience and trial & error in studios, homes, theaters, and other rooms I have worked in, as well what I have learned from dozens of reference books and discussions with some the leading authorities on acoustics. Here is a short article discussing what I consider to be some important things to consider when treating a room.

Types of Acoustics Issues

Every acoustical environment is fraught issues caused by reflected and absorbed sound. There are issues caused by standing waves which generate nulls, or absence of sound, at certain bass & lower midrange frequencies and nodes, or increased output, at other bass & lower midrange frequencies. There are also issues with reflected sound in the midrange and treble frequencies causing the stereo imaging from the speakers to suffer. These higher frequency reflections also cause the sound to lose detail and seem distorted.

This article will discuss the mid to high frequency reflections that tend to hinder detail and stereo imaging.

LEDE

The LEDE room (Live End Dead End) is a well thought-out approach to treating a room for stereo reproduction. The concept is based on preventing all first order reflections from the front of the listener and diffusing all rear reflections. The most important reflections to treat in the midrange are the first order reflections that arrive at the listener’s ears less than 9mS, or so, after the initial sound from the speaker reaches those same ears. Any reflected sound that arrives in that initial 5mS to 9mS window will not be interpreted by the brain as an echo. Instead, it will be interpreted as part of the original signal and the listener will hear the audio as smeared, fat, lacking detail or fuzzy (see charts 1 & 2).

 

[Flint]

In the LEDE approach, the initial reflections are absorbed by wide band absorbers that cover the entire front of the room right up the point where the listener’s head is placed in that room (see diagrams 1 & 2). The area behind the listener is fully reflective, but diffusers are used to reduce the localization of the reflections and to reduce echo, replacing it with reverb. In diagrams 3 & 4, I show the primary reflections before and after treatment in a LEDE room.

 

[Flint]

I have designed the acoustics in dozens of rooms that take advantage of the LEDE principle and can attest to the absolute and stunning detail you hear in such a room. However, there are several drawbacks to LEDE rooms. For one, the expense of acquiring and installing the acoustic treatments which need to cover a very wide frequency range (200Hz to over 10kHz) is very prohibitive. For home users, dedicating an entire room to acoustic treatments can be very difficult to pull off. And, multi-channel audio can be difficult to make sound good in a LEDE room where the speakers in the rear of the room have a different acoustical environment that the front speakers.

So, a new way of treating a room is needed that can benefit from what was learned from the LEDE design concept yet is capable of being affordable, attractive, and practical for multi-channel audio.

 

[Flint]

Update to LEDE Concept:

First let’s address cost. By reducing the quantity of acoustic treatments and reducing the required effective bandwidth of the treatments, we can greatly reduce the total costs to treat a room. Since our ears are most sensitive to sound in the midrange, and since out brains use midrange information to localize sound (things like twigs snapping and rocks falling are what our brains instinctively locate when we hear them), using absorbers and diffusers that are most effective from about 800Hz to about 8kHz is sufficient. That could translate to standard pyramid or wedge acoustic foam at least 2 inches thick or products made from 1 inch thick Owens Corning 703 compressed fiberglass. Most commercial diffusers that are at least 4 inches thick will also be effective for this requirement. Now, instead of covering the entire front of the room with absorbers and the entire rear with diffusers, simply treating the primary points of reflection is likely to be all you need (see diagram 5).

Secondly, let’s look into the appearance of the treatments. Even though it conjures up images of Star Trek or high tech spy movies, I find that standard wedge or pyramid foam is not usually the most attractive product to use in a HT – especially if your significant other cares about appearances. There are some attractive foam products on the market which are flat or have a more design oriented pattern on them, and they can be effective both acoustically and fiscally. For a little more money, there are the more effective fiberglass products which offer a wide range of fabric coverings that should please most people. As for diffusers, there are fewer options, but some are attractive, though few are affordable. One company makes a diffuser that looks identical to their fiberglass absorber in that it is covered with fabric offered in several colors.

As for multi-channel audio, this can be the most challenging. Treating the first order reflections on the main Left & Right speakers should take priority since most serious listening takes place with 2-channel stereo music. However, a few strategically placed absorbers can improve the sound of the rear speakers for critical discreet multi-channel music recordings (see diagram 6).

 

[Flint]

Reality

Let’s look at an example of this new LEDE inspired philosophy in my own Home Theater, which is a dedicated room. The primary reflection points are treated with various forms of absorbers, primarily home made panels with Owens Corning 703 as the absorption material. I also used several 2 x 2 tiles of acoustic foam from Audiotec, USA, to remedy some key issues in the room. See diagrams 7 through 10 for references to how I addressed the reflection issues in my HT.

First off, I use acoustic absorbers (foam or fiberglass) to absorb the initial reflections off the walls to the side of my speakers. I then applied very large amounts of foam to the wall behind my speakers to absorb as much bandwidth as possible – down to 100Hz, or so. On the ceiling between the speakers and the listener, I hung a slanted reflector that reflects sound to the rear of the room over the head of the listener where it is diffused, absorbed or reflected. This addresses all of the primary reflection point for the front speakers (except the floor, which is not practical to treat).

Next, I applied diffusers on nearly all of the surfaces not already covered with absorbers. This virtually eliminates any echoes from the room and creates a nice smooth, diffuse decay for the reflected sound in the room.

The resulting sound is astonishing. My own DIY speakers sound better than every in this room with detail and resolution I have rarely heard from any speaker. Listening tests with studio monitors and even moderately priced common bookshelf speakers are just as amazing in this room.

Thus the LEDE concept is still alive and well, though modified for cost, appearance and appropriateness for surround sound.

 

[PaulyT]

Great info, thanks Flint! I notice you have a lot of diffusers on the side walls as well as the rear, is that just to make the most use of available space, since much of the back wall is taken up by the cabinets and door? That is, is the location of the diffusers particularly important or can they just be used to fill in space around - the obviously more critical placement of - the absorbers?

 

[DIYer]

Thanks for the effort, Flint. :handgestures-thumbup:

 

[Flint]

Great info, thanks Flint! I notice you have a lot of diffusers on the side walls as well as the rear, is that just to make the most use of available space, since much of the back wall is taken up by the cabinets and door? That is, is the location of the diffusers particularly important or can they just be used to fill in space around - the obviously more critical placement of - the absorbers?

All the diffusors are placed to address specific reflection issues. By placing them high in the rear my side & rear surround speakers sound better with their reflection points being diffused by them. The front high side diffusors address non-direct reflection points throughout the room and eliminate slap echo from the two very large side walls being perfectly parallel to each other.

Also, my cylindrical diffusors are also mid-bass absorbers with open backs and filled with fiberglass. This makes them both bass traps and reflection diffusors. With bass traps, more is always better.

 

[MatthewB]

Flint having had the pleasure of sitting in your acoustically treated room, your design is proof why when I make it rich, I'm hiring you to come to Casa de Mateo and sound proof my HT room.

For those who haven't had the pleasure of being in a room likes Flints, just imagine a room with no reverb, no echo and sound that seems to be flawless and right there with you.

Thanks again Flint for these articles.