Loudspeaker Basics 101 - Part 3 - From Vinyl To Plastic

Loudspeaker Basics 101 – Part 3

In Loudspeaker Basics 101 part 3, this final post in the series, we examine where your stereo/satellite speakers should be placed, and in particular the optimum locations for your sub-woofers.

Introduction

Life was so much easier years ago when we just had monophonic reproduction that required just one speaker cabinet that barely needed to reproduce any frequencies beyond 40Hz – 15KHz. Then came stereo, multi-channel surround sound and now we have immersive audio. Not to mention as many as four subs or more! On top of all that modern recordings and movie sound tracks require significantly increased frequency bandwidths and powers to create those heart pounding visceral moments.

So how many speakers is enough and why do we need so many, that’s the question? Where should they all be positioned in order to produce the optimal sound field experience?

Lets start with good old stereo, still the major format for most music, and audiophiles like myself.

Stereo Speakers

Stereo is still the most common and popular speaker setup. Most CD’s (plastic) and all vinyl only contains a left and right channel. These two channels are more than sufficient to create the 3D aural illusion of instrument placement between and outside of the speakers and in-front of and behind them. Some complex phasing processing during recording can even produce very convincing left and right surround channels – listen to Paul Abdul-Spellbound on the Virgin label.

Good stereo is all about symmetry, both electrical and acoustical. Good room acoustical treatment is still a requirement to get great stereo sound and imaging, no matter how expensive your stereo system. In fact even low cost stereo systems can sound outstanding in a well acoustically treated room.

This isn’t the post to discuss room acoustics but the diagram below shows the simple basic principles of a good stereo setup. The speakers and the MLP should create an equilateral triangle. Minimize all first reflections, position speakers at least 2-3 feet in from the front and side walls, don’t sit too close to your rear wall or place lots of deep absorption or broad band diffusion on it, adding as much bass absorption as your partner will allow!

Make sure that your ears are sitting on the acoustical axis of each speaker. That may mean “toeing” them in and up to point directly at your ears. Consult your speaker manufacture on how much toe in maybe required as the speakers design will affect this. The degree of toe in also has a profound effect on stereo imaging.

Finally isolate your speakers from their supports/floor/shelf in order to prevent early early sound – see part 2 or here.

Multi-Channel Speakers & Surround Sound

DVD movies brought us into the world of multi-channel (digital) audio despite several failed attempts in the 50’s, 60’s and 70’s to introduce multi-channel sound via various analog media formats to include radio and tape and quadraphonic sound on vinyl. That introduction relied on techniques and technology that just weren’t ready for prime time, so eventually the formats were withdrawn from the market as new digital standards took over the multi-channel world.

The number of discrete fixed channels has continued to grow over the years and is now up to seven satellite and one sub (7.1). This number of channels can contain data that can be used to create both height and extra width channels for a maximum total of 11.1 dedicated channels. The data for these extra four channels may be encoded into the 7 channels or derived from them by using sophisticated algorithms within your receivers DSP processing engine. With this number of fixed channels the placement of the speakers needs to reflect that of the original mix environement in order to enjoy an aural experience that reflects what the mix engineer and director were trying to achieve.

Surround Formats

Typical Speaker Layout

Abbreviations:

LF/RF – left & right front
C – center
LFH/RFH – left & right front height
LFW/RFW – left & right front wide
LSS/RSS – left and right side surrounds
LBS/RBS – left & right back or rear surrounds
SUB 1/2 – sub woofers (as many as you can get away with)

The above speaker configuration will provide full coverage for all none immersive audio and music formats. Personally I am not a great fan of width channels, nor do I have the space to support them, so I use that processing to provide the front height channels instead.

Surround speaker configurations:

5.1 = 1+2+5+7

7.1 = 1+2+5+6+7

9.1 = 1+2+3+5+7 or 1+2+4+5+7

11.1 = 1+2+3+4+5+6+7

The good news is that this speaker arrangement can provide a good starting point for moving to immersive sound. What is immersive sound I hear you say? Read on.

Immersive Audio

So the movie makers (and manufacturers) decided that 11 channels weren’t good enough to immerse you in the sound field, ONLY surround you in one; enter immersive audio. This technology that is available from Dolby, DTS and Auro tries to put you inside a sphere of sound rather than just surround you in two dimensions. It achieves this in two ways:

With additional channels both at ceiling height and above in the ceiling and
The channel information is not dedicated to a given channel but is an “object” that according to metadata contained in the audio bit stream can be panned between speakers by your receivers audio processing.

The above techniques in conjunction with additional speakers can create quite impressive and realistic special and real world sound effects.

Immersive formats

Dolby Atmos
DTS-X – compatible with the basic 7.1 speaker setup with the addition of front height speakers (9.1) and width speakers (11.1), and is claimed to support any channel layout.
Auro-3D (or here for the pdf) or here.

(Clicking on the above formats will take you to the appropriate manufacturers information that discusses the various speaker placement options.)

The original and more popular Dolby Atmos can support up to 34 speakers in a residential environment with a maximum of 128 audio ‘objects’. The current maximum number of audio channels supported by a high end processor is 64 (Trinnov Altitude), for home theater receivers it is 13 plus 1 sub-woofer channel (13.1). Some receivers like the Denon AVR-X8500H and audio processors like the Marantz AV8805 provide two additional selectable outputs (15.1) to support the selection of speakers in different locations for the alternative immersive sound formats.

DTS-X will support up to 11.2 and 32 channels.

Auro-3D will support up to 13.1 and 64 channels.

So what do all those numbers mean for those immersive sound channel configurations A dot B dot C? Here is a quick run down:

A= the number of conventional surround channels.
B= the number of sub-woofer channels.
C= the number of overhead or immersive enabled channels.

The more common available channel combinations for home use are:

5.1.2 or 5.1.4
7.1.2 or 7.1.4
9.1.2 or 9.1.4
11.1.2 or 11.1.4

So 5.1.2 would indicate that you are using 5 conventional surround channels, one sub channel and two overhead/immersive channels. DTS-X can support two sub-channels and would be indicated this 5.2.2. As of today no media supports two independent sub channels. Processors and receivers that provide two or more sub outputs do so to help provide better bass coverage for a room. They are NOT independent sub channels. I.E. 5.2 would indicate that you have two subs in the room and 5.4 would indicate that you have 4 subs in the room.

The following drawing shows what would fully support all three immersive techniques. Given a short room up to four of the top ceiling speakers would be removed. Even the width speakers in narrower rooms are not necessary in order to maintain a good immersive sound field.

Typical Speaker Layouts

Abbreviations:

CH – center ceiling height
LSH/RSH – left & right surround height
LRH/RRH – left & right rear height
LFT/RFT – left & right front top
LMT/RMT – left & right middle top
‘VOG’ – ‘Voice Of God’ for Auro 3D only
LRT/RRT – left & right rear top

The biggest issue is that the three manufacturers listed above do not conform to exactly the same speaker placements so if you want to support all three some degree of optimization will be required.

The layouts shown above represent the ideal speaker placements to support all formats in large room. However, all three techniques have several speaker arrangements based upon your room size, speaker type, available channels and receivers processing ability. Please refer to the provided links above.

Most immersive enabled receivers will process the immersive audio to provide the best effect with the speakers that have been assigned to it. This means that ALL the speakers shown are not required in order to support all three formats. Some very high end audio processors like Trinnov & Datasat, once they have been told what speakers are available and where they are placed will optimize the placement of the audio objects in order to generate the ‘best’ immersive audio experience for each format.

Remember that ALL immersive formats contain a core set of channels that are compatible with the standard 5.1, 7.1, 9.1 and 11.1 surround speaker layouts and most modern immersive enabled home theater receivers and processors will decode all three formats to drive what speakers you have assigned.

Sub-Woofers

The location of your sub-woofer(s) is very important. At frequencies below about 300Hz rooms start to generate noticeable standing waves. These static sound pressure levels give rise to boominess, bass coloration and tremendous variations in bass level as you move around the room. The idea of placing the sub is to minimize the creation of these standing waves or to try to cancel them.

So how many is enough? One will work, two is good but four is much better. Be warned DO NOT mix sub woofers of different frequency response, size, power handling or from different manufactures or types; sealed or ported. Most equalization systems cannot deal with, or do not optimize well, mixes of different sub models. These equalization systems are also designed to not overdrive the ‘poorest’ of the subs so the better subs power and extended frequency response will generally not be available to the user. Keep them all matched or you will regret it; see my post here.

So how do we position subs as most of us only have one. Typically it gets put in one of the room corners but if you wish to optimize its position use the crawl method to determine its optimum location. What was the crawl method I hear you say?

In short, place the sub where you sit, drive it with some bass heavy music that you know well, and crawl around the floor until you hear the most extended, cleanest and tightest bass. That is where you put your single sub….hopefully!

For many, two subs will provide an optimal bass experience in most rooms, even large ones. The advantage of four subs is that beside providing lower distortion, as they are driven less for a given sound pressure level, they will provide a more uniform spread of the bass throughout the room and make equalization an easier process. This is more important in a large home theater with many seats.

Examples of some of the preferred sub-woofer locations

While the above sub placements will generally provide the most even and cleanest bass distribution for the room, each room is different, especially if it is not symmetrical and it depends upon the amount of bass absorption it contains. So experimentation is always the name of the game.

From Vinyl To Plastic

One man's quest for audio perfection

One man's quest for audio perfection

Loudspeaker Basics 101 – Part 3