The Science of Audio - a series of lectures by Floyd E. Toole, Ph.D. Vice President Acoustical Engineering
Harman International Industries, Inc.
8500 Balboa Boulevard, Northridge, CA 91329

818 895 5761

ftoole@harman.com

harman/kardon
Loudspeakers andRooms forMultichannel Audio
Reproduction
byFloyd E. Toole
Vice President Acoustical Engineering
Harman International

Part 1 – How many loudspeakers? Whatkind? Where do we put them?
Here we look at the basic theory ofmultichannel audio systems, leading us tounderstand why certain loudspeaker designs
and room arrangements work better thanothers.
67 years ago, in1934:
Bell Telephone Laboratories research concluded that
there were only two ways to properly reproduce
a realistic sense of direction and space:
1. Binaural – dummy head recordings reproduced
through headphones. O.K. but not commercial.
2. Multiple channels –• How many? More are better!
• A realistic minimum? Three across the front toreproduce only the soundstage, not ambiance.

So, how many did we get?
3

In the beginning, there was monophonic – single channel – sound.
Compared to nothing, it was impressive, but eventually serious
people began to wonder what more might be possible. Bell Labs, oneof the world’s greatest research institutions, looked into what was
required for accurate directional reproduction, and concluded that, forloudspeaker reproduction, multiple channels were needed. While
many channels were desirable, they were not very practical, certainly
not at that time, so they looked into a practical minimum number.
They came up with three front channels, just to reproduce thesoundstage – no ambiance, no sense of spaciousness. For a single
listener, they thought that two channels would be sufficient.
So, in the 1950’s we got two channels.

Multi-channel Sound - 1st Try
TWO CHANNEL STEREO*Better than mono
*Antisocial - sweet “line”*Poor sense of ambianceor envelopment

31 January, 2002

The reason was that there were no practical methods, at the time, toget more than two channels into and out of the groove of an LP
record.
We have put up with this limitation for over 50 years, and it is time tomove on to better things. The idea that we only have two ears, andtherefore need only two channels, applies only to headphone
listening. In normal hearing our two ears and brain give us aremarkable three-dimensional sense of direction and space. To
reproduce this, we need many channels.
1

The Science of Audio - a series of lectures by Floyd E. Toole, Ph.D. Vice President Acoustical Engineering
Harman International Industries, Inc.
8500 Balboa Boulevard, Northridge, CA 91329

Multi-channel Sound - 2nd Try
QUADRAPHONICS*More directional effects
are possible*Better sense of ambianceor envelopment*REALLY Antisocial –Sweet SPOT!!

Dolby Stereo for Cinemas

ftoole@harman.com

In the seventies, there was an attempt to do better. It failed because
of industry disagreements over which of several competing systems
should become the standard. Frankly, I am glad it failed, because it
was the wrong arrangement of channels. With no center channel, theannoying stereo sweet spot remained. Most of the systems had a lotof crosstalk, or leakage among the channels, so that even the frontback impressions depended on the listener being half way between
the front and back speakers. With no side channels, the sense of
amb ience and spaciousness was less than it could be.
A lot of paraphernalia for another antisocial system.
The multichannel matrix technology underlying quadraphonics wasquite clever, and Dolby seized on it for Cinema applications. They
revamped the active matrix system into four new channels: a left,
center and right front array, and a single surround channel that wassent to several speakers distributed down the sides and across theback of a cinema. They included features that allowed the system tofunction with the imperfections that pervaded optical sound tracks –then the basis of the film industry. One of the compromises was asevere limitation in the high frequency response of the surroundchannels.

Multi-channel Sound - 3rd Try
Dolby ProLogic - for homes* Social ! It can be shared.
*A real center image*Only fair ambiance andenvelopment (mono surround)*Surrounds not broadband*Directional information only
across the front

Multichannel Sound - 3rd Try
Dolby ProLogic - for homes
And the common-channel
subwoofer arrives on thescene . . . and stays!

8

Embellishments to ProLogic
Home THX*low-pass and high-pass filters for
sub and satellites.
*decorrelation of surround channel
speakers*”timbre matching” of surroundspeakers – a dubious feature!
*re-equalization (treble
attenuation) of sound tracks.
9

31 January, 2002

818 895 5761

Naturally, the system migrated to the home, with the simplificationthat the surround channel was sent to a pair of speakers located at thesides of the listening area.
One of the most common mistakes is to locate the surround speakers
behind the listeners, at the back of the room. The perception of
spaciousness, or surrounding ambiance, is greatest when the sounds at
the two ears are uncorrelated (different from each other). This ismost effectively achieved when sounds arrive from the sides. Thisputs you in the hall, not between the band and the hall.
Recognizing that customers would react badly to being told that theyneeded five big full-range speakers, a subwoofer channel (the .1 in5.1) was added.
However, in the beginning, no provision was made for a proper
crossover between the subwoofer and the five “satellites”. As aresult, there were many unhappy experiences in trying to make agood sounding acoustical transition between satellites and subwoofersthat were not necessarily designed for each other. Because of this,many people thought that subwoofers were inherently less good thanfull-range systems. They were often right.
Recognizing that cinemas were not always doing justice to theirambitious sound tracks, Lucasfilm set out to standardize soundquality in cinemas with their THX program. Then, sensing thatthings were not always right in the consumer world, they created aHome THX program, licensing a number of features and functions to
manufacturers of surround processors. The loudspeaker crossoverfeature was welcomed, as was the electronic decorrelation of thesurround channel, making the single channel sound less“monophonic”. The timbre matching of surrounds was inappropriate,
and the re-equalization was not always what was needed, but thesefunctions could usually be switched off in the processors.
2The Science of Audio - a series of lectures by Floyd E. Toole, Ph.D. Vice President Acoustical Engineering
Harman International Industries, Inc.
8500 Balboa Boulevard, Northridge, CA 91329

Embellishments to ProLogic
Home THX*vertical directivity control ofloudspeakers - dubious*”dipole” surround speakers -10Beyond 5.1:
Logic -7 (Lexicon, Harman/ Kardon), Dolby ProLogic II*FIVE or SEVEN Steered
channels*Directional informationin all channels*Superb ambience andenvelopment*Great for MOVIES & MUSIC

Beyond 5.1:
Logic -7 (Lexicon, Harman/ Kardon), Dolby ProLogic II*Sounds good overmost of the room. You
can look out thewindow!

12

2 channels in / 5 or 7 channels out• INPUTS: any stereo source– CD’s, tapes, LP’s
– Television, VCR, Laserdisc, DVD– Stereo radio or satellite/cable audio channels– Games• OUTPUT:– 5.1 or 7.1 multichannel audio
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31 January, 2002

818 895 5761

ftoole@harman.com

To add differentiation in loudspeakers, THX promoted the idea ofrestricted vertical directivity. In my opinion, if one wishes to restrict
directivity, it should be done in both vertical and horizontal
directions. Directional control is very difficult to achieve with coneand dome drivers, and still retain high sound quality, but it is easily
done with horns and waveguides. Some basic performance standards
were also a part of the THX program.
To help two monophonic loudspeakers to sound like more, the ideaof “dipole”(bi-directional out-of-phase) surrounds was introduced. It
helps in many situations. And, so do other multidirectional designs
that spray sound in many directions.
As time passed, the limitations of four-channel ProLogic became
more evident. It was, after all, designed when optical sound tracks infilms were the norm, and several features catered to problems withthat signal source. A very successful effort to improve reproduction
from film sound tracks was Logic 7, a sophisticated digital active
matrix system that translated two channels into five or seven. In
addition to all-channel steering, all channels were full bandwidth.
Designed by Lexicon with a special sensitivity to the needs of music,
Logic 7 has modes that tastefully extract and reproduce ambiencefrom stereo recordings. Some of the modes are extensively user
adjustable to accommodate personal tastes or system peculiarities. In
my personal experience, the majority of stereo recordings benefit
from a well-done multichannel conversion.
Recently, Dolby has introduced an upgraded ProLogic II with someof these features.
The traditional 5.1 channel arrangement evolved from movie theaters,
in which everyone is regimented into rows, facing the screen, in thedark. While there are occasions to do this in homes, the reality is thatmost of our entertainment does not require such restrictions. For
more casual video entertainment and for music, we may have somelight in the room, open the blinds to see the garden, and face awayfrom the screen. With only two loudspeakers at the sides, thesurround illusions suffer and one may be aware of the limited numberof channels. With four surround channels, appropriately processed,
the sense of space can be remarkably seamless for listeners seatedover much of the listening area. And, because there are now four
speakers, there is less of a need for them to be multidirectional. In
very small rooms, multidirectional speakers may still be less easy tolocalize, but in larger rooms that problem disappears and we canmake all channels absolutely equal in sound quality, something that is
important with multichannel music recordings.
The entertainment business does itself a disservice by overemphasizing the “home theater” theme. We all watch movies, to be
sure, but we spend far more time watching TV, listening to FM radio,
CD’s, satellite/cable music channels, etc.
We need to acknowledge the idea that a multichannel audio systembrings a higher level of enjoyment from ALL sources of sound. A
high quality two-channel to multichannel conversion system makes
this possible.
The often gimmicky and mostly bad sounding “hall”, “stadium” and“club” reverberation devices are not what I am talking about. Thesecan be fun, but a tasteful multichannel conversion relies on ambience
extraction, digging out the spatial information in the originalrecording, not adding synthetic reverb that may not be appropriate.
3

The Science of Audio - a series of lectures by Floyd E. Toole, Ph.D. Vice President Acoustical Engineering
Harman International Industries, Inc.
8500 Balboa Boulevard, Northridge, CA 91329

Multichannel Sound - 4th Try
5.1 channel Digital Discrete
Dolby AC-3
MPEG2
DTS
SACD
Audio DVDetc.

MOVIESMUSIC

Multichannel Sound - 4th Try
5.1 channel Digital Discrete*Social - group listening*A real center image for stability
and better sound quality*A good sense of ambiance andenvelopment as required

818 895 5761

ftoole@harman.com

Eventually it had to happen, and it has. Technology caught up with
our desires to have a fully discrete multichannel system, with allchannels equal and independent. Now that it has, because it is digital,
and therefore flexible, we have several systems. Not all of them needto survive, but at least all of them sound very good. SACD and DVD
Audio, in certain of their modes with very high sampling rates anddynamic ranges, can be better than any human listener needs them tobe. These excesses waste bandwidth, but it can certainly be said, withassurance, that the delivery medium is not a restriction to good sound.
The recorded music itself is as variable as ever, so, don’t shoot themessenger if you don’t like the message.

The industry is still learning. The medium is in good shape, it is themessages that are mixed.
Still, if five channels are good, more would be better.

*FIVE discrete channels*Directional informationin all channels*All channels broadband
and equal power
15

Beyond 5.1 Digital Discrete
Dolby/THX EX, DTS ES, DTS 6.1 ES*A single center rear,or a pair of side rear,
Loudspeakers for a matrixderived 6th channel or, in DTS
6.1 ES, a discrete 6th channel.

Resurrecting a feature of ProLogic and using it to extract a center rearchannel from the surround channels of a Digital Discrete 5.1 mix is alogical embellishment. The channel can be reproduced from a single
center speaker or a pair at the rear sides.

Beyond 5.1 Digital Discrete
Logic-7 – Lexicon, Harman/Kardon

As always, if one is good, two are better. Logic 7 retains
compatibility with the derived center idea, but improves on the senseof surround by adding some separation between the rear channels.

*SEVEN Steered channels*Directional informationin all channels*All channels broadband* Great for movies* Great for MUSIC

Get a life!
Logic-7 – Lexicon, Harman/Kardon

Again, one can break away from the rigid cinema -seating format, andhave a chance to visually share a dramatic or comedic moment withyour fellow viewers. When listening to music, one can swivel the“lazy -person” chair around and look at the garden view, without
sacrificing the sound illusions.

Great for MUSIC
And, guess what?
You don’t have to look at a
blank screen!

18

31 January, 2002

4

The Science of Audio - a series of lectures by Floyd E. Toole, Ph.D. Vice President Acoustical Engineering
Harman International Industries, Inc.
8500 Balboa Boulevard, Northridge, CA 91329

818 895 5761

ftoole@harman.com

Always remember –We are selling more than Home
Theater.
We are selling ENTERTAINMENT
19

Multichannel Audio Entertainment• Movies• Multichannel music recordings• Stereo music converted to Multichannel• Television converted to Multichannel• Games

20

The challenge is to get people into situations where they can heargood demonstrations of multichannel entertainment, using goodequipment. The entry-level in-wall/in-ceiling systems, and systems
with inadequate tiny surround speakers can be impressive if one iseasily impressed. However, those of us in the business need to show
customers that much better sound is possible, and sometimes for only
modest cost increases. Good in-wall speakers exist, but they are notinexpensive. Ceiling speakers are for background music systems instores (I exaggerate, but only slightly). Ceiling speakers should never
be used for the front channels, and used for surround channels only inhomes that I am never invited to.

But, how were they recorded?
What are we trying to reproduce?

21

Film Sound Mixing/Dubbing Stage

22

Some folks make a big deal about replicating the cinema experiencein the home. For me, I want better than that. First, I want quieter
surroundings, better sound quality, my own volume control, theability to pause the experience, a fridge and a bar for nibbles anddrinks, and the ability to choose my company.
That said, the rest is easy - no big deal. Buy a popcorn machine.
Film sound tracks are put together in a dubbing stage – a scaled down
version of a cinema. The rooms are usually larger than we typicallyhave at home, but otherwise there is nothing difficult about imitating
the experience. Multiple or multidirectional surround speakers would
be appropriate, and they would be located high on the walls.
For music the rules change.

Music Mixing – the “standard”
A nice theory, but how
many will do it?

We are still in the early stages of multichannel music, and theindustry is still figuring out what to do. One standard has surfaced,suggesting this arrangement of speakers. It is a reasonable approach.
All five speakers are intended to be identical. Some hard liners insist
that all five be full range – not a good idea, as we will see later. They
would all be at ear level.

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31 January, 2002

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The Science of Audio - a series of lectures by Floyd E. Toole, Ph.D. Vice President Acoustical Engineering
Harman International Industries, Inc.
8500 Balboa Boulevard, Northridge, CA 91329

Music Mixing – “retro” surroundA hangover from stereo“sweet- spot” listening

24

Music Mixing – Quad lives!
Back is behind,
Envelopment suffers

25

The only agreement is about the
LCR arrangement.

818 895 5761

ftoole@harman.com

Old habits die hard, and entrenched stereo music engineers find it
difficult to work with a center channel. So several of them simply
don’t use it, or use it very sparingly. The sweet spot is back.
They invent “reasons” like: the customer can turn off all of the otherchannels and then can hear “the talent” naked, without the backing.
Another is that customers might buy small cheap center channel
speakers, thus degrading “the talent”.
Since 80% or more of what we hear in a movie is from the centerchannel, it seems to me that film people are the ones who should beconcerned. In reality there is little to be concerned about. This is aregrettable practice that, I hope, will cease.
In looking for material to remix or “repurpose” into a mu ltichannel
format, it is natural that old master tapes from the quadraphonic era
should find favor. Along with the music have come some old habits,
namely placing the surround speakers towards the rear of the room,
mirroring the fronts. If this is done for the engineering of the album,
customers would have to move their surround speakers to the back of
the room in order to hear what was mixed in their specific album.
That is not going to happen, and this is a bad idea.
Repurpose the recording engineer!

THX introduced the idea of dipole speakers to surround systems, butthere are many other ways to spray sound around a room. Actually,
the THX speakers are not real dipoles, since the front and rear
speakers are separated in space. Consequently they do not produce a
sharp null in the direction of the audience, something that probably
would be a disadvantage in any event.

The Surrounds are up for grabs.
No one solution will work
perfectly for all programs!
26

Surround Speaker Options• Conventional forward -facing ‘monopoles’• ‘dipoles’
Multidirectional radiators• ‘Bipoles’
attempting to spray moresound away from the• ‘Tripoles’
listeners than at them.
• ‘Quadrapoles’• Multiples of the above (more than one on eachside wall, operating in parallel).
• Any of the above, used with additional derived
and steered ‘channels’.
30

Multichannel Systems: Surrounds-Option 1

31

31 January, 2002

Since the THX “dipole” null is a soft one, it means that othermultidirectional designs can achieve similar effects. Some of the JBL
Synthesis dipoles go further and use horn loaded mid/high drivers to
improve on the directional performance by reducing comb filtering in
the null region.
With digital discrete and the new generation of active matrix systems,
multidirectional speakers are less advantageous, and with sevenchannel systems, the need is even less.
Music oriented systems should use similar or identical speakers in all
channels.
Conventional forward-facing loudspeaker systems work well in large
rooms but, in small rooms, it may be too easy to localize them.
Placing them high on the walls was an answer in the days of Dolby
ProLogic, when surrounding sounds were mostly ambient andreverberant sounds and aircraft flyovers. Nowadays, they may needto come down closer to ear level for those occasions when we play
multichannel music.
In-wall speakers? These can work, if you choose ones that soundreally good (not cheap). Ceiling speakers? Not for me! Be careful,
in-wall speakers can also entertain the room next door. Back boxes(enclosures for in-wall speakers) can help.
6

The Science of Audio - a series of lectures by Floyd E. Toole, Ph.D. Vice President Acoustical Engineering
Harman International Industries, Inc.
8500 Balboa Boulevard, Northridge, CA 91329

Multichannel Systems: Surrounds-Option 2

32

A common Problem

The wall is an
acoustic mirror
Too muchsound comesfrom the rear.

818 895 5761

ftoole@harman.com

For small rooms, or when the surround speakers need to be close to
the listeners, multidirectional speakers can be useful.
However, there is a problem that is not much discussed. Most of thesound in the surround channel in Dolby ProLogic encoded films is
intended to be ambiguously localized – to come from everywhere andanywhere. Atmospheric music and reverberation were common.
However, nowadays, all kinds of sounds are sent to the surroundchannels. Impulsive sounds, like gunshots and ricochets, are very
unlike “atmospheric” sounds. They are usually intended to appear tocome from certain directions, and they can be especially well
localized by our keen ears and brain.
It is annoying when these sounds appear to come from the front(reflected from the screen or front wall) or rear, when they wereintended to be at the side.
Lots of homes force us to sit with our backs to the wall. This is aproblem, because the wall behind the head is an acoustic mirror. This
setup often leads to sounds that were intended to be ambiguously
localized, or deliberately to one side or the other, being localized
behind the listeners. I have heard this in installations where theyshould have known better.

33

And a solution:

Moving the speakers forward pushes the mirrored speakers further
back, improving the situation. Then be careful about reflections fromthe front surfaces.

34

Behind the Listeners:

A thick absorber.

It is sometimes suggested that the solution is to absorb the front andback wall reflections from the dipole surround speakers. Doing thisleaves the listener in the direct sound field from the side speakers,
which is just what we were trying to avoid by using dipoles.
Incidentally, the sound quality to the sides of dipole surroundspeakers may not be as good as it could be.
Let’s focus on the front speakers for a minute. A reflective wallbehind the listener is a huge problem, because very shortly after the
direct sound from the front arrives at the ears, there is a powerfulreflection from the wall. Comb filtering degrades the s ound quality,
and the temporal confusion degrades the soundstage imaging.
An absorber, like cushions, fiberglass, an acoustic foam panel, or the
like, behind the heads brings the front soundstage into sharper focus.
How thick an absorber? Not less than 2 inches. Thicker is better.
This is something that really works for stereo, where most of thesoundstage is created from “phantom” images. Try it.

35

And a solution:

A diffuser behind the heads confuses the imaging even more. Not agood idea. It is well known from scientific research that uncorrelated
sound at the ears (what the diffuser creates) delocalizes (makes lessclear) auditory images in the front soundstage.

A diffuser.

36

31 January, 2002

7

The Science of Audio - a series of lectures by Floyd E. Toole, Ph.D. Vice President Acoustical Engineering
Harman International Industries, Inc.
8500 Balboa Boulevard, Northridge, CA 91329

818 895 5761

ftoole@harman.com

Behind the Listeners:

A combinationof both.

A bit of both can work though, with the absorber maintaining the
integrity of the front soundstage, and the diffusers at the sides
contributing to an enhanced sense of space.

37

Four Surrounds-Option 3

Surrounds oneach side areconnected inparallel.

In a large room, multiple speakers can be useful. Just hook them inparallel, the way they do in cinemas.

38

Four Surrounds-Option 4

Surrounds oneach side areconnected inparallel.

Multidirectional speakers can also be used, but watch out for sounds
appearing to come from unusual directions because of reflections.

39

Four Surrounds- Additional Derived Channels

Sides

Rears

Still better, use a surround processor that creates two new channels
with the appropriate delays and spectral processing. The two
additional speakers now have their own signals, and the additional
benefits are profound.
It works with multidirectional speakers . . .

40

Four Surrounds- Additional Derived Channels. . . and arguably even better with conventional speakers. In a largish
room, this is a superb solution. If the side speakers are very close tolisteners, one might use multidirectional speakers on the sides only.

For
Movies
And
Music

41

31 January, 2002

8

The Science of Audio - a series of lectures by Floyd E. Toole, Ph.D. Vice President Acoustical Engineering
Harman International Industries, Inc.
8500 Balboa Boulevard, Northridge, CA 91329

818 895 5761

ftoole@harman.com

Four Surrounds- Additional Derived Channels

The big bonus, from my perspective, is the liberation of listeners fromthe rigid theater-seating plan.
And it is sociable!!

42

A Solution to a Common Problem: it WORKS!

43

It amazes me that so few people think to do this. If you are notbuilding a special entertainment room from scratch, the probability isvery high that there will be a fireplace, a door, a window, or an
opening, precisely where the video display should go. It has
happened in my last two houses, in spite of my attempts to avoid it.
So, just go with the flow, and put the system on the room diagonal.
Those bothersome side-wall reflections go away, the side channel
loudspeakers are a long way from the listeners – if they are in thecorners, equalization will be necessary, otherwise move them down
the walls a couple of feet or more.

A Solution to a Common Problem: it WORKS!

Having lived with this, I have come to really like it. The spacebehind the TV can hide a subwoofer, or a rack of equipment, or both.
It is excellent for a general-purpose entertainment room. Now, I
would not hesitate to deliberately design a room in this configuration.

And it is sociable!!

44

Since choosing a surround speaker type and configuration is difficult,
one would hope that the choice for the left, center and right fronts
should be much easier. It is.

What is the ideal loudspeakerdirectivity?

So, what kinds of loudspeakers are best for the purpose? In the stores
there are several options vying for our attention, and some aredramatically different in design from others.

Let’s look at the options.

1

Directivity of Forward-Facing
Direct Radiator Systems

LOW BASS
MID FREQUENCIESHIGH TREBLE

2

31 January, 2002

The traditional form of loudspeaker is the forward-firing direct
radiator configuration. Woofers handle the low frequencies, tweeters
the highs and, in some designs, a midrange fills in the intermediate
frequencies. All drivers are arranged on the front face of theenclosure, facing the listeners. Because of their long wavelengths,
low frequencies radiate equally well in all directions. As the
wavelengths get shorter, in the middle frequencies, the radiated soundbegins to favor the forward direction. At the very highest
frequencies, the short wavelengths tend to radiate rather specificallyin the forward direction. This is by far the most popular of all
speaker design configurations.
9The Science of Audio - a series of lectures by Floyd E. Toole, Ph.D. Vice President Acoustical Engineering
Harman International Industries, Inc.
8500 Balboa Boulevard, Northridge, CA 91329

Directivity of Forward-Facing
Systems with Horns or Waveguides

LOW BASS
MID FREQUENCIESHIGH TREBLE

3

818 895 5761

ftoole@harman.com

We now know that it is important that speakers exhibit uniform
directivity through the middle and high frequencies. Consequently,
one increasingly sees systems with horns or waveguides. A true horn,
driven by a compression driver, is derived from high power
professional systems. Waveguides (really they are horns) on tweeters
are becoming more common as designers learn the advantages they
offer. No longer need we concern ourselves with the old idea thathorns have a “megaphone” kind of sound. Good horn designs do
nothing but allow us to control the dispersion of the sound, making it
wide or narrow, as we wish, in order to arrive at a superior systemdesign. Designed properly, a horn can improve a system design.

Directivity of Bidirectional InPhase (Bipole) Speakers
Putting two conventional systems back to back in the same enclosure,and letting them radiate in phase with each other, creates a systemthat is almost omnidirectional at low and middle frequencies. It
generates a lot of reflected sound in a listening room, which, forstereo, is often a very good thing.

+
FRONT

BACK

LOW BASS
MID FREQUENCIESHIGH TREBLE+
4

Directivity of Bidirectional outof-phase (dipole) Speakers
Flat panel loudspeakers of all kinds, electrostatic or electromagnetic,
and some systems that use cones and domes, radiate sound equally inboth directions, but do so in opposite polarity. This means that,
where the sounds from front and rear meet, at the sides, they cancel
each other out, creating a null.

-+

BASS ANDMID FREQUENCIESHIGH TREBLE

5

Directivity of Omnidirectional
Speakers
And finally, there are systems deliberately designed to be as truly
omnidirectional as possible.

ALL FREQUENCIES

6

All of these . . .
+

-

+

+
. . and More . . .
7

31 January, 2002

10

The Science of Audio - a series of lectures by Floyd E. Toole, Ph.D. Vice President Acoustical Engineering
Harman International Industries, Inc.
8500 Balboa Boulevard, Northridge, CA 91329. . . evolved during the era of
two-channel stereo.
• They sound different from each other.
• Reflected sound from the multidirectional designs
enhanced the sense of air, space and depth fromstereo recordings.
• What suits one style of recording, may not suit
another.
• Only forward -facing radiators are used by theartists and engineers creating the recordings.
8

Multichannel Audio Changes the Rules• With multichannel sound, impressions of direction,
space and envelopment are in the recordings &sound tracks. The artists have more control.
• Customers at home have a higher probability ofhearing what was created.
• Some loudspeakers, designed to embellish stereo,
are not appropriate for multichannel audio.

9

Speaker Mythology
Aren’t some speakers better for movies thanthey are for music?
THIS IS THE WRONG QUESTION.
The real issue is which speakers are appropriate forstereo and which for multichannel reproduction.
The same multichannel systems can – indeed they
MUST - work superbly for both movies and music.
10

818 895 5761

ftoole@harman.com

Those of us who grew up with stereo, remember the numerous
devices and techniques that were developed in attempts to create amore convincing illusion of space and envelopment. At the other endof the chain, in the studios, recording engineers experimented withmicrophone techniques and electronic processing to try to achieve thesame end. All of this was trial and error. Among the variables that a
consumer could play with was loudspeaker directivity. By spraying
sound in more directions it was possible to create a greater sense of
space, width and depth. Sometimes, with some recordings, it may be
a bit too much. But that is stereo, always a bit of a gamble whether
the recording technique and the playback technique match.
Multichannel audio promises more. The existence of more channels
is a big step in the right direction, but there are still opportunities to
confuse the issue. With more discrete channels, the listening
experience should translate from the studio to the home with muchless distortion. The effects of the listening room should be less thanin stereo, assuming that the loudspeakers don’t unnecessarily
aggravate the situation. In cinemas lis teners are placed in a strong‘direct’ sound field through the use of directional horn speakers up
front, and acoustically damped rooms. In homes this argues forforward firing, if not horn/waveguide-loaded, speakers for the
Left/Center/Right locations.
In music recording, it is assumed that all monitoring will be donewith conventional forward firing systems in all five locations.
But what of the persistent assertion that some speakers are better formovies than they are for music? The implication often is that we canget away with less ‘refined’ sound in movies than we can in music.
But . . . there is music in movies – sometimes a lot of it. Sometimes,
as in a concert video, the music is the entire point of the production.
The assertion is silly. Good sound is good sound, whether it is inmovies or music -only performances. The only special considerations
for film sound are maximum loudness and power handling. In films,
things occasionally can get very loud, especially in the low bass.
What about directivity?
Multidirectional speaker designs tend to alter the spatial aspects ofwhat we hear by adding reflections to the sound field. Is this a goodthing for multichannel audio?
The evidence suggests that using anything other than forward facing
speakers is a deviation from the principle of recreating the recorded“art” as it was intended.
My vote goes for conventional forward-facing systems.

And the answer is:• Forward-facing radiators– The overwhelmingly popular choice for stereo
reproduction.
– The only choice for monitor loudspeakers inmaking music and film sound recordings, bothstereo and multichannel.
– The logical choice for L,C & R in multichannel
music and movies, and for surrounds in music.
11

Locating Loudspeakers –some details that can matter a lot.

So, now we know what kind of loudspeaker to use in each channel,
and we know, more or less, where to put them. But there is more.
The details of placement also matter if truly good sound is desired.
Loudspeakers acoustically interact with the floor, and walls that are
nearby. Small loudspeakers can be put on shelves with their backs
against the wall, or even inside cavities in a cabinet. Doing thischanges how they sound, sometimes seriously.
There are also on-wall or in-wall speakers that require no visible
support, and are visually less obvious. Are these good choices?

• Adjacent boundary interactions• Speakers in bookcases and A/V furniture• On-wall designs• In-wall, in-ceiling mounting

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31 January, 2002

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The Science of Audio - a series of lectures by Floyd E. Toole, Ph.D. Vice President Acoustical Engineering
Harman International Industries, Inc.
8500 Balboa Boulevard, Northridge, CA 91329

Adjacent Boundaries:
Optimizing Locations

Three dimensions are relevant.
Each one is measured from thecenter of the woofer to one of
the adjacent boundaries – floor
and walls.
Each of these dimensions isrelated to a dip in thefrequency response of thesound we hear in the room.
To minimize the effect on what
we hear, try to make all three
dimensions different from eachother.
The general exception: woofersand subwoofers operating
below about 80Hz.
More about this later.
2

The adjacent boundary effects are in
addition to those of the room resonances.
For a specific speakerlocation:
Relative Amplitude (dB)

The calculated effect of
the adjacent boundaries.
The superimposition ofmany ‘room curves’
measured within thelistening area. These
include resonances.
The average of all ‘room
curves’, showing basic
similarity to the shape ofthe boundary effect curve.

Frequency (Hz)3

4p and 2p conditions

4p steradians = full free space
No reflecting surfaces nearby.
Most freestanding speakers aredesigned with this in mind.

2p steradians = half space e.g.
speaker mounted in a wall.
Low bass will be boosted comparedto 4 p conditions, and this isincluded in the design of in-wallspeakers.
4

An Adjacent Wall
A conventional free-standing speaker
used in this manner will exhibit
increased low bass and colored upperbass. The speaker has been“redesigned”, and needs equalization.
However, speakers designed
specifically for on- wall use will be rightat home.
For example: Infinity OWS1
( On Wall Speaker)
5

Freestanding Speakers get Built In

Were these properly measured and equalized after installation?
I’ll bet they weren’t!
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31 January, 2002

818 895 5761

ftoole@harman.com

The location of a full-range or satellite speaker with respect to theadjacent boundaries determines the acoustic “loading” of the woofer.
The consequence for listeners is that, at frequencies determined by
the distances from the woofer to the reflecting surfaces, there will bevariations in the sound radiated by the woofer. This usually shows up
as peaks and dips in the frequency response – certain sounds being
more or less loud than they should be. These effects cannot be
eliminated, but they can be considerably reduced by making the
distances to each of the nearby surfaces different from one another.
The worst situation is if they are all equal. We will deal withsubwoofer locations later in this paper.
In listening rooms there are also resonances or standing waves. We
will spend a lot of time on these later in this paper. However, for now
it is important to realize that this is an additional effect. In thisdiagram can be seen a calculated curve showing the ups and downs infrequency response contributed by the adjacent boundaries. Under
this can be seen the ‘grassy’ looking dark area which is the
combination of many measurements made at different locations in thelistening area of the room. The spikey ‘grass’ is evidence ofvariations caused by the room resonances – they are very large. An
average of all these curves has been calculated and it can be seen tofollow the general shape of the predicted curve. It is a real effect.
When speakers are designed, it has been traditional to evaluate themin what we call the ‘free field’, a space without any reflections.
Technically, this is called a 4π environment, one where the soundfrom the speaker is free to radiate in all directions. We know this is
not how they get used, but it is important to understand how the
speaker functions alone, before adding the complications of the room.
If a speaker is flush mounted into a wall, it is said to operate in a
‘half space’, or a 2π environment. This means that all of the soundthat would have radiated to the rear is reflected back towards thefront. If you look back a page or so, to the directivity of forwardfacing speakers, you will find that the bass is omnidirectional.
The middle and high frequencies have a more forward directional
bias. Consequently, mounting such a speaker in a wall causes thebass to be increased, something that must be compensated for in thedesign of speakers used for this purpose.
If a conventional box speaker is placed adjacent to a wall, the bass
will also be increased and, depending on whether this was theintended location, the sound may or may not be correctly balanced. If
the speaker is thick or deep, there may also be upper bass/lower
midrange coloration due to interference of the direct sound withsound reflected from the wall behind.
Speakers designed for on-wall mounting avoid the problems entirely.
Custom cabinets often put speakers into specially made cavities. In
the picture shown here, the speakers are, in effect, 2π mounted. The
notes accompanying this photo tell us that the speakers were models
that would normally be used in a free-standing manner. Thisinstallation substantially changes their performance. Low frequencies
would increase in level, and since the cabinets no longer have any
front edges or corners, the directional effects of diffraction are absent.
To restore them to anything like their original sound quality requires
equalization of a kind that is difficult to do in a room. This forces theinstaller into the role of speaker designer, something both he and theoriginal speaker manufacturer might be very uncomfortable with!
12

The Science of Audio - a series of lectures by Floyd E. Toole, Ph.D. Vice President Acoustical Engineering
Harman International Industries, Inc.
8500 Balboa Boulevard, Northridge, CA 91329

Some speakers are designed to bebuilt into custom cabinetry –or not, it’s your choice.

Plain black boxes, with flat fronts anddirectional control to minimize diffraction
effects. Final acoustical compensation for
mounting differences are accommodated in
the final installation step – equalization done
by a trained installer using a real measuringsystem loaned by the manufacturer.

7

Some “bookshelf”
speakers actually work
properly when placed on
bookshelves . . .

818 895 5761

ftoole@harman.com

Speakers designed with large front baffles and some directional
control are less susceptible to changes in performance due to theiracoustical surroundings. Such speakers make life easier for custom
installers, who have to adapt to the conditions of each customer’staste and physical circumstances. These ‘black box’ speakers are
extremely versatile. The final step is to measure and to
parametrically equalize the system after it is in place. In the JBL
Synthesis systems this is done by a trained installer, using acomputer-based multi-microphone measuring system that is loanedby the manufacturer. Knowing the ‘target curve’, how each model of
speaker should measure is the key, and only the manufacturer knows.

Small speakers are sometimes called ‘bookshelf’ speakers because
they can fit into a bookshelf – albeit sometimes a rather large one. In
actual fact, most such speakers are recommended for use on stands,
positioned some distance from the walls.

. . . and have adjustments
to allow them to work
properly when on stands.

However, if the speaker is powered, and if it has the appropriate
equalization options built into it, the customer has the option of using
it in either manner.

7

These got built in with a wood mesh
grill in front. Two problems!

Good woodworkers like to work wood. Here the craft has been taken
too far, because of the coarse wooden grating over the speaker
openings at the top of the cabinet. The open area is too small for thisto be an acceptable grille.
The second problem is the elevation. Front channel speakers should
not be so far above ear level.

It looks O.K. but good speakers would be seriously corrupted!
8Each speaker has its own little
resonant cavity. B-a-a-a-d idea!
Manufacturers take care to
avoid resonances in speaker
drivers and enclosures. Herethe empty space around the
speakers in the cabinet add
numerous unwanted
colorations, honks andbooms.
Done properly, the empty
space would be filled withsound absorbing material anda grille cloth would cover the
opening.

Speakers surrounded by empty space in an overly large cavity will
have their sound degraded by the acoustical resonances in the open
spaces. To avoid this, fill the empty space with fiberglass wool, orother fibrous material, foam, cloth or even old socks.
Cover the filling, or the entire opening with a porous, open weave,
grille cloth. This is available in many colors at specialty shops or atany fabric store as polyester double knit.

9

Most center speakers find themselves sitting on top of a television set.
Such speakers need to be designed with this in mind. Sadly, many
are not.

Center-Channel Speakers
Can we use regular speakers?
It depends. Center channel speakers are (or should be)
designed to sound good when placed on a television. The
television modifies the performance of the speaker.

The television set is an acoustical obstacle in the sound field of the
speaker, and it modifies the sound of the speaker, mainly at lower
frequencies.

And some classy ones
have adjustments tooptimize theirperformance on different
sizes of televisions!

Certain speakers can be adjusted to perform properly when used in afree-standing fashion, or sitting on TV screens of different sizes.
10

31 January, 2002

13

The Science of Audio - a series of lectures by Floyd E. Toole, Ph.D. Vice President Acoustical Engineering
Harman International Industries, Inc.
8500 Balboa Boulevard, Northridge, CA 91329

818 895 5761

ftoole@harman.com

Speakers intended for in-wall or in-ceiling installation began as
relatively inexpensive devices intended for distributed soundapplications – music everywhere. As such, sound accuracy was not
the first requirement. Many were direct descendants of car audio
speakers.

In-wall and in-ceiling speakers

Times have changed, and now we find installations in the main
entertainment area of a home that are completely or partially outfittedwith these speakers.
11

The virtues of in-wall speakers

Out of sight, out of mind, the saying goes. Invisible speakers don’t
exist, but these come the closest, and as such fill a need for customers
who just want the sound, but not the sight, of speakers. Let’s put
them in the ceiling. We rarely look up, right?

• They are less visible

12

The problems with in-wall speakers.
• Many of them don’t sound all that good.
• Walls make poor speaker enclosures.
– They resonate mechanically and acoustically– They can buzz and rattle• Sound leaks through the wall and throughceiling/attic spaces into adjacent rooms.
• Many of them are located in the wrong
places for good sound.

I accept ceiling speakers in airline terminals, and tolerate them in“music everywhere” uses in homes.
I do not endorse ceiling speakers in multichannel music or hometheater systems.
Mounting a speaker on a wall may be asking for trouble. Mounting
one in a wall is worse. Of course the wall panels will vibrate, andradiate sounds that were never intended, corrupting the sound fromthe speaker itself. The vibration will occur on the opposite side of thewall as well, radiating sound into the adjacent room. A ceilingspeaker will fill the attic with sound, letting it leak into all of therooms on that floor.
The constraints of building frames and services often restrict wheresuch speakers can go, and customers often ask for them to be put
where they are least visible. Many end up in the wrong locations.

13

Applying the same design criteria to in-walls as get applied to top line
free-standing speakers, it is possible to make them into truly hi-fi
products. Naturally, the problems of walls are still there, so it is agood idea to get inventive and find ways to mount the speakers sothat the vibrations generated by them are not well coupled to the wall.
This moves this category of product into the front line, able tocompete with conventional speakers for multichannel soundapplications. Place the front speakers close to ear level, and thesurround speakers slightly higher. Please keep them out of theceiling.

Hi-Fi in-walls + a difference
These competewith the sound of
free-standing
speakers, in part
because of built -in
mechanicalisolation to reduce
the wall vibrations
generated by them.

15

A good idea?

These poor in-walls are not just too low, but they are built into
cavities. The additional touch of sculptures and ornaments thatfurther obscure the sound truly tells us that sound quality was not apriority. Pity.

Only if the customer
is not interested inhigh quality sound.

It looks good though.

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31 January, 2002

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The Science of Audio - a series of lectures by Floyd E. Toole, Ph.D. Vice President Acoustical Engineering
Harman International Industries, Inc.
8500 Balboa Boulevard, Northridge, CA 91329

Back boxes
These are special enclosures designed
for in-wall speakers create morepredictable acoustical environments.
They also can reduce the amount ofsound that leaks through the wall.
Do they guarantee good sound? No.
Are speakers without back boxes
necessarily less good? No.
Back boxes are an option. Follow the
manufacturer’s recommendation.
15

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ftoole@harman.com

The enclosure is an important part of a conventional free-standing
speaker. It seems logical, therefore, to add an enclosure to an in-wall.
There are difficulties, because the space is very limited in the space
between the studs and wallboard. Adding a substantial enclosure
made from thick materials, as is done conventionally, reduces thespace further, and shallow enclosures are acoustically not a goodthing, as a rule. Small enclosures are also problems if good bass
output is required. So, some systems recommend back boxes, andothers choose to live with the reality of walls, encouraging builders to
add a few more screws and perhaps some glue, and filling the wall
cavity with fibrous material. Both, demonstrably, can work.

Coming up in Part 2
What makes a loudspeaker good? Can the same loudspeaker sound good in
different rooms? What about “acoustical” treatment of rooms? Can I
customize a room to suit my personal tastes?
Loudspeakers and rooms operate as a system. One cannot be separated fromthe other. Knowing this allows us to design better loudspeakers, ones thatare ‘friendly’ to different rooms. Knowing something about room acoustics
allows us to use furnishings and acoustical devices to improve stereo andmultichannel reproduction, making the listening experience more pleasurable.

31 January, 2002

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