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 2 – Making a good loudspeaker Imaging, space and great sound in rooms.
Loudspeakers can be designed to be “roomfriendly” so that they can sound good in avariety of different rooms. Controllingreflections can optimize imaging and spatial
effects.

Some of what we think weknow about audio is BS!
Before Science
BS

Audio is an industry that is unfortunately burdened with a lot of ideas
and beliefs that are not based on physical facts. Our business iscomplicated enough without adding the confusion of half-truths andfolklore. An understanding of at least the fundamentals of room
acoustics, and how loudspeakers and rooms interact with each other,takes us a long way towards our goal of truly excellent sound. This
benefits everyone, the customer, the installer/consultant/retailer, andthe manufacturers of the equipment.

2

The fact is that audio is a technology with a firm scientific grounding
under most of it. It is not an art, and audio products do not possess
artistic characteristics, except in the visual sense. The better weunderstand and use the real science to achieve excellent performancefrom our products, the more often the true audio artistry, the sound ofmusic and movies, will be heard by us and by our customers.

Music and Movies
are ArtAudio is a Science

Science in the Service of Art
is our Business!!
3

31 January, 2002

The existence of custom audio installers and consultants is something
relatively new to our industry. For the first time in the history ofaudio, there are professionals whose job it is to help the customer, in
his or her own home, to achieve the best possible sound.
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

The Goal: To deliver highquality sound to our customers’ears.

818 895 5761

ftoole@harman.com

The traditional problem in audio has been that the room, the final
audio component, is not within our control. Customer satisfaction,
assuming that it is based on good sound, has been, therefore, a matterof chance.

The Problem:
ROOMS, the final audio component.

This can change. With the selection of the appropriate loudspeakers,
the application of some fundamental room acoustical knowledge and,if necessary, the right kind of equalization, we can greatly increase
the odds in the favor of the customer and thereby ourselves.

They affect sound quality and imaging
They dominate bass quality
They do this during the making of the recordings, andduring their playback at home.
They are all different.
4

Loudspeakers should sound good . . .
. . . and that is part of the problem.

No matter what measurements tell us, a loudspeaker isn’t good until itsounds good. Complications in determining what is “good” include
variations in rooms and recordings. The latter is something often
ignored as we go about our daily businesses.

How do we judge what is “good”?
5

We listen, of course .
But when we do . . .

6

The Audio Industry is in a “Circle of Confusion”
LOUDSPEAKERS
which are evaluatedby using

MICROPHONES, EQ,
REVERB & EFFECTS
which are evaluatedby using

RECORDINGS
which are madeby using

which are then used
to evaluate audio
products
7

The Audio Industry is in a “Circle of Confusion”
Professional Monitor
LOUDSPEAKERS
which are evaluatedby using

MICROPHONES, EQ,
REVERB & EFFECTS
which are evaluatedby using

RECORDINGS
which are madeby using

which are thenauditioned throughConsumer
LOUDSPEAKERS
8

31 January, 2002

When we listen we are instantly trapped in the “audio circle ofconfusion”.
Loudspeakers are evaluated by listening to recordings.
Recordings are made using microphones that are selected andpositioned, equalized and processed in a variety of ways using themasses of equipment in a recording studio.
All of this is done while listening through loudspeakers in a room – arecording control room or movie dubbing stage.
The quality of the sound in a recording is very much dependent on thequality of sound from the monitor loudspeakers in that particularroom.
The recording industry has NO critical standards relating toloudspeakers for monitoring and for the rooms in which they areused. Consequently, recordings are extremely variable in quality,
even in the gross characteristics of bass and treble balance. Yet, we
try to evaluate audio products using such recordings. It is likemaking a technical measurement with an undefined test signal. The
result is that mistakes are made. We cannot tell whether a goodsound is the result of a truly good loudspeaker/room combination, or
whether it is a case of compensating errors: a recording with, forexample, too much bass being combined with a playback system thatis deficient in bass.
Some control rooms sound superb, while others are back in the darkages of loud “mid-fi”. Some even go out of their way to use badmonitor loudspeakers that they think represent what people arelistening to in their homes and cars. It is obvious to anyone who
listens carefully that all loudspeakers are getting better, and that thegood ones are sounding more and more alike, and much more like the“real thing”. However, bad loudspeakers can be bad in an infinite
number of ways. No two are alike, and they can be dramaticallydifferent. How, then, is it possible for one “bad” monitor loudspeaker
to represent the huge variety of sounds from clock radios, boom
boxes, mini-systems, headphones and entry level car audio? It isn’t!!
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
The Audio Industry is in a “Circle of Confusion”
Professional Monitor
LOUDSPEAKERS
which are evaluatedby using

THE CREATION
OF THE ART

MICROPHONES, EQ,
REVERB & EFFECTS
which are evaluatedby using

RECORDINGS
which are madeby using

which are then
APPRECIATION
auditioned through
OFConsumer
THE ART818 895 5761

ftoole@harman.com

All of us need to exercise whatever influence we have to elevate thequality of sound everywhere. Then, and only then, will we havesome assurance, when we listen at home or in our cars, that we arehearing what was intended by the artists. The enemy in this effort isignorance and apathy. Most customers are intimidated by these kindsof decisions, and some truly say they don’t care. Yet, I have never in
my life demonstrated a truly good sound system to anyone who was
not impressed, if not absolutely “blown away”.

LOUDSPEAKERS

9

Professional Monitor
LOUDSPEAKERS

Ironically, the problem exists at both the professional and consumer
levels. Both need to be aware of the genuine advances in acoustic
science and technology.

The “circle of confusion”
is broken,
if these loudspeakers
sound like each other.
Consumer
LOUDSPEAKERS

10

Professional Monitor
LOUDSPEAKERSPRESERVATION
OF THE ART

The “circle of confusion” is broken,
if these loudspeakers
have the same sonic signature i.e. if they are similarly good.

Only then can we say that we are working within an industry thataims to preserve the audio artistry.

Consumer
LOUDSPEAKERS
11

This is a problem to which there
is not a single, or a simple,
solution.

If we cannot totally rely on our ears, what else is there?

12

Science Involves Measurements,and We Need to Do Two Kinds:
OBJECTIVE
1. Frequency Response on and off axis, spatial
averages including
sound power.
2. Phase Response
3. Non-Linear Distortion- THD, IMD, noise
4. Power Compression

SUBJECTIVE
1. Sound Quality timbre, bandwidth
2. Directional and spatial
effects in stereo andmultichannel systems
3. Distortions and noises
4. Dynamic Capabilities

The scientific method requires data. Data of all kinds, and the more
the better. In this case, we must use technical measurements, because
they are the essential tools of the engineers designing the products. It
is necessary to measure everything that we think might be relevant to
how something sounds. This is more than is commonly thought.
However, we also need subjective data, relating to listeners’ opinionsof the many perceptual dimensions of sound quality as well as spatial
and directional attributes.

13

31 January, 2002

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

818 895 5761

ftoole@harman.com

If listeners like or dislike something, it is important to try to identify
what, technically, was responsible. By taking the listening reportsback to the lab, it is possible to learn the relationships between what
we measure, and what we hear. This is the science of
psychoacoustics, and the better we understand it, the better we will beat delivering good sound to our customers’ ears.
Technical measurements must be accurate, or else they lie to us.
Getting accurate acoustical measurements without good facilities isvery difficult to impossible. Much of the data floating around theloudspeaker industry is not accurate.

Scientific Design Requires:(a) carefully controlled listening tests:
subjective measurements
combined with(b) accurate and comprehensive
technical measurements
combined with(c) knowledge of the psychoacoustic
relationships between perceptions andmeasurements .
14

What are Subjective Measurements?
They are LISTENING TESTS in which the
OPINIONS OF LISTENERS relate to the
SOUNDS OF THE LOUDSPEAKERS rather than:
How they look
How much they cost
What reviewers think of them
What listeners think of the manufacturer
How loud they are
How the room is physically arranged
Etc. . . .

These areNUISANCE
VARIABLES-they MUST be
controlled
15

Mono, Stereo and Multichannel
Tests

Most listening tests are valid only at a specific time and place, for the
specific recordings that were listened to, and for the people offering
their opinions. This can be acceptable if it is you choosing your own
system in your own home. It is not acceptable for a loudspeaker
manufacturer, who is trying to design products that can sound good tomany listeners, in many rooms, with many recordings.
Consequently, we get scientific about it, and start to remove some ofthe variables that have nothing to do with the sound from theloudspeaker, letting the listeners focus as much of their attention as
possible on the sound, and the sound alone. Purveyors of ‘magic’ in
the audio industry find these double-blind tests very threatening.

SHUFFLER

One of the most problematic nuisance variables is the position of theloudspeaker and the position of the listener in the room. To control
this variable at Harman, we have created a room with a “shuffler” thatphysically moves speakers around, bringing them always to the same
locations when they are being auditioned. It is pneumatically driven,
quiet, and computer controlled so that the positions are precise. It
takes about three seconds to switch the positions of the active andinactive loudspeakers.
16

A pair of speakers ready for listening.

17

And, about three seconds later, a second pair can be heard.

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

The listener (and we prefer to use one listener at a time) sees none ofthis, of course. Here we show a video display on a large perforated
screen. For the evaluation of most products this is not used.
The tests are controlled by the listener, who takes as long as is neededin order to form a satisfactory judgment. A computer randomizes the
choice of music, and the coded identity of the test loudspeakers foreach musical selection, so that the opinions must relate as much as
possible to the sound itself.
Listeners are selected for normal hearing and aptitude, and then aretrained to be really fussy. They yield remarkably consistent opinions.

What the Listener Sees

19

“Where the rubber hits the road”, in the customer’s home, we have nosuch conveniences, so we must develop products and techniques thatallow good sound to prevail even when the local acoustical conditions
are less than ideal.

In the Real World
The room is the final audio
component,
and it is not under control!

This is where knowledgeable custom installers, consultants and audio
specialists come to the rescue.
20

The Room - Direct SoundThe first sound to arrive at a listener’s ears is the “direct” sound. If
the loudspeakers have been angled to face the listener, this will be theon-axis sound, often the best possible sound from the loudspeaker.

21

The Room - Early Reflections
However, following only a few milliseconds behind, and only slightly
less loud, will be the early reflections: sounds that have been reflectedfrom only one surface in the room.

22

Still later, come the multitudes of reflections that have been reflected
more than once, perhaps many times. These are individually much
lower in amplitude, but collectively loud enough to be a powerfulfactor in our impressions of sound quality, space and imaging. In
small rooms, typically furnished, this sound field, although often
called ‘reverberation’ is not the directionally diffuse and temporally
complex reverberation that we hear in a concert hall, or many otherlarge, acoustically ‘live’ spaces. Some would argue that it deserves adifferent name.

The Room - Reverberation

23

31 January, 2002

5

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

The Room – Resonant Modes

818 895 5761

ftoole@harman.com

Rooms also have resonances that emphasize certain frequencies,
attenuate others, depending on the dimensions and shape of the room.
And, they do so in a manner that depends on where the loudspeakers
and listeners are located within the room. These effects are strongest
at low frequencies.

LENGTHWIDTH
HEIGHT

24

A Problem Loudspeaker10
woofermidrange

tweeter

0dB-10

ON AXIS30 DEGREES OFF AXIS
60 DEGREES OFF AXIS

-20

20

50 100500 1K
5K
FREQUENCY (Hz)10K 20K

Using a loudspeaker that we know, in retrospect, had a design flaw,
let us see what happens in a room. This loudspeaker was designed
using the philosophy that the direct sound, the on-axis sound, is themost important. The top curve is the on-axis measurement, and it isvery smooth and flat, a credit to the design team.
The second and third curves, moving downward, are the 30- and 60degree off-axis measurements, showing that these sounds are notnearly so neutral; the output varies with frequency. What happens tothis in a room?

25

What Happens in a Room?
20
PREDICTED“ROOM CURVE”

10dB
0
DIRECT SOUNDEARLY REFLECTIONSREVERBERANT SOUND

-10

20

50 100500 1K
5K
FREQUENCY (Hz)10K 20K
26

Real vs. Predicted Room Curves
30
20
Three “typical” locations
close to a room corner.

dB
10
0
-10

20

50 100500 1K
5K
FREQUENCY (Hz)10K 20K
27

There is a region where the room dominates,and one where the loudspeaker dominates.
30
20
ROOM

dB
10

SPEAKER

0-10

20

50 100500 1K
5K
FREQUENCY (Hz)31 January, 2002

10K 20K

The data in this slide are derived from many measurements made in alarge anechoic chamber. This is a room having no “echoes”, used foracoustical measurements. All surfaces are covered with highly
effective acoustical absorbing material that is, in this case, about fourfeet thick. The color coding of the curves is not visible here, so it is
not possible to see which curve is which. However, one mightrecognize the flat on-axis curve representing the direct sound, and see
also that none of the other curves is remotely smooth or flat. This
tells us that all of the sounds arriving at the ears do not convey thesame message about the sound quality, or timbre. The top curve is acalculated prediction of what a measured “room curve” might be.
The loudspeaker was then placed in a typical left or right channel
location in a real room. It was measured at the listening position,then moved to two other locations within a radius of two feet, andmeasured at each location. The fourth curve, the top one, is thecalculated room curve from the previous slide. Obviously, little ischanging at frequencies above about 300-400 Hz, and the prediction
is right on target. However, below these frequencies, there areconsiderable location-dependent changes, and the prediction fails
completely. The reason? Room resonances and boundary effects
that are specific to that particular room. These can only, with
precision, be evaluated by measurements in the room itself.
However, it is also clear that through the middle and high frequencies
the anechoic measurements made in the laboratory have done an
excellent job of predicting what happened in the room. However,doing so required many, many measurements at positions all aroundthe loudspeaker.
If we are to try to anticipate how a loudspeaker will sound in a room,
it is necessary to measure everything, and not just a few curves
around the on-axis measurement.

28

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

818 895 5761

ftoole@harman.com

At Harman, the engineers have dubbed our basic measurement of
frequency response, the Spin-o-rama, since it involves spinning theloudspeaker on two axes, and accumulating a total of 72
measurements.

29

DIRECT SOUND

The collection of raw data is computer processed to generate a set of
curves showing estimates of the distinctive regimes of sound arriving
at a listener’s ears in a typical room. To do this, a large survey of realrooms was undertaken, and a statistical analysis of angles anddistances led to the algorithm that generated these curves. All
measurements have a frequency resolution of 1/20 octave, from 20
Hz to 20 kHz.
The top curve is the on-axis curve, representing the direct sound for a
person in the “sweet spot”.

30

LISTENING
WINDOW

The second curve is a spatial average over +/ - 30° horizontal, and +/10° vertical, representing the direct sound for listeners seated in a row
of chairs or a large sofa, and possibly standing and sitting.

31

EARLY
REFLECTIONS

The third curve is the energy sum of the set of early reflections.
Ideally, these should look a lot like the on-axis curve, so that itconveys the same timbral information.

32

SOUND POWER

The fourth curve is a calculation of the total sound power radiated by
the loudspeaker in all directions (this is NOT a simple average or sumof all 72 measurements). Again, this curve should be smooth andflattish.

33

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

The uppermost of the bottom pair of curves is the Directivity Index,
or DI. This is an indication of the angular uniformity with which theloudspeaker radiates its energy into a room as a function of
frequency. It is a measure of the uniformity of its dispersion as afunction of frequency.
DIRECTIVITY
INDEX

34

The bottom curve is an invented DI, this time just for the earlyreflections.

EARLY
REFLECTIONSDIRECTIVITY
INDEX
35

The Complete Set –A Picture of Performance

10 dB
5 dB DI
0 dB

Here we see the complete set as they are presented for visualinspection. The whole idea of this is to present to the eyes, a set ofdata that can be interpreted in a way that allows one to anticipate how
a loudspeaker might sound in a room.
The curves shown here describe a truly excellent loudspeaker, notperfect but, currently, a good example of the state of our art. Note thesmoothness of all of the curves, and the basic similarities in all of thecurves, from the single on-axis measurement, through to the estimateof the total sound radiated in all directions, the sound power.

36

Performance in an “average” room
can also be predicted
1/20-octave resolution.

In the same way that the earlier example was calculated, we cangenerate a curve that tries to anticipate a room curve. Here we havenot included all of the possible refinements, but it suffices to get us
within a couple of dB over most of the frequency range.

Measured average of 4 different rooms

Predicted from anechoic data

37

Performance in an “average” room
can also be predicted
1/20-octave resolution.

125 Hz to 8 kHz
8-octaveswith useful accuracy

38

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

Now, how do we interpret themeasurements?
• Frequency response curves are not flat andsmooth. Does this matter? How much doesit matter? What is the ideal shape?
• Can we hear phase shift?
• What about time-domain behavior: transient
response, “speed”, etc.?

O.K. So we get some curves. The real proble m is that they ARE
curves, and not straight lines. What is the ideal shape? How muchdeviation from the ideal is audible? Is there more to this than justfrequency response?

39

For Example: What is in aFrequency Response?

0 Degrees / On Axis
40

What is in a Frequency Response?
Spectral Balance
and Bandwidth

Resonances

Acoustical
Interference
41

Let’s start with the most basic of all measurements, the frequency
response. In the case of a loudspeaker we would begin with a look atwhat happens on the major axis.
Incidentally, such a measurement should be made at a distance of 2 m(6 feet) or more. The industry standard specifies loudspeakersensitivity at one meter, however, the standard also requires themeasurement to be made in the “far field” of the source, and if
necessary, for the measurement to be calculated back to 1 meter.
Many people mistakenly do not do this, and also make frequency
response measurements at 1m. For loudspeaker systems of typicalsize, these measurements can exhibit large errors.
Of the features that our eyes can extract from a curve like this, it isobvious that spectral balance and bandwidth are important. Try
playing with the bass and treble controls, and you will find that smallchanges are audible.
Resonances are REALLY important because our perceptual system(the ears and brain) is highly sensitized to them. The reason:
resonances are the “building blocks” of all of the sounds that we arereally interested in listening to – voices and musical instruments.
Resonances can cause peaks and dips in a frequency response curve.
However, so can acoustical interference, a phenomenon that turns out
to be much less audible under normal listening circumstances.

The components
must be separated

So, we need a measurement system that allows us to separate,visually, those features in a curve that are caused by each of these
phenomena. Only then can we be truly analytical, and make good
judgments about how good or bad the device is.

Spectral Balance
and Bandwidth

Resonances

Acoustical
Interference
42

A Traditional, On-Axis
Frequency Response
Measurement
Once upon a time, it was thought that a single curve told us useful
information.
0 Degrees / On Axis

43

31 January, 2002

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

818 895 5761

ftoole@harman.com

Simple Spatial Averaging
10º UP
30º R
20º R
0º

30º L

10º R

10º L

20º L

10º DN

Then we learned that spatial averaging allows us to separate thosepeaks and dips caused by resonances from those caused by acousticalinterference. The explanation is really simple: those featuresassociated with resonances tend not to change when the microphone
location is changed, while those associated with interference do.

THE LISTENING
WINDOW
44

A lot of Spatial Data
When we average a lot of measurements made at a lot of different
locations, and certain visual shapes do not disappear, we can be quite
confident in concluding that those are resonances, and not the resultof acoustical interference.

45

Spatial Averaging in Rooms• Helps to reveal the presence of resonances,which can be equalized.
• Attenuates the effects of acousticalinterference, which cannot be equalized.
• Helps to eliminate visual evidence of dips in
the frequency response, and thereby thetemptation to try to fill them withequalization – which does not work!

In rooms, there is an abundance of acoustical interference, caused bymultitudes of reflections. Therefore, spatial averaging, i.e.
combinations of measurements made at several locations, can help toisolate resonances. This is important because it turns out that we canequalize resonances (about which, more later), and we cannot
equalize the effects of acoustical interference.

46

Resonances are major problems!
So, we expend a great deal of effort eliminating resonances fromloudspeaker systems, and when they are installed in rooms, we needto spend some time and effort to identify and eliminate serious
resonance problems.

• In loudspeaker drivers– cone flexure modes– suspension and frame modes• In enclosures– mechanical resonances in panels and surfaces– acoustical resonances in cavities• In rooms
47

Resonances of Different “Q”
Q=1
10 dB

Q = 10Q = 50

100

200

500
1K
2K
FREQUENCY (Hz)

5K

10K
48

31 January, 2002

Resonances are differentiated by their “Q”, or quality factor. A highQ resonance is one that is very frequency specific and that rings along time. An example of a high-Q resonance is an empty wine glass,held by the stem, and tapped with a finger nail. It emits a clear tonethat rings. If one places a finger on the side of the glass and taps it
again, the ringing is shorter. The finger has taken some energy out of
the resonant system, and the ‘quality’ is reduced. If the entire glass isgrasped by a hand, and the tap is repeated, there is almost no ringing
at all. A tone is still recognizable, but it is a low ‘quality’, or low-Q,
resonance. High-Q resonances have sharp peaks, and low-Q
resonances are broader when they are seen in frequency responses.
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

SOUND LEVEL (dB)The just audible variations in spectrum
or frequency response:
High-Q resonance Q = 50
20

PROGRAM: VOCALIST & JAZZ COMBO
10 dB

10
0
-10
-20
20

50

100

200

500

1K

2K

5K

10K 20K

FREQUENCY (Hz)49

SOUND LEVEL (dB)The just audible variations in spectrum
or frequency response:
High-Q resonance Q = 50
20

PROGRAM: SYMPHONYORCHESTRAPROGRAM:SYMPHONY
ORCHESTRA10

818 895 5761

ftoole@harman.com

Sean Olive and I, when we were at the National Research Council, in
Canada, published a paper in which we showed the shapes ofdeviations in frequency responses that corresponded to the justaudible thresholds for resonances of different Q, at different
frequencies, for different kinds of music or sounds. The effect of
frequency was secondary, so here I show only what happens at 500
Hz. The results at different frequencies are similar. It shows that, formultimiked pan-potted, low reverb, pop or jazz, the threshold ofaudibility corresponds to a 10 dB spike in a frequency response
curve. It looks bad, but it is just barely audible!

With a big band or symphony orchestra (complex orchestration) in areverberant hall, the threshold is lower (we are more sensitive).

6 dB

0
-10
-20
20

50

100

200

500

1K

2K

5K

10K 20K

FREQUENCY (Hz)50

SOUND LEVEL (dB)The just audible variations in spectrum
or frequency response:
High-Q resonance Q = 50
20

PROGRAM: PINK NOISE

10
3 dB
0
-10

Of all the signals we tested, pink noise was the most revealing ofresonances. It produced the lowest thresholds. Such low-amplitude,
narrow, spikes are difficult to measure with precision at all
frequencies.

-20
20

50

100

200

500

1K

2K

5K

10K 20K

FREQUENCY (Hz)51

SOUND LEVEL (dB)The just audible variations in spectrum
or frequency response:
Medium-Q resonance Q = 10
20

PROGRAM: VOCALIST & JAZZ COMBO

When the Q is reduced, the pattern of audibility is much the same, but
the thresholds are lower.

10
6 dB
0
-10
-20
20

50

100

200

500

1K

2K

5K

10K 20K

FREQUENCY (Hz)52

SOUND LEVEL (dB)The just audible variations in spectrum
or frequency response:
Medium-Q resonance Q = 10
20

PROGRAM: SYMPHONY ORCHESTRA

10
4 dB
0
-10
-20
20

50

100

200

500

1K

2K

5K

10K 20K

FREQUENCY (Hz)53

31 January, 2002

11

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

SOUND LEVEL (dB)The just audible variations in spectrum
or frequency response:
Medium-Q resonance Q = 10
20

PROGRAM: PINK NOISE

10
1.5 dB

0
-10
-20
20

50

100

200

500

1K

2K

5K

10K 20K

FREQUENCY (Hz)54

SOUND LEVEL (dB)The just audible variations in spectrum
or frequency response:
Low-Q resonance Q = 1
20

PROGRAM: VOCALIST & JAZZ COMBO

10
2.5 dB

0
-10
-20
20

50

100

200

500

1K

2K

5K

10K 20K

FREQUENCY (Hz)55

When we get to really low-Q resonances, the ones that ring very little,
it turns out that we can hear them at very low measured amplitudes.
What, then, of the arguments that the ringing of high-Q resonances“smears” sounds, making them less articulate? These are argumentsthat are most likely based on visual interpretations of measured data,
not on actual subjective tests of the audibility of the effects. Theysound as though they should be true but, except at very low
frequencies, they are fanciful. Good engineering should attempt toeliminate resonances of all kinds, but it is important to understandwhat is and is not audible.

SOUND LEVEL (dB)The just audible variations in spectrum
or frequency response:
Low-Q resonance Q = 1
20

PROGRAM: SYMPHONY ORCHESTRA

10
1.5 dB

0
-10
-20
20

50

100

200

500

1K

2K

5K

10K 20K

FREQUENCY (Hz)56

SOUND LEVEL (dB)The just audible variations in spectrum
or frequency response:
Low-Q resonance Q = 1
20

PROGRAM: PINK NOISE

10
0.3 dB

0
-10
-20
20

50

100

200

500

1K

2K

5K

10K 20K

FREQUENCY (Hz)This “curve” looks almost like a straight line. Our eyes are telling usthat it is almost perfect, yet our ears are telling us that there might still
be something audibly wrong.
So, in this case, what our eyes tell us does not intuitively correspondwith what we hear. This is why it is so important to do the science,and to establish what the real psychoacoustic relations are. Our
instincts can be wrong.

57

SOUND LEVEL (dB)The just audible variations in spectrum
or frequency response:
These are all EQUALLY audible!
20

PROGRAM: SYMPHONY ORCHESTRA

10
0
-10
-20
20

50

100

200

500

1K

2K

5K

10K 20K

FREQUENCY (Hz)Is there an explanation? It is probably because music and speech are
ever-changing. Also, voices and many musical instruments areplayed with vibrato – a modulated pitch. High-Q resonances taketime to build up, as well as to decay. We tend to talk about theringing, overhang or decay of resonances after the signal has stopped,
ignoring the front-end effect. High-Q resonances are narrow, veryfrequency-specific, and musical sounds must be sustained longenough to energize them. Few are. Low-Q resonances are wide
enough that they respond to everything, and they take almost no time
to reach full amplitude.

58

31 January, 2002

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

818 895 5761

ftoole@harman.com

In order to make any sense at all of a frequency response curve, or aset of curves, they must be capable of revealing to our eyes
everything that is audible.

Measurements must have enoughresolution to show what we canhear

The belief, still widespread in this industry, that we cannot measure
what we can hear, has its origins in situations where the measured
data were erroneous or incomplete. Such situations are common in
the loudspeaker business.
59

A Resolution Test

Let’s create a test. Suppose we had an imaginary system in which
there were high-Q resonances uniformly distributed from low to highfrequencies. A competent measurement system would reveal them toour eyes as they truly are.

10
dB
0
20

50 100500 1K
5K
FREQUENCY (Hz)10K 20K

Q=50 resonances at the threshold of audibility
60

A Partially “Blind” Measurement
10

True Level

dB
0
CANNOT MEASURE WHAT WE HEAR

20

50 100500 1K5K 10K 20K
FREQUENCY (Hz)Measurement of Q=50 resonances at the threshold ofaudibility using a time window of 17 ms (60Hz)

However, not all measurement systems are equal. Many very
commonly used ones “gild the lily”, making the curves smoother thanthey really are. All systems that use time windowing, or theequivalent (MLSSA, TEF, and any FFT-based system) can do this IF
the measurement window is not sufficiently long. Here I show what
happens with a quite long window (17ms), more than is used by many
manufacturers and reviewers. It is clear that the measurement does
not reveal the existence of the high-Q resonances in the middle andlow frequency regions. It does not show things that we know we can
hear.

61

A Totally “Blind” Measurement
10

True Level

dB
0
CANNOT MEASURE WHAT WE HEAR

The popular one-third-octave measurements, very common in roommeasurements, simply fail. These give only a very “broad-brush”
view of what is happening, and are of very limited use. One needs ananalyzer capable of at least 1/10 octave resolution in order to reveal
what we need to see.

20

50 100500 1K5K 10K 20K
FREQUENCY (Hz)Measurement of Q=50 resonances at the threshold ofaudibility using a 1/3-octave analyzer
62

A 4 π (full space)anechoic chamber

4-foot long wedges:
anechoic to 60Hz,
calibrated to 20 Hz

Automatic positioning:motorized, computercontrolled platform

63

31 January, 2002

For loudspeaker measurements, very long measurement windows arenecessary, and these can be accomplished only in anechoic spaces.
Outdoors, away from all reflecting surfaces, is free, but impractical.
Anechoic chambers, such as the one shown here, are very practical,
but also very expensive. However, this is the price of entry if you areseriously in the business. The length of the wedges determines how
low in frequency one can measure accurately. These 4-foot wedges
create a reflection-free environment down to about 60 Hz. We havecalibrated it down to 20 Hz for specific measurement locations within
it. With a large enough measurement time window, ANY
measurement system should yield accurate data.
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

AND the most commonly used
specification for frequency
response is useless . . .
. . unless it is accompanied bya graph!!!
64+320 Hz to 20 kHz +/- 3 dB

dB 0

Every audio device has a specification for frequency response. A
tolerance of +/- 3 dB is sufficient to describe a range from junk tojewels. By itself, it is meaningless window-dressing. A curve, andthe ability to interpret it, are necessary. If the tolerance is smallenough, then it does have meaning, of course.

-3
+3
dB 0
-3
+3
dB 0
-3
+3
dB 0
-3
20

50

100 200

500

1K 2K

5K

10K 20K65(Hz)Frequency and Time Domains• Related by Fourier Transformation• Behavior in one domain is paralleled bycorresponding behavior in the otherSo far, we have talked about frequency response as though it were theonly important factor. What about the all-important transient
response, speed, punch, drive, and all of those descriptors of what
happens in the time domain?
Well, it turns out that the two domains are related to one another, by
the Fourier transformation.

66

A perfect linear system would be described either by a clean
uncluttered transient, or by a pair of flat straight lines portraying a
constant amplitude vs. frequency characteristic (we call this thefrequency response, although it is really the amplitude response), anda constant phase vs. frequency response. The flat amplitude responsetells us that the signal level at all frequencies is constant. The flat
phase response tells us that everything is happening at precisely theright instant in time. The combination of flat amplitude and phas e
responses correspond to a perfect impulse, or transient, response.

Frequency and Time Domains
A
A

TIME

FREQUENCYøFREQUENCYA PERFECT SYSTEM

67

Frequency and Time Domains
4 dB

A
A

TIME

FREQUENCYø
AN OTHERWISE PERFECTSYSTEM WITH A Q=50
RESONANCE AT THE
THRESHOLD OF AUDIBILITYFREQUENCY68

31 January, 2002

Here we have disrupted the perfect system with a single high-Q
resonance. The narrow “footprint” in the amplitude response, as seen
earlier, is repeated in the phase response. In the time domain, thecorresponding effect is extended ringing (the empty wine glass).
If we measured the amplitude and phase responses, a computer could
perform a Fourier transform and give us the transient response. If wemeasured the transient response the computer could calculate theamplitude and phase responses. So the information on the left side of
the slide is the same as that on the right side, only displayed in adifferent form.
14

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

Frequency and Time Domains
2 dB

A
A

TIME

Here is a medium-Q resonance. The frequency-domain ‘footprint’ islarger, and the time-domain ‘footprint’ is smaller.

FREQUENCYø
AN OTHERWISE PERFECTSYSTEM WITH A Q=10
RESONANCE AT THE
THRESHOLD OF AUDIBILITYFREQUENCY69

Frequency and Time Domains
0.5 dBAA

TIME

And a low-Q resonance. Note the convenient relationship: as the
footprint in the frequency domain gets larger, that in the time domain
gets smaller.

FREQUENCYø
AN OTHERWISE PERFECTSYSTEM WITH A Q=1
RESONANCE AT THE
THRESHOLD OF AUDIBILITYFREQUENCY

70“Minimum-Phase” Systems
A

TIME

A
FREQUENCY

IN “MINIMUM-PHASE”
SYSTEMS THE PHASERESPONSE CAN BE
CALCULATED FROM
THE AMPLITUDE RESPONSE.

øFREQUENCY

71

There is a class of systems that behave as “minimum-phase” systems.
In such systems, if one has measured only the amplitude response –our familiar “frequency response”, it is possible to calculate the phase
response from that data. Now, if we know both the amplitude andphase responses, we can calculate the time response. So, in aminimum phase system, a measurement of the frequency response,
allows us to predict the time response. A bump in the frequency
response means that the system must ring. A flat, smooth, frequency
response means that there is no ringing. The previous data show that
we are able to measure the visual evidence of audible resonances infrequency response curves. This is really important.

What Systems Exhibit Minimum
Phase Behavior?
Several very important devices are minimum phase systems, meaning
that, for these devices, the frequency response curve is the single
most important measure of audible performance in the linear domain.
Of course we do measurements of non-linear effects as well, but ingeneral these are much less troublesome.

• Many common functions in analog anddigital electronics.
• Loudspeaker transducers – conventional
woofers, midranges and tweeters.
• Room resonances at low frequencies.

This is a HUGE advantage!
72

A minimum-phase system with aproblem resonance.
A
A

TIME

If a minimum phase system has a resonance, and we wish to get rid of
the audible effects, we can choose to do it electronically.

FREQUENCYøFREQUENCY73

31 January, 2002

15

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

Address the resonance with an equaland opposite parametric EQ filter
A
WHEN THE CORRECTAMPLITUDE RESPONSE
IS “DIALED IN”, THE
PHASE RESPONSE IS
AUTOMATICALLYCORRECTED.

FREQUENCYøFREQUENCY

74

A
TIME

ftoole@harman.com

Simply design a minimum-phase filter, in either analog or digitalelectronics, that exactly matches the shape of the bump in thefrequency response, but is inverted. When the two are added, we geta straight line. The filter, because it is minimum phase, will have aphase shift that mirrors the phase shift in the resonance, so that asummation yields another straight line.
How often have you heard that equalizers are bad because they add
phase shift? Here we show that it is a good thing – assuming that it
has all been done properly, with the necessary precision.
Two flat lines on the right, we know, correspond to a perfect impulse
response on the left.

And everything is fixed!

A

818 895 5761

This is a very simple form of pre -distortion, a well known techniquethat, with the advent of digital processing, is likely to become morewidespread. If we know what an electromechanical or acousticalsystem is doing wrong, there are some things that can be corrected bymodifying, or pre-distorting, the signal so that what is eventually
radiated as sound is correct.

FREQUENCYøFREQUENCY

75

Which is one reason why
active/amplified loudspeakers are
attractive.
A good loudspeaker without
equalization can be an even betterone with the right kind of
equalization.

It is not magic, but it certainly seems like it. Good loudspeakers canbe made better. Room resonances can be tamed (for specific listeners
at least).
However, in order for it to work we need accurate, high resolution,
frequency response data, and parametric filters.

76

Spatial Averaging ADDS Information
LISTENING WINDOW ± 30º HOR. ± 10º VERT.

ON AXISTHESE ARE NOTRESONANCESSOUND POWER

AND THEY SHOULD
NOT BE EQUALIZEDWhich brings us back to measurements and spatial averaging. Here
we see a portion of a spin-o-rama for a loudspeaker showing somebumps in the on-axis curve. The bumps are attenuated, and evendisappear, with spatial averaging. This tells us that the bumps arecaused by acoustical interference (in this case diffraction from thecabinet edges). They are not resonances, and they should not be
equalized.

77

In contrast – these are resonances,and they can be equalized

In this example, the series of bumps penetrate all of the curves,
surviving even the 72-curve calculation of sound power. These truly
are resonances, and they can be treated with individually designed
parametric filters.
Spatial averaging ADDS information. Spectral averaging(smoothing) takes it away.

78

31 January, 2002

16

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

Measurements make a nice story,
but
can people really hear thedifferences?

818 895 5761

ftoole@harman.com

If all of this really means anything, we should be able to prove itusing our carefully conducted listening tests.

Let’s test them with four“high-end” speakers.
79

Four expensive and highly regarded loudspeakers are evaluated in the“shuffler” room, in a double-blind test. These are the measurements.
The listeners, of course, do not get to see them until it is all over.

This one looks good: $10,000/pr

This one is well behaved. It has smooth, flattish curves, very wide,
very uniform dispersion, and excellent low-frequency extension. The
slight sag in the upper middle frequencies is something that issometimes done to compensate for the numerous excessively brightrecordings out there. The trade-off, others might sound a bit laid
back.
80

This one too: $8000/pr

Here is a ‘tell it the way it is’ speaker. Unabashedly flat, verysmooth, superb low bass extension, being only about 5 dB down at 20
Hz, with a directivity that smoothly and gradually rises withfrequency. A small dip in sound power around 2 kHz might be barely
audible, but likely only in quite ‘live’ rooms.

81

A bit wobbly: $8000/pr

82

A Mountain Skyline: $5000/pr

83

31 January, 2002

This one is likely to have a ‘personality’. The undulations in theupper mids/lower treble are everywhere, including the directivity. It
is possible to play detective, and guess that this is a three-way system,with the woofer crossing over to the midrange around 300 Hz, and the
midrange crossing over to the tweeter around 3-4 kHz. How do weknow? Look at the directivity curves. Very low frequencies areomnidirectional. The curve rises as the woofer becomes more
directional until it crosses over to the smaller midrange when the
directivity drops. It then rises again with frequency until it crosses
over to the small tweeter, at which point the cycle begins again. The
low bass is fair, but a small bump just below 100 Hz spoils it.
This speaker has a bunch of things going on. Clearly the designers
didn’t believe that flat was necessary, or they didn’t know how toachieve it. Not only are the general trends not flat, but superimposed
are peaks and dips suggesting resonances. The proof that they areresonances is in the fact that the patterns are repeated in all of thecurves. The directivity is interesting, being zero up to 100 Hz (thewoofer) and then abruptly rising to about 5 dB and hovering aroundthat all the way to 20 kHz. Since 4.8 dB is the directivity of a dipole,
we could suspect that this is a hybrid system with a panel loudspeaker
operating above about 100 Hz. The woofer exhibits a significant
bump and then rolls off below about 60 Hz. No subwoofing here.
17

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

Cell Bar Chart
Grouping Variable(s): Loudspeaker
Error Bars: 95% Confidence Interval
8
7
6
5
4
3
2
1
0
Brand X

Brand B

ftoole@harman.com

After several rounds of listening to different kinds of music, several
listeners yielded subjective preference ratings that were processed ina statistical analysis program. One of the results is a bar graphshowing the average rating for the group of listeners, for each of theloudspeakers. The tiny lines on top of the bars show the 95%
confidence intervals. If the differences in the ratings are greater thanthese lines, the differences are probably statistically significant, andnot due to chance. The two top-rated speakers are not significantly
different from each other, according to this rule. The other two are
truly less good.

The Listeners Speak

Brand Y818 895 5761

Brand A

84

When we combine the subjective with the objective data, it is clearthat the loudspeakers that yielded the best set of technical data, also
were preferred by the listeners.
Cell Bar Chart
Grouping Variable(s): Loudspeaker
Error Bars: 95% Confidence Interval

It works!

Brand Y

Brand X

Brand B

Brand A 85

At Harman, we do hundreds of such listening evaluations, using
competitors’ products that we purchase on the open market. It isessential to know where our new products stand with respect to thecompetition.

What do listeners say they like?
$2000/pr

The results are monotonously the same. Loudspeakers that look goodin the spin-o-rama measurements are the ones that are subjectively
preferred.
86

What do listeners say they like?

This is one of those pricey ‘high end’ bookshelf-sized speakers thatsome reviewers have raved about. The measurements suggest that it
is a slightly dull sounding, moderately colored system with no real
bass. The listeners agreed.

$8000/pr

87

What do listeners say they like?
Note that price sometimes has nothing to do with sound quality.
There may be material value to justify the high price, a sexyappearance, or just a lot of hype. This is the audio business, andreason and the laws of physics seem not to be universally applicable.

$10,000/pr

88

31 January, 2002

18

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

What do listeners say they like?
$1800/prTruly excellent sound can be found at moderate prices.

89

What do listeners say they like?
$1800/pr

90

What do listeners say they like?
$700/pr91

What do listeners say they like?
$900/pr92

What do listeners say they like?
Even entry-level products can sound good. They will come in plain
enclosures, and they may not play as loud as the bigger boys, but they
are eminently capable of preserving most of the artistic integrity.

$300/pr93

31 January, 2002

19

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

What do listeners say they like?
Here’s one to avoid! It is amazing that anyone, especially a welladvertised brand as this one is, would actually let something like thisout into the marketplace. For the same price, they could be selling
good sound. Obviously they don’t care. Instead they sell a slick
package and a story.

$460/pr94

Conclusion:
Listeners don’t like resonances!!

ALL of the most preferred loudspeakers are ones that exhibit theflattest, smoothest families of curves.
They exhibit the fewest, and the lowest level, resonances. They havethe flattest, smoothest, widest bandwidth frequency responses whenmeasured from all angles.
They have similar shapes in all of the curves – i.e. they have quite
constant, or at least smoothly changing, directivity as a function of
frequency.
Can we measure what we can hear? No, but we sure have made agood start.

95

This is a powerful position to be in, when it is possible to demonstratethat the right set of accurate measurements has a consistent
relationship with listener evaluations.

Equivalent Measurements

SUBJECTIVE

We do not claim to have mastered everything, at this stage. However,some things are understood. They even make logical sense.

OBJECTIVE

96

So, let’s assume that, to a first approximation, we understand how todesign loudspeakers that have the potential of sounding good in aroom.

The Rules for Good Sound inRooms
At middle and high frequencies:ü Start with a loudspeaker that was designed tofunction well in a variety of different rooms.
Ø Use geometry, reflection, diffusion, and absorption toachieve good imaging and ambiance.
At low frequencies:
Ø Maximize the output from the subwoofer(s).
Ø Achieve a uniform performance over the listeningarea.
Ø Equalize to achieve good performance.

The second rule for good sound requires that we look at somespecifics of the room itself.

97

Reflections alter bothSound Quality and Imaging
Reflected sounds can be controlled by:(a) controlled-directivity loudspeakers,(b) absorbing or diffusing objects onreflecting surfaces in the room,(c) the shape of the room,(d) some of each.
98

31 January, 2002

20

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

Here is a cartoon description of what happens with reflected sounds
in a room.
We start with only a floor, no walls. Brunhilde, of opera fame, issinging in the right speaker only.
BRUNHILDE IN THE RIGHT CHANNEL99

Adding walls, the one next to the right loudspeaker is some distanceaway, produces a nice warm “spatial” illusion. It sounds a little
richer.

ADD WALLS, THE RIGHT ONE FAR AWAY
& WE HEAR A ROOM AROUND THE IMAGE

100

If the wall is moved a bit closer to the speaker, we note that the lady
is a little bit “smeared”, putting on some weight, and maybe leaning a
bit to the right.

MOVE IT CLOSER AND THE FATLADY SINGS

101

If the wall is too close, the truly fat lady is singing.

STILL CLOSER AND THE REALLY
FAT LADY SINGS

102

Why? Because the wall is an acoustic mirror, creating a second
acoustical loudspeaker, just as it would create a second visual one if
the wall were optically reflective. No wonder things got a bit fuzzy.
THE REASON:
THE WALL IS AN ACOUSTIC MIRROR,CREATING A DUPLICATE SPEAKER

31 January, 2002

103

21

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

SOUND ABSORBING MATERIAL ON THE WALL
REDUCES THE “SIZE” OF THE REFLECTION
AND THE LADY IS SLIM AGAIN

104

NARROW DISPERSIONWIDE DISPERSION

FOCUSES SOUND ON
LISTENERS, MINIMIZINGWALL REFLECTIONS,MAXIMIZING CLARITY
AND INTIMACYAS INTENDED IN MOVIES.

“ILLUMINATES’ THE
ROOM, CREATING MANYREFLECTIONS WHICH ADDSPACIOUSNESS, EVEN IN
SCENES WHERE IT IS NOTAPPROPRIATE.

818 895 5761

ftoole@harman.com

Placing some sound absorbing material on the wall, at the reflection
point (have a helper hold a mirror against the wall and find thelocation where you can see the loudspeaker tweeter from the mostimportant listening position). The reflected sound is attenuated, andthe lady loses a bunch of weight.
What material? Acoustic foam or rigid fiberglass board, with orwithout acoustically transparent fabric covering.
How thick? Not less than one inch, preferably two to four inches.
How large? To be really effective, a patch at least 3 to 4 feet on aside is necessary. Tiny little “cushions” are more psychological thanacoustical. Heavy, velour drapes, densely folded also work well.
Nowadays, we know enough about horn design to be able to makethem sound really good, and take advantage of their directional
control. The days of horns that are just loud and sound like
megaphones are past – for good engineers at least.
If the room is acoustically live (the way many interior decorators likethem), then the only option is to use horns, or waveguides, to control
the radiation from the loudspeakers. This way the energy is focusedon the listeners, and kept away from the reflecting surfaces,
improving the intelligibility and directional effects.

105

The create and destroy method

Movies, especially, are designed for listeners in a strong direct soundfield. Some people use wide-dispersion loudspeakers, and then cover
the walls with sound absorbing material. This gets the job done, but
in doing so it makes the entire system work harder, first to create thesound, and then turning it into heat in absorbers. The result, dynamic
range is sacrificed. Not necessarily a good tradeoff.
Acoustically dead rooms are also not very pleasant places in which tospend time, conversing or anything else. Some custom home theaters
are like this. It is not a recommended solution.

ADDING SOUND ABSORBING
MATERIAL ALONG THE SIDEWALLS CAN
CREATE MUCH THE SAMEEFFECT AS DIRECTIONALLOUDSPEAKERS.
HOWEVER, IT IS VERYWASTEFUL
TO CREATE THE SOUND
ENERGYAND THEN TURN IT INTOHEATIN AN ABSORBER!
106

A Basic Imaging Test

Whether it is a classic two-channel stereo system, or a multichannel
system, one of my first tests is to play monophonic pink noise(available on numerous test CD’s) through the front left and rightspeakers, sit in the “sweet spot” and listen. What should be heard is acompact image of noise, floating midway between the loudspeakers.
As you move backwards in the room the image should stay. As youlean left or right, the image should move left or right. This is normal.
It is a phantom stereo image.

THE SIGNAL: BROADBAND MONOPHONIC PINK NOISE

107“Normal” Stereo Imaging
Now, put on some music. The featured artist in pop and jazzrecordings should float in the middle location. The band should be
across the front creating a solid sound stage (the success of this is
greatly dependent on the recording, so be sure to try a few). In
recordings with “ambiance”, like most in the classical repertoire, you
might sense an acoustical spaciousness around you. This is good.
108

31 January, 2002

22

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“Tightening” the Image
Some two-channel customers like to “get into the image”. For them
you can suggest some absorbing material, even heavy drapes will do,
along the side walls. This attenuates the side wall reflections and theimage “tightens” up nicely. Moving the curtains away, opens up the‘space’ again.
109

Opening it up with Reflections
Other customers like to think that they are in the concert hall. Forthem room reflections are not necessarily a bad thing. In fact, you
might consider adding a few more, using some of the commercial
diffusing elements on the market. Just be careful not to overdo it.
The test is that the center image stays intact even when you move tothe rear listening locations.
110"Confusers" and Confusion

Too much of anything can be bad
111

Even good things can be taken too far. I have been in recording
control rooms where so much diffusion has been added that the center
image is completely destroyed! The noise ‘image’ was the entire
front wall. And recordings were being made in this situation! This
design was fashionable – yes there are ‘fashions’ in acoustics too – a
few years ago. Just as in many things, some fashions are just silly.
This one was aided by the other fashion of that period: the liveend/dead-end room, another case of an idea taken to excess. It helped
some bad studio monitor speakers sound better, but it is not
something to be recommended, certainly not for recreational
listening, and not for multichannel sound.

"Confusers" and Confusion
This becomes
especially
important if theback wall is close
to the listener(s).

Many homes do not allow us the luxury of sitting away from the backwall. In those cases the last thing one would do is put diffusers
directly behind the listeners’ heads. Even a hard flat wall can disrupt
the front soundstage. A simple demonstration can convince you, or
your customer that something is wrong. While listening to the monopink noise, just hold an upholstered cushion or pillow behind the headof a listener in the stereo seat. Usually the image tightens right up.

112

Designing the Listening Experience
This becomes
especially
important if theback wall is close
to the listener(s).
In such cases,
some absorption
is appropriate.

A patch of absorbing material is a much better solution. Use diffusers
on the sides, if you like.

113

31 January, 2002

23

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

What is a “Diffusor”?
Commercial diffusers are highly specialized devices, designed toaccept sounds arriving from any angle, and then to re-radiate them inall directions. Such diffusers, then, need to be considered as
distributed sound sources.

It can be a special
surface designed
to “reflect” sounds that
arrive from any direction,
off in all directions.
It becomes a “distributed source”
with all parts sending some soundback towards the listener.
114

To a listener, these surfaces send a large number of individualreflections to the ears, from all parts of the device.

115

Or, it can be a shaped surface

Here, the conventionallaws of reflection areobeyed116

The classic “polycylindrical” diffuser, is nothing more than a curved
surface intended to break up large flat surfaces. As diffusers theywork very well indeed, and they are inexpensive. They can also be
incorporated into interesting looking architectural features, possibly
including lighting effects. If you want to get creative, there are many
regular and irregular geometrical shapes that work well. A good drywall artisan will love you for giving him something interesting to do.
Remember to bounce some of the sound vertically too. If the
diffusion is to be effective over middle as well as high frequencies,
some of the shapes must be a foot or more deep. The notion that
textured paint does anything consequential is another fantasy.

A listener receives only one reflection from each of the curved
surfaces.

117

Or it can be furnishings: bookcases,
cabinets, fireplaces, etc.

If the listening room is also a normal living space, it may not benecessary to use any special acoustical devices at all. With a little
thought, bookcases, display cases, paintings, fireplaces, etc. can all dothe job without making the room look at all “technical”.

118

31 January, 2002

24

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

In contrast, a wall . . .

Flat empty walls not only look stark, but they sound that way too.

119

120

An acoustical consultant walks into a room, stands by the door, claps
his hands, furrows his brow, and pronounces that this room has reallybad flutter echoes and you need his (expensive) help to fix it. If thishappens, say goodbye.
The only flutter echoes that are important to the quality of reproduced
sounds in the room are those that are excited by the loudspeakers
themselves. Have an assistant clap hands at the loudspeaker locations
while you listen from the relevant locations in the room. If there is aproblem then fix it. It matters not that flutters can be heard from the
top of a step ladder.

Evaluating Flutter Echoes

121

Geometric irregularities on walls,
furniture and diffusing elements
are excellent cures for flutter
echoes.
They eliminate the problem
without absorbing sound.

It is amazing how little it takes to cause an audible flutter, and it isamazing how little it takes to get rid of one. I have seen a picture,hung on a slight angle, do the job. Moving a bookcase, adding a wallbulge over a fireplace, a two-foot square patch of diffuser or absorberin a large wall, all have solved annoying problems without absorbing
significant sound.

122

Coming up in Part 3
Perfecting the low frequencies. How many subwoofers? Where do I put them?
Where do I sit? What about “bass traps”? How do I get rid of “room boom”?
Here we look at what it needed for truly excellent bass performance in rooms.
An understanding of room modes, or resonances, is essential to achieving
uniform bass over a listening area. The right kind of equalization can help to
31 January, 2002
25

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

make that bass sound good, but it cannot do everything. Some traditional
forms of equalization have a good chance of getting it wrong. Interestingly,
two or more subwoofers, strategically located, can be very beneficial.

31 January, 2002

26