kooncewright2932.pdf

Tube, Solid State,

Loudspeaker Technology

Article prepared for www.audioXpress.com

Dayton Audio WT3 Woofer Tester

By G.R. Koonce and R.O. Wright, Jr.

IntroductIon (By GrK)

When I agreed to do an equipment review

on the WT3, I recalled that many times

coauthor Bob Wright had purchased a

WT3, so we agreed to do the review to-

gether. Bob has considerable experience in

measuring V as via the added-mass method

and will concentrate on that topic. This

gives results from two WT3 units and two

users—a more balanced review because I

tend to be a nit-picker.

agreement stating what you can and can-

not do with the software.

2. Readme.Txt—Tells where to get up-

dates and current information. An item I

found confusing is this document lists the

operating system requirement as Windows

98SE, while other places list Windows 98.

If running early Windows 98, you should

check with Parts Express before ordering.

3. WT3 Quick Start.Pdf—A copy of the

two pages on “Quick Start for the WT3

Woofer Tester” that you can view with

Adobe software.

4. WT3_setup.Exe—The program that

contains and installs the WT3 software.

The Dayton Audio WT3 Woofer

Tester is available from Parts Ex-

press, 725 Pleasant Valley Drive,

Springboro, OH 45066-1158.

The catalog price is $99.88 1 . See

daytonaudio.com and parts-

express .com. No serial number

was found on the units .

GRK operated the WT3 from an HP Pa-

vilion zv6000 laptop computer, 1.99GHz

processor, 512MB of RAM, and MS

Windows XP Home Edition version

2002. ROW’s computer is a custom-built

64-bit CAD/graphics computer using MS

Windows XP as an operating system.

HIStory

The “Woofer Tester” name has been ap-

plied to several earlier and current devices.

In 1994 Dickason 2 reviewed one sold by

C&S Audio Labs (Model DSP-126), and

in 1997 Weems 3 reviewed a revised version

(Version 4.4) sold by Parts Express. These

AC powered devices both used DOS-

based software. Currently a variety of de-

vices with this name, having additional

options and higher cost, are available.

SySteM reQuIreMentS

To operate the WT3 you need an IBM

PC or compatible computer with the fol-

lowing characteristics:

1. 64MB memory RAM or more.

2. 500MHz Pentium III processor or

higher.

3. A USB port, USB 2.0/1.1 compliant.

4. MS Windows 98(SE), Me, NT, 2000,

XP, or Vista.

WHat WILL It do?

While you may use the WT3 to measure

impedance of any passive electrical cir-

cuit, we are interested in its application

to speaker work. The WT3 directly mea-

sures the input impedance of a driver or a

speaker system. With the WT3 you may

do the following:

1. Directly measure the f s and Qs of

a driver.

2. Directly measure the driver V as by

any of three techniques.

3. Directly measure inductors of the

size used in crossover work.

4. Develop driver zobels and other

impedance correction networks for

crossover design.

5. Help tune a vented-box design by

locating the impedance minimum

frequency which approximates the

box tuned frequency.

6. With the proper modeling or pre-

vious-test results, you can verify the

WHat you Get (By GrK)

The WT3 arrives in a box about the size

used for handheld DVMs. Inside

are the WT3 tester, a calibration

resistor, the software CD, and an

instruction sheet covering how to

get started. The WT3 itself is a

rather small unit with built-in cables

( Photo 1 ). The computer side has

a 4′ cable with a USB connector,

while the test side is a short pair of

red and black wires with alligator

clips. The WT3 is self-powered via

the USB cable with a blue LED

indicating power on.

There were four files on the CD:

1. License.Txt—The licensing

PHOTO 1: WT3 unit setting on a laptop computer.

audioXpress 2008 1

FIGURE 1: Test Lead Calibration with and without leads twisted.

FIGURE 2: WT3 main screen.

proper wiring of crossover networks.

7. With additional software you can calcu-

late how severely a speaker system stresses

an amplifier.

8. With simple software you can predict

the bass response curve for a closed-box

speaker system.

9. With additional software you can design

all the standard bass enclosure formats.

playing! The program will print the help

information, so I did this for off-line study.

Figure 2 shows the very busy main

screen for the WT3 software. It is in color,

which helps a lot. You need some study of

the help information to really understand

program operation. Once you have learned

how, the operation is simple and straight-

forward.

The program has multiple file options

as follows:

1. It will save and recall project files that

save what you have committed to the 20

available memories. This allows retrieval of

information from saved testing should

some question arise.

2. It will save and recall impedance curves

to/from export files. It offers the option of

files with the extension of .TXT or .ZMA.

These are both text files and look identi-

cal. Unfortunately, the .ZMA files do not

match the original format developed by

Liberty Instruments for the IMP tester.

While certain software may properly read

the WT3 .ZMA files, the post-processing

programs I had written would not. They

objected to the long text header before

the actual data. If you have this problem,

you can use a text editor to put the WT3

.ZMA files into a format your software

can read. The quickest solution was for me

to write a filter program that takes in the

WT3 .ZMA file and writes it out in the

classic format of a Liberty Instruments’

.ZMA file.

3. It will save and recall driver files (exten-

sion .DVR) that are common with the

format used by the WinSpeakerz program.

ability to output the graphs to files as pic-

tures. This would be useful when using the

WT3 to prepare material for publication,

such as this review.

The program also offers several options

relative to the printed output. You should

check the options under both the File ->

Print Setup... and the Edit -> Preferences

menus.

GettInG Started (By GrK)

The “Quick Start...” instructions tell you

how to install the WT3 program and get

the unit running. For me installation went

without a problem. Once installation is

accomplished, you do two calibration pro-

cedures, as noted in the instructions. This

calibrates the WT3 with a precision 1kΩ

resistor and establishes the resistance of

the test leads.

When running the test lead calibration,

I noted that the plotted lead “resistance”

showed a rising magnitude above 2kHz.

The rising phase shift indicates this is due

to inductance, which is related to loop

area, so I repeated the calibration with the

two leads mildly twisted together. Figure 1

shows that this helps slightly, and for the

following testing I kept the leads twisted.

repeataBILIty (By GrK)

One of my first applications for the WT3

was to measure the impedance of a baffle

mounted woofer several times. The results

for ten tries are shown in Table 1 .

I think this repeatability is very good.

I no longer beat the box design out to

several decimal places because the pa-

rameters will vary with temperature and

humidity, and so on, and the box response

changes slowly with minor parameter

changes. The WT3 is so quick you should

run the free-air test on a driver a few

times to verify that the parameters you

are going to use for the V as extraction are

a typical set.

taBLe 1: MeaSureMent of repeataBILIty

of tHe Wt3

optIonS (By GrK)

When I first started the WT3, I did not

like the dark background on the plots (a

nit-pick). It made the curves hard to see.

After viewing the information in the help

section, I found I could change the plot

background color. The program has many

options of what you can display and print,

and it is recommended you study the help

files once you have finished your initial

Tr y

R e

f s

Q ms

Q es

Q ts

L e

5.95

49.8

1.66

0.62

0.45

0.59

5.96

49.8

1.66

0.62

0.45

0.59

5.98

49.8

1.66

0.63

0.46

0.59

5.93

49.8

1.66

0.62

0.45

0.59

5.95

49.8

1.66

0.62

0.45

0.59

5.98

49.8

1.66

0.62

0.45

0.59

5.96

49.1

1.63

0.62

0.45

0.59

5.97

49.1

1.64

0.62

0.45

0.59

5.96

49.8

1.65

0.62

0.45

0.59

5.95

49.8

1.65

0.62

0.45

0.59

The one missing file option seems to be the

2 audioXpress 2008

www.audioXpress.com

FIGURE 3: Input impedance to the woofer measured by WT3

and Audiosuite.

FIGURE 4: Input impedance to the Mid/Bass measured by WT3

and Audiosuite.

coMparISon WItH LIBerty

InStruMentS’ audIoSuIte

(By GrK)

In an attempt to determine the accuracy of

the WT3, I free-air tested the three drivers

in Table 2 with both the WT3 and Au-

diosuite. Figure 3 shows the comparison

results for the woofer mounted on a small

baffle. The two curves match extremely

well; only near the resonance and around

20kHz can you identify that there are two

curves on the plot. The same comparison

is shown for the mid/bass driver in Fig. 4

with, again, a near perfect match. Figure

5 is the comparison for the tweeter also

showing very good agreement. The slight

differences seen at high frequency may be

due in part to the WT3’s lead inductance

as discussed earlier.

I had both units compute the driver

T/S parameters. Only the woofer had V as

extracted via the closed-box method. Table

3 compares the test results obtained with

the catalog 4 data.

The two measurement systems show

reasonable agreement with each other but

not always with the catalog data. With

two different test instruments some vari-

ation is to be expected. For the woofer,

either measured set of parameters should

result in a reasonable enclosure design. It

might not be the same size as one based

on the catalog data.

For the mid/bass and tweeter the mea-

sured data is in better agreement with

the catalog data. These drivers were all

new and had no break-in. My experience

is modern drivers don’t change as much

with break-in as we saw in the past.

The WT3 allows you to extract the

driver V as the “easy way,” by specifying the

driver SPL in dB/W/M. This technique

must be used very carefully. First, some

manufacturers will specify the SPL in

dB/2.83V/M. If your driver is truly 8Ω,

then the two specifications are the same,

because 2.83V RMS will put 1W into an

8Ω load. If the driver is other than 8Ω,

you must correct the dB/2.83V/M rating

according to the following:

dB/W/M = dB/2.83V/M + 10 *

Log 10 (R n /8)—where R n is the nominal

driver impedance

For example, a 4Ω driver with a

91dB/2.83V/M SPL would add 10 *

Log 10 (0.5) or -3dB yielding SPL = 88dB/

W/M. This makes sense because 2.83V

RMS would deliver 2W to 4Ω, which is

3dB higher than 1W.

I had the WT3 extract the V as in cubic

feet for the woofer and mid/bass drivers

based on the catalog SPL ( Table 4 ). The

value for the woofer does not agree well

with the closed-box measured value of

0.43-0.45 or the catalog value of 1.23. The

mid/bass result of 0.079 is much closer to

the catalog value of 0.09ft 3 . If the free-air

f s and Qs reported by WT3 agree well

with the catalog values, then this tech-

nique is probably safe. If those values do

not agree with catalog data, then I would

not use the SPL approach to predict V as .

If the catalog f s and Qs agree with your

driver, then you might just as well use the

catalog V as value.

taBLe 2: tHree teSt drIverS uSed By

GrK (aLL neW WItH no BreaK-In)

Woofer

Vifa* P17WJ00-08 6½ ″

Mid/Bass

Vifa* MG10SD09-08 4 ″

Tweeter

Vifa* XT25TG30-04 1 ″

dual concentric ring

*Note: Tymphany Peerless now calls these V-Line drivers

taBLe 3: coMparISon of Wt3 and audIoSuIte reSuLtS

6½ ″ Woofer

4 ″ Mid/Bass

Tweeter

WT3

AuSw

Cat.

WT3

AuSw

Cat.

WT3

AuSw

Cat.

Units

R e

6.01

5.97

5.8

5.42

5.23

5.5

2.93

3.08

3.0

Ohms

f s

49.8

48.6

91. 5

92.0

513

500

Q ms

1.56

1.40

1.55

5.34

5.22

3.40

2.84

2.74

2.5

Q es

0.61

0.57

0.45

0.88

0.82

0.73

0.62

0.63

0.71

taBLe 4: v aS extractIon BaSed on SpL

Driver

Q ts

0.45

0.41

0.35

0.76

0.71

0.60

0.51

0.55

V as

0.45

0.43

1.23

0.09

Cu. Ft.

SPL

V as - Cu. Ft.

Woofer

0.74

L e

0.59

0.55

0.22

0.33

Mid/Bass

0.079

SPL

85.8

85.7

88.5

dB/W/M

audioXpress 2008 3

obtain good results. It,

too, is a “power” func-

tion in the WT3 pro-

gram equation and

must be done quite ac-

curately.

When I was search-

ing for a suitable

added mass for the

driver, I found that

“poster caulk” obtain-

able at any office sup-

ply store turned out to

be an ideal choice. It

is low-cost, easily ob-

tainable, does not stick

to the driver cone, and

stores well in a sealed

container. The required added mass is al-

most always 200 grams or less (1 gram =

0.035274 ounces).

To accurately weigh the added mass, I

found in my research that there are scales

on the open market referred to as “pocket

scales” that have a weighing capacity of 0

to approximately 200 or 400 grams and

are accurate to within 0.1 grams (0.1 gram

= 0.00022046 pounds). This accuracy is

excellent for driver measurement work.

The cost of these scales is approximately

$25.

When executing this method shape the

poster caulk into a “donut” and apply it

to the driver cone. This allows the added

mass weight to be evenly spread over the

FIGURE 6: Effective piston diameter

calculation for a circular driver.

FIGURE 5: Input impedance to the tweeter measured by WT3

and Audiosuite.

MeaSureMent of drIver

v aS vIa tHe added-MaSS

tecHnIQue (By roW)

In the WT3 program under “Driver Pa-

rameters,” the second of three methods

provided for in the program for determin-

ing the driver characteristics is named the

“Added Mass Method.” This method uses

two physical parameters to generate the

basic outputs of the program. These are

(1) the effective piston diameter designat-

ed “D” in the program input and (2) the

added mass designated “M” in the pro-

gram input. The speakers used in this test

were not “broken in.” It has been my expe-

rience that the T/S values do not change

significantly under these conditions.

It is very important to get a very accu-

rate effective or active cone diameter and/

or equivalent cone diameter. It is generally

accepted that the mean piston diameter

includes one-half of the speaker surround.

This measurement must be done as ac-

curately as possible, because the area of

the driver cone is a “power” function in the

WT3 program. Any inaccurate input to

the program will make a more than pro-

portionate difference in the results gener-

ated. Figures 6-8 give real and equiva-

lent mean diameters of the most common

driver cone shapes found on the market

today.

The mass to be added to the speaker

cone must be weighed very accurately to

FIGURE 8:

Effective pis-

ton diameter

calculation for

rounded paral-

lelogram driver.

FIGURE 7: Effective piston diameter calculation for ellipsoidal

driver.

4 audioXpress 2008

www.audioXpress.com

surface of the cone, emulating a balanced

cone. The amount of weight to be added

is determined by the WT3 program it-

self. However, there are some suggestions

in the help file of the WT3 program.

The program will analyze the amount of

weight added to the cone and determine

whether it is adequate to properly calculate

the driver parameters and, when necessary,

instruct the user as to what remedial ac-

tion should be taken. When all of the

input parameters have been satisfied, the

program will produce a driver parameter

datasheet.

The drivers selected for testing were

two 8″ woofers and two 6½″ woofers.

Based on a recommendation by George

Augspurger, we tested each woofer with

three different values of test weight, then

averaged the V as values. The results of

these tests are listed in Table 5 .

In the 6½″ unit 1 of the woofer testing,

the V as variations from three different test

weight values were 0.958 (44.4g), 0.901

(59.5g), and 0.808 (74.7g), with the aver-

age = 0.889 the others are not listed.

ment of such shunt networks.

A corollary application of the WT3

is in verifying that you have connected

the components in a crossover correctly.

If you know, from previously testing or

modeling, what the impedance into the

crossover should be, then the WT3 can

quickly verify you are still showing that

impedance.

Last summer I was simultaneously

working with three pairs of breadboard

three-way crossovers. This involved mak-

ing changes in them and trying them on

speaker systems. The WT3 would have

been ideal in verifying I had the pairs

wired the same and that I had not mis-

taBLe 5: reSuLtS for dayton dc160S-8 6½" Woofer and dayton dc200-8" Woofer

6½ ″

Catalog 8 ″ Catalog

Unit 1

Unit 2

Data

Unit 1

Unit 2

Data

Units

f s

31.6

30.3

Q ms

2.04

2.28

3.89

2.23

1.93

3.23

Q es

0.34

0.33

0.34

0.53

0.47

0.44

Q ts

0.29

0.32

0.43

0.38

V as

0.89

0.92

0.78

1.28

1.31

2.04

Cu. Ft.

L e

1.54

1.49

2.40

2.08

1.86

1.60

SPL

86.0

84.9

dB/W/M

ZoBeL and otHer

coMpenSatIon netWorK

deveLopMent (By GrK)

The WT3 is ideal for developing zobels

and other impedance compensating net-

works. Figure 9 shows the free-air im-

pedance into the test woofer with and

without a simple R-C zobel as printed by

the WT3. It is easy to play with the zobel

values and have the WT3 retest because of

its speed.

Last summer I was developing a system

using a first-order crossover with a tweeter

having a very high impedance peak at

resonance. This was causing trouble with

the low slope of the first-order high-pass

network. Figure 10 shows the bare tweet-

er impedance and the impedance with a

zobel and series L-R-C network across the

tweeter to tame the resonance peak. Note

that not only do these networks improve

the magnitude, but they also improve the

phase angle.

My experience is that correcting such

effects by networks shunted across the

driver results in a better-sounding system

than doing it with networks placed in se-

ries with the driver. My rule for crossover

design is to keep the number of compo-

nents in series with the driver to a mini-

mum. The WT3 is ideal for the develop-

FIGURE 9: Input impedance to woofer with and without zobel.

FIGURE 10: Input impedance to tweeter with and without compensation networks.

audioXpress 2008 5

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