AudioXpress 2006 07.pdf

(8932 KB) Pobierz

AUDIO SCENE IN THE NETHERL ANDS

JULY

2006

US $7.00 • Canada $10.00

Back

The Audio Technology

Authority

This TL Sub Is

SPEAKER COLUMN

UPGRADE

“Outta Sight”

HP Test Box

Restored

A FRESH LOOK AT

PHASE INVERTERS

Constructing the

KT88-

Based Amp

www.audioXpress.com

plus

Parts Upgrade for

the CLASSIC SCOTT AMP

speakers

Back

The Hideaway TL Sub

A Danish blueprint for an “invisible” subwoofer.

By Bjorn Johannesen

nspired by Rick Schultz articles,

I made a successful transmission

line to replace a vented sub. How-

ever, I was surprised that the reso-

nance frequency was lower than I had

expected—lower than the driver’s Fs. I

had the impression this was “forbidden,”

but I could see it was working very well

and much better than the same driver

in a vented box. Therefore, I started to

study Martin J. King’s quarterwave de-

sign. My own experience and the useful

information and supportive help from

Martin led to this article.

DISAPPOINTING SUBWOOFER

Some years ago, I made two subwoofers

enclosure was a bass reflex with 81 ltr

internal volume. The driver was a 10

Beyma 10B60 (Fs 31Hz, Qts 0.36, Vas

115 ltr, and Sd 380cm

). You can find all

the data for this discontinued driver on

www.thielesmall.com.

I was not happy with the result: noth-

ing below 30Hz, port noise, and prob-

lems with uneven frequency response due

to room resonances. Furthermore, these

cabinets are not easy to hide away. Actu-

ally, they were hidden away. Far away.

Twenty years ago, I built horns like ev-

erybody else at that time. The horns

were quarterwave designs with expand-

PHOTO 1: Completed unit, with the line

painted black to make it “disappear.”

TRANSMISSION LINE SUB

ing pipe and a driver offset down one-

third of the line. Quarterwave designs

are not outdated at all, as I found out by

reading articles by Rick Schultz. I want-

ed to design and build a transmission

line (TL), which is the term I use for all

types of quarterwave design. Because I

had two bass reflex speakers, I would be

able to compare the TL sub against the

vented enclosure.

My idea was to mount the driver in a

small box and let the pipe itself be small-

er, so I could hide it under the couch.

Of course, this would reduce the volume

of the cabinet, and you could expect a

weak bass. But because both the open-

ing and the driver are close to the floor, I

FIGURE 1: Simple and practical sub design.

hoped to get a lift in the deep bass. With

the opening at the front, you could even

place the sub in a corner to get maximum

room gain. I ended up with this design,

using a small opening, to be able to place

the cabinet just about anywhere (

Fig. 1

The box itself should not be very vis-

ible. The size of the rest of the cabi-

net was dictated by the space under my

couch. I made the opening small, set

to about 20% of the cross section (and

thereby 10% of the derived Sd).

The total length of the pipe is 230cm,

and I assumed, the pipe was tuned to

about 37Hz. But I soon found out this

was not the case.

I used relatively thin material and in-

cluded a cross bracket in the box. The

front baffle is 22mm MDF. The stuffing

is relatively light in the box and for the

first line of the pipe only.

THE RESULT

In comparison with the vented box,

the Hideaway bass is lower, clearer, and

Photos 1

and

show the working unit.

audioXpress

7/06

www.audioXpress .com

PHOTO 2: The visible

part is only 32

32cm.

tighter. There is no noise caused by tur-

bulent air in the opening.

The cone motion is reasonable down

to 20Hz, but at 16Hz the movement

increases. There is still output at 16Hz,

even though the output drops from 20

to 16Hz. This surprised me. It was clear

that the tuning frequency of the pipe

was lower than I had expected and lower

than the driver Fs. I wanted to find out

what was happening.

I started to study the

J. King, who has been

patient with my

questions (

see

sidebar).

I could

see that the tun-

ing frequency was

about 25Hz, and

by the use of Mar-

tin’s worksheets,

which can be used

for private and

non-commercial

purposes, I learned

what was going on.

If the line had

been a straight

line, the tuning

would be around

37Hz. Actually,

the Hideaway en-

closure behaves

like a mass-loaded

TL with a port,

which lowers the tuning frequency of

the cabinet.

The plot from Martin’s worksheets,

Acoustic impedance of the cabinet,

showed the tuning frequency was about

25Hz (

Fig. 2

). The first peak is at 25Hz,

which is the tuning frequency of the

cabinet alone.

TUNING FREQUENCY

work of Martin

very helpful and

The dotted line in

Fig. 3

is the driver in

an infinite baffle (IB). The SPL might

look poor, but because the driver and

the opening are at the floor, there is a

SOUND

PRESSURE LEVEL (SPL)

FIGURE 2: Cabinet tuning is 25Hz.

FIGURE 3: Room gain makes it sound better than it looks.

audioXpress

July 2006

good deep bass performance in the lis-

tening room. Because this is a sub, I do

not worry about the uneven frequency

response above 100Hz, which will be

eliminated by the crossover filter.

Fig. 4

, the solid line is the driver,

and the dotted line the opening. As you

can see, the opening is adding at lift to

the bass where the driver is rolling off.

The output from the aperture is broad

and flat. Also notice the fall output from

the opening, with a rolloff starting at

about 85Hz. The relatively small volume

and the mass loading by the small open-

ing causes this unique behavior of the

output from the opening.

In the

Fig. 5

plot, the enclosure is not

stuffed, to make the impedance peaks

more visible. With stuffing the imped-

ance curve becomes very flat, and some

of the peaks hard to see.

The dotted line is the driver in an IB.

The solid line is the combined enclosure

and driver, forming the new resonance

system. The first resonance (impedance

peak) just below 15Hz is a combination

of the air mass in the pipe combining

with the moving mass of the driver cone

and voice coil oscillating on the driver’s

suspension. Very much the same as the

old trick of lowering the driver Fs by

adding some physical mass to the cone.

The second resonance peak about

60Hz is the original cabinet resonance,

but with the influence of the driver. The

rolloff from the opening is about 85Hz,

which corresponds to the dip in the im-

pedance curve between the second peak

at about 60Hz and the third peak at

about 98Hz.

IMPEDANCE

PHOTO 3: The cabinet is

relatively thin, but there are

little vibrations.

A GENERIC DESIGN

I have modeled several other drivers in

the Hideaway enclosure, and while this

versatile design does not necessarily get

the very best of each individual driver,

the drivers I have simulated are all doing

fine. The tuning frequency of 25Hz is a

reasonable value for drivers with a Fs of

35Hz and below. A driver with the same

Fs, but higher Qts and lower Vas has a

little improvement in the deep bass, due

to the later rolloff.

FIGURE 4: The contribution from the

opening is shown by the dotted line.

FIGURE 5: Impedance shown with no

stuffing.

audioXpress

7/06

www.audioXpress .com

The Peerless 830452, with the very low Qts of 0.17, is also

doing well. In this case the cabinet tuning is higher than the

driver Fs of 18.9Hz, which is a good combination for a low

Qts driver, which has an early rolloff.

CONSTRUCTION

line length is 230cm, internal dimensions of the box are 29 ×

29 × 29cm, and cross section in the pipe is 12.8 cm (width) ×

15 (height). The footprint is 30 × 140cm using 10mm par-

ticleboard. Front baffle is 22mm MDF, which then extends

the footprint in this end.

The box uses 10mm particleboard, with 10mm MDF

added inside to reduce vibration. The box also has a cross

bracket inside. The “labyrinth” walls measure 150 × 14mm. I

added triangular pieces of wood in all corners. The labyrinth

is closed on the top with 10mm particleboard (

Photo 3

The box measures 32 × 32 × 32cm externally, and by plac-

ing a piece of glass on the top, the box becomes a nice little

table.

Photo 4

gives you an idea of the cabinet size. Stuffing

in the box and the first half of the labyrinth is 200g of poly-

ester wool.

Designing a TL is a challenge, because you have many pa-

rameters to work with. But this is also the reason TL design

is great fun. I use a design idea as a starting point; I do not

choose a driver, and try to find the best design possible. Try-

ing to achieve maximum performance of a driver most likely

would result in a cabinet size and shape you probably would

not like to place in your listening room. Designing a TL is a

recursive process, with the goal of finding the best compro-

mise.

The most important design parameter is defining the tun-

ing frequency of the cabinet. If you increase the tuning fre-

quency, the output level from the opening will increase. And

when you lower the tuning frequency, the output level will,

not surprisingly, decrease.

The output from the opening is a desired property of the

TL, adding a lift to bass. If your driver has a high Qts, you

Figure 6

gives the unit’s external dimensions. In addition, the

THE NEXT GENERATION CAPACITORS

DESIGN CONSIDERATIONS

Optimized Winding Geometry

Extremely short, low-loss signal transmission

Extremely reduced residual-resistance (ESR)

Considerable low residual-inductivity (ESL)

Polypropylene capacitor-foil, alu metallized

Massive connection plate

Best value for money

MCap® RXF (Radial Xtra Flat)

Oear

ap Y

C he

•

MCap Supreme

Silver/Gold

Please visit www.humblehome

madehiﬁ.com and ﬁnd "The Great

Capacitor Test" featuring a liste

ning comparison of 20 caps. Aurally

the very best: MCap

Supreme

Silver/Gold followed by MCap

Supreme Silver/Oil on 2

place.

•

TubeCap

- as compact as an

electrolytic capacitor, as fast and

voltage-proof as a ﬁlm capacitor.

Due to its considerable low ESR

and lower residual inductivity the

TubeCap contributes to a ﬁner,

livelier sound. Advanced no drying

out and self-healing properties.

•

MLytic HV

- electrolytic capacitor

with the latest technology and

best characteristics. Following

many classic designs, these

capacitors are offered as double

capacity and are perfectly suitable

for repair and upgrading.

•

MLytic Audio Grade

- compact

electrolytic capacitor providing

remarkable low ESR and ESL

values as well a low inner-sound

developement.

Recommended US Dealer

MADISOUND SPEAKER COMPONENTS

www.madisound.com

Exclusive Canadian Distributor

AUDIYO INC.

www.audiyo.com

Exclusive Australian Distributor

SOUNDLABS GROUP PTY LTM

www.soundlabsgroup.com.au

TUNING FREQUENCY

More audiophile innovations on

�½�½�½�½�½�½�½�½�½�½�½�½�½�½�½

PHOTO 4: The small opening at

the driver end does not make any

noise from air turbulence.

�½�½�½�½

�½

�½�½�½�½�½�½�½�½�½�½�½

�½�½�½

�½�½�½�½�½�½

OEM and dealer inquiries invited

audioXpress

July 2006

Plik z chomika:

gasma

AudioXpress 2006 07.pdf

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: