Open baffle study, August-2003

The aim of this study is to investigate the impact
of baffle size and shape on frequency response.

Setup:
SEAS W15CY001 + Point75A crossover, 12 dB high-pass @ 300 Hz,
18 dB low-pass @ ~3000 Hz
Basic baffle size = 110 x 24 cm,
15WCY001 mounted at 70 cm height.

Additional baffles available:
2 pcs 110 x 2 cm
2 pcs 110 x 6 cm
2 pcs 110 x 9 cm
2 pcs 5cm/25 cm sloped baffles.
All tests performed with vertical baffle.

Microphone distance: 50 cm unless otherwise stated.
Signal start = 1.3 ms, signal stop = 5.6 ms (5.6 ms
is the equivalent of 1.8 m, i.e. signals from rear
and front should be fully integrated). Lower reliable frequency =
216 Hz. Disregard readings below 200 Hz in the following
presentations.

All response graphs show 100-20,000 Hz.

Example:
The distance from front to rear of the mid-driver of the Point75 is
45 cm. Corner frequency = 34400/45*2 = 382 Hz.
The point of crossover for bass and midrange in the Point75
is ~300 Hz and we can take advantage of the natural roll-off
in dimensioning of the crossover network.

Width of baffle for given lower point of crossover =
34400/Fc*2, where Fc = corner frequency.

In practical terms the width can be somewhat smaller.

Baffle = 24 cm, flat, symmetrical

The driver is supposed to have a flat frequency response in the
300-3000 Hz range.

Baffle too narrow to give sufficient response at both ends.

Baffle = 24 cm + 2 cm + 2 cm = 28 cm, flat, symmetrical

Slightly improved response at low end but suck-out at 1800 Hz.

Baffle = 24 cm + 6 cm + 6 cm = 36 cm, flat, symmetrical

A severe dip is seen at 13-1400 Hz.

Baffle = 24 cm + 2 x 6 cm = 36 cm, flat, asymmetrical

Improved response in 1-2 kHz region compared to C.

Baffle = 24 cm + 9 cm + 9 cm = 42 cm, flat, symmetrical

This is not much better than D. Severe dip at 1300 Hz.

Baffle = 24 cm + (9 cm + 9 cm, 45^o), symmetrical.

Bending the side-panels 45^o to the rear improves the
situation considerably, but a small peak at 1600 Hz occurs.

Baffle = 24 cm + (9 cm + 9 cm, 90^o), symmetrical.

Bending the side panels 90^o to the back does not improve the
situation compared to F.

Baffle = 24 cm + 9 cm, flat, asymmetrical.

This situation appears to be the best response obtained so far.
Flat response from 500 - 2000 Hz. Smooth roll-off at both ends.

Baffle = 24 cm + (1 x 5/25 cm) side panel, flat, asymmetrical.

Suck-out at 1200 Hz compared to H.

Baffle = 24 cm + (2 x 5/25 cm) side panels bend 60^o to the back.

This situation is close to resembling the P75 cabinet and produces
a reasonable response from 400 - 2000 Hz.
Response is approximately 6 dB down at 300 Hz as supposed to be,
and 6 dB down at 2500 Hz approximating an 18-24 dB slope upwards.
The electrical slopes are 12 and 18 dB respectively.

Point75 cabinet.

As can be seen the response here resembles the J-situation with a
smoother roll-off characteristic at 2.5 kHz.

^{Red = W15CY001 response in P75 cabinet
with P75 crossover vs. same driver on
simple baffle: 28 cm x 110 cm,
added 9 x 110 cm side-panels at 90o.
(200-5,000 Hz)}

What can be learned from this little exercize?

• The baffle size can be of reasonable dimensions if a point of
  crossover around 300 Hz is used.
• In an open baffle situation, the response curves should be
  avaluated with caution as the overall perceived sound will be a
  mix of direct and reflected sound. This makes crossover con-
  struction and final "voicing" of the total system considerably
  more difficult.
• The addition of side panels to the main front panel has a
  significant impact on the overall frequency response and should
  be based on careful testing of a variety of baffle components.
• If we use a lower point of crossover like 100 or 200 Hz, like seen
  in the Orion loudspeaker by Linkwitz, equalisation is needed to
  overcome the natural roll-off characteristic.

Obviously this study should have been performed before construction
of Point75! Hmm.... The design was more or less given, being
derived from a commercial design that seemes to work properly
with the chosen points of crossover.

Does this study tell everything about a 35 cm wide "curved" baffle?
It doesn't. What you measure is only derived from the choices made
when it comes to measurements. Distance, angle, etc.
But I think it gives a fairly good picture of how the driver is
performing within the chosen framework and it helps insuring
proper performance within +/- 2 dB.

As can be seen from the graph to the left, a more simple construction
may be applied to the W15 driver with much the same response as
seen from the P75 cabinet.

Further measurements below: Vifa P13WH mounted on an open baffle
with no crossover.

Vifa P13WH driver as dipole:

Basic baffle dimensions: 24 x 110 cm
+ additional side-panels.

P13WH, 100-20,000 Hz

H:

Vifa P13WH mounted on a 24 + 6 cm baffle, asymmetrical.

No croosover is used here and this well-known driver exhibits a smooth
response all the way from 300 Hz to 6,000 Hz. No wonder this driver has
has gained widespread acceptance for well-behaved performance and ease
of use when it comes to crossover construction.

G:

Vifa P13WH mounted on a 28 + 9 + 9 cm baffle, symmetrical.

A major part of the dipole constructions I have come across utilises this
panel construction. If used below 250 Hz some driver equalisation may be
necessesary to reach a possible subwoofer at 100-150 Hz. Very smooth
response between 500 and 5000 Hz.

.

Vifa P13 mounted on "Point75" baffle

Only minor deviations are seen from this driver without crossover,
mounted on three quite different baffles.
Too bad the resolution of the P13 driver is somewhat limited.

Vintage stuff:
Read 1956 review of Wharfedale dipole speaker.
Download zip file, 700 KB