OA-129

“OA-129” Floorstanding three way speaker

OA-129

The design idea
These were the first speakers adapted for stereo from Stig Carlsson, they were intended to be placed against a wall playing diagonally into the listening room. The difference compared to earlier omni directional models is that here a quota of direct radiated sound is introduced, the quota of direct sound needs to be two to four times the reflected sound.

Placing the midrange near the back wall tilted towards the listening position results in a quota that is not changed with the frequency.

Tweeters directional properties are determined by the size of the speaker and the size of the membrane. The directional pattern is that high frequencies have smaller dispersion and low frequencies have higher dispersion. Therefore, omni directional speakers need to use several tweeters oriented in different directions. To get the desired result of increased quota of direct sound there has to be at least one tweeter facing the listener and one for the reflected sound. The direct sound needs to be between 1/2 and 7/9 of the total sound. The direct sound shall be within 90 degrees horizontally. The tweeter(s) for reflected sound needs to be placed minimum 30 cm, preferably 40 or 50 cm, from the back wall.

It is a characteristic property of an omni directional loudspeaker that it gives an integrated and relaxed sound that is similar to a natural sound source placed freely in a room. This property has been maintained in this design. This indicates that an omni directional loudspeaker is suitable also for stereo use. According to Stig Carlssons ideas the correct quota of direct- and reflected sound is between 1/2 (OA-12) and 7/9 (“OA-129”). For more information on this see The theory behind OA.

This is the design that most likely became the OA-2212 in production. All other models from this patent can be easily recognised as OA-12, OA-14 and OA-116. The design of “OA-129” is described as a modification of the OA-116 design.

It was an interesting design as it did not require production to produce left and right models, the idea is to turn the speaker 90 degrees to get left or right.

TECHNICAL DATA

Measures
Box volume: 65 litres estimated
Measures W x H x D: 45 X 45 X 55 cm

Drive units, estimated
Woofer: 1pc. 23 cm woofer
Midranges: 2 pcs 16.5 cm, probably SC-165
Tweeters: 9 pcs. 5 cm, probably Peerless MT 20 HFC

Crossovers
Cross over frequencies: 400 and 1800-2500 Hz (OA-116)

Explanations to figure
1a: Midrange
1b: Midrange
2: Seven tweeters*
3a: Tweeter for reflected sound
3b: Tweeter for reflected sound
4a: Front side of left speaker
4b: Front side of right speaker
5a: Inner side of left speaker
5b: Inner side of right speaker
6: Baffle with raised part for midrange
7: Baffle for woofer
8: Bass reflex port
9: Feet
10: Grill
16: Woofer

* In a modified version of this speaker Stig Carlsson suggest the use of one electrostatic tweeter instead of the seven tweeters for direct sound.

Martin Dellstig – Co-Editor

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