Acoustic design of the speaker 75GDN-3 in a band-pass loudspeaker

Improving the sound quality of modern acoustic systems is achieved mainly through the use of new materials and technologies. In this article, we will analyze the manufacture of a homemade subwoofer based on the 75GDN-3 speaker.

The designers of the foreign firms “Jamo” and “Paradigm” are developing loudspeakers, consisting of a separate low-frequency link (subwoofer) and two high-frequency loudspeakers.

Subwoofers are a housing with an acoustic panel located in it and dividing it into 2 volumes with a dynamic woofer head installed on it. This acoustic design is called a band-pass loudspeaker (6th order).

This head with the front surface of the diffuser works for one closed volume of the housing, and with the rear – for the second, with a bass reflex tuned to a certain frequency. Designs with two bass reflexes tuned to different frequencies are called dual tuning systems.

Some subwoofers have two push-pull woofers positioned opposite each other in acoustic coupling.

Some designs use dual voice coil woofers. This allows the speaker to be connected to the two channels of a stereo bass amplifier.

The frequency range reproduced by these ACs is 25 … 150 Hz.

I have made several subwoofers of a similar design. Used domestic low-frequency heads 75GDN-3. The result exceeded expectations!

Subwoofers do give, as the ad says:

… unsurpassed, deep and penetrating reproduction of bass tones

I would like to share my experience of making the most successful subwoofer design for repetition with one woofer head.

The body (Fig. 1, 2) is made of 20 mm thick chipboard. The walls of the case are connected to each other with 20×20 mm rails using glue and screws. The acoustic panel with a hole for the woofer is mounted inside the speaker cabinet.

The rear wall of the case is removable; a low-pass filter and terminals are installed on it (low-pass filter – typical for multi-band speakers).

Bass reflex pipes – with an inner diameter of 75 mm, lengths of 50 and 250 mm, their walls must be at least 3 mm thick. The diameter of the holes for the phase inverters is not indicated in the drawing and depends on the diameter of the pipes used.

The inner surface of the speaker case is pasted over with batting or foam rubber. The woofer is fixed to the panel with screws. The slots between the panel, the woofer head and the walls of the case are sealed with plasticine and foam rubber. The back wall is attached through a foam pad.

The speaker cabinet is pasted over with veneer or synthetic foil. Bass reflex openings are decorated with decorative rings in the form of rings.

Source: magazine Radio amateur, No. 1, 1999.