# Vd: Sd x Xmax = Vd

#### OffGridKindaGuy

Help me understand this..

If a driver has Sd 54.1 cm² with Excursion of 2.5 mm, would a passive radiator be considered 2x Vd if the Sd is 54.1 cm² and the "Possible" Excursion is 9 mm?

Is the area of the passive radiator the factor or is the the excursion plus the area of the passive radiator the factor?

**Would I need 2 passive radiators to double the Vd or will one with over twice the drivers stated ability of excursion provide double the Vd?

Dayton Audio DMA105-4 4" Dual Magnet Aluminum Cone Full-Range Driver 4 Ohm

Dayton Audio DMA105-PR 4" DMA Series Passive Radiator

#### Scottmoose

Volume displacement is normally provided in cm^3. You already have the figures. For a fixed cone area, 2mm travel will provide double the nominal air volume displacement as 1mm travel. To double again requires 4mm travel.

Two radiating surfaces of identical area moving 1mm will provide double the volume displacement within their operating range as one of half the area.

Passive radiators are essentially just substitutes for vents & primarily of use when a duct is either too long to fit in the desired enclosure size (or will suffer issues with self-harmonics), or of too small a cross-section to keep size down & suffers excess air velocity as a result. Their excursion therefore depends on the specifics of box tuning & how much you turn the wick up.

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#### Lojzek

There is a very good excel spreadsheet "Woofer Box and Circuit Designer" which simulates PR's. This would give you answers you seek. Assuming the TSP of your driver are correct, in a 9 litre vented box you can achieve exemplary loading with an ordinary 1" vent. Still, if you preferred a PR for whatever reason, the one you chose would be a poor one, lacking Sd significantly.

#### OffGridKindaGuy

Many thanks to the both of you. I'm still trying to gasp the physics of this..

I've never ventured into PR's. Just another toy adventure to mess with..