[Parts]

As per title,

Just wondering why two ports are required to be longer than a single port?

For instance if I tune a cab with a single 10" port, but the port velocity is too high and I add another port, these ports are then required to be 22.5" long each?

 

[dyohn]

It's because they are Helmholtz resonators. Using multiple ports is similar to a single larger port with the same surface area. Larger ports = longer lengths for any given enclosure volume and tuning frequency.

 

[Parts]

Thanks!

So in some builds it may well be better to tune the sub to a higher Hz? And is it worth using something like bitumen dampening on a port? Or what about multiple straw filled type ports, do these behave in a different manner?

 

[fusseli]

I wouldn't ever sacrifice tuning frequency for a shorter port, unless onyl absolutely necessary. Another hugely important note is that the cross-sectional area of the port will allow enough airflow for the sub(s) of the given design. I think air velocity should not exceed 30 m/s. Anybody correct me if I'm wrong here...

 

[talunceford]

I wouldn't ever sacrifice tuning frequency for a shorter port, unless onyl absolutely necessary. Another hugely important note is that the cross-sectional area of the port will allow enough airflow for the sub(s) of the given design. I think air velocity should not exceed 30 m/s. Anybody correct me if I'm wrong here...

You are correct there. I typically try and use 15-16 sq in of port area per cubic ft of air space. It keeps the air velocity down to a min.

 

[bknights]

I was thinking of building a sub box similar to the svs with 3 ports, but winisd gives me ridiculous port lengths. How does svs get away with having three ports?

 

[Moonfly]

The driver and cabinet size both factor into the design, and they design and built their own driver, so they dictate a lot of what the final design will be like.

We have to use off the shelf drivers, and depending on what tune you want, and what size cabinet you use, this will determine the port dimensions. Companies like SVS have a little more input in how the ports will eventually be, than you or I do. Still, it would be possible to build something similar to the the SVS Ultras using say a 12" driver. With a good 15 your going to need a huge cabinet.

 

[spasticteapot]

With a good 15 your going to need a huge cabinet.

Not necessarily - some 15" drivers work quite well in a relatively small (~2 cubic feet) box. However, the efficiency is pretty horrible - expect to use a 1kW+ amp to get the most out of 'em.

 

[Mike P.]

Not necessarily - some 15" drivers work quite well in a relatively small (~2 cubic feet) box. However, the efficiency is pretty horrible - expect to use a 1kW+ amp to get the most out of 'em.

I'm sure Moonfly was referring to ported, it would be a challenge to port a high excursion 15" driver in 2 cu.ft.

 

[Moonfly]

Not necessarily - some 15" drivers work quite well in a relatively small (~2 cubic feet) box. However, the efficiency is pretty horrible - expect to use a 1kW+ amp to get the most out of 'em.

I'm sure Moonfly was referring to ported, it would be a challenge to port a high excursion 15" driver in 2 cu.ft.

Yeah I was talking about ported. Even something like the AV15-H, which is directly targeted at small enclosures with its design, still requires in the order of 150 litres volume to become an EBS sub, more for a true LLT. Considering the same driver can work in only 40 litres sealed, then 15" LLT's or -3/-6EBS subs (like the SVS Ultras) are very very hard to get into tiny cabs, even external porting wont make the cabs tiny.

I should clear up though, US huge and UK huge do differ a little. In the UK, a 600mm cube is considered pretty large, and anything even approaching an 800mm cube would be considered huge. So while 50 litres might be considered smaller in the US than where I live, its still not that small considering.

In respect of BKnights comments, I had a play with WinISD last night, and managed to get port lengths to around a meter each with a 15" driver, its still a lot for a box though, and I think a Sonotube would be the only realistic way to build an SVS Ultra type sub with a 15" driver.

 

[bknights]

In respect of BKnights comments, I had a play with WinISD last night, and managed to get port lengths to around a meter each with a 15" driver, its still a lot for a box though, and I think a Sonotube would be the only realistic way to build an SVS Ultra type sub with a 15" driver.

I was modeling a dayton 12in hf in a 4.2ft³ and wanted the look of the svs with 3 ports. I like how proportional the box looks in regards to the driver and 3 ports.

but it seems i won't be able to get that look with this sub, guess i'll have to save this look for a 15in driver like the tempest which was more reasonable with three ports

 

[Moonfly]

Ill try take a look at the model(s) tomorrow and see if I can come up with anything of use, if no one beats me to the punch.

 

[bknights]

Ill try take a look at the model(s) tomorrow and see if I can come up with anything of use, if no one beats me to the punch.

Thanks I was planning on using the 4 in flared ports from PE which can be up to 17 in long.
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[Mike P.]

I was modeling a dayton 12in hf in a 4.2ft³ and wanted the look of the svs with 3 ports. I like how proportional the box looks in regards to the driver and 3 ports.

5 cu.ft. tuned to 21 hz would get the most performance from a RSS315HF with a 500 watt amp. Three 3" flared ports each need to be 24.5" long. 4" flared ports wouldn't be practical as they would each need to be 44" long.

​

 

[talunceford]

Yeah, I used 3 3" ports on my 18hz tuned enclosure. It worked out perfectly. BUT my enclosure started out as a 7 cu. ft. enclosure. By the time you take into account for port, sub and various other displacements, it worked out to where the usable volume was 5 cu. ft.

I use bass box pro 6 to model my projects. Not sure if that makes a difference or not, but it might.