port air speed

[Hakka]

A few years back I built 2 12" shiva based sontube subs, 170lt cabinets, 17.5hz tune, single 4 inch diameter port.

After bottoming out the other night for the first time I am thinking of building 2 more the same for a bit more headroom.

I have been running some models in WinISD, and the port air speed looks way too high using the 4 inch port. I have attached a graph comparing the 4inch port to a 6inch port. Its been a few years since I built these subs but I used WinISD the first time round, and I can't really work out why didn't go with a 6 inch port. Port resonance is around 190hz with the 6 inch.

I don't think I can hear any 'chuffing' from my subs but then again I don't really know what to listen for.

I'm planning on making the new ones with a 6 inch port and making new top endcaps for the existing ones also with a 6 inch port.

Am I missing anything here? Is there any downside to using a 6inch port?
I realise that my cabinet volume will be reduced by a few litres.


Hakka.

[Exocer]

Hi,

Port noise must not be a huge problem with your system if you're unable to hear anything abnormal at high SPL levels. You may be able to get away with that 4" port.

I am not sure if its my version of unibox or something else but when I throw a 6" port into a box that size with a tune of 17.5hz port resonances are down to around 175hz. IIRC 175hz is a little bit lower than one would want due to possible interactions with resonances and your choice of crossover frequency.

[Hakka]

Exocer wrote:

Hi,

Port noise must not be a huge problem with your system if you're unable to hear anything abnormal at high SPL levels. You may be able to get away with that 4" port.

I'm assuming port noise cannot be measured by REW, can you describe what it sounds like. I've heard it referred to as 'chuffing' but I'm not sure what chuffing would sound like. All I hear is an insane amount of deep bass

Here's a REW measurement from the listening position that shows my housecurve, I usually have my AVR set to -12.5db when watching a movie, so there's some serious SPL going on around the tuning point.

The WinISD graphs use a 230w input signal (behringer A500) and I often clip the amp, so I'm surprised I'm not hearing it.

Hakka.

[brent_s]

Your air speed is below most recommended levels down to 17ish hz. If you're not hearing any noises you don't like from the sub, you're fine. With the added headroom of two more subs, you'll probably never push these guys any where near their limits.

If you want some insurance, use something like the 4" Precision Ports that are flared on both ends. Supposedly breathes like a 6" straight port. Even if it doesn't quite match the hype, it'll lower your air speed below the WinISD models for a straight port. I've got a single 4" PSP in my DVC15 and haven't heard any port noise with 105-110db in room peaks, but I did down fire the port into the carpet which will help some with port noise.

-Brent

[collo]

I've done some research on "chuffing" for different sized ports and flares.

As brent_s points out, your velocity in the 4 incher is fine till you get to the lower end of operation.

As frequency is decreased, audible turbulence occurs at lower speeds. At 39 m/sec, you are well past the point where turbulence is occuring in the core of the port.

From my work.....
For a 4inch port "core" turbulence & compression occurs @ 20 m/sec
For a 6inch port "core" turbulence & compression occurs @ 26 m/sec

Moving to a 6 incher drops you to 18 m/sec which fixes this problem.

The next area of concern is turbulence occuring as the "boundary" layer of air exits the port. This can be addressed by flares.

For 18 m/sec, you would need a 25mm radius flare on the port exit. The intake could be 5mm smaller. This could be done with a rollover bit in your router, or you could use commercial offerings, such as the PSP flares.

All of this assumes that you're wanting to feed your sub with 17hz at the same level as content higher up the spectrum. In practice, content this low is usually at a lower volume. In this case you could drop your flare radius to 19mm, which is more in line with what sonosub builders have been doing for ages now.

See the links on my Port Flares page for more detail.

If you're building a sonosub, grab a copy of sonosub.exe - it will tell you your port resonance at a glance.

[Hakka]

collo wrote:

For 18 m/sec, you would need a 25mm radius flare on the port exit. The intake could be 5mm smaller. This could be done with a rollover bit in your router, or you could use commercial offerings, such as the PSP flares.

All of this assumes that you're wanting to feed your sub with 17hz at the same level as content higher up the spectrum. In practice, content this low is usually at a lower volume. In this case you could drop your flare radius to 19mm, which is more in line with what sonosub builders have been doing for ages now.

I have flares routered into the port ends, either 19mm or 25mm radius. If you take my housecurve into account 17hz is probably the highest peak in my my response.

I use your sonosub program Collo, it's great.

I might just build the next 2 subs with a 6inch port, if I can hear a difference I'll make up some new endcaps for the existing subs.

Thanks guys.

Hakka.

[collo]

You'll have to resurrect this thread when the time comes, and let us know what you found....

[Blaser]

collo wrote:

I've done some research on "chuffing" for different sized ports and flares.

As brent_s points out, your velocity in the 4 incher is fine till you get to the lower end of operation.

As frequency is decreased, audible turbulence occurs at lower speeds. At 39 m/sec, you are well past the point where turbulence is occuring in the core of the port.

From my work.....
For a 4inch port "core" turbulence & compression occurs @ 20 m/sec
For a 6inch port "core" turbulence & compression occurs @ 26 m/sec

Moving to a 6 incher drops you to 18 m/sec which fixes this problem.

The next area of concern is turbulence occuring as the "boundary" layer of air exits the port. This can be addressed by flares.

For 18 m/sec, you would need a 25mm radius flare on the port exit. The intake could be 5mm smaller. This could be done with a rollover bit in your router, or you could use commercial offerings, such as the PSP flares.

All of this assumes that you're wanting to feed your sub with 17hz at the same level as content higher up the spectrum. In practice, content this low is usually at a lower volume. In this case you could drop your flare radius to 19mm, which is more in line with what sonosub builders have been doing for ages now.

See the links on my Port Flares page for more detail.

If you're building a sonosub, grab a copy of sonosub.exe - it will tell you your port resonance at a glance.

By the way I will be using practically 36 mm flares for my 8" port exit...Is this value acceptable or should I use more/less radius?

B Rgds

[collo]

36mm flares on an 8inch port would be perfect.

That size flare will keep "chuffing" at bay all the way up to the core limit. There is no advantage in going any larger than this

You could safely run up to around 30 m/sec - in an 8 inch port, that's a lot of air!

[Blaser]

Thank you Collo!!

Indeed my similations give me 39 m/s @ 15 Hz , but very low above 18 Hz...
I forgot to say that it will be a single flared port. Does it change much?

Thanks

[collo]

I used to think that you could get away with a much smaller flare on the inside due to masking by the inrush of air

After doing a bit of testing, I found that the inside flare needs to be nearly as big as the outside.

If you have enough clearance, you could fit a donut ring



A few sono builders have used these, and they work well.....

[Chrisbee]

collo wrote:

I used to think that you could get away with a much smaller flare on the inside due to masking by the inrush of air

After doing a bit of testing, I found that the inside flare needs to be nearly as big as the outside.

If you have enough clearance, you could fit a donut ring



A few sono builders have used these, and they work well.....

I have been suggesting a donut ring on the inner end of the port after using a similar device decades ago in the air cleaner on a tuned sports car engine. Much the same problem applies. A resonant airflow trying to enter a round orifice in a cramped space with a plate right in front of the opening.