Bass reflex port length and potential own vibrations

[Pete1]

Hi All,

I'm new to the DIY speaker building and hope I am posting in the right forum.

I'm in the process of constructing a 3-way floorstanding speakers with a tweeter, mid driver and two 6,5" woofers each (low to mid crossed at around 350-400Hz). I'm aiming to tune the box at 43Hz which using WinISD gives me about 20,5cm lenght of the port with 10cm width at 45L inner box volume. When taking away 18mm thickness of mdf panel which this port will be mounted in it gives me roughly 19cm of the port lenght inside the enclosure. Port is made of 2mm thick PVC pipe.
Therefore my question is, do you think at that lenght (and width) the port inside the enclosure might have some unwanted vibations at higher levels? Is the port too long to be just on it's own without any additional support i.e. some sort of bracing for the port itself to stop any potential vibrations? I'm guessing it wouldn't hurt it but is it necessary as it would obviously take extra work and additionally take away the enclosure's inner volume which I've not accounted for?

Or are the vibrations I am concerned about simply a "first port resonance" which WinISD is showing at above 700Hz (I don't remember exactly as I don't have my laptop with me atm)? If that's the case then I don't think I should worry as the woofers shouldn't be having much output above 500-600Hz given the ~350-400Hz Xover point...

Taking to account Xmax of the speakers I will be feeding it a maximum of 60W per channel.

I guess I still have a lot to learn regardless...😄

I would greatly appreciate any input on the matter😉

Thanks in advance!

 

[markbakk]

No worries. Although you possibly could do with a 7cm wide port, a 10cm wide and 19cm long one won’t resonate much. The port resonance is there though, but as you point out, it’s outside the passband of the woofers. Still, you ought to check the result with a port measurement.

 

[Pete1]

Thanks for your input markbakk!

I was originally thinking about using 7cm port, however the air velocity of the port would be close to 30m/s at maximum power of 60W which would most likely create port noise and even with flared ends I think that's still risky. Even at half the power air velocity would be at 21m/s. The 10cm one is at air velocity of 15m/s at max power so I don't think it will even need any flared ends... But obviously bigger the diameter longer the port needs to be and that's where my original concern comes from.

Unfortunately, at least here in UK, it is tricky (if not impossible) to find any other port sizes between 7cm and 10cm range so it's either 7cm or a jump all the way to 10cm. I am looking around on the web for PVC pipes in sizes between the two and I think I managed to find one at around 8.3cm inner diameter. If I choose to use that one it will give me (at max power) 21m/s port air velocity.
As I understand the general rule of thumb for the air velocity's limit (chuffing free) is at around 17m/s, is that correct? Therefore would I need to flare the ends to avoid a port noise at 21m/s? Would a flare at just one end be enough?

 

[planet10]

http://p10hifi.net/FAL/downloads/Changing-Port-Size.pdf

I typically just use a slot vent.

dave

 

[DrBoar]

Both ends, and if only one the inner end not the one on the baffle.

 

[Pete1]

http://p10hifi.net/FAL/downloads/Changing-Port-Size.pdf

I typically just use a slot vent.

dave

Thanks Dave,

I've considered the slot port initially but as this is my first build I've opted for the round/ tube port as once I would commit to certain slot size built in to the enclosure I wouldn't be able to change it without major work to be done to the construction of the box. With the pvc tube there's usually much easier way to change the dimensions (at least the lenght). Also as my enclosure is at slightly above 100cm in hight there might be an issue of standing waves creating inside the box, therefore if I'd place the slot at the bottom of the box (only practical way I see doing it) I wouldn't be able to place any damping material at the bottom to try and mitigate it.
That is why I chose to go with the round pvc port placed at the back at around 1/4 to 1/3 of the hight of the speaker (from the bottom) to give me the best chances to make any adjustments if needed.

Cheers
Pete

 

[Pete1]

Both ends, and if only one the inner end not the one on the baffle.

Thanks DrBoar,

Would you be able to elaborate a little bit on the reasons why the inner end is more important in terms of having a flare as opposed to the one on the baffle, please?

As I've mentioned earlier I am still learning the ins and outs of DIY speaker design/ building and I'm curious what is the difference in terms of where the flare is used?
I think I understand the basic principle of flares having a positive effects on high air velocity causing the port noise but it's usually lot easier to create or have the flare on the baffle end of the port, be it port bought from the shop or creating a flare on the baffle itself etc.

Cheers
Pete

 

[Joel Wesseling]

You Can read about flares in the attached AES paper

 

[Pete1]

You Can read about flares in the attached AES paper

Thanks Joel.

That is some beefy read😅
It'll take me a while to digest all of it but it's seems like an awsome paper on the subject from the quick glance through it!👍

Thanks again!

Cheers
Pete

 

[Joel Wesseling]

You are welcome!

FWIT, I'm using ABS pipe with a coupling that is adhered to the front baffle with construction adhesive. The 3" pipe extends through the baffle and the coupling is pressed against the back of the baffle. The 3" piece is of course attached with ABS cement. The final length can be inserted into the coupling inside and gives me a range of alignments as shown in the sim. The ABS is thick and rigid, likely no vibrations can occur.

I won't flare the wood on the baffle. The openings can accommodate custom made tapered tubes with flares on both ends. The flare on baffle side can be used to mount the vent to the baffle.. Maybe 3D printed is possible for this plan.

 

[kipman725]

You Can read about flares in the attached AES paper

I implemented the findings of this paper with NFR=0.5 ports for a stage wedge and am very happy with the ports performance. I don't have any quantative data though.

 

[Pete1]

You are welcome!

FWIT, I'm using ABS pipe with a coupling that is adhered to the front baffle with construction adhesive. The 3" pipe extends through the baffle and the coupling is pressed against the back of the baffle. The 3" piece is of course attached with ABS cement. The final length can be inserted into the coupling inside and gives me a range of alignments as shown in the sim. The ABS is thick and rigid, likely no vibrations can occur.

I won't flare the wood on the baffle. The openings can accommodate custom made tapered tubes with flares on both ends. The flare on baffle side can be used to mount the vent to the baffle.. Maybe 3D printed is possible for this plan.

Nice work this one!
Quite clever with that coupling for adjustable port.
I did look into getting an ABS pipe but in the end opted for PVC as I could get it from my local Screwfix for £1.69 so it was a no-brainer for me especially considering how much the ports cost in the diy audio shops (around 10 times that). It's not that thick (about 2mm) but from the advice I got from here and some other sources looks like that shouldn't be a problem in terms of vibrations.

Cheers
Pete

 

[Pete1]

I implemented the findings of this paper with NFR=0.5 ports for a stage wedge and am very happy with the ports performance. I don't have any quantative data though.

Hi kipman725,

Yeah, it is a really good paper with an excellent insight into the port's workings, physics etc.! It's quite techy but I can still get a lot of useful info out of it.


Cheers
Pete

 

[MrKlinky]

You Can read about flares in the attached AES paper

With compelling data like this available, I am constantly astonished that after hours agonising over simulations, DIY builders still simply stick a piece of drainpipe in the baffle and think the job's done...

 

[MrKlinky]

I typically just use a slot vent.

dave

Cheap, quick, and structurally simple, but woefully inefficient compared to a properly designed and implemented circular port...

 

[kipman725]

With compelling data like this available, I am constantly astonished that after hours agonising over simulations, DIY builders still simply stick a piece of drainpipe in the baffle and think the job's done...

if the port air velocity is always low this is fine it becomes a problem when high excursion subs are used, low tunes, small box etc. The loudest sub on databass uses a simple drainpipe, its just huge! https://data-bass.com/#/systems/5aa4a25261feb10004121e18?_k=xo6mey

 

[Joel Wesseling]

With compelling data like this available, I am constantly astonished that after hours agonising over simulations, DIY builders still simply stick a piece of drainpipe in the baffle and think the job's done...

Yes, I don’t understand why some do that.

 

[MrKlinky]

if the port air velocity is always low this is fine it becomes a problem when high excursion subs are used, low tunes, small box etc. The loudest sub on databass uses a simple drainpipe, its just huge! https://data-bass.com/#/systems/5aa4a25261feb10004121e18?_k=xo6mey

It's actually ludicrously tiny given the displacement of the drivers and will produce huge port compression and distortion. The best plan is to start with a port equal to Sd and compromise, compromise, compromise until it fits in the box and is not too long. Clearly things are a little different when dealing with higher order enclosures like this one but small ports suck on so many levels.

 

[planet10]

but woefully inefficient compared to a properly designed and implemented circular port...

I don’t know about effeiciemcy but the added R in a high aspect slot vent can be used to push a BR towards aperiodic and make the system less sensitive to dynamic change sin T/S.

dave