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funkydoug

Port tuning and velocity

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Posted (edited)

Any input much appreciated on this one folks...

I am toying with the idea of adding a port to a sealed cab, mainly to get some more bottom-end but also as a bit of a learning project. I am modelling it in WinISD and can't seem to arrive at a port config that works. Can anyone help?

Box size: 41 Litres

Driver: 1 x 10inch SICA (I know, never going to sound huge... but it is what I have got to work with on this project).

The puzzle: I set the port diameter at something sensible like 7.5cm which seems okay in that it will fit on the baffle, is about the diameter of a lot of other ports, and I can buy a ready made port of this diameter from Blue Aran.  Doesn't strike me as an unusual choice. This all looks ok, WinISD tells me it should be a reasonable length to achieve the tuning I am modelling (for 45hz the 7.5cm port should be cut to 10.4cm). All good. The issue comes when I look at the air velocity charts for the port. It says air velocity down at the tuning frequency will be over 100ft per second with a 100w signal. That speed is more than twice what google tells me I should aim for. My questions:

Am I missing or mis-understanding something? Probably.

Is it a non issue because there is rarely much power in those low frequencies?

What is normally done to resolve what looks to be a basic tension between the competing priorities of wanting a port of about this size (most cab designs don't permit enormous ports), of practical length (can't have one longer than the depth of the cab!) and with air velocity that doesn't get into 'chuffing' territory?

For now I'll keep reading and modelling and trying to work it out. I am not going to reach for the hole-cutter and ask questions afterwards  - as I have been known to do in previous 'projects'!. 

Thanks in advance, Doug.

Edited by funkydoug

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The port area is too small, increase the diameter by ~2cm and see if the length remains reasonable.

With that driver in that size box, you may not be able to achieve an acceptable tuning.

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Posted (edited)

Thanks @agedhorse, kind of reassuring to know that "it's not me, it's the box"!

If I go up to 10cm diameter it needs to be about 20cm long to maintain the tuning, and this brings the port speed down quite a bit. That is longer than I 'imagined' it being, but you're (of course) right that it would do a lot to reduce the problem. The cab is about 30cm deep, so assuming I can find a 10x20 port - or make one -  and that it fits the baffle, this should work.

On the other hand ... maybe this is why they made it a sealed cab! 

Much appreciated.

Edited by funkydoug

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They made it sealed because it had to be when that small. When using a driver that has specs suitable for either sealed or ported the ported will always be larger, not only to allow the port to fit inside, but also to allow tuning low enough to make porting worthwhile. That's the trade off to realize the lower extension and higher mechanical power handing that ported gives. While on the subject of mechanical power handling, check the excursion chart. There's no point in using a larger port area to keep port velocity down with more power than the driver can take before exceeding xmax. I won't say I'd be shocked if the SICA can take more than 50 watts, but I'd be surprised.

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Thanks for you input Bill.

I'll check when I'm back home. It has Xmax of 4mm, so its not too bad in that department. 

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Welcome to the world of speaker design, you squeeze a parameter somewhere and a new problem pops up. Rarely do you come up with the ideal design where your speaker never exceeds excursion limits, can handle all the the thermal power it is rated at goes down to 40Hz or whatever your target is and has a tiny practical cabinet with a conveniently sized port.

There are a couple of things in your favour however. The output from a bass pickup isn't flat and the lower frequencies are always attenuated, how much varies and is determined by how close to the bridge the pups are positioned. That means you'll get some chuffing at quite low powers from a signal generator but less from a bass played at decent volumes. As you've noticed the ports in a lot of commercial cabs are sub-optimal for the same practical reasons you are encountering. They are successful if the compromises are carefully chosen ( and nobody notices :) )

One thing to try is to see if an HPF might help, WinISD lets you model that. 45Hz also sounds quite low for a 10" speaker with a modest magnet in a cab that size. Try other tunings and keep looking at excursion and port velocities. Obviously this will also affect the frequency response and you'll lose deep bass but with a modest speaker I'd be more concerned with the response at 80Hz than 40. Get the second harmonic right and the cab is going to sound better than worrying about the barely heard fundamental.

In the end you'll learn a lot from doing this, you may not end up with a great speaker though, it's almost certain there was a reason for putting this speaker in a sealed box and if you want a great ported 10 you probably wouldn't start here. Equally though you should get a bit more bass and you can always cover the hole over and go back to square one if you don't like it.

Good luck

 

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@stevie and @Phil Starr have authored a number of speaker cab threads, it's really worthwhile reading through them to benefit from their vast experience. The recent 10 ins Lockdown Easy build cab had a target tuning of 50Hz which maybe more suited to your project. 

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Thanks Phil.

Yeah, I am almost enjoying the experience of finding myself wrestling with the trade-offs. Its a good learning process for sure.

HPF is definitely part of the solution for the cab, whether sealed or vented as excursion is a problem either way. 

One thing I noticed was that the driver has QTS of 0.33, pretty low for a sealed cab?? Might explain why there is so little bottom end. Maybe replacing the driver would be a better idea.

This is the driver:1061876160_Z006900Cspecs.thumb.jpg.39d3b0567ef9ccefdb309d660de9d375.jpg

 

 

 

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Posted (edited)

Its an Ashdown combo, like this:

https://ashdownmusic.com/products/abm-c110h-400

 

Very nice little amp, and very handsome too, so I'm not in a hurry to mess with it, just exploring options as to how to get the most out of it. 

I think I've underestimated the volume ... It must be more like 55-60litres but I can't measure it just now. Apologies to those who've already contributed based on it being 40L! 

Edit: I've added this later in the thread,  but to avoid confusion here, the volume of the cab is 42 litres minus the driver itself

Edited by funkydoug

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Posted (edited)
48 minutes ago, JohnDaBass said:

@stevie and @Phil Starr have authored a number of speaker cab threads, it's really worthwhile reading through them to benefit from their vast experience. The recent 10 ins Lockdown Easy build cab had a target tuning of 50Hz which maybe more suited to your project. 

Yeah I've really enjoyed those threads, and was impressed by your combo version too! 

I don't have tools or any space for a project like that (yet!) so I'm thinking that modding and / or modelling might be a good way to start tinkering.

This one is actually a nice and quite rare little combo so I'm not going to hack into it unless I am sure it's a good idea. I've done that before ... some here might remember... and although it worked out ok in the end, it was pure luck!

Edited by funkydoug
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51 minutes ago, funkydoug said:

One thing I noticed was that the driver has QTS of 0.33, pretty low for a sealed cab?? Might explain why there is so little bottom end.

0.33 is too low for a sealed cab, but that alone isn't why it lacks bottom end. The high Fs along with being sealed is. Sealed cabs don't go low unless you load them with low Fs drivers, and then you trade off sensitivity. If I had to use it I'd use a 45 liter net cab tuned to 50Hz.

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Might put an Eminence SC10-16 in there. On the face of it, it looks like a downgrade but it seems like a better fit for the box and amp. Would see no more than 100w from the amp at 16ohms, and is designed for sealed cabs.

Won't sound massive but might sound right for a nice small 10. Hmmm...

 

 

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According to the Ashdown blurb, the C110H-400 combo is supposed to be ported. The Sica driver definitely belongs in a ported rather than a sealed box. Perhaps Ashdown forgot to put the port in the baffle.The only other explanation is that the tea boy designed the cab. 

The Sica driver is a decent budget driver and the Eminence would definitely be a downgrade. I think the OP's original idea was sensible. However, before anyone can advise on  porting, we really do need accurate internal measurements in order to work out the cabinet volume. I doubt that it's 55-60 litres. Is there space on the front baffle for a 4-inch port?

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Posted (edited)

Thanks Stevie,

I'm not with it just now to measure up, but I took this shot of the baffle when I was inspecting the driver. Think a 4inch port might just fit .

20210430_135850.thumb.jpg.d7aa851922309d7f1dc75e0304f27353.jpg

20210430_140349.thumb.jpg.eff6ab755f61b17ddb7ba2c0553b6558.jpg

 

Will measure it when I can. Very much appreciate the input guys.

 

Also, you'll spot that while the other cabs and combos in this NEO range have APT80 tweeters the baby of the bunch got what looks like a pretty basic piezo. No complaints on how the tweeter sounds though.

Edited by funkydoug

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Posted (edited)

Looks like a 4" port will fit. It should make a big difference. Keep it as far away from the corner as you can. Was there any damping material in the box when you looked inside? Ashdown don't normally bother to fit any.  If not, it would be worth putting some in.

Edited by stevie
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Totally unlined. Yep I'll add some. Did that to one of my other Ashdowns lately, easy job and quite satisfying. That one sounds great now. I put a new crossover in it and lined it, having learned a lot from you and a few others here.

 

This one remains in factory condition and while the amp is great and the combo is ideal in lots of ways there are a few modifications in mind: lining it, maybe bracing it and working out what to do about the low end, hence this thread.

I'll measure properly this weekend and report back.

👍

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1 hour ago, funkydoug said:

I'm not with it just now to measure up, but I took this shot of the baffle when I was inspecting the driver. Think a 4inch port might just fit .

Put it on the back, or if that doesn't allow the necessary length put it on a side. It will sound the same no matter where you place it.

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According to a rough & ready winISD run-through, the SICA drive unit works best in an internal volume of 20-25litres.


An an example, using winISD's net volume of 23 litres, a 4" port you'll about 19.30cm long will give you tuning of 62Hz. So make the volume about 25.50 litres and allow 2.5litres for the space occupied by the drive unit and the port.


My suggestion would be to reduce the internal volume to the  above size using a number of hard polystyrene packing pieces or any other solid but lightweight material.

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Posted (edited)
2 hours ago, stevie said:

 Keep it as far away from the corner as you can. 

What are the disadvantages of the port being too close to the corners?

 

1 hour ago, Bill Fitzmaurice said:

 Put it on a side. It will sound the same no matter where you place it.

+1

Always used my BF One10 on it's side with the port pointing out the side. 

Edited by JohnDaBass

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2 hours ago, Balcro said:

According to a rough & ready winISD run-through, the SICA drive unit works best in an internal volume of 20-25litres..

'Best' is a relative term. It has maximally flat response in that size cabinet, but with weak low end. It won't be as flat in a larger box, but it will go lower. A smaller box with higher tuning would be appropriate for a PA top that's used along with subs, but not for an electric bass cab.

Quote

What are the disadvantages of the port being too close to the corners?

There are none. For that matter in reflex boxes I don't use panel mounted ports, I use corner ports in all four corners. They work better than panel mounted ports, and they brace the cabinet baffle, top, bottom and sides as well.

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2 hours ago, JohnDaBass said:

What are the disadvantages of the port being too close to the corners?

Port midrange leakage, particularly of standing waves, is worst when you locate ports in the corner. They're OK for subs where you're not reproducing midrange frequencies. It's the same principle as locating your loudspeakers in the corner of a room. I avoid doing it. 

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I'd run the Sica through winISD and come to the same conclusion, the cabinet is over-sized and it isn't suitable for a sealed cab. Tuning it higher makes more sense 55Hz looked a good compromise to me but that's a matter of what you are trying to achieve. TBH It's a bit too good to waste in an over sized cab, it's a nice speaker. 

What would probably work really well there would be the Celestion Pulse 10 assuming you have the volume about right. I also notice you have a horrible piezo tweeter in there. it would be a considerable improvement to fit a decent horn and tweeter. Why not put in the components we used in the Lockdown Easy Build project? You'd end up with a really nice sounding combo for a modest cost. If you decide to go that route we'll check the dimensions for you. 

Oooh looks like Stevie has replied, wonder what he's saying?

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1 hour ago, Bill Fitzmaurice said:

'Best' is a relative term. It has maximally flat response in that size cabinet, but with weak low end. It won't be as flat in a larger box, but it will go lower. A smaller box with higher tuning would be appropriate for a PA top that's used along with subs, but not for an electric bass cab.

 

Agreed, but in this case is looks like the OP is trying to make the best of a bad job.

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True, but if he makes the cab smaller and tunes it to 62Hz, as opposed to leaving it as is tuned to 50-54Hz, he'd lose 4 to 5dB of sensitivity from 35 to 55Hz.  I wouldn't want that myself.

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