WinISD results suspect - can I have a second opinion please

[Mottlefeeder]

I had in mind to build a 'Joe Blogs Briefcase' using FaitalPro 5 inch drivers, but when I feed their parameters into WinISD I'm getting port lengths of over 300mm which does not look right.

Can anyone be bothered to input the TS parameters for the FaitalPro 5FE120, for a cab with two 4 ohm drivers and a volume of 9900cc tuned to 70Hz with a 12dB/octave HPF with a 30Hz turnover.  My clculations give - 

        For 30 watts I need a slot port 150 mm*20 mm by 132 mm long giving me 17 m/s air flow.

        For 80 Watts I need 150 mm*35 mm by 260 mm long giving me over 16m/s air flow.

        For 150 Watts (my maximum amplifier power) I need a slot port 150 mm*45 mm by 345 mm long giving me just over 17m/s air flow.

Do you agree with these figures?

Alternatively, can anyone tell me how they split the power between the fundamental and the harmonics so that they don't have to design for full power at every frequency?

Thanks

David

[stevie]

Your figures are certainly in the ballpark. I checked for 150W using a different program. I'd be tempted to tune to 60 and use a single round port if possible - 69mm drainpipe x 200m long. Perhaps add a couple of extra litres to account for the space the drivers take up.

I'd expect this to sound really good.

[Mottlefeeder]

Thanks for that - I'll check it out with the new figures. Win ISD suggests 330cc's per speaker, so with an internal width of 150 mm, and the amp and battery strapped on the back it's looking like this -  Briefcase speaker calculations.pdf

[stevie]

Two things on the vent. Thin ports are generally bad for turbulence. If you can't use a round port, try increasing the height of the port from 25mm. Say, 90 x 50mm.

The 70Hz tuning frequency concerns me a bit. I'd ideally like to see it at 60Hz (or lower), as a low-E 42Hz signal could overwhelm the drivers. This is the big problem with porting small, high power cabs. The length of port you've specified is more or less ideal for the internal depth you're working with.

My home-use-practice cab is a similar size to this and it has a 50mm vent. At home practice levels, it's not a problem. What I'd tentatively suggest is that you build as-is, then reduce the width of the vent by adding pieces of wood to it to lower the tuning frequency. Then see what it sounds like. As I'm sure you know, a lower tuning frequency will improve power handling at low frequencies but the smaller vent will make chuffing worse. This way, you get to choose which you prefer based on the volume levels you intend to play at.

Edited August 25, 2020 by stevie
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[Mottlefeeder]

My original plan was to use a slot port between the two speakers, so it would double as bracing for the cab sides, so I may have to rethink that. 

My existing rig uses Eminence Basslite S2010 speakers each in a 0.9 cu ft cab with ports tuned to 60Hz  (based on an Eminence design). They used twin 3 inch ports, and I went for a 12 inch wide 1 inch high 7.5 inch long port instead. I haven't had any problems with my sound at the volume I play, so  I was aiming to get a similar response curve with this design.

I play a 5-string with a low B, and often use an HPF at about 50-60 Hz, but I tend to go for punch rather than rumble, so that may be why I haven't heard any problems.

David

[Dan Dare]

If it's any help, my PJB C4s (which are 4x5, so twice the number of drivers as a Briefcase) have two rectangular ports, each 3cm high, 14cm wide and 24 cm deep). The internal dimensions of the cabs (not including ports) are 29x29x27 cm. I appreciate the Faital drivers won't have the same requirements as the PJBs.

[Mottlefeeder]

On 22/08/2020 at 14:08, Dan Dare said:

If it's any help, my PJB C4s (which are 4x5, so twice the number of drivers as a Briefcase) have two rectangular ports, each 3cm high, 14cm wide and 24 cm deep). The internal dimensions of the cabs (not including ports) are 29x29x27 cm. I appreciate the Faital drivers won't have the same requirements as the PJBs.

Thanks for that - a commercial 3:14 ratio of port height and width is a useful benchmark. Stevie 's suggestions of 69mm diameter or 50:90 don't sit so well in the baffle size I had in mind, although that may change as the design matures.

David

Correction - since there are two ports, the 'combined port' ratio is 6:14, or slightly over 1:2.

Edited February 14, 2021 by Mottlefeeder

[Balcro]

After coming across quite late to your post and playing with the various options available under winISD I've found what  I think is a compromise you might care to look at, but only for an input signal of 80 watts. As Stevie has said, getting good venting with high-powered small speakers in compact lightweight enclosures is very difficult. In my opionion it's a real P.I.A.

Set the volume to 11.30 litres and tuning to 64Hz (same as the Fs).

Following your references to a 3:14 ratio between port height and width I went back and checked my own figures which I had calculated earlier, but they didn't match. I found that using 3.8cm x 11.6cm I could get the air velocity down to 20m/s @55Hz. Port length is down to 23cm. With the 3:14 ratio for the port, the air-velocity is 1m/s higher.

Without a filter, the frequency response is down - 4.9dB @ about 55 Hz. Put in a high-pass filter set to 50Hz and the 55Hz response drops to - 7.2dB. In these circumstances I think that is for the better as it is acting at the point where the air velocity is highest.

With a 150 watt input, like you, I found it impossible to achieve good air velocity figures which ever way I changed the enclosure volume. At 150 watt input there is a small dip in power handling between 80 - 115 Hz (the xmax / cone excursion figure corresponds) so I would consider limiting your input to about 110 watts.

Best of luck.

[Mottlefeeder]

5 hours ago, Balcro said:

After coming across quite late to your post and playing with the various options available under winISD I've found what  I think is a compromise you might care to look at, but only for an input signal of 80 watts. As Stevie has said, getting good venting with high-powered small speakers in compact lightweight enclosures is very difficult. In my opionion it's a real P.I.A.

Set the volume to 11.30 litres and tuning to 64Hz (same as the Fs).

Following your references to a 3:14 ratio between port height and width I went back and checked my own figures which I had calculated earlier, but they didn't match. I found that using 3.8cm x 11.6cm I could get the air velocity down to 20m/s @55Hz. Port length is down to 23cm. With the 3:14 ratio for the port, the air-velocity is 1m/s higher.

Without a filter, the frequency response is down - 4.9dB @ about 55 Hz. Put in a high-pass filter set to 50Hz and the 55Hz response drops to - 7.2dB. In these circumstances I think that is for the better as it is acting at the point where the air velocity is highest.

With a 150 watt input, like you, I found it impossible to achieve good air velocity figures which ever way I changed the enclosure volume. At 150 watt input there is a small dip in power handling between 80 - 115 Hz (the xmax / cone excursion figure corresponds) so I would consider limiting your input to about 110 watts.

Best of luck.

Thanks for your observations and effort on my behalf.

I'm on holiday at the moment and have no access to WinISD but I will be checking it out when I get back. 

David

[Mottlefeeder]

And I'm back in the room...

My existing 10 inch speaker box has a thin slot port, but very low air speeds, which may be why I have never heard any problems due to turbulence. Moving away from a thin slot in this design, to something like 1:4 or 1:3, and big enough to bring the airflow down below 17 m/s, gives me a port that is longer than any of the box dimensions, so I'm now looking at a port turning a corner inside the cab. One option is a drainpipe with a 92.5 degree angle, and the other is a rectangular  extractor fan duct with a 90 degree bend. 

Should I be aiming for a 'middle of port' length as per WinISD, or a port volume as per WinISD? And are there any rules of thumb for compensating for whatever problems are caused by bends in ports?

Thanks

David

[Balcro]

In a reply to one of my posts elsewhere, Bill Fitzmaurice used the words "keep the velocity below 18m/s within the pass-band". From that, I assume he meant the pass-band for bass generally, would be something "above" 40 / 50Hz, with most (average) bass speakers managing a flattish frequency response down to 80Hz and then declining perhaps -7dB by 40Hz. So for example, if the port velocity rises at frequencies at or below e.g 40Hz, then air-velocity wouldn't be an issue.

If you're aiming to keep velocity below 17m/s then you only need to do that over the pass-band, which, with the little Faital-Pro's is about 50Hz & upwards. As you have stated that you have use of a 50/60Hz HPF, I would use the filter/graphic equaliser  all the time. It pulls down the air velocity over 4m/s in the 50/60Hz region.

I've seen some fairly extreme home hi-fi set-ups with curving pipes, and it may be usable in your circumstances, but in doing that you need to make the enclosure bigger to accomodate the larger volume of internal pipework. As port calculations seem to be based upon the cross-sectional area x length I expect that "middle of port" is the answer.

I can't comment on the use of the drainpipe or the ventilator duct, apart from to say that a right-angle will cause some turbulence. Over to Stevie on that one.

 

[Mottlefeeder]

5 hours ago, Balcro said:

...I've seen some fairly extreme home hi-fi set-ups with curving pipes, and it may be usable in your circumstances, but in doing that you need to make the enclosure bigger to accomodate the larger volume of internal pipework. As port calculations seem to be based upon the cross-sectional area x length I expect that "middle of port" is the answer.

I can't comment on the use of the drainpipe or the ventilator duct, apart from to say that a right-angle will cause some turbulence. Over to Stevie on that one.

 

If the port calculations are based on area times length, that implies that the volume of air is the important bit?

Yes, a sharp 90 degree corner will cause turbulence, but the prefabricated pipes have radiused corners to reduce that.

David

[Phil Starr]

I think the answer is in your original question. 

You ask how the power is 'split' between the fundamental and the harmonics. The answer is that there isn't very much fundamental and it's likely to be 6-12dB down in terms of what comes out of the pickup and which note you are thinking of. The fundamental is greatest at the twelfth fret and much reduced at the position of the pickup. By wisely using a filter you are also reducing the chances of over excursion and chuffing  in the ports. http://www.buildyourguitar.com/resources/lospennato/index.htm

All this goes out of the window of course if you use 12dB of bass boost and an octave pedal!

Any cab like this is going to be a compromise, you are using small speakers and want a compact cab so looking for full power with no port noises at all frequencies is something you are going to expect to compromise on. It's useful to know the points where you expect problems and port velocity is just one of them. Have a look at commercial 'briefcase' designs and see what compromises they have made. If your port dimensions are coming out close to theirs you are probably in a safe enough place with your design.

I've looked at this for some of my own designs and you can come up with something where the port can end up with you needing to considerably increase the size of the cab. In the days before Win ISD I never calculated port velocity and some of my older cabs have quite restricted port dimensions. You can show the chuffing with a signal generator easily enough but I used them for years without noticing anything playing live..

[Mottlefeeder]

7 hours ago, Phil Starr said:

I think the answer is in your original question. 

You ask how the power is 'split' between the fundamental and the harmonics. The answer is that there isn't very much fundamental and it's likely to be 6-12dB down in terms of what comes out of the pickup and which note you are thinking of. The fundamental is greatest at the twelfth fret and much reduced at the position of the pickup. By wisely using a filter you are also reducing the chances of over excursion and chuffing  in the ports. http://www.buildyourguitar.com/resources/lospennato/index.htm

All this goes out of the window of course if you use 12dB of bass boost and an octave pedal!

Any cab like this is going to be a compromise, you are using small speakers and want a compact cab so looking for full power with no port noises at all frequencies is something you are going to expect to compromise on. It's useful to know the points where you expect problems and port velocity is just one of them. Have a look at commercial 'briefcase' designs and see what compromises they have made. If your port dimensions are coming out close to theirs you are probably in a safe enough place with your design.

I've looked at this for some of my own designs and you can come up with something where the port can end up with you needing to considerably increase the size of the cab. In the days before Win ISD I never calculated port velocity and some of my older cabs have quite restricted port dimensions. You can show the chuffing with a signal generator easily enough but I used them for years without noticing anything playing live..

Just to make sure I understand - the output from the pickup is 6dB down at 30-40 Hz because there isn't as much signal to pick up, so to model that into a port calculation, if I have say 10 watts at 60 Hz, putting me on the limit of port air-speeds, I should check that at 30 Hz, the port can cope with 4 watts without reaching that air-speed limit?

Talking of compromises, and refining my design slightly, I was aiming for a similar sound and volume to my 8 ohm Eminence basslite S2010 in a small cab, taking about 10 watts from one channel of a stereo amp. This two speaker box will do it, with two 4 ohm drivers taking 20 watts each  - i.e. half the size of box , but taking 4 times the power to reach the same volume. The next step up in volume would be to add a second S2010 and run one per channel, taking 20 watts in total.  But  the increase in output is only 3dB, and I am now carting a mini stack around.

The alternative next step is to use my bigger battery-powered amp - 80 watts into 8 ohms / 150 watts into 4 ohms. At this point, the single S2010 will deliver 114 dB but the twin speaker box, with the drivers in series, will only deliver 108dB, so it is 6dB quieter. Beyond that, a pair of S2010s will deliver 116 dB, but I would also be carrying a bigger battery (27 kg!), so it would no longer be a portable rig.

On that basis, there is no point in designing the twin speaker box to take more than 20 watts per speaker - if I want it louder, there are better ways to do it.

David

[Phil Starr]

Hi David,

there's a lot in your response! I'll have a think and maybe reply in detail later.

There's an online emulator somewhere where you can slide the position of the pickup and it shows the frequency response. Basically it confirms what we already know that you get more bassy as you move away from the bridge and bridge pickups are always more tinny and neck pups more bassy. If i find it I'll send it across to you. It's in one of the 12" design diaries in this forum.

The 6-12dB figure was illustrative only. It'll be much more than 12 for a bottom B and a bridge PUP and much less for an E played on the G string. You could theoretically do the calculations for air velocity at all the frequencies for any particular PUP position but I don't think that would be useful. If you are getting wind noise form the ports your poor little 5" drivers are going to be in over excursion anyway. If you tune to 70Hz you aren't going to have any discernible output at 30Hz or even at bottom E (41Hz). Far better to do what you said and use a 50Hz filter, take out the subsonics that you won't hear anyway and use your speaker with confidence. 

Your smallest slot port is 3000mm² Stevie's 50mm circular port is even smaller than that and is satisfactory. I think you can go ahead and build.

[Mottlefeeder]

Thanks for that - I'll be away for the next few days, so I may not respond to further comments. 

David 

Edited September 8, 2020 by Mottlefeeder

[Mottlefeeder]

And I'm back in the room.

My basses are all 5-string, and vary from two-pup designs to a bridge Piezo acoustic bass, so I'm not planning to do any further analysis on output vs frequency. I'm in the right ball park so I'll stop there.

So, firming up the design, with an 18dB/octave HPF set to 60 Hz, and limiting the input power to 20 W per speaker, and using a vent with a 2:1 cross section, I get an 80 mm x 40 mm vent 160 mm long giving me a maximum air flow of 15 m/s at 65 Hz. So far, so good.

For an overall box depth of 305 mm, with an amp on the back, and a grill on the front, I have an internal depth of 220 mm for a port of 160 mm. Is the port end too close to the back wall?

David

[Balcro]

Pm to OP for clarification.

[Mottlefeeder]

And replied^.

David

[Mottlefeeder]

Just a quick update, and thanks to all of you who responded. 

I wasn't sure whether to go make a rectangular port or buy in a round one,  so (amongst other things) I measured the internal diameter of the port pipe I would use, and it is 63 mm, not the 68 mm default in WinISD. This means that my port will be shorter than originally thought. Also, in calculating how much room I had in the box, I forgot that the port comes through the box wall, which means there is less of it in the box. The combination of those two means that I have room to use a straight port, and it finishes more than one diameter from the back wall, which seems to tick all the boxes.

One of the things I have learned came from this thread - http://www.diymobileaudio.com/forum/technical-advanced-car-audio-discussion/156355-winisd-slot-ports.html  - in summary, WinISD gives the option of modelling a port (a) finishing flush with a baffle, or (b) finishing hanging in free space, but a shelf port is neither of those. The 5th poster reported that a port finishing by being boxed in by the enclosure base and two sides, needs to be about considerably shorter than WinISD calculates it. He misremembered it as 40%, and later corrected that to 20%.

So, if any air movement restriction in the vicinity of the port causes the port to act as it it was longer, this suggests that if you have to put a bend in your port, then you need to have at least a diameter's length of port after the bend to avoid the pipe before the bend affecting the air flow.

Thankfully not something I need to worry about this time.

David