Help with Bass port calculations on speaker build

Try winisd , it's very simple .

I am maybe looking in the wrong places, but almost everywhere I look up calculations for bass ports are for giant subwoofers rather than speakers.

The calculations are the same. Somewhere in the calculation is a variable for the cabinet volume. It can be small or large.

If I build the speakers to the interal volume reccomended, does the port placement, size and lengh matter depending on the shape I make it in?

The placement of the port is not critical, as long as the exit is not restricted or blocked, it can go wherever is convenient. The front baffle is most common followed by the rear baffle.

Bass reflex cabinets act like Helmholtz resonators. The port size is determined so as to resonate at the chosen frequency for the cabinet volume. As port area increases, so must the length to maintain a fixed tuning frequency. Too small a port area results in high air velocity, and too big a port area can result in either the length of the port acting like a transmission line or the cabinet not behaving like a Helmholtz resonator. From the book Transducers and Sound Fields, “The volume of air in the port VP, which is simply the product of its cross-sectional area SP and its length t, should be chosen to be several times greater than the amount of air it has to displace in order to produce the maximum sound pressure at full power”. In many cases the port size is restricted to what will physically fit in the cabinet. It is not overly critical.

Ideally, a round port will give best results, but with compact cabinets, where baffle space is restricted, larger ports can be achieved with other shapes, which offsets any disadvantage due to the shape. With the cabinet you propose there should be plenty of space on the front baffle. The internal depth will probably limit the maximum port size.

With the latest version of the software being from 2016, and others still available from 2002 and 2004, even if it still works, I don't really want software this old on my PC.

The software works fine, with no detriment to computers of any age. However if you feel that it is not for you, it is possible to do the calculations using some paper, a pencil and a calculator. You only need the equation for the port tuning, which shouldn’t be too difficult to find online. I’m pretty sure I have included it in a previous post on this forum.

I'm just after some advice specifically regarding bass port tuning, and from my perspective, it isn't at all easy to get simple answers from this software.

Not easy! I would say it is impossible for you to get answers from the software if you don’t install it on your computer.

As for advice, using winisd is about the most simple way to design bass reflex loudspeakers. Dayton already provided the cabinet size and tuning frequency for the drive unit so, by advice, are you really asking for someone to calculate the port size for you? Other than that, it might be possible to find the plans for an existing design using this drive unit.

You could always try emailing Dayton and asking what size port they recommend for their design.

The software was (in my opinion) designed for much lower resolution monitors from years ago. It was almost impossible to read anything on it and scaling the display settings to a much higher percentage only changed the window. I respect the program function will still work, but it could do with an update to support modern 4k monitors.

What software are you using? Winisd should scale. The same as any other windows program, you just click in the corner of the window and drag. Due to advancing years, my eyes are not the best, but I have no trouble using a 32” monitor with 2560 x 1440 resolution.

I think I must be doing the calculations wrong though as it suggests the port length to be nearly 20 inches.

What port area, or diameter if it is round, are you using? Reducing the port area will reduce the length.

The drive unit that you mention has an overall diameter of about 180mm. Is there any reason you need the internal width of the cabinet to be 300mm? Reducing the width and height of the cabinet enables you to increase the depth and allows a larger port. Essentially, you would be rotating your design so that the width becomes the depth.

I now realised that the information from Dayton gave the f3 frequency not the actual tuning frequency,which I’d guess would need to be about 32HZ. Using a standard, round, plastic gutter pipe of 68mm dia, you would need a port of between 260 and 300mm long.

A cabinet with a height of 440mm, and width of 220mm would give a depth of 356mm which would be plenty deep enough for the above port.

Port size i first tried to calculate was 3" which I knew was a little overkill, but it still surprised me just how long it reccomended it being.

Ignoring end correction, the area and length of the port are proportional to each other. With a round port, the area is proportional to the square of the radius so doubling the diameter requires four times the length.

I will see if I can think about redesigning the size and shape, or would two front ports solve the issue with my current internal measurements? I just didn't want the speakers to be all that deep (not sound wise!) but respect it can be triky to get the right sound without making some compromises.

Using two ports won’t solve your problem. The length of each port is mainly determined by the combined area. There is an end correction factor that will slightly alter things, but putting two 3” ports would double the length compared to a single port. Multiple ports can be useful where it is difficult to fit a single port of the desired size onto the baffle, or getting an acceptable port area using a smaller standard pipe size.

The port does not have to be on the front baffle. If the cabinet is lifted off the floor, the port exit could be on the bottom. I have some small computer speakers where the port is on the top. Position is more of a practical (or aesthetic) consideration. With PA speakers, it is highly likely that they will be used in multiples, stacked together, or placed against a wall, so forward facing ports make most practical sense.

Re: winisd. The version I have is 0.7.0.950

Yea i could consider putting the port on the base if i add some spiked feet or something like that. I mainly wanted them on the front as it has least potencial to sound boomy when close to a wall.

Spiked feet, depending on how long they are, might not give enough clearance. I would suggest a gap equivalent to the diameter of the port. The wavelength at 32Hz (the port tuning) is about 10m. The difference in distance from any wall due to placement on the cabinet is negligible. The boominess due to speaker placement is more likely to be at higher frequencies

Does adding corners to the port complicate the calculations? E. G. Have it in my intended position under the driver, but have it bend 90 degrees and go upwards. You mentioned a standard gutter pipe, which has plenty of angled adapters that I will be able to seal fine. Would the length along the centre internally if it has a 90 degree corner still be 260 – 300mm.

Sort of and yes. Adding a bend slightly lifts the tuning frequency, but not significantly. Most people add the bend to enable  using a larger port. The assumption is that it reduces air flow velocity and thus turbulent flow. However, according to paper titled:

“Acoustic Mass and Resistance as Function of Drive Level for Straight, Bent, and Flared Loudspeaker Ports”


Another thing to consider is port resonance. Most of the sub designs discussed on this forum cross over below 100Hz, so port resonance is generally above the operating range. The drive unit you are using is designed to be used well up into the high midrange, so resonances are in its operating range. The port is considered a pipe open at both ends so the resonant frequencies occur at:

    f = nc/2L

where n is an integer, c the speed of sound and L the effective length of the pipe.

Damping material in the cabinet can reduce the effects and using a smaller, shorter, port can raise the frequency of the fundamental resonance, thus reducing the number of resonant frequencies in the operating range.

In the end, there is no one correct solution. For you, the proportions of the cabinet seem to be a priority, so other design aspects must fit to that. Most participants of this forum high spl is a priority so they look at the predicted air velocity in their favourite software program  and aim to minimise that. If that requires a bend in the port, they probably don’t consider the detrimental effects of the bend to be significant.

Whilst a smaller port does increase the air velocity relative to a larger port, its actual value is level dependent. In a home system, if the system is rarely used at maximum, a smaller port could be a viable option.

Having fired up winisd, it predicts that a 33 litre enclosure (based on your dimensions minus a bit for the port and drive unit), tuned to 32Hz  will require a port length of 272mm with a diameter of 68mm. The first port resonance is at 632Hz. The f3 frequency will be 27Hz. The max spl will be limited by Xmax to 95dB which will require 12W to achieve. With that input power the maximum port air velocity will be 11m/sec.

I would increase the tuning frequency to 38Hz, the transfer function predicts the f3 frequency would rise to 32Hz, but the max spl would be increased by 5dB around the 40Hz region. This tuning gives a couple of dB boost to the response centred on 45Hz, compared to the 32Hz tuning, which depending on taste, could be flattened with some eq. This tuning would also reduce the port length to 178mm which fit in your design with no bend.

If you are not averse to some experimentation, it s possible to fit the port temporarily, as long as where it fits the baffle is sealed. It is not difficult to try various port lengths. It is also worth revisiting winisd.

I don't think it will need to be the apsolute optimal setup, so long as doing it differently won't sound "bad". But then I know that the location and surrounding / accoustics of the room and equipment driving them all make a difference as well.

This design is not overly sensitive to the port dimensions. Both the 32Hz and 38Hz tuning work OK so anything in-between will work too.


There is a lot of rubbish information about room acoustics in the hi-fi world. For some people, the music is a necessity to listen to the hi-fi. They are constantly fiddling and tweaking their setup. Unfortunately, now-a-days they create a YouTube channel preaching to fellow obsessives. There are lots of gurus, well meaning or not, on the internet peddling their own ideas if you want to go down that route.

As for equipment, In my opinion, unless the signal is deliberately manipulated, any modern electronics that imparts its own sound on an audio signal has been badly designed. If kept within its linear operating range it should not have its own sound.

I have in the past found that the depth of speakers when placing them is a little more of an inconvinience than their width, but that is just preference. I would be OK if it made a big enough difference to keep the height the same, and make the cabinates more rectangular and have the width and depth more or less the same.

One of the benefits of designing and building your own speakers is adapting them to your own requirements. Generally it is not best practice to have similar dimensions for height/width/depth. The reasoning is that any cross resonances will occur at the same frequency, making them more audible.

Regarding a smaller port, I did find this on CPC, which has an inner diamitor of 66mm

The link didn’t work for me, but the difference between a 68mm and a 66mm port will be negligible, especially if its length is adjustable. For a round port, I just use the closest pipe size to what I calculate and adjust the length accordingly. I usually cut the baffle hole with a router, with the pipe fit from the back and front hole flush with the inner diameter. If finish is not so important (behind a grille) I just router or laser cut a hole and fit the pipe flush with the front baffle. I guess a flange covers up a less than perfect cut-out, but it is not necessary acoustically.

I respect that I should try this program again. I have reinstalled it with a different version and it is better now regarding readability. However, I still feel blank with what I should do, and after watching a tutorial, I still feel rather confused. Software is not my strong point, and I do have difficulty with learning stuff like this. It depends how patient you can be with helping me with it as I may appear a bit clueless!

One word of caution is to be careful with the included library of drive units. A few have incorrect parameters and others have some parameters missing, which leads to errors. The best thing is to fiddle about with the program and ask if you run into any specific problems.

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