Playing around with a few things and am curious to know what your thoughts are on cylindrical vent diameters, vent mach and port noise.

Say, for example, a 3" port requires X" length... of which is too long for the enclosure, but mathematically provides an ideal vent mach.

If, hypothetically speaking, four 1" ID ports will satisfy the depth of the box as well as provide a vent mach below a 'noiseless' 0.04... will the small ports still somehow create a whistling/chuffing noise of some sort despite the optimum calculated vent mach?

If so, how would someone calculate that? My assumption has always been that so long as the desired vent mach has been attained, there shouldn't be any noise. However, internal port diameters smaller than 3" are typically discouraged for noise reasons.

Can anyone care to shed some light on this?

Thanks!

 

It isnt just the speed of air in the vent. It's also how that air is controled while moving in the vent. Entry and exit also play large role.

 

4 1" vents only have the area of a single 2" vent, and less than half the area of a single 3" vent. You need 9 1" vents to have the same area as a single 3" vent

If the area is the same, the length will be the same, regardless of how you got that area (with an exception for extremely small vent dimensions)

Controlling the air entry is also important. Vent edges should at least be chamfered or rounded, if not well radiused.

As for mach limits. The general limit in a house or a single cab car sitting still is 5% while a trunk or any car that moves while listening can usually tolerate 9% without much fuss. Something like an aeroport can handle up to 13%.

 

In my experience as well, a smaller vent always seems to have a tendency to have noise, especially at higher outputs or lower notes.

 

What do you mean by 'how that air is controlled while moving in the vent."? What other methods in vent design can be implemented to further control air movement outside of said entry/exit considerations?

I just plugged in a driver into Bass Box Pro and WinISD. The suggested box size (computed by BBP) is 1.3cuft and Fb is 21.2Hz. It also suggests a vent that is 4.124"ID x 58.38" in length. Clearly there is no way to to fit a 58" port into a 1.3" cuft box. However, if four 1" ID ports are plugged into the program, each port length comes out to be 13.29"... much more feasible than 58". Now, understand this was just plugged into the program very quickly and nothing has been plotted, but I think it gets my point across that according to the program, four 1" ID ports of 13.29" in length is equivelent to a single 4.124" ID x 58.38" port. Considering that all attempts to ease air entry and exit have been accounted for, outside of measuring vent speeds, how would someone know what the minimum diameter of the port should be?

How small were your vents? Was vent mach something you calculated in your enclosure designs as well?

Thanks for the comments

 

A 4" diameter port has an area of 12.57" square inches.

A 1" diameter port has an area of 0.785" square inches, thus four of them is 3.15 square inches. Not so coincidentally, this is the same area as one 2" vent. If you double the diameter, you quadruple the area.

This is basic math.

It is true that 1.3cuft tuned to 21hz results in (1)4" vent ~58.38" long. It is also true that 1.3cuft tuned to 21hz results (4) 1" vents ~13.29" long. While tuned to the same frequency, the two are completely different.

Thus, NO, 4 1" vents are NOT comparable to a single 4" vent.

The fundamental problem here though is trying to tune 1.3cuft to 21hz. IMHO no driver that works in a volume that small should be tuned that low, but that's just me..

As for your minimum vent diameter question, bookmark this link: PORT Size Calculations