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Topic No. 5

Questions on Vents
by
John L. Murphy
Physicist/Audio Engineer

 

Q: "How critical is the placement of the port(s) in an enclosure?"

To understand the sensitivity of a vented (ported) speaker to the location of the vent it is first necessary to understand why it would matter at all. In a vented box type of woofer subsystem all of the acoustic out put of the system is from the woofer driver itself at frequencies well above the vent tuning frequency of the box, that is, the F(B). At lower frequencies, specifically a narrow band of frequencies centered on the box tuning frequency F(B), all of the acoustic output is from the vent. Thus the speaker system consists of two acoustic sources: the driver and the vent. The total output of the speaker system is the acoustic sum of the outputs of these two sources. In order for these two sources to sum as intended the two sources must be largely "in phase". It is this phase relationship between the two sources that is affected by the location of the vent. The phase difference is also affected by the location of the listener.

What really matters is the DIFFERENCE in the distance of the listener from each of the two sources. As long as the listener is the same distance from the driver and the vent the space between the driver and the vent doesn't matter. But as a general rule we want the frequency response to be constant no matter where the listener is located. Therefore we want the vent and the driver located "close" to one another. In this case "close" means within a small part of an acoustic wavelength at the box tuning frequency. Now, since most vented systems are tuned below 100 Hz that means that the wavelength at the box frequency is generally longer than about 10 feet (the approximate wavelength of a 100 Hz tone). So as long as your vent is within, say, 1/4 of a wavelength the location of the vent will not matter much. If the vent is more than about a 1/4 wavelength then the frequency response in the bass range will become dependent on how the listener is positioned with respect to the driver and vent.

So, if your speaker is tuned to 100 Hz you want to keep your vent no more than about 2.5 feet (1/4 wave) from the driver. But if your box is tuned closer to 50Hz (a more realistic figure) then the vent only need be within 5 feet of the woofer. Since this is an easy criteria to meet for most systems it seems fair to say that for most speakers the vent placement is not critical.

Below is a table of frequencies and corresponding 1/4 wavelengths.

Frequency Wavelength 1/4 Wavelength
200 Hz 5.65 ft 1.41 ft
100 Hz 11.30 ft 2.83 ft
80 Hz 14.13 ft 3.53 ft
60 Hz 18.83 ft 4.81 ft
50 Hz 22.60 ft 5.65 ft
40 Hz 28.25 ft 7.06 ft
30 Hz 37.67 ft 9.42 ft
20 Hz 56.50 ft 14.13 ft
10 Hz 113.00 ft 28.25 ft


Q: "When multiple ports are used, how should they be spaced or placed in relationship to each other? (minimum spacing requirements?)"

I think the "1/4 wave" rule described above applies here as much as to vent-driver spacing.

Q: "Is it beneficial to have the ports on the baffle with the driver(s)?"

As long as the vent is within about 1/4 wavelength of the driver it doesn't really matter which face of the cabinet it is on in order for the vent output to sum correctly with the driver output. However, there are some other points to consider when selecting the vent location. Such things as midrange leakage from the vent, the possibility of the vent getting blocked in normal use, the available space on the baffle, etc. Most designers tend to locate their vents on the front baffle close to the woofer. For most speaker systems I see no reason why the vent should not be located on the rear of the enclosure, and in fact, the midrange leakage from the vent of a rear vented speaker will tend to be less noticeable. Put your ear near the vent of most vented boxes and you can hear a significant amount of "other stuff" coming out of the vent. This "other stuff" is typically the upper frequency material from the rear side of the woofer. Ideally it should be held captive inside the box so any of it that leaks out constitutes "distortion". A rear located vent reduces the audibility of this "distortion". For rear vents just make sure you don't violate the "1/4 wave" rule described above.

Q: "Does a port have to be inside the enclosure or can it protrude from the enclosure?"

Other than looking a bit weird I know of no reason why the vent should not protrude from the box. For a box of given dimensions, the net internal volume will be decreased by the volume of the vent when the vent is located internally. Where the vent volume is a tiny part of the net internal volume this will not really matter. But for "micro" sized speakers where the vent volume may be a significant part of the net internal volume this choice may make a big difference in overall system performance. (Imagine a tiny speaker where the vent took up half the internal volume.)

Q: "Can you shed some light on the laws of port flaring, and wouldn't it have horn-like characteristics."

Port flaring helps reduce the rushing noise that can occur when the air velocity in the port exceeds about 5% of the speed of sound. I doubt that any reasonably sized port would have much of a horn-like effect considering the very low frequencies at which most ports operate. For example, consider a speaker where the vent (port) is tuned to 40Hz (a typical tuning frequency). In order for a horn to be effective at 40Hz it would have to have a mouth area bigger than most speakers ( at the very least, several feet by several feet). The flared vents I have seen don't come anywhere close to these dimensions. So I doubt that there is any significant increase in output due to the "horn loading" of the vent. A more practical problem I have with port flaring is that I don't have a precise method for predicting the box tuning frequency using a flared vent. The vent length is well defined but the vent surface area is NOT well defined. Maybe taking an average of the inside area and the outside area would give a reasonable representation of the effective vent area. The only way to know for sure would be to build the vented box (with flared vent) and then measure the box tuning frequency.

Q: What should the minimum port area be for a bandpass box? 
Possibly S(v)=.02*Fb*Vd ?

Your formula looks good provided S(v) is given in square inches and V(d) is given in cubic inches. In fact, this is the same formula used in The Speaker Design Toolbox to give the minimum recommended vent area for both vented and bandpass type enclosures.

The origin of this relationship is Dick Small's excellent paper on vented boxes:

"Vented-Box Loudspeaker Systems Part II: Large Signal Analysis"
Journal of the Audio Engineering Society, Volume 21, Number 6
July/August 1973

For vented type speakers operated at full power that is, full excursion, it is possible to end up with audible noise from the air rushing in and out of the vent. For a given speaker system, the smaller the vent opening the greater the velocity of air in the vent and the greater the vent noise. Small wanted to know what the smallest vent area was that would be free from this noise. In the above paper Small states that based on listening tests he determined the maximum air velocity which avoids excessive noise generation is about 5% of the speed of sound. This is provided that the inside surface of the vent is smooth and that the edges of the vent are rounded off to a "reasonable radius". Small came up with an approximate formula which limits the velocity of sound in the port to 4.5% of the speed of sound for a wide range of vented box alignments. The formula he gave is as follows:

S(V) >= 0.8 * f(B) * V(D) (read ">=" as greater than or equal to)

where the area of the vent, S(V), is in square meters, and the peak volume displacement of the cone, V(D), is in cubic meters. After converting S(V) to square inches and V(D) to cubic inches this formula becomes:

S(V) >= 0.02032 * f(B) * V(D)

jlm

 


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