What's all this about port compression?!? (1 Viewer)

[Chris_Campbell]

Hello all, I don't mean to be disrespectful to those individuals who have spent years in the industry, but I don't understand the logic behind port noise. With the decision to build my tempest sonosub, I downloaded team linear's WinISD. This nifty little program was telling me that it wasn't safe to have a vent mach of over .08. I've heard from some on this board that this is because you get compression of the air, which causes unwanted noise. However, in the field of aerodynamics, any air flow velocity under mach .3 is still safely considered in the INCOMPRESSIBLE mach regime. Even beyond this up to .8 mach is considered subsonic, with some regions approaching mach 1 possibly because of localized accelerations of flow over a body. In addition, when taking into consideration the effects of elevation and temperature and humidity, this still doesn't bring a mach number of even .1 remotely close to compressible. Now I'm really not looking to rock the boat with this one, and I will say that many elements of speaker design are beyond my knowledge and experience, but this matter has piqued my interest. I apologize for the long post, and thank you for any thoughts you may have.

 

[John E Janowitz]

Chris,
There are great differences between speaker ports where the air flow moves in one direction, stops and reverses direction, and in aerodynamics where there is a one way air flow. Techniques such as dimpling the surface to reduce fluid friction, which have positive aerodynamic effects in some cases, also do not work in ports and actually have a negative effect.
BassBox pro calculates vent velocity in meters/second, not in mach. I am not sure the exact correlation between the two. However, in the research done at Klipsch, it was found that at 10 m/s the vent velocity began to contribute to audible distortion, and compression begins to set in. According to bassbox, they set the limits of vent velocity at 35m/s. I believe this is the point at which your port is compressed beyond useable limits, and increasing the input power will no longer give any increase in volume.
The other factor not typically taken into account is the fluid friction in the port. The more surface area in the port, the more fluid friction there will be. This lowers the vent velocity(good), but decreases output due to compression(bad). None of the models I am aware of for ported systems take into account the internal friction in the port directly. By changing the QL of the port you can get a good idea of what is happening though. When I get a chance here I'll post an example.
John

 

[Chris_Campbell]

I see what you're saying, and I guess there's no disputing emperical tests. Like I said, I really don't know how the flow is behaving, but typically there is no possibility for air compression under .3 mach (approx 102 meters/sec). You see I'm building a sub right now, and I'm really trying to make sure that my ports won't give any problems. I'm using PVC 4" diam (internal) 23.5" long. I believe this will tune my enclosure (a little over 9 ft^3) to 20 hz. Hopefully this will give me nice bass extension right down into the sub 20 hz range. Does this sound right?

 

[Tom Vodhanel]

a single 4" non-flared port will begin to measureably compress around 50-75w input(using one of the common 12" drivers(shiva,titanic,PE,lambda,mass12,ect).It will audibly compress around 100w input(but you may not notice---depends on your experience and if you have a *non compressed* unit nearby to compare).
For low power apps, this can be ok.
If you plan to use >100w---I go with a widely flared 4"---flared at both ends.
TV

 

[Trenton McNeil]

How does the flared end combat port compression? I thought
the operative was the diameter of the port inside, anyway?
Just curious, I'm about to build one myself, and bought 4"
PVC. I'm gonna be driving 400watts to a 15" tempest.

 

[Patrick Sun]

The slug of air being pushed through the port will "seem" bigger as the flared opening will give more volume for the air to go through the port. I have a flared port on Sunosub III.
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PatCave ; HT Pix ; Gear ; Sunosub I + III ; DVDs ; Link Removed

 

[Chris_Campbell]

Actually, I meant to say that I have two of such ports. Each 4" diam 23.5" long. Thanks though

 

[TerryC]

"Techniques such as dimpling the surface to reduce fluid friction, which have positive aerodynamic effects in some cases, also do not work in ports and actually have a negative effect."
Interesting. B&W has implemented just that: Link Removed

 

[Chris_Campbell]

Actually, in the case of a golf ball, dimpling the surface doesn't actually reduce skin friction, it increases it. But instead of a completely laminar flow over the body, you have a turbulent flow, which resist separation much better than a laminar flow. Separation is what will give you an incredible amount of turbulence and drag. I'm not sure how this applies to a speaker port, but this may be why they implement the dimpling effect.

 

[John E Janowitz]

There was some discussion on port dimpling and B&W's marketing of the technique on the basslist some time ago. Basically it doesn't work to any advantage over a smooth port, and Deon Bearden explained why, quoting AES pre-print number 4855(G-6). Since he said it much better than I could, and also copied most of the article I will just copie most of his email here:

In addition, golfball dimpling works (Bernoulli's Law) under a DC condition, not AC. Keep in mind, a golfball see's essentially direct current flow, while a port see's AC, or Alternating current flow. Under this condition, where the air flow is literally halting mid track, then colliding with
itself, and reversing direction, much of any improvement "potentially" seen with 'GolfBall' dimpling will be negated. Mark is 100% correct in saying a larger port is a much better means of achieving this.
See also AES pre-print number 4855 (G-6). This paper was presented by JBL Proffesional, and Infinity Systems at the 105'th AES convention in 1998. The title is "maximising Performance from Loudspeaker Ports, and was written by Alex
Salvatti and Doug Bacon of JBL, and Allan Devantier of Infinity Systems.
In it they examined 'dimpling' of ports, and had this to say:
"One might think that smoother textures in ports would directly result in higher performance. However since Coulomb's experiments from the 1800s it has been known that surface roughness has an effect on friction resistance.
Interestingly the effect is negligible in laminar flow, but not if the flow is turbulent, i.e., surface roughness effects would be evident only at the higher port velocities. If reduced drag is desired, a rough surface will actually perform better due to boundary layer effects. This is the reason why golf balls have dimples -- the surface roughness is intended to "trip" the boundary layer so it will go turbulent at a lower Renoylds number (in flight the Re of golf balls is about 100,000). The turbulence causes the
separation point to move from the front to the back of the golf ball, thereby reducing drag and allowing further flight. There are now even commercially available subwoofer speakers which use a flared port with dimples similar to
a golfball. Fig. 36 of a bowling ball entering the water at 25 fps demonstrates how much larger the wake is on the smooth ball, vs that of the surface roughened ball on the right. ( considerably) Notice also the separation point has moved further back.
Another example of intentionally induced roughness is often seen on the top surface of an airplane wing near the leading edge. These "vortex generators" are also used to prevent boundary layer separation, which could cause the wing to stall under high lift conditions such as during landing.
In fluid mechanics, surface roughness is characterized by the dimensionless 'roughness ratio'
( formula given I can't do in ASKII )
Small changes in the roughness ratio can lead to very large effects in the turbulent flow region. To test this hypothesis, we constructed 5 copied of the best performing port (NFR=0.5) then affixed precision glass beads of
various sizes ranging from 1mm to 2.5mm to the inside port walls using a spray adhesive. This corresponds to a roughness ratio range from .01 to .042 on the Moody chart. These ports were manufactured such that the volume occupied by the beads was accounted for, see fig. 37 for picture of a typical textured port from the study. ( It's textured ). These ports were then subjected to the same distortion and compression tests described earlier. ( An exhaustive study of ports flare rates, and other things, very good material, you should purchase the pre-print. I believe it's 6.50 to non members.. )
Contrary to expectation, over the range of roughness examined, rough ports were generally inferior to the smooth walled ports. Rough ports had more harmonic distortion above 95 dB at 1 meter. Only in a very narrow range between 90-95 dB did wall roughness give a marginal improvement in odd harmonic distortion. At all other levels, the smooth walled port performed better ( see fig 38 ) Based on fluid mechanics literature [22], we expected to see the benefit of the rough walls in the acoustic compression measurement. Unfortunately, roughened port walls failed to show any
advantages here as well. In fact, fig 39 ( a compression chart ) shows all rough ports were consistently compressing about 1-1.5dB more than the smooth port. These negative results may be explained by noting that at even the highest Renoylds numbers near 100,000, the Moody chart predicts that we are only JUST entering the transition region and have not yet reached the fully turbulent region where roughness would be expected to make a large impact. Based on these results, it does not appear that roughening the wall in this range buys any extra performance."
I would not have said something as heavy handed towards 'B&W's" marketing department such as "Golfball 'dimpling' is a marketing gimmick. At 45m/s it won't help." if I did not have a GOOD reason.
Deon
So that basically sums up why the dimpling of ports doesn't work. In theory it looks good, but when you are looking at system where the airflow is in one direction, then stops and reversed direction, there are no measureable advantages.
John

 

[Eric S]

Without having an extensive background in fluid dynamics as others here do, My first guess is that B&W is banking on the "coolness factor" for the appearance of the dimpled port. The cynic in me says its the oldest trick in marketing - something that functions the same, but somehow looks different or has different packaging is used to generate new interest in an existing product.

 

[Chris Popovich]

Anyone here know what effect adding a 90degree elbow to the port will have on compression?
Chris

 

[Chris_Campbell]

Interesting, thanks for posting that. I think you're right. But hey, it does look really cool

 

[John E Janowitz]

Eric,
Yes I have to agree with you. It's basically a gimmick to make their product stand out. Even though the performance is the same, they can claim that it is better, and nobody can really prove otherwise. And it does look different.
Chris P,
I'm not sure the exact effects of a 90 degree elbow, but it is not good. Basically instead of having air just go in one direction, stop, and reverse direction, now you have air that flows in one direction, hits a wall, makes a 90 degree turn, goes a little further, then turns around and does it again. I'm sure there are some AES papers on it somewhere, but I don't know offhand. I do know that two 45 degree bends will be much better than a single 90 degree bend. Also the bends should be more gradual and rounded, not just a sharp 90 degree angle.
John

 

[Stan Marcewicz]

By port compression we don't (or I don't ) mean that the air is compressing per se.
When subs are played loud, they start to produce less output than expected based upon the increase in the input signal sent to them. For example a sub might be playing at 85 dB. If you add 3 more dB of input it might produce 88 dB. If you added 15 more dB (above the 85), maybe only 99 dB will be produced. This gets worse rapidly as you go up if you have a high output sub. A loss in dynamic range. There are various reasons for this such as running out of power, driver issues etc. One limiting factor can be the ports or passive radiators.
Some commercial subs such as Velodyne use limiters whose ills are seldom exposed in reviews. For example suppose a particular version maxes out at 110 dB at some low freq. It takes many extra dBs of input to get it to go from 109 to 110 db. This doesn't sound that bad, but is very easy to hear if you compare it with a sub without such characteristics. Personally, I'd consider such a sub to have less than 110 dB of max output at that freq since you can't get their without distortion. Not THD but distortion of the dynamics. But at least it will never fart.
The different factors contribute to compression in different ways. For example a driver thermal issue may require large amounts of power at certain freqs for a certain amount of time. A hot voice coil thing - resistance increase. Generally the problem freqs will correlate with the impedance curve as expected. Port compression kicks in at those freqs where the port is very active. Without any real time delay.
When a port is just a straight tube, the ends can act as though they are smaller in diameter as far as the air flow is concerned - turbulence near the circumference.
If you are designing a sub and anticipate pushing the limits I think it makes sense to get an idea of the various limiting factors so that you can max out your result.
Stan

 

[Chris_Campbell]

Thanks Stan, so would you recommend a flare for the port? I understand this will improve the airflow through the port, but I've been hesitant to put one on because I bought such thick PVC piping (about .25" thick), and i'm not sure that I'll be able to fit a flare on it. Any recommendations?

 

[Patrick Sun]

Chris, unless you can fabricate a flared opening for the PVC pipe (not impossible, need to be resourceful), the other option is to spend some money and buy a Precision Flared Port from a number of places, like Parts Express , Madisound , Adire Audio , and maybe Stryke , etc. If you need up to 18" of a 4" wide port, you won't need to get additional port and coupler ring. Otherwise, if the port needs to be longer, then you'll have to get more port and a couple ring. Then you just glue them together using the coupling rings. The flared opening on the endcap side will take up a space of a 7" wide circle. The precision ports are thin, not like the PVC 1/4" pipe, maybe 1/16" thick.
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PatCave ; HT Pix ; Gear ; Sunosub I + III ; DVDs ; Link Removed

 

[DanWiggins]

One thing to keep in mind is that you can make a vent too long, and it ends up no longer being a vent! Basically, you go over a certain length, and the vent starts to lose output, because it's too long.
In a Helmholtz resonator, the idea is like the mass on the spring idea. The spring is the compliance of the air in the box, and the mass is the air in the port. This requires that the air in the port moves as one "solid" mass. Because there IS a finite speed of sound in air, long ports will end up with the inside end of the vent starting to move considerably earlier than the outside end of the vent. The two ends can start to move "out of sync", and thus the whole "solid" mass is no longer true.
Taken to the extreme, you end up with a resonant pipe, like in an organ or flute. Basically, the vent is no longer a homogeneous narrow-band device; it's now a broad-band device with multiple resonances. Those resonances are driven/excited by the fundamental frequency of the pipe. This means energy from the fundamental is used to excite upper resonances. Result? Less output at the fundamental (which is the opposite of what a vented speaker is trying to do) and considerable output at higher frequencies (and because this is excited by the fundamental, it will occur regardless of the crossover setting!).
Taking the vent area to the extreme (vent area is equal to one side/end of the cabinet), you end up with a true pipe. The compliance of the spring in the box drops off as the area of the vent increases, until you have the area of the vent equal to one side/end of the box. Then the compliance in the box is pretty much gone. You're in a dipole mode at best, long pipe at worst.
What's a good upper length limit for a vent? From the literature I've seen, anywhere from 1/10th to 1/30th the wavelength of the tuning frequency. I believe Dick Pierce recommends 1/20th of a wavelength. For a tuning frequency of 20 Hz, this translates to a length around 34". Longer than this, and your vent moves towards "non-vent"ness...
As a result of the above, there's actually a recommended MAXIMUM diameter for a vent! You can, in fact, hurt performance by making a vent too long; there is a sweet spot for maximizing the resonance of the box, in terms of coupling between the mass in the vent and the springiness in the box.
One last thing: what Stan posts is sage advice! Power compression in drivers is a real problem, and is typically at least an order of magnitude greater than what you get in typical ports. Power compression in a driver on the order of 2-3 dB is not that uncommon when driven by 100+ Watts; that much compression in a typical DIY port is rarely seen.
Dan Wiggins
Adire Audio

 

[Stan Marcewicz]

>>>>Thanks Stan, so would you recommend a flare for the port? I understand this will improve the airflow through the port, but I've been hesitant to put one on because I bought such thick PVC piping (about .25" thick), and i'm not sure that I'll be able to fit a flare on it. Any recommendations?

 

[Hank Frankenberg]

FWIW, the instructions that came with my PNR Aero Port kit (that I've installed in my Tempest sonosub)say to put the larger diameter flare INSIDE the sub and mount the smaller diameter flare on the baffle. The larger flare is quite a bit larger, BTW.
Dan: what's the reason for the inside flare being larger?