So there are all sorts of calculators for port length/area/volume etc. but we're tuning for some very low frequency such as 32Hz which has a wavelength of 30-ish feet but the ports are designed to something much higher frequency. They aren't 30ft in a typical vented enclosure design. Sure, you deal with half wavelengths in room gain, but how in the heck can you somehow boost 32Hz with a tiny (relative to 30ft) port? If I guess correctly, the "1st resonance" is the actual Helmholtz resonant frequency of the port which makes sense...200Hz, etc., but it doesn't seem like you can get a resonance from ~3 octaves below that. A Helmholtz resonator resonates at integer multiples of half the wavelength (right?) and is why you get the crazy resonances at increasing frequency in (for example) Hornresp. Every frequency that is an integer multiple of the Helmholtz (1st resonant) frequency is a resonant frequency, but they have to be higher than the 1st resonant frequency. But to get a resonance at 32Hz from a resonator designed for ~200Hz means you're dealing with fractional (not whole number) multiple of the half wavelength.
I'm sure the explanation is simple. But what is it?
Link to the inevitable thread discussing this is welcome.
Playing with my new table saw with my son's new sub enclosure and still have 10 fingers (born with 10)...
I'm sure the explanation is simple. But what is it?
Link to the inevitable thread discussing this is welcome.
Playing with my new table saw with my son's new sub enclosure and still have 10 fingers (born with 10)...
Mack,
Keep those fingers, I lost two and half a thumb, being digitally challenged is not fun!
The volume of the air in a bass reflex box in relation to the port length and width determines the Fb (box tuning, Helmholtz resonant frequency).
If a box is of the aspect ratios normally used (not long and skinny..), the wavelengths associated with those dimensions have almost nothing to do with the Fb.
The port duct itself will have it's own resonant frequency too, usually so far out of band that it's excitation is of little concern.
Art
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Not replying takes less time and is equally helpful. I know the equation relating port area and length and enclosure volume, speed of sound in air, end correction factors, etc., calculating hydraulic equivalent diameter for rectangular/slot ports. If you could share a link that answers my question, please share; I've looked and haven't had any success. So I posted here.
Thanks Art. Is there a mass/spring analogy or something? Like the air mass in the port is coupling to the spring of the enclosure volume? I'm trying to grok what's going on physically in a bass reflex design. I know it works, I've built a couple of these, but I don't know why and it bugs me.
No, my suggestion to Google could actually have been very helpful
how does a helmholtz resonator work - Google Search
Unfortunately, that article (which I had already read) did not answer my questions. It starts with the resonator equation and goes from there. There's a footnote for the source of the resonator equation, but it links to an unrelated chapter on thermodynamics (with broken markup for the equations). My question is about why that equation works and that page didn't answer it. I've found that equation all over the place and had to re-derive the one that floats around on every audio website (that I've been *searching* for weeks) to find out how to correct for altitude. I live at ~7500ft where air pressure and density are noticeably lower than sea level and am more often at even higher altitudes. Most of the equations you see have the speed of sound in air *at sea level* "baked in".
You can assume I didn't search, but, aside from being wrong, it doesn't help to say so. FYI.
Fortunately, the PDF that rayma posted is exactly what I was after.
Thanks toulou! It's getting much clearer in my mind now. I was thinking of the port as a horn but it's a mass/spring system where the port's air mass is resonating with the "spring" of the enclosure's volume of air. The end corrections account for the extra air that is just outside of the port but plays a mass role nevertheless. I need to wrap my brain around the mechanical analogies of the electrical concepts of capacitance, inductance, resistance and impedance. I think the PDF rayma posted will help a lot with that.
Thanks! I still have to figure out something. It can't just be the mass in the port, because you can have narrow, short ports or wide, long ports that, aside from velocity/choking/chuffing issues, work the same but obviously have different masses. I think I've got what I need to get to the heart of this now.
Kenamond,
After recently thinking that the 6200 foot altitude of Madrid, NM may have affected my measurements of various cabinets (tapped horn and bass reflex) due to lower air pressure, was relieved to find out that the speed of sound remains the same for all intents and purposes at altitudes we normally would exist at.
Speed of sound in air temperature barometric pressure calculator without no table air density of air formula temperature calculation mach 1 acoustic impedance room temperature propagation air density sea level velocity ideal gas 20 degrees or 21 degr
"It is a wrong assumption that the speed of sound decreases with altitude above sea level, because the density of air decreases with height. The changing of atmospheric pressure does not change the speed of sound.
Only the colder temperature (!) lets decrease the speed of sound at higher altitudes.
The speed of sound has nothing to do with "sea level".
For sound pressure without the medium air there is no speed of sound. Speed of sound is dependent nearly only on its temperature.
It is not dependent on the sound amplitude, frequency, or wavelength."
Most "speed of sound" calculators assume a reduced temperature at a higher altitude.
Even when adjusting for temperature swings from "room temperature" down to freezing or up to very hot (110F/38C), the speed of sound change would only amount to about 1Hz variance from a 35Hz tuning.
That said, the low humidity in the high desert definitely affects high frequency absorption losses in addition to inverse distance loss:
Calculation method of absorption of sound by atmosphere air damping dissipation absorbtion high frequencies attenuation sound during propagation outdoors outdoor - sengpielaudio Sengpiel Berlin
Compared to "living in the swamp" here in Florida, the high frequency losses in the mountain desert at long distance can be more than double.
Art
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Thanks Art!
c=sqrt(g*P/rho) is the speed of sound of an ideal gas (air) where g=1.4 for air, P is pressure and rho is density. Both P and rho drop by 30% at my altitude, but the ratio will be the same, so you're absolutely right! I knew that pressure was 30% lower but I didn't realize that density was ~30% lower as well.
Any worry about the humidity difference between the swamps and Madrid (inexplicably pronounced mad'rid) and what it does to the wood in the cabinet?
My dad bought a nice acoustic guitar in South America back in the late 60's and the back split like crazy (cupped) the first winter he had it back in the northeastern US. Still sounds great!
-Mack
c=sqrt(g*P/rho) is the speed of sound of an ideal gas (air) where g=1.4 for air, P is pressure and rho is density. Both P and rho drop by 30% at my altitude, but the ratio will be the same, so you're absolutely right! I knew that pressure was 30% lower but I didn't realize that density was ~30% lower as well.
Any worry about the humidity difference between the swamps and Madrid (inexplicably pronounced mad'rid) and what it does to the wood in the cabinet?
My dad bought a nice acoustic guitar in South America back in the late 60's and the back split like crazy (cupped) the first winter he had it back in the northeastern US. Still sounds great!
-Mack
I've since sold most all the cabinets built in Madrid, but they and about 8 guitars survived going from the desert to the swamp with no problem.
Guitar builders that expect their products to travel well used kiln dried wood, which stays stable if temperature and humidity swings aren't too extreme in too short a time period. That said, my only acoustic guitar is an Ovation, the plastic back could be used to carry water ;^).
Guitar builders that expect their products to travel well used kiln dried wood, which stays stable if temperature and humidity swings aren't too extreme in too short a time period. That said, my only acoustic guitar is an Ovation, the plastic back could be used to carry water ;^).
I need to wrap my brain around the mechanical analogies of the electrical concepts of capacitance,
inductance, resistance and impedance. I think the PDF rayma posted will help a lot with that.
Here's also another of his books that you might like to see. Olson was the "horse's mouth" on acoustics.
http://www.tubebooks.org/Books/intro_Olson_1943_Dynamical_Analogies.pdf
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Yup! Like a car wheel bouncing out of control on its worn out shocks. OK maybe that is not too relevant. 😛 The genius of Thiele and Small (and let's not forget J.E. Benson) was to recognize that the physical quantities of a speaker could be analogized to electric filter circuit elements. The mass is of the cone and coil and part of the suspension and some air load...the spring is the suspension elements plus the air in the box.
The port mass IIRC is not just a mass, it is multiplied by the cross-sectional area because that is what the force is applied across and therefore...I could be off, but it's *something* like that and frankly I'm feeling too lazy to dig Small's thesis out of my cold garage to check 😉 Ah, if the shape of the port changes, but the cross-sectional area and length remain the same that IS the same mass come to think of it (end corrections that differ for different shapes may change the length versus shape).
Just to clarify, the acoustic mass of the port tube is equal to the mass of the air inside the port tube plus the mass of the air in the associated end correction extensions, divided by the cross-sectional area squared. The SI unit of acoustic mass is kilograms per metre^4.
I've got plenty of solid body electrics which haven't had a problem at all...except the one I built without any luthier knowledge back in undergrad; it warped a bit, but it was unfinished (in terms of frets (sounds like a zither), pickup(s) and finish). And all my acoustics are cheap but did fine. My son's Taylor is doing fine, but I got it for him in Santa Fe.
Thanks rayma, but that's just the table of contents. Not that I don't already have enough dense reading from you already, but do you have the rest? Sounds interesting!
I get your meaning. The mass in the port acts on the area of the port, stretching and compressing it, so mass per port area makes sense. Not sure why it's squared yet, but I'll get there. rayma gave me a bit of a reading chore.
Thanks for the clarification David. Before you know it, I'll be able to write a horn response code that everyone can use! Oh, wait...
Sorry about that. Olson's Dynamical Analogies book is from 1943, by the way. The other book was from 1957.
http://www.tubebooks.org/Books/Atwood/Olson 1943 Dynamical Analogies.pdf
http://www.tubebooks.org/Books/Atwood/Olson 1943 Dynamical Analogies.pdf
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