Is there a way of measuring/verifying tuning frequency for a vented enclosure?
And what kind of air velocity do you consider "too much" when designing a port?
And what kind of air velocity do you consider "too much" when designing a port?
Tuning frequency/port velocity
- Thread starter Helge A. Bentsen
- Start date
Re: Tuning frequency/port velocity
JR
It seems like there are multiple terms involved. You might be able to empirically determine that vent tuning is peak for the driver/box combination, but I don't know how to easily isolate the port for measurement.
I am not a speaker guy but when turbulence from air flow becomes audible, could be a problem.
JR
Re: Tuning frequency/port velocity
I thought that the best test is an impedance sweep. For two reasons. One is that you can see where the peaks are, and two, you can see how the two peaks compare to each other. If you are forcing the box, the peaks will differ, and if the driver is fitting into the box, and tuning, the peaks will be more similar. What I have been taught.
I don't know that I have ever experienced this phenomena. Was just contemplating this on Saturday. Has anyone else? What does it sound like?
Is it defined as harmonic, or intermodulation distortion?
I thought that the best test is an impedance sweep. For two reasons. One is that you can see where the peaks are, and two, you can see how the two peaks compare to each other. If you are forcing the box, the peaks will differ, and if the driver is fitting into the box, and tuning, the peaks will be more similar. What I have been taught.
I don't know that I have ever experienced this phenomena. Was just contemplating this on Saturday. Has anyone else? What does it sound like?
Is it defined as harmonic, or intermodulation distortion?
Re: Tuning frequency/port velocity
Impedance sweep, or by looking at the cone excursion minima. The former is more informative if you have the tools, the latter can be done for free.
For the air velocity, the port will start undergoing substantial compression at a Reynolds number between 50,000 and 100,000. These effects are dependent on the port geometry. The reynolds number can be calculate for the port geometry, but for common geometries a reynolds number of 50K corresponds to approximately 7 m/s. Usually port velocities up to 15 m/s can be had without too much port noise, but of course there will be port compression happening.
Further, the velocity of air in the port increases, the tuning frequency drifts upwards. This can be observed by making impedance sweeps of the cabinet at ever increasing levels, using something like the LinearX VI Box.
Impedance sweep, or by looking at the cone excursion minima. The former is more informative if you have the tools, the latter can be done for free.
For the air velocity, the port will start undergoing substantial compression at a Reynolds number between 50,000 and 100,000. These effects are dependent on the port geometry. The reynolds number can be calculate for the port geometry, but for common geometries a reynolds number of 50K corresponds to approximately 7 m/s. Usually port velocities up to 15 m/s can be had without too much port noise, but of course there will be port compression happening.
Further, the velocity of air in the port increases, the tuning frequency drifts upwards. This can be observed by making impedance sweeps of the cabinet at ever increasing levels, using something like the LinearX VI Box.
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Re: Tuning frequency/port velocity
JR
PS: Phil is a speaker guy so listen to him not me.
I said I wasn't a speaker guy, but back in the '80s I designed a piece of cheap test equipment that allowed simple impedance sweeps. Basically drive the speaker cabinet with a signal sweep through a fixed resistance, and measure the voltage drop across the resistor. This signal doesn't need to be very loud, just enough to get a clean voltage drop measurement (1v or so works),
Is this what they mean when they say the port is being "choked"?
JR
PS: Phil is a speaker guy so listen to him not me.
Re: Tuning frequency/port velocity
I'm just beginning to try to digest speaker building.... here's a paper on ports that i found quite technical, but still fairly readable...
http://koti.kapsi.fi/jahonen/Audio/Papers/AES_PortPaper.pdf
I'm just beginning to try to digest speaker building.... here's a paper on ports that i found quite technical, but still fairly readable...
http://koti.kapsi.fi/jahonen/Audio/Papers/AES_PortPaper.pdf
Re: Tuning frequency/port velocity
I'm looking at a design using a high excursion 18" driver in a cabinet with a large port for a tuning around 28.5Hz.
The cabinet is simulated in WinISD, and it looks like "everybody" on the internet agrees that you'll have to verify your port tuning with a prototype build because WinISD gives you a port length that is too long.
So, using this approach, I was thinking about constructing a prototype,measure the actual tuning and adjust the port length if/as needed.
With the search function, I noticed that several people on this forum have recommended Dayton Audios woofer tester for doing impedance sweeps. It's affordable, but is it reliable?
http://www.parts-express.com/dayton-audio-dats-v2-audio-test-system--390-806
I'm looking at a design using a high excursion 18" driver in a cabinet with a large port for a tuning around 28.5Hz.
The cabinet is simulated in WinISD, and it looks like "everybody" on the internet agrees that you'll have to verify your port tuning with a prototype build because WinISD gives you a port length that is too long.
So, using this approach, I was thinking about constructing a prototype,measure the actual tuning and adjust the port length if/as needed.
With the search function, I noticed that several people on this forum have recommended Dayton Audios woofer tester for doing impedance sweeps. It's affordable, but is it reliable?
http://www.parts-express.com/dayton-audio-dats-v2-audio-test-system--390-806
Re: Tuning frequency/port velocity
Helge,
You cannot think of port tuning as precisely as something like 28.5Hz. The tuning frequency may shift 10Hz depending on the output level. Generally it is prudent to tune the box slightly low so that the tuning frequency at high volume is where you want it. Here is where taking impedance sweeps at different volume levels really works in your favor, to quantify the shift upwards.
Further, to help with the port overloading, it can be prudent to set corner frequency of the high pass to the tuning frequency, or just above. If the filter is a Butterworth, then the knee is -3dB with respect to the nominal, and the port gain is usually sufficient to bring this response back close to flat down to the knee frequency. Because the driver is -3dB electrically at Fb, that helps reduce your port velocity at Fb. You can take this even further by also considering the shift in port tuning frequency when picking the tuning frequency and the high pass knee.
That's how it is done for serious designs.
Bennett likes his. I think he posted a review of it in the product review section here. I use a LinearX VI Box to take impedance transfer functions.
Helge,
You cannot think of port tuning as precisely as something like 28.5Hz. The tuning frequency may shift 10Hz depending on the output level. Generally it is prudent to tune the box slightly low so that the tuning frequency at high volume is where you want it. Here is where taking impedance sweeps at different volume levels really works in your favor, to quantify the shift upwards.
Further, to help with the port overloading, it can be prudent to set corner frequency of the high pass to the tuning frequency, or just above. If the filter is a Butterworth, then the knee is -3dB with respect to the nominal, and the port gain is usually sufficient to bring this response back close to flat down to the knee frequency. Because the driver is -3dB electrically at Fb, that helps reduce your port velocity at Fb. You can take this even further by also considering the shift in port tuning frequency when picking the tuning frequency and the high pass knee.
That's how it is done for serious designs.
Bennett likes his. I think he posted a review of it in the product review section here. I use a LinearX VI Box to take impedance transfer functions.
Re: Tuning frequency/port velocity
A cheap and simple method to measure the tuning frequency - face the cone upwards, place a small pinch of saw dust (or similar material) on the cone and do a sine wave sweep. You will see quite easily where the point of minimum excursion is ... The saw dust will almost stop jumping up and down at that point.
... And plus 1 for parts express / Dayton Audios tester.
A cheap and simple method to measure the tuning frequency - face the cone upwards, place a small pinch of saw dust (or similar material) on the cone and do a sine wave sweep. You will see quite easily where the point of minimum excursion is ... The saw dust will almost stop jumping up and down at that point.
... And plus 1 for parts express / Dayton Audios tester.
Tim Weaver
Senior
Re: Tuning frequency/port velocity
Yes the woofer tester is great, but it won't tell you how the port behaves at high velocity. That is new territory for most of us diy'ers. We have only just begun to see woofers that will really test a port's ability to breathe in the last few years.
Yes the woofer tester is great, but it won't tell you how the port behaves at high velocity. That is new territory for most of us diy'ers. We have only just begun to see woofers that will really test a port's ability to breathe in the last few years.
Re: Tuning frequency/port velocity
Excuse me, my highly technically affluent comrades, but why does the port tuning shift as volume increases?
Excuse me, my highly technically affluent comrades, but why does the port tuning shift as volume increases?
Re: Tuning frequency/port velocity
A whole bunch of fluid dynamics that I erased from my brain immediately after getting my college diploma
http://www.fohonline.com/home/20-features/6031-tech-feature-subwoofer-ports.html
A whole bunch of fluid dynamics that I erased from my brain immediately after getting my college diploma
http://www.fohonline.com/home/20-features/6031-tech-feature-subwoofer-ports.html
Re: Tuning frequency/port velocity
Man, you do live forever on the internet... That was while I was trying to do aerospace consulting and audio consulting.
That old article is mostly correctly descriptive of what happens in a port for the layman, but doesn't really answer Max's question.
---
Max, as Spencer says, there is a lot of fluid mechanics to discuss to give a deep answer to this, but let me see if I can cut to the essence. Two effects are in play, and both tend to reduce the apparent inertia of the air in the port:
-The first is that as the velocity of air in the port increases, the air "feels" the walls of the port less, and can move more easily (i.e. lower inertia).
-The second is that the air immediately beyond the two port ends behaves differently at different air velocities. You can think of this extra air being like virtual port length, and you lose some of this length as the air in the port moves faster.
Both of these effects combine to cause a shift in tuning frequency upwards.
A whole bunch of fluid dynamics that I erased from my brain immediately after getting my college diploma
http://www.fohonline.com/home/20-features/6031-tech-feature-subwoofer-ports.html
Man, you do live forever on the internet... That was while I was trying to do aerospace consulting and audio consulting.
That old article is mostly correctly descriptive of what happens in a port for the layman, but doesn't really answer Max's question.
---
Max, as Spencer says, there is a lot of fluid mechanics to discuss to give a deep answer to this, but let me see if I can cut to the essence. Two effects are in play, and both tend to reduce the apparent inertia of the air in the port:
-The first is that as the velocity of air in the port increases, the air "feels" the walls of the port less, and can move more easily (i.e. lower inertia).
-The second is that the air immediately beyond the two port ends behaves differently at different air velocities. You can think of this extra air being like virtual port length, and you lose some of this length as the air in the port moves faster.
Both of these effects combine to cause a shift in tuning frequency upwards.