Do ported speakers really act like an IB above the port resonance frequency? I’ve seen this quoted, but is it true? Has it been researched and is there any evidence? I’m wondering about this for the following reasons: an IB enclosure volume has an effect on the bass units performance (f3, transient response, etc.) At the port resonance frequency the hole in the enclosure will be “plugged” by the oscillating air in the port, but what if the speaker was reproducing a solo piccolo piece? Would there be enough low frequencies in order to set the port into resonance? If not, would the speaker essentially be an IB with a massive leakage?
I'm not familiar with the abbreviation IB, but I gather from the context that you mean a closed box.
Above the port resonance, the box behaves approximately as a closed box - not exactly, because the port still does something, especially in the first few octaves above resonance. At normal sound pressure levels, the box behaves (to a good approximation) as a linear system, so the piccolo will be processed (essentially) the same with or without other sounds.
Above the port resonance, the box behaves approximately as a closed box - not exactly, because the port still does something, especially in the first few octaves above resonance. At normal sound pressure levels, the box behaves (to a good approximation) as a linear system, so the piccolo will be processed (essentially) the same with or without other sounds.
Closed box = IB althou it usually refers to a really big box.
Except for any vent resonances, a BR will act like the same driver in almost any other box than a front-mounted horn.
dave
Except for any vent resonances, a BR will act like the same driver in almost any other box than a front-mounted horn.
dave
I don't like the term IB in this situation because that term implies (at least to me) an enclosure at least 10xVas.
Thanks for your replies gents. You’re quite right, I really meant a sealed enclosure, of any size.
Right, though more correctly, above its EBP' frequency, i.e. based on the system's total DCR (Qts') insertion losses.
[Qts']: [Qts] + any added series resistance [Rs]: http://www.mh-audio.nl/Calculators/newqts.html
[Rs] = 0.5 ohm minimum for wiring, so may be higher if a super small gauge is used as a series resistor and/or there's other series resistance.
The air in the port doesn't move much at higher frequencies, it gets too heavy to have much effect. So the box acts closed.
Look at any front ported speaker test on Stereophile magazine and you will see port output measured.
Frequently there is significant output up to around 1 kHz, so it is affecting the balance
Frequently there is significant output up to around 1 kHz, so it is affecting the balance
Right, but the much worse problem here is that if there's enough wide BW signal to energize the vent and the vent's long enough, then it's the pipe harmonics comb filtering with the higher BW that can audibly 'color' piccolos/whatever.
Anytime there is a midbass being used past 200 or so hz on up, the inherent and even the most subtle amount of pipe resonance and/or reflective midrange output from the port will pollute and ruin the impulse response of the mid frequencies on up.
Unless its a PA type application, I would never use a ported cab for a midbass driver. Just the phase response alone will ruin the integration with a sub, but thats only the case when the cab is being crossed over lower around the port tuning frrquency. You can plug the port with foam and that will help alot, but the dampening requirements for a ported vs sealed cab will sort of negate the double duty use via a port plug. There's always a tradeoff that way.
The other thing to be aware of is the lossy cab won't provide a restoring air spring for the driver if the compliance is very high. Most midbass drivers won't have this issue, but it can become a problem without the safeguard of a highpass filter if you're playing vinyl or sources with significant DC offset.
Unless its a PA type application, I would never use a ported cab for a midbass driver. Just the phase response alone will ruin the integration with a sub, but thats only the case when the cab is being crossed over lower around the port tuning frrquency. You can plug the port with foam and that will help alot, but the dampening requirements for a ported vs sealed cab will sort of negate the double duty use via a port plug. There's always a tradeoff that way.
The other thing to be aware of is the lossy cab won't provide a restoring air spring for the driver if the compliance is very high. Most midbass drivers won't have this issue, but it can become a problem without the safeguard of a highpass filter if you're playing vinyl or sources with significant DC offset.
I'd say not - as if the port length in resonance condition can "block" higher frequencies that happen to be inside the enclosure from coming through. Port resonance isnt a butterfly valve, right?
If the port is really short; I've had one design with the port an inch above the carpet, under a stand and still you can hear more frequencies than just the bass emissions I want there.
If the port were constructed as a laminar flow element - basically stuffed with a fist-full of drinking straws - wouldnt the arrangement of parallel smaller diameters impede the high frequencies from making it through? How about if you built the port like a muffler / silencer; a large outer diameter, some sound absorbent material, then a screen of the port diameter? So instead of solid walls, the port has absorbent walls?
If i remember, a ported has the same group delay as a sealed at twice its tuning frequency.
I like 27-30hz tuning, but I've never tuned lower.
I like 27-30hz tuning, but I've never tuned lower.
Not massive leakage but leakage.
The massive leakage happens below port resonance, otherwise known as the typical unloading of a reflex cabinet.
You can observe the impedance response in a model with very low leakage around 15 to 20 for Ql
then see how the impedance curve changes with more leakage.
Far as we know the leakage formula is fairly accurate.
But as noted by many in practice. And found by Small
The models tended to more realistically match real world measurements using a Ql around 7 to 10
Meaning the models were closer to real world practice by assuming more leakage.
The speaker surround itself and the cone can also be a source of leakage.
but I feel likely yes the port is technically leakage.
far as proving it would require a lot of work building real boxes, then comparing to ideal models.
Which in a way has already been done. the leakage wasnt pointed at the port.
But the models did tend to behave closer to real life assuming more leakage.
Hello All,
Theory is good to a point.
Sometimes you need to put your baseball cap on with the bill in the back.
Think about it, if the bass reflex installed driver is moving sound is coming out the front of the driver and out the port in the enclosure. If you do not believe it put a 9 volt transistor radio inside the reflex enclosure and turn it on.
Take a look at any of the speaker reviews where they place a microphone at the reflex speaker port, all kinds of cavity resonances and noise propagate from that port all the way up the frequency scale up to and past the crossover frequency.
The reflex speaker port is good for some cheap dB's and not much else.
Thanks DT
Theory is good to a point.
Sometimes you need to put your baseball cap on with the bill in the back.
Think about it, if the bass reflex installed driver is moving sound is coming out the front of the driver and out the port in the enclosure. If you do not believe it put a 9 volt transistor radio inside the reflex enclosure and turn it on.
Take a look at any of the speaker reviews where they place a microphone at the reflex speaker port, all kinds of cavity resonances and noise propagate from that port all the way up the frequency scale up to and past the crossover frequency.
The reflex speaker port is good for some cheap dB's and not much else.
Thanks DT
Let's say your driver is playing a bass rhythm, this will produce a resonance inside the box due to the port, resulting in the air pressure pushing on the back of the cone. Now at the same time a higher frequency needs to be transferred, that resonant pulsing of the bass will be superimposed on the higher frequency. Not my idea of an ideal speaker. But this is just a theory, I'm not sure how much it effects the sound in real life.
[Linkwitz about vented boxes]
I would say 90% of loudspeakers out there today use this concept to get a little more bass. Because if you put the resonance [of the vent] below the resonance of this driver in the box, and it seems that you get more bass. I find you now have bass ALL the time!
I would say 90% of loudspeakers out there today use this concept to get a little more bass. Because if you put the resonance [of the vent] below the resonance of this driver in the box, and it seems that you get more bass. I find you now have bass ALL the time!