Does using foam plugs raise a speaker's impedance?

See, now you've gone and exhausted my fount of knowledge!



I'd have to first read up on what aperiodic loading is, since I either have never known about it, or once did but have forgot.

So the honest answer is that I don't know.

Jeff

 

I am using my speakers aperiodic now. So there's still some flow from the port. I'm very pleased with the result.

 

This is how I use my Wharfedale W60. With a tube amp the bass is a little too prodigious unless I stuff the port just a bit.

 

Fundamentals of HVACR: Understanding Centrifugal Fan Motor Performance

The most convincing way to teach this concept is to have students figure it out for themselves using a centrifugal blower. Have them operate a centrifugal blower in free air with no restriction and measure both the amp draw and the fan RPM. Note that most centrifugal blowers cannot operate in free air for an extended time without overheating, so try and keep the free air operating time to a minimum. Next have them block one side of the air intake with a piece of cardboard and recheck the amp draw and RPM. Typically the increase in RPM is immediately obvious, but measurements prove the point. Have them slide the cardboard to block the intake only half way while watching the amp draw. A few minutes of experimentation will convince the students that blocking the intake actually causes an increase in RPM and a decrease in the motor amp draw. Next have them partially block the fan outlet while checking the amp draw. Once again, the amp draw will decrease. Allow them a few minutes of play time to convince themselves. This experiment does more to explain centrifugal blower motor performance than a week’s worth of lectures.

 

With the hose open the vac will try to draw in all the atmosphere it is "seeing " and therefore drawing more amperage.With the hose closed it has very little air to move and draws less amperage.Same applies to home heating systems....open up your ductwork on the intake side and it will burn out your fan motor.

 

Happy to report that some of us remember our basic electricity and fluid dynamics courses.... Now ask me to demonstrate laplace transforms and I'll suddenly remember a prior engagement...

Cheers,
Robert

 

I suspect we're close to the same age but I appreciate being called a pup!

In the shop vac case it will help to understand that the motor simply turns a fan within a housing where there is a single air inlet. The fan creates a low pressure zone and this causes air to angrily rush up the hose in a vain attempt at showing it's contempt. The motor worked hard at that low pressure zone and insists on it, which is not easy, but that's why it gets all the joules . When you block the hose it lets the motor simply create and maintain the low pressure zone without having to deal with angry air pixies lousing up the joint.

Cheers,
Robert

 

It is best to leave the transforms to the math geeks, you know, so they can feel all superior and stuff.

 

Textbook quality reply!

I will, however, purchase one of those plug power meters to either drive a stake through the heart of my vampiric opinion once and for all, or have it rise from the grave, thirsty to suck... (bad vacuum metaphor, dies off...)

Jeff

 

A vacuum is a centrifugal fan in reverse.
The hose is the inlet, it discharges into the bag.
When you cover the suction end it speeds up because it is doing LESS work, moving no air. No load.
Current decreases as does power assuming V is constant.
HP = V x I x (eff x PF)/746

The total pressure across a fan = suction + discharge pressure (relative to atm P)
If a fan can do 10" WC and it takes 2 on the suction, net is 8.
On a vacuum the suction has little restriction, the discharge more (the bag/filter) so most fan pressure is on the suction side. When you block it off it all shifts there but moves no air, so power drops off.

look at the fan curve: as flow CFM decreases to 0 HP drops to minimum.

 

We were all enjoying a harmless thought experiment, then someone had to go all Poindexter on us with his fancy formulas and graphs...

Cheers,
Robert

 

you can still enjoy

BTW the formulas are all of ours, not just mine.

 

Free-wheeling is the opposite of what you describe. If you take a blower from a furnace, and don't provide air resistance in front of it, it will actually use more power, while moving less air volume. It is designed for a particular motor load and back-EMF.

For a ported speaker, there are too many analogies in this thread that are not appropriate.

The best analogy one might use, is to compare the woofer porting to a spring with a weight on the end. It will have a natural resonant frequency where it will bounce up and down. If you hold the top of the spring, you don't have to move it much to excite a large movement at this frequency. Likewise, you'll continue feeling that bounce in your hand when you stop. Shaking too fast will produce little movement in the weight.

The box volume and the mass and velocity of air in the port create the spring and weight.

Here's the response of a very real subwoofer I own. Blue is with the port, and white is with the same volume box with port blocked (no port).



The first effect of plugging the port is the loss of flat bass extension in the frequency response. White drops off earlier, giving less total output. The second effect is the significantly increased cone movement at 25Hz, motion exceeding the driver's capabilities instead of being reduced by the port reactance.

Concerning impedance, you'll see at the bottom, the high-impedance blue peaks are gone. The white line with lower impedance shows that more current will flow from your amp with the port blocked. The total impedance is similar, so you don't have to worry about using a different terminal on your amp or such, it's merely academic.

 

Around here,if you put up an industrial building in the winter you rent a giant portable furnace and attach it to the ductwork until the real furnace is installed (workers get cold)We often get service calls for the 5 or 10hp motor kicking out on overload.Pretty much every time,a walk around inspection finds an opening in the ductwork somewhere...close the hole and all is well again

 

I think I should have avoided the word "freewheeling," as it always catches at least one individual that knows a little about the topic.

Dictionary definition of freewheeling: characterized by a disregard for rules or conventions; unconstrained or uninhibited.

So in this case, a centrifugal fan that is moving no air is, in fact, freewheeling, it is not constrained by airflow.

There are also freewheel blowers, these are designed to be used w/o a scroll enclosure w/o exceeding maximum draw for which the motor was designed.

I hope that helps.

 

Yep...we actually call it "running away" at work ..but yeah cut a blower off from it's intake or block it's exhale and it "freewheels"

 

Were the SPL's the same? port/blocked
How does it calculate Z?
What were the V and I at Z peak.?

plugged 40 Hz 18 Ohm
Not 32 Hz 12 Ohm

if the amp was constant V as most are voltage gain devices

P = V^2 / Z
Cancelling V and taking a ratio the plugged took 12/8 or 50% more power/current at that point for less output. It would be interesting to see that plot with both making the same SPL, basically cranking it up when plugged.

 

Ported is the blue plot. Sealed (or plugged up) is white.

The frequency response plots above are directly comparable as SPL output, as they are referenced at the 1KHz level, not affected by port loading. Voltage goes in, this response comes out.

The sealed driver demonstrates near the expected 1st order 6dB per octave roll-off, here, at a -3dB point of about 40Hz. The box volume and the driver parameters are not ideal for a sealed box - but that's what you get when you simply plug up a port on an existing design. The ported enclosure is significantly more efficient in the 20Hz-85Hz range a sub is asked to reproduce.

The voltage output of a voltage-source amplifier doesn't change. With the port plugged, though, all SPL must come from solely the piston movement of the driver. Sealed-box or infinite baffle SPL that has exact relation between max driver displacement, frequency, and max SPL. Below, the white line is not limited by the 500W speaker rating, but by low-frequency excursion limits.




Plugging up the port changes the peculiar reactance around port frequencies, with a different effect on the magnitude and phase of current flows. Like my bouncing spring example, there are ported enclosure driving frequencies that not only excite the resonance, but also resist the driving force.

At particular low frequencies, the impedance the amplifier drives may be either higher or lower than before. It does not change the nominal impedance of the speaker or create new dangers.

 

So, there is (1) closed box, (2) BR box and (3) resistive-damped box (in which the port is not completely closed off).
Some say that (3) can improve low end as compared to a BR box... Would like to see graphs how that works.
I guess that people don't bore a hole into a closed box, then partially block the port.

 

3) is not a thing unless you like port noise.
4) is a passive radiator, which acts like a port's mass, but also has excursion limits and distortion.

 

Well, it gets complicated and graphs don't tell the entire story. For example, the shallower roll-off of (1) and (3) often compliment room acoustics/gain better than the steeper roll-off of (2).

Also, the impedance curve of (3) means the amplifier may be able to exercise better control of the woofer's motor.

I'm not knocking any alignment, they can all sound great, the key is proper implementation. Commercial ported speaker designs often go with undersized enclosures (to keep costs down), resulting in an alignment which can accentuate the bass a little bit, at the expense of extension.

 

Ports "chuff," passive radiators do not. Also, ports tend to allow more leakage above the tuning frequency, when compared to passive radiators. This, of course, can be problematic.

 

I used to have B&W DM4 speakers (early 70's), one of the first vented box designs I guess,
aka resistive-damped design,
since it features lots of damping material (real sheep wool) and a hole in the baffle.
Some suggested to insert a pipe into the port hole, turning it into a BR design.

 

A ported enclosure has 2 nodes centered on the resonance frequency. As you increase the tightness of the enclosure the nodes converge on the Fo. The impedance peak must be higher.
Although the nominal Z remains ~ the same for a given power input and SPL remain ~ the same.

If you drill a large hole in a sealed speaker will the efficiency change? SPL vs fixed input power?

 

A small hole in a large speaker volume is also a port. There are many PA speakers that are just that, in the range of 3'x3'x2', and a 6" diameter port. The port length of plywood thickness makes for 40Hz tuning, good for a stiff-suspension speaker like a 18" JBL. A larger hole and the tuning frequency becomes higher, until you are approaching an open baffle with out-of-phase cancellation like an open guitar combo amp.

The larger the port area, or the shorter the port, the higher the tuning frequency. You get increased output there, but losses below. Efficiency depends on measurement methodology and design goal. A box with 90Hz porting would be a bad subwoofer, but may be a quite efficient midrange, employing digital crossover and EQ to level the frequency response and overcome phasing challenges.

A small hole in a a small speaker is a leak, you get turbulent noise, although it also could be port-like in the leak output being in-phase. It affects the Q of an enclosure in the same way that lots of poly fiberfill does.

A very long large port with almost no box volume is a pipe organ. Its behavior is dictated by the standing wave that can form in the pipe, though.