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Does using foam plugs raise a speaker's impedance?

Discussion in 'Audio Hardware' started by Phil12, Jan 12, 2021.

  1. Phil12

    Phil12 Radiant Radish Thread Starter

    That's what I read somewhere. Couldn't find it back though. The question is simple: does inserting foam plugs in the BR port raise the impedance for the amplifier?
     
  2. No.
     
    caracallac likes this.
  3. formbypc

    formbypc Forum Resident

    No. Impedance/resistance is an electrical property, unaffected by cabinet construction etc.
     
  4. MusicNBeer

    MusicNBeer Forum Resident

    Location:
    Florida
    It doesn't raise impedance but it completely changes the impedance in the bass region where box/woofer interaction occurs. Depending on how much the foam obstructs the port, the resulting impdance is some hybrid between the original dual peak ported response and a single peak sealed response.
     
    jupiterboy, Mr.Sign and Rick58 like this.
  5. Bananajack

    Bananajack Phorum ... wat Phorum? Where am I?

    Location:
    Singapore
    It changes the dynamic impedance curve in the bass region
    But it doesn’t raise impedance like 1Ohm higher, that’s technically impossible

    Your amp doesn’t mind and doesn’t feel it, no worries
     
    Shawn likes this.
  6. Morbius

    Morbius Forum Resident

    Location:
    Brookline, MA
    I believe this statement is false. From reading published test reports (Stereophile) cabinet resonances and port tuning can be clearly delineated in plots of a speakers impedance curves. They clearly define port tuning and the vibrational behavior of the cabinet walls as it correlates to the speakers electrical impedance.
     
    Mr.Sign and Phil12 like this.
  7. RhodesSupremacy

    RhodesSupremacy yeah man

    Location:
    Northern Europe
    It'll increase impedance at the resonant frequency and the resonance itself will go higher in frequency.
     
    Mr.Sign, Phil12 and Rick58 like this.
  8. Rick58

    Rick58 Forum Resident

    Location:
    Dublin, CA, USA
  9. Ingenieur

    Ingenieur Forum Resident

    Yes
    You are changing the resonant frequency
    You are changing a ported enclosure to a (poorly) sealed one.
     
    Mr.Sign likes this.
  10. Phil12

    Phil12 Radiant Radish Thread Starter

    Not complete closing off the port (resistive damping) does improve the sound though.
    I am just relistening various types of music.
     
    Rick58 likes this.
  11. Phil Thien

    Phil Thien Forum Resident

    Location:
    Milwaukee, WI
    I'd think it would reduce the impedance peak, similar to an aperiodic/variovent port.
     
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  12. Phil12

    Phil12 Radiant Radish Thread Starter

    You might be right when going from BR to aperiodic.

    Found this about going from closed box to aperiodic:
    Aperiodic Loudspeaker Enclosure Design .

    "The purpose of an aperiodic design is to allow a smaller enclosure size than would normally be possible in a sealed box. If a driver is placed into a sealed box that is too small, the result will be a high Q, which will cause a peak in the lower frequency response and a high impedance peak. By allow air to leak from the enclosure, both the frequency response and impedance peaks are tamed. This results in clearer, better defined bass, with more amplifier power and control into the lower frequencies. The impedance response of a sealed versus an aperiodic enclosure is shown in Figure 01 below."

    [​IMG]

    Also interesting (long live internet): Bass-reflex to Aperiodic Conversion? - diyAudio
     
    Last edited: Jan 13, 2021
  13. Blank Frank

    Blank Frank King of Carrot Flowers

    Can someone explain for, at least, my benefit how changing a physical characteristic by blocking a port changes an electrical characteristic, like impedance? What physics I studied was quite a while ago and I've forgotten most of it. Thank you.

    PS This is a genuine question.
     
  14. WildPhydeaux

    WildPhydeaux Picture withheld by The Authorities

    Location:
    A Parade in Dallas
    Analogy: A shop vacuum with a single suction hose open at one end. Now block the end of the hose with your hand. Hear the motor pick up speed? Since the house voltage didn't change the current must have changed.

    Cheers,
    Robert
     
  15. Rick58

    Rick58 Forum Resident

    Location:
    Dublin, CA, USA
    A speaker is an electro-mechanical device. Movement of the cones WRT the coils that drive it actually generate current among other things ... which is why 'damping factor' can be important in an amplifier, BTW ... and why a speaker can be used as a microphone ... I'm not an EE nor a physics major and it's 4 in the morning. Looking at curves like I posted above shows 'resistance vs. frequency' but there's also 'phase' which is involved ... 'negative' phase = capacitance, 'positive' phase = inductance, IIUC. Maybe it's the other way around. These things all change whether the speaker is in free air/space or in an enclosure (and whether the enclosure is sealed, vented in some way, or whatever).

    KEF Q350 loudspeaker Measurements LOOK AT THIS ...!

    So it's not a resistor ... it's a complex assembly.

    PS/Edit: these things also change the actual frequency RESPONSE (output) of the speaker too. Mainly seen in the bass - how much sound the speaker puts out and how fast the output falls off as frequency drops.

    PPS: the curves further down show this too ... altho he plots the complex sum (black) only for the ported option. The acoustic phase (different from electrical phase) changes too. With the port open, the output 'changes sign' below/above the resonance (sharp dip). This affects how a powered subwoofer sums up with the main speakers as well - if you block your ports you'll likely have to change the phase on the sub ... OK that's enough physics for 4am ... and I might be wrong! YMMV, no warranties expressed nor implied, you get what you pay for, etc. ... :D

    PPPS: the red trace does show the output with the port blocked ... the system puts out MORE low bass (<30Hz) with the port blocked but less bass above this ...

    PPPPS: the reason the bass rolls off faster <30Hz with the port open is that the acoustic phase IS inverted, so the output of the port actually causes 'subtraction' (nulling) of the output of the speaker ... I think ... ZZZZZZzzzzzz ... :D Above the resonance, the port output is IN phase with the speaker output, so ADDS.

    PPPPPS: I'm going back to bed ...
     
    Last edited: Jan 13, 2021
    Blank Frank likes this.
  16. Larry I

    Larry I Forum Resident

    Location:
    Washington, D.C.
    The speaker voice coil moving in the magnetic field actually generates a current which is flowing in the opposite direction of the flow from your amplifier to the voice coil. This "back electromotive force" (back EMF) manifests itself as an increase in impedance. If you change the characteristics of the cabinet, you are changing the movement of the speaker diaphragm and voice coil, hence back EMF is also changing.
     
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  17. RhodesSupremacy

    RhodesSupremacy yeah man

    Location:
    Northern Europe
    This is correct, I misspoke in my earlier comment! Also, count me in as a fan of aperiodic bass.
     
  18. formbypc

    formbypc Forum Resident

    Surely it's changing speed because the flow of air has been restricted, leading to less air pressure around the fan, and less air resistance to it?

    Not because increased current has been applied.
     
    Ingenieur and Kyhl like this.
  19. WildPhydeaux

    WildPhydeaux Picture withheld by The Authorities

    Location:
    A Parade in Dallas
    Of course it is, and this moves you a few steps closer a "no load" condition, the result of which is increased efficiency. Less load, less effort. The motor is drawing only the current required to turn it and the fan at its designed RPM, more or less. Take it a few steps further and disconnect the motor leads: zero load, zero effort (current).

    Cheers,
    Robert
     
  20. Phil Thien

    Phil Thien Forum Resident

    Location:
    Milwaukee, WI
    The blocked airflow means the fan in the vac is what is called "free wheeling." So voltage stays the same, amperage drops, even though the shopvac seems louder.
     
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  21. jeffmackwood

    jeffmackwood Forum Resident

    Location:
    Ottawa
    I don't think so. There's a difference between a pump pulling and maintaining a vacuum (pressure, not the device) and a pump working in a vacuum (a space where a vacuum (no pressure) exists). The former is high load, the latter can be close to no load (aerodynamic load drops to zero but there's still likely some friction-related load (like bearings)).

    But it's been nearly 40 years since I got my mechanical engineering degree, so I'd be happy to defer to a younger pup who knows better. :)

    Jeff
     
  22. RhodesSupremacy

    RhodesSupremacy yeah man

    Location:
    Northern Europe
    This is how I understand it as well.

    Full disclosure my last physics class was more than 10 years ago and I likely flunked it as well.
     
  23. Phil Thien

    Phil Thien Forum Resident

    Location:
    Milwaukee, WI
    Well in terms of a wet dry vac, power consumption falls as the airflow is restricted and is at a minimum with the airflow blocked.

    When it comes to other types of devices, their performance characteristics will typically be dictated by manufacturer-supplied curves, generalizations will likely produce exceptions.
     
    WildPhydeaux likes this.
  24. jeffmackwood

    jeffmackwood Forum Resident

    Location:
    Ottawa
    Can you point to data (manufacturer's spec sheet perhaps) that backs that up?

    My critical thinking spidey sense is tingling really hard.

    :)

    Jeff

    ps. I'm willing to take one for the team and buy one of these devices and put theory to test - unless someone out there already owns one and can do the deed for us.
     
    Last edited: Jan 13, 2021
  25. RhodesSupremacy

    RhodesSupremacy yeah man

    Location:
    Northern Europe
    Would this be analogous to putting a loudspeaker on the end of a long pipe (high impedance) versus operating in free air (low impedance)? This is how aperiodic loading decreases the impedance peak by letting air leak out and having a weaker restoring force against the cone.
     

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