Bass reflex box resonance

[costin]

How many resonance frequecies does a bass reflex box have ???
I know only one, between the 2 impedance peaks, at the minimum impedance of the box.Please correct me if i'm wrong.

Cheers,
Costin

[richie00boy]

Yes only the one between the peaks as you point out. It might not be the minimum impedance across the band, but it will be quite low and always between two peaks.

Assuming of course that you are on about a traditional single vented box, not a double chamber reflex or any other derivative, or bandpass box.

[costin]

Thanks, this is what i needed to hear.

[Svante]

Well, there are also the undesired tube resonaces in the vent, when the tube length is a multiple of ½ the wavelength. This makes small bass-reflex boxes tuned to low frequencies troublesome, since this calls for long tubes which moves the tube resonances down towards too low frequencies.

[jwatts]

Poor construction can lead to leaks that can lead to additional peaks. Just a thought.

[Collo]

If you are interested in finding the resonances that can occur between opposite walls, and between the driver and a wall, I have some software available to work these out for you.
Grab "boxnotes" for free at:
http://www.users.bigpond.com/bcolliso/freesoft.htm

It runs under windows and now supports imperial measurements as well as metric.

regards
Collo

[costin]

Ok....thanks for all your help guys.........but

It was suggested to me that because the bass reflex box has 2 imepdance humps, it certainly has to have 2 resonances.This puzzles me for shure.My mind keeps telling me that this can't be true.The resonance is based upon the mass of the air in the port and the volume of the box.It's just like a plain and simple spring and mass oscillator......I am kindly asking for some extensive explanations and comments

[MJK]

Using simple lumped mass and stiffness models :

A driver has one resonance.

A ported box has one resonance.

A driver in a ported box has two resonances.

The two peaks in the impedance plot are the two resonances of a ported box system. The minimum between the peaks (usually at the frequency where the individual box and driver have resonances) is not a resonance for the combined system. The minimum between the two peaks is the sum of the two mode shapes that cancel driver motion and accentuate port output.

If you do the detailed math treating the driver as a mass and spring in series with a mass and spring for the box, and solve for the eigenvalues and eigenvectors, you can prove to yourself that a driver in a ported box system has two resonances that form the two impedance peaks. You can also look at the mode shapes and understand why a ported box rolls off at 24 dB/octave.

[Svante]

Originally posted by costin
Ok....thanks for all your help guys.........but

It was suggested to me that because the bass reflex box has 2 imepdance humps, it certainly has to have 2 resonances.This puzzles me for shure.My mind keeps telling me that this can't be true.The resonance is based upon the mass of the air in the port and the volume of the box.It's just like a plain and simple spring and mass oscillator......I am kindly asking for some extensive explanations and comments

Ah, ok, so I see the reason for your question. Ok looking at the electrical impedance there is actually three or even four or more resonances. A resonance occurs when a mass m is connected to a spring c. The resonance frequency is f=1/sqrt(mc)

The lowest resonance can be estimated by adding the port mass and the driver mass and form a resonator with the driver compliance.

The second resonance (which occurs at the low between the two peaks in the impedance curve) is the helmholtz frequency. This frequency can be estmated by forming a resonator with the port mass and the box compliance.

The third resonance occurs at the second peak in the impedance curve. Its frequency can be estimated by the resonance between the driver mass and the added effect of the driver and box compliances. This resonance is very similar to the single (Ah well...) resonance of the closed box.

The fourth resonance has a slightly akward origin, in that the two reactive components is the voice coil inductance and a capacitance that originates from the driver mass as seen "through" the driver motor. It occurs at the second low to the right of the second peak in the impedance curve, and it is not particularly important.

On top of these resonances, there are also the pipe resonances in the vent.

Now, the great number of resonances is not necessarily bad, most of them have a relatively low Q, which means that they "die" quickly if they are excited. On the contrary, these resonances are used to form the desired response of the speaker.

[Svante]

Originally posted by MJK
The minimum between the peaks (usually at the frequency where the individual box and driver have resonances) is not a resonance for the combined system.

Hmm, so the helmholtz resonance is not a resonance in the bass reflex speaker?

I think you'd have to define what you mean by a resonance to make that statement hold. In my world a resonace occurs when two reactances cancel one another.