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Impedance chart, need explanation
Impedance chart, need explanation
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Old 8th May 2016, 06:27 PM   #1
lup31337 is offline lup31337  Romania
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Default Impedance chart, need explanation

I don't know how to interpret an impedance chart for a subwoofer in a bass-reflex enclosure. Does it give any information ?

I know that for free-air, the peak in impedance indicates the resonance frequency of the speaker.

Before measuring the impedance, I made a nearfield frequency measurement and I could accurately tell the resonance frequency of the box (Fb), which is 52 Hz.

I have attached the impedance curve. Why does it have 2 peaks ? 52 Hz corresponds to the lowest point between the 2 peaks. Is this correct ?
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File Type: jpg impedance chart.jpg (271.4 KB, 394 views)
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Old 8th May 2016, 07:13 PM   #2
sreten is offline sreten  United Kingdom
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Hi,

The minimum impedance between the
peaks indicates the port tuning frequency.

Vented has two peaks, sealed just one.
Related to 2nd order and 4th order.

Interpretation of the two peaks is not so simple.
Generally overdamped the lower peak is lower
and underdamped the lower peak is higher.

Without running off to check I think classically
maximally flat they should be equal, but I don't
like maximally flat at all, I like overdamped
that maximizes F-6dB to F-10dB.
Maximally flat optimises F-3dB.

I like room gain friendly alignments.

rgds, sreten.

Note that 52Hz tuning is speaker, not subwoofer territory.
All vented subs IMO need tuning < 41Hz. Low E on a bass.

Last edited by sreten; 8th May 2016 at 07:35 PM.
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Old 9th May 2016, 01:12 AM   #3
Ron E is offline Ron E  United States
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Electrically, the vented box speaker looks like a series LCR filter in parallel with a parallel LCR filter. The parallel LCR is the speaker, and the series is the box/port. A parallel notch filter (speaker) causes an impedance peak. The series notch of the box/port then splits this single peak into two peaks.

If you can measure that same box sealed and vented with no damping material, you can determine many things, mostly from the maxima and minima. The best simple reference for this is D'Appolito's Testing Loudspeakers.

The heights of the peaks are strongly dependent on damping material and flow resistance in the port. The lower peak is affected mainly by the the port - more port flow resistance (or more velocity) means a lower peak, for this reason this peak will also change with drive level. The upper peak is affected mainly by box damping, more damping means a lower peak. Leakage losses from driver leaks or poor construction will reduce both peaks and raise the minima between. The minima is the port tuning frequency in the absence of driver inductance. The minima can be quite broad, and its exact frequency (and that of the zero phase point) can be shifted away from the actual port tuning frequency by driver inductance. For these two reasons, close miking the driver and finding the minimum is the most accurate way to determine tuning frequency.

Proper box volume and tuning frequencies are very close to (Small-Margolis):
Vb=Vas*20*Qts^3.3
Fb=Fs*0.42*Qts^-0.96
or (Keele):
Vb=Vas*15*Qts^2.87
Fb=Fs*0.42*Qts^-0.9

You can lower tuning from this and get a drooping response, or raise it and get a peak.
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Last edited by Ron E; 9th May 2016 at 01:15 AM.
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Old 9th May 2016, 01:15 PM   #4
lup31337 is offline lup31337  Romania
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Now that you mention it, the tuning frequency seems a bit high. It's an 8" car woofer (with 6.5" effective diameter) in a box made by the manufacturer. I don't know why they chose the Fb so high.
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Old 9th May 2016, 01:50 PM   #5
Ron E is offline Ron E  United States
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Quote:
Originally Posted by lup31337 View Post
I don't know why they chose the Fb so high.
Likely boosted bass, a little more thump. Hard to say, though, without knowing parameters.
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Old 9th May 2016, 02:22 PM   #6
minkuni is offline minkuni  Norway
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The paper on the frequency dependency of driver damping by Claus Futtrup is an interesting read when trying to make sense of the impedance peaks.

Scientific papers
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Old 9th May 2016, 05:16 PM   #7
Zero D is offline Zero D  United Kingdom
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@ Ron E

I would like to congratulate you on Post # 3 = Perfect compact explanation

@ minkuni

Thanx for the link
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Old 9th May 2016, 05:32 PM   #8
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by lup31337 View Post
Now that you mention it, the tuning frequency seems a bit high.
It's an 8" car woofer (with 6.5" effective diameter) in a box made
by the manufacturer. I don't know why they chose the Fb so high.
Hi,

Try lining the port with porous foam about 3/8" thick.
Last time I did it I used a cheap washing up sponge.

Cut the sponge into a rectangle port length x 3xdiameter,
Fold it into a tube and insert into the port. This will tidy
up any tendency to boom and go deeper, tighter. The
foam also helps with chuffing by smoothing airflow.

rgds, sreten.
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Old 10th May 2016, 04:36 AM   #9
bentoronto is offline bentoronto  Canada
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Impedance chart, need explanation
At low frequencies, the impedance curve precisely reflects the motion of your cone. That's what it tells you. That's wonderful.

In the case of using a resonant box to counter-act the natural motion of the driver, it (viewed in isolation) tells you not much about sound output in the range around the bumps. Odd to try to make accurate sound when the cone motion is poorly related to the sound output, eh.

Ben
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Old 10th May 2016, 05:27 AM   #10
just a guy is offline just a guy  Canada
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Quote:
Originally Posted by bentoronto View Post
At low frequencies, the impedance curve precisely reflects the motion of your cone. That's what it tells you. That's wonderful.
What on earth are you talking about now? The impedance curve doesn't look even vaguely similar to the displacement curve. Here's the impedance and displacement of a random ported box.

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Quote:
In the case of using a resonant box to counter-act the natural motion of the driver, it (viewed in isolation) tells you not much about sound output in the range around the bumps. Odd to try to make accurate sound when the cone motion is poorly related to the sound output, eh.

Ben
A resonant box does not counteract the natural motion of the driver. The resonance provides a little bit of boost a frequencies where it's needed.

The sounds resonant boxes make are plenty accurate when properly designed, when tuned below 40 hz transient response and group delay are largely inconsequential. What you hear is frequency response and if frequency response if fine it will sound fine.

Even with sealed boxes the displacement curve doesn't reflect the impedance curve OR the cone motion so basically everything you posted is wrong and based on a misconception of how sealed and ported boxes work.

Last edited by just a guy; 10th May 2016 at 05:33 AM.
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