Strange dip in low-end response

So I got around to a project I have been wanting to do mainly for fun: build a larger floorstanding speaker.

But not just any floorstanding speaker - I wanted to try a ported design.

I had this so-so Focal woofer laying around from a component car audio set. Not the greatest thing in the world - but half-decent.

The bigger point of this project was: expiriment with WinISD. I have always built sealed enclosures - ALWAYS. I thought.. why not try a vented enclosure with this little driver just for the fun of it?

Basic Driver specs are:

6 inch woofer
4 ohm
FS = 58hz
(I can post all the TS params if needed)

I measured the TS parameters with Dayton DATS (v3). The specs were pretty close to the factory stated details. And YES, this woofer had a LOT of prior break-in.

So... I went through the tutorial for WinISD. Plugged in the TS params for the woofer. Started a new project: Vented. And off I was - playing around with the numbers.

I found that - according to WinISD - I could set the tuning frequency at 36hz and stay within (less than) 3db all the way. I choose a 2.2 sq ft enclosure and was told (by WinISD) that if using a 3 inch port, it would have to be 4.45 inches long. Done and done.

That was my assumption - and what WinISD showed me.

First - the good news: the bass is DEEP. Surprisingly deep. I actually would say it was a great success except for... I noticed certain songs with various bass notes just did not sound right. So I started to measure.... Somewhere between 52hz and 54hz I get a dip. And by dip, I mean - easily 12db (or more) down. Like... you can clearly see the cone is vibrating hard, but almost hear NO SOUND AT ALL. This only happens within this very narrow band. Below 52, it is solid down to 32hz (lower than the WinISD simulation!). And above 54hz, all is normal.

A tiny 2-4hz gap where sound literally goes mute.

My theory is: this has something to do with trying to push the driver too far below the FS? Perhaps I should have tuned this for something higher?

Perhaps some sort of box resonance?

If this IS likely what happened, could someone give me some pointers when tuning a vented enclosure in regards to the tuning freq and the driver FS?

This project was my first time playing around with vented enclosures like this - and was primarily to get my feet wet.

I am certainly impressed with the results for the most part. I just want to be sure I do not repeat the issue that turned up above.

My next project is moving my dual (sealed) Dayton 15 inchers into a proper dual vented setup. Their FS is something like 19 or 20hz. I have WinISD simulation showing that I can tune the enclosure for these at 14hz with a 14 cu ft box. But after the above, I am worried that this may be too low (??).

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Here it is:

* This data was exported from the Dayton Audio Test System: DATS
  • Piston Diameter = 131.3 mm
  • f(s)= 58.81 Hz
  • R(e)= 2.778 Ohms
  • Z(max)= 16.33 Ohms
  • Q(ms)= 4.689
  • Q(es)= 0.9608
  • Q(ts)= 0.7974
  • V(as)= 12.9 liters (0.4555 cubic feet)
  • L(e)= 0.3333 mH
  • n(0)= 0.2604 %
  • SPL= 86.26 1W/1m
  • M(ms)= 14.63 grams
  • C(ms)= 0.501 mm/N
  • BL= 3.953
  • K(r)= 0.02488
  • X(r)= 0.5372
  • K(i)= 0.005363
  • X(i)= 0.7069

According to Focal, the XMAX is 6 mm.

The measured diameter (includes one-third the suspension) is 131.3 mm.
Calculated Sd is 135.4 (cm^3)

Here is what I entered into WinISD:

Box Volume = 2.204 (ft^3)
Tuning Freq = 36 HZ

Vent Number = 1
Vent Diameter = 3 (in)

Everything else is pretty much default values in WinISD.

- Dean
Meaning: reaction with the room? It does this in my bedroom, the dining room (4 times the size), in the living room (similar to dining room), and at my wifes tiny store (smaller than the bedroom).

Even does it when sitting against a very large brick wall outdoors.

I can pretty much assure this is not specific to a room if that is the assumption.

Again - to call it a "dip" in response would be an understatement. This is more like a NULL in the output alltogether. At this narrow band, the entire speaker is almost silent.


Joined 2003
  • Q(ts)= 0.7974
Here is your problem, Qts is much too high for a basic vented alignment that limits Qts to ~0.403 where Vb = Vas, Fb = Fs, so while you can use a higher Qts, the box design needs to change to a higher aspect ratio transmission line (TL) or vented variant (MLTL):

T/S max flat alignment:

Vented net volume (Vb) (L) = 20*Vas*Qts'^3.3

(Ft^3 = (Vb)/~28.31685)

Vented box tuning (Fb) (Hz) = 0.42*Fs*Qts'^-0.96

F3 (Hz) = Fs*0.28*Qts'^-1.4

(Qts'): (Qts) + any added series resistance (Rs)

FWIW, IME and proved to my satisfaction when 'proofing' MJK's MathCad design programs against some of my own Tower/Column alignments from 'way back when', the vented TL [closed pipe] alignment [aka MLTL] range from ~0.312 - 0.624 Qts' seems technically good enough, i.e. from Keele's 6th order assisted bass reflex to his Extended Bass Shelf [EBS] alignment.

While the 'sweet spot' [mean] is a ~0.4412 Qts', a 'close enough' simple guideline is:

~0.403, Vb = Vas, Fb = Fs [MLTL]

< ~0.403, Vb = < Vas, Fb = > Fs [inverse tapered MLTQWT]

> ~0.403, Vb = > Vas, Fb = < Fs [expanding taper MLTQWT/MLhorn]

From all this we see that ideally you need a long, vented horn shaped cab (often folded in half to make it a typical box shape/size) designed using a horn program such as Hornresp.