How do aperiodic enclosures generate more bass?

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#22
I thought that there was some calculation involving the VAS of the driver. I've been fiddling around with an online closed box simulator (Leo Beranek and Tim Mellow), and it seems I'd need a huge box to get anything close to a 100Hz response, if I go for a 11-litre volume it has a little hump, that I presume would be flattened by a vent. You can vary the input impedance, but not to negative figures on this particular simulator. I was hoping to have something close to a 100Hz hump (like the BBC LS35a) so I could use the speakers on their own but having a choice of opening a vent and using a sub - but that dosen't seem doable with this driver, I guess it's the price to pay for the high sensitivity.
 
#23
Briggs:

Here is information from an article by Steve Hutton published by Hi-Fi News & Record Review in December 1974 which refers to the distributed port (DP) enclosure which was a type of aperiodic enclosure famously employed by Gilbert Briggs of Wharfedale fame. The DP consisted of a series of narrow slits or small holes in the rear of the enclosure which were covered internally by a thin, felt like cloth to form a resistive vent.

The article states that, as the DP area increases from zero, the Q and resonant frequency fall. However, after the Q has reached unity it starts to rise again with the figures becoming multi-valued, hence unpredictable, below Q = 1.

Most interestingly, the author gives an empirical formula for calculating the DP or resistive vent area:

A = 5.5 x V [0.818 (Fs/25) + 0.182]
Click to expand...

I have also grabbed the same a couple other times.

This might be useful as well:
The defining characteristics if the A25 were definition, tight bass, and a flat impedance curve. The last two resulted from the 'aperiodic' tuning. I question whether this can be replicated in the DIY remakes and 'aperiodic ports' out there. It wasn't as simple a just putting some fiberglass in a port. Dynaco tuned the port in each system by feeding a low frequency square wave into the system and observing the output on an oscilloscope. The amount of stuffing was adjusted to get the resistance in the port and thus the transient response just right. I'm guessing the average end user lacks the gear to do that. A labor intensive process that no speaker maker would invest in a moderately priced model today.
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"basic aperidoid math”:

There is very little difference mathematically between a leaky sealed box and a damped ported box. The effects of stuffing are difficult to model. In order to characterize the stuffing you would have to measure the effects and make assumptions that won't hold true in all cases.

Empirically, you can:

Model it as a sealed box with a loss Q of ~2-3
That is Q =1/(1/Qa+1/Ql)

Model as a ported box with a similar loss Q
Q=1/(1/Qa+1/Ql+1/Qp) with port losses dominant

Qa = absorptive loss component ~=5-10 in a stuffed box, 15-30+ in an empty box.
Ql=leakage losses ~=10-20 depending on box size
Qp=port losses ~5ish in a stuffed port, ~30-50 in a good port (Oh I forgot to mention, it's level dependent )

If you wish to tighten up your models, build a box, measure impedance and vary Q components until the measured and modeled impedance are similar. Compare close-miked measurements with modeled results. Scratch your head... ROTFL!

Life is not deterministic, neither should your speaker be....
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dave
 
#24
If you put BG 17 in a 7 liter box with medium stuffing (15 grams per liter of polyester wadding) total system Q wont be over 1.
So I dont see the reason to go aperiodic, unless your amp has high otput impedance.

Theres nothing magic about it, aperiodic box is just leaky closed box, which further lowers the total Q of the system if its way to high, like over 1,2 or something.

BG 17 has Fs=120Hz, any kind of closed box (including aperiodic) will raise that. So your expectations of getting a hump at 100Hz are not realistic.
If you wish to stretch to 100Hz or lower, 16 liter vented box tuned to 85Hz should give nice results with power handling reduced to half.
Also, you may try with ML-TL or ML-Voigt box using Hornresp or similar tool.
 
#25
planet10 said:
Briggs:
Click to expand...

'Twas I that supplied the Steve Hutton empirical formula for calculating the total area of the narrow slits or small holes in a distributed port enclosure.

Allow me to add the units pertaining to the formula:

A = 5.5 x V [0.818 (Fs/25) + 0.182]

Where A is the total area in sq in, V is the enclosure volume in cu ft and Fs is the resonant frequency, in Hz, of the driver in free air.

The Wharfedale cabinet construction leaflet shows that the total area, A, can be made up of the appropriate number of 12" x 0.25" slits, placed 2" apart. Each slit can be replaced by fifteen holes of 0.5" diameter if preferred.
 
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#26
In case it is of interest, here's what Gilbert Briggs had to say about his Distributed Port (DP) enclosure:

"The low frequency cut-off of a well designed DP enclosure is in the order of 1.5 to 2.0 x Fs, not as low as a reflex but better than a closed box of the same volume, while its transient behaviour is appreciably better than both, especially the reflex."
 
#27
Galu said:
Each slit can be replaced by fifteen holes of 0.5" diameter if preferred.
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damps3-photo-gif.159947


dave
 
#31
Davor D said:
If you put BG 17 in a 7 liter box with medium stuffing (15 grams per liter of polyester wadding) total system Q wont be over 1.
So I dont see the reason to go aperiodic, unless your amp has high otput impedance.

Theres nothing magic about it, aperiodic box is just leaky closed box, which further lowers the total Q of the system if its way to high, like over 1,2 or something.

BG 17 has Fs=120Hz, any kind of closed box (including aperiodic) will raise that. So your expectations of getting a hump at 100Hz are not realistic.
If you wish to stretch to 100Hz or lower, 16 liter vented box tuned to 85Hz should give nice results with power handling reduced to half.
Also, you may try with ML-TL or ML-Voigt box using Hornresp or similar tool.
Click to expand...
IMG_20220930_072215~3.jpg

I had never used a simulator before, so it was interesting to see how far the possibility of a hump close to 100 Hz was, something a little over 120 Hz wouldn't be too bad, but I wanted a reasonably small box. I'm hoping to get away with very little stuffing, using the shape of the box to reduce internal reflections, I'm also thinking of carpet tile/cork on the inside to dampen the walls, that should help as well.
 
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