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Port assisted subwoofer design for PA |
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Crashpc 
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Joined: 26 February 2008 Location: Czech Republic Status: Offline Points: 539 |
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 Topic: Port assisted subwoofer design for PAPosted: 02 August 2024 at 9:01pm |
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I have touched that topic in few discussions, but never got enough good arguments, so here it is. Modern subwoofers have enough motor force and displacement, that the bassreflex port on a ported bin is often almost not needed. At least not fully in band. Now I use it just for cone displacement control, impedance shaping and cooling, otherwise it works for me almost out of band. As it is out of band, it can be small, it doesn't get the efficiency hit of small port, port noise is low, power compression is low, what's not to like? Speaking about 18" subwoofer in a 95l/3.55cu.ft box, tuned at roughly 28Hz, used between 34 and 85Hz. I first did it with 21"s. Seemingly it works great for its box volume. Only the price per woofer is somewhat higher. Is here someone willing to discuss and object to get to the bottom of things? Why noone is doing it ever? Many thanks. |
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Conanski
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Joined: 26 January 2006 Location: Ottawa, Canada Status: Offline Points: 2757 |
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Posted: 03 August 2024 at 7:30am |
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What does the system response look like in this config? Isn't a 28hz port still contributing something to output at 34hz? If it's grossly undersized then aren't you are throwing away LF output potential.
An extended shelf response can be useful when the full output of the subs in the traditional 40-100hz range isn't needed but I have never found myself in that situation, the subs are always pushed for all they are worth.
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Crashpc
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Posted: 03 August 2024 at 8:14am |
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The response looks like rolling off towards the low end. Yes, the port is atill somewhat contributing at 34Hz, but way less than at the tuning, so it can be smaller, and there is no issue with efficiency and chuffing, and also power compression is avoided, as the driver is only used around the impedance peak, not in the impedance valley of the port.
Seemingly LF potential is theown away, but only from the angle of view of the driver. Not from angle of view of the compact box design. One cannot pull such SPL from such box size anyways. To me it looks even after Hornresp sims, that this design wins over conventionally tuned solution. LF18X451 in 95l/3.55cu.ft bin tuned at 28Hz, properly HPF'd and EQ'd gives more output at 40Hz than when it is tuned at 38Hz. Exactly fue to the port loading (efficiency) and power compression at tuning frequency.
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vertx
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Joined: 14 March 2012 Location: Sydney Status: Offline Points: 599 |
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Posted: 03 August 2024 at 9:38am |
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If we're talking modern drivers in DSP Assisted Reflex enclosures the port is everything, and when properly tuned and processed will allow you to use you're high power high xmax drivers to their full potential.
Once a port chokes up you get no further gain or efficiency from it, only more distortion. If you have a 3600w peak power driver with 16-19mm linear xmax you could easily choke you're ports up at half that power and xmax at which point you've wasted money on big boy drivers in enclosures that are velocity limited.
Edited by vertx - 03 August 2024 at 9:39am |
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Crashpc
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Posted: 03 August 2024 at 9:55am |
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Yes, but it is generic mumbo jumbo - in the design, frequency response and SPL are always a chosen compromise. More SPL comes at a price, at given box volume.
The driver in my case will be used to its full potential by cone excursion. Some power will be left on the table, but power and heat dissipation is not equal to sound, so I am inclined to discart it as irrelevant. Port in this design is not to be choked up. Top velocity is set at about 14-15m/s. As mentioned, from the point of view of the driver and maximum SPL, it is a waste. But that's the price to pay for more power dense compact design. The high excursion driver here is tasked to move most air by itself in order to exactly not choke the port. In conventional design, the port WOULD get choked. Not here. The driver works more instead, and the port just past the edge of selected usable frequency response, somewhat assisting. Yep it is expensive to do it like that, but heck it is a lot SPL in reasonable frequency response in a very compact box that can be grabbed easily. //Edit: grammar.
Edited by Crashpc - 05 August 2024 at 1:47pm |
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fudge22
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Joined: 26 July 2022 Location: UK Status: Offline Points: 263 |
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Posted: 05 August 2024 at 10:46am |
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The following graphs show the predicted maximum output of
the drive unit you mentioned in a 95 litre cabinet. The green line is a bass reflex tuned to 40Hz, the red line
is tuned to 28Hz and the blue line is a closed box. In my opinion, no-one is doing it because they consider the extra of output from 35Hz upwards is more beneficial than sacrificing the overall output to extend the response.
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vertx
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Posted: 05 August 2024 at 11:09am |
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There's a few interesting bits in this https://audioxpress.com/files/attahment/2721#:~:text=DSP%20Assisted%20Reflex%20lets%20you,%2C%20extension%2C%20and%20dynamic%20range.
Edited by vertx - 05 August 2024 at 11:10am |
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Crashpc
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Posted: 05 August 2024 at 1:54pm |
Yep. People look at simulated graph with "dollar eyes look" thinking they instantly get this performance in real life. Well?! Port compression is ignored, speaker power compression is ignored. Once you account for that, the situation nearly reverses. It depends. For dynamic signal, live music, It doesn't have to be far off, but for synthetic music and bass heavy music, it is the case. Once we simulate with all kind of losses and power compression in place, we see very different story.
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Xoc1
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Posted: 05 August 2024 at 9:21pm |
Forte Audio Q18 Q15 Q12 Tony Wilkes (RIP)
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Crashpc
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Posted: 06 August 2024 at 5:50am |
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I have heard that name. Dangit, it's even called the same thing! Many thanks assuring me that I'm onto something here.
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fudge22
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Posted: 06 August 2024 at 8:06pm |
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Is this the same Q12 that is discussed in this old thread? https://forum.speakerplans.com/q12-12-sub_topic28555_page1.html?KW=BMS12 If so, it seems to be a 40l box tuned to 38Hz, or there
abouts. That is just standard 4th order tuning, comparable to the
green plot in the graphs I posted above. If the other cabinets are similar
designs (presumably using 15” and 18” drive units) they are not tuned similarly
to that which the OP is referring. If the OP wants some sort of validation to brilliance of his
design, try searching for sixth order or B6 alignment cabinets. Yes, they work,
but on the whole, the cabinets are no smaller, they require additional eq boost
to create a flat response, and subsequent additional power requirement at low
frequencies to achieve that boost. As I mentioned above the maximum output is
significantly reduced compared to fourth order cabinets. Check out the book “Bullock on Boxes” for more background on
the original alignments used. |
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Xoc1
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Posted: 06 August 2024 at 9:58pm |
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The Q18 was 100 litres an used an 'assisted alignment' The Q15 was described as being similar to a B6 allignment. Not many details left as Tony passed away 10 years ago.
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Is here someone willing to discuss and object to get
to the bottom of things? Why noone is doing it ever?