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Port Resonance Suppression: Inspiration from Technics

Ilkless

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You may have seen the measurements I posted here of the Technics SB-C700, a coaxial 2-way standmount. Despite being an innovative design with superior measurements to the LS50 (especially wrt directivity <7kHz, bass SPL and extension) for not much more cost, its market prospects were torpedoed by a baseless What Hi-Fi review that gave it 2 stars. It has since been discontinued. Several users that I've recommended the SB-C700 to on Reddit, DIYAudio and this forum corroborate that the subjective impression matches the measurements.

In the DIYAudio thread on the Purifi PTT6.5, HifiCompass released their own reference design for the midwoofer using a waveguided Bliesma beryllium tweeter. In the documentation for the design, much was written about port resonance suppression to allow a reflex enclosure that lives up to the extraordinary performance of the midwoofer. Ultimately, the builder gave up trying to optimise a port (which included heroic efforts like drilling holes to depressurise the port), and opted for the Purifi passive radiator.

The reason why I mention the Technics is because it has the best port resonance suppression I've ever seen measured, and implementation looks straightforward compared to other approaches. Look at the textbook rolloff of the port (red curve) down to -30dB (credit: Stereophile):

1215TC700fig3.jpg


To really underscore how impressive this is, the LS50 has an extremely sophisticated port with semi-permeable walls that had tons of computer modelling behind it and they got this:

1212KEF50fig3.jpg


Some resonances, but 20dB down, which is still good. To highlight how good the LS50 is, and how extraordinary the Technics is, here are a couple more indicative measurements of what is more typical port performance, even in vastly more expensive standmounters by engineering-focused firms (Q Acoustics Concept 300 and Dynaudio Special 40):
120Q300fig05.jpg


918DS40fig04.jpg


The KEF approach is infeasible for DIY, given the modelling and special design needed. The Technics approach, on the other hand, is a "padded partition" with a more conventional flared port. It looks like this on cutaways:

technics_c700_09.jpg
P1460683.jpg



MUb3zu_P6DA0hg76P_AIhKRSqcvTKBkrTdUWSFQMRtK2O4gmkuP5iNpxh5rBnmM5B8mPnD7ZlIEpM1L8LQ4DuB2GEusUNwf-hJ_KbgyEAaA5XJB4guU0sMiZvDtcJaZyMc4LqSVm92yO6oP5h2sJOSNe9_riAsI7RjTLdQ


technics-sb-c700.jpg


What it seems like is a solid horizontal brace right across the internal opening of a flared port, wrapped thickly with insulation (looks like conventional fibreglass), as well as polyfill wrapped around the port. This might be a relevant approach for compact standmounters with modern high-linear excursion drivers such as the Purifi, which seem particularly susceptible to port resonances. Of course, I don't doubt that much care was taken by Technics to optimise the flare, the brace size, the materials used and their placement, but it provides some guidance toward excellent port suppression in a way that is still achievable in DIY (unlike KEF).
 

tecnogadget

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I was always impressed by the stellar build quality of these speaker based by their cutaway promo photos. There is a lot of “substance” into the drivers and cabinets.
But since I’ve never found really positive reviews from reliable sources I didn’t gave them enough attention. This would be great measurement material for ASR.
Really clever way of reducing port artifacts without needing rocket science, my respects for Technics.
 
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Ilkless

Ilkless

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I was always impressed by the stellar build quality of these speaker based by their cutaway promo photos. There is a lot of “substance” into the drivers and cabinets.
But since I’ve never found really positive reviews from reliable sources I didn’t gave them enough attention. This would be great measurement material for ASR.
Really clever way of reducing port artifacts without needing rocket science, my respects for Technics.

Stereophile gave it a class A rating, making it the second cheapest speaker to be ranked Class A after the LS50. It has been measured by several sources across the US, Poland and Germany. All show exemplary measurements.
 
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Ilkless

Ilkless

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Nah.. I'm not so sure - What HiFi didn't rate them well. :D

That was the first review that went to press. And 2 stars is conspicuously low for What Hi-Fi. Wrecked any chance of getting a foothold in the market.
 

tecnogadget

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I remember being a child and how my father and other friends had Technics in very high regard.
So funny to watch their vintage photos showing racks with phono,cassette decks, tunners, amplfiers, etc :p.
I don’t know why, but Technics has some sort of “magical aura” & mysticism in Argentina for audio enthusiasts into their 50’s to 70’s. Maybe because they were the best equipments imported from Japan at that time, and people could actually afford them if they save up.
 

DDF

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The reason why I mention the Technics is because it has the best port resonance suppression I've ever seen measured

Thanks for pointing this out llkless, Technics did a nice job with this port. Placing a blockage so close to one port end doesn't seem to have diminished it ability to belt out the bass (thanks to ilkless for the link) https://www.diyaudio.com/forums/multi-way/279383-coaxials-flat-woofers.html#post4441726
1589048042925.png
1589047967154.png


One potential issue is that the padded brace should cause some asymmetric port flow which usually leads to increased even order distortion. From http://www.aes.org/e-lib/browse.cfm?elib=11094 :
1589049251989.png
...........
1589049158031.png


It would be a good learning opportunity to see distortion measurements as well. It may not be an issue given how tolerant the ear is to distortion at low frequencies.

Another innovative but somewhat impractical way to get rid of port resonances is to hang resonator chambers off the pipe:
http://techtalk.parts-express.com/f...should-i-be-looking-for?p=1384969#post1384969
That design uses an usually large vent, but the technique works for smaller vents as well. Wider vents need to be longer for same tuning, and longer vents causes stronger sport standing waves. So larger vents would benefit more from this technique (of course, you could also just add more ports...)
 

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Ilkless

Ilkless

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Thanks for pointing this out llkless, Technics did a nice job with this port. Placing a blockage so close to one port end doesn't seem to have diminished it ability to belt out the bass (thanks to ilkless for the link) https://www.diyaudio.com/forums/multi-way/279383-coaxials-flat-woofers.html#post4441726
View attachment 62693View attachment 62692

One potential issue is that the padded brace should cause some asymmetric port flow which usually leads to increased even order distortion. From http://www.aes.org/e-lib/browse.cfm?elib=11094 :
View attachment 62697 ........... View attachment 62694

It would be a good learning opportunity to see distortion measurements as well. It may not be an issue given how tolerant the ear is to distortion at low frequencies.

Another innovative but somewhat impractical way to get rid of port resonances is to hang resonator chambers off the pipe:
http://techtalk.parts-express.com/f...should-i-be-looking-for?p=1384969#post1384969
That design uses an usually large vent, but the technique works for smaller vents as well. Wider vents need to be longer for same tuning, and longer vents causes stronger sport standing waves. So larger vents would benefit more from this technique (of course, you could also just add more ports...)

The proof is in the listening. It has belatedly gained a small following (see here, here, DIYAudio for a positive review by Jonathan Carr of Lyra cartridge fame and a couple of owners further down and ASR itself for a glowing review).

As the SB-C700 appears to be significantly less distorting in the (mid)bass, I'd take the even order distortion. To have lower distortion AND the possibility of a more even-order-skewed harmonic profile is win-win in my books.
 

GelbeMusik

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Another innovative but somewhat impractical way to get rid of port resonances is to hang resonator chambers off the pipe:
http://techtalk.parts-express.com/f...should-i-be-looking-for?p=1384969#post1384969

I issued a patent on this and similar techniques, only to learn later, that it is already patented by Bose. It is a ( intentionally? ) small addendum to a patent addressing a ( useless! ) chimney effect with dual ports. That's why, I assume, Neuman and KEF can't come up with a real solution. DIY is not affected, though. It works.
 

Objectivist01

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That was the first review that went to press. And 2 stars is conspicuously low for What Hi-Fi. Wrecked any chance of getting a foothold in the market.
Technics was trying a re entry to the market with a news series of product line back then. What Hi-Fi rated ALL of the products low where the same products were recommended by all less known magazines and websites. Seriously some players were scared of them. It’s sad, that the c700 is an excellent speaker , which I would prefer any day over LS50 shit. I listened to both in a store and the R3. R3 is the Kef to buy and technics is in also a similarly nice consideration. Technics somehow is totally “unboxy” with its sound.
 
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Ilkless

Ilkless

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Technics was trying a re entry to the market with a news series of product line back then. What Hi-Fi rated ALL of the products low where the same products were recommended by all less known magazines and websites. Seriously some players were scared of them. It’s sad, that the c700 is an excellent speaker , which I would prefer any day over LS50 shit. I listened to both in a store and the R3. R3 is the Kef to buy and technics is in also a similarly nice consideration. Technics somehow is totally “unboxy” with its sound.

I found the LS50 sibilant, with a treble haze from the off-axis flare and elevated FR in the low treble.
 

Objectivist01

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I found the LS50 sibilant, with a treble haze from the off-axis flare and elevated FR in the low treble.
I too found the upper midrange not flat on them. But you can eq it down. But then many lower end speakers too be eqed to sound better.
 
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Ilkless

Ilkless

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I too found the upper midrange not flat on them. But you can eq it down. But then many lower end speakers too be eqed to sound better.

You can EQ down some of the odd balance, but it still leaves the wonky dispersion. I really tried very hard to like it because I think it's a very modern and rational design but was let down by voicing + driver integration.
 

KSTR

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This is leakage control, not damping port resonances. Afforded mostly by sound velocity absorbtion layer between the compartment sections. Internal damping works best away from the walls. At the walls sound velocity is (close to) zero and lining the walls only is ineffective.
 

Objectivist01

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This is leakage control, not damping port resonances. Afforded mostly by sound velocity absorbtion layer between the compartment sections. Internal damping works best away from the walls. At the walls sound velocity is (close to) zero and lining the walls only is ineffective.
I am a noob to this, can you explain why the velocity of sound is close to zero near the walls, so effective damping could be glass wool spread like feathers ?
 

KSTR

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I am a noob to this, can you explain why the velocity of sound is close to zero near the walls, so effective damping could be glass wool spread like feathers ?
The air cannot move right at the walls, simple as that. Therefore light polyfill wadding is most effective where air particle velocity is at max, which definitely is: away from the walls. At the walls, sound pressure maxes out, so you need pressure absorbers there to be any effective (thick heavy felt lining).
 

Koeitje

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