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Sealed, ported, transmission line phase shift?

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  • Sealed, ported, transmission line phase shift?

    I was talking to guy about the porting of the old Royd revelation series. It uses a long, flexible port and Is described by the designer as "compound reflex loading". He said one of the advantages was to do with phase shift. I Googled "Royd and compound reflex loading" and found this:

    Quote "The use of compound reflex bass loading in these loudspeakers reduces the phase shift compared with standard reflex bass loading by more than 75%, and is 3 times less than a closed box or acoustic suspension type cabinet. A transmission line cabinet has more bass distortion than a closed box, because it adds a further 180 phase shift to that produced by a closed box. Compound reflex bass loading also ensures that all bass frequencies are critically damped and that the Revelation range of loudspeakers reproduce frequencies down to 20Hz in the listening room."

    Could someone explain what he means? How does a closed box have phase shift?

  • #2
    All speakers have phase shift. Model any speaker using a program that has phase response charting, like WinISD 0.7 or HornResp, and you can see it.
    www.billfitzmaurice.com
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    • #3
      I guess that would be "System phase:" In Jeff Bagby Woofer box and Circuit designer?. Would be Interesting to know what's considered good and bad, and how that would affect the sound.

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      • #4
        Originally posted by fatmarley View Post
        ...Would be Interesting to know what's considered good and bad, and how that would affect the sound.
        That is a source of a lot of debate, involving things like perception of low frequency aspects.
        How does a closed box have phase shift?
        "A driver-box assembly may be represented as a high pass filter. A sealed box form equates to a second-order network"
        Capacitors and Inductors create phase shift.
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        • #5
          Phase doesn't audibly affect the sound unless you have two or more sources that sum at or in the vicinity of 180 degrees apart. In the case of cabs that utilize both the front and rear waves, bass reflex for instance, the front and rear waves may act as two sources and may sum at or near 180 degrees apart, which may or may not be a problem. With a sealed or IB alignment you don't have two or more sources that can sum, so phase shift is inaudible. Whether or not the Royd claims are accurate would probably be confirmed or disproved by modeling, but to do that you'd have to see what their compound loading is. Since they claim it works better than a sealed cab with respect to phase I'm very skeptical.
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          • #6
            Originally posted by billfitzmaurice View Post
            With a sealed or IB alignment you don't have two or more sources that can sum, so phase shift is inaudible.
            That's what I thought but had to check to see If I was missing something.

            It's a shame we can't ask Joe Akroyd himself (He died). He was way ahead of his time In some ways (Invented slit cone technology before Scanspeak IIRC) but maybe his description of "compound reflex loading" was a bit of sales speak?

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            • #7
              One can't say, since all that appears in the ad copy is the term, with no technical details. It could be something as simple as a dual chamber reflex, in any of it's various permutations, and that's hardly groundbreaking.

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              • #8
                If you do a Google Image search for 'Royd RR3', you can see It's only a small, standmount speaker. You can also see pics of the flexible port and material over the back of the mid/bass driver. Unfortunately, you can't see the pics In the actual thread because Photobucket has removed them.

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                • #9
                  Originally posted by fatmarley View Post
                  Quote "The use of compound reflex bass loading in these loudspeakers reduces the phase shift compared with standard reflex bass loading by more than 75%, and is 3 times less than a closed box or acoustic suspension type cabinet. A transmission line cabinet has more bass distortion than a closed box, because it adds a further 180 phase shift to that produced by a closed box. Compound reflex bass loading also ensures that all bass frequencies are critically damped and that the Revelation range of loudspeakers reproduce frequencies down to 20Hz in the listening room."

                  Could someone explain what he means? How does a closed box have phase shift?
                  No one can explain what that nonsense means . . . seriously. Reflex systems (including transmission lines) are known to REDUCE bass distortion by better controlling cone motion at and near the enclosure tuning frequency.

                  Anyone can make all kinds of claims and they understand that the consumer of their product is completely ignorant as to the terminology involved. If we could see the actual frequency response of the system we could calculate the phase response. If it's truly a reflex system with a 4th order roll off below tuning, then the claim that it has less phase shift than a sealed system is absolutely false.

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                  • #10
                    Originally posted by Pete Schumacher View Post
                    Anyone can make all kinds of claims and they understand that the consumer of their product is completely ignorant as to the terminology involved.
                    In this case one must wonder if Akroyd himself was aware of what critically damped means.

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                    • #11
                      I can't make sense of that quote, either. I hope some salesman said that, not an engineer.
                      Francis

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                      • #12
                        Originally posted by billfitzmaurice View Post
                        In this case one must wonder if Akroyd himself was aware of what critically damped means.
                        I'd be more than a bit surprised If he didn't.

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                        • #13
                          I wouldn't be so sure. He also endorsed first order crossovers.
                          Successful marketing doesn't always mean knowing what you're talking about. Appearing that you do to your potential customers can be sufficient.
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                          • #14
                            Lol

                            EDIT: Should explain why I assumed It was a joke.

                            Quote from Wikipedia - "Prior to starting Royd Audio, Joe Akroyd worked for Goodmans in the early 1960s, where he contributed to the Maxim mini monitor and the Audiom and Axiom bass drivers. In 1970, Joe Akroyd joined Wharfedale, where he was involved in the manufacture and design of the Denton, Linton, Melton, Triton, Doredale loudspeakers. During the mid-1970s Joe Akroyd joined Decca, in their newly reorganized Speaker department.[2] However in 1979, Racal bought Decca,[3] at which point Joe Akroyd left to set up Royd Audio."
                            Last edited by fatmarley; 09-19-2017, 10:10 AM.

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