posted
If you adjusted the group delay from a ported speaker so that: group delay = .5 * (1/resonant frequency)
Would you maximize output at the resonant frequency (I'm thinking that this would perfectly align the phase of the output of the front and back sides of the cone)?
-------------------- 2004 Saturn Vue Red Line Edition Alpine DVA9860 Alpine H701 MB Quart QTD-25 B&C 6MD38 6.5" Mid B&C 8NDL51 8" Midbass IDMAX (3) MTX 81000D (3) MTX 8302 (2) MTX 6304 (2) Posts: 282 | From: Huntsville, AL (go to BFE and take a left) | Registered: Oct 2003
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posted
Yes, I realize that if there is any delay there is not a perfect alignment of output. However, the whole point of a ported enclosure is to delay the back wave of the speaker so that it can constructively interact with the front wave. To do that you need a 180 degree delay in phase because the original back wave of the speaker is already 180 degrees out of phase.
The reason I ask the question is that it seems like no one ever takes this into account. It seems to be an excellent way to pick up a db or two. I thought that my understanding of group delay must have been flawed or incomplete since no one ever mentioned it.
-------------------- 2004 Saturn Vue Red Line Edition Alpine DVA9860 Alpine H701 MB Quart QTD-25 B&C 6MD38 6.5" Mid B&C 8NDL51 8" Midbass IDMAX (3) MTX 81000D (3) MTX 8302 (2) MTX 6304 (2) Posts: 282 | From: Huntsville, AL (go to BFE and take a left) | Registered: Oct 2003
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posted
Hmm, after thinking it over, I retract my previous statements.
The phase difference between a woofer and port is ~90 degrees at tuning, greater than 90 degrees at frequencies below tuning, and less than 90 degrees above the tuning frequency.
^This changing phase correlates with the added group delay that a ported enclosure has over a sealed. (the negative derivative (slope) of phase with respect to frequency)
If the output of a port can approach a 360 degree phase difference, then I dont think its a ported box anymore.
quote:Originally posted by SSSnake: However, the whole point of a ported enclosure is to delay the back wave of the speaker so that it can constructively interact with the front wave. To do that you need a 180 degree delay in phase because the original back wave of the speaker is already 180 degrees out of phase.
The wave exhibited by the backside of a speaker is opposite polarity of the speaker's frontside wave. Different than 180 degrees of difference in terms of time as you stated above.
posted
First of all, saying the whole point of a ported enclosure is to delay the back wave of the speaker so that it can constructively interact with the front wave is highly misleading. By comparing the wavelength of a 40 Hz wave to the dimensions of your average ported enclosure you should be able to see this. Also, suppose it were about delay- it wouldn't make sense that enclosures of vastly different sizes can be tuned to the same frequency. A ported speaker is Helmholtz resonator- an acoustic spring/mass system driven by the transducer. The increase in output over sealed is from the resonance in the spring (box) mass (port) that is excited by the transducer.
Secondly, group delay is a SYSTEM property not a property of just the cone...adjusting this characteristic does not directly affect the relative phases of the port and cone.
Finally, group delay is the delay on a wavepacket centered at the frequency of interest...it is NOT the delay at a certain frequency when you play a sine wave; that is called phase delay. Group delay concerns the change in phase delay with respect to frequency. When playing a single tone, group delay isn't an applicable concept.
Posts: 3952 | From: Bloomington, IN | Registered: Sep 2000
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posted
Ryb - First thanks for the response. The information below is NOT meant to be argumentative but only contribute to this discussion...
quote: First of all, saying the whole point of a ported enclosure is to delay the back wave of the speaker so that it can constructively interact with the front wave is highly misleading.
Other than reducing excursion at and around the tuning freq. what are the other dominant charactersistics of a ported enclsoure?
quote:By comparing the wavelength of a 40 Hz wave to the dimensions of your average ported enclosure you should be able to see this.
The delay, in a ported enclosure is NOT due to a labrynth of passages, as found in a transmission line enclosure, but a result of the interaction of the driver and mass/spring system represented by the enclosure and the port.
quote: Also, suppose it were about delay- it wouldn't make sense that enclosures of vastly different sizes can be tuned to the same frequency.
It makes sense because of the variation in the enclosure sizes versus port volumes & lengths.
quote:A ported speaker is Helmholtz resonator- an acoustic spring/mass system driven by the transducer.
I agree completely but this does NOT indicate that phase, and therefore delay, is unaffected. As a matter of fact a hemholtz resonator DOES in all cases introduce phase shifts
quote:The increase in output over sealed is from the resonance in the spring (box) mass (port) that is excited by the transducer.
Agreed, because you are allowing the backwave produced by the driver to constructively interfere with the front wave.
quote:Secondly, group delay is a SYSTEM property not a property of just the cone...adjusting this characteristic does not directly affect the relative phases of the port and cone.
I in NO way TRIED to indicate that this was a property of the cone (I apologize if I did this somewhere). How can group delay not have an effect on the relative phases of the port an cone (this is really the cruxt of my original question)?
quote: Finally, group delay is the delay on a wavepacket centered at the frequency of interest...it is NOT the delay at a certain frequency when you play a sine wave; that is called phase delay.
I agree with the initial definition of group delay and offer this definition as a more straightforward explanantion that agrees with your concepts.
From various Audio Engineering Society sources -
quote:Group delay refers to the delay experienced by one group of frequencies with respect to another
(this is the most layman's definition that I could find).
from RANE (I could not readily find a good source definition for pahse delay)
quote: phase delay A phase-shifted sine wave appears displaced in time from the input waveform. This displacement is called phase delay and is usually constant for all frequencies of interest. Used as another name for group delay; however there are instances where they are not the same, for example systems exhibiting ripple in their phase vs. frequency characteristics.
Again the question that I am looking to answer is the relationship, if any, that exists between group delay and output level.
quote:Group delay concerns the change in phase delay with respect to frequency. When playing a single tone, group delay isn't an applicable concept.
I understand your comment but group delay is measured from a reference freq (a point where phase is not changing relative to freq ((I believe)). Group delay IS increased when using a ported enclosure. Therefore there must be at least a LOOSE coupling between the terms.
Maybe I should ask the question in a different way... What parameter in loudpspeaker enclosure design captures the difference in phase between the cone and the sound radiated by a port?
-------------------- 2004 Saturn Vue Red Line Edition Alpine DVA9860 Alpine H701 MB Quart QTD-25 B&C 6MD38 6.5" Mid B&C 8NDL51 8" Midbass IDMAX (3) MTX 81000D (3) MTX 8302 (2) MTX 6304 (2) Posts: 282 | From: Huntsville, AL (go to BFE and take a left) | Registered: Oct 2003
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quote: First of all, saying the whole point of a ported enclosure is to delay the back wave of the speaker so that it can constructively interact with the front wave is highly misleading.
Other than reducing excursion at and around the tuning freq. what are the other dominant charactersistics of a ported enclsoure?
I'm not sure what you are looking for here...what exactly do you mean by "characteristics?" The best I can do is to say that most ported enclosures loaded with most transducers behave like the standard lumped parameter model (speaker as a driven spring-mass-damper system coupled to an acoustic spring-mass system) suggests. When working within the assumptions of this model, the speakers seem to behave remarkably close to the model, but if you push things beyond the assumptions of this model, it can be inaccurate. That to me is the fundamental description of the standard ported loudspeaker...if you try to boil it down any more, it is oversimplified.
quote:Originally posted by SSSnake:
quote:By comparing the wavelength of a 40 Hz wave to the dimensions of your average ported enclosure you should be able to see this.
The delay, in a ported enclosure is NOT due to a labrynth of passages, as found in a transmission line enclosure, but a result of the interaction of the driver and mass/spring system represented by the enclosure and the port.
This is not typically understood as "delay" within the context of the ported box model, in the same way you would say a sound wave is delayed on its way to you. The dimensions of the enclosure are assumed (and rightfully so most of the time) to be small compared to a wavelength so that we aren't concerned with wave type of phenomenon. In this environment it is difficult to define the way a signal propagates. I don't really have an answer other than to suggest that this isn't the most appropriate (both in difficulty and utility) way to view the situation physically.
quote:Originally posted by SSSnake:
quote: Also, suppose it were about delay- it wouldn't make sense that enclosures of vastly different sizes can be tuned to the same frequency.
It makes sense because of the variation in the enclosure sizes versus port volumes & lengths.
quote:A ported speaker is Helmholtz resonator- an acoustic spring/mass system driven by the transducer.
I agree completely but this does NOT indicate that phase, and therefore delay, is unaffected. As a matter of fact a hemholtz resonator DOES in all cases introduce phase shifts.
quote:The increase in output over sealed is from the resonance in the spring (box) mass (port) that is excited by the transducer.
Agreed, because you are allowing the backwave produced by the driver to constructively interfere with the front wave.
You're going to have to more clearly define what you mean by this before I can reply. What do you mean by backwave? Are you conceptualizing the signal leaving the rear of the cone as propagating out of the port, thinking of the ported chamber as some sort of filter on the signal?
quote:Originally posted by SSSnake:
quote:Secondly, group delay is a SYSTEM property not a property of just the cone...adjusting this characteristic does not directly affect the relative phases of the port and cone.
I in NO way TRIED to indicate that this was a property of the cone (I apologize if I did this somewhere). How can group delay not have an effect on the relative phases of the port an cone (this is really the cruxt of my original question)?.
quote: Finally, group delay is the delay on a wavepacket centered at the frequency of interest...it is NOT the delay at a certain frequency when you play a sine wave; that is called phase delay.
I agree with the initial definition of group delay and offer this definition as a more straightforward explanantion that agrees with your concepts.
From various Audio Engineering Society sources -
quote:Group delay refers to the delay experienced by one group of frequencies with respect to another
(this is the most layman's definition that I could find).
from RANE (I could not readily find a good source definition for pahse delay)
quote: phase delay A phase-shifted sine wave appears displaced in time from the input waveform. This displacement is called phase delay and is usually constant for all frequencies of interest. Used as another name for group delay; however there are instances where they are not the same, for example systems exhibiting ripple in their phase vs. frequency characteristics.
Again the question that I am looking to answer is the relationship, if any, that exists between group delay and output level.
I don't think you're going to find anything directly correlating the two, but if you want to explore this what I would suggest is writing a box program that displays more thorough phase information about the port and driver independently so that you can view them at the same time as group delay while changing parameters. If I had the spare time I'd pull out my Matlab code and throw something together, but I just don't suspect you'll find anything.
quote:Originally posted by SSSnake:
quote:Group delay concerns the change in phase delay with respect to frequency. When playing a single tone, group delay isn't an applicable concept.
I understand your comment but group delay is measured from a reference freq (a point where phase is not changing relative to freq ((I believe)). Group delay IS increased when using a ported enclosure. Therefore there must be at least a LOOSE coupling between the terms.
NO, group delay is not measured relative to any reference frequency! It is the derivative of the phase curve! Looking up formal definitions (the only ones that can be trusted) will likely clear up a lot of this for you. This is a start: http://en.wikipedia.org/wiki/Group_delay
quote:Originally posted by SSSnake: Maybe I should ask the question in a different way... What parameter in loudpspeaker enclosure design captures the difference in phase between the cone and the sound radiated by a port?
The best way to view this is to look at their individual phase curves. This can be done easily if you write your own box program. I don't know that anything you find in most programs out there tells you this information.
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