So... what *disadvantages* can waveguides have?

[Dennis Murphy]

If you are comfortable doing so, could you elaborate?

I know The Abbey has its enthusiasts, but I have one here and it suffers from a bad case of the dreaded "AWK" horn midrange coloration. It does give studio vocals a very up front and personal presentation, and I wonder whether that's what people are enjoying about it. But on complex instrumental material, the problem is obvious. And I know you're not supposed to listen to them on axis and I don't, but that doesn't solve the problem.

 

[ROOSKIE]

I know The Abbey has its enthusiasts, buy I have one here and it suffers from a bad case of the dreaded "AWK" horn midrange coloration. It does give studio vocals a very up front and personal presentation, and I wonder whether that's what people are enjoying about it. But on complex instrumental material, the problem is obvious. And I know you're not supposed to listen to them on axis and I don't, but that doesn't solve the problem.

This is with the foam waveguide inserts installed?

 

[ernestcarl]

Regarding the "horn sound" issue...

Probably because the designs talked about are passive types(?), but for modern active designs, why not just shove a computer in there and get rid of the additional unwanted horn coloration as much as possible by using advanced FIR filtering?

Wavelet image example is from Fulcrum Acoustic

 

[617]

Regarding the "horn sound" issue...

Probably because the designs talked about are passive types(?), but for modern active designs, why not just shove a computer in there and get rid of the additional unwanted horn coloration as much as possible by using advanced FIR filtering?

View attachment 108031Wavelet image example is from Fulcrum Acoustic

I find it difficult believe that a FIR filter can attenuate HOMs given their complex nature. The foam plug used by Geddes is a an ingenius way of doing it though. Is that what this diagram is showing?

 

[ernestcarl]

I find it difficult believe that a FIR filter can attenuate HOMs given their complex nature. The foam plug used by Geddes is a an ingenius way of doing it though. Is that what this diagram is showing?

Fulcrum's designs are specifically for compact coaxial horn loaded waveguides -- most of them I think anyway. It likely does not eliminate all resonances from all types of horns, but shouldn't FIR filtering be able to at least ameliorate some of the colouration from other horn designs? I am no expert in waveguides so cannot really comment about whether this is even applicable to those large waveguides found in Geddes speakers.

 

[Dennis Murphy]

This is with the foam waveguide inserts installed?

Yes, although I don't think it makes much difference.

 

[mitchco]

Yes, although I don't think it makes much difference.

I would agree having played around with this in a few waveguides I have owned over the years. I must say, I have yet to measure or hear "HOM's" in a reasonably designed waveguide like the JBL 2384's in my current system. The best attribute of a CD waveguide is that once eq'd on axis, the off axis response is predictably smooth. I have some examples of off axis measurements on ASR, here are a couple.

 

[briskly]

A waveguide is a duct for conducting waves efficiently, a transmission line in acoustics. That was how I (half)learned about it.
The association of acoustical waveguides and horns comes from Earl Geddes wanting to distinguish his method from that outlined by Webster's horn equation.
A horn by its most useful property is an acoustic load transformer. It extends the effective radiating area of the driver.
So we go back to the primary drawback of a horn. A horn needs to be large to extend its useful bandwidth.
There are finer points of increased HF nonlinear distortion and nonlinear compression, but these are not too relevant beyond PA use at extremely high SPL.

Regarding higher order modes, or reflections within the horn, my understanding is that they are present in all horns.

Among the exact propagation solutions, there exist only 3 one-parameter horns for which we can totally avoid the excitation of modes besides the fundamental mode. For a spherical wave expansion, this is exclusively the conical horn excited by a spherical wavefront. This should have come up several times in discussing KEF's "ideal" compression horn design and their metamaterial absorber.
The OS horn is optimal for transforming plane waves into spherical waves, but there are necessarily modes excited beyond the fundamental. Geddes didn't want to bother with the problem of figuring out how to create a spherical wavefront in reality and addressed the remaining modes one at a time.
As to whether HOMs are necessarily bad, that depends on how you view things like the diffraction slots in the Image Control Waveguide or Seas DXT tweeter. You can extend that to dual compression drivers, for which the negative effects seem more apparent. The cross chamber resonances create aberrations in the magnitude response that a single compression driver could eliminate.

In recent interview Earl Geddes at 01:03:40

52 minutes.
Although what I think Geddes is taking particular interest in is the modification of the OS profile into a finite horn. The OS solution of wave propagation assumes an infinite length horn. The OS-SE profile is finite, closed-form, and C^infinity. The discontinuity at the horn mouth has caused some problems for horns, particularly the axisymmetric OS horn.

advanced FIR filtering?

Spatially independent filter being applied to spatially dependent response functions.
To consider a different HOM problem, this is like saying EQ fixes room reflections from the sidewalls. The propagation of the wavefront is the problem, so the response function correction is only applicable within some limited region of space.

 

[ernestcarl]

To consider a different HOM problem, this is like saying EQ fixes room reflections from the sidewalls. The propagation of the wavefront is the problem, so the response function correction is only applicable within some limited region of space.

Maybe I wasn't clear... but I was replying to the comment of Dennis's description of a very distinct "AWK" midrange sound colouration. Sure, I was not implying that this will physically solve the problem inherent in certain waveguide designs, but why not try to eliminate/ameliorate it -- even if only partially -- through FIR correction from the very start? Is this not a sensible course of action to take in the world of hi-fi speakers? I cannot understand why FIR corrections are not even considered as part of the solution -- again, even if it only partially solves the problem "in some limited region of space". It does not make any sense to me why FIR correction should be completely off the table at all, other than there being a lack of technical know how on how to implement it effectively.

 

[ernestcarl]

I find it difficult believe that a FIR filter can attenuate HOMs given their complex nature. The foam plug used by Geddes is a an ingenius way of doing it though. Is that what this diagram is showing?

David Gunness from Fulcrum Acoustic doesn't use the term HOMs or higher-order modes, but I think his technique of FIR filtering does work in reducing such distinct resonances attributed as a "honk" or similar type of distinct horn resonances.


Here's an excerpt taken from: Improving Loudspeaker Transient Response with DSP


3.2 Horn Resonance

A second loudspeaker behavior, which yields well to digital preconditioning, is horn resonance. A wavefront progressing down any horn will encounter one or more discontinuities in the area expansion. All horns present a discontinuity at their mouths. Constant directivity horns often employ a diffraction slot to achieve a wide coverage pattern at high frequencies. The exit of this slot represents a severe discontinuity. A discontinuity in a horn’s expansion produces a reflection. A fraction of the sound power reverses course and returns to the compression driver where it is partially absorbed and partially re-emitted, often several milliseconds late. This process is, of course, regenerative, once again producing a decaying series of arrivals. Low frequencies tend to reflect more strongly than high frequencies, so the reflections are most prevalent in the lowest octaves of the horn’s usable range. It is this precise phenomenon that produces an audible artifact commonly described as a “honk”. The wavefront does not return to the compression driver in a perfectly coherent fashion, but it appears that the bulk of the reflected energy does converge back at the driver. To the extent that it does, the phenomenon acts as a two-port system, and is correctable via signal preconditioning.


*For more detail on the wavelet spectrogram display being used: A Spectrogram Display for Loudspeaker Transient Response

 

[oivavoi]

David Gunness from Fulcrum Acoustic doesn't use the term HOMs or higher-order modes, but I think his technique of FIR filtering does work in reducing such distinct resonances attributed as a "honk" or similar type of distinct horn resonances.


Here's an excerpt taken from: Improving Loudspeaker Transient Response with DSP


3.2 Horn Resonance

A second loudspeaker behavior, which yields well to digital preconditioning, is horn resonance. A wavefront progressing down any horn will encounter one or more discontinuities in the area expansion. All horns present a discontinuity at their mouths. Constant directivity horns often employ a diffraction slot to achieve a wide coverage pattern at high frequencies. The exit of this slot represents a severe discontinuity. A discontinuity in a horn’s expansion produces a reflection. A fraction of the sound power reverses course and returns to the compression driver where it is partially absorbed and partially re-emitted, often several milliseconds late. This process is, of course, regenerative, once again producing a decaying series of arrivals. Low frequencies tend to reflect more strongly than high frequencies, so the reflections are most prevalent in the lowest octaves of the horn’s usable range. It is this precise phenomenon that produces an audible artifact commonly described as a “honk”. The wavefront does not return to the compression driver in a perfectly coherent fashion, but it appears that the bulk of the reflected energy does converge back at the driver. To the extent that it does, the phenomenon acts as a two-port system, and is correctable via signal preconditioning.

This sounds interesting, and the technical explanation is remarkably clear for an AES paper. But I still struggle to understand exactly how such FIR correction works. How is the signal altered in order to cancel or weaken these soundwaves which are re-emitted from the driver?

 

[More Dynamics Please]

Not sure if this will be any help but RCF published a white paper on their FIR correction implementation that they call RCF FiRPHASE as used in their pro and studio monitor speakers:

https://drive.google.com/file/d/0B3n4tG6v30B_N01NT183WmZoWDA/view

 

[617]

This sounds interesting, and the technical explanation is remarkably clear for an AES paper. But I still struggle to understand exactly how such FIR correction works. How is the signal altered in order to cancel or weaken these soundwaves which are re-emitted from the driver?

Right and for it to work it would have to be effective at multiple measurement points.

 

[ernestcarl]

This sounds interesting, and the technical explanation is remarkably clear for an AES paper. But I still struggle to understand exactly how such FIR correction works. How is the signal altered in order to cancel or weaken these soundwaves which are re-emitted from the driver?

A mirror of the expected backwards reflecting signal is created, or the subsequent outgoing signal is preconditioned or something along those lines... I don’t know enough of the details of this kind of signal processing and the math is most definitely beyond me. I have not heard any of their speakers personally to know whether they “awk” or “honk” like others of the same kind without any DSP. But I can tell you that the Sceptre S8 (which Dave also helped design) does not sound like it has a horn. Out-of-the-box it may sound too mid-forward for some folks, but it can be EQ’d easily. I attribute that to Presonus’ “voicing” choice rather than to any kind of left-over audible HOMs from an incompetent FIR filtering design.

 

[ernestcarl]

Right and for it to work it would have to be effective at multiple measurement points.

Which Dave acknowledges and discusses in the paper. If something is unclear why not give the designer or the company itself a call? Why would Fulcrum Acoustic be producing speakers that do not work as designed? It seems rather odd to assume that their signal processing expertise (baked in all their products) is there just for show.

 

[Wombat]

Which Dave acknowledges and discusses in the paper. If something is unclear why not give the designer or the company itself a call? Why would Fulcrum Acoustic be producing speakers that do not work as designed? It seems rather odd to assume that their signal processing expertise (baked in all their products) is there just for show.

"Why would Fulcrum Acoustics be producing speakers that do not work as designed"?

This is called 'begging the question'. What is required is verifiable information that they do, indeed, work as designed.

Of course working as designed does not guarantee high performance if the design is flawed.

 

[ernestcarl]

"Why would Fulcrum Acoustics be producing speakers that do not work as designed"?

This is called 'begging the question'. What is required is verifiable information that they do, indeed, work as designed.

Of course working as designed does not guarantee high performance if the design is flawed.

Then why not ask them for verification or a specific review model to test or whatever.

I have said that with the S8 (which I own) there is not any kind of audibly distinct horn type of sound or resonance. What kind of “verification” do want?

 

[Wombat]

Then why not ask them for verification or a specific review model to test or whatever.

I have said that with the S8 (which I own) there is not any kind of audibly distinct horn type of sound or resonance. What kind of “verification” do want?

n=1

I can't ask every Tom, Dick or Harry for basic information they could/should provide.

 

[ernestcarl]

n=1

I can't ask every Tom, Dick or Harry for basic information they could/should provide.

Since you haven't answered, let me guess... Do you want comprehensive, detailed high-resolution before and after DSP spectrograms of every speaker they have in their catalog? At all angles within their specified coverage pattern? Well, I don't have that kind information readily at hand with me -- nor do I know of any other company that does. I only have a pair of S8s which I can take very amateur measurements of, but there is no way for me to disable the FIR processing inside. But my sample of 1 and anecdotal experience is quite meaningless and devoid of value to someone like you, it seems. I can take that, no problem.

 

[Wombat]

Using anecdotal mfr information to support your subjective view or support the mfrs limited info is the issue.

If you prefer your speakers, that is fine.

Going beyond that requires more than personal subjective opinion. Who can provide relevant information on performance, up-front, if not the manufacturer?

I avoid being emotionally attached to stuff. It does the job well for me or it doesn't. Preference is final over the opinions of others. I just don't claim to be superior re my personal preference or need to forensically defend it