I thought the minimum phase question was central. If horns of any kind turn out to be minimum phase systems then response issues, including those caused by higher order modes, can be equalized out. Wasn't that the issue?
David S.
No not at all. It's GD that is audible, not MP. And since the HOM are 3 dimensional, it is not possible to equalize them out. We have had this discussion before - because something is MP at one point does not mean that it is MP wiith regard to all spatial points, i.e. correct one point does not correct all points in space. Basically the concept of MP does not apply to acoustics, only one dimension circuit type problems.
So basically even if diffraction is minimum phase, that is only at a single point in space. diffraction is not globally correctable even if every point in space is MP wrt to the source. Perhaps this is where I object to the concept of MP - it does not apply globally in a 3 dimensional sense, only point to point and correcting that one point does not guarantee that all points are corrected. This is why it is so essential to sort out what in a loudspeakers response is correctable and what is not.
So basically even if diffraction is minimum phase, that is only at a single point in space. diffraction is not globally correctable even if every point in space is MP wrt to the source. Perhaps this is where I object to the concept of MP - it does not apply globally in a 3 dimensional sense, only point to point and correcting that one point does not guarantee that all points are corrected. This is why it is so essential to sort out what in a loudspeakers response is correctable and what is not.
But minimum phase group delay should be totaly correctable for one axis. Yes? (Actually, I don't understand the notion that "it is group delay". We are talking about delayed arrivals, much like cabinet or room reflections but internal to the horn, correct?)
And if correctable for one axis then, yes, an issue remains in how quickly it varies off that axis. This is much like room correction, you can EQ for one seat or EQ for a seating area. If the response varies slowly away from the central seat then it can still be helpful.
Can EQ fix the audible effects of higher order modes, even at one point in space?
David S.
And if correctable for one axis then, yes, an issue remains in how quickly it varies off that axis. This is much like room correction, you can EQ for one seat or EQ for a seating area. If the response varies slowly away from the central seat then it can still be helpful.
Can EQ fix the audible effects of higher order modes, even at one point in space?
David S.
The main aspect of the 3-D (non)correctability is related to having more than one source that are separated in space. (not related to MP or not). When there is more than one source separated in space, optimizing one point in space will make other points in space not optimum. In case of diffraction there are virtual sources along the edges plus the main source from the speaker driver.
There could be two equal real sources equal distance from observation point, even in that case (even though there is no delay involved at observation point as the signal from both sources arrive at same time) correcting the sum in that single point will make other points in space not correct. So again my point is, 3-D correctability is related having a single point source or multiple.
Only when correctability on a single point is MP or non-MP becomes relevant.
I am going off the top of my head, but wasn't Olive or some other's studies resulted showing that on-axis response of loudspeakers was the most important one in perception? Not saying others are not important either, but this one has the highest weight in general. So it may be important to be able correct on axis.
There could be two equal real sources equal distance from observation point, even in that case (even though there is no delay involved at observation point as the signal from both sources arrive at same time) correcting the sum in that single point will make other points in space not correct. So again my point is, 3-D correctability is related having a single point source or multiple.
Only when correctability on a single point is MP or non-MP becomes relevant.
I am going off the top of my head, but wasn't Olive or some other's studies resulted showing that on-axis response of loudspeakers was the most important one in perception? Not saying others are not important either, but this one has the highest weight in general. So it may be important to be able correct on axis.
Diffraction and HOM are correctable only at a single point or along an axis if that axis is in the far field. The problem is that the correction fails in space beyond about a quarter wavelength (identical to the sphere of convergence of an active noise system). Hence at the HFs where this is the biggest issue the corrections are not even good for the size of a head. This is very limiting.
Yes, we are talking about delayed arrivals, they have group delay and amplitude effects and can be MP or not, I don't see that it matters. The GD effects are audible even if there is no amplitude variation (per Moore - which is non-MP). When there are both effects then its a more complex issue, part of which my research studied, but we did not sort out the amplitude effects from the GD effects. It wasn't necessary to do that, although it is an interesting concept.
Yes, we are talking about delayed arrivals, they have group delay and amplitude effects and can be MP or not, I don't see that it matters. The GD effects are audible even if there is no amplitude variation (per Moore - which is non-MP). When there are both effects then its a more complex issue, part of which my research studied, but we did not sort out the amplitude effects from the GD effects. It wasn't necessary to do that, although it is an interesting concept.
Yes, Olive did show that the direct response (not always the axial response) is the most important, but a simple test will show that it isn't a dominate thing. Listen to your speakers and have some walk across the room in front of them while your eyes are closed. There is a mild effect, but nothing monumental. The direct response matters and the reverberant response matters - it all matters.
I'm not sure that this is actually a valid test when it comes to determining the relative importance of direct vs reflected sound.
In the typical case you have the direct signal arriving first and generally with greatest amplitude (provided that you're not past the critical distance) so you have a positive direct to reflected ratio, as well as the time delay of the reflections which helps the ear sort out the first arrival from later arrivals.
In this case the first arrival does dominate at least at high frequencies, and more so the higher in frequency you go as the effective windowing time decreases. The perceived tonal balance at high frequencies is therefore predominately from the direct arrival.
As soon as someone walks in front of the speaker the situation changes completely - the direct signal drops in amplitude greatly particularly at high frequencies, in fact the direct signal might now be 10dB or more below the reflected path signal - so now the reflections are likely to dominate the perception of tonal balance despite the delay time as they are now much greater in amplitude than the first arrival. (We're well beyond the effective critical distance now)
If the off axis response has a similar spectral balance to on axis (as is likely to be the case in a CD design) then you might not even notice much of a shift in tonal balance as someone walks in front of the speaker, just a drop in amplitude.
That doesn't mean that the reflections play a significant contribution to perceived tonal balance at high frequencies when someone isn't standing in front of the speaker though...just that they do when the first arrival is significantly attenuated.
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Another test you can do is to change your speaker's toe slightly either way, noticing what changes and what doesn't.
@ Earl (OT), would you have described your ceiling diffuser previously? My wife informs me I need something low profile if I'm to venture near the ceiling (could be time to convert the garage
) and I want to go low in the mids and stay broadband.
@ Earl (OT), would you have described your ceiling diffuser previously? My wife informs me I need something low profile if I'm to venture near the ceiling (could be time to convert the garage
) and I want to go low in the mids and stay broadband.
So I am not sure - does this support the direct sound as being dominant or doesn't it? Seems to me that if the direct field were dominant that removing it would have to strongly affect the result, and yet it doesn't. Something not logical here. It sounds like you are saying that the direct field isn't all that necessary as long as the speaker is CD.
I did describe it in detail at my forum (somewhere!) Geddes on Audio.
The problem here is that it's misleading to look at loudness perception, localization and timbre in isolation. Hearing is a highly complex process which is not sufficiently understood (yet).
Once a perception is established, a person walking across the room in front of the speakers will not disrupt perception. But when you start with a person standing in front of the speaker, you will get a different result.
Is there also a picture of it?
The original question is how dominant is the direct field relative to the reflections in dominating the perceived sound, particularly the tonal balance.
Obviously that depends on frequency, (hearing integration/windowing time decreases as frequency goes up making the direct field dominate more) the delay time of the reflections and the direct to reflected ratio, all are factors.
If the delay time of reflections is "sufficient" and there is a reasonable direct to reflected ratio then the direct field will indeed dominate at high frequencies at least for tonal balance, and the whole "the direct field dominates" position pre-supposes favourable conditions in terms of delay to first reflections and direct to reflected ratios to be valid.
However as a test you then suggested someone standing in front of the speaker - this will dramatically cut down the direct field at high frequencies. (low frequencies will just diffract around the object)
If the direct field at high frequencies is now way below the reflected field that is arriving around the blocking object from walls etc then this change in direct to reflected ratio is obviously going to affect which field dominates.
If you just keep turning down the amplitude of the direct field you must eventually reach a point where the reflections become the first arrival...and will both be localised as such and their tonal balance perceived.
Think of a real world situation outdoors where a sound can be heard both directly and from a reflection from a large building - so long as there is line of sight to the sound source we'll localize the first arrival, but if we move so that there is an object sufficiently blocking the direct sound but not the reflection from a building on the other side the localisation switches to the direction the reflection is coming from, because it is now the first arrival of any significant amplitude.
My point is, cutting down the amplitude of the direct signal by standing someone in front of the speaker tells you nothing about whether direct or reflected sound is dominating our perception when someone is not standing in front of the speaker - you're changing the test conditions, in particular the direct to reflected ratio.
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Human sound perception isn't simply determined by the physical sound field. It's a cognitive process that tries to make sense of the input. Once a perception is established, it's sometimes hard for our brain to "let go". You probably know the Franssen effect.
Someone standing in front of the speaker will results in fewer cues so it probably will alter the perceived sound more dramatically than someone walking across the room. Our hearing is no measuring instrument, it's an interpreted process.
I'm not saying that direct sound is irrelevant, I'm just saying that there's more to it. Reflection patterns, D/R ratios, etc. can only be a means to an end. We're running around in circles when the goal is undefined [insert link to Toole's Circle of Confusion here].
Someone standing in front of the speaker will results in fewer cues so it probably will alter the perceived sound more dramatically than someone walking across the room. Our hearing is no measuring instrument, it's an interpreted process.
I'm not saying that direct sound is irrelevant, I'm just saying that there's more to it. Reflection patterns, D/R ratios, etc. can only be a means to an end. We're running around in circles when the goal is undefined [insert link to Toole's Circle of Confusion here].
You are getting close to the group that is advocating pillows in front of their speakers to spread the stereo stage!
If you obscure the direct response it will still retain its earlier arrival time. As I understand, you can drop the first arrival until it is about 10dB below the later arrivals before the ear lets go of it as the source of the sound. This is certainly true for direction of arrival tests.
As we discussed considerably in "the big thread" the interesting question is which arrival determines frequency response. I beieve everything points to the primacy of the direct/early sound. The only question is whether later power response has any effect, and it appears it has some, but not much. Speakers with different power response can sound different. Speakers with different axial response will always sound different.
That an object blocking the speaker is audible, even well past the critical distance, is evidence that the direct sound is disproportionately important. The last time I did the ruler test (had a helper place and vary a reflective edge near the tweeter of my speaker) I could hear it at the farthest corner of my fairly reverberent home theater.
We hear much differently than a non-time selective spectrum analyzer.
David
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