OK, Art, you can beat your chest, jump around, do whatever. But your arguments are still just flat-earth arguments. Sorry, but you're throwing around misinformation and I wish you would stop.
I post actual test data in post #235 clearly showing the inverse square law applies equally to BR and FLU from one meter to 32 meters.
You provide no actual supportive information refuting or even addressing the data, and talk of flat-earth arguments ?
That is worth a laugh 😀😀.
Art Welter
I provide no supporting evidence that the Earth is round either. 🙄
But by the way, the inverse-square law is not debated here. It's the position and relevance of the acoustic center that we're talking about.
But by the way, the inverse-square law is not debated here. It's the position and relevance of the acoustic center that we're talking about.
Last edited:
That’s good, lets keep the debate "centered".
I have provided data showing that the position of the sub's acoustic center is not relevant regarding the SPL measurement of subs from 1 to 32 meters.
You choose to ignore the data because it does not fit your preconceived "flat earth" view.
Unfortunately, no one but you can change your ignorance.
Too bad about that.
Attachments
Funny you mention this, I was just starting to think along these lines after writing my previous post. Most of my experience is with direct radiators, so I hadn't really considered before that there would be a difference between time of flight for time/phase alignment purposes, and the reference point for inverse square law fall-off.
If you look at a driver + cabinet as a large black box with an opening in it on one side, it doesn't matter (at least for bass frequencies) how deep inside the driver is - whether it's right at the opening like a direct radiator, or way down inside it at the base of a horn throat, what matters is the point where the wave emerges from the constraining angles of the box and folds around the outside of the box into 360 degree "full space", so the mouth of the box/horn becomes a virtual source as far as wave front expansion is concerned. (Again only at low frequencies - at high frequencies where the wave does not expand to a greater angle than the mouth after exiting the situation may be different)
What I said about inverse square law not tracking accurately if you measure too close still applies though as far as I can see, except now in the case of a bass horn it's the size of the mouth that counts instead of the driver size.
While a single diaphragm that large obviously isn't realistic, (I was giving an extreme example as a thought experiment) it could in principle be approximated by a 2 dimensional wall of many cabinets, direct radiators or horns.
In that case with a large stack of cabinets, measuring one metre from the middle most cabinet would be in error compared to measuring that one cabinet on it's own at one metre - the stack of cabinets together wouldn't show the full expected increase over one cabinet, because you're making the radiating area much larger relative to the measuring distance, and now you're measuring "too close" due to the change in proportions.
Both the single cabinet and the wall of cabinets would measure correctly at a much greater distance though, so I would suggest that measuring further away is still the more "foolproof" way to get good accuracy without any ambiguity, especially when comparing speakers of radically different topology or size.
(For example imagine a speaker with woofers both on the front and rear of the box - the distance to microphone will be different for each, so measuring close will give a significant error due to the large distance differential, while measuring further away will progressively eliminate the error, as the distance difference becomes a smaller percentage of the measuring distance)
Last edited:
You made a good point in post 236 that the inverse-square law applies to point sources, but that when the source is acoustically large, it is not a point source. This is part of the acoustic center problem.
There are two definitions of the acoustic center:
1. The position from which outgoing wavefronts appear to diverge in the far field, and
2. The position from which the sound pressure varies inversely as the distance
The first definition is primarily concerned with phase, and the second is primarily concerned with amplitude. Both are important.
The phase definition is shown mostly in the path length and is why we have to delay the mains to match the subs. There are also electro-mechanico-acoustic parameters that affect phase, i.e. driver parameters, cabinet shape, configuration, etc. The amplitude definition includes both path length and radiator size. It is the position where the sound source begins to act as a point source, and so it is influenced by both the the path length and the radiating area. There is expansion of the wave within the horn, it is a developing wavefront.
The acoustic source position is really kind of complex. It can be measured, and then a very precise microphone-to-DUT distance can be setup. But another solution presents itself, which is to simply make the measuring distance greater, which then reduces the acoustic-center/path-length error. This makes the SPL measurement more accurate even when the acoustic center position is only approximated, i.e. using the cabinet front as the radiator position and the microphone tip as the receiver position.
There are two definitions of the acoustic center:
1. The position from which outgoing wavefronts appear to diverge in the far field, and
2. The position from which the sound pressure varies inversely as the distance
The first definition is primarily concerned with phase, and the second is primarily concerned with amplitude. Both are important.
The phase definition is shown mostly in the path length and is why we have to delay the mains to match the subs. There are also electro-mechanico-acoustic parameters that affect phase, i.e. driver parameters, cabinet shape, configuration, etc. The amplitude definition includes both path length and radiator size. It is the position where the sound source begins to act as a point source, and so it is influenced by both the the path length and the radiating area. There is expansion of the wave within the horn, it is a developing wavefront.
The acoustic source position is really kind of complex. It can be measured, and then a very precise microphone-to-DUT distance can be setup. But another solution presents itself, which is to simply make the measuring distance greater, which then reduces the acoustic-center/path-length error. This makes the SPL measurement more accurate even when the acoustic center position is only approximated, i.e. using the cabinet front as the radiator position and the microphone tip as the receiver position.
- A note on the concept of acoustic center, by Jacobsen, Figueroa and Rasmussen
Last edited:
I never once said that. I said that I have not messed with FLH's myself. There are MANY other enclosure styles, sizes, arrangements.
Yes I have looked at it. It did nothing to change my opinion that the acoustic center has absolutely nothing to do with the real world measurement or use of large bass systems. For calibration to the nth degree with microphones and small distances it would be a different matter.
Yes I have done a lot of measurements and seen some strange results. Most notably at close 1m distances. True but the results are still valid.
#1 I have never argued that the acoustic center cannot be different between cabs. That is not the point.
#2 The actual acoustic center is irrelevant as regards real world results. That is the point I am saying. You have provided exactly no data or counter argument as to why it does. I don't believe that logically there is one.
You maintain that you cannot get an accurate 1w/1m measurement of a large horn cab up close. The microphone does not lie. If the microphone is placed at 1m from the nearest enclosure face or mouth of the cab it will report what the results are there. It is that simple. You are saying that the microphone and electronics are lieing about what the results are at that position. That is completely illogical. The results are perfectly valid and whether they are strange or whatever due to the effects of a large enclosure face, acoustic centering, multiple radiation points or whatever, the microphone is only reporting what the results are with that particular 1m set-up. These factors may influence the close response but they are part of the design and are real so they should not be dismissed as inaccuracies IMHO. Repeat the measurement 15 times and it will be the same. They are the real response of the cab at that distance, there can be absolutely no denying this.
Now that said, if you want to know the actual 10m/100w response you need to measure it you can't just scale the 1m result and expect it to be dead on. That is the issue it seems is being argued about. Otherwise expect it to only be the same general shape and +/- a few db. Same goes the other way you can't back calculate the 10m measurement by subtracting 20db and assume that it is going to be dead on as what the actual 1w/1m response is with all of those other factors involved in the close mic measurement. Instead of back calculating either way and claiming it as accurate I would suggest a real measurement at whatever distance you need.
I'm really surprised at the level of misunderstanding some people seem to be having with this concept, especially since most of us agree that measurements taken some distance away are the right way to take measurements.
Why do you think it is suggested by most industry professionals to measure a good distance away to get accurate SPL readings?
The concept of the acoustic center - of source location - is really key here. You cannot use a close mic'ed measurement and get accurate SPL unless the acoustic centers of both the microphone and the the DUT are known very precisely.
Why do you think it is suggested by most industry professionals to measure a good distance away to get accurate SPL readings?
The concept of the acoustic center - of source location - is really key here. You cannot use a close mic'ed measurement and get accurate SPL unless the acoustic centers of both the microphone and the the DUT are known very precisely.
Simon,
I referenced your example exactly because near field measurements of an acoustically large source don’t hold up.
We would not accept a measurement of an array of cabinets with a 5 meter mouth diameter measured at one or two meters as indicative of anything other than what a security guard at a concert might encounter standing between the subs and the crowd barricade.
That said, the data I presented in post #237, clearly shows that cabinets in the size range of what have been discussed in this thread with differing acoustic origin distance, with frontal area around one meter or less, when tested in one or two units, conform to the inverse square law as measured from 1 to 32 meters from the cabinet front, not some distance emanating from meters within the cabinet.
Wayne’s transparent agenda is simply trying to discredit any sub measurement not taken at 10 meters, thereby dismissing any data that was not collected at that distance, even though lesser distance measurements clearly are valid for cabinets with frontal area of around one square meter or less.
By attempting to discredit ( or ignore) data presented by others, (unless it supports his product) Wayne avoids the preponderance of evidence showing BR or TH of equal cubic volume and bandwidth, may have output equal or greater than FLH.
Art Welter
So, Art, you propose one meter measurements of subwoofers are acceptable.
If one meter is OK with you, how about one foot?
As I asked before, why do you think industry professionals choose to measure subwoofers at ten meters?
If one meter is accurate enough, why bother to go to the trouble of setting up at ten meters?
If one meter is OK with you, how about one foot?
As I asked before, why do you think industry professionals choose to measure subwoofers at ten meters?
If one meter is accurate enough, why bother to go to the trouble of setting up at ten meters?
Wayne,
Replying to your goofy questions in order :
One foot would be a poor choice for measuring subwoofers unless they were tiny, like something used in a computer monitor, and tested indoors, where boundary bounce is a problem.
Testing headphones at one foot would decidedly be too long of a distance, though.
If ten meters is good, why not test at 100 🙄?
One meter, four feet, and various other distances dependent on test facilities are and have been common with “industry professionals”.
A 10 meter distance with a 100 watt signal (far more valid than a 1 watt signal for high power subs) has been adopted by some industry professionals because it requires no math to be equivalent to one watt one meter, and reduces differences that may be associated with large cabinet frontal area when multiple subs are tested.
I agree that 10 meters is a valid distance for testing subs, and small sub arrays, if the test facilities allow it.
My observation of measured facts has been shown in the data I presented in post #237, which clearly shows that cabinets in the size range of what have been discussed in this thread, with differing acoustic origin distance, with frontal area around one meter or less, when tested in one or two units, conform to the inverse square law as measured from 1 to 32 meters from the cabinet front, not some distance emanating from meters within the cabinet.
The data shows that even one meter is an acceptable measurement distance for this size of sub.
In an effort to discredit any sub measurement not taken at 10 meters, you have repeatedly dismissed any data that was not collected at that distance, even though lesser distance measurements clearly are valid for cabinets with frontal area of around one square meter or less.
Rather than asking absurd questions, why don’t you simply acknowledge the facts ?
Art Welter
Maybe this will help shed some light, I don't know:
Right, that's the point. It's a key point, a clue as to what I'm talking about and why it is better to measure at a greater distance.
You can measure one meter from the face, sure. But what does that tell you? Why is that interesting? Will anyone be standing three feet away from your hornsub?
Maybe you're thinking if you measure at one meter and one watt, you'll have the 1W/1M value. But if it is 6dB higher or lower than the 100W/10M value, isn't that important information too? Which information do you think is more important for a hornsub?
You can measure at one foot and know the precise SPL there too. Or you can put the microphone in the throat if that's interesting to you. Is any of this useful for comparing other subwoofers with?
Or would it be better to have the 100W/10M measurements, which calculate back to being 1W/1M equivalents? This is the accepted way to do it, because it scales the acoustic center problem and makes it insignificant.
If you put the microphone too close to the speaker, the inverse-square law doesn't apply. That's the whole point of the acoustic center, with respect to SPL measurements. The acoustic center is the location where the inverse-square law begins to form.
The further away you get, the more accurate you'll be able to see the true SPL. It averages in the "too-close region" with the "far-enough away" region and you get comparible values as a result.
That's the whole point of the acoustic center problem. That's what those guys were studying when they wrote that paper. They were trying to find accurate ways to set the distances so they could get reliable SPL measurements. They were trying to be able to rate the sensitivities of microphones, which made accurate position/distance measurements very critical.
This applies to our discussion too, and is exactly why we measure at a distance for accurate SPL results.
I would say the "strangeness" makes close-mic measurements unusable, at least if honest SPL comparison is the goal. If you want to map the "bubble" of SPL values around a particular speaker, that's probably interesting and all but I think the only way to get accurate SPL figures is to know the acoustic centers with enough accuracy to truly map to 1W/1M. An easy way to do this is to measure at distance and scale back.
Yes, repeat the measurement 15 times and it will be the same. But it will be the same inaccuracy, in my opinion. I mean, if you want to know what the sound is one meter away from the face, then that's OK. But if you want to really know the 1W/1M value, then you have to know the exact position of the acoustic center. Or, again, you can scale back.
Where SPL is concerned, the acoustic center is defined as "the position from which the sound pressure varies inversely as the distance". That is where the speaker is radiating as a point source. This is the true sound source location. So if you want to accurately know the SPL at some distance, you have to know this location.
You can define the front face as the source location if you want. But you could also define the center of the box. Or you could use the acoustic center.
I suggest that the best thing to do is to measure far enough away that there is basically no difference. If you measure far enough back, you can be certain of the accuracy, because the SPL is within a decibel whether or not the source position is at the front face, the physical center or the acoustic center. You gain accuracy by scaling.
Frankly, there appears to be agreement on this matter pretty much everywhere, but for some reason, you and Art seem to be arguing against this practice.
Art accuses me of trying to discredit any sub measurement not taken at 10 meters, because I want to discount mounds of "evidence" or some such thing. That's nonsense. He doth protest too much, methinks.
Your arguments seem to be a little more inquisitive than accusatory, so I hope maybe these comments have made some sense to you.
Right, that's the point. It's a key point, a clue as to what I'm talking about and why it is better to measure at a greater distance.
You can measure one meter from the face, sure. But what does that tell you? Why is that interesting? Will anyone be standing three feet away from your hornsub?
Maybe you're thinking if you measure at one meter and one watt, you'll have the 1W/1M value. But if it is 6dB higher or lower than the 100W/10M value, isn't that important information too? Which information do you think is more important for a hornsub?
You can measure at one foot and know the precise SPL there too. Or you can put the microphone in the throat if that's interesting to you. Is any of this useful for comparing other subwoofers with?
Or would it be better to have the 100W/10M measurements, which calculate back to being 1W/1M equivalents? This is the accepted way to do it, because it scales the acoustic center problem and makes it insignificant.
If you put the microphone too close to the speaker, the inverse-square law doesn't apply. That's the whole point of the acoustic center, with respect to SPL measurements. The acoustic center is the location where the inverse-square law begins to form.
The further away you get, the more accurate you'll be able to see the true SPL. It averages in the "too-close region" with the "far-enough away" region and you get comparible values as a result.
That's the whole point of the acoustic center problem. That's what those guys were studying when they wrote that paper. They were trying to find accurate ways to set the distances so they could get reliable SPL measurements. They were trying to be able to rate the sensitivities of microphones, which made accurate position/distance measurements very critical.
This applies to our discussion too, and is exactly why we measure at a distance for accurate SPL results.
I would say the "strangeness" makes close-mic measurements unusable, at least if honest SPL comparison is the goal. If you want to map the "bubble" of SPL values around a particular speaker, that's probably interesting and all but I think the only way to get accurate SPL figures is to know the acoustic centers with enough accuracy to truly map to 1W/1M. An easy way to do this is to measure at distance and scale back.
Yes, repeat the measurement 15 times and it will be the same. But it will be the same inaccuracy, in my opinion. I mean, if you want to know what the sound is one meter away from the face, then that's OK. But if you want to really know the 1W/1M value, then you have to know the exact position of the acoustic center. Or, again, you can scale back.
Where SPL is concerned, the acoustic center is defined as "the position from which the sound pressure varies inversely as the distance". That is where the speaker is radiating as a point source. This is the true sound source location. So if you want to accurately know the SPL at some distance, you have to know this location.
You can define the front face as the source location if you want. But you could also define the center of the box. Or you could use the acoustic center.
I suggest that the best thing to do is to measure far enough away that there is basically no difference. If you measure far enough back, you can be certain of the accuracy, because the SPL is within a decibel whether or not the source position is at the front face, the physical center or the acoustic center. You gain accuracy by scaling.
Frankly, there appears to be agreement on this matter pretty much everywhere, but for some reason, you and Art seem to be arguing against this practice.
Art accuses me of trying to discredit any sub measurement not taken at 10 meters, because I want to discount mounds of "evidence" or some such thing. That's nonsense. He doth protest too much, methinks.
Your arguments seem to be a little more inquisitive than accusatory, so I hope maybe these comments have made some sense to you.
Last edited:
In reply to post # 228
Wayne, all of my findings I openly explained in readable text and verifiable data. I haven’t used assumptions but instead I used verifiable computer models. Anyone who doubts the HornResp data I have used can check for themselves since I used the Xoc1 TH18 data that can be found in the corresponding threads.
Although I have a personal preference how PA cabs should be measured LINK it doesn’t mean 1 meter measurements are useless or way too far off to be used in comparisons.
I haven’t compared measuring data with models, since I only compare measurements with measurements or models with models. Theoretical models can be used as long all data is verifiable.
Wayne, all of my findings I openly explained in readable text and verifiable data. I haven’t used assumptions but instead I used verifiable computer models. Anyone who doubts the HornResp data I have used can check for themselves since I used the Xoc1 TH18 data that can be found in the corresponding threads.
Although I have a personal preference how PA cabs should be measured LINK it doesn’t mean 1 meter measurements are useless or way too far off to be used in comparisons.
I haven’t compared measuring data with models, since I only compare measurements with measurements or models with models. Theoretical models can be used as long all data is verifiable.
Actually, it does. I've seen them be several decibels different than the 10M/100W values, some higher and some lower.
You can measure 1M from the face if you want, but it won't necessarily be comparible with measurements made at 10 meters.
Ahhh but I never said it wasn't better ideally.I just disagree that 1m outdoor measurements are invalid.
It tells me how that particular sub performs at 1m gp in comparison to any other also measured in the same manner.
You would have the real 1w/1m value. The back calculated one from a 100w/10m measurement does not actually exist as something measurable by the microphone or experienced by the listener in some cases. That is why I say the measured 1w/1m is the real one.
Yes of course. Both are important which one is more would depend on the application of the sub. Not all horn subs are behemoth pro sound units.
I made this same point earlier from the other point of view as to why the back calculated 1w/1m result is invalid.
I find this accepted way to be fundamentally flawed. Why the hell would we use a 100w/10m measurement then back calculate 1w/1m specifications to provide a "true" spec for 1w/1m which may be nothing like what a real world 1w/1m measurement would actually be? 😕 And this for subs which as you say are more likely to be used at 10m to begin with? Why not provide the 100w/10m spec which is more useful. If you want a 1w/1m spec measure it there. Ideally provide both. 😱
Agreed. However the further away the measurement distance the more difficult it is to get a suitable test area and also the to get background noise low enough for measurements to get a workable SNR. I typically test at 2 meters as the best compromise between distance and SNR. Also the subs I usually work with are home audio or HT subs or DIY stuff for car audio. It also approximates 1m anechoic results roughly. 2 meters works better for the majority of tests for me.
I still disagree with this. I would rather just have the data for the measurement at distance presented as exactly what it is and allow back calculation to other distances be done by the interested party. Maybe that seems like hair splitting but it is what it is. Whether you have a 1,2,5 or 10m measurement set as long as the measurements were taken the same way for each sub being compared, they are valid in a relative sense between the subs. The issue comes when comparing a 100w/10m with a 1w/1m from different data sets as directly comparable. Now you can do this in a limited sense but I would assume there to be a +/-3db or so on the whole deal.
This is not true as I see it, all you have to do is measure at that distance. That would be an approximated or scaled response measurement from another distance being presented as trumping the real, at that distance measurement, as "right". This is the central point of disagreement in out discussion. I simply cannot accept that as correct.
The point I mentioned directly above is the big bone of contention for me. I have no problem with 10m measurements. I also agree that technically if you can pull them off they are better. I do not agree that 1m measurements are fundamentally flawed for large subs as long as each is measured in the same manner. 10m measurements have issues of their own namely that the SNR can become an issue for lower volume measurements and the required extra space from large objects can also give issue. Not all subs are huge pro subs. 😉
BTW what smoothing is used on most of your measurements?
Maybe we're getting somewhere. You noted earlier that sometimes your 1W/1M values were "strange". Maybe you don't see that as "invalid" but you do admit them to be unexpected sometimes.
I think you do understand the problem of the acoustic center, yes? If so, do you see how the two results (1W/1M and 100W/10M) might be different? From your comment below, I am assuming the answer is "yes".
I would generally agree with that. That's 6dB varience though, a pretty big spread.
I think many people make the assumption they are the same, but they're not. For example, look at this fellow's reaction to the discrepancy between the 1W/1M SPL value and the 100W/10M value:
"The 1w 1m sensitivity test resulted in a curiously low result, much lower than simulation suggests it should be. The impedance minimum measured with the drivers in series was 7.26 ohms at 3hz and an input of about 2.7 volts was used to approximate 1w into the minimum. The fact that the drivers were side firing and slightly further from the mic could account for some of it but not to the extent seen. I tested with one driver firing towards the mic and one away without much difference. Oddly when the mic was moved back for the 10m 100w test there was a large increase in the recorded result bringing it closer to what would be expected, but still lower than simulation suggests it should be."
Because this solves the acoustic-center/path-length error problem.
I use that distance sometimes too, mostly for home hifi speakers. But I do realize that the true SPL may not be shown. For the most accurate SPL figures, I measure at greater distance.
None, they are the raw sweeps.
Last edited:
Wayne, could you show one example of a comparison of a PA sub where the 1w/1m ends higher then the 100w/10m?
Last edited:
Wayne I am that fellow and those are all my tests. Data-Bass is my site with a partner who is a transducer engineer and helps out.
- Status
- Not open for further replies.