Well, if you call "changes more in volume level with distance" an advantage...
All speakers get louder as you get closer to them, but some increase less as you get quite close to them than others, for the two reasons discussed in this thread. (Eg acoustic centre offset, and radiating area relative to listening distance)
I would have thought a speaker which doesn't get loud quite as rapidly as you got very close to it would actually be an advantage in the scenario you describe, as it would allow you to turn up the volume for people far from the speakers without blasting those really close to the speakers nearly as much - ie less total change in SPL with listening distance, so a more uniform SPL through the room.
(One reason long line arrays are sometimes used at concerts - as they only fall off at 3dB per distance doubling until you get really far from them - vs 6dB per distance double of a point source, allowing them to still be quite loud at a distance without being overpowering close up. Coupled with vertical height and some angle and you can get a fairly uniform SPL over a wide range of distances compared to a point source)
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Thats actually a good point wich a haven't thought about
Guess the only real way to find out what works best for me on the type/size of jobs i do, is to test it for myself.
I have always had BR and BP speakers before, so my quite new THs (SS15 from here) is something i have to learn about (and horns in generel). Both advantages an disanvantages, and this thread just caught my attention. But thanks for some nice replies you guys
Simon,Ok that does clear up a few things, particularly "Away" vs "Towards", which I had completely misinterpreted.
From that I've opted to graph the "Away" figures which were level matched at 1 metre, both for consistency with your measurements, as well as the fact that one of the "towards" measurements is missing. I've also only bothered with a logarithmic scale this time, as I think it's the more useful of the two scales visually.
I've used all the figures as presented with one slight adjustment - although Phil has level matched the JBL and the Growler to 115dB at 1 metre as individual units, according to his description and as is apparent in the figures, the levels have not been matched again after switching to pairs of units.
Since we're looking for any differences in fall off rate between the JBL and Growler with both starting at the same SPL at 1 metre, I've subtracted 1dB from the 60Hz Growler pair figures, and 2dB from the 90Hz Growler pair figures so that they all start at 121dB at 1m.
(As an interesting aside, I assume the fact that the 2x bass reflex cabinets only gain the expected 6dB over one cabinet, whilst the 2x horns gain 7dB and 8dB at 60Hz and 90Hz respectively is because the larger total mouth area is making the combined horn less compromised, thus raising their individual efficiency
So what do we see ? In the first graph there does appear to be a difference in the slope of the fall off between 1 metre and 2 metres, with the JBL showing a steeper line which would suggest that the measurement position was closer to the acoustic centre. (Since the rate of increase of SPL with distance accelerates as you get closer)
I would caution though that since all the figures are rounded to the nearest 1dB and the difference at 2 metres is only 1dB, it's within the experimental error...
The other thing I notice is the wiggles in response from 2 metres onwards which in some cases amount to a 2dB p-p variation. If the measurement was done well away from any buildings this wiggle shouldn't be there, unless there was a ground reflection.
So do we know whether Phil had the microphone on the ground to eliminate the ground reflection, or was it up in the air a bit ? Because the error is +/- 1dB it could again be rounding error, or +/- 1 error from the SPL metre if it only had a 1dB minimum step size. If the SPL metre did report fractional values, it's a shame we don't have those more accurate figures to see if the wiggle is really there or whether its just a +/- 1 digit error. Their periodic wave shape does tend to make me think they are real and caused by a "floor bounce" ground reflection though, especially when the 60Hz and 90Hz peaks and dips occur at different distances.
The second graph shows similar but different fluctuations, reaching 3dB p-p in one case. None of the graphs show the correct figure at 32 metres - in most cases the difference between measured and predicted value is 2dB, with the measured figure at 32 metres always being too low.
While interesting, I'm not sure that we can conclude anything from these graphs. While there are differences between the speakers, the magnitude of the difference we're looking for is probably in the order of 2dB or so, yet we only have figures rounded to the nearest dB, along with what looks like a ground reflection.
I'd have to call it inconclusive.
Thanks for taking the time to plot the information from Phil’s tests.
Without contacting Phil, I can’t say for sure whether he had the microphone on the ground or hand held. In prior distortion testing, he placed the microphone on the ground.
That said, if a "floor bounce" ground reflection problem were to occur, it would be most noticeable closer rather than farther away.
Even with buildings being far enough away to not affect the SPL directly, distant buildings still affect wind flow, as I can attest to from hundreds of ultralight airplane landings
.I would tend to interpret the 2dB p-p deviation from the inverse distance law primarily to wind variation and rounding errors.
In looking at your graphs, it is obvious that the both the FLH and the BR cabinets conform fairly closely to the inverse distance law, considering the experimental errors you point out.
Plotting the “towards” figures would give more data to average, even though one test was forgotten.
I have no doubt that using a system such as LMS as Wayne used in his sub shootouts could reduce the errors to the point where the measurements of a similar test would conform even closer to the inverse distance law.
Wayne has suggested that the tests were done with “a preconceived notion”.
I can’t speak for Phil, so I will only speak for myself.
Over the course of forty years professionally listening to and testing hundreds of BR, FLH, and recently TH, I have found the various cabinet types conform to the inverse distance law.
In single units of the above, this has been true even from one meter onward, while in large arrays, it has been true from a distance roughly the diagonal of the array on out when the array has been roughly square .
So, in a manner of speaking, the fact that Phil’s tests conform fairly closely to the inverse distance law did conform to my “preconceived notion”.
Similar to when the sun sets each day, it fits my preconceived notion that the earth continues to rotate around the sun
.Art Welter
Art, you really need to quit pressing this. Your data is wrong. Do you understand me? It's wrong. It doesn't really matter to me if it was due to faulty equipment or operator error, the fact is, it is wrong. I have measured the same things you have - too-close for acoustic-center error - and I disregard it because it is so far off. The fact that you don't have this feature in your data is what proves it to be flawed.
Wayne,
I have no idea what “this feature in your data” you are talking about, measuring a sine wave tone with a dB meter is a straight forward operation, as long as the DUT is not heavily distorting, and wind is not gusting to the point where the meter does not settle down.
Having tested the Growler and the JBL SRX 728 (quite similar to the 718), you are aware that they would not have enough distortion to affect Phil’s test at the level and frequency he tested at.
I have compared many dB meters, even though they may read a different absolute level with a sine wave tone, they all show the same level increase or decrease when the tone is increased or decreased.
You really need to show some data tested in a similar fashion rather than insisting that a simple dB meter somehow is “faulty”, or that a person that can read said meter somehow is operating it wrong.
I find it amusing that you continue to find dismiss data that has some “wiggles”, yet post no test data backing up your theories or claims
.We are all still waiting for your measurements of your 12Pi at with the same drive level at various distances, done with your fine test equipment.
Until you come up with data, Phil’s inverse distance data is the best we have.
Art Welter
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The feature I'm talking about is a change in the rate of change of SPL at close distances.
The fact that your data doesn't show it proves your data to be false.
Very interesting thread, but it starts to become a bit sterile.
Here is a test made by Rog Mogale. It is the designer of Void products, and a talentuous designer. I know he makes his measurements with LMS. He knows what it says and it does: http://www.voidaudio.com/pdf/lffaq.pdf
Rog is on the freespeakerplans forum (if necessary).
Here is a test made by Rog Mogale. It is the designer of Void products, and a talentuous designer. I know he makes his measurements with LMS. He knows what it says and it does: http://www.voidaudio.com/pdf/lffaq.pdf
Rog is on the freespeakerplans forum (if necessary).
We have gone over this way too much, ad nauseum. I think probably we should put this to rest.
In conclusion, what the reader should understand is that the shift of the acoustic center can have significant influence on the SPL measured up-close. A close measurement is often several decibels different than a distant measurement, scaled back to being equivalent (like 1W/1M compared with 10M/100W). That's why SPL reference level measurements are usually taken at a distance and scaled back.
Here are a few reference links that discuss the acoustic center, its movement and how that affects SPL measurements:
In conclusion, what the reader should understand is that the shift of the acoustic center can have significant influence on the SPL measured up-close. A close measurement is often several decibels different than a distant measurement, scaled back to being equivalent (like 1W/1M compared with 10M/100W). That's why SPL reference level measurements are usually taken at a distance and scaled back.
Here are a few reference links that discuss the acoustic center, its movement and how that affects SPL measurements:
- A note on the concept of acoustic center, by Jacobsen, Figueroa and Rasmussen
- Sound system engineering, by Don Davis, Eugene Patronis (Third Edition)
- Far Field Criteria for Loudspeaker Balloon Data, Pat Brown (Syn-Aud-Con)
- Comparing Loudspeaker Specs, Rick Kamlet, Pat Brown and Dan Field (Pro AV Magazine)
Originally Posted by weltersys
I have no idea what “this feature in your data” you are talking about
You would have us believe that a 2 meter path length horn measured at one meter, would only drop 6 dB at 6 meters (double the distance of the 2 meter path length and one meter first measurement distance) then only 6 dB more for each additional doubling at 12, 24, and 48 meters, with no supporting test data of your own.
Then, because Phil’s measurements don’t fit your (wrong) view, you claim his measurements are false.
I expect you will claim the same thing of Rog Mogale of Void Acoustics, who did the same basic test as Phil and I did, (Rog's test started at 116 dB, and used 50 and 100 Hz ) using a small 4th order bandpass sub that contains a single 12" woofer compared to an 18” horn loaded design, and found it a whopping 1 dB louder from 8 meters and beyond.
http://www.voidaudio.com/pdf/lffaq.pdf
You will probably continue to doubt everyone’s measurements until you do your own, then you will realize what the rest of the pro community already knows, that cabinet path length has little or no effect on the inverse distance drop we measure from 1 to 32 meters and beyond.
Art Welter
I have no idea what “this feature in your data” you are talking about
You would have us believe that a 2 meter path length horn measured at one meter, would only drop 6 dB at 6 meters (double the distance of the 2 meter path length and one meter first measurement distance) then only 6 dB more for each additional doubling at 12, 24, and 48 meters, with no supporting test data of your own.
Then, because Phil’s measurements don’t fit your (wrong) view, you claim his measurements are false.
I expect you will claim the same thing of Rog Mogale of Void Acoustics, who did the same basic test as Phil and I did, (Rog's test started at 116 dB, and used 50 and 100 Hz ) using a small 4th order bandpass sub that contains a single 12" woofer compared to an 18” horn loaded design, and found it a whopping 1 dB louder from 8 meters and beyond.
http://www.voidaudio.com/pdf/lffaq.pdf
You will probably continue to doubt everyone’s measurements until you do your own, then you will realize what the rest of the pro community already knows, that cabinet path length has little or no effect on the inverse distance drop we measure from 1 to 32 meters and beyond.
Art Welter
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Rog's measurements are primarily concerned with extreme far field. He is examining the effects of directivity at very large distances, not the near field to far field transition. To quote his words, "That is to say that for any given distance beyond near field operating conditions a horn loaded design will exhibit a greater SPL than a reflex design even if both have the same starting SPL." Whether of not one might agree with the concept of "throw", certainly all would agree that directivity has a large influence on the SPL at a distance.
However, this is an entirely different concept than the near-field/far-field transition, which is what we have been talking about in the last dozen pages or so. This is where the position of the acoustic center can make a large difference in scale. There is a large change in the rate of change when you get too close to the speaker, and that's the point.
Rog does not show the expected drooping of SPL increase as the measurement difference enters the near field in his charts. I am not sure if this is because he did not actually measure at one meter or if he did but then disregarded it when plotting his data. I think probably he chose not to include the near-field droop because he wanted to bring attention to the extra SPL at distances, not the reduced SPL at extremely close range.
The SPL curve you'll see when measurements are made up close is best shown in (Syn-Aud_Con) Pat Brown's chart below, from his "Far Field Criteria for Loudspeaker Balloon Data" article:
However, this is an entirely different concept than the near-field/far-field transition, which is what we have been talking about in the last dozen pages or so. This is where the position of the acoustic center can make a large difference in scale. There is a large change in the rate of change when you get too close to the speaker, and that's the point.
Rog does not show the expected drooping of SPL increase as the measurement difference enters the near field in his charts. I am not sure if this is because he did not actually measure at one meter or if he did but then disregarded it when plotting his data. I think probably he chose not to include the near-field droop because he wanted to bring attention to the extra SPL at distances, not the reduced SPL at extremely close range.
The SPL curve you'll see when measurements are made up close is best shown in (Syn-Aud_Con) Pat Brown's chart below, from his "Far Field Criteria for Loudspeaker Balloon Data" article:
- A note on the concept of acoustic center, by Jacobsen, Figueroa and Rasmussen
- Sound system engineering, by Don Davis, Eugene Patronis (Third Edition)
- Far Field Criteria for Loudspeaker Balloon Data, Pat Brown (Syn-Aud-Con)
- Comparing Loudspeaker Specs, Rick Kamlet, Pat Brown and Dan Field (Pro AV Magazine)
Wayne,
Rog Mogale is very clear in his article on how the measurements were made.
They happen to correspond exactly with the way Phil and I conducted our tests.
Rog writes :
“Fig 1. and 2. show the results of measurements I made with two different LF enclosures
at 50Hz and 100Hz. The test was carried out on a large flat test area that measured 50 x 40 meters. The test areas surface was concrete and there were no boundaries or obstructions within 30 meters of the devices under test. The enclosures and measurement mic were ground located giving half space loading. Both enclosure types were set to produce 116dB at 1 meter at 50Hz and 100Hz, then the measurement mic was moved out to 2m, 4m, 8m and finally 16meters from the enclosure.”
Wayne, it is time you accept what the rest of the pro community already knows, that cabinet path length has little or no effect on the inverse distance drop we measure from 1 to 32 meters and beyond.
If you still don't believe Rog, Phil, Josh Ricci, on that point, do the measurements yourself.
Art Welter
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Don't bring Rog Mogale and Josh Ricci into this because I do not believe they agree with your position. I'm not sure about Phil, but I suspect that he would question his measurements too after seeing this thread. If they want to voice an opinion, they're free to do so. But I don't think you'll get many that agree with your flat-earth argument.
The near-field/far-field transition is well-known and understood. It is in acoustics textbooks and various technical journals. The acoustic center is rarely at the face of the speaker or microphone, so if you measure from speaker face to microphone tip, the values don't scale at very close range the same as they do at greater distance. The transition happens at a distance which is further away for acoustically large sources. Lots of things affect the position of the acoustic center, such as mouth size, path length and loudspeaker configuration and shape.
The near-field/far-field transition is well-known and understood. It is in acoustics textbooks and various technical journals. The acoustic center is rarely at the face of the speaker or microphone, so if you measure from speaker face to microphone tip, the values don't scale at very close range the same as they do at greater distance. The transition happens at a distance which is further away for acoustically large sources. Lots of things affect the position of the acoustic center, such as mouth size, path length and loudspeaker configuration and shape.
it's easy to find the the near-field/far-field transition with the formula used for line array, you can find here: http://www.jblpro.com/catalog/support/getfile.aspx?docid=1009&doctype=3
So imagine the mouth of the horn on the rog test is 1,20m large.
For 100Hz: (1.2*1.2*100)/690= 0.20cm, so far field begins at 20cm
So imagine the mouth of the horn on the rog test is 1,20m large.
For 100Hz: (1.2*1.2*100)/690= 0.20cm, so far field begins at 20cm
There's more to it than just mouth area, but yeah, the concept is the same. You can't assume that the measurements of one speaker taken at close range will match another speaker at an equally close distance from the front face. Their sizes have an affect, so it is best to measure at a distance and scale back.
- A note on the concept of acoustic center, by Jacobsen, Figueroa and Rasmussen
- Sound system engineering, by Don Davis, Eugene Patronis (Third Edition)
- Far Field Criteria for Loudspeaker Balloon Data, Pat Brown (Syn-Aud-Con)
- Comparing Loudspeaker Specs, Rick Kamlet, Pat Brown and Dan Field (Pro AV Magazine)
Wayne,
Is your memory failing?
Here are some opinions Josh Ricci and I agree on and you still disagree with:
Josh Ricci wrote in #221
“I agree with Weltersys here.
The diaphragm may be further inside the horn but that is completely irrelevant to the listener or end user other than perhaps for setting delays between cabs. When you simulate something in HR or another program it calculates the acoustic performance at some specified distance from the final radiation point of the cabinet, not from a driver sitting 3m or whatever inside. “
From #230
“Wayne see my last post please. I simply disagree that in the case of a basshorn that the acoustic center matters at all as regards its real world use and performance. I also disagree that 2 meter or 1 meter measurements are flawed for basshorns or other subs....
The horn affect on the driver output doesn't mature fully and take shape until it reaches the mouth and exits the cab anyway. “
From #232 Josh writes:
“You seem to be implying that the long path from the driver to the mouth of the horn is already engaging the inverse square law such that if the acoustic center of the FLH is back 2.5m in the horn then the output should only drop by 12db at 10m instead of the 20db a direct radiator would see which would place the FLH at advantage at long distances and disadvantage or on equal footing, depending on your view point at shorter distances like 1 meter. I have not seen this born out in any measurements yet. The ones I have seen seem to trend the other way.”
As far as Rog Mogale opinion goes, no need to speculate, I'll simply quote what he wrote:
http://www.voidaudio.com/pdf/lffaq.pdf
"What can be seen from the data is that whilst the small single 12" bandpass enclosure follows the inverse square law, the single 18" horn loaded enclosure does not.
The horn loaded design is 1dB louder from 8 meters and beyond.
To correctly follow the 6dB per doubling of distance law we should see 92dB out at 16 meters, but instead we see 93dB. So where has this extra 1db come from. I believe it is due to the larger surface area behaving more like a cylindrical radiation pattern than a spherical radiation pattern. "
The small single 12" bandpass enclosure in his test followed the inverse square law, the single 18" horn loaded enclosure deviated from it by one dB.
Both the bandpass enclosure and the horn loaded cabinet in Rog's test followed the inverse distance law within one dB over a range of 1 to 32 meters.
Wayne, you like to cite Pat Brown, a guru of sound measurement, he wrote :
"It is often thought that a remote measurement position is necessary for low frequencies since their wavelengths are long. Actually the opposite is true. It is more difficult to get into the far-field of a device at high frequencies, since the shorter wavelengths make the criteria in Item 4 more difficult to satisfy."
Item 4:
"4. The distance from the source where the path length difference for wave arrivals from points on the device on the surface plane perpendicular to the point of observation are within one-quarter wavelength at the highest frequency of interest ."
This is an important distinction between high frequency and low frequency measurement, criteria #4 can be satisfied at 95 Hz and below for a subwoofer of one square meter mouth area measured at one meter.
I most certainly agree with Pat Brown.
Of interest, the only sub you gave as a reference to a one meter test result not following the inverse distance law was Josh Ricci's dual 21" side firing home stereo sub, which happens to not fit Pat Brown's criteria #4. Testing that sub (long before you got involved with this thread) obviously did not make Josh agree with you, as can be seen in his statements above.
During the course of this thread you have basically called me a liar, do you also consider Josh Ricci, Phil Lewendowski and Rog Mogale liars because their test results conflict with your misguided belief that a horn's path length affects adherence to the inverse distance law?
Art Welter
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Art, you just don't stop, do you? Your argument has been that there is no deviation from the inverse-square law whether measurement is made at 1M or 32M, or anywhere in between. My argument is that is absolutely false, that you cannot compare a measurement made at very close range with another made at a distance because of the acoustic scaling problem.
You mention three people, Pat Brown, Rog Mogale and Josh Ricci. Each of these people has stated that measurements are different at a distance than they are up close. That's the real point here, and it is the opposite of what you have been trying to say. You cannot take their words out of context and try to make people think it is acceptable to compare close measurements with scaled distant measurements, because each has said exactly the opposite. You cannot use "sound bites" from them, because their positions do not support your argument.
Pat Brown always measures at a distance, in order to reduce path length error. It is well understood that the problem is greater at high frequencies, but that doesn't necessarily make it insignificant at low frequencies. It's a matter of acoustic scale. Large horns have large mouths and long path lengths, and they definitely have a different acoustic center than direct radiators. That's why they are commonly measured at distance and scaled back.
Rog Mogale is also saying that he measures something different at distances than he does up close. He makes the case that this is due to directivity and "throw", which is not the same thing as the acoustic center ambiguity problem, but it is still a near/far difference that he cites.
When Rog made his directivity/throw chart, I suspect he simply rounded the up-close measurements with a linear trend, like smoothing. He wasn't really looking at the acoustic center problem, which is an issue at very close distances. Rog was focused on the influence of directivity, and was mostly interested in the extreme far field. And whatever his explanation was, the point is that he also found a difference in the scale of the close measurements compared to the distant measurements.
As for Josh Ricci, I think he initially thought the attenuation falloff did not begin inside the horn until he saw the Davis & Patronis textbook that shows it does. There's area expansion inside the horn, and that's why there is attenuation along its path length. Once Josh saw this, I think he backed off the argument. So I don't think you can point his earliest posts, because I think he changed his mind after that.
I think Josh's position is that 1M measurement is potentially useful, not that it is the same as the scaled 10M measurement. In fact, he has stated that he measured a difference in the 1W/1M value and the 10M/100W value. See post 258. So at this point, I don't think you can use him as someone that agrees with you on this matter any longer.
You mention three people, Pat Brown, Rog Mogale and Josh Ricci. Each of these people has stated that measurements are different at a distance than they are up close. That's the real point here, and it is the opposite of what you have been trying to say. You cannot take their words out of context and try to make people think it is acceptable to compare close measurements with scaled distant measurements, because each has said exactly the opposite. You cannot use "sound bites" from them, because their positions do not support your argument.
Pat Brown always measures at a distance, in order to reduce path length error. It is well understood that the problem is greater at high frequencies, but that doesn't necessarily make it insignificant at low frequencies. It's a matter of acoustic scale. Large horns have large mouths and long path lengths, and they definitely have a different acoustic center than direct radiators. That's why they are commonly measured at distance and scaled back.
Rog Mogale is also saying that he measures something different at distances than he does up close. He makes the case that this is due to directivity and "throw", which is not the same thing as the acoustic center ambiguity problem, but it is still a near/far difference that he cites.
When Rog made his directivity/throw chart, I suspect he simply rounded the up-close measurements with a linear trend, like smoothing. He wasn't really looking at the acoustic center problem, which is an issue at very close distances. Rog was focused on the influence of directivity, and was mostly interested in the extreme far field. And whatever his explanation was, the point is that he also found a difference in the scale of the close measurements compared to the distant measurements.
As for Josh Ricci, I think he initially thought the attenuation falloff did not begin inside the horn until he saw the Davis & Patronis textbook that shows it does. There's area expansion inside the horn, and that's why there is attenuation along its path length. Once Josh saw this, I think he backed off the argument. So I don't think you can point his earliest posts, because I think he changed his mind after that.
I think Josh's position is that 1M measurement is potentially useful, not that it is the same as the scaled 10M measurement. In fact, he has stated that he measured a difference in the 1W/1M value and the 10M/100W value. See post 258. So at this point, I don't think you can use him as someone that agrees with you on this matter any longer.
- A note on the concept of acoustic center, by Jacobsen, Figueroa and Rasmussen
- Sound system engineering, by Don Davis, Eugene Patronis (Third Edition)
- Far Field Criteria for Loudspeaker Balloon Data, Pat Brown (Syn-Aud-Con)
- Comparing Loudspeaker Specs, Rick Kamlet, Pat Brown and Dan Field (Pro AV Magazine)
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Three more papers on the subject:
"This paper focuses on uses for the acoustic centre concept, which in this paper represents a particular point for a transducer that acts as the origin of its low-frequency radiation or reception. The concept, although new to loudspeakers, has long been employed for microphones when accurate acoustic pressure calibration is required. A theoretical justification of the concept is presented and several calculation methods are discussed. We first apply the concept to subwoofers, for which the acoustic centre is essentially a cabinet dimension away from the centre of the cabinet."
There are so many papers like this, written on the subject of the acoustic center, it's nearly impossible to reference them all. Many of them are on the differences between different cabinet shapes, like rectangular boxes, rounded-edge boxes, spheres, etc. Those shift the acoustic center by diffraction, some putting it pretty far out in front of the speaker. Other configurations put the acoustic center far behind the cabinet. All are frequency dependent, meaning the acoustic center moves with frequency.
I encourage anyone reading this to search for academic papers on the subject. Even a quick Google search turns up a lot of useful information:
- Applications of the Acoustic Centre, Vanderkooy, John, AES:122 (May 2007) Paper:7102
- The Acoustic Center: A New Concept for Loudspeakers at Low Frequencies, Vanderkooy, John, AES:121 (Oct 2006) Paper:6912
- On the Movement of a Horn's Acoustic Center, Ureda, Mark, AES:106 (May 1999) Paper:4986
"This paper focuses on uses for the acoustic centre concept, which in this paper represents a particular point for a transducer that acts as the origin of its low-frequency radiation or reception. The concept, although new to loudspeakers, has long been employed for microphones when accurate acoustic pressure calibration is required. A theoretical justification of the concept is presented and several calculation methods are discussed. We first apply the concept to subwoofers, for which the acoustic centre is essentially a cabinet dimension away from the centre of the cabinet."
There are so many papers like this, written on the subject of the acoustic center, it's nearly impossible to reference them all. Many of them are on the differences between different cabinet shapes, like rectangular boxes, rounded-edge boxes, spheres, etc. Those shift the acoustic center by diffraction, some putting it pretty far out in front of the speaker. Other configurations put the acoustic center far behind the cabinet. All are frequency dependent, meaning the acoustic center moves with frequency.
I encourage anyone reading this to search for academic papers on the subject. Even a quick Google search turns up a lot of useful information:
- Google Search: Acoustic Center Concept for Loudspeakers at Low Frequencies
Wayne,
No, I don’t stop.
This “discussion” may go on forever until you pull your head out (of your textbooks) and actually do an inverse distance test yourself on a horn cabinet and a front load cabinet, instead of arguing against other's findings with no substantiating evidence of your own.
When you do the test, you will then see for yourself that what Rog Mogale, Phil Lewandowski, me, and probably hundreds of others that are unaware of your opinion to the contrary already know, that the acoustic center is irrelevant as to the inverse distance fall off from 1-32 meters.
Pat Brown measures at a distance to get out of the near field of the what he is testing, which requires a further distance for full range devices than subs.
There certainly is nothing wrong with measuring subs at similar distances to full range speakers.
As far as “attenuation falloff .. inside the horn” neither Josh nor I ever contested that, anyone who has ever stuck their head (or a dB meter) inside a bass horn knows that it gets louder at the throat than the mouth.
What we are concerned with when measuring a speaker is what it does in the listening area, not what happens inside the cabinet.
You can “suspect” whatever you want about Rog’s findings, but his measurement protocol was clear, if he was going to round the up-close measurements with a linear trend, the test would have been useless.
From:
http://www.voidaudio.com/pdf/lffaq.pdf
Rob writes:
"What can be seen from the data is that whilst the small single 12" bandpass enclosure follows the inverse square law, the single 18" horn loaded enclosure does not.
The horn loaded design is 1dB louder from 8 meters and beyond.
To correctly follow the 6dB per doubling of distance law we should see 92dB out at 16 meters, but instead we see 93dB. So where has this extra 1db come from. I believe it is due to the larger surface area behaving more like a cylindrical radiation pattern than a spherical radiation pattern. "
As the test stands, it clearly shows that the internal path length of the cabinets tested makes virtually no difference regarding the inverse distance rate of fall, the small cabinet fell off at the expected rate, the large horn cabinet deviated by one dB.
Rob makes no mention of the acoustic center affecting the measurements anywhere in his article.
The acoustic center location, whether near the front baffle, or some distance behind the cabinet front, simply makes no difference regarding the adherence to the inverse distance law as measured from 1 to 32 meters.
None of the academic papers you repeatedly cite ( and I encourage readers to check for themselves) contest Rob’s findings.
Art Welter
Maybe you missed it, or maybe you just want to argue. But I have said numerous times that I have measured up close. Sometimes a close measurement is useful for amplitude response, looking for nulls, etc. It's a convenience thing, and I do it often. But the SPL is almost always off, sometimes way off, exactly because the acoustic center isn't at the face. In fact, this is why I studied the issue, because it surprised me just how far off the 1M value is.
That's precisely the feature that tells me your meaurements are wrong. If you actually measured at 1M, you would have several decibels difference. You can adjust the drive voltage up or down a half volt or something to make it match, but when you send a signal that produces 1W, then the SPL measured 1M from the face isn't the same value as it is at 10M/100W.
Well, but that's the point, or at least part of it. If there is attenuation falloff inside the cabinet, then the acoustic center is somewhere behind the mouth. That's one of the definitions of the acoustic center.
You don't have to believe what has been written in the literature, you can stick your head in the sand if you want. You can look at the horizon and see that it appears flat; You can measure the epicylces of the planets and argue that the Earth is the center of the universe. But at the end of the day, you're spreading misconceptions and that's just not cool with me.
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