Khorn attenuation/distance less than inverse square

[Poindexter]

Gil: I found some articles on the IEEE website but I'm not sure they are the ones that you recommended (Geddes, Murray, etc). Do you have a link to those papers readily available?? If not, don't worry about it, maybe I'll ask you again later after I've glaced over the one's I already have.

I have already theoretically built a horn in one of my labs (had to do the calculations but not build it) to direct a very high frequency (microwave) to a dipole array. So. . . I guess. . . I kinda know some of the design theory but I'm looking for some nitty-gritties. Maybe I'll find some on IEEE soon.

Thanks Gil, you're awesome!!! I'm learning a lot.

This board rocks

[JohnA]

Just call him William F. "Buckley" McDermott, minus the politics!

No offense intended, Gil. I haven't figured out what you may think of Buckley.

[WMcD]

I'll take that as a complement, John. Thanks. I would be a conservative except some of the company is unacceptable. Smile.

Perhaps John is saying I talk (type) too much. I'm fairly introverted unless talking about something I love.

Gil

[artto]

First of all, the Radio Crap SPL meters are not using a calibrated microphone. I have one. They can easily be off by +- 2 to 3 Db.

Two, white noise is not going to reduce the room effects and standing waves. Those things are a function of the room proportions, the room surface treatment, and the objects in the room.

Last but not least, the inverse square law applies to free space, not necessarily enclosed rooms with reverberant sound fields, especially as the room gets smaller in size. Youll find that in an enclosed space, especially those in the size range that most people listen to recorded music, the sound level doesnt fall off very much after 6-12 feet away from the speaker (depending on room size, could be even less), because of the reflective sound from room boundaries still contributing to the measurements in the reverberant sound field.

Also.....line sources do not fall off following the inverse square law. I've forgotten what the rate is, but it's less than it is with "point source" or dipole or whatever else.

John....you have mail......and how much do we owe you for the Friday night mixer treats. I saw that bill ring up on the register.....$85 or something? Did Klipsch pick up the bill for that too?

[KhornKerry]

First when I say white noise, it is more likely a pink noise source.

A white noise source will not produce standing waves but may still have some room effects (reflections). In order to produce standing waves the source frequency has to be a continuous tone.

If you look at the measured data it shows no hint of standing waves, no peaks or valleys it falls off in a very smooth way. If you try this measurement with a pure tone the plot will look like a saw tooth wave.

Even if the meter has errors over the frequency range it should not matter because the source signal does not change its frequency content only the amplitude should change with distance. I was careful not to over drive the meter, always set the scale to the middle of the range.

Most if not all home theater receivers use a white noise generator to set the speaker level because it does not generate standing waves.

I plan to take data this weekend using a small Radio Shack speaker. I will place the front of the RS speaker where the mouth of the Khorn squawker was (grill cloth) and make measurements at exactly the same distance points as the Khorn data was taken. Now any difference in attenuation will be a result of the speakers because the room will be the same, the SPL meter will be the same, the distances from the speaker will be the same and the source will be the same. Will plot results on Monday and put it on this forum.

Remember that some of the most important discovers in science and physics were often made with the crudest of instruments.

As Gil pointed out I am only trying to learn something here.

In my opinion: Kerry

[WMcD]

Yeah, I figure I owe John A. something too. I took home the left over plates, cups, napkins, and some of the remaining food. Plus I nibbled.

John mentioned he liked a recording by Jesse Colin Young with Grey Day on it. I found the remastered version, recently released.

So, John, I'll see if I have your postal address, and send it off. If not I'll ask you for it by e-mail.

Thanks,

Gil

Now I also did a little cerimonial tasting of Atto's libation. And somebody's cooler supplies! Thanks.

[JohnA]

" I'm fairly introverted unless talking about something I love." - Gil

And you're *fairly* articulate, too.

Actually, guys, you don't owe me anything. It was my treat! I was happy Andy brought his Heresies. That added a lot to the atmosphere. OTOH, I really would like to get my hands on a copy of that old Jesse Colin Young recording; ripped if not an original.

[KhornKerry]

First let me correct a previous error, I said the SPL meter weighting was A actually it was C weighted (more Flat).

I have had a lot of complaints about using a RS SPL meter. I checked the booklet and the accuracy was stated at + - 2 dB at 114 SPL. Also I compared the meter reading to change in volume of my Denon receiver which reads out in dB, they agreed within 1 dB over a 40 db change of volume. Even if the meter has errors over the frequency range it should not matter because the source signal does not change its frequency content, only the amplitude should change with distance. I was careful not to over drive the meter, always set the scale to the middle of the range.

I took measurements this weekend using a small Radio Shack cone speaker. I removed the top of my Khorn and placed the RS speaker where the center of the midrange horn had been at. The Radio Shack speaker was about 15 or 16 dB less efficient and I could only get a SPL of 99dB at 1 meter, but that is close enough. I took measurements at the same distance points as with the Khorn. Every thing was the same except the speakers. I forgot to measure the .75 meter point.

So it looks like the attenuation verses distance of the 2 speakers is the same. My guess is that room reflections keep it from falling off at the inverse square rate.

The blue line is the Khorn, Red is the Radio Shack speaker and the green is calculated inverse square.

Kerry

[artto]

Kerry, I dont mean to be rude. But I think you really missed the point and have a few issues confused. I dont know where you got the idea that white noise, or pink noise, or sine waves or whatever, has anything to do with producing standing waves. Id like to know your source of documentation.

Standing waves are a function of the room boundaries, the room proportions, and affected to some extent by any objects in the room. Obviously, one needs to produce sound, in order to hear or measure the acoustic effect of standing waves. But those sounds, sine wave, white noise, or even music, etc. are not producing standing waves, nor are they the cause of them.

Furthermore, standing waves in a ROOM do not have anything to do with the inverse square law other than altering the results from location to location within the room, both by reinforcement, and cancellation, because of the ROOM BOUNDARIES.

The inverse square law applies, in absolute terms, only in free space. And that means getting the speaker far enough away from any boundary, including the ground plane, relative to the frequency being measured. A 32Hz tone for instance, has a wavelength of aproximately 35. So to accurately measure the speaker in free space at 32Hz without any boundary reflections to alter the measurement results, the speaker really needs to be at least 35 above the ground in addition to not having any other boundaries near it.

Your comment Most if not all home theater receivers use a white noise generator to set the speaker level because it does not generate standing waves is also not true. If it said that in the manual somewhere, whoever wrote it is wrong. White noise is a type of noise that is produced by combining sounds of all different audible frequencies together. If you took all of the imaginable tones that a human can hear and combined them together, you would have what is called white noise. White noise is noise whose amplitude is constant throughout the audible frequency range. It is often produced by a random noise generator in which all frequencies are equally probable. Because white noise contains all frequencies, it is frequently used to mask other sounds. Thats why many home theaters have a white noise generator to set the speaker level. Because by generating all audible frequencies simultaneously, the measurable and audible effects of standing waves (as well as other noise, HVAC or a nearby refrigerator for example) are somewhat reduced (masked) and therefore wont affect setting the level as much. But the white noise (or pink noise, etc) DOES NOT have anything to do with not producing standing waves. The standing waves are in fact still there, they are simply masked to some extent.

White noise is defined as sound with equal power per Hz in frequency, where as pink noise is filtered to give equal power per octave or equal power per 1/3 octave, usually for use with an equalizer. Since the number of Hz in each successive octave increases by two, this means the power of pink noise per Hz of bandwidth decreases by a factor of two (3 dB per octave).

Consequently, pink noise has relatively more bass than white noise, and sounds more like the roar of a waterfall than like the higher hissing sound of white noise.

You also said In order to produce standing waves the source frequency has to be a continuous tone. Wrong again. You need BOUNDARIES. A sine wave in truly free space will not produce any standing waves. A continuous sine wave will show up the effects (reinforcement as well as cancellation) of standing waves more readily than random noise will. In a live room, its very difficult to get any semblance what the room or speakers frequency response is with a sine wave. Even moving an object in the room as small as the microphone or SPL meter will alter the results.

And your idea of using the Radio Shack speaker in exactly the same location as the mouth of the Khorns mid horn is invalid as well. Speakers such as horns, tend to have what is known as controlled directivity. This can also be used to throw the sound, not only across a desired area or angle of coverage, but also, more or less with regards to distance too. There are, for instance, horns designed specifically, as long throw or medium throw, etc. which is useful in large concert situations. However, you may still experience some of that effect when comparing something like a Khorn with a small direct radiator speaker which the Radio Shack probably is.

In any case, you are not going to get consistent results making these measurements in a live room, especially one the size you are doing this in.

[artto]

And.....actually, if you want to reduce the effects of standing waves caused by the ROOM BOUNDARIES, the 'A' weighting would be more useful as it primarily responds to frequencies in the 500 to 10KHz range where as the 'C' weighting will go down to 32Hz and definitely show up any bumps in the frequency response caused by standing waves which occur primarily & increasingly with lower frequency.

[KhornKerry]

I agree you need boundaries or reflections to produce standing waves in air but you also need a coherent source or continuous tone, noise dose not have that quality. You are correct that white noise is made up of all the tones BUT there is no phase relationship between the frequencies at all.

I should have ran the test A weighed but I started with C as a mistake because the meter defaults to C when you turn it on and I didn't realize it until a was more than half finish.

I you look at the plots there is no sign of any peaks or valleys that you would see if it were done using a signal tone.

I will ask you a question, what conditions are required to produce standing waves?

[artto]

RE: but you also need a coherent source or continuous tone, noise does not have that quality

Wrong again Kerry. You do not need a coherent source or continuous tone. In fact, music itself is not a coherent source or continuous tone. It is changing constantly with regards to frequency, amplitude and phase relationships between various instruments & voices & other sounds. Yet the effects of standing waves are easily audible.

RE: white noise is made up of all the tones BUT there is no phase relationship between the frequencies at all

Thats the point. Actually they use things like white or pink noise to measure system response because noise, by its very nature, it typically random. It, as a CONSISTENTLY random (oxymoron?) test signal, most resembles music, which is also changing constantly with regards to frequency, amplitude and phase.

Therefore, if you can hear the effect of standing waves with music, you dont need a sine wave to show it up. In fact, in simple, rectangular rooms, the frequency and strength of standing waves are completely predictable. I have gone to great lengths in my room to minimize these effects (keyword=minimize).

RE: what conditions are required to produce standing waves?

Standing waves are modes of vibration associated with resonance in extended objects like piano strings and air columns We call these characteristic patterns standing waves. These standing wave modes arise from the combination of reflection and interference such that the reflected waves interfere constructively with the incident waves. The term standing wave is often applied to a resonant mode of an extended vibrating object, in this case, the room, and the air inside of it. This occurs (relative to the listening position) when, for instance you have a room dimension of 18.833 feet in all three dimensions. Do you know what will happen? 18.833 is the wavelength of 60Hz. We all know what 60Hz noise & hum sounds like. With a room dimensions of 18.833 feet in all directions, any 60Hz noise will become more audible (exaggerated) because the room modes for that wavelength coincide in all three dimensions.

Standing waves can be produced in any enclosed space with boundaries however they are most obvious, prominent and problematic the smaller the room gets. Using the above example, if we changed one of the room dimensions to one half of 18.833, or 9.42, we would still have a problem with 60Hz tones being exaggerated. However, it would not be quite as strong.

Take the example of a speaker in a wall and a second wall at distance L in front of it. As the cone vibrates it will send out an acoustic wave which gets reflected back by the second wall, returns to the first wall, gets reflected again back to the second wall and so on. If the frequency of vibration is such that the distance L corresponds to half of a wavelength, then the cone movement is in phase with the reflected wave and the sound pressure keeps building up. Eventually an equilibrium is reached between the energy supplied by the cone movement and the energy absorbed by the two walls and the air in between. This is a standing wave resonance or mode condition.

I have an Excel spreadsheet program that will allow you to calculate the room mode resonances & their relative strength in simple rooms. The Forum wont let me upload it. I can email it to you if you want. Its also available from Linkwitzlab.com http://www.linkwitzlab.com/rooms.htm The file you want is modes1.xls. Its highlighted in the text.

[KhornKerry]

Artto

RE: In fact, music itself is not a coherent source or continuous tone.

Wrong again Artto. Music by isn't very nature is full of coherent sources. Guitar strings are tuned aren't they, the flute produces a very clean coherent tone. When I say continuous I mean for several cycles.

Lets drop the standing wave argument for now.

My original thought was that sound from a horn may not fall off with distance the same as a cone speaker would, but the data I took ( khorn RS fall off.jpg ) indicates that they are virtually the same. The room starts to dominate the attenuation after a few meters.

So I have a few questions.

Would transmitted radio waves from the spacecraft Voyager with its parabolic antenna fall off at inverse square?

In free space is there any attenuation curve different that inverse square?

Will a laser beam fall off inverse square?

[JohnA]

Laser beams and EM transmissions from parabolic antennae do expand in area with distance from the source. I suspect they may follow the inverse square law, but do not appear to since they begin as highly focused, narrow beams. However, I have seen demomstrations in a laser lab where beam spread hurt the experiment.

By substituting a direct radiator in your test, you have proved that room reflections heavily influence the SPL fall-off indoors.

We may be quibbling over the definition of "coherent". Still, I was taught a great lesson about standing waves in my L-shapped bedroom in my father's house. I had a low voltage AC adapter I could connect my speakers to. The 60 Hz tone was deafening in some places and a couple of feet away, it was inaudible. The room was about 12 x 18 + 10 x 12 feet. I had noticed the bass was different in different places and proved why with the adapter. It was kinda cool at 17.

The room dimensions set the resonant frequencies of the room (standing waves). That's why I built my room with one non-parallel wall. I now must have standing waves over a broad range of frequencies rather than a just a few.

[artto]

Kerry, you have great deal of misunderstanding about acoustics, physics & how music is recorded and reproduced. Just because a guitars strings are tuned doesnt mean the strings are coherent. Each string does not occupy the same space as any of the other strings. They are not even struck simultaneously. The sound from each string reaches your ear at slightly different time intervals. In a true stereo recording, the various instruments & vocals are at different distances and locations. An accurate playback system should be able to easy discern those differences. And, if you look even more closely at the strings, or the even physical instrument itself, you will also see various resonant modes occurring. Its the phase differentials that allow us to aurally localize where the sounds are coming from.

Earlier on in this thread several Forum members (including myself) told you outright that the inverse square law wouldnt apply very well in a closed room because of boundary reflections creating a reverberant sound field which is contributing to the measurements.

From my previous post: the sound level doesnt fall off very much after 6-12 feet away from the speaker (depending on room size, could be even less), because of the reflective sound from room boundaries still contributing to the measurements in the reverberant sound field. At least you finally figured that out. Albeit, your reasoning of how or why this occurs under various conditions is still somewhat muddled.

RE: In free space is there any attenuation curve different that inverse square?

Yes. Gil posted the other variations further up the thread. I guess you didnt read that either.

RE: radio waves & lasers. Yes. However keep in mind that we are talking about the speed of light here. The wavelengths are can be many miles long even at very high frequencies. This is not a perfect world that we live in. Even light can be affected by other objects in free space. For instance, light has been found to accelerate when approaching a very large mass like the sun, and de-accelerate as it passes due to gravitational pull. It will even bend it. So technically speaking, there is no such thing as perfectly free space.

[WMcD]

John A.

I think the inverse square law and its variation is poorly explained. It works because of the two concepts of conservation of energy, and that intensity is measured per unit area.

The real issue is how the energy is distributed over larger areas as it propegates. There must be conservation of energy, but less intensity in any given area as the area "painted" increases as the the square of the distance.

For what ever reasons, a laser beam is projected pretty much like a tube with no distribution to a wider area, then there is no fall off in energy distribution (intensity) as the distance increases. Of course conservation of energy is honored. However, the inverse square law is about intensity in a given area.

In classic field theory we have three conditons.

One is a point source. Being a point is not much of the issue. Rather, it expands into three dimensional space where the area painted expands as the square.

The infinte line source has a fall off directly as the distance. This requires some math which it too complicated to explain. Meaning (ahem) I couldn't hope to comprehend it. My visualization of the three dimentional geometry is as follows.

Suppose you had a very long neon tube (infinite line source) running through the center of a spaced stack of big 33.3 rpm records which are mirrors on each side. Any given point source would try to propigate in three dimensions but any ray would be reflected off the mirror. Say one is going 45 degrees north. It is reflected back south at 45 degrees. However, this matches the point source and reflection going 45 degrees south in the portion of the stack above. So they are equivalent. The mirrors are not doing anything, or they are creating an equivalency of propagation space.

The result of having all these mirrors is that the little sections of the line source paint a very small pancake area which is increasing only as a function of distance. We could take away the mirrors because there is symetry of the rays which impinge on the top and bottom sides of the mirrors defining the area..

In my head, there is a similar analogy in the infinite plane source.

You have some very big plane of a light source. In front of it is a a system of packed soda straws which are mirrored. Here, any given point source is forced to keep the light in the tube. But say, any given single source is shooting off a photon at plus 1 degree on any axis, there is a similar one in the adjacent combination of sources and refective soda straws sending one off at minus one degrees. They bounce off the mirrored soda straw. Again, since there is symetric photon distribution because of the light pipes, there is no fall off from the infinite plane.

Err, it is pretty deep. In my studies there was a prof explaining it in three dimentional wave mechanics. It took me a few years of pondering to come up with something which made sense to me. It may well make no sense to anyone else. But I get to vert.

Gil