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Old 8th June 2007, 04:22 PM   #991
MBK is offline MBK  Singapore
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Actually now that I just wrote this - the interesting point comes in from the fact that we don't have point sources in real life. Therefore to some extent none of these formulas really apply, and the benefits of large speakers precisely come from the fact that they are less of a point source, in general, even ignoring line sources.

In addition to that in a room reflection and reverberation prevent some of the power losses (through restricted radiation space) and that again makes the SPL falloff smaller than predicted by point source theory.
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Old 8th June 2007, 09:36 PM   #992
mige0 is offline mige0  Austria
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Default cinema "sound" reality check

Hi


I haven't read all I must admit, but for sure a really nice overview from Lenard Audio about cinema though with some flaws as panomaniac already has pointed to.

To give an even more correct explanation the inverse square law IS valid also with lenses in use. At double the distance you always get the illumination given the SAME lens applied. The size of the picture gets 4 times bigger then of course. (no cylinder wave front with light !)

Where Lenard is definitely wrong is that there is no such virtual origin of light ( the projector in his explanation) that corresponds to the perception of a BIG picture. With very few exceptions screen materials have pretty uniform reflection directivity. Lenard's explanation would imply a mirror like screen ( with respect to reflection directivity ) which is not really worth your ticket if you didn't catch the only sweet spot in your cinema.

The subjective perception of a BIG picture is based on two factors only :
1) the size of the screen with respect to the viewing distance ( the field of view covered by the screen, in other words )
2) the absence of other light sources than the picture on the screen itself

One thing to keep in mind whenever drawing parallels to cinema "sound" systems is that this is just the ART of NOISE and by no means the ART of SOUND that Dolby, THX and Co are providing.

Many, many cinemas are equipped with 2 way passive main speakers - a double 15" in an undampened box and a 2" CD on a really BIG horn.
Not kidding this is 90% of cinema reality and even at Dolby Studio I was surprised to find this horrible speakers behind the screen.
Beside that the predominant majority of movies in Cinema or at Home Cinema

playing Dolby Digital ( AC3 ) sound format which is nothing better than iPod or MP3.


#########

Quote:
IIRC in line sources there is also a falloff of SPL with frequency at -3dB/decade
What is the explanation behind that?


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Michael
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Old 8th June 2007, 09:46 PM   #993
Pano is offline Pano  United States
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Quote:
Originally posted by MBK
I don't think horns act as lenses do. A horn acts as an impedance transformer (increasing efficiency, not relevant in this context) and a waveguide (restricting radiation into a fraction of full 4 PI space, possibly relevant).

I believe that's true. A horn and an optical lens are not the same, and there are many types of lenses. But it does seem that there is more going on with a wavefront in a horn than simple wave guidance.

The question would be, is the shape of wave front coming out of a horn significantly different from the shape of a wavefront coming from a point source at the distance of the compression driver?

And if so, is there an audible difference? Is the shape of the wave from a given diameter horn different from that of wave from a cone speaker of the same size? Can the difference be heard?

Reflectors are a different matter, and work much the same in light as sound. Hemispherical and parabolic reflectors can make significant changes in radiated sound. But that's another subject.
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Old 8th June 2007, 10:11 PM   #994
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Quote:
Originally posted by MBK
IIRC in line sources there is also a falloff of SPL with frequency at -3dB/decade, which would overlay with the proposed changed room reflection pattern, and of course the line source character depends on the length of the line vs. frequency, so I am not sure if this effect would be so clearcut. In any case line sources seem to be one of those terrible to measure but pleasant to listen to things...

The discussion on "natural sound" from large format speakers seems to apply to long linesources as another solution to getting realistic radiating area by gowing vertical instead of horizontal.

Tweeter ribbon = 90" long and 0.5" wide = 45 in^2
covers 3K - 30K Hz ~8" speaker area

Midrange ribbon = 90" long and 3" wide = 270in^2
covers 80 - 3K Hz ~18" speaker area

Woofer line array = four 15" woofers
covers 20 - 80 Hz


I believe it is critical to get the entire human voice range on one speaker and selected a narrow source to control beaming plus a large radiating area to keep Xmax small.

I live with the phase issues of steep analog LR8 crossover slopes while waiting for full digital crossovers with room equalization. Room equalization for the bass seems more valuable than a flat frequency servo woofer.

The -3dB/decade falloff of SPL with frequency in the nearfield mentioned by MBK makes linesources seem more dynamic then their 96db/m SPL spec.
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Old 8th June 2007, 10:30 PM   #995
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Default Wide Angle View

The 100th page is a good place to step back and look at what we're trying to do. To start, let's compare a monopole, say, a 12" driver with a HF horn, with a similar dipole. For now, let's set aside Baffle Step Compensation, and just look at the difference between the monopole and the dipole.

Relative to the monopole, there's a peak around 400~500 Hz, and response starts dropping off at a 6 dB/octave rate below 220 Hz. By playing around with the baffle shape, edge treatment, and size, we can smooth out the peak to some degree, but the onset of the 1/f rolloff region, at some frequency, is inevitable.

Rather than let the baffle become really large, we can transition to a monopole subwoofer at an arbitrary frequency. Since much of the benefit of a dipole is lack of box standing-wave modes, that says we should avoid W and H-baffles, and the subwoofer box should be small in comparison to the wavelengths at the crossover frequency. Picking a number out of thin air, the 12" sealed Rythmik subwoofer in a 1.5~2 cu. foot box crossed over at 80 Hz looks like a good compromise.

So we need to cover the range between 80 and 220 Hz. That's 1.5 octaves. 110 to 220 Hz is 1 octave. With the 1/f rolloff rate of 6 dB/octave, the dipole needs to make up between 6 and 9 dB, relative to the monopole, which is flat through the same frequency range.

6 to 9 dB doesn't sound like much, until you consider the impact on amplifier power, and most important, IM distortion from the driver. In the monopole, driver excursion is increasing at a rate of 12 dB/octave. In the dipole below the 1/f frequency, excursion is increasing at a rate of 18 dB/octave. In a horn, diaphragm excursion increases at a rate of 6 dB/octave. (Note that a full-sized, non-truncated straight bass horn in the 80 to 220 Hz region is gigantic, since the diameter of the mouth, and the length of the horn, need to equal an 80 Hz wavelength - about 14 feet! Folding a horn reduces its size but also drastically reduces its bandwidth.)

This comparison of monopole, dipole, and horns shows us that a conventional equalized dipole is taking us in the wrong direction, towards more distortion, not less. If we care about distortion, the area of the driver must increase when the driver goes below the 1/f frequency.

How much? Well, the true (power) efficiency of a driver doubles when the number of drivers doubles. Compared to one driver, two are 3 dB more efficient, four are 6 dB more efficient, eight are 9 dB more efficient, and so on. Not surprisingly, when the areas of the drivers approach horn dimensions, efficiencies become comparable as well.

The prospect of eight drivers per channel isn't very appealing. Fortunately, if we mount them next to a boundary, an image will appear, doubling the number of drivers. This gets us down to four drivers - large, but within reason. Raising the crossover frequency of the subwoofer to 110 Hz, the requirement drops to two woofers - but they both need to be at the floor boundary, otherwise you'll need three drivers, one at listening height, two at the floor boundary.

The Visaton NoBox BB is about the simplest possible way to get from here to there, with the least number of drivers. The wideband driver is 8", the bass driver is 15", and mounted close to the floor. The presence of the big 15" driver is not quite what it looks like - remember, this dipole system has the about the same excursion performance as a single 8" driver in a sealed box. In terms of excursion performance, the NoBox represents the lower bound of what we'd like to do.

So if we want "conventional" dimensions, we must raise the frequency of the subwoofer, equalize and boost the power going into the 1/f region, and accept (much) more distortion than the equivalent monopole system. Sorry, that's how it is. The harmless-looking requirement for a 6~9 dB of equalization has a very substantial tradeoff of increased driver area or increased IM distortion.

Bastani, in his Apollo and Prometheus systems, chose a very high subwoofer crossover of 220 Hz to meet the 12" widerange driver. The Visaton NoBox chooses a different tradeoff, with excursion and distortion comparable to the 8" widerange driver. Linkwitz chooses moderately small driver areas, audiophile drivers selected for low distortion (with excursion limits) and fairly heavy equalization.

Horn systems are different. The distortion is (much) lower, thanks to the 6 dB/octave increase in excursion vs frequency, compared to the monopole 12 dB/octave and the dipole 18 dB/octave. But most all-horn systems cheat - once the frequency gets below 200~300 Hz, wavelengths get really awkward and big, so we see folded and truncated bass horns, with passbands of little more than an octave.

Many so-called "horn" systems run the bass horns right through and below horn cutoff, the Altec A7 being the most famous example, although the Klipschorn does as well. When you run a horn below its cutoff, the excursion performance is no better than the sealed-box direct-radiator equivalent - but no worse either, which is still pretty respectable for a prosound 15" driver.

We obviously can't expect a dipole of any reasonable size, short of an entire wall, to equal what horns do. But we can surely ask that dipoles equal the distortion performance of an equivalent monopole, instead of settling for less. That's why I've been nudging the system in the direction of large bass arrays, with a variable-geometry overlapping crossover to maintain uniform directivity and constant power efficiency. There's been a temptation to use 18" drivers, but nearly all of them are intended for professional subwoofer use, with very heavy diaphragms and outright bad performance above 1 kHz.

By comparison, there seem to lots of 12 and 15-inch professional midbass drivers, with light cones, low distortion, and quite respectable midrange performance. Arrays of 12 and 15-inch drivers have a long and distinguished track record in stacks of bass guitar amps on stage and full-sized movie theater speakers.

Not a bad starting point for the bass array of a dipole speaker, which needs all the help it can get from low-distortion, linear-excursion, and well-behaved midbass drivers. The less crossover trickery and fancy equalization, the better it's going to sound. The unique dipole virtue of "hearing the speaker as it really is" - without box or horn coloration - applies to the bass array as well as the rest of the range.
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Old 8th June 2007, 11:51 PM   #996
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Default Progressive-Loss Mesh ... Not A New Idea

Right here in the original Rice and Kellog patent of 1925, a cabinet with lossy mesh on the sides and rear.

Excellent article on the history of loudspeakers - highly recommended. This underlines the point of interesting ideas that are lost over time - the original patents and papers make for interesting reading, with all sorts of subtle design details.
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Old 9th June 2007, 03:10 AM   #997
jacq. is offline jacq.  Canada
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Here is a link to a new product now being advertised.

http://www.emeraldphysics.com/EP_1.html

Regards to all for a wonderfull thread.

Jacq.
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Old 9th June 2007, 03:30 AM   #998
DougL is offline DougL  United States
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Lynn,

I think you are on the right track. However, some of your baffle calculations I believe are in error.

Quote:
Relative to the monopole, there's a peak around 400~500 Hz, and response starts dropping off at a 6 dB/octave rate below 220 Hz. By playing around with the baffle shape, edge treatment, and size, we can smooth out the peak to some degree, but the onset of the 1/f roll-off region, at some frequency, is inevitable.
My prototype OB has a 48" by 18" flat baffle. I calculate Feq to be 140 Hz, based on a baffle area of 864 Sq inches (.56 sq Meters). Without doing the math, the baffles in the thread are considerably larger than that. Feq is the Frequency where a monopole and dipole have equivalent output. This is about an octave from your planned crossover point of 80 Hz. With the floor bounce, the output of a Qts ~ .7 driver is relatively flat to about 70 Hz, rolling off after that. By using a moderate Q notch filter on the DCX2496 to remove the 6 db "hump" above Feq, I found that I needed only about 3 db of equalization at 40 Hz to be subjectively flat. This lowers the required driver displacement from insane to merely large.
My sense it that 2 * 15 will be in proportion to the rest of the design.

Regards,

Doug
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Old 9th June 2007, 04:36 AM   #999
soongsc is offline soongsc  Taiwan
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Default Re: Re: Geometry of Sound Source

Quote:
Originally posted by panomaniac


Thanks for the link, Lynn. A very interesting site.
Though I understand what he is trying to convey about sound, and agree with him, I have to take a technical quibble with his inverse square law.

Click the image to open in full size.

Once passed thru a lens, light no longer falls off in the inverse square. Not at all. If it did, you'd lose the major benefit of using a lens! Surprisingly, most technicians don't understand this.

Does the same apply to sound passed thru a "lens" such as a horn? Maybe. From what I've read and the diagrams I've seen of wavefronts in horns - it seems to have some bearing.

Light coming from a lens does not radiate the same way as light coming from a point source, nor does sound coming from a horn radiate in the same way as sound coming from a point source.
I sort of wonder about the sound image = screen image thing. One thing I am sure of is that most of the time the video changes views, and recordings normally don;t change accordingly.
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Old 9th June 2007, 05:49 AM   #1000
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Quote:
Originally posted by DougL
My prototype OB has a 48" by 18" flat baffle. I calculate Feq to be 140 Hz, based on a baffle area of 864 Sq inches (.56 sq Meters). Without doing the math, the baffles in the thread are considerably larger than that. Feq is the Frequency where a monopole and dipole have equivalent output. This is about an octave from your planned crossover point of 80 Hz.

With the floor bounce, the output of a Qts ~ .7 driver is relatively flat to about 70 Hz, rolling off after that. By using a moderate Q notch filter on the DCX2496 to remove the 6 db "hump" above Feq, I found that I needed only about 3 db of equalization at 40 Hz to be subjectively flat. This lowers the required driver displacement from insane to merely large.

My sense it that 2 * 15 will be in proportion to the rest of the design.

Regards,

Doug
I'm largely in agreement with your post. With the 1/f set to 220 Hz, I was being extremely conservative about the 1/f power requirements - as you mention, with larger baffles, the transition frequency goes lower, decreasing the area requirements - although the area requirement always remains larger than the equivalent monopole.

What happens if the bass radiating area is "excessive"? You get a little closer to the horn condition, with its far lower distortion, but without the frightening size requirements, or the single-octave limitations of folded horns.

Most of the dipoles we've seen so far are underpowered compared to their direct-radiator monopole equivalents, much less horns. I'm advocating going just a little way in the opposite direction - a system that has a bit more capability than the equivalent monopole, not less. If we have too much bass efficiency we can always series-connect the LF drivers or use EQ to decrease output, for a change. It is quite a luxury to be able to crank back on the bass power, reducing excursion at the same time.

There is a broad spectrum of bass arrays we can build - heavy EQ with drivers that are too small, a lightly equalized array with performance broadly equivalent to a monopole, or a lightly equalized array with performance a bit better than a monopole. No matter what we do, dipoles still have to contend with that 18 dB/octave excursion vs frequency rolloff, while monopoles are at 12 dB/octave, and horns are at 6 dB/octave (above cutoff).

The tradeoff of size+cost vs distortion performance and bass headroom is ours.
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