Design Article
Loudspeakers: Objective evaluations - Part 1: Sound source radiation patterns
Floyd Toole
11/22/2010 7:41 AM EST
Griffin (2003) gives a comprehensive and comprehensible presentation of what is involved in designing practical line sources that approach the performance of full-height lines using less hardware. Smith (1997) describes a commercial realization and explains why it does what it does.
Keele culminates a series of papers on constant-beamwidth transducers (CBTs) in a collaboration with Button, in which they examine the performance of several variations of truncated lines: straight and curved, "shaded" (drive power reduced toward the end), and unshaded (all transducers driven equally), all standing on a plane-reflecting surface (Keele and Button, 2005). It is a masterpiece of predictions and measurements that provide many answers and suggest many more possibilities. Figure 18.3 shows a small sample of the informative sound field simulations in the paper.
It is rare to see such clear illustrations of what is right and wrong with certain aspects of sound reproduction. In Chapter 12, we looked at adjacent boundary interactions, pointing out that the immediate surroundings of loudspeakers affect how they function and that some of the effects are not subtle.
Figure 18.3a shows how just a single reflecting surface, the floor, disrupts an omnidirectional point source. Instead of tidy expanding circular contour plots, we see an example of gross acoustical interference with alternating lobes of high and low sound levels. The constant directivity of the source, indicated on the right, means that this problem exists at all frequencies, but the patterns will be different because of differing wavelengths.
FIGURE 18.3 Illustrations of the near-sound fields generated above a ground plane by several sound sources. The shading gets darker as sound levels drop; adjacent contour lines represent sound levels that differ by 3 dB. The original paper displays results for several frequencies; all of those shown are for 1 kHz. The words and graphics on the left explain the sources. On the right are far-field directivity indexes. Data from Keele and Button (2005).
Additional boundaries - ceiling, side walls - add more of the same, of course, and the merged combination usually ends up being more satisfactory than this single-dimensional perspective suggests. This is, after all, another perspective on comb filtering, discussed in Chapter 9.
Chapter 12 finished with examples of loudspeakers designed to interface with room boundaries. Illustration 18.3b and those that follow show how much better things can be if a boundary is considered as part of the loudspeaker design. Figure 18.3b shows that a simple truncated line seems to be an improvement over the elevated point source, but note that uniform directivity has been sacrificed. The directivity index has a sharply rising character, indicating high-frequency beaming.
Figure 18.3c shows that shading the output, reducing the drive delivered to the transducers closer to the top of the line according to a Hann contour, greatly simplifies the pattern, but it still beams at high frequencies. We are not there yet.
Curving the line, as shown in (d), seems to be a step in the right direction. The contour lines are not yet smooth, but there is an underlying desirable order to them. The constancy of the directivity index tells us that it applies over a wide bandwidth.
Shading the curved line using the Legendre contour yields a set of plots that have a sense of order and beauty, (e). The constant directivity index indicates that it will be similar at most frequencies. This is the kind of thing we like to see.
If the marketing department thinks that the customers might prefer a straight line, applying the right delays to the drive signals can, in effect, contour the line (f). When shaded, the result is very similar to (e) - and good.
Scanning from (a) to (e) and (f), it is easy to see that there are improvements that can be made in the delivery of sounds from loudspeakers, through rooms, to listeners. This is a two-dimensional example of what is possible. Interfacing the source with the floor benevolently uses that reflection, and directivity control reduces the effect of the ceiling reflection. Line sources, by their nature, have a narrow frontal aspect, so horizontal dispersion can be wide and uniform.
How did (e) and (f) sound? Excellent - at least that is the author's opinion from a biased, sighted test. It was distinctive in how little the sound level and timbre appeared to change with location in the room and how the loudspeaker did not get "loud" as one walked up to it. Note that the sound level contours around ear height (just under 2 m) are only gently sloped.
Any of these line radiators can be positioned at the ceiling interface - for example, as surround loudspeakers - or positioned between floor and ceiling. In the latter situation, they lose the boundary reflection and will need to be physically lengthened to regain comparable radiation performance. The shaded versions would have the lower half inverted so the acoustical output would decline toward both ends, top and bottom. So as we move into the detailed characterization of loudspeaker performance, it is important to keep in mind that directivity and propagation characteristics are important parts of the data set.
REFERENCES
Beranek, L.L. (1986). Acoustics, Acoustical Society of America, New York.
Cox, T., and D'Antonio, P. (2004). Acoustic Absorbers and Diffusers, Spon Press, London & N.Y.
Griffin, J.R. (2003). "Design Guidelines for Practical Near Field Line Arrays," http://www.audiodiycentral.com/resource/pdf/nfl awp.pdf.
Keele, D.B., and Button, D.J. (2005). "Ground-Plane Constant Beamwidth Transducer (CBT) Loudspeaker Circular-Arc Line Arrays," 119th Convention, Audio Eng. Soc., Preprint 6594.
Lipshitz, S., and Vanderkooy, J. (1986). "The Acoustic Radiation of Line Sources of Finite Length," 81st Convention, Audio Eng. Soc., Preprint 2417.
Smith, D.L. (1997). "Discrete-Element Line Arrays - Their Modeling and Optimization," J. Audio Eng. Soc., 45, pp. 949–964.
Printed with permission from Focal Press, a division of Elsevier. Copyright 2008. "Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers and Rooms" edited by Floyd Toole. For more information about this title and other similar books, please visit www.elsevierdirect.com.
Related links:
Acoustics and Psychoacoustics Applied - Part 1: Listening room design
Acoustics and Psychoacoustics: Introduction to Sound, Part 1: Pressure waves and sound transmission | Part 2: Sound intensity, power and pressure level | Part 3: Adding sounds together | Part 4: The inverse square law | Part 5: Sound Interactions | Part 6: Sound Interactions (cont.) | Part 7: Time and frequency domains
Using the Decibel - Part 1: Introduction and underlying concepts | Using the Decibel - Part 2: Expressing Power as an Audio Level
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sharps_eng
11/23/2010 7:01 PM EST
Brands of loudspeakers are recognisable by their characteristics, in one case in my experience by their lack of charateristics - I once walked past a London shop doorway and for a second or two felt a bubble of clear sound round my head as I passed; sure enough, at the back of the store was a Tannoy 15in monitor pointing outward. Its outstanding phase-coherence provides a unique and (sadly) unusual experience. As the author points out here, however, room placement and loudspeaker format interact hugely in making the LS more or less sensitive to listener position and movement, and any improvement through this work will be welcomed by audiophiles everywhere.
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selinz
11/25/2010 1:46 PM EST
It's interesting that the whole phase linearity thing came and went in the late 70s. Recently the "big thing" is frequency response customization. Most high end recievers have Audyssey (you set specially calibrated microphones in the primary listening areas)MultiEQ room calibration or equivalent to take care of room characteristics, but generally at the cost of phase linearity. There's no denying that it sounds great, by my 1977 vintage phase linear speakers still have the effect of turning your head to see if it's real or not, particularly with brass and drums...
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Ralex
11/26/2010 6:55 AM EST
It's interesting how little emphasis it is about what sounds natural and good to the ear. Too many people are into technical mumbo jumbo and the results are too many products doing room correction sounding real bad. I investigated this during the 2000s and wrote about it in AudioXpress 2004 issue august, september and october. Basically, I found the the room messes up the direct sound, meaning that the only thing that can be corrected sounding good in the mid and high frequency domains is the sound coming from the loudspeaker. A corrected loudspeaker sounds great and very realistic, the same goes for a phase linear loudspeaker! The low end can sound great by removing offending peak resonances in a room. Any parameteric eq can do this. Ask any live sound engineer. Easy.
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WKetel
11/26/2010 4:25 PM EST
What is unfortunate is that for a large portion of our citizens it will not matter in a few years. The practice of playing sounds that I will not call music at very high levels has been demonstrated to cause permanent hearing damage, to the point where it will not matter what the phase relationship is for the sound reproduction system.,The very non-linear frequency and amplitude response of the hearing of these individuals will assure that nothing that they hear will be undistorted or sound good. Truly a tragedy, but truer a reality.
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kdboyce
11/26/2010 6:13 PM EST
WKetel has a good point, and if I may amplify (pun intended) it a bit.....
Audio professionals and audiophiles alike bemoan the fact that the vast majority of the sound buying public has no clue as to what good sound really sounds like. (Do you think I got the word sound in there enough? )
Now the market is flooded with so much MP3 material that the younger generation thinks this is what it should sound like. Couple that with so many other types of audio codecs (MP3Pro, OggVorbis, AAC, FLAC, to name a few), the digital audio world is more like alphabet soup than audio and are hard to distinguish between.
I was fortunate enough to work in a group with people who did know high quality and how to get it on the amplifier side of the equation. And they listened to this on really good speakers.
What would spell doom for good music would be when the sound recording engineers that mastering the recordings to take advantage of MP3. UGH!
As for me, I won't stick earbuds in my ears. There is no faster way to ruin your ears than earbuds and an MP3 player. Well maybe being in a performing Rock band is faster....
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K6TVM
12/1/2010 12:12 PM EST
Around the 1940's a group (Bell Labs?) did a study to find out what kind of sound the public actually likes. They compared in a blind test really good sound systems against really bad sources, like the telephones and table-model radios of the day - the five-tube AC/DC things. Turns out the most popular source was also the most restricted - the little AC/DC radios with the three-inch speakers! You wonder why they like MP3 and think cell phones sound okay?
I wouldn't worry too much about the public at large - they never did like good sound!
John Amos
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kdboyce
11/26/2010 6:19 PM EST
Additional comment:
Really good speakers and amplifiers make a BIG difference.
In Taipei, there was a restaurant run by an audiophile who had set aside part of his establishment as a kind of studio which could be seen by patrons. Also, huge vacuum tube amplifiers and a massive speaker stack was visible at the front. They had to be capable of several hundred watts per channel at a minimum.
This guy would play very high quality recordings over that system at a volume level that was quite low compared to what it could be. The sound that came out of that set up was glorious and was really appreciated by all. So much so that it was the quietest Chinese restaurant I had even been in. :-)
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sharps_eng
11/27/2010 4:29 PM EST
My Tannoy DL2000s have an annoying response 'bump' at 50Hz (an original feature) which I have always intended to iron out with acoustic treatment. Hasn't happened yet - (a) I'm an electronic engineer so in true style my hifi is only ever half-finished and (b) the room keeps getting remodelled by the C-in-C :-)
So by being flexible and easily repeated, audio analysis and pre-compensation can keep pace with changes in a room. This research should benefit the effectiveness of that DSP acoustic code in receivers.
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divide_by_zero
12/8/2010 6:12 PM EST
Sigfried Linkwitz (of Linkwitz-Reilly crossover fame) has a web site where he not only sells speaker systems, plans and kits, but also discusses all of the parts of the audio reproduction chain. This includes a discussion of what live, unamplified music in a good venue sounds like and how most listeners are accustomed to badly recorded, close-miked, over-produced recordings played back by systems designed for these inferior recordings. He makes some compelling arguments. I'll pass judgment when I complete my Pluto speakers. See LinkwitzLab (dot) com.
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Cookie Jar
2/4/2011 1:50 PM EST
Shame on loudspeaker engineers using pink noise to measure frequency response "to avoid troublesome resonances". Unfortunately, music consists of sustained frequencies which tend to excite every resonance in your transducers, enclosures and room. If you measure loudspeaker response with sine-waves, you get a very wild response curve indeed. That is in fact your real world response curve, but it's bad news for marketing.
There's a lot of resonating going on. You have the mass of the cone and its spring. The Q of the enclosure resonances are commonly over 20!
Don't forget the resonances of the room.
The solution? Make your room dimensions a multiple of the cube root of 2 to spread the resonances evenly and use an open back full range electrostatic speaker. Nirvana!
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Sasquatch884
3/24/2011 11:00 PM EDT
I worked as a recording engineer from 1965 to 1980, the last 12 years of that period at Columbia Records studios in Manhattan. I worked on pop, jazz, rock, blues, R&B and classical music, with artists as diverse as Barbra Streisand, Bill Evans, Edgar Winter, Junior Wells, J.J. Jackson, and the Budapest String Quartet. I never once gave a rat's you-know-what about speaker specs. Here was the test I used - I placed a high-quality condenser mic, a Neumann or Schoepps, a few feet away from an open grand piano, put my head near the mic and had somebody competent play the piano. I would then walk into the control room, open the fader for that mic, get the volume up to about what I had heard in the studio. If the piano sounded different, the speakers failed. If it sounded really close, the speakers passed. This is all without equalization - not on the mic channel and definitely not in the monitor system. The only speaker that ever passed that test - unequalized and in different control rooms - was the original Electro-Voice Sentry II. I started using them at independent studios and then had Columbia buy a couple of pairs when I worked there. I still own a pair. They are simple, classically-designed two-way bass-reflex speakers, with a stiff 15-inch woofer, a small-throat horn tweeter crossed over pretty high and a cabinet with the resonance of a wet brick.
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Nic Cohen
7/27/2012 4:06 AM EDT
I do believe that the quality of sound in relation to the majority of the general public is lost when it comes down to the brand and functionality of the device they choose to use, as the quality does not always correspond to the ""quality"" of the product.....
Nic
www.kdweb.co.uk
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