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Well over on a different forum there was some talk about line arrays. I figure some here might be interested too. I guess I've had enough experience with line arrays to give some meaningful info on the benefits of line arrys. There are several benefits to a line array system over a conventional point source type speaker if it is made right. The main differences are between a conventional speaker, which is a point source, and a line array, which is a line source.
First off, a line source will typically project it's near field response for a distance of about 8-10 times its length. So an 8ft line array would have a nearfield of 60-80ft. What this means is that within the nearfield the line source will only decrease in output by 3dB per each doubling of distance. On the otherhand, with a typical speaker(point source) will have a nearfield response of only about 1 meter.
We can compare a 90dB efficient line array to a conventional speaker 90dB efficient. At 1m their responses will be equal. Doubling the distance to 2m will give 6dB less for the conventional speaker, but only 3dB less for the line array. At 4m away, the conventional speaker will be -12dB, and the line array only at -6dB. It is for this reason that line arrays tend to "fill" a room much better.
The next advantage of a line source is the controlled room interaction. I've often heard that 75% of the problems with speakers are due to room interaction. Reflections off of walls and the floor and ceiling can greatly change the response of a speaker. Where line arrays help is in eliminating problems with vertical reflections from the floor and ceiling. However McIntosh found this only works if you approximate a true infinite line source. This means designing a line array that runs from floor to ceiling to get a true cylindrical wavelaunch. So a shorty 4ft line array would not give these benefits.
If the line array is also made as a dipole, the horizontal reflections are also greatly eliminated. In a dipole you will have nulls at each side of the line. In a perfect world the front and back waves would completely cancel. In reality this is not possible, but in a dipole line array measuring 40dB less output in the "null" spots is common. What this means is that the reflections off the side walls are virtually eliminated as there is very little output at the sides.
Another advantage I find with line arrays it that they have no sweet spot vertically. With typical speakers you need to be at ear level with the tweeters to have the best sound. Stand up and they will sound different. With a line array, as long as you are not right near the ends of the line, the output is equal. So whether you are sitting or standing the sound is the same. I find this very important because I like to have good sound whether I'm sitting or standing.
There are also disadvantages. Unless you can get your drivers very close together your high end response will suffer slightly. This depends on the distance between the drivers. For example we'll take a 4" driver, assuming the edges of the drivers are touching so centers are right at 4" apart. Taking this into account we can find the frequency point where the line will begin to lose directivity, and I believe at approximately four times this frequency(1/4 wavelength frequency) the line will begin to exhibit audible comb filtering and the high end response will begin to decrease. Now, to do this we need to find the frequency with a 4" wavelength. We use this equation:
speed of sound = freq x wavelength
Now the speed of sound at STP = 331.45 m/s at STP with dry air, but to get a more realistic number, calculating at 70degrees F with 50% relative humidity the speed of sound would be around 344 m/s. This gives a difference of about 120Hz in this case compared to using the STP value.
So, the wavelenght is 4" and converting to meters we get .1016 m. Plugging into the equation we get:
344m/s = freq x .1016 m
freq= 3391Hz
This is the point where the line will begin to lose directivity with a 4" driver, and at double this point, 6738Hz, comb filtering will begin to take effect. I haven't been able to do any expermineting yet to determine at what point the comb filtering becomes audible though in relation to this frequency. I have been taking to someone with more real world experience with line arrays, and his would guess say that likely around 13,500Hz, double the frequency where the comb lines begin to take effect they would be come audible.
The other problem is often the high cost of multiple drivers, and the time required to assemble a line array. Believe me, cutting a total of 48 holes for a bunch of small drivers is not a fun experience.
All in all if you can find inexpensive drivers to use, I believe the advantages far outweigh the problems. My shipment of 2" x 3.5" line array drivers should be here within the next 2 weeks. I'll give you a little background on these drivers.
Awhile ago I set out to find an inexpensive driver for line array use. Steve Sedmak had done quite a bit of looking into tall narrow drivers for his line arrays. Typically a tall narrow driver will have very good off axis response. This is a very good thing. So I went out to find some narrow drivers. They needed to have a very smooth frequency response. Inexpensive was a main concern, but they also needed to sound good. My first thought was to find a driver slightly shorter than 4" tall. A 2" x 3.5" tall driver pushed the 1/2 wavelenght frequency up to almost 8000Hz and the quarter wavelength up to almost 16000Hz. Although not that different from the 4" drivers, to me this was acceptable. EQ should be able to compensate for the high frequency rolloff, and above 16000Hz comb lines should be less apparent. When I measured these drivers, I found that they also had a very smooth off axis response, which like I said, was very important. In addition these drivers have a nice response up to nearly 20KHz. They actually measured flatter up to this range than many tweeters I had played with. Finally the nice thing about these 2" x 3.5" drivers is that they are affordable.
Someone mentioned the Zalytron Axom array on the other forum. As far as I can see there are a few problems associated with the short line of only 4 tweeters. First is the distance between centers on the tweeters. Being apart as far as they are you will still have the problems with high frequency rolloff. Second is the vertical dispersion problem. You really need to be within that range of the 4 tweeters for things to sound right. As you go above or below the 4 tweeters things will sound different. That basically takes away one of the main advantages of line arrays IMO. Having the short line of tweeters will also not give the benefits of canceling reflections from the floor and ceiling as a full line would. Finally is the problem with the rolloff as you exit the nearfield. Like I said before a line typically has a nearfield equal to about 8-10 times it's height. So 4 tweeters would have a nearfield of around 100inches at most. Beyond that the high end will roll off at 6dB per doubling of distance and the midrange drivers would still only roll off at 3dB per doubling in distance. In a small listening room this would not be a problem, but in a large hometheater it could be. This is also a problem seen with their other line array using the Raven tweeter.
If anyone is interested in any more info on the small drivers, or has any questions on line arrays in general, feel free to contact me.
Also check out this page http://www.stryke.com/current.htm if you are interested in seeing pics of a few line array's I've played with. None of them use my drivers, but the pics may still be interesting.
John
First off, a line source will typically project it's near field response for a distance of about 8-10 times its length. So an 8ft line array would have a nearfield of 60-80ft. What this means is that within the nearfield the line source will only decrease in output by 3dB per each doubling of distance. On the otherhand, with a typical speaker(point source) will have a nearfield response of only about 1 meter.
We can compare a 90dB efficient line array to a conventional speaker 90dB efficient. At 1m their responses will be equal. Doubling the distance to 2m will give 6dB less for the conventional speaker, but only 3dB less for the line array. At 4m away, the conventional speaker will be -12dB, and the line array only at -6dB. It is for this reason that line arrays tend to "fill" a room much better.
The next advantage of a line source is the controlled room interaction. I've often heard that 75% of the problems with speakers are due to room interaction. Reflections off of walls and the floor and ceiling can greatly change the response of a speaker. Where line arrays help is in eliminating problems with vertical reflections from the floor and ceiling. However McIntosh found this only works if you approximate a true infinite line source. This means designing a line array that runs from floor to ceiling to get a true cylindrical wavelaunch. So a shorty 4ft line array would not give these benefits.
If the line array is also made as a dipole, the horizontal reflections are also greatly eliminated. In a dipole you will have nulls at each side of the line. In a perfect world the front and back waves would completely cancel. In reality this is not possible, but in a dipole line array measuring 40dB less output in the "null" spots is common. What this means is that the reflections off the side walls are virtually eliminated as there is very little output at the sides.
Another advantage I find with line arrays it that they have no sweet spot vertically. With typical speakers you need to be at ear level with the tweeters to have the best sound. Stand up and they will sound different. With a line array, as long as you are not right near the ends of the line, the output is equal. So whether you are sitting or standing the sound is the same. I find this very important because I like to have good sound whether I'm sitting or standing.
There are also disadvantages. Unless you can get your drivers very close together your high end response will suffer slightly. This depends on the distance between the drivers. For example we'll take a 4" driver, assuming the edges of the drivers are touching so centers are right at 4" apart. Taking this into account we can find the frequency point where the line will begin to lose directivity, and I believe at approximately four times this frequency(1/4 wavelength frequency) the line will begin to exhibit audible comb filtering and the high end response will begin to decrease. Now, to do this we need to find the frequency with a 4" wavelength. We use this equation:
speed of sound = freq x wavelength
Now the speed of sound at STP = 331.45 m/s at STP with dry air, but to get a more realistic number, calculating at 70degrees F with 50% relative humidity the speed of sound would be around 344 m/s. This gives a difference of about 120Hz in this case compared to using the STP value.
So, the wavelenght is 4" and converting to meters we get .1016 m. Plugging into the equation we get:
344m/s = freq x .1016 m
freq= 3391Hz
This is the point where the line will begin to lose directivity with a 4" driver, and at double this point, 6738Hz, comb filtering will begin to take effect. I haven't been able to do any expermineting yet to determine at what point the comb filtering becomes audible though in relation to this frequency. I have been taking to someone with more real world experience with line arrays, and his would guess say that likely around 13,500Hz, double the frequency where the comb lines begin to take effect they would be come audible.
The other problem is often the high cost of multiple drivers, and the time required to assemble a line array. Believe me, cutting a total of 48 holes for a bunch of small drivers is not a fun experience.
All in all if you can find inexpensive drivers to use, I believe the advantages far outweigh the problems. My shipment of 2" x 3.5" line array drivers should be here within the next 2 weeks. I'll give you a little background on these drivers.
Awhile ago I set out to find an inexpensive driver for line array use. Steve Sedmak had done quite a bit of looking into tall narrow drivers for his line arrays. Typically a tall narrow driver will have very good off axis response. This is a very good thing. So I went out to find some narrow drivers. They needed to have a very smooth frequency response. Inexpensive was a main concern, but they also needed to sound good. My first thought was to find a driver slightly shorter than 4" tall. A 2" x 3.5" tall driver pushed the 1/2 wavelenght frequency up to almost 8000Hz and the quarter wavelength up to almost 16000Hz. Although not that different from the 4" drivers, to me this was acceptable. EQ should be able to compensate for the high frequency rolloff, and above 16000Hz comb lines should be less apparent. When I measured these drivers, I found that they also had a very smooth off axis response, which like I said, was very important. In addition these drivers have a nice response up to nearly 20KHz. They actually measured flatter up to this range than many tweeters I had played with. Finally the nice thing about these 2" x 3.5" drivers is that they are affordable.
Someone mentioned the Zalytron Axom array on the other forum. As far as I can see there are a few problems associated with the short line of only 4 tweeters. First is the distance between centers on the tweeters. Being apart as far as they are you will still have the problems with high frequency rolloff. Second is the vertical dispersion problem. You really need to be within that range of the 4 tweeters for things to sound right. As you go above or below the 4 tweeters things will sound different. That basically takes away one of the main advantages of line arrays IMO. Having the short line of tweeters will also not give the benefits of canceling reflections from the floor and ceiling as a full line would. Finally is the problem with the rolloff as you exit the nearfield. Like I said before a line typically has a nearfield equal to about 8-10 times it's height. So 4 tweeters would have a nearfield of around 100inches at most. Beyond that the high end will roll off at 6dB per doubling of distance and the midrange drivers would still only roll off at 3dB per doubling in distance. In a small listening room this would not be a problem, but in a large hometheater it could be. This is also a problem seen with their other line array using the Raven tweeter.
If anyone is interested in any more info on the small drivers, or has any questions on line arrays in general, feel free to contact me.
Also check out this page http://www.stryke.com/current.htm if you are interested in seeing pics of a few line array's I've played with. None of them use my drivers, but the pics may still be interesting.
John
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