long throw

[widowgobo]

just found this little nugget of info.

 

Does throw exist with regard to low frequencies

It has widely been accepted that throw does not exist and that to achieve a greater SPL at any given distance requires a

higher source SPL, so most people will reply with no, as sound must obey the inverse square law. But for anyone who

has experience in setting up sound systems that have employed different types of LF enclosure design, most will reply

with a yes, certain types of enclosures seem to be louder at a greater distance. Is this phenomenon really down to a

higher starting SPL or is something else going on.

The classic debate is that horn loaded LF designs seem to throw more than reflex loaded designs. A horn loaded design

is normally capable of a higher starting SPL and so should always achieve a higher SPL at distance, but this is not what

seems to happen in practice. Most seem to find that the reflex design's SPL seems to fall away very rapidly after 10

meters whilst the horn loaded designs SPL carries out much further. This should not be as if the starting SPL of the horn

loaded design is 5dB greater than the reflex design then out at 20 meters the measured SPL of the horn loaded design

should also be 5dB greater. Another phenomenon noted by many is that larger arrays of reflex enclosures extend the 10

meter drop off point and exhibit as much throw as a similar number of horn loaded enclosures.

I'm just starting my investigations into LF directivity and throw and have still to carry out many tests and experiments,

but even at this stage can confirm that throw with regard to low frequencies and enclosure design does exist. I have

found 3 main reasons why some designs and the quantity in use can exhibit a greater SPL with distance. First and most

important is the radiating area of the LF design or enclosure. Second is the scattering effect brought about by

turbulence within a reflex port or duct. And lastly, the directivity gained from multiple enclosure arrays.

Are certain types of LF design capable of greater throw

My preliminary research has discovered that horn loaded designs do throw further than reflex designs. 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. This would appear to break the 6dB per doubling of distance

inverse square law and theoretically be impossible, but a very basic measurement test with the two different enclosure

types seems to contradict this. 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 16

meters from the enclosure.

 

 

 

[Stan Hope-Streeter]

<br />just found this little nugget of info.

 

Since you haven't cited the source of that quotation, we have no idea whether to take it seriously. Although from the style it could be one of Roger Mogale's articles.

 

The best explanations for this phenomenon come from Tom Danley, and he has posted them several times on the ProSoundWeb message boards.

[mackerr]

<br />just found this little nugget of info.

 

Since you haven't cited the source of that quotation, we have no idea whether to take it seriously. Although from the style it could be one of Roger Mogale's articles.

 

The best explanations for this phenomenon come from Tom Danley, and he has posted them several times on the ProSoundWeb message boards.

 

Dave Gunness, formerly speaker designer and head of R&D at EAW, and now at Fulcrum Acoustic, has also posted at PSW about this topic.

 

Dave's explanation is that a given SPL at a given distance requires a certain number of acoustic watts. When measured up close the horn loaded box will measure lower because those acoustic watts are spread over the whole opening of the horn mouth. This would mean that for the lower measured SPL up close the level held up better with the horn loaded system. I think he means it is more a matter of how the horn measures up close rather than the horn somehow defying the inverse square law.

 

Some of it may have to do with where you start your measuring. Is it the front of the box, or the driver? They will be different for the horn loaded box.

 

 

Mac

[widowgobo]

yes, it was Rog, I ment to put that in.

[Rob_Beech]

As stated, it is simply that you are hearing and measuring a smaller portion of the sound when you are close. They are just as loud at the back if they're just as loud at the front. What we actually see is they are quieter or as loud over a GIVEN MEASUREMENT AREA and then as loud or louder at the back. What we actually find is that if we match the level at the back to say 90dB, over the SAME measurement area it's quieter at the front. It does make it a useful type of design to use, you can often get a more even coverage with this type of enclosure if it is deployed correctly, but at the expense of other things. We can't get away from physics.