 Read up on Huygens theory and imagine how it would affect sound.If the source is small compared to the wavelength, the source will become omnidirectional. If the source is much larger than the wavelength, it will automatically be directional.
Typically you have 3 regimes that control directivity. - Low frequencies where the wavelength is much lower than the size of the horn in the measurement plane. In this area directivity is primarily controlled by horn size and efficiency/frequency response by expansion profile. Since your horn is wider than it is tall, it will be more directive down low in the horizontal plane than it will be in the vertical plane. This is contradictive to what you want if you for instance want a 60Hx40V horn. Below a certain frequency, the horn will become more directive in the plane with the bigger size, in this case the horizontal plane. This is a phenomenon commonly refered to as pattern flip.
- Midband frequencies where the wavelength of the sound is roughly equal to the size of the horn. In this area, both directivity and and frequency response are HEAVILY influenced by horn geometry more than it is influenced by the size. Since the wavelength is roughly equal to the horn dimensions, you can get all sorts of reflections and therefor standing waves inside your horn if the mouth does not perfectly terminate in a smooth transition towards the open space. This is because the sound undergoes a change in acoustic impedance and if this change is abrupt, it will cause the sound to be reflected. Usually in this area is where you see edge effects that cause comb filter like effects in the frequency response, and long, slow decaying trails in a waterfall plot. The effects you see in a directivity graph are what is commonly known as midrange waste-banding.
- High frequencies where the wavelength is bigger than the sound. In this region, size does not matter anymore. The primary effects that affect the dispersion is simply the angle of the horn walls, and how well the sound can follow these walls. If you have a horn with a rapid expansion profile, the higher frequencies will ''break'' free from the walls very soon which means that for increasing frequency, the directivity increases and dispersion angles decrease.
To make sure that even the highest of frequencies are ''dispersed'' throughout the intented coverage angle, many horn designs use a diffraction slot. If the width of this slot is small enough compared to the wavelength, the slot will be omnidirectional. If the horn angle after that is for instance 90 degrees, the sound will be dispersed throughtout the full 90 degrees. (again, read up on Huygens). The diffraction slot however is a distortion generating beast since like i said in 2, it offers a HUGE abrupt change in acoustics impedance > reflected sound > standing waves > resonances> high distortion. (This is why i love my JABO KH53 horns on the BMS horns, no diffraction slot. These horns sound leagues better than the JBL 2385A i also sometimes use. (See the topic : http://forum.speakerplans.com/2-tractrix-horn-any-interest_topic97925_page1.html )
In your horn, which i guess is around 50cm wide, the point where the flip between 1 and 2 occurs is roughly around the 400~500Hz ballpark horizontal and higher for the vertical. This means that you will never, ever with these horn sizes, get a horn that is directive to below this frequency region. This shows in your simulations. What your simulations also show, is that in this area (500Hz), the directivity decreases, typical midrange waste-banding behavior. I do need to add, that this might be exaggerated a bit since around 700Hz your on-axis sensitivity is higher. What your simulations also show is that this effect is much worse in the vertical plane than in the horizontal plane. This is logical, since your horizontal termination to the open air is WAYYYY more smooth than that for the vertical, which is basically an open ended pipe.
The funniest part of this all to me, is that most people have no idea that these effects exist. If you listen carefully to a sound system, you can spot them though. Especially when you stack multiple horns side by side, things like this can become problematic pretty fast. It is THE main difference between a professionally engineered cabinet and a home built DIY system. Measuring frequency response never tells you the entire picture, and you have found that out for yourself, kudos! Maybe this clarifies why i always recommend people, that if you can stick with one speaker per side with a wide coverage angle (they are way easier to design, and the horns are automatically more constant directivity since the transition from 1>2>3 is way more smooth (physics, yeah!!)), than it is something you should always do i.m.o. Hope you find some helpful point in this post. |
Post Options
Thanks(0)
Quote 
Posted: 12 January 2018 at 9:28am
Topic Options