The effect of leakage on FR goals.

["Q"]

In the case of designing a speaker with directionality in mind how does one deal with the fact that you will get more bass reflections than higher end reflections? I guess you could just measure and tweak for flatness but that seems kind of crude when so much effort was put in to making other parts of the spectrum more directional. Plus, if more of the bass is coming from reflections doesnt that introduce phase problems?

["Q"]

Bump. I thought this was a good question.....did I phrase something wrong? <_<

[godfrey]

Hi Q

Yes, a good question, although a good answer would probably fill a book. Maybe the title is a bit misleading, though. (leakage? )

I'll give my thoughts fwiw, and some references at the end.

When we listen to music through loudspeakers in a room, what we hear is a combination of the clean direct sound from the speaker and the mucky reflected sound, which has bounced around the room a few times before reaching our ears.

Listening from a few meters away in a "typical" room, the reflected sound is at least as loud as the direct sound, if not louder. The frequency response of the overall sound is a total mess due to it being a combination of direct sound and lots of reflections with different time delays, all out of phase with each other to different extents at different frequencies.

Trying to equalize this to get a flat overall frequency response is probably a BAD idea because our ears/brain pay most attention to the direct sound, and we would have to mess that up to straighten out the overall sound.

Higher directivity is nice as it lets us hear more of the direct sound and less of the reflected sound. Ideally, we want constant directivity so that both the direct sound and the reflected "room sound" will sound natural.

One of the main problems with normal box loudspeakers is that the bass tends to be omni-directional, the midrange only radiates into the space in front of the speaker, and the treble is even more directional. If it's engineered to have a flat on-axis frequency response, then the total sound energy radiated into the room will be bass-heavy.

The problem is worse if we use e.g. horns to get more directivity in the mid and treble without any improvement in the bass. What we want is to get some directivity down to at least 100 or 200 Hz.

Low bass is heavily influenced by room resonances etc. A good option here is to have two or more sub-woofers spread around the room. At those frequencies our ears aren't very directional, timing is not critical, and using more than one sub helps break up the resonances and give a more "spacious" sound.

One way to get directivity down into the bass is to use dipoles, e.g. open-baffle or planar speakers. These radiate strongly to front and back, but not sideways or vertically. Surprisingly good, strong, deep bass can be had e.g. from an "H-frame" dipole sub.

Another approach is to use corner horns. With the bass speaker right in the corner of the room, there will be negligible delay between the direct sound and the reflections from the two adjacent walls, so the bass radiation is effectively covering only 90 degrees horizontally. Unfortunately, corner placement tends to be the worst for stimulating room resonances in the bass. Horn-loading the bass PROPERLY can reduce the room resonance problem, but introduce a significant time delay in the bass.

In the end, there's no "right" answer - you get to pick your poison, though!

Many people don't like normal box speakers because somehow, no matter how expensive, they always end up sounding like boxes. Most people don't know the difference though, because they've never heard anything except box speakers.

Here's some other stuff to read:

A) Siegfried Linkwitz's website is an absolute goldmine of information about sound reproduction in general and loudspeakers and loudspeaker design in particular (with lots of emphasis on directivity / directionality issues). It's here: Linkwitz Lab - Loudspeaker Design

B) Pi Speakers' website is interesting as well. The website's navigation is a bit of a mess, but in the forums Wayne Parham shares a lot of information about his designs and design process, what he's trying to achieve, how he goes about it, prototypes and measurements etc. He's running a business, but very helpful towards DIYers as well. That's here: Pi Speakers - unmatched quality and state-of-the-art performance

C) This website! There's lots of knowledgeable people here, both DIYers and professional loudspeaker designers. If you search through the threads, you'll find a lot of good info.

Cheers - Godfrey