Explaining BSC
I have some ideas on what will happen with our spare room. I became aware of some articles on BSC I think over at Avaserfi's site which is now gone but it like a lot of other stuff is over my head. I plug away at stuff but it's always an uphill fight. I tell people that it's like learning a language. Some things are easier when you're 5 years old.
Don't sell yourself short. For me, the difficult part of speaker building has been the woodwork. You're an experienced finish carpenter, and building speaker cabinets should be easy. I made a lot of sawdust learning the hard way how to make good boxes without gaps. Veneering is still something that makes me nervous. I didn't have any tools to speak of. So I had to learn about that too. From what I've seen from your photos while you built the BR-1s, you have good instincts about using tools and using your hands in general.
OK on to baffle step compensation (BSC). Think about a bare light bulb hanging on a long length of wire. Light travels from it in all directions, up, down and sideways . If you raise the bulb closer to the ceiling, you get more light reflecting off the ceiling. If the bulb is right up against the ceiling, light now travels only down and sideways. It actually makes the light brighter because all the light that used to travel up, gets reflected down.
The same goes for sound from speakers. Instead of a ceiling, think about speakers sitting in front of a wall. The closer the speakers are to the wall, the more reflected sound you hear. There is one big difference between light and sound, and that is wavelength. Sound waves are much much longer than light waves. Light wavelengths will always be much smaller than the size of the light bulb. Sound waves can be larger or smaller than the size of the speaker.
Sound travels at 1130 feet/second. 1130 ft/sec ÷ frequency (Hz) = wavelength in feet.
20 Hz = 56.5 feet
200 Hz = 5.65 feet
2,000 Hz = 0.565 feet = 6.8"
20,000 Hz = 0.0565 feet = 0.68"
When sound wavelength is larger than the width of the speaker cabinet (the BR-1 front baffle is 8⅝" wide), it acts like light coming from the bare light bulb hanging on a long wire. It travels outward in all directions of a sphere, around the sides and to the rear of the speaker. As the wavelength gets shorter than the baffle width, it acts like light from a light bulb up against the ceiling. It reflects off the front baffle in a hemisphere pattern - and
is about twice as loud as sound with longer wavelengths.
If you look at a graph of frequency response of the speaker, you get a step up in loudness of about 4-6 dB. Hence the name baffle step response. In the example of an unidentified but well-known commercially available speaker, you can see the baffle step beginning between 500 and 1 kHz and ending with a dip just beyond 2 kHz.
Our hearing is very sensitive in this range. Depending on what you're listening to, this exaggerated midrange can sound too forward. Some people have described this a nasal or honky sound. Voices and brass will be overemphasized in a way that smears the sound, loosing overall clarity and detail. I'm always surprised at how many commercially available speakers fail to deal with this.
It can easily be corrected in the crossover with BSC as explained in
this article. There are simple to use
online calculators that make the math easy.
When BSC is built into a crossover, the designer has to make an assumption about where a speaker will be located relative to the wall behind it. If it will be far away, more than 2 feet, add in 6 dB of BSC (see page 6 of that BSC article). For 1 to 1½ feet add 4 dB. For less than a foot, you may want to omit the BSC. That is why I was asking you if you plan on putting your speakers on a bookshelf. That would be the same as keeping a speaker close the wall. With BSC, such a location will make a bass-heavy sound. Without any BSC, it will sound more natural.
