...and I thought designing and building speakers would be easy ! That was until I started doing some research into the science of loudspeaker design; an exercise that raised more questions than it answered !
I've been lurking around the Mission Possible DIY section for a few weeks now and have been totally amazed by the designs and craftsmenship of all the knowledgable forum members. I've learned a lot by reading the various threads, but I've also amassed a list of many questions that I haven't really found answers for.
This thread is all about my questions regarding enclosures. Some of them already have answers based on my "research", but most of them are still unanswered.
I would really appreciate any help with answering these questions I'll update this orignal post with answers as they come in. Hopefully other beginners like me will find the information in here useful. Thanks !
Questions
1. What does it mean to "tune the enclosure/box to 37Hz" ?
Not really sure. I'm guessing here, but I think its an exercise in setting the output frequency of the port. If so, does that mean that the Port will always output 37Hz regardless of the frequency being emitted by the woofer ?
1a. How do you do that ?
Increasing the port length decreases the tuning frequency. There's a formula here: http://www.clubknowledge.com/Car_Audio_FAQ/?t16
However I also noticed that reducing the port diameter decreases the tuning frequency as well. I would have thought that the opposite is true. Do any issues arise by reducing the port diameter ? Does it decrease the amplitude of the sound waves (lower volume) ?
1b. Does it apply to ported and sealed enclosures ?
I believe it only applies to ported enclosures.
1c. Why would you do that ?
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1d. Does it only apply to dedicated subwoofers, or can it apply to regular speakers as well (i.e. 3 way speakers) ?
I assume it applies to both subs, and regular speakers.
1e. Some speakers have 2 ports, and I assume they're tuned to different frequencies. What effect does that have ?
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2. What are the basic good enclosure design principles ?
- Box should be well braced
- Internal walls should be lined, especially woofer areas.
- Baffle should be have smooth round edges
- Anything else ? I'm sure there are a few more.
2a. Is there an ideal H x W x D ratio for enclosures ?
One suggested ratio from "Designing, Building, And Testing Your Own Speaker System, Chapter 3" is 0.37:0.68:1 (internal). Width x Depth x Height for imaging OR Depth x Width x Height for bass response. Although to me that seems a little too extreme. Most of the DIY designs here follow a 0.5:0.55:1 (WxDxH) ratio. Side Note: I haven't read that book, just the excerpt from Google Books.
3. Bass response or lower frequency response. With all other things being equal, will the bass response of a single woofer in a 20L enclosure be equivalent as 2 woofers in a 10L enclosure ? This question is trying to ascertain whether extra woofers can compensate for lower enclosure volume.
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4. Why do ported enclosures have a lower frequency response than sealed enclosures, when both are using exactly the same woofer ?
I'm not sure, but I think it's because the waves generated by the negative movement of the woofer are channeled through a port which forces the sound waves to travel a farther distance, in essence "stretching" them out, resulting in longer wavelengths = lower frequencies. That's why longer port lengths lead to lower Tuning Frequencies (according to the formula). Can someone please point out if my "analysis" is wrong !
5. I've noticed some 3 way designs (TMWW) have the WW sealed away from the TM part. Other designs have an "open" chamber design ? What's the difference ? Why would you choose one over the other ?
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6. What is baffle step compensation ?
Took me a while to understand get my head around this, but this is what I know so far. When the wavelength of sound is smaller than the baffle width, the sound is directional. When the wavelength is greater than the baffle width (generally from woofers), it becomes omni-directional at those frequencies and it seems those frequencies are attenuated (usually by 3dB). To compensate for this effect, the frequencies from the tweeter and midrange are attenuated to match the output of the woofer.
However, if you have your speaker placed on the wall or very close to a wall, you have a sort of infinite baffle scenario, so BSC is not required.
Judging when you need BSC seems to me to be more art than science. But a general rule is that the further you place your speaker from the wall, the more likely it is that you will need BSC circuitry.
6a. What if you have a woofer that is crossed over at 500Hz. According to your calculations, frequencies below 250Hz will be attenuated because they're longer than your baffle width. However if you go ahead with BSC circuitry to attenuate the tweeter and mid sound, won't you end up with a spike between 250Hz and 500Hz in the frequency response curve ? What do you do in a situation like this ?
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7. I've read that it's best to scallop the back of the front baffle to allow sound waves to exit the rear of the driver smoothly. I'm not sure what that means, and what problem "scalloping" aiming to resolve ?
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8. Passive woofers: Side firing vs Front firing. What are the things that need to be taken into consideration when designing a speaker ?
Bass has some directionality, so frequencies between 100Hz and 250Hz will be better represented by a front firing woofer. Below 100Hz, sound is more omni-directional and a side firing woofer will work just as well. Any other considerations ?
8a. Why would you choose one over the other ?
One reason to choose side firing would be to keep the width of the speaker narrow. But that's really a superficial issue. I'm more interested from a perfomance perspective.
9. Why do some speakers have the tweeter and the midrange off-axis, and others have them in straight line ? What are the advantages / disadvantages of the off-axis design.
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10. Is there an absolute minimum amount of space that should be left between a woofer and the sides of the enclosure ? E.g. If I had a 7" woofer, would it be problematic if I mounted it in an enclosure with an internal width of 8" ?
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11. Why are tweeters usually placed above midranges, and midranges above woofers ? Is it all about having the higher frequencies closer to ear level ?
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12. What is floor bounce ?
Floor bounce in an interference problem that occurs when sounds wave reflected from the floor cause destructive interference with sound waves travelling directly from the bass / midrange driver to the listener's ear.
12a. What should you listen for when trying to discern if your speaker suffers from floor bounce ?
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12b. Does it only apply to hard floors ? Or does it affect carpeted areas as well ?
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12c. How do you prevent it from happening ?
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13. What effect do Acoustastuff and similar products have ?
The basic premise is that stuffing a box with Acoustastuff or something similar (e.g. wool) will lead to a smaller enclosure exhibiting characteristics of a larger enclosure, because it absorbs and releases air molecules when the cone moves in and out respectively. This leads to lower than usual air pressure when the cones moves in, and higher than usual air pressure when the cone moves out - much like a enclosure with a larger volume.
13a. How do you know when you need to use it ? Is it only used for sub enclosures ?
Haven't been able to find a formula out there that dictates when to use it. It seems to be a subjective matter (like many things in speaker design). If you'd like a lower frequency response from your speaker, that's when you'd use the stuff.
13b. How do you know how much to use ?
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14. Isobaric design versus Bipole design. What are the differences in sound characteristics produced by each design ?
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14a. Under what circumstances would you use an Isobaric or Bipole design ?
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