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I'm planning to use a 4 inch 4 ohm 25W local subwoofer from amazon for my diy project. This one - https://www.amazon.in/Soumik-Electricals-Subwoofer-4-Inch/dp/B077BFKZZV/ref=mp_s_a_1_3?ie=UTF8&qid=1534886410&sr=8-3&pi=AC_SL180_FMwebp_QL65&keywords=4+inch+subwoofer

  1. Will it produce good and deep bass for home and loud thumping bass for outdoors when hooked up with 2x20w tweeters in a box ?

  2. My Class D amplifier TDA3116D2 has seperate knob for subwoofer gain. I can control sub volume or turn it off competely when I have less battery left. So in that case, a passive radiator will produce good bass with only tweeters left. My quistion is when the subwoofer will be playing.. Will it damage the passive radiator with its frequencies or blow it out completely when the bass drops ?

  3. Will the passive radiator effect the sound (improve or distort) of subwoofer ?

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First of all, you need to find out the technical data for your planned speaker. This will lists things like fs Qes Qts Sd Vas and many more.

I tried to find this data for the speaker you mention, but I cannot find them. Hence let me give you an example of a branded 4" woofer, click here. Scroll down till you see "Technical Data" then click on "Show all data".

All these details can be used to design your enclosure, either vented/ported or sealed. There are many applications out there. Let me give you an example, click here, and you will find three freely downloadable applications to design speaker enclosures for subwoofers, "Bandpass Designer", "Sealed Designer", "Vented Designer". (DO read the instructions before use)

Using applications like that, and with the technical data of your speaker, you can comfortably design an enclosure which does what you want (within the limits of the speaker)

Now to address your questions:

Will it produce good and deep bass for home and loud thumping bass for outdoors when hooked up with 2x20w tweeters in a box ?

The terms "deep bass" and "loud thumping base" are a little bit subjective to use for designing speakers.

You can generalize some of this, but the following 3 bullet points are subjective to personal taste, and with that also influenced by the specific design of your enclosure:

  • A sealed enclosure has a more natural roll off of bass, which some might find pleasant when listening to music. Say in your sitting room.
  • A vented enclosure has a more "boomy" character, which could enhance the experience of certain types of music, mainly dance music.
  • A band pass enclosure has more deep bass, to more accurately reproduce only the "band-passed" frequencies.

But ofcourse you can make a sealed enclosure boomy, and a vented enclosure with deep bass, and a bandpass enclosure with natrual roll off... it is all in the design.

To create something which is "this thing inside" and "another thing outside" is going to be very hard. It probably can be done with multiple active crossover circuitry, with a switch which can switch from "behaviour A" to "behaviour B"

My Class D amplifier TDA3116D2 has seperate knob for subwoofer gain. I can control sub volume or turn it off competely when I have less battery left. So in that case, a passive radiator will produce good bass with only tweeters left. My quistion is when the subwoofer will be playing.. Will it damage the passive radiator with its frequencies or blow it out completely when the bass drops ?

For the purpose of this answer, lets make a simple definition for passive radiator:

  • It is a speaker,
  • without voice coil,
  • in the same enclosure of the (sub)woofer,
  • which moves in opposite direction of the (sub)woofer,
  • due to the air pressure build up from the (sub)woofer.

So "turning off the sub woofer" at the amplifier, will reduce the bass, and with that the movement of the actual (sub)woofer. If the (sub)woofer is no longer moving, the passive radiator will not move either.

Enclosure design with passive radiators are quite tricky, but in general will follow some design features of the vented enclosure.

Will the passive radiator effect the sound (improve or distort) of subwoofer ?

As detailed in the previous section the passive radiator (with a correctly designed enclosure) will aid the (sub)woofer, to accomplish the sound you want with the enclosure. It follows (some) features of a vented enclosure.

Final remarks

Sub-woofer enclosure design is a tricky thing. It requires a lot of formula's to get the design right. Many hobbyists will use applications like mentioned above to get started, and improve experimentally their designs.

I have built some in the past, some with reasonable results, some with not so reasonable results. But for what I am listening to; I have purchased a known branded speakers, with known characteristics.

If you are doing this as a hobby, go ahead and do it ! It is certainly fun, and you will learn a lot about sound reproduction. You will need to search online for more information, there are lost of good websites about speaker design for hobbyists. And there are many video tutorials as well.

Be prepared that some of your builds may produce different results then initially thought. Good Luck.

  • Thank you for the detailed answer. Well I have decided to not use the subwoofer as I have found that a 20w subwoofer will require a good wood box with sealed enclosure for that deep bass Im looking for. But that will make the speaker heavy and less portable. Also power consumption will be more. So I have decided to go with 2 JBL 120SI speakers which produce good amount of bass in small box with 2 passive radiators. – user196370 Aug 25 '18 at 6:38
  • "which produce good amount of bass in small box with 2 passive radiators" -- how do you know ? – Edwin van Mierlo Aug 27 '18 at 6:58
  • Cuz I made it just like the Boat Stone 1000 speaker. 2x 4 inch 22 watt speakers have good amount of bass when encloused in wooden box and enough room to resonate the bass radiator. See this speaker for example amazon.in/Zoook-Rocker-Torpedo-Bluetooth-Speakers/dp/B01N9ZMT6X/… – user196370 Aug 27 '18 at 7:30
  • oh... I see... (hand to forehead moment) – Edwin van Mierlo Aug 27 '18 at 8:41

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