2 liter (0.07 ft^3) bass speker challange - diyAudio
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Old 9th May 2016, 01:04 AM   #1
rbt is offline rbt  Romania
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Default 2 liter (0.07 ft^3) bass speker challange

I would like to make a portable bass speaker for which there are two major limitations:

  1. The enclosure volume is 2L (10cm*20cm*10cm) (0.07 ft^3).
  2. It will be completely portable so it will run on batteries.

I was thinking of using a 3" Monacor speaker. According to the specs the Vas is 1.9 liters so it fits the bill.

The purpose of this bass speaker is to be used in addition to portable bluetooth speakers or in addition to the computer/TV builtin speakers to make the sound much better.

As an amplifier I was thinking of using either a very compact one based on TPA3118 or one based on TDA8932.

As the low pass filter, I just ordered this board to see how it performs.

For power I was thinking of using 16V coming from a 4S LiPO pack.

Do you think this would work? Should I spend the time and money to build this? What suggestions do you have for me?
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Old 9th May 2016, 07:51 AM   #2
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Quote:
Originally Posted by rbt View Post
Do you think this would work? Should I spend the time and money to build this? What suggestions do you have for me?
The real question is are you actually wanting a SUBwoofer? If you want to cover below say 80hz, I'd say no unless your typical listening volume is fairly quiet. Any kind of social situation (which is usually what it seems portable bluetooth speakers are good for) and you're unlikely get loud enough for any appreciable difference. Bass requires a certain level of volume displacement to produce, and while those tiny sub drivers certainly can produce low frequency bass, it won't be very loud at all.

Now on the other hand a lot of small bluetooth speakers start rolling of fairly high, say 200hz or so. You could put a woofer in a small box and run it between 80-200hz. Especially if you are able to incorporate DSP to EQ an undersised box and and cross the two units over smoothly, you might actually have something worth building.

Edit:
http://www.parts-express.com/tang-ba...oofer--264-909
I've actually been reading a few of these reviews and a lot of people sound pretty happy with doing something fairly similar to what you're looking for. So maybe I'm just a bit cynical. Could work okay. YMMV

Last edited by zettairyouiki; 9th May 2016 at 07:55 AM.
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Old 9th May 2016, 03:59 PM   #3
rbt is offline rbt  Romania
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My goal is to enhance the sound produced by a portable speaker with a very small portable sub which could make an actual difference so much that I would bother using it.

I went ahead and ordered the SPH-75/8 monacor driver because the tang would really difficult to get in Romania. Also, I got the TDA8932 amp which should produce ~15W at 16V with that speaker.

Now I need to make the box until the parts arrive. I would most likely use 8mm MDF for that.
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Old 9th May 2016, 04:52 PM   #4
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There are some fundamental physics that govern how much bass (how loud and how low) you can get from a speaker. The relationship is called "Hoffman's Iron Law" and basically it goes like this:
There are three quantities that you can trade off: bass extension, cabinet volume, and efficiency.
Efficiency is related to how loud the system can play above F3.
You can increase power to make up for low efficiency, but this is not likely possible for your battery operated system.

You have a very small cabinet. Thus, you are left to trade off bass extension and efficiency. But since you probably want to be able to hear the bass, you need some efficiency and your bass extension will be very poor. This will be compounded by the fact that with such a small cabinet you can only use small drivers and these will in general have higher Fs than a true subwoofer. As another posted mentioned, you may need to live with a F3 of 80Hz because that is all you can get from a direct radiator system of this size and modest input power.

One thing that you might have considered before you bought the drivers is whether you could use a bandpass system to increase SPL. With a bandbass (e.g. 6th order bandpass) you can create a peaking SPL (a narrow passband) with higher output compared to a wide band direct radiator type application.

Another possibility is a tuned duct e.g. a transmission line. This can work well for small drivers but might be larger in total volume than your 2L limit. On the other hand it is essentially an open stuffed tube and you can make it very lightweight and cheaply if the drivers are small. One example of this is the "Bose Wave Radio", which has two 3" or 4" drivers with a TL. I recall hearing this back in the 1990's and being amazed how much bass came out of the thing.
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Old 9th May 2016, 05:55 PM   #5
bjorno is offline bjorno  Sweden
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Hi All,

FYI:

PS: Agree with CharlieLaub..

b
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Old 9th May 2016, 06:08 PM   #6
Zero D is offline Zero D  United Kingdom
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@ bjorno

Good info & nice design, as usual

I think for this purpose relaxing the 10ms AV would be ok ! Say up to maybe 40ms or so ?
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Old 10th May 2016, 11:15 AM   #7
rbt is offline rbt  Romania
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Thanks very much for the feedback!
I've been studying in the past few hours and I am blown away how great the community is here.

@CharlieLaub, the Bose Wave Radio is really interesting. I wonder why we are not seeing more of that.

Sure, I want to use a filter. I've chosen this one because it's cheap, adjustable and tolerates 20V. When it comes we'll see if it's actually any good. It is clearly not 6th order but I'll keep an eye for a better filter.

@bjorno Your math looks really good. I appreciate it. I understand probably half of it but it's educating me.

So, for starters, I see you start from the driver sensitivity and X-max. I completely ignored X-mas and I see that it's terrible in the driver I choose. I feel I burned 20$ The driver you chose looks awesome but it would cost me ~110$ to get it to Romania so I don't think I should get it.

Perhaps I should reconsider to use Monacor SPH-100C . I'm not a Monacor fan but this is what I can get here. The thing with this driver is that I am not sure I can get away with using a small box. According to my calculations, the smallest box I could use is 3L and still wouldn't give me great specs (f3 = 80.93, fb = 87.5 and Qtc = 0.77).

Do you think this might make a world of a difference? What does the fb/f3 ratio tell me?
What other things are the most important that I should consider from those calculations?
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Old 13th May 2016, 05:58 AM   #8
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The Bose wave radio uses active filters to optimize bass response. Virtually all modern "compact stereos" or "tabletop hifis" employ this method. In an integrated system, you can optimize bass response for a non-ideal woofer, even in a cabinet that's way too small.

Perhaps you've taken some cheap speakers that sounded pretty good with the unit they came with and hooked them up to a component system. They probably didn't sound very good at all. This is why.

I've reverse engineered a few of those compact "hi-fi" systems and this is when I discovered this. They all had bass optimization filters.

I decided to experiment with this and it works very well. As an example, I have some old Design Acoustics "point source" 8" 2 ways that I got for free. The woofers were trashed. I had some woofers that matched efficiency (more powerful and more xmax too) but weren't really the best for the enclosure size. The f3 frequency was about 85 Hz, and turning up the bass control boosted midbass way too much and bottomed out the woofers too. So I built a line level bass boost 3 dB/octave + 3 dB at 85 Hz filter, which is theoretically perfectly complementary to the response of the woofer in the enclosure. I also built an 18 dB/ octave Butterworth high pass filter -3dB at 40 Hz. The woofers don't do much below 55 Hz so no sense driving them at those frequencies. I used metal film resistors and MKP capacitors in the filter circuits and cascaded the two filters. The bass response is now balanced and strong with the receiver's bass control at flat, and I can turn the bass up a little bit if I want to without wasting a lot of power overdriving my woofers.

So this shows that this concept can be applied to "classic"audio systems, which are really quite different from most modern consumer stuff. I think it will work for your mini subwoofer too. You need to do the math and find out what your f3 is in your enclosure, and look at xmax, and decide on some turnover frequencies.
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Old 13th May 2016, 12:42 PM   #9
Zero D is offline Zero D  United Kingdom
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@ Fast Eddie D

Hi, yes active EQ can work, as you've discovered. Did you mean 6db/octave for the EQ you made ? If not, can you show the circuit ?

If the f3 of the Design Acoustics was around 85Hz, & they are a closed box, the roll off should be about 12db/octave. If reflex, more like 24db/octave. So even though you made them sound more to your liking, it "seems" they are not as you thought ! I would try a 6db boost @ say 65Hz instead. The HPF is a good idea if relexed, but might not be needed if closed box.
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Old 13th May 2016, 05:21 PM   #10
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Quote:
Originally Posted by Zero D View Post
@ Fast Eddie D

Hi, yes active EQ can work, as you've discovered. Did you mean 6db/octave for the EQ you made ? If not, can you show the circuit ?

If the f3 of the Design Acoustics was around 85Hz, & they are a closed box, the roll off should be about 12db/octave. If reflex, more like 24db/octave. So even though you made them sound more to your liking, it "seems" they are not as you thought ! I would try a 6db boost @ say 65Hz instead. The HPF is a good idea if relexed, but might not be needed if closed box.
My simulation showed a gentler slope than 6 dB/octave for a closed box. You only get about a half octave extension below 85 Hz anyway. Below 60 Hz it dropped off fast.

My circuit improves bass extension down to 60 Hz and balances the overall sound of the speakers. It also attenuates the last octave of bass by 18 dB/ octave (classic Walt Jung filter) which greatly improves power handling and lowers distortion.

The whole circuit was based on the simulation and was built to correct the bass rolloff in the simulation. I did zero empirical measurements. Of course you could do a whole lot better with a microphone and measurements, but this circuit is a quantum improvement in subjective performance.

And the speakers were 100% free. I had the woofers (I replaced the surround and dust cap 3 years ago) and I got the speakers for free. The cabinets are in great shape, grill cloth and all. The tweeters are great, the crossover is decent, the cabinet is very rigid and the front baffle is felt covered. Free.
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