Small computer Sub for music

kman42

Member
2016-03-07 8:55 am
Hi guys,
I'm planning to build a subwoofer to complement the bookshelf speakers I have for computer audio. The speakers get a nice flat response down to about 100hz, after which they gradually taper off. I'll specify that the setup is for music ONLY, and I'm aiming for accurate reproduction of audio as this setup will see a bit of recording use, and I'm a fan of a flat response curve. Before you suggest it, I'm steering away from studio monitors due to expense and the extra equipment that I'll need. Honestly, I'm very happy with the bookshelf speakers - apart from the bass. As this is for music only, I only need bass response down to around 32Hz, with anything lower than that being a bonus.

Firstly, the amplifier I'm using is a 2.1 channel amplifier I found here:

LP168HA CAR Amplifier Audio Stereo 2 1 Hifi Channel 2X40W SUB Output Super Bass | eBay

It looks dubious, but is an excellent little unit that gives crystal clear, transparent amplification with plenty of control over the signal. It produces around 40W each channel, and around 68W in the sub channel. It includes an adjustable crossover and subwoofer output level. Honestly, it is a great little amplifier that does exactly what I need.

Anyway, I'm looking to build a sealed, passive, downfiring subwoofer with about 30 Litres internal volume. I'd really like to avoid bigger than that if possible, as it will sit under my desk (where my feet go). I'm planning to use an 8" driver - I'm considering either this one:

QTX Sound 8 Woofer With Kevlar Cone | eBay

Or this one:

8 Paper Cone Woofer | Component Speakers | Hi Fi Speakers & Accessories | Sight & Sound - Home | PRODUCTS | CW2196 | Jaycar Electronics

I'm leaning towards the QTX woofer, as it has a lower advertised frequency range and the extra power handling will make a more versatile subwoofer.

My questions are

1. Since I'll only be running maybe 30-50W through the sub, are there any drawbacks to using the QTX with a 250W RMS rating? I'm wondering if the driver mass might be an issue.

2. Can I expect a frequency response down to the low 30Hz range in a 30 Litre sealed box? Or am I too optimistic?

3. In this application, is higher sensitivity or lower sensitivity more desirable? The QTX woofer advertises 91dB @1W/1M, while the Jaycar woofer advertises 89dB @1W/1M.
 
Hi,

The QTX driver has no useful specifications to go on, and is
quite frankly likely to be very poor, going on my experience.
I've tried the 5" version, which was dirt cheap at the time,
they are low quality cheap drivers independent of price.

The Jaycar has more useful information, and its likely to
be fine in a sealed and stuffed 28L / 1 cuft box, it is very
likely the recommended 42L box refers to a vented box *.

I'd go with the Jaycar driver every time, and its cheaper.

rgds, sreten.

* A Qts of 0.33 indicates vented is the typical best option.
Unfortunately Vas of the driver is not specified. However
I'd hazard a guess 42L tuned to 33Hz is near optimum.
 
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kman42

Member
2016-03-07 8:55 am
Thanks Sreten, this is good news, as I am quite poor lol. I actually thought QTX made decent drivers, but I'll remember this.

Bentoronto, I have no way of measuring frequency response. I arrived at this conclusion through use of audio sweeps, test tones and various test songs that I have used to compare many different sound systems. This in particular is very useful:

Online Tone Generator - Generate free low frequency tones to test your subwoofer.

It might seem like a flawed method but I am a musician with a good ear and I can tell you that output drops off steadily below 110-120Hz.
 
It might seem like a flawed method but I am a musician with a good ear and I can tell you that output drops off steadily below 110-120Hz.

Right. But if you ever do entertain the possibility, you can be into measurement for $40. Of course as a musician, you must have access to all the great mics you need for free.

About my discovery of REW honorware software, I think of the line for the bass from Handel's Messiah, "they that dwelleth in darkness hath seen a great light...."

Ben
 
I actually thought QTX made decent drivers, but I'll remember this.

No clue about quality or its published specs accuracy, but based on the specs I just posted: per Hornresp it's only ~85.2 dB efficient and near enough a ~0.73 Qtc, so a bit stuffed would make it a max flat 0.707 with a ~48.8 Hz Fb, making the low 30s a good 8 dB down, all ignoring any room gain, etc., so the cab could conceivably be too small, requiring EQ to flatten it out.

GM
 
Hi All,

FYI:
Surprise:, :cool: The T/S Parameters of the QTX Driver are trustworthy:

b:)
 

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kman42

Member
2016-03-07 8:55 am
Hmm, thats annoying. What size sealed enclosure do you suggest would get me decent response in the mid to low 30s?

I've also been toying with the idea of a vented box. a 30L box with a 20cm long, 5cm diameter (with flares) port should tune the sub pretty well right on 30Hz... but I'm not sure how that box would behave with the drivers I'm considering.