The mobile subwoofer sideshow...

I've been pootling along in my van a lot recently, usually on the way to a beach to indulge in some windsurfing and possibly subsequent fish and chips, and enjoying listening to my iPod on Shuffle - what an amazing collection of music!

However, the stereo is currently limited to two small Infinity "digital ready" speakers mounted in the dash. As usual, "digital ready" means raucously shouty and harsh in the top end, whilst being little and dash-mounted, the bottom end is non-existent. This means I have to have them far too loud for any useful effect. So...

I realised that I have two Rear channels on the stereo not currently in use. So let's put them to use! Rather than an actual subwoofer, how about something that adds a bit to the stereo effect whilst supporting much better bass, albeit not necessarily too thumps? Make sure that the boxes involved are sufficiently small, and they can go under the seats in the van, for simplicity of re-use and effectiveness of bottom end. No need for tweeters, I'm not looking for full-range, just something to fill out the bottom and mid a bit.

A fair bit of research at Falcon Acoustics and Wilmslow Audio websites, lots of runs of ISD Online (since WinISD is necessarily only available for Windows!), and some more searching, usefully including the hifitest.de website with a build using the selected mid/bass unit, and I'm trying out something self-brewed.

The Monacor SPH-5M appears to offer a reasonably priced woofer-ish object that can be used in a sealed box or a ported box to taste, with reasonable power handling and sensible box sizes i.e. sealed 6 litres, ported 10-20 litres, so comfortably sized for stashing behind the seats in the van. There are others available that might do better, but hey... I can always construct a pair of bookshelf speakers with them if I don't like it.

And I even got them cheaper than expected at Audiomate.co.uk, being delivered tomorrow. All I have to do is source some MDF (hifitest used 16mm MDF, but my choices are 12 or 18 - too light or too heavy!), some bass port tubes (probably Wickes finest black plastic drainpipe, 40mm), and some terminals (I suspect whatever I can find cheapest, possibly at the Shop on the Bridge). I already have wire. Hurray. And I can dick about measuring them, maybe even build both sealed and ported versions to see which I prefer!

Setting up the speakers in active mode

Having (almost) got myself an active speaker setup (need to check out the effectiveness of my DPDT switch for passive/active switching, suspect inductance still across woofer :-( ), now it's time to set the whole thing up.

I'm using the approach suggested by this blog post, which is basically

• Measure the chassis speakers separately in the box
• Easy to do this now, since I can easily turn one or the other off
• For each speaker
• Adjust response using speaker-specific equalisation, noting carefully the off-axis response and keeping this as smoothly degrading as possible
• Set up the crossover, and measure both together
• Some additional effort is required to ensure they are integrated properly
• Make timing adjustments to ensure the impulse response is the same on both units

Here's my first pass at a setting for the tweeter - I started with this because the tweeter is definitely not affected by any remaining passive crossover components. I shall move the testing outside to reduce reflections, and probably re-do it, but it's illustrative and good practice (as is "I need to practise doing this").

REW plots for tweeter response before/after

You can see that there is a pretty stonking (5dB!) rise from 6k to 10k, which I've EQ'd out. I've also flattened out the bottom end a bit - the blogger mentioned above reckons you need to be pretty flat to 2 octaves above/below crossover frequency, but this tweeter isn't going to do that! Hey, take what you can...

Tweeter on and off-axis response after EQ

Looking at the off-axis response, it looks ok to me. It's quite surprising how the top end holds up.

EQ settings

I've been using AULab to do this, because it's simple (hah!! relatively...) and has graphical interfaces. I tried several EQ tools - High Shelf filter, Parametric, but Graphic EQ provided the most flexibility. I can't believe how easy it is to tweak it and re-test - just imagine how horrible it would be with any kind of passive arrangement! Bleuch.

However, the last measurement I made produced extremely distorted sound, so I saved the AULab document, presumably with all in/out and plug-in settings, and restarted it. However, on reloading the document, I discover that AULab is insisting on Soundflower or Saffire in/out, rather than Soundflower in/Saffire out - to get that I would have to create a new document, which means recreating all the Graphic EQ settings. Sigh. Oh for a text config file and some CLI! The AULab config file is actually XML, but it's pretty impenetrable, and of course, it presumably reflects the same in/out arrangement. Maybe I should raise a bug with Apple?

Right, time to move outside, do some more measurement.

Routing Audio on a Mac - :-) or maybe :-( - no, :-) !

I am using Apple's AULab to implement my first pass at crossovers for the A100s:

• Define an AULab document with one stereo input, two stereo outputs each 
• Input so far has been AUGenerator, which plays sound files; great as a first pass
• Outputs are set to my Focusrite Safire, channels 1-2 (woofer) and 3-4 (tweeter)
• Configure an AUCrossover on each of the outputs
• HiPass for the tweeter
• LoPass for the woofer
• Wire Safire
• Channel 1 to amp LH channel (woofer)
• Channel 3 to amp RH channel (tweeter)
• Play audio file to check configuration

AULab Document for 2-Way Crossover, Soundflower or MP3 input

AU Crossovers (currently Butterworth!)

This seems to work ok, as far as I can tell there are different signals coming out of the appropriate inputs.

However, on closer listening, ALL inputs are coming out of ALL outputs - not good if I'm looking for two separate sets of signals, especially for the tweeter!

How to fix this??

<<<<<<<< Long pause (many days!!) whilst I calm down, amongst other things >>>>>>>>

OK. I've located the Saffire Control application, and looked at that. I have to click on the "soundcard" button, which makes the Saffire behave just like a simple 8-way soundcard. Hurray! The button is down on the right hand side of the panel below.

Setting Soundcard mode in the Saffire Control panel

Now to get the REW software to route its testing signals to the AULab.

Lots of struggling with this... Hours, in fact! Until I notice that there is a choice in the AULab software for configuring the input, which has as its last two channels Soundflower 1&2... Sorted. Set REW output to Soundflower 1, job done. The other channels are from the Saffire inputs.

Selecting Soundflower as input in AULab

Next problem - got a great setup, lots of good equalised curves for tweeter, but some distortion on last couple of runs. Saved AULab document, exited AULab, restarted, reloaded doc -  and now it's lost the Saffire output!! Everything is via Soundflower - suboptimal. If I make the default Saffire, then I can't set the input to Soundflower. Bah!!