Cheap-ass sub shakedown

This is a Dayton SUB-120, a dirt-cheap, value for your money subwoofer.

It’s not a product you buy to show off, but to save $$$. The scratches on mine witness a lot of use. See how I have a rolled-up sock sticking out of the port in back? That was recommended online by other users, and how I’ve used it for years. I’ve always been curious how the sock changed things, and decided to shake one down with proper measurement.

I like having a bunch of subwoofers to minimize room modes and EQ changes when walking around. I have three of these, from once upon a time when there was a good deal. (I actually bought four, and gave one to a friend.) All have performed similarly to my ear.

Inside

It’s a simple device, a reinforced particle board box with a plate amp on the back, a big driver, and two unflared cardboard port tubes. The tubes extend most of the way through the body, and the mass of air they hold makes the box a Helmholtz resonator: an oscillator tuned to exaggerate low frequencies. Blocking one or both ports changes the frequency response.

Below you can see it opened up. The back plate amp is very heavy (with a substantial heat sink) and securely mounted with fourteen screws. The electronics are well secured and padded. The driver is generic and unremarkable. The walls are adequately braced. There is some fiber glued to the walls inside, which is standard for dropping speaker frequency response, presumably by reducing the speed of sound, making the cabinet effectively larger.

The amp itself has uncalibrated GAIN (volume) and FREQ (crossover frequency) knobs. The terminals are super-cheap spring loaded.

The high-level outputs are connected to the inputs through a capacitor. This is a cheap way to get a high pass filter, but has nothing to recommend it electronically, and the filter frequency depends on the output impedance. Note that whatever the FREQ knob does isn’t setting a proper crossover point, because the pass-through isn’t affected by it. These controls are toys, and real EQ will need to be handled upstream.

On the plus side, the auto-off seems to work as one would hope. There’s a inrush thump when turning on the device, but not when it turns itself on.

Measurements

(I used an ECM-8000 placed at one foot, on-axis, and a Tascam M-164UF for ADC to a PC running REW. Tests were conducted in my untreated garage with minimum movement between measurements so room coloration is comparable.)

I have often noticed that these subs have a very nonlinear volume response with the GAIN knob, so I started by looking at that. Shouldn’t volume change smoothly with knob position? Apparently that’s too much to ask:

sub120gain

The top five lines are all equal-angle turns of the GAIN knob, and should be equally spaced on the y axis. This lurching response is pathetic. I find the frequency shift of the dip around 28hz troubling. Fortunately, in practice this knob can be usually be set once and left alone.

Moving on to the FREQ knob,

There are a lot of wiggles here, but it isn’t necessarily the sub’s fault. To neutralize the room coloration here I’ve divided the responses by the curve with FREQ set at max (all the way clockwise):

sub120freqnormalized

It appears to be a -12db/octave low pass filter (but this is a different slope than the built-in 6db/octave high pass on the outputs!). The slope of the fall-off varies with frequency, and like the GAIN knob the FREQ knob lurches between 10 and 11 o’clock. Yuck. Another knob to be tolerated by setting once and leaving alone.

Mods

I tried blocking one and then both ports, with and without extra polyfill stuffing. In theory blocking ports converts it to a sealed box, bringing the resonant frequency of the enclosure up, while possibly making it quieter at lower frequencies.  More fill should bring the resonant frequency down.  In practice one can never tell what will happen. We are detuning a (hopefully) carefully tuned system, which might be bad.

A tennis ball is a little small to temporarily block a port, but these nerf balls are perfect.

Here are the results, absolute and then normalized to the vanilla state with factory filling and both ports open:

There is nothing here to recommend blocking a single port with no stuffing, the mode I’ve been using. So much for trusting online user reviews.

Blocking both ports or stuffing produce similar curves. As expected, the sensitivity at the lower end is reduced, but not badly (-5db). In exchange the response smooths out a bit. In particular, the troubling dip at around 28hz vanishes.

Whether stuffing or blocking ports is the right thing depends on all kinds of things, not least being the room one is in. This particular sub I chose to leave stuffed and block both ports. I’m interested in musicality and not high volume, and use an external DSP equalizer to flatten the curve. This makes the most important constraint the least sensitive frequency. The dip at 28hz bothers me more than the roll-off at 20hz, because I can hear 28hz more than 20hz. I also didn’t like that measurements suggested that dip frequency varying with gain.

Here you can see it filled and then closed up again. Although this is a pretty trashed exterior, even my low standards couldn’t stomach leaving Nerf balls sticking out the back, so I glued some wooden squares primed black over the ports.

Based on these numbers I also went ahead and closed the ports on the two other subs I have. I didn’t bother to stuff them.

Conclusion

If I had to pick one word for this sub, it would be: adequate. It is solidly built and reproduces low frequencies at moderate volumes, but also looks and measures cheap. The built-in crossovers are not to be trusted. For musicality they really need upstream DSP to correct the response (which all subs need anyway to remove room color).

I’m a cheapskate and am going to continue using them – in fact, I kind of enjoy the duct-tape, anti-audiophile aesthetic. If you have a favorite cheap sub, let me know.

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