Sub Project: Humble Beginnings

I had a lot of fun building the Overnight Sensations, so I decided to start a new DIY speaker project. This time I'll be building two subwoofers using the 15" Eminence MFW-15 driver. They will be a sealed design at about 2.8 cubic feet of internal volume. I'll be updating the blog with my progress as I work on these.

There isn't much to show right now, but I did finally get my feet wet with some modelling in Google Sketchup to test out a design I was toying around with. It turned out pretty nicely and I like the way it looks. I wanted to try something a little more special than the standard flat-black cube to set these apart since I'd be making them myself. The grey will probably end up being a dark gunmetal or something similar, though I'm contemplating veneer.

In any case, there's a long way to go before I start worrying about aesthetics, so I'll leave it at that for now.

More Fun With Crossovers

I decided to play around with the crossover frequency between my sub and the main left and right speakers today. It seemed to make a little bit more of a difference this time around. I think I may have messed something up in the process last time. The biggest changes occurred in the region from 90-100Hz. 40Hz seemed to flatten out that area the most, but also led to a little less output in the lower range and a sharper drop at about 115Hz. It looks like 60Hz is the happy medium in this case, so that's where I'll keep it for now.

How To Listen

Here's one thing that's been taking up a little of my time lately. It's a program from Harman Audio called How To Listen. It's used by Harman to train people on how to critically listen to recordings and evaluate their quality. I haven't been able to play around with it too much yet, but eventually I plan to take some time every day to complete some of the exercises.

So far, I've looked at the band ID module, and it starts out fairly easy, but as the number of bands increases it gets much more difficult. Around 5 bands, my scores started dropping pretty quickly.

To learn more about the program, you can head over to this link: How To Listen: A Course on How to Critically Evaluate the Quality of Recorded and Reproduced Sound.

To download the latest version of the program go here: How To Listen Version 2.04

Just as a side note, you'll need some fairly nice headphones for this to work well. Speakers probably won't be adequate.

Sub Placement

I got a little curious today and decided to play around with my subwoofer placement to see if I could flatten out the frequency response a little bit. Living in a dorm means there aren't too many placement options, but I tried out a nice little spot underneath one of my desks, and the results were pretty interesting. Here's an overlay of the response next to the listening position in the corner (in red) and under the desk (in blue):

The first thing to notice is that the vicious null I had in the corner is almost completely gone with the subwoofer placed under the desk. That's great news, but there are a couple of other issues to take a look at. You'll notice that the extension has decreased a bit, dropping off more steeply around 35Hz. That's probably due to the loss of the corner loading from the previous position. On the upper end, the slope has gotten a little steeper as well. I'm not entirely sure why, but that might also be a result of not having the boundary gain from the corner. I attempted to remove that issue by lowering the crossover for my left and right main speakers.

As you can see, changing the crossover had little to no effect. The green plot is the front left and right crossed over at 80Hz, while the blue is with them crossed over at 60Hz. Since it doesn't make a difference, I'm switching it back to cross over at 80Hz to remove a little bit of the load from the main speakers.

I'm not sure if I'm happy with the changes based on the data, but I'm going to give the new setup a shot before I move anything around anymore, because I'm not sure which one I'll like better subjectively. I just thought the results were interesting and decided to share.

Room EQ Wizard

Another post, and some more new toys to play with. I recently got an external USB sound card (Behringer UCA-202) and an extra long RCA cable so I could play around with some room measurements. The main reasoning behind the external sound card was cabling issues, and also to add a second output so I could drive either my desktop speakers or my main 5.1 setup with a signal from my computer. Cabling-wise, my laptop's internal card only has two 3.5mm stereo jacks (one for mic/line in and one for output), which meant that I would need an RCA adapter for each one. With the sound card there are left and right RCA plugs for both the input and output, which makes hookup a little simpler with cables that are A) more versatile and B) already in my possession.

Before I get into the measurement process I'll go ahead and give a brief answer to the question you might have: "What are you measuring, and why?" At the most basic level, this process measures sound level. However, the purpose of this exercise is not to determine how loud I can play back the Death Star explosion (louder than is comfortable, if you were wondering), but to see how accurate that sound reproduction is. Accuracy can be determined by comparing the relative sound levels of all of the frequencies from 20Hz to 20Khz (the audible range). An ideal system would output the exact same sound level for every frequency over that entire range, but due to speaker characteristics and room interactions that is not the case. The purpose of measuring the frequency response is to see what is happening with the frequency response so that equalization can be applied, or to determine if components need to be moved to achieve a more even output.

As I write this, I see that I'm beginning to stretch the definition of brevity, so I'll leave that explanation as it is and have a future post dedicated to my opinion of the characteristics of an good audio system and the challenges we're faced with in trying to obtain it.

Now, onto the measurements. For these measurements I'm using some free software called Room EQ Wizard (REW). It works by playing frequencies over a range that you choose (through your speakers) and simultaneously measuring the sound level at each frequency (through a microphone or Sound Pressure Level (SPL) meter). It then plots the sound level as a function of the frequency, giving you the combined frequency response of your speakers and the room. For these measurements I'm using a Radio Shack Analog SPL meter, which isn't very reliable above about 3KHz, so I'm just going to be measuring my subwoofer.

The first step of the process is to calibrate the sound card. To do this, I fed the output of the sound card directly back into the input and made a measurement from 0Hz to above 20Khz. I don't have the original graph, but it looked similar to this one with better extension below 10Hz.

From there, REW created a calibration file which cuts or boosts certain frequencies to achieve a flat curve. This way, the sound card has no (read: negligible) effect on the final measurements. Here's the calibrated response of my sound card:

After the sound card calibration I was ready to start measuring. I hooked up the SPL meter to the input on the sound card, and the output of the sound card to the AV receiver that drives my subwoofer.

You may notice that there are a couple of unconnected banana plugs sitting in the picture below. I disconnected the main left and right speakers so I would only get the output from the subwoofer.

Next I set up a measurement in REW to sweep from 15Hz to about 150Hz, and this is the result:

The line at 75dB is the level of the signal that was fed to the speaker, and as you can see, that's not exactly where the output ended up. I have the subwoofer crossed over at 80Hz, (that's when the left and right speakers take over) which explains the steep rolloff on the upper end. On the bottom end, I start to lose usable output around the low 30's. The speaker is rated from 24Hz to 200Hz, which means I either have a pretty brutal room mode in that area, or the published values are pretty optimistic (or both). The other large peaks and dips are all due to room modes, or resonances in the room which are a function of the room dimensions as well as speaker and listening positions. There are some pretty low dips at 40Hz and 50Hz. For those of you unfamiliar with a decibel scale, -3dB equates to about half of the original power, while -10dB is about 1/10th. In terms of perceived loudness, -10dB is about half as loud, which is a huge difference over such a small range. I'll be playing around with my subwoofer placement in the future to try to mitigate some of these modes.

Overnight Sensations Build Log

My first DIY speaker project was this pair of small bookshelf speakers designed by Paul Carmody called "Overnight Sensations." Not having much experience with this before, they were hardly done overnight. After about 20-25 hours of work spread over a month, I managed to complete the speakers to my satisfaction.

As I worked, I compiled a build log documenting all the steps I took to complete these guys. You can find the log here: Overnight Sensations Build Log.

They sound fantastic, and have some surprising output for such small speakers. They image extremely well (Almost too well. It's a little unnerving sometimes), and have an impressive bottom end. Since I had a lot of fun making these, I've decided to start work on a new custom subwoofer (or 2). I'll detail that process in future posts.

Why I'm Here

I've tried blogs before, and they've often failed. And by "failed" I don't mean that they failed to draw a massive web-following. I've never aspired to have millions of readers hanging on my every word, or even to have any readers at all. I'd just like to have a place to share some of my thoughts or projects related to a specific topic. My previous blogs failed quite simply because they were slowly (or not so slowly) subjected to ever increasing neglect on my part. Who's to say if that will happen with this one or not? Certainly not me, but I'll make an effort to consistently update the blog with my latest audio escapades.

This brings me to the actual topic of the blog: my obsession with home audio. This will be a place for me to document various home audio related projects I'm working on or fiddling around with--which could include DIY speaker building and design, equipment measurement and room response analysis, as well as equipment placement and calibration. If there's nothing currently going on project-wise I'll still try to maintain weekly updates with some links to interesting audio theory, and my take on it.

With that said, welcome to "Sounds Good To Me." If you'd like to know more about anything I post, or have any suggestions, please leave a message in the comments section.