Appendix Frequency Response Graphs
How good something sounds is the most important part of any home cinema audio or Hi-Fi setup but the fastest way to get their is to trust in a little science and measure the in-room frequency response. This involves playing frequency sweeps and recording and displaying how these sound in your seating position.
Curious as to what the AS-EQ1 is up to, I plotted several graphs with the subwoofer in two separate positions.
The 'Waterfall' graphs below show how my room sounds in my favourite seating position. The vertical axis depicts the frequency range from approximately 16hz up to just over 250hz, which safely covers the useful range of most subwoofers. The horizontal axis shows the time in milliseconds and the colour depicts the SPL or volume (pink being loudest and dark blue, the quietist) Because the 'theoretical' ideal output is a flat response with very little 'ringing', the perfect graph should be a consistent colour from top to bottom with very little 'warm' colour across the right hand side of the graph.
Caveats: I would add that these graphs were taken with domestic equipment in far from controlled conditions, they are provided for interest more than to prove scientifically one thing over another. Also bear in mind that these graphs were taken at one single fixed point in the hot-spot (my head) and Audyssey especially, aims to improve the frequency response over a much larger area so they are not telling the whole story.
No Room Correction - Subwoofer Position 1 (beside main listening position)
As you can see with no room correction this is a terrible spot with a massive room mode around 30-40hz and other smaller ones around 60hz and 90hz. The 40hz mode also rings with a lot of energy for a considerable amount of time. If you used an SPL meter and pink noise to balance your subwoofer and speakers in a room like this, the chances are you would be very disappointed, with almost no bass being delivered except around the 40hz area and even that would likely sound bloated and not very musical.
Velodyne SMS-1 Manual Room Correction - Subwoofer Position 1 (beside main listening position)
EDIT: I am not convinced this is not an eroneous reading as the 'stretched' looking reading for the first 50ms is not repeated in the later SMS-1 reading.
This position is very difficult to EQ with the SMS-1 due to the very large peaks which take a lot of cutting to bring anywhere near acceptable. This rather humped response was the best I could manage in the few minutes I had but you can see already that after 75ms or so the SMS-1 rapidly reduces the ringing effect of those nasty natural peaks. Trust me this may be far from perfect but it sounds much better than the natural response. What does seem to be occurring is the SMS-1 is adding its own ringing to the output before the 75ms drop off, although this is my supposition as I am far from qualified to explain exactly what is happening here.
Onkyo SR TX-876 onboard Audyssey MultEQ XT - Subwoofer Position 1 (beside main listening position)
This graph shows what onboard Audyssey can do on its own and delivers a much flatter response than my feeble efforts with the SMS-1 including a much deeper bottom end. The observant among you will notice that it does something I avoided when using the SMS-1, which is to boost the entire subwoofer signal by around 10db (to bring the bottom end back up) and then cut everything down to even things out. I make note of this because normally you would avoid this with most other EQ devices, as boosting a low signal asks the amplifier of the subwoofer to do a tremendous amount of work and can push subwoofers beyond their limits and cause damage. Ed Mullen from SVS has assured me that the AS-EQ1 can detect this and keep the signal within the safe limits of a given subwoofer, although I have read 'claims' of subwoofers without internal limiters being damaged by onboard MultEQ XT.
There does appear to be some ringing but it appears to tail off slightly quicker than the SMS-1 after 50ms.
SVS AS-EQ1 integrated with onboard Audyssey MultEQ XT - Subwoofer Position 1 (beside main listening position)
The AS-EQ1 plot does not look night and day better than the onboard Audyssey but it does still have a flatter response and in the majority of the frequency range, the ringing appears to be reduced earlier.
How does the AS-EQ1 perform in more favourable conditions?
Ok, let's assume you have a more benign room, is it worth spending £650 and will it make a noticeable difference? To help answer this I tried my usual subwoofer position to the right of my right hand speaker at the front of the room. All the Room Correction methods I have tried have allowed me to produce an almost flat response here without too much effort. However anyone who has spent some time doing this will know there are many combinations of filters that will give you a response that looks pretty much the same on a 2D Frequency / SPL chart but they often sound quite different.
No EQ - Subwoofer Position 2 (Right of FR speaker)
As you can see this response still has a large peak at 40hz but doesn't suffer from the null at 20hz making it a much simpler proposition to manually EQ.
Velodyne SMS-1 Manual Room Correction - Subwoofer Position 2 (Right of FR speaker)
Now the SMS-1 is in its element and an almost perfectly flat response is achievable with only a few minutes effort.
SVS AS-EQ1 integrated with onboard Audyssey MultEQ XT - Subwoofer Position 2 (Right of FR speaker)
The graph doesn't look as smooth as the SMS-1 but as I stated in my caveat these can never be directly comparable because the Audyssey EQ is applied over a large area, what is apparent though is how much the ringing is reduced with the AS-EQ1 and listening bears this out. The overall difference between the two methods is not huge but it is still there and more noticeable with music which always tends to emphasis subwoofer integration issues.