Many contacted me how to integrate the subwoofer with the main speakers after Marvel’s post. What are the protocols? First of all, read the specs of the main speakers for general reference. What are the crossover points? If you read the spec of all speakers, they are pretty much the same particularly on the frequency responses, especially in the lower octaves. The claims are almost universal from 40hz to 20,000hz regardless of two way monitor, floor standing, and all the way to gigantic horn. They are meaningless pretty much because one don’t know if there is adequate energy particularly below 60hz. In some cases, intense roll off behavior begins at 80hz.
That is why your ears are still the judge. Why do we need to add a subwoofer? If one only listens to a few vocal recordings and string solo, I‘d recommend to forget all the troubles. On the other hand, if your music preference is wide encompassing from rock, heavy metal to big symphonic scores, then the lowest two octaves matter very much. Also, the overall improvement brought from the subwoofer is not more attack in the upper bass, but rather on the completeness of soundstage, palpability of imaging and enrichment of ambience wherever they are. It is a 3D concept. It is NOT an arithmetic concept. For example, you can’t claim 80hz below is unimportant because our ears consume mid-range the most. If this logic holds, then composers would not add double bass, timpani and other bass instruments in the formation of an orchestra. Taking them out is not the same music composition anymore. It is true that they represent a lesser nunerical percentage of the whole frequency range. But they lay a solid foundation upon everything above. Please digest this session before reading the next paragraph.
The first step of setting up the sub in our approach is to determine the LOW-PASS point. Should we cross at 80hz, 70hz or 60hz or 50hz? The first two octaves are respectively 20hz to 40hz, and 40hz to 80hz. For me, I usually begin under 80hz because underneath this threshold subbass has no directionalty theoretically speaking.
In the case of Azzurri‘s Cessaro Wagner 2-way horn system, we began the LOW-PASS point at 65hz. Then my father asked me to play Beethoven Violin Concerto No.1. After the first 3 minutes, he told me to bring it up to 70hz. Listen again. There was some improvement on the perceived soundstage height, width and depth. Then we knew the direction was right and brought it up to 75hz. The percieved size of the orchestra was larger. It was time to swap with another recording with more bass punch and attack - the Dave Brubeck, the Take 5 track. My father decided to push it up to 80hz on this recording as he felt energy was not enough in the upper bass (energy of upper bass also derives from the subbase to a certain degree). Still, he wanted more energy and hence we elevated the low pass further to 85hz. But then resolution began to blur forcing us to bring it back to 80hz where overall resolution returned to normalcy. The low pass was set at 80hz was the conclusion. But how do we extract more energy without altering the low pass? This is the time when we began working on the subwoofer volume, which were set at default of 0 db.
The subwoofer volume setting is determined technically by the INPUT SENSITIVITY of the amplifier driving the main speakers. In the case of Azzurri, the input sensitivity of the ARNE amplifer by Engstrom is on the high side. If the input sensitivity of the amp inside the ”Xiang Xu“ subwoofer is LOWER than ARNE, then it must be the case we need to raise the input sensitivity of the amp inside the sub to match with ARNE. The subwoofer volume inside the ”Xiang Xu“ subwoofer is thus equivalent to input sensitivity adjustment.
Since we couldn‘t cross above 80hz, we began raising the subwoofer volume (input sensitivity) to reach the ear-perception of my father in this case. We ended up raising the sub volume to +4.5db. (Each increment of the sub volume is very finely divided in 0.25db step.) Then we switched back to Beethoven Violin Concerto No.1. The energy of the orchestra projected much more. Each turn of the bow on the violin radiated microscopic energy. Azzurri’s biggest complaint in the past was very narrow soundstage blaming entirely to the narrow physical living room. Now the orchestra penetrated beyond the side walls. The addition of the subwoofer proves our judgments were wrong in the past.
What is next? Then we began adjusting the HIGH PASS. It was preset at 35hz, which means the sub gnerates NOTHING below 35hz. Frankly speaking, the full wave length of such low freuquency is extremely long that ordinary household apartment could not accommodate. But still adjusting it impacts the sound. We lowered it to 30hz first. Height of the soundstage was seemingly higher with more depth. Then we brought it down gradually by 29hz, 28hz...... to 25hz, but no obvious improvements were observed. We hammered it further to 20hz, and Azzurri immediately detected resonances coming from the glass wall behind his listening position. Then we brought it back to 25hz to seal the HIGH pass location.
We were using a few more complicated music passages to confirm the settings. Before we left, Azzurri played a cut of Carol Kidd‘s famous song - ”When I dream“ for image palpability testing. We pushed the Wagner backward (not even half an inch) to make sure both speakers are aligned symmetrically. Then we also relocated the listening chair. Then we played again. That was the time when my father said, ”Excellent！“
Appendix: For the setting of LOW PASS and HIGH PASS, there are 3 types of filters to choose. Each filter has their own sound character. For the sake of simplicity, I had chosen the Bessel curve 18db in both positions. I usually prefer sharp cut at 24db to 18db because the former generates less room interaction based on my many field trip experiences. In the case of Azzurri, I picked 18db bessel for better energy transition. If we used 24db, my father felt there was a small hole leaking energy. For more advance setting, you can choose different filter curve for both parameters. Last but not least is the application of Revopod underneath the subwoofer is a must because it brings palpable silence to our senses.
The integration of ”Xiang Xu“ subwoofer with Cessaro Wagner was much easier than the case of my home because sensitivity of Goebel Epoque Reference speakers is much lower compared to the ”Xiang Xu“ subwoofer. It was a much long drawn exercise for them to dial in seamlessly. In both cases, the performance of ”Xiang Xu“ subwoofer is truly high fidelity. There are so many parameters for adjustment to accommodate all kind of situations. The disappearing act of “Xiang Xu” is magnificent.