Room modes

  • Big BenD Bass Horn: Installation

    Big BenD Bass Horn: Installation

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    Finally, the last article about the Big BendD bass horn! This part will cover the installation and setup of the horn, with some comments on the subjective performance at the end.


    Although the DIY 12" driver showed the most promise during outdoor testing, I still wanted to try the Altec 515-8G when setting up the horn. It was mounted in the rear chamber, the chamber was filled with wool, and the rear wall covered with pieces of an acoustic celing tile. 


    Then came the process of carrying all the parts into the living room, setting it up, adding gaskets, bolting it all together, installing the drivers, wiring it up and put the rest of the system back together. In the process I had help from my good friend Harry. This is really a two-person job, because carrying the big parts into the house isn't easy to do by oneself. The parts were all designed to fit through a standard door, but they are still a bit difficult to move around. 





    With two people working, the setup was done in a couple of hours. There are a lot of bolts, about 70 per horn, so or ratchet spanners got a real workout. 

    The complete setup is shown below. The white middle/throat sections blend in with the walls, making the bass horn less dominating in the room. It does work, some people have not recognised it as part of the bass horns.


    The First Test: Where's the Bass?

    Just setting up the horn and putting some music on, without any crossover or EQ, left us wondering: Where's the bass? Isn't this supposed to be a bass horn? There's a lot of lower midrange, but the sound was a bit thin. 

    Altec95dBInRoomThe measurements made it obvious what the problem was: the output fell below 70Hz, and apart from a peak at 50Hz, the response was more like a midbass horn than a 30Hz bass horn. What was going on? 

    We swapped the Altec for the DIY 12", and that helped significantly; the better impedance match between the horn and driver was definitely an issue in the presence of room modes. Which turned out to be the real problem.

    Room Modes

     My living room is about 7m long and 3.5-4m wide. This means that the second mode in the length direction and the first mode in the width direction are both at about 50Hz, which is also clear from the measurement above. Between these two modes, energy transfer in the room is quite limited. A wider room would definitely be an improvement, but you have to work with what you've got. 

    In the modal region in a room, the response can vary quite a lot from position to position. Some listening positions have a smoother response, while others place the listener at peaks or nulls of modes. To find the best starting point for EQing the response, I measured the response in the room at a 2.5 by 3m grid, every 0.5m. The results for all positions are shown below. 


    It turned out that the response was quite smooth about 3m from the horns mouths, somewhat behind the middle of the room. The responses at this line are shown below. Annoyingly, the response drops quite sharply below 45Hz, due to the lack of modes between 25Hz and 50Hz, but apart from rebuilding the room (which I'm sure the landlord wouldn't like), there's not much to be done. 


    I have applied some EQ to lift the response a bit, but one should be very careful with EQing dips caused by room modes. And I'm not going to spend the rest of my life in this house, so hopefully my nest home will have a more beneficial distribution of modes int eh listening room!

    Listening Tests

    So, after all this work, how does it sound? The lack of output below 45Hz is only noticable on music where you know there should be something down there. Apart from that, the response in the listening position is quite smooth, and it has the traits of bass horns that I have been missing for so long: proper impact - even at low volumes -, and responsive, detailed and tight bass. Low frequency details in the recordings are quite easy to hear, and there's no overhang or resonance. It sounds effortless even at very high volumes too. The horn integrates well with the fast and detailed midrange of the Axi2050, making it a good combination. I also tried a delay-derived subtractive crossover, as described in the Horn Book, and it made a worthwile improvement to the coherence and naturalness in the lower midrange.

    All in all, I'm very satisfied with the performance. 

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