Bass horn: sleeping on the job

[Mr Evil]

I'll be building a subwwofer at some point in the future. Not for a while yet, but I like to have a design sorted out long in advance. I want to build something unusual, with the priority being on low distortion and wide bandwidth, and it must use one or both of the two woofers I already have (very nice 12" drivers from the unfortunately now defunct McKenzie).

Initially I experimented with motional feedback on a normal sealed sub to achieve this, but I didn't like the way the complexity kept increasing as the system approached a worthwhile improvement. The next choice is a horn. I've not designed a horn of any type before, but the theory doesn't seem too hard to grasp and Hornresp makes it easy to see what might happen!

Anyway, the obvious problem with bass horns is the huge size. This problem can be overcome if the space taken up by the horn is space that would have been taken up by something else anyway, if the horn can somehow be dual function. Luckily, it just so happens that in the room where the sub will be is a bed. Thus the horn must be a bed...

The two woofers have Qt=0.21 and Fs=30Hz, which I gather is fitting for horn loading. They will be connected in parallel, both electrically and acoustically. I roughly calculated the horn parameters and then plugged them into Hornresp and tweaked until the response was reasonably flat and sensitivity still high at 20Hz. It fits well into the size of the bed with some folding, assuming corner loading.

I have also modelled construction using Unreal Ed (UEd > real CAD) to ensure it is physically practical to construct. Overall dimensions are 1.0*2.2*0.5 metres.

Since I lack experience with horns, any comments are most welcome. In particular, I was wondering if the narrowness of the throat chamber would be a problem. In order to keep the volume fairly low (Hornresp says nasty bumps happen if it's too large) it is only ~10mm deep (a fair amount of volume is taken up by the baffle cutouts and cone).

Following are some attachments. First the parameters entered into Hornresp.

[Mr Evil]

Next the predicted frequency response. It is reasonably flat from 40Hz-250Hz, and still at 100dB@2.83V@20Hz. Room gain will probably lift that up enough to be flat to 20Hz, or a little EQ may be in order (predicted excursion reaches a maximum at 20Hz, but I don't listen too loud and the efficiency is high, so I think it would be ok).

[Mr Evil]

Next the UEd model. The top is removed to show the interior. The red line indicates the path of airflow. The last section is split in half to provide mechanical support at that end.

[Mr Evil]

And another pic of the model from the mouth end. I think I may have to cover it to prevent curious cats from entering and getting stuck...

[Mr Evil]

This is the final pic. I plan to place it directly onto the carpet, without a panel underneath so that the carpet will help damp the standing waves that will occur between the parallel top and bottom. Do you think this can be done without leakage being too much of a problem? It's a short-pile carpet and the thing will be very heavy, so it should sit very firrmly, but Im not sure.

[BillFitzmaurice]

A few comments. Lose the second driver; it only gives you 3dB extra, and if the horn needs that 3dB then the design is botched anyway. Flat to 40 is fine, unless your room is 50 feet long. There won't be any standing waves between the top and bottom, the distance is too short compared to the horn passband. The dimension of the throat chamber doesn't matter much for the flow of low frequencies, but keep it as small as possible, as a 30 Hz fs is rather low for a 20 Hz horn, and the larger the front chamber the more it acts as a HP filter that pushes the box frequency down. Same for the rear chamber with a horn as large as this one.

[Mr Evil]

Is there any disadvantage to using two drivers? Since I already have a pair and don't have any other use for them, I would be inclined to just stick them both in even if it doesn't have any significant advantage, as long as there is no particular disadvantage. Hornresp does suggest that the frequency response will be smoother with two though.

[Centauri]

As Bill mentioned, the standing waves won't be a problem, and the rear chambers should be sealed well - so a base plate at least in this area may be in order. Compression ratio is very high, but if the drivers have a stiff cone and not too much power is fed into it, may be ok.

The chambers in front of the drivers seem to be a bit narrow compared to the width at the start of the horn proper - I have some reservations as to whether firing the drivers straight into a solid wall this close will cause problems. I would be more inclined to continue the throat section of the horn right to the back of the box (bed head) and position the drivers on each side facing each other ala LabHorn style.

I like the idea of making use of usually wasted space like this - could have an interesting effect if fed with low frequency sine wave whilst bed is used for alternate functions......

Cheers
Graeme

[johninCR]

I think your real problem is going to be directivity, especially going up to 250hz. You have plenty of volume under the bed to make it 2 horns with the mouths on one side. You'd give up some efficiency, but it sounds like you don't really need it. It would also give you flexibility later if you build them as separate units, and currently to run them in stereo. Loading the room more evenly with 2 is likely to help with the directivity issue. You can also decrease some of the vibration by having the woofers work in opposing directions and net that energy to 0.

Regarding the cat, 100 watts at 40hz will resolve that issue once and for all.

[Joules]

Is there any disadvantage to using two drivers? Since I already have a pair and don't have any other use for them, I would be inclined to just stick them both in even if it doesn't have any significant advantage, as long as there is no particular disadvantage. Hornresp does suggest that the frequency response will be smoother with two though.

Buy a twin bed and make a stereo pair