Help with folded bass horn in a small box (instrument)

Unseen

Member
2016-02-26 9:42 pm
Greetings von australien!
Im making a cajon with a folded horn...the membrane is of course low excursion but high compression so it should work. Have attached a quick sketch...predominant fundamental is usually 100-110hz but therer is also a lower harmonic around 80. So im aimimg at 65-70hz as target sweet spot. Any help would be amazing.
Cheers!
 

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just a guy

Member
2006-05-12 6:59 pm
Help with what? This can be pretty easily simulated with a variety of software programs. But there's no dimensions and no driver in the picture so it could be either an offset driver front loaded horn or a tapped horn.

What is in the picture is your name and full mailing address (or the name and address of someone who sent you something). Not sure if you care but it's easily legible.
 

Unseen

Member
2016-02-26 9:42 pm
Thanks for the reply...hmm...and the heads up on the pic...not my details but...will sort that.
The cajon dims are there in mm. That's kind of the point in asking for help...it doesn't use a driver but the actual laying surface which is a rectangular membrane. So in essence its all acoustic. I just wanted to see, if anyone could critique the application as a pre project check ie any major reason why not...and using an experienced designer vocab as I'm just not across those details. Cheers
 
I think it will work but I would recommend the throat not faced down. I am a drummer and familiar with the cajon, but honestly a pzm mic or transducer with amplification is the way to go unless it is out of the question. It would be neat to fool with a horn in a cajon, after all, the cajon is the perfect diy instrument.
 

turk 182

Member
2012-10-26 3:03 pm
being that the principle of the cajon is relying on the resonant properties of a box i would like to compare a sealed version to the few i've seen with what appers to be a bass reflex port.
i could be wrong but i think the sealed version would have more overall level (or the BR types may be better in terms of depth i.e. lower note for box size)
horn loading one i think would be a challenge.
if one thinks of the "striking area" as the loudspeaker in a horn and creates a resonant chamber that is then horn loaded perhaps something useful could be achieved.
 
Greetings von australien!
Im making a cajon with a folded horn...the membrane is of course low excursion but high compression so it should work. Have attached a quick sketch...predominant fundamental is usually 100-110hz but therer is also a lower harmonic around 80. So im aimimg at 65-70hz as target sweet spot. Any help would be amazing.
Cheers!
Your sketch looks basically OK. For a 65 Hz target the horn path length should be around 4.3 feet (1.34 meters) long.

I'd suggest building the box out of stiff plywood, around 12mm or 1/2", but use thin wood for the "membrane"(playing face). Old cabinet drawers are a good source for that type of wood, 1/8" and 1/4" Luan or Mahogony being fairly common.

The many horn bends will reduce the "snap" of the snares, but should kick up the low end over a standard cajon if you make it a greater volume. If it is the same size, a regular cajon would likely outperform it in all respects. To increase the volume, the box could be made wider and deeper than usual so you can still sit on it comfortably. For ease of micing and better top end a rear horn exit would be preferable to the down firing horn.

The sound of a cajon can be captured well with one mic in the port pointed at the snares, with a "tapped horn" cajon you may want as many as three- front "head", inner "snares", and horn mouth exit.

Art
 
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Unseen

Member
2016-02-26 9:42 pm
Thanks for the inf.
@Zwiller yeah I agree...in the studio an R88 ribbon is brilliant if close enough but I was thinking a boundary type half cardioid for live. Also heard an amazing transducer on a tongue drum on the K&K site...almost like you are inside the wood.
I was thinking down firing as I have a sub that is front ported but the driver is down firing and actually works nicely...I calibrated with and RTA and its splits at 90hz with some atcs
So the larger box sounds critical...after mucking around a bit...I have settled for 77hz. I looked at a reflex accessory (cajonport)...the demos were all over the place, mic wise but there was one example that nicely rounded out the bass and separated the tones quite a bit.
One other question...in a djembe, there is a compression chamber/cone/throat/reverse cone that seems to do the job...not hifi but a bandpass of some sort that really thumps the fundamental...can anyone explain the mechanics/physics of it?
Cheers
 
So the larger box sounds critical...after mucking around a bit...I have settled for 77hz. I looked at a reflex accessory (cajonport)...the demos were all over the place, mic wise but there was one example that nicely rounded out the bass and separated the tones quite a bit.
One other question...in a djembe, there is a compression chamber/cone/throat/reverse cone that seems to do the job...not hifi but a bandpass of some sort that really thumps the fundamental...can anyone explain the mechanics/physics of it?
Cheers
The djembe and the cajon both use the same principle as a bass reflex cabinet (or a guitar, or bass) the Helmholtz resonance of the enclosed volume and port.

https://en.wikipedia.org/wiki/Helmholtz_resonance

The djembe uses a longer duct on the port, the cajon, stringed instruments (and early bass reflex cabinets) generally use only the thickness of the wood as the port length.

A larger enclosed volume (box) has a lower (and louder) resonance than a smaller box.
The larger the port, the higher the tuning. The longer the port duct is, the lower the tuning.

Ports that are shallow (like the cajon) get "blown out" at higher volume, so the sound becomes "thinner" the harder you play. The djembe, having a much larger slug of air in the port, does not sound as thin when played hard.

A horn is an acoustical impedance matching device, effectively turning a small radiating surface into a large one, a "bigger paddle". To go low requires a long horn and a big mouth, and to be louder than a Helmholtz resonator it must be larger.

To support a 77 Hz wavelength, the horn must be 1/4 wavelength long.
Speed of sound is 1130 feet per second, 1130/77 = 14.67 feet. 14.67/4 =3.67 feet (44 inches, 1.129 meters.

The manufacturer "North" made "horn loaded" drums, though they were too short to reinforce the drum head fundamental much, other than on the smaller toms. They were loud, but being made of plastic, they did not have a very harmonically rich sound, the last I recall micing up was about 35 years ago.

Art
 
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Unseen

Member
2016-02-26 9:42 pm
The djembe and the cajon both use the same principle as a bass reflex cabinet (or a guitar, or bass) the Helmholtz resonance of the enclosed volume and port.

https://en.wikipedia.org/wiki/Helmholtz_resonance

The djembe uses a longer duct on the port, the cajon, stringed instruments (and early bass reflex cabinets) generally use only the thickness of the wood as the port length.

A larger enclosed volume (box) has a lower (and louder) resonance than a smaller box.
The larger the port, the higher the tuning. The longer the port duct is, the lower the tuning.

Ports that are shallow (like the cajon) get "blown out" at higher volume, so the sound becomes "thinner" the harder you play. The djembe, having a much larger slug of air in the port, does not sound as thin when played hard.

A horn is an acoustical impedance matching device, effectively turning a small radiating surface into a large one, a "bigger paddle". To go low requires a long horn and a big mouth, and to be louder than a Helmholtz resonator it must be larger.

To support a 77 Hz wavelength, the horn must be 1/4 wavelength long.
Speed of sound is 1130 feet per second, 1130/77 = 14.67 feet. 14.67/4 =3.67 feet (44 inches, 1.129 meters.

The manufacturer "North" made "horn loaded" drums, though they were too short to reinforce the drum head fundamental much, other than on the smaller toms. They were loud, but being made of plastic, they did not have a very harmonically rich sound, the last I recall micing up was about 35 years ago.

Art

Thanks Art...that all makes sense but what about the effect of the horn shape ie expansion?
 
Prob is what parameters do you enter for driver?? :)
You could use the basic parameters of a stiff, heavy cone driver like the B&C 18SW115, Vas should be reduced, Mms and Fs increased to get a basic "feel" for what various expansion rates and types do to the response.

Obviously, a piece of wood won't have the same response as a loudspeaker, but you may get an idea of what type of horn will give the response you are looking for.