Design Study --- Front-Horn Loaded Reflex Cabinet

Design Study --- Front-Horn Loaded Reflex Cabinet

In this blog post I look at the front-loaded bass reflex cabinet. This design features a shallow front horn mounted into a bass reflex cabinet. The idea is to provide bandwidth coverage from around 45Hz to 800Hz. To summarize the advantages of this particular design:

  • Eliminates the need for a separate bass cabinet 
  • Physically time aligns the bass driver with the midrange 
  • Provides pattern control down to the 250Hz region 

If the design is optimized correctly then the entire system can sum to a flat frequency response. The shallow front horn provides a moderate sensitivity gain of +6dB from around 100Hz and up. This contrasts with a fully dedicated front horn which targets +9dB sensitivity gain. Below 100Hz there is a shallow falling response that helps sum with the port output. Again, a fully dedicated front horn would fall sharply below cutoff. Also, the design goal is to achieve a flat response without requiring corner placement. 

This design can be simulated in Hornresp by using an exponential front horn with a mouth area of 6400 square centimeters and a depth of 30cm, with a throat area of 530 square centimeters. I've chosen the Acoustic Elegance TD15M for the simulation.

The resulting output for horn only is shown below. 

Below is the reflex vent output (black) superimposed over the horn's output (grey).

If we sum the two responses together (horn + vents) we get the following response (black). I've shown the vent output (grey) for reference. 

The internal volume is optimized at 137 Liters and I'm using 1pii space (rear wall boundary reinforcement). 

If the cabinet is placed in the corner of the room we get the resulting response graph below. Bass output jumps +5dB but we don't see improved bass extension. So overall the system is flexible for room placement. 

 

Past Examples 

This approach has found use in the past with products from JBL, Altec, and TAD. Perhaps the most commonly known is the Altec 817A shown below. It measures 95cm wide x 85cm tall x 68cm deep and utilizes two 515-8G woofers.

Altec published the response shown below. We see where the horn provides the increase in sensitivity starting at around 125Hz. There is a shelved down bass response extending to around 60Hz. 

However the above response is anechoic and does not take into account boundary reinforcement of a typical listening room. To highlight this I've provided the simulation results of my design in open space (anechoic). As you can see my response closely matches the response of the 817A. 

The 817A shows a tuning frequency of about 52Hz. My design has the tuning at 40Hz as shown below. 

Another popular cabinet is the JBL 4550A bass cabinet shown below. It is about 50% wider at 152cm and about 28% deeper than the 817A 

However the larger horn seems to reach down to 100Hz versus 125Hz of the 817A as you can see from the published data. 

TAD Exclusive 2301 

 

The TAD Exclusive 2301 uses the EW-302 cabinet which features the front-horn reflex design. It's the smallest of the three and measures only 63cm wide x 75cm tall x 48cm deep. TAD claims bass extension down to 50Hz and a sensitivity of 97dB. We don't get published test data on this system unfortunately. 

But wait...there's more. 

G.I.P. laboratory is a custom builder out of japan and they mainly use the bass reflex front horn design in most of their products. The 9700 system uses the 9320 cabinet measuring 124cm wide.

Additionally they use an “Acoustic Drape" louver system to allow a controlled amount of rear wave to exit the cabinet.

This is an interesting paradigm shift and essentially makes the cabinet a quasi open baffle. It is possible that this takes advantage of the benefits of open baffle which is reduced output to the sides of the cabinet which minimizes harmful room modes in the bass frequencies. G.I.P. states that the louvers help to adjust the bass output for the specific room conditions or "self favorite". 

Coming up with a design.

Taking the Hornresp conditions and designing a cabinet took some thought. The enclosure volume is quite small compared to the horn size and so it required a trapezoidal cabinet shape. I didn't want the vents to interrupt the horizontal horn flare geometry and so I placed them top and bottom similar to the TAD EW-302. This to me would help distribute the sound a bit and reduce room modes. I also decided to keep the horn flare as smooth as possible around to the sides of the cabinet which always helps. 

I decided to introduce the louver system since I am very curious on how they may affect the performance. If the system doesn't work then it can be replaced with a solid cover.

The design accommodates full CNC manufacture from hardwood as shown below. This provides a more suitable appearance for home use. 

The side ribs are fully CNC machined from one block of hardwood. The louvers are not shown in this model. 

Conclusion

The design is at the point where I can now build a prototype for evaluation. There is a strong history for this design configuration. I've spoken with others in the industry about the topic, however I would love to hear from you if you have insight or other ideas before I start cutting wood. So please feel free to reach out to me at joseph_crowe@josephcrowe.com. For reference this cabinet is design No.1798. 

 

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2 comments

A much better alignment than the Altec A7 and A5, congratulations! As for the rear louvers, it would be fun to aim for a cardioid polar pattern, which would give directivity control down to low frequencies.

Lynn Olson

If the rear vents were in an extended ‘tube’, that portion would resemble a u- frame open baffle (which as I recall might have a cardioid type response?). Could you tune this sort-of-uframe to reinforce the same frequency as the ports?

Peter

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