The rear cover is fully sealed. I did not evaluate the cloth louvers. I plan to test this at a future date.
The design seeks to achieve the following goals:
- 100dB + sensitivity at 1w
- Bass extension below 40Hz
- Physical time alignment with midrange driver
- Pattern control down to 180Hz
I focused on making the cabinet construction as straightforward as possible without compromising sound quality. The build went relatively well with no issues during construction.
I changed the design compare to the previous blog post by increasing the number of straight sections in the horn from two sections to three sections. This smooths out the horn flare geometry even further towards the ideal ES curve.
The driver of choice for this specific project is a made to order unit, the TAD TL-1601B from Japan. The woofer has a good set of parameters which makes it ideal for the project.
The cabinet is also relatively easy to maneuver. I can lay the cabinet face down with my own strength. Two people can carry the cabinet quite easily and the it fits through door ways with ease since it is only 55cm
Tests and Measurement
With the final cabinets built and drivers mounted I was able to get some measurements at my brother's place.
I positioned the speaker away from the room corner as shown in the picture below. The ES-290 Biradial sits on top awaiting a compression driver.
I began by conducting an impedance sweep to observe the resulting tuning frequency of the cabinet.
As you can see from above the tuning frequency is 33Hz as predicted by my simulation software.
The next step was to measure the frequency response. Since it's winter here in Canada with 30cm of snow outside, the only option at this time is to measure indoors.
I set up a 100Hz test tone and calibrated the sensitivity to the standard 2.83V which represents one watt of input power. This establishes the speaker's sensitivity. The measured speaker sensitivity for in-room is 101.5dB.
With the measurement microphone placed 1m from the front of the speaker, the resulting raw in-room frequency response for speaker No.1798 is shown below.
We see that we get an F3 of 33Hz. There are still room reflections occurring in the response even after I've applied 1/3 octave smoothing. However we are still getting a very linear response across it's bandwidth.
If we apply even more smoothing to the repsonse we can see the general trend and the effect of the front horn becomes a little more clear.
For the next stage of testing I decided to install a 4mH inductor to flatten the rise from the front horn.
The resulting effect of the inductor is shown below overlaid against the raw response.
I continue to use the inductor in place for the duration of the distortion measurements which follow.
I begin by conducting a basic harmonic distortion test at 85dB SPL at 1m. The microphone is 20cm from the horn mouth.
H2 is very low at 100Hz at only 0.048%.
Showing the same result as above however with the vertical scale changed to dB is shown below. H2 is -65dB at 100Hz.
Increasing the test SPL to 95dB is shown below. H2 remains low at only 0.24% at 100Hz.
Showing the same however in dB scale is shown below. H2 is still only at -52dB for 100Hz. Good by any standard.
Looking at intermodulation distortion we see that it remains low at -60dB at 100Hz using an 85dB test signal.
Increasing to 95dB is shown below. Distortion remains low at -52dB for 100Hz.
I moved the bass cabinet to my father's farm's heated shop which is a large space allowing off-axis measurement. I constructed a large rotary table to get precise measurements. I also setup the ES-290 Biradial on with a 500Hz crossover point to get a picture of the total system response off-axis.
Below is the resulting polar map for the system. We can see that the bass cabinet short horn is providing great pattern control in the 100Hz-500Hz region and blends seamlessly with the ES-290 above that.
For interest's sake here is the frequency response overlay of the system with an ungated response using a 4m mic distance (black trace).
Bass cabinet No.1798 provides a linear frequency response down to 33Hz with an in-room sensitivity of 101.5dB at 1w. Subjectively the bass sounded well defined without any hint of boominess. The front horn provides a very focused sound that is free from any coloration. Distortion is the lowest I’ve measured for both the 85dB and 95dB output levels, setting a new benchmark for low distortion performance. Judging by the test data the driver is clean up to 1kHz where it encounters some pretty high IMD at 1.8kHz due to a breakup. I would want to use a steep filter at 800Hz to keep the breakup region well below the noise floor.
It remains to be seen how other drivers perform. I will be testing other drivers in the near future which I’m sure will interest most people. For custom build inquiries please contact me at Joseph_crowe@josephcrowe.com