Acoustic Design Goals
My acoustic design goals for the Annex speaker were as follows.
- smooth frequency response within the listening window with a 1dB to 2dB tilt from 100Hz to 10KHz
- smooth in-room mic position averaged frequency response with a 5-6dB tilt from 100Hz to 10KHz
- smooth sound power response with directivity rising with frequency
- appropriate balance between sensitivity and low frequency extension
- distortion and noise below audible limits at reasonable playback SPLs ( 80-85dB at 2M, C weighted)
The transducer performance, transducer position, crossover, damping, cabinet volume and cabinet tuning all play an important role in achieve these goals. You may expect to see flat on-axis frequency response in this list. I’ve heard good sounding speakers that didn’t measure that well on-axis and I’ve also heard fatiguing speakers that measured flat on-axis. The directivity of the speaker substantially influences the optimal on-axis frequency response. If the speaker is highly directional, a high frequency boost may be appropriate in the on-axis frequency response to compensate for the high frequency attenuation in the off-axis and vice versa.
Cabinet Design Considerations
For adequate low frequency extension and SPL capability from a 2-way stand mount speaker, a ported or bass-reflex low frequency design is required. Whenever possible, I locate the port on the back of the cabinet. Front firing ports can cause frequency response deviations and be the source of noise at lower frequencies when driving the speaker at high levels. A port tube is a pipe and just like a pipe organ, it has a resonant frequency. The woofer excites this resonance and it interferes with the woofer’s direct forward sound. The interference is drastically reduced if the port is located on the back of the cabinet.
Cabinet damping was optimized to eliminate reflections behind the woofer and standing waves. This requires a careful approach in a ported cabinet to ensure the damping is sufficient but not excessive which reduced the contribution of the port to the low frequency SPL of the speaker.
A traditional rectangular cabinet shape was chosen for a few reasons including ease of assembly and finishing, low cost and efficient flat-pack packaging. The builder can of course modify the shape of the top, bottom, back and sides of the cabinet however the fundamental dimensions of the front baffle should not significantly (<1 inch) change. This would alter effect that the baffle has on the transducer’s frequency response, also known as baffle step or 2pi to 4pi transition. Adding a ½ inch radius to the front baffle perimeter edges is acceptable because it will only affect frequencies with smaller wavelengths and by less than 1dB.
Below is the effect of adding 3/4″ radii to the sides of the front baffle. The SPL difference as confirm with the cursor and legend is only 0.5dB! This is an on-axis frequency response with the mic at 1M centered on the tweeter.
Below is the frequency response of the tweeter and woofer mounted in the cabinet with the target acoustic crossover frequencies responses. You make look at this and wonder how we’ll be able to achieve the targets responses with the addition of various inductors, capacitors and resistors. Keep in mind that these are single on-axis measurements which are considerably affected by diffraction especially across the tweeter’s target bandwidth of 2KHz to 20KHz.
Anechoic frequency response measured at 2M, graphed at 1M, 2.83V.
Below is the impedance measurement of the woofer in the damped cabinet and the tweeter.
Crossover design and voicing info in the next post…