Annex 2way Bookshelf / Monitor Speaker Kit Design

Specifications

Designer Mark Mason
Configuration Bass Reflex, Wide Dispersion
Frequency Response, +\- 3dB 33Hz to  23KHz
Sensitivity, Anecohic, 2.83V at 1 meter, 300-3kHz 83.5dB
Sensitivity, In-Room, 2.83V at 1 meter, 300-3kHz 86.5dB
Voicing SPL, 2M, A weighted 80dB
Impedance, Nominal
Impedance, Minimum 4.2Ω, -0.1° at 209Hz
Recommended Amplifier Power 10 to 150W
Cabinet
Material CNC Routered 3/4″ MDF
Width, inches 9
Height, inches 15
Depth, inches 12
Weight, assembled speaker, lbs 18

Design Philosophy

Another 2 way stand mount speaker kit? Not just another.

There is no shortage of speaker kits in the marketplace using this configuration. It seems to be considered the quintessential go-to kit for the new speaker builder. And that is precisely why I thought it was appropriate to begin my MakeAudio journey targeting this configuration.

 

Choosing the Transducers

Tweeter

A high performance wide dispersion soft dome tweeter can offer exceptional performance with careful design. Metal dome tweeters may have some advantages but the audibility of those advantages is questionable. Some builders and audiophiles dislike metal dome tweeters because they describe their sound as harsh. This harshness is more of a product of the crossover design but I likely won’t be able to convince the critics. There is generally no such negative attribute associated with soft dome tweeters and speakers using them are found to be acceptable to a larger audience. The tweeters chosen for the Annex speaker are of the wide dispersion variety and do not utilize a horn or waveguide. I consider this to be the preferred target for the masses because room treatment and speaker position can be used to manipulate unwanted room reflections. Increasing the dispersion of a directional speaker is typically not an option.

Other important attributes are the resonant frequency, linear excursion, amplitude response linearity, dispersion linearity, impedance linearity, power handling, sensitivity, manufacturer quality assurance, materials, and of course cost to performance ratio.

Woofer

As it relates to low frequencies, I define “full range” as a speaker that generates bass sufficient for most music. It may not be able to reproduce the lowest organ notes or your car audio bass demo CD but you typically won’t notice any missing bass when listening to most of your music at a typical SPL.

I have found that a nominal 6.5” diameter woofer yields just about the best balance between low frequency extension, output capability and uniform dispersion for a 2 way speaker. A smaller woofer can result in a smoother off-axis amplitude response around the crossover frequency however the smaller woofer easily reaches its excursion limit when driven with music containing moderate bass content. If used with a subwoofer, the smaller diameter woofer is a better choice however this is a “full range” speaker designed to be used without a subwoofer.

Other important attributes include the T/S parameters, linear excursion, amplitude response linearity, dispersion linearity, power handling, sensitivity, voice coil diameter, overdrive behavior, spurious noise, manufacturer quality assurance, materials, and again, cost to performance ratio.

 

Acoustic Design Goals

My acoustic design goals for the Annex speaker were as follows.

  1. smooth frequency response within the listening window (on-axis, +\-15degV, +\-15,30H,V) with a 1dB to 2dB tilt from 100Hz to 10KHz
  2. smooth in-room mic position averaged frequency response with a 5-6dB tilt from 100Hz to 10KHz
  3. smooth sound power response with directivity rising with frequency
  4. appropriate balance between sensitivity and low frequency extension
  5. 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

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.

 

 

To the right is the simulated anechoic frequency response of the woofer in the optimized ported cabinet at 1M with 2.83V input. Resonant frequency of this bass reflex system is 42Hz.

 

 

 

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 reduces 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.

 

Shown here is the effect of adding 3/4″ radii to the sides of the front baffle. The SPL difference as confirmed 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.

 

Transducer Measurements

Below is the frequency response of the tweeter and woofer mounted in the cabinet with the target acoustic crossover frequencies responses.

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

 The Annex crossover is of the 12dB/octave LR variety with baffle/diffraction, impedance, level and phase compensation.  Both transducers are wired in phase.

The crossover design allows the builder to configure treble levels of +1dB and -1dB relative to the reference level. This allows for differences in personal preference, listening SPL, and room reflectivity.

High quality film capacitors and air core inductors are used throughout the design. An iron core inductor is used in the woofer crossover as this will result in the lowest cost effective resistance to ensure the woofer sensitivity is not excessively reduced.

The crossover is fully assembled and potted. A schematic diagram is not included to help deter others from copying the design.

 

Speaker Voicing

My goal with the Annex speaker is neutral sound. No excessive bass or hyper accentuated treble allowed. The initial voicing session consisted of critical listening to a single speaker in an IEC calibrated room (average of most listening rooms) at an average level of 80dBA with about six to eight music and speech tracks. Subsequent voicing sessions include comparisons with well received reference speakers behind a blind screen and a broader range of source material. These were also conducted with single speakers to ensure the differences aren’t masked as is the case when comparing pairs of speakers in stereo mode. Final voicing was conducted in stereo mode.

 

Measurements

On-Axis frequency response, listening window and early reflections. Mic at 1M centered on tweeter, 2.83V input. Anechoic measurement spliced with ground plane measurements at 250Hz.

Priority was given to the most accurate sound over the flattest measurement. The overall frequency response tilt is typical when using a wide dispersion tweeter without a horn or waveguide. No tilt results in an overly bright balanced sound in a typical listening room. The speaker’s low frequency extension is 35Hz at -3dB from the 83.5dB sensitivity rating.

 

 

Horizontal Frequency Responses, relative to on-axis, mic at 1M, 2.83V, 1/24th octave smoothing

 

 

 

 

Impedance and Phase

The Annex speaker cabinet is tuned to 41Hz with the impedance reaching a minimum of 4.25Ω at 209Hz and phase is an amplifier friendly -0.1°. The Annex can easily be driven by an amplifier rated for use with 4 or 8Ω speakers.

The slight inaudible impedance wiggle at 250Hz is a woofer resonance that appears when the woofer is measured in free air. It is not cabinet panel resonance.

 

Cumulative Spectral-Decay from the output of an accelerometer mounted to the middle of an Annex speaker cabinet side panel

10HZ high pass filter, 100mS window, 0.1mS rise time, 7.5V signal level

 

 

 

 

Quasi-Anechoic Spliced On-Axis Frequency Response at 2M

With +1db and -1dB crossover treble options

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