The Open Source Monkey Coffin repository

The Monkey Coffin Loudspeaker is an open source loudspeaker design, which is being developed by members of the diyAudio community. Full details of the Monkey Coffin Loudspeaker project can be found at the corresponding diyAudio thead. The idea of this page is to keep the central information and measurement data of the Monkey Coffin in a somewhat organized way. The development of the Monkey Coffin does not come for free. We would greatly appreciate your financial support. Important note: DIYers may profit from this open-source project for their own private purposes, for example by building and enjoying a copy of the Open Source Monkey Coffin speaker. Please do not use the information developed in this open source project on a larger scale (for example by selling speakers based on the Open Source Monkey Coffin design or substantial parts of it) without written permission.


CAD drawing of the first prototype (version 20181219):

Monkey Coffin enclosure version 20181219

Monkey Coffin enclosure version 20181219


Woofer (FaitalPRO 12PR320):

  • FaitalPRO 12PR320 factory datasheet: PDF
  • Impedance in free air was measured at cold and warm ambient temperatures (approximately 10°C and 30°C) to show the effect of the variation of the compliance of the suspension. Raw data (cold): FRD file Raw data (warm): FRD file Faital_impedance_warm_cold
  • Thiele Small parameters (determined from above free-air impedance curves and added mass measurement; mean values from “cold” and “warm” measurements): fs = 43.1 Hz, Qes = 0.44, Qms = 8.2, Qts = 0.42, Mms = 50.8 g, Cms = 0.27 mm/N, Vas = 91.1 L
  • Distortion tests: harmonic_distortion_FaitalPRO_12PR320gedlee_distortion_FaitalPRO_12PR320

Midrange (Volt VM752 / 8Ω)

  • Volt VM752 / 8Ω factory datasheet: PDF
  • Impedance in free air Raw data: FRD file Volt_VM752_impedance
  • Thiele Small parameters (determined from above free-air impedance curve): fs = 386 Hz, Qes =  0.75, Qms =  10.7, Qts =  0.70, efficiency (2.83 Vrms, 1 m) = 93.5 dB-SPL

Tweeter: The choice of the tweeter is not yet finalized. We are currently looking at the Scan-Speak R2904/7000 in a waveguide.

System analysis

Crossover filters

  • First x-over design in diyAudio post 119. This is a purely hypothetical concept study based on published driver data, assuming Satori TW29BN as tweeter.
    Monkey Box Schematic LR4 500 - 2500 HzMonkey Box SPL and targets LR4 500 - 2500 Hz

Bass-reflex tuning (woofer in 20181219 box):

  • Measurement shows cone excursion minimum occurs between 34 Hz and 35 Hz
  • Impedance Raw data: FRD file impedance_Faital_12PR320_20181219Box
  • Nearfield SPL Separate SPL measurement of woofer and port is not possible due to acoustic crosstalk. Combined woofer and port output was therefore measured following the method described in the Klippel Application Note AN39. The artefacts at 65 Hz, 130 Hz and 260 Hz are due to room modes. Raw data: FRD filenearfield_box20181219_klippelAN39

Acoustic centers, time alignment:

  • The “driver depth” is defined as the distance of the joint of the voice coil to the baffle plane. The depths are follows:
    • Scan-Speak R2904/7000 in modified WG148 waveguide: 20 mm
    • Volt VM752: 28 mm
    • Faital 12PR320:
  • Measured relative acoustic centers (“AC”, see here):
    • AC of Scan-Speak R2904/7000 in a modified Visaton WG148 is 10 mm behind the AC of the Volt VM752.
    • AC of Faital 12PR320 is 39 mm behind the AC of the Volt VM752.

Acoustic far-field measurements:

  • Single drivers measured with microphone at 1 m distance from the baffle, on-axis and horizontal off-axis at 0°, ±15°, ±30°, ±45°, ±60°, ±75°, ±90° (Faital 12PR320, Volt VM752, Scan-Speak R2904/7000 in WG148). Raw data files (TMD and FRD): ZIP archive

AOS Studio 30

The AOS Studio 30 (S30) is a closed box MTM loudspeaker (Photo 1). The S30 is in many ways a bigger brother of the S24. I built this Studio 30 for use in a large room, so I changed the «bookshelf» format of the original AOS design to a full-size floor-standing loudspeaker. The S30 uses high-quality driver units made by ScanSpeak (Photo 2).


Photo 1: The AOS Studio 30


Photo 2: Close up of the tweeter and the midwoofers

Peerless Subwoofer (active)

I designed this subwoofer (Photo 1) for flexible use in high-quality audio systems. In particular, I use it in my own system in combination with the S24. The subwoofer uses two Peerless XXLS-P830845 12-inch long-stroke woofers. Each woofer is mounted in a separate closed box. The woofers are mounted opposite to each other to cancel the vibrational forces acting on the enclosure.


Photo 1: The Peerless subwoofer

Each woofer is driven by a dedicated power amplifier. An active DSP with adjustable low-pass filters is used match the subwoofer to the main loudspeakers. In addition, the DSP provides a Linkwitz pole-zero equalisation, which is tuned yield a flat acoustic frequency response down to a -3 dB point of 21 Hz. The quality factor was set to Q = 0.65, which results in a very well-controlled impulse response. Peerless_Subwoofer_drawing


Impedance curves of the two Peerless XXLS-308-8 woofers mounted in the box, and theoretical impedance of a woofer-box system with f0=45.5 Hz, Q=0.94, and RDC=5.8 Ω.


Frequency response of the Peerless Subwoofer, measured in the nearfield to gate out room echoes. Linkwitz pole-zero equalisation from f0=45.5 Hz, Q=0.94 to f0=21 Hz, Q=0.65. High-pass filter (18 dB/oct) is a cascade of a second order filter (fc=47 Hz, Q=0.7) and a first order filter with f0=118 Hz (determined experimentally for use with the AOS S24).


mangerbox_3This is my implementation of a loudspeaker with the unique and famous Manger MSW driver. Unlike with conventional loudspeaker drivers with rigid membranes, the Manger MSW is a bending-wave driver. The movement of the voice coil at the center of the highly flexible membrane induces a wave that travels radially towards the edge of the membrane. This is similar to the surface waves that result from throwing a stone into a pond. The Manger MSW has a flat frequency response from 300 Hz and higher, so it’s highly suitable for a very high-quality FAST (full range and subwoofer technology) loudspeaker system. I designed this small floor-standing loudspeaker around a Manger MSW WO-5 and a Visaton TIW 200 XS woofer mounted on the side of the enclosure. Both the MSW and the woofer are driven by a dedicated power amplifier using DSP filters. The crossover-frequency between the MSW and the woofer is about 300 Hz. Detailed notes and measurements taken during the development of the Mangerbox are available in a separate PDF document.mangerbox_1 mangerbox_2 mangerbox_plan

Tube Speaker

tubespeakerThis is a small speaker which I designed as an improved clone of a Bang&Olufsen designer speaker. Unlike the B&O model, however, the Tube Speaker is made of real metal and sounds really good. It uses two 11-cm Seas-Excel mid-woofers (yes, the ones with the magnesium membranes) and a Scan-Speak D2905/930000 soft-dome tweeter mounted in a d’Appolito arrangement. The metallic outside is backed with a 2-cm thick layer of sand-filled epoxy, which results in an extremely «dead» enclosure.


Since these speakers are only 15cm wide I had to use very small woofers. The only good woofers in this size (about 12cm max. outer diameter) I could find at the time were the Seas Excel drivers. Their outer diamter is about 11cm and there’s even a model with the famous magnesium-membrane by Seas. Because these woofers are so small I had to use at least two of them in each speaker to get reasonable bass-response – so why not mounting them in d’Appolito style? I never built a d’Appolito speakers before so I decided to try it. Well, it was worth it: the sound qualtiy and the 3D imaging of the Tube-Speakers turned out the be very good!

The magnesium membranes have a very strong resonance peak at at about 11 kHz. This is way higher thant the cross-over frequency, so this resonance is not a problem in this design. The tweeter should be of the same good quality as the mid-woofers. I ended up using the ScanSpeak 9300 tweeter, befcaus it is a very good tweeter and it’s not too difficult to design a suitable cross-over filter network for it.


The design of the metal enclosure is where the Tube Speakers are different from every other speaker I have come across so far. But when you get off the beaten track, things can get rough…

The first major problem was to get a suitable metal tube. I called a metal-working companies, who told me that a suitable tube with cut-outs and a flat face plate to mount the drivers would cost a fortune. That means two fortunes for a stereo pair. Finally, a friend who works as a metal-working teacher took up the project and built the tubes for me. Thank you Erwin!

The second major problem with a metallic enclosure is that it would resonate like a church bell, very bad for a speaker cabinet. My first idea to solve this problem was to fill the metallic tube with concrete from the inside, but I was afraid the concrete wouldn’t stick to the metal. So, what else? Epoxy is really sticky… so I mixed epoxy with as much sand as possible. This gives a very heavy and solid material after the epoxy is hard. I put about 2 cm of my epoxy-sand mixture on the inside of the metal-tube. To do this I put a carton-tube inside the metal-tube. The carton-tube’s outer radius was about 2 cm less than the radius of the outer metal-tube which left the required gap to pour my mixture in. Epoxy has some disadvantages, though. Firstly, it is expensive. And secondly, it gets very hot when hardening and after becoming hard it cools down again which also means it contracts a little. This contraction caused the epoxy-sand-mix to loose contact to the metal-tube so I had to pour some more epoxy into these gaps. The result of my epoxy treatment is a very rigid and perfectly air-tight speaker-cabinet with virtually no vibrational resonances.

Crossover filter network

I used a 3rd order low-pass filter for the woofers (both woofers in parallel) and a 2nd order high-pass filter for the tweeter. Together with the natural low-frequency roll of the tweeter, this results in a 3rd order slopes in the frequency responses of both the woofers and tweeter, as required for a school-book d’Appolito design. The cross over frequency is about 2 kHz. The figure below shows the frequency response (measured with MacSpeaker using an MLS-signal), which is very flat (±1.5 dB between 500–8000 Hz, and ±3 dB throughout the entire frequency range of the measurement).


Anechoic frequency response of the Tube Speaker

AOS Studio 24

The AOS Studio 24 BE (S24) is a closed box MTM loudspeaker (Photo 1). The S24 is sold as a kit by Axel Oberhage Starnberg (AOS) in Germany. The original AOS design is a “bookshelf” format box. However, free-standing operation of the S24 on a stand is acoustically better than stowing it away in a bookshelf. The S24 uses high-quality driver units made by ScanSpeak.


Photo 1: My AOS Studio 24 BE, which I re-desinged as a full-blown floor-standing loudspeaker

A unique feature of the S24 is that the woofers protrude by 18 mm from the front baffle, whereas the tweeter sits flush with the baffle. Most speaker designs lack this offset, which, due to the recessed woofer cone(s), causes a lag of the sound wave(s) emitted by the woofers relative to that of the tweeter. The additional offset used in the S24 therefore avoids the typical misalignment of the different drivers and therefore allows time-coherent sound emission from the woofers and the tweeter.


Photo 2: Close up of the tweeter and the midwoofers, which are mounted on aluminum rings to achieve an offset relative to the front baffle

I decided to modify the original AOS design a little when I built my S24. In particular, I used a full-height floor-standing enclosure for the S24, revised the crossover filters a bit, and added an active subwoofer. A detailed description of my tests, modifications, and extensions to the S24 are available as a separate PDF document.

Cheap Trick 154

cheap_trick_xyA small two-way floorstanding speaker, developed by Bernd Timmermanns (then the editor of the german DIY-HiFi magazine Klang & Ton). This speaker uses the a Morel MDT 29 soft-dome tweeter and a Monacor SPH 135C 13-cm midwoofer.


scanspeakerI designed the ScanSpaker as a mid-sized but high-end two-way speaker using Scan-Speak drivers. This floorstanding speaker uses the Scan-Speak D2905/990000, a 1″ soft-dome tweeter, and the Scan-Speak 18W/8545-00 mid-woofer, which has a 17-cm diaphragm made out of carbon fibre and paper.

1. Drivers

As the ScanSpeaker name suggests, all the drivers are made by ScanSpeak. I used the 8545 woofer (17cm diameter) and the 9900 tweeter (aka «The Revelator», 28mm dome). These are very good, but expensive drivers. The 8545 midwoofer has the reputation of very smooth reproduction of voices. This wasn’t true in my speakers at all until I added impedance-compensation for the impedance-rise caused by the voice-coil. The 8545 has a strange “bump” at about 600–700 Hz. It doesen’t look like a membrane-resonance at the waterfall-diagrams so it can be corrected with an LCR-network. The 9900 tweeter has a special frontplate which is supposed to improve lobing behaviour. I heard and read rumors suggesting that this special frontplate design causes a slight resonance somewhere between 10–20 kHz, but I couldn’t see it on my measurements. The dome diameter is 28mm and the resonance-frequency is at 530 Hz, so the Revelator goes lower than most other tweeters. The 9900 has a reputation to acts a bit like a «diva» when designing the crossover, which I attribute to the very pronounced impedance peak at resonance.

2. Enclosure


Drawing of the ScanSpeaker enclosure

I used a bass-reflex construction for the ScanSpeaker. I put two bass-reflex channels in the bottom of the case, «aiming» at the floor. This works because the speaker stands on spikes at about 2 cm above the floor. The internal volume of the case is about 18 liters. The front is 18.8 cm wide which is about the minimum possible with the 8545 woofer. An ideal case does not vibrate at all – the driver’s membranes are the only thing that should move. To get a «dead» case I built the case from 18 mm particle-board (inside) and 10 mm MDF (outside). Enclosure vibrations are further dampened by a layer of ceramic «bathroom» tiles and a layer of «Hawaphon» on the insides of the enclosure walls. A layer 10 mm felt and some wool  dampen internal sound resonances and eflections in the case. In addition, I installed a piece of cardboard at the top of the case with some foam glued to it. The cardboard is angled at about 30° relative to the face plate, which helps to avoid standing waves along the long axis of the enclosure.

3. Crossover network

The final crossover filter networks for the midwoofer and the tweeter are shown in the figures below. The initial design started out by calculating theoretical parts values, which were then optimized by acoustic measurements and by ear. As mentioned above, the 8545 woofer frequency response shows a slight «bump» at about 600–700 Hz. This is compensated using an LCR network. Also, for proper operation of the tweeter filter, the impedance peak of the 9900 needs to be compensated. This is achived by an L-pad in between the tweeter and the filter network. The L-pad is also matches the tweeter SPL to that of the woofer.


Midwoofer crossover filter network


Tweeter crossover filter network

The parts values for the midwoofer filter are:

  • L1 = 1.8 mH
  • L2 = 15 mH
  • C1 = 10 µF
  • C2 = 15 µF
  • C3 = 3.9 µF
  • R1 = 5.6 Ohm
  • R2 = 4.4 Ohm (tot. resistance of LCR including L2)
  • R3 = 1 Ohm

The parts values for the tweeter filter are:

  • C4 = 3.9 µF
  • L3 = 0.82 mH
  • R5 = 10 Ohm
  • R6 = 8.9 Oh