DIY Harbeth 40.1

I understand how musical instruments work. I have been a musician most of my life. That doesn't mean that is what they are doing. They do not say the cabinets operate like musical instruments, only that they use the same construction techniques. That could mean a myriad of things and is really used as a marketing piece.

A speaker is not a musical instrument and striving to vibrate like one doesn't make any sense. It is designed to reproduce the recording of an instrument and vocals as accurately as possible. Any cabinet vibrations only server to color the sound being reproduced by the drivers themselves. Alan Shaw writes in and interview about Harbeth speakers, "I certainly don't see this as an extension of an 'art form' - I'm not trying to enhance reality. Some loud speaker designers, quite legitimately, say that they are trying to express a certain kind of sound, in the same way that an instrument maker would want a certain sound. But that's not what I'm about."

Also from Harbeth's site:

CABINET SYSTEM: Harbeth cabinets are crafted from thin (typically 12mm) panels which, by virtue of their profile, lend themselves to being internally acoustically damped in a way that is impossible with conventional 'thick wall' panels such as 18mm (3/4 inch) chipboard or MDF. We work harmoniously with the knowledge that the human ear is intolerant of buried resonance in the middle frequencies to steer panel resonances out of the critical band and into the Region of inaudibility.

The 'thin-wall' box concept was thoroughly researched by Harbeth's founder at the BBC and we've stuck with it because it works despite the high cost and difficulty of assembly from individual wood panels - and the cabinetmaker's insistence that we should abandon the removable front and back and go for an 'all-in-one' locked solid cabinet. We could - but it would then NOT sound like a Harbeth.


Again, Harbeth never once mentions that their speakers are supposed to vibrate like a musical instrument. I understand the concept of pushing cabinet vibrations out of the midrange where the ear is more sensitive, but this has led to people creating this cult around the cabinet design and the everyone saying of them "vibrating like a musical instrument." This is simply not the case.
 
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The (BBC) Lossy cabinet design is made of relatively thin (Baltic) Birch plywood panels, about 12mm, a trademark of all Harbeth loudspeakers. There is not much bracing inside so it's built more like an instrument. Think, an acoustic guitar's body... From my research, I have come to the conclusion that natural wood will add no coloration where as MDF will absorb and dead'n the sound.

One reviewer wrote: ~~"The M40.1 have their own distinct character which is completely different from what seems to be the norm for most modern high-end designs. Even so they are one of the best loudspeakers I ever listened to in my room. It came as a kind of surprise or even shock because they represent everything I hate about other designs – plastic cones, soft domes, lively cabinets that actively contribute to the sound. Based on all my prior experience there was just no bloody way a speaker like the Harbeth should sound any good. Yet it sounds fantastic!"~~

All interior walls are covered with "bituminous damping mats" then remaining areas are filled with quite stiff foam and some tight rolls of natural wool. I've read that by stuffing the cabinet it lowers standing waves and also increases the box volume size and damping. Whats funny is, heavy stuffing is more commonly used in sealed box designs, yet the 40.1 is ported, with two ports.
 
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I've read that by stuffing the cabinet it lowers standing waves and also increases the box volume size and damping. Whats funny is, heavy stuffing is more commonly used in sealed box designs, yet the 40.1 is ported, with two ports.

Stuffing the cabinet does lower standing waves and it will also lower the amount of reflected waves coming from the back wall back through the cone. It does not actually increase apparent box size though. It merely lowers the Q of the driver in the given cabinet. There was a really good discussion on the Parts Express forum a month or two ago about this.

It is weird that they would use this in a ported design. Most ported designs would only line the walls of the cabinet with foam or some other insulation. Stuffing the entire cabinet will diminish the output of the port significantly. I can see at least one reason why they might do this. They are using a higher tuning than is optimal, creating a peak in the low end response (maybe trying to use a smaller cabinet than the woofer wants?). By stuffing, you could potentially reduce this peak back to flat.

I will concede that the M40.1 vibrates like a bell at low frequencies, but most of their other cabinets seem to be much better damped by looking at all their measurements in Stereophile.
 
Custom, proprietary drivers - check
Custom, optimized crossover - check
Custom cabinet with "the right" resonance - check

It should be that you can get "similar" drivers, and through trial and error get a cabinet and crossover tweaked to "close".

DIY work is fun. I have tried speakers in the past, and suck at it. I have having a blast with a soldering iron making some amps and dacs. There is a lot of help available here and at DIYAudio.

If you wanna spend your time and money to try and recreate some rather expensive commercial speakers, more power to you. Make it work, and publish the plans here or diyaudio and become famous. There are certainly commercial speakers that have "unobtainium" or other hard to source materials, but the classic BBC type speakers all rely on a resonant box, and carefully chosen drivers and crossovers.

I tend to lower power amps, many of which are very uncomfortable below 6 ohm. While some folks drive 80db sensitive speakers with 20 watts, I prefer speakers at least in the mid 90s. Therefore I have never looked at getting the 40.1, which are very well regarded, are quite expensive, but hold their value well.

Keep up your interest, study, and be prepared to do a few trial attempts. I have found it best not to keep track of what you spend while DIY-ing.:thmbsp:
 
Look at how similar these are yet different. i wonder if these designs will unlock some 40.1 secrets.
 

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The 40s thin wall box is not an easy box to build, it may look like it but it is a fasteners nightmare to get bang on. Also stuffing them just right is very time consuming. Going line or digital active is the way to go, passive just sucks ass. Having a detachable front baffle makes for an endless drive choice. The box volume can be tuned for many 10" and 12" bass drives. The thing to remember here, is how the box reacts mechanically in the room when playing tunes. Zero ringing like a bell with a crack to it. The grills and stands are also huge to the overall thin wall box sound ;). The center side braces are to never touch the inside side walls. It would defeat the boxes breathing action. Likewise the speaker to the stand relationship must be supported at the 4 outer points, in the middle of the baffle and back holders. My 40s volume eats any Harbeth or Spendor 40 sized speakers I have heard. An activated amp set per drive is WILD. My next upgrade to them may be one of the 12" ATCs with the now set in stone 380Hz-3.5KHz. I hope this helps the original OP. Never let people get you down. PS: I have many more pics of all the build. Have a GREAT MUSIC DAY EH :)
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