Interview with High-Quality Horn Loudspeaker Systems authors, Bjørn Kolbrek and Thomas Dunker

Horn loudspeakers have existed for over a century, attracting much interest from

enthusiasts and engineers. Bjørn Kolbrek and Thomas Dunker recently released the

3rd printing of what many consider the most complete book on the subject, so much

so, that many refer to it as “the horn bible”.

High Quality Horn Loudspeaker Systems covers the history, theory and design of

horn speakers and the book has become a definitive reference for anyone serious

about the art and science of horn loudspeakers. .

https://hornspeakersystems.info/

The first edition of the book was launched in 2019 at the 147th AES Convention in

New York with the second printing following three years later in 2022.

Both authors have such special dedication to audio history and horn loudspeaker

technology both with interesting paths into the world of audio.

We have known Bjorn and Thomas for over twenty years and have been following

this project since the beginning. We are humbled and thrilled to be able to celebrate

and promote the third print, together with Bjørn, Thomas, Ojas and Celestion.

We managed to catch up with them recently and have a chat.

Bjørn Kolbrek (BK)

How did you get into audio?

It all started with radio around the age of 8 or so, but I think I got into audio for real

when I was given a proper cassette deck and connected it to the aux input of an old

radio. Over the next few years I built amps and played around with some of the lower

cost equipment from Hifi-klubben, before building an OTL amplifier around the time

when I entered engineering school.

Around that time I read the claim "once you hear a horn loudspeaker you will never

be the same again". I started looking into horns, found Thomas' Horn Speaker Home

Page, and at that time also

connected with a German student who was also into horns.

The developments from here are detailed on my homepage:

https://kolbrek.hornspeakersystems.info/index.php/my-systemIn terms of education, I first took Electronics Engineering (BSc level), and later, after

working for about 7 years in the electronics industry, I did my MSc and PhD in

acoustics before working for Celestion. Now I'm back in electronics again.

My interest in audio has always been a mix of enjoying music, building my own

equipment, and learning about how it all works and how we perceive it. I think going

back and forth between these three has been very useful. Also doing a little bit of live

sound and playing bass guitar in various amateur groups has been inspiring.

What was the background, goal and vision for this enormous book project and what

made you do this in collaboration?

We first got in touch around 2003, and found we were both on the same page

regarding audio, speakers and amplifiers. And one thing we started discussing was

how to design a wideband midrange horn. The common rule of thumb was that horn

speakers can be made to cover about 3 octaves, but the old curved Western Electric

horns covered six. How did they do it? We started digging up what we could find of

horn related material from the 1920s and 30s, which turned out to be quite a bit. And

more recent material as well, of course. And at some point, in 2005 from the looks of

it, we had collected so much material, much of which was only to be found in

obscure papers and patents that few people knew about, that we thought we should

try to collect it in a systematic way in form of a book. Because there was no book

about horn speakers, only a couple of chapters in certain textbooks, like Beranek

and Olson, and hundreds of articles and papers, many behind paywalls. The goal

was primarily to collect the available knowledge about horn speakers, and then it

grew. At the time of publication, we had collected around 1200 references. We also

added a lot of our own research.

What were the biggest challenges and how did you overcome them?

One of the biggest challenges we faced, was finding time to do the work. Fortunately

I was able to put in a lot of time during his time at the university, and Celestion also

sponsored us by allowing me to write during work hours a few days a week in the

last 6-7 months before the deadline. Other challenges included, how to organise our

huge pdf and image library and how to share manuscript files. We found out dropbox

worked best.

What is the path going forward, will there be new revisions, new books or anything

else?

The third Edition has recently been released in May 2025 But apart

from that... The project was quite exhausting, so it takes some time for the

motivation to build up to the point where we can actually do either a revision or

another book. But we do see a need for a smaller 'engineering handbook' type of

book covering more directly the task of designing various types of horns.

I am also working on horn simulation software that will hopefully reach the point

where it can be publicly released. But don't hold your breath!-Bjørn

Thomas Dunker (TD)

How did you get into audio?

I have been interested in technology and electronics since early childhood. In

particular the older, more classic pieces that would be found in museums, which was

explained in old books and not found in normal households. I built speakers in grade

school and put my first stereo system together using a pair of identical Tandberg

radios for tube amplification. More serious audio projects were initiated with time,

and audio has now been my main hobby and pursuit for 35 years. Although I did not

become an engineer for various reasons, for decades I researched into a whole lot of

papers and books on various audio related technologies.

My fascination with horn speakers grew out of learning about them in audio journals

in about 1990, and also because of many papers on distortion mechanisms in

speakers which I also studied in the early 1990s.

I built a partly horn based system in 1993-94 at about the same time that I created

The Horn Speaker Home Page in what was the early years of the WWW. This, and

being part of a discussion forum that has been active for more than 30 years now,

got me in contact with a lot of people around the world with similar interests.

Including Bjørn, who first contacted me in 2003.

What was the background, goal and vision for this enormous book project and what

made you do this in collaboration?

Bjørn and I initially did not plan to write a book, merely to review and revisit theory

and papers we had both collected on the subject to prepare ourselves for more

ambitious horn designs, which we felt that most amateurs would shy away from.

Given the access to interesting libraries and wanting to dig deeper into what

literature might exist. Over several years we had collected and studied hundreds of

papers and while this continued on for years, we began to realise that writing a book

based on our extensive archive would be welcomed, for us as well as to others,

given the otherwise scattered and hard-to-find nature of existing theoretical and

historical knowledge on horn speakers.

We decided it would simply be the best way to access and navigate information on

the subject. It was a project that sort of found its makers. It also motivated Bjørn to

resign from his engineering position to study acoustics and specialise in horn theory

and computer simulation methods at the NTNU in Trondheim, which culminated in a

doctorate thesis on the mode matching method for simulating horns. A more

extensive background story on the book project is found on our web page and in the

preface of the book itself.

https://hornspeakersystems.info/index.php/the-authorsAs we had already worked closely together for years studying and building the

growing library of material it became clear to us that it would be difficult to

impossible to bring any further people up to speed to help us write the book. It also

seemed obvious to us that it would have to be a joint effort, as the scope and size of

the project had also grown progressively with the amount of interesting material we

would want the book to be based on.

What were the biggest challenges and how did you overcome them?

For the whole duration of the project we always lived quite far apart geographically

and could not meet very often. We would keep in touch by phone and e-mail and

shared access to a file sharing server meant that the archive could be continuously

managed and accessed by the both of us. The book manuscripts were also kept on

this server so that we could easily look at each other's recent writing and editing, and

we would have regular phone chats about recent developments and ideas that

needed to be discussed.

Bjørn had suggested we use the LyX editor which simplified the integration of the

bibliographies and images in our archive with the book manuscripts. In effect, the

book writing process made our archive of material much easier to navigate and make

use of.

We decided not to partner with a publisher, as we wanted to be completely in charge

of editing and decide how exactly we wanted the book to ultimately end up. We did

engage some more or less voluntary proof readers in the scramble to get the book

ready for printing in 2019, but it was only after David McBean's and Phillip Newell’s

meticulous reading of the already published book during the pandemic that most of

the final slips and misprints were weeded out, in time for the second print run of the

book.

What is the path going forward, will there be new revisions, new books or anything

else?

Some ideas have been discussed in this regard, but as the published book was both

exhaustive in scope and exhausting work to complete, we do not have plans for

further publications so far. We were extremely excited with the access we got at the

AT&T Archives, which we visited twice while Bjorn was studying his Phd. We were

granted access to Bell Telephone Labs technology, and we know that a huge wealth

of information on related technologies such as vacuum tubes and amplifiers, sound

recording technology and phonograph developments exist in that archive. However, I

think it proved difficult for us to be admitted there for further research after Bjorn

completed his Phd. If we had had the chance to go back there, we would have

jumped at it, and that might have resulted in "unearthing" more exciting material that

might have inspired us to at least write something new. To me however, it is also a

dizzying and a bit of a terrifying prospect!

-Thomas