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Gundry Dip or BBC Dip?
Folks,

I've been aware for a while that many loudspeakers intentionally reduce the "harshness" range around 2 to 4 KHz to sound smoother and less fatiguing. I've seen data for speakers having dips as large as 8 dB! I really hate that, especially in speakers sold to the pro audio market. If everyone mixes on speakers like that, those of us who care about accuracy and have flat speakers will have more harshness than usual. Plus, it's dishonest.

Anyway, I learned the other day that this "dirty little secret" of the speaker industry actually has a name - the Gundry Dip, also called the BBC Dip apparently related to some early BBC research.

I'm trying to learn more about the origin of this intentional dip, but all I get from Google is links to people discussing it in audio forums. I'm trying to find something more authoritative. Does anyone know where I can read more about this?

--Ethan
I believe in Truth, Justice, and the Scientific Method

Gundry Dip or BBC Dip?
Reply #1
Folks,

I've been aware for a while that many loudspeakers intentionally reduce the "harshness" range around 2 to 4 KHz to sound smoother and less fatiguing. I've seen data for speakers having dips as large as 8 dB! I really hate that, especially in speakers sold to the pro audio market. If everyone mixes on speakers like that, those of us who care about accuracy and have flat speakers will have more harshness than usual. Plus, it's dishonest.

Anyway, I learned the other day that this "dirty little secret" of the speaker industry actually has a name - the Gundry Dip, also called the BBC Dip apparently related to some early BBC research.

I'm trying to learn more about the origin of this intentional dip, but all I get from Google is links to people discussing it in audio forums. I'm trying to find something more authoritative. Does anyone know where I can read more about this?

--Ethan


The only thing I can find is the original spec for the LS3/5a mini-monitor (www.bbc.co.uk/rd/pubs/reports/1976-29.pdf). That dip is not specified there. So, it either happened before or after this, and the published data online is sketchy. Alan Shaw at Harbeth inherited all the R&D work of Dudley Harwood from the BBC and he's probably both the source of information and the author of many of the discussions in audio forums you saw on Google.

I've counted at least four different explanations for the dip, however. So maybe Alan Shaw knows and isn't telling! Shame... he seems rather sensible and distinctly free from the usual audiophile disorders.
  • Last Edit: 10 October, 2009, 06:49:09 PM by Gag Halfrunt

Gundry Dip or BBC Dip?
Reply #2
Thanks, that's more info than I had before.

Anyone else?

--Ethan
I believe in Truth, Justice, and the Scientific Method

  • ArgNostic
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Gundry Dip or BBC Dip?
Reply #3
I didn't know it had a name, nor that it was back. In the early 1970's, when I was a teenager and my father (Arthur A.) was developing a $130/pr ESL hybrid, he explained to me in tones that combined resignation with a bit of retained incredulousness and amusement that some of the popular speakers against which the new one would have to compete had a midrange dip, and that people described the sound as "smooth". Around that time, there was also a minor proliferation of gear that had a midrange control, usually called "presence", in addition to bass and treble, presumably used typically for diminishing the range. He attributed the trend to a combination of sonic fashion and a longstanding tendency for electrodynamic speakers to have clearly bothersome distortion in this range.

David Janszen

Gundry Dip or BBC Dip?
Reply #4
When you first started this thread.
I thought a Gundry Dip was a special
chocolate coating on British custard,
but I have thought about the issue
and finally have almost two cents to contribute,
or maybe no sense?:
.
The Fletcher-Munson effect tells us
human ears have difficulty hearing
deep bass and high treble,
relative to midrange frequencies. 
.
The ear's "frequency response" seems realtively smooth only at high volumes. 
.
So when audiophiles listen to music at less than "high volumes",
they may not hear the deep bass and high treble
as well as the recording engineers intended.

The best fix might be equalization,
but the settings would vary with average SPL of the music,
(like the Fletcher-Munson curves do)
so an EQ setting that works with one CD,
might be counterproductive with another CD.
.
An alternative to doing nothing would be to reduce output at the frequencies
where the ear is most efficient = The Gundry Dip.

By reducing the SPL of frequencies where the ear is most efficient,
you would reduce the Fletcher-Munson problem
where mid-range frequencies seem subjectively too loud
relative to bass and treble.

If my hypothesis is right,
and all I remember about "hypothesis"
is that it has something to do with triangles:
-- The Gundry Dip would probably
be good news at low volumes,
but not when listening to music
at high average SPL's.

Gundry Dip or BBC Dip?
Reply #5
Thanks Richard. It's not only that we hear more efficiently at 2-4 KHz, but those frequencies are also tend to be more irritating when loud.

--Ethan
I believe in Truth, Justice, and the Scientific Method

Gundry Dip or BBC Dip?
Reply #6
Folks,

I've been aware for a while that many loudspeakers intentionally reduce the "harshness" range around 2 to 4 KHz to sound smoother and less fatiguing. I've seen data for speakers having dips as large as 8 dB! I really hate that, especially in speakers sold to the pro audio market. If everyone mixes on speakers like that, those of us who care about accuracy and have flat speakers will have more harshness than usual. Plus, it's dishonest.

Anyway, I learned the other day that this "dirty little secret" of the speaker industry actually has a name - the Gundry Dip, also called the BBC Dip apparently related to some early BBC research.

I'm trying to learn more about the origin of this intentional dip, but all I get from Google is links to people discussing it in audio forums. I'm trying to find something more authoritative. Does anyone know where I can read more about this?

--Ethan



I can certainly understand that you wouldn't want this sort of thing  in a control room. I wouldn't either. But as a consumer product I fail to see the problem much less the alleged dishonesty. If this dip is preferable to some consmers why is it wrong? Unless the speakers are specifically advertised as not having this dip how is it in any way dishonest?

  • Axon
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Gundry Dip or BBC Dip?
Reply #7
To some degree, Harmon/Sean Olive's research strongly asserts that listeners will universally prefer a flat frequency response under blind listening conditions. That alone is a pretty good reason.

Beyond that, as far as individual preferences are concerned, the adjustment of preferences via a change in speakers seems less preferrable than getting a speaker with the flattest response and then applying eq. I'd imagine that the latter approach will generally yield a final response that is less ragged and lower Q in nature, although I don't have any real evidence to back that up.

Gundry Dip or BBC Dip?
Reply #8
I can certainly understand that you wouldn't want this sort of thing  in a control room.


Right, and that's my main concern.

Quote
Unless the speakers are specifically advertised as not having this dip how is it in any way dishonest?


There's the rub. Have you ever seen a speaker vendor admit their speakers are intentionally skewed to sound "better?"

--Ethan
I believe in Truth, Justice, and the Scientific Method

Gundry Dip or BBC Dip?
Reply #9
I can certainly understand that you wouldn't want this sort of thing  in a control room.


Right, and that's my main concern.

Quote
Unless the speakers are specifically advertised as not having this dip how is it in any way dishonest?


There's the rub. Have you ever seen a speaker vendor admit their speakers are intentionally skewed to sound "better?"

--Ethan



actually yeah, Beveridge. But I'm not terribly concerned as a consumer about ad copy nor do I expect to find 'confessions" there. Such skewing is easy enough to measure no? I would think that anyone who makes flat frequency response a priority will simply pass on any speakers that don't demonstrate this desired quality with published measurements. If one picks a skewed speaker by ear then it stands to reason that said person might actually prefers such skewing.

Gundry Dip or BBC Dip?
Reply #10
To some degree, Harmon/Sean Olive's research strongly asserts that listeners will universally prefer a flat frequency response under blind listening conditions. That alone is a pretty good reason.


Pretty good reason for what?

Beyond that, as far as individual preferences are concerned, the adjustment of preferences via a change in speakers seems less preferrable than getting a speaker with the flattest response and then applying eq. I'd imagine that the latter approach will generally yield a final response that is less ragged and lower Q in nature, although I don't have any real evidence to back that up.



How do you draw the conclusion of which is less preferable? Less preferable for whom?  If flat frequency response is such a significant force in preference then ultimately the speaker makers who prioritize this will win the market share no? I say let the market decide. Choice is IMO a good thing. The existance of speakers with deliberate dips in the frequency response don't prevent audiophiles who prioritize flat frequency response from seeking out and buying speakers from manufacturers who share that priority. So if some audiophiles like those speakers with the dip leave em alone! let em have what they want.

  • solive
  • [*][*][*]
Gundry Dip or BBC Dip?
Reply #11
Folks,

I've been aware for a while that many loudspeakers intentionally reduce the "harshness" range around 2 to 4 KHz to sound smoother and less fatiguing. I've seen data for speakers having dips as large as 8 dB! I really hate that, especially in speakers sold to the pro audio market. If everyone mixes on speakers like that, those of us who care about accuracy and have flat speakers will have more harshness than usual. Plus, it's dishonest.

Anyway, I learned the other day that this "dirty little secret" of the speaker industry actually has a name - the Gundry Dip, also called the BBC Dip apparently related to some early BBC research.

I'm trying to learn more about the origin of this intentional dip, but all I get from Google is links to people discussing it in audio forums. I'm trying to find something more authoritative. Does anyone know where I can read more about this?

--Ethan


Neither I nor Floyd Toole had never heard about the Gundry dip until about 2 months ago when an audio reviewer used the term in an email to us. Many poorly designed 2-way loudspeakers already have dips in the sound power response in the cross-over range 1-3 kHz where the directivity of the woofer is too high compare to the directivity of the  tweeter at those frequencies. As a result, this produces  a notch in the sound power response of the loudspeaker, usually followed by a peak.  Depending on the bandwidth and depth of the notch, it is the peak that is often heard as sounding objectionable (harshness, hardness or excessive brightness).  The extent to which this a problem depends on whether you are sitting on or off axis, and the reflectivity of the room. Some room correction products, by default, have a dip in their target curve in an attempt to compensate for this sound power problem, essentially trying to second guess whether or not the loudspeaker is well-designed. As I've shown in a recent AES paper - this doesn't always lead to good results.

You can see this notch/peak in the BBC frequency graphs of the LS3. I also wonder, if people are not hearing distortion when these small loudspeakers are overdriven.

Cheers
Sean
  • Last Edit: 24 October, 2009, 03:47:38 PM by solive
Sean Olive
Audio Musings

  • ajinfla
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Gundry Dip or BBC Dip?
Reply #12
Neither I nor Floyd Toole had never heard about the Gundry dip until about 2 months ago when an audio reviewer used the term in an email to us. Many poorly designed 2-way loudspeakers already have dips in the sound power response in the cross-over range 1-3 kHz where the directivity of the woofer is too high compare to the directivity of the  tweeter at those frequencies. As a result, this produces  a notch in the sound power response of the loudspeaker, usually followed by a peak.  Depending on the bandwidth and depth of the notch, it is the peak that is often heard as sounding objectionable (harshness, hardness or excessive brightness).  The extent to which this a problem depends on whether you are sitting on or off axis, and the reflectivity of the room. Some room correction products, by default, have a dip in their target curve in an attempt to compensate for this sound power problem, essentially trying to second guess whether or not the loudspeaker is well-designed. As I've shown in a recent AES paper - this doesn't always lead to good results.

Hi Sean. Would it be unfair to categorize this as applying aspirin and band aids to the cancer of poor acoustic source design?
What are your thoughts on balancing acoustic absorbers (aka room "treatments") and spaciousness? If I understand correctly, Toole recommends none above schroeder?
You can see this notch/peak in the BBC frequency graphs of the LS3. I also wonder, if people are not hearing distortion when these small loudspeakers are overdriven.

Cheers
Sean

Probably both. The power issue should not be quite as prominent in the very near field (which is where these are supposed to be used IIRC). But driver non-linearities that manifest themselves as distortion would certainly be heard at monitoring levels. Maybe even diffraction effects?

cheers,

AJ
Loudspeaker manufacturer

Gundry Dip or BBC Dip?
Reply #13
Depending on the bandwidth and depth of the notch, it is the peak that is often heard as sounding objectionable (harshness, hardness or excessive brightness).  The extent to which this a problem depends on whether you are sitting on or off axis, and the reflectivity of the room.


Thanks Sean, your posts are always highly informative!

I've seen third-party on-axis measurements of ten "pro audio" loudspeakers showing nulls being more prominent than peaks. So I assume the nulls are intentional. One popular speaker (Dynaudio BM-15) had a null at 3 KHz about 8 dB deep! My main interest in asking here is to learn about the history of this, since it was "new information" to me that speaker makers might do this on purpose.

--Ethan
I believe in Truth, Justice, and the Scientific Method

Gundry Dip or BBC Dip?
Reply #14
Folks,

I've been aware for a while that many loudspeakers intentionally reduce the "harshness" range around 2 to 4 KHz to sound smoother and less fatiguing. I've seen data for speakers having dips as large as 8 dB! I really hate that, especially in speakers sold to the pro audio market. If everyone mixes on speakers like that, those of us who care about accuracy and have flat speakers will have more harshness than usual. Plus, it's dishonest.

Anyway, I learned the other day that this "dirty little secret" of the speaker industry actually has a name - the Gundry Dip, also called the BBC Dip apparently related to some early BBC research.

I'm trying to learn more about the origin of this intentional dip, but all I get from Google is links to people discussing it in audio forums. I'm trying to find something more authoritative. Does anyone know where I can read more about this?

--Ethan


Reviving this a touch, I found this from the F.A.Q. on the Harbeth website

Quote
There is much myth, folklore and misunderstanding about this subject.

The 'BBC dip' is (was) a shallow shelf-down in the acoustic output of some BBC-designed speaker system of the 1960s-1980s in the 1kHz to 4kHz region. The LS3/5a does not have this effect, neither in the 15 ohm nor 11 ohm, both of which are in fact slightly lifted in that region.

According to Harbeth's founder, who worked at the BBC during the time that this psychoacoustic effect was being explored, the primary benefit this little dip gave was in masking of defects in the early plastic cone drive units available in the 1960's. A spin-off benefit was that it appeared to move the sound stage backwards away from the studio manager who was sitting rather closer to the speakers in the cramped control room than he would ideally wish for. (See also Designer's Notebook Chapter 7). The depth of this depression was set by 'over-equalisation' in the crossover by about 3dB or so, which is an extreme amount for general home listening. We have never applied this selective dip but have taken care to carefully contour the response right across the frequency spectrum for a correctly balanced sound. Although as numbers, 1kHz and 4kHz sound almost adjacent in an audio spectrum of 20Hz to 20kHz, the way we perceive energy changes at 1kHz or 4kHz has a very different psychoacoustic effect: lifting the 1kHz region adds presence (this is used to good effect in the LS3/5a) to the sound, but the 4kHz region adds 'bite' - a cutting incisiveness which if over-done is very unpleasant and irritating.

You can explore this effect for yourselves by routing your audio signal through a graphic equaliser and applying a mild cut in the approx. 1kHz to 4kHz region and a gradual return to flat either side of that.


The Gundry dip. What does it mean?

Hope that helps
  • Last Edit: 22 November, 2009, 06:08:50 PM by Gag Halfrunt

Gundry Dip or BBC Dip?
Reply #15
Excellent, thanks very much!

--Ethan
I believe in Truth, Justice, and the Scientific Method

  • Ken G
  • [*]
Gundry Dip or BBC Dip?
Reply #16
Well, of course having found this, I have to jump in.  My father, Dick Gundry, who spent almost all his working life in the BBC and was for many years responsible for maintaining technical standards in BBC Radio (which have sadly gone down since his retirement in about 1971), and who was known behind his back as golden ears, would not have been pleased to have his name attached to a deliberate departure from a flat frequency response in loudspeakers.  Has anyone any idea on how this term arose?  It must have been much more recent than 1971.

One of my father's responsibilities back in the late 1950s and early 1960s was the development of stereo techniques in preparation for a means to broadcast it.  (Some of those early experimental recordings have more recently been issued on CD by the BBC).  At that time the BBC developed its own monitoring loudspeakers on the grounds that commercially available ones were generally not very good.  I used to say that loudspeakers were either good or loud but not both!  During early stereo experiments it became apparent that the best BBC monitoring speakers of the day did not perform well in pairs for stereo because they did not match each other closely enough, particularly in phase response, so central images tended to be diffuse.  A major reason was that to accommodate variations in the drivers each and every cross-over network was adjusted for a flat amplitude response.  A new range of speakers was developed, but it is possible that at least for those first ones, the uniformity was considered more important than perfect flatness, and thus the speakers may have shown the "Gundry dip".  However it would not have been a design aim but a side-effect, and in any case my father would have had no input to the designs, which were developed at the BBC Research Department (Dudley Harwood, Spencer Hughes et al.)

Kenneth Gundry, San Francisco 

Gundry Dip or BBC Dip?
Reply #17
Wow, that's fabulous. Thanks very much for posting Kenneth!

--Ethan
I believe in Truth, Justice, and the Scientific Method

Gundry Dip or BBC Dip?
Reply #18
I have just been pointed to this thread, better late than never.

A bit of input from my time at the BBC, 70/80s.

BBC desks had a small elliptical speaker to emulate the type found in typical television sets of the time.  They were not intended to be used for monitoring but only to check that the sound would be sensible when heard through a domestic set.
I have known of sound mixers who believed that equalizing for a good sound through  a typical television set was more important than a flat response and would use devious means to achieve this.
They couldn't get away with simply using the small speaker since production staff would complain so they would sneak some equalization into the monitoring chain rolling off the high and low ends to closer match the small speaker.
This would then require the program material to have the highs and lows boosted to compensate leading to a relative dip in the mid-range.  The production staff would hear a flat mix and be unaware of the hidden effect on the program.

Usually they would just patch in the equalization and hope no one would notice but I have found equalizers hidden under desks.

  • Ken G
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Gundry Dip or BBC Dip?
Reply #19
I can confirm Rob Wansbeck's recollection of some TV studios, at BBC Television Centre at least where I was for two or three years in the mid-1960s, having available small loudspeakers representative of typical TV sets to permit a check that the programme remained intelligible when reproduced over poor audio equipment.  However, I am quite sure that has nothing to do with the "Gundry dip"!  I still would like to know how my father's name became associated with something that I am sure he would not have endorsed.

Kenneth Gundry