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Why do dynamic headphones get away with only one driver?

post #1 of 17
9/11/11 at 1:29am
Thread Starter 

Sorry if this has a really obvious answer... This applies only to dynamic Moving Coil driver designs. Planar, ribbon, electrostatic, bending wave, balanced armature, piezo, need not apply. 

 

Many dynamic headphones sound great (such as the T1, HD800, etc) and have a very even frequency response. 

 

I realise that multiple drivers in such a full sized dynamic headphone design would be a bad idea for many reasons. But why do they get away with a single driver when when dynamic floor/bookshelf speakers need subwoofers, woofers, midrange, tweeters, etc, to get a good frequency response range? Floor sized speakers can only dream of such an even response.

 

For dynamic floor speakers, is the full-range dynamic headphone driver design just not loud enough and is very directional? For example, could you just have many parallel (say, 100) identical T1 drivers put into a floor standing speaker enclosure (to simply get a room-filling volume) and get a good result without having the traditional subwoofer/woofer/midrange/tweeter/supertweeter system? Ignoring the fantastic amount of money that would require, of course. 

 

Picture is of a T1, for example. Thanks, Headroom, for the graph.  Single driver, doing better than many three/four driver floor speaker systems. 

 

 

 Beyerdynamic T1

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post #2 of 17
9/11/11 at 1:51am

Wow this is a good question. Unfortunately i don't have an answer but i am curious as to why as well. Subscribed.

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post #3 of 17
9/11/11 at 2:38am

I'm not exactly an authority on the subject, but this is what springs to mind as a likely explanation.

Speaker drivers are bigger and have to produce a far louder sound. Hence, outside specific frequencies the distortion of a given driver rises abruptly compared to headphone drivers - to resolve this issue they have multiple drivers for different frequency ranges. Headphone drivers don't have to go anything like as loud and thus distortion is dramatically reduced.

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post #4 of 17
9/11/11 at 9:07pm

About the 100 t1 drivers thing, that's what a line array basically is. 

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post #5 of 17
9/11/11 at 9:59pm

I am a speaker designer, and have not ventured into anything other than mods.  It will certainly be its own science, and a dedicated can designer could probably correct much of this, but as I've never seen any on HF, I'll put a few thoughts down, as this IS a good question.

 

The biggest difference is how much air you must move to ensure the listener perceives adequate volume.  Speakers must move exponentially more volume of air to achieve a given sound level because they are far further from your ear.  This creates an immediate problem: moving enough air at bass frequencies requires a large driver that is large and has very large excursion, and probably weighs a lot in order to be rigid enough to not flap wildly during large excursions.  Heavy, large drivers are not ideal to reproduce high frequencies because the diaphragms break up and create standing waves, distortion and other problems at high frequencies.  Hence you need a separate driver to handle treble.  

 

In a headphone, the driver is very close to your ear, moving a small volume of air.  As such, a small driver is able to produce adequate bass with minimal excursio via a small and light driver (since there's not so much air-mass pushing back on it) which is thus also able to reproduce high frequencies.  

 

A key benefit of this is that this eliminates the crossover, so in general the system will produce superior phase response, and avoid other problems such as driver sound-signature mismatch.

 

If you took a bunch of T1 drivers, you would either need a LOT of them to move enough air in the bass regions, and they would have to have longer excursion (otherwise more surface area, thus recreating the need for tweeters).  Plus you get interference patterns from multiple drivers spread over a line or an area, which creates horizontal and/or vertical lobes in the frequency response.

 

 

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post #6 of 17
9/11/11 at 10:40pm
you know the best crossover is no crossover. you should know that if you are a speaker designer since every limitation to a speaker driver is always in the crossover network,always.

even fullrange drivers for speaker building is idea if looking for the most accurate reproduction of sound. Fostex is still the big leader in Fullrange drivers and have schematics for builders on their website for cab designs since one of the biggest limitation of a fullrange speaker is it's enclosure you put it in. fostex has special complex designs to allow their fullrange drivers to hit sub-bass regions if the user wants it to. also full-range drivers are known to be best re-creating stereo image and soundstage.

headphones are the most practiced in all fullrange designs. manufactures spent countless hours since the 1950's to find ''how can this one driver re-create this frequency range''. they did countless and crazy innovative designs. also it has lot to do with ''human hearing'' limitations and how close the driver sits. the speakers are very close so say your speakers are 88db@1w and headphones are 88db@1mw, the same 88db will sound much louder with the headphones due to how close they are to your ears so basically meaning every single frequency is boosted as well cause the driver is producing soundwave directly into your ear drum with much less work compared to a actual loudspeaker.

if you think the t1's and hd800's are impressive then you must check out some vintage studio headphones as well. you'll be surprised how deep and high they can extend as well with precise accuracy. believe it or not,not much tech has changed since the golden era of the 70's when coming to headphone designs.
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post #7 of 17
9/11/11 at 11:13pm
Thread Starter 

So perhaps some of it might have to do with the fact that headphone drivers only need to make very small excursions (so floppiness etc. isn't as large of a problem), as they have a very low power output?

 

As far as working power...

 

A Denon D7000 (going sealed as the below speaker has a sealed cabinet) as a 50 mm headphone driver is designed to handle 900mw (per driver). That's about 20cm2, or 45 mw/cm2.

 

A single Bower and Wilkins 800 has 2x250mm, 1x150mm, and 1x25mm driver, and they are supposed to handle up to 1000w. That's about 1160cm2 of driver area, or 860mw/cm2.

 

 

These numbers are rough (and I know that drivers are cones, not circles, and all that, and maximum power handling doesn't mean it has to not have any distortion) but in this example,  a floor speaker needs to put out almost 20x the power/unit area. 

 

Hmm... interesting idea, too, about the interference patterns of a uber-driver-array...

 

We need to find a professor from "Beyer University" or the "Sennheiser Institute" to help us out.

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post #8 of 17
9/12/11 at 7:27am
Quote:
Originally Posted by RexAeterna View Post

you know the best crossover is no crossover. you should know that if you are a speaker designer since every limitation to a speaker driver is always in the crossover network,always.

Did you actually read what I wrote, because having no crossover was what I described as a "key benefit."
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post #9 of 17
9/12/11 at 7:57am
Quote:
Originally Posted by mrspeakers View Post


Did you actually read what I wrote, because having no crossover was what I described as a "key benefit."

actually not really till now. i didn't realize you mentioned it as well. i just saw op's question and tried answering it myself.
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post #10 of 17
9/12/11 at 12:10pm
The question to me is why many popular high-end IEMs use multi balanced armature designs. I guess it's for the advantage of balanced armature transducers, and putting multiple of them in there is a band-aid for the limited (good) frequency range. But on first glance it seems odd to go from multiple drivers in most speakers to single drivers in headphones to multiple drivers in many IEMs.
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post #11 of 17
9/12/11 at 4:20pm

Balanced armature has some unique and powerful attributes.  

 

1) The driver has a symmetric magnetic field and this generally means lower distortion

2) The nature of balanced armature drivers means they have restricted bandwidth 

3) They are extremely efficient and need little power

4) They can be very, very small which is ideal for IEMs

 

For all these reasons, they are excellent, though they do introduce the evil of a crossover, again.  Also, they are so small they store very small amounts of energy while in motion, and tend to have excellent transient response.

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post #12 of 17
9/13/11 at 7:34am

As ety has stated, you don't need more than one balanced armature. It is a very simple device with a very simple purpose: compress air in the ear canal. When you simplify audio reproduction to this level, demands on the driver ease up in some ways. Bass, for instance, becomes less problematic and it is fairly easy for IEMs to get a flat response out to about 1 or 2k.

 

Traditional headphones have a larger volume of air to work with, along with fit and earpad considerations, hence a more complex electroacoustic coupling that designers have to consider when tuning the (dynamic) driver. Many companies from the 60s to the 80s attempted to solve the problem by using two drivers, but the benefits of splitting up the bass/mids from the highs were outweighed by the increase in size, weight, complexity and marketability of some huge, butt-ugly headphones. The advent of Quadrophonic sound didn't help either.

 

To get back to the original question: you're dealing with radically different acoustic environments between speakers and headphones. I read a good chapter about this topic a while back, I'll try to dig it up and quote a few sections of it later.

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post #13 of 17
9/13/11 at 4:32pm

OK, the following are relevant excerpts from the Loudspeaker and Headphone Handbook (3rd) by John Borwick. I've omitted most of the technical parts.

 

This about headphone acoustics in general, not driver selection in particular.

Quote:
The first difference that comes to mind when comparing how loudspeakers and headphones
produce a sound signal at the ear is that in one case the ear is immersed in
a propagating sound field, and in the other it registers the SPL (sound pressure level)
in a leaky pressure chamber. ‘Since the ear drum is essentially a pressure detector,
pressure gradient, particle velocity and other effects do not influence the final sound
image.’ This is a daring claim, since much research has been performed dealing
precisely with this point, for example such elusive phenomena as ‘the missing 6 dB
between LS (loudspeaker) and HP (headphone) listening’...

Unlike loudspeakers, which produce a propagating sound field around the head of
the listener, the sound field of headphones is confined to a relatively small volume
of up to about 30 cm3. Since the ear is essentially a pressure detector, this difference
in mechanism is of little consequence as far as the subjective sound image is
concerned...

For a flat SPL response, the acceleration should be independent of frequency. To
fulfil this, the excursion must drop by 12 dB/octave on raising the frequency. This
happens automatically above the main resonance, where compliance is unimportant
compared with inertial mass...

In a closed headphone, the force pumps on a cavity, giving a sound pressure proportional
to excursion. Up to about 2 kHz, where the wavelength of sound is still large
compared with the dimensions of the cavity, the sound pressure is distributed
uniformly in the volume, even in the presence of mild leaks. The SPL and phase of
this uniformly distributed pressure does, of course, depend on the extent of the leaks.
In practice, leaks are always present whose influence dominates at low frequencies.
For intermediate frequencies (1 kHz), where their influence is very low, the cavity
can often be regarded as a pressure chamber. Here the pressure is in phase with the
volume displacement of the transducer membrane and its amplitude is proportional
to it...

Circumaural headphones are characterized by a relatively large coupling volume in
excess of 30 cm3, and an inner diameter of the cushions of at least 55 mm. The upper
limit for the validity of the lumped-element approach is about 2 kHz, since standing
waves in the coupling volume occur above this frequency, associated with a nonuniform
sound-pressure distribution. The upper limit for supra-aural headphones is
higher, but not as high as one would expect from the diameter of the coupling volume
alone, which can be as small as 20 mm, since the length of the ear canal, about 25 mm,
now contributes to the largest dimension of the coupling volume. However, improving
on the lumped element approach is much easier for the ear canal
than for the three-dimensional coupling volume....

The reproducibility of the given frequency response of a supra-aural headphone
is lower than that of its circumaural counterpart, due to the relative ambiguity of the
positioning of the earpiece. The advantage of supra-aural headphones is their lightness.
The extreme of this is seen in the popular mini-earphones, which can have
excellent sound quality, including bass, whose reproducibility is still a problem.
Between the two extremes – namely the circumaural and mini-earphones – the supraaural
headphones attempt to cover the whole surface area of the ear, thus reducing the unwanted leaks to a minimum.

 

And this graph was too awesome to pass up:

Image1.png

 

 

edit:

 

The chapter goes on to take apart the design of typical variants of various types of transducers. Sadly, it would be impractical to dump the whole thing here. Suffice to say that once you know the Thiele-Small parameters the design process is analogous to that of loudspeakers, except with headphone-specific components and much less room to work with smily_headphones1.gif


Edited by anetode - 9/13/11 at 4:49pm
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post #14 of 17
9/13/11 at 5:04pm

John Grado has tried with this prototype using 'unknown' Grado drivers :-

 

grado-33.jpg

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post #15 of 17
9/13/11 at 6:16pm
Quote:
Originally Posted by Gwarmi View Post

John Grado has tried with this prototype using 'unknown' Grado drivers :-

 

grado-33.jpg

 

I thought they were RS1 drivers? Old ones, since they look pink.

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