Les Cargill wrote:
Adrian Tuddenham wrote:
Les Cargill wrote:
[...]
Adrian - how do you couple a headphone transducer to a microphone for
measurement? By using a dummy head?
Make up a stack of thick hardcover books about the same thickness (and
density!) as the space between my ears. Allow the headphones to clamp
themselves over the stack and slip a couple of electret pressure
microphone capsules on thin wires into the cavities.
Ah. So for some models of 'phones, there is effectively no cavity, and
I suspect making one would munge the measurements severely. So I wonder
is a piece of whitewood with a hole drilled to have the electret's
surface flush with, or slightly under the plane of the wood would
work?
I expect it would, although the shape and volume of the human ear cavity
varies enormously between individuals - so anything which relied heavily
on exact cavity dimensions for its performance would give rather
unpredictable results in practice even if it showed up well on
standardised tests.
The sound reflecting properties of the lightly-embossed covers of
hardback books are probably nearer to human skin than the surface of
whitewood, but I haven't noticed any significant difference between
measurements on the 'dummy head' and the same measurements made on a
real head.
Something which can affect the response of headphones is the poor seal
caused by locks of hair or spectacle frame sides spacing the ear pads
off the side of the head. Pressing the pads firmly onto the ears can
often make a noticeable difference due to the improved sealing, whereas
further pressure, which compresses the pads enough to decrease the
cavity volume, has much less effect.
Connect the
headphones to an audio signal generator, connect the mic capsules to a
small recorder with suitable phantom power (Tascam DR-04) and read the
sound level off the recording level display. The measurements are good
to a dB or two - and are going to be far more accurate than the accuracy
of most budget headphones (and some expensive ones).
If there are any acoustic artefacts that need investigation, they will
change if the mic capsule positions are changed or the air cavity shape
is altered by compressing the earpads. If they don't change, then they
can reasonably be attributed to the headphones themselves
That's interesting - thanks for that. I would have thought you'd want
the geometry of things to be pretty rigidly controlled; didn't think of
moving the elements as a control like that.
It's a very powerful tool in acoustics; change the air space and see if
the problem changes. "Stuff a sock in it" can be a quick and valid
scientific way of tracking down the source of air cavity resonances.
--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk