MKH Story

The MKh STory
Page 4
Difference frequency distortion of studio condenser microphones
(cardioids)
The blue curve shows the improvement yielded by the push-pull capsule
design of the MKH 40 compared with other top-quality studio condenser
microphones
Sine generator
f1 = 200-20000
Sine generator
f2 = 280-20080
f2, 2f2, 3f2 etc.
f1, 2f1, 3f1 etc.
1. f1, f2
2. 2f1, 2f2,
f2-f1, f1+f2
3. 3f1, 3f2
2f1-f2, 2f2-f1,
2f1+f2, 2f2+f1
etc.
Band-pass
filter f2-f1
f2-f1 = 80 Hz = const.
Difference frequency test set-up
The microphone under test is simultaneously exposed to two tones of 104
dB SPL via two loudspeakers. The distortion component at f2-f1 is selected
by a narrow band-filter and measured.
80 Hz
New non-linearity analysis
A new challenge for microphone development arose at the beginning of the 1980s when digital recording found its
way into the studios, and the LP was, over time, replaced by the CD. Now the music connoisseur, for the first time,
could hear the filigrees of sound in the same way that the recording engineer could. But now any imperfections in
the microphones, which were formerly concealed by the noise and distortion of the tape and the vinyl record, were
now detectable by anyone with decent equipment. At that time Sennheiser made investigations on the sources of
the tonal differences between studio microphones for optimising a new line of studio microphones. Not only the
frequency responses and directional characteristics of established studio microphones were reviewed but their
non-linear distortions were also analysed.
At that time distortion measurements on
microphones were regarded as problematic because
sound sources with sufficiently low distortion were
not available. Therefore the specified distortion
exclusively concerned the microphone amplifier.
Capsule distortion was not included. Sennheiser
eliminated these difficulties by measuring the
difference-frequency distortion instead of the
more common harmonic distortion (THD). Two
sounds of equal pressure (104 dB SPL) were
applied simultaneously and separately via two
loudspeakers to the microphone under test. The
frequencies differed by 80 Hz. The distortion
component generated by the microphone at the
difference frequency 80 Hz (difference tone) was
filtered out and measured. The distortion of the
loudspeakers did not affect the test results because
each loudspeaker produced only a single tone, and
the harmonic distortions (THD) of the loudspeakers
were far beyond the pass-band of the filter. The
twin-tone signal was swept from 200 Hz to 20 kHz
with an 80 Hz frequency offset. Contrary to THD
measurements, the difference-frequency method
enables measurements in the whole upper audio
frequency range as the difference-frequency
component is kept inside the audio frequency
domain.
The measurement results revealed very individual
distortion characteristics of the various microphones.
The distortions were low at low frequencies but
increased remarkably at higher frequencies. The
onset frequency was lower for large capsules and
higher for small ones. The distortion increases almost
linearly with the sound pressure. As the sound
pressure level of the test tones was more than 20 dB
below the overload level of the microphones, at least
ten times higher distortion can be expected near the
overload limit.