HD238
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published: Sep-4-2013, updated Feb-20-2023
NO SMOOTHING is applied to the shown plots. Most measurement sites have some smoothing applied which ‘irons flat’ sharp peaks and ‘wiggles’. I do not use smoothing because some info about sound quality is lost when plots are smoothed.
Aside from a small correction of the microphone itself also some correction in the lowest frequencies is applied to the plots to compensate for the perceived loss of bass when using headphones. This is described HERE in more detail.
A ‘horizontal‘ frequency response curve on the shown frequency response plots on this website thus indicates a perceived ‘flat’ tonal signature.
ALL measurements are made with a good SEAL on a flatbed measurement rig.
The shape of your head, bone structure, pad size, pad ‘softness, (compliance), hair or no hair and or wearing glasses may (drastically) change the frequency response of some headphones, so… your personal experience may differ substantially from these plots.
Frequency response (tonal balance) is the most sound-determining aspect of headphones. A horizontal line shows audible neutral response in the plots on this website. Deviations in different severity at different frequency bands have an effect on the sound character.
The bigger the deviation the stronger the effect.
Below an aid to help determining the sound character of headphones with relation to the frequency response.
Sennheiser HD238
A decent sounding on-ear headphone with a more luxurious fit and finish and higher price tag is the Sennheiser HD238. It was sold between 2010 and 2014 for around € 80.- and was succeeded by the HD239.
specifications
Type: On-ear, open
Usage: Home, Portable
Isolation: decent
Driver type: dynamic
Pads: replaceable, soft regular foam, velours outside, ear part is pleather.
Outer pad dimensions: Height: 65mm, width: 49 mm, thickness: 16mm
Collapsible: yes, but can fold flat
Headphone connector: fixed.
Cable: 1.4m terminated in nickel-plated 3.5mm TRS plug
Driver size: ø 36mm
Nom. power rating: 0.1W
Max. voltage: 2V
Max. current: 55mA
Max. S.P.L.: around 120dB
Impedance: 35 Ω (measured)
Sensitivity: 114dB @ 1V
Efficiency: 99dB @ 1mW
Weight: 102 g.
Color: black with silver accents.
Clamping force: low (2N)
sound
A Warmish sound with highs that are quite present. The peak at 3kHz is narrow and gives it everything a bit of an ‘edge’ to it.
Bass sounds good and is not overblown or rolled-off. The sound is not very dynamic and upper mids and treble are not of good quality. Replacing the foam insert in front of the driver with some wool reduced the overly present treble making it sound less sharp.
measurements
Below the frequency response of the HD238. (Left, Right)
Channel matching is excellent ! The sound is warm with a present treble but is lacking a bit in sound quality.
Below the phase response of the HD238.
There are no real surprises here. Only above 3kHz some sharp changes usually indicative of problems in clarity.
Below the Group Delay of the HD238
No pad bounce only some issues visible at around 3kHz.
Below the distortion plots of the HD238:
Below the same distortion plot but with the vertical scale in percentages instead of level differences. 
The distortion is quite high and is partly responsible for the bit ‘woolly’ bass reproduction and point towards compression at higher SPL (above 80dB). At lower listening levels the performance is a lot better.
CSD below shows some around 3kHz ringing which is not as bad a s it seems as our ear canal resonates also at this frequency so the brain tends to ignore slight ringing in that area of the frequency range. (Left, Right overlaid)
Some resonances around 1.5kHz and between 3kHz and 5kHz.
Below the step response of the HD238.
There is some overshoot and ringing visible. The wiggles up to 2ms show the upper mids and clarity is a bit less than desirable.
Below the squarewave and pulse response plots of this headphone (foam replaced by wool).
conclusion
An enjoyable but slightly ‘colored’ headphone. Not colored in an annoying way though. Do bear in mind that some of the plots above are NOT from an entirely stock HD238 !
DIY-Audio-Heaven 