K371

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Published: Apr-19-2020, updated: May-17-2020

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 severities 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.

AKG K371

The AKG K371 is a closed headphone with a 50mm dynamic titanium coated driver. It is available in black with very dark grey accents. The MSRP is around €199.- but retail price (2020) is between €110.- and €150.-

The headband can easily adjust over a wide enough 32mm range and is padded with a soft rubbery feeling part resting on the head.
L and R markings are located on the inside of the headband. A give away for the left cup is the cable entry as well.

The straight 1.2m and 3m cables aren’t thin, but also not ‘audiophile thick’. The 3m coiled cable is a bit more heavy in weight. The cables are mildly microphonic. The cables have a 3-pin mini XLR on the headphone side and a goldplated 3.5mm TRS with a gold plated screw-on 6.3mm adapter.
The 3 different cables that come with it ensures this headphone can be used in portable conditions (no mic/remote) and home and studio applications. This saves you from having to buy more or different cables.
As the headphone itself has a 3 pin mini-XLR connector this headphone can NOT be used with balanced amps. Nor is this possible with adapters.

Given the high efficiency (114dB/V) a high output power is not needed and is easily driven well even from a phone.
The low impedance 34Ω (measured, 32Ω specified) also is good for portable equipment but less suited for high output impedance OTL tube amplifiers.

The pads are large enough in inner height of 60mm and 40mm width and have a decent depth (19 mm once on the head) so most ears will fit. The pads are pleather (fake leather) with very soft memory foam inside. This creates a good seal which is essential for good sound (bass extension). The clamping force is very good between low and medium and quite comfortable even for longer periods. The K371 stays put and doesn’t move around when moving (your head) around.
When you receive the headphones the bottom side of the pads are squished. Also the first time you squish the pads the foam makes a ‘crackling’ sound. Just push the pads all the way down 2 or 3 times and it stops doing that and retains the correct shape. It may take a bit of positioning and not moving around too much to get good seal.

This headphone is quite comfortable and can be worn for long periods without getting too ‘hot’ on the ears. The headphone is light in weight.

The headphone feels and is quite sturdy. The mechanism for adjusting the ear cups is quite simple but works quite well. On both sides the cups can rotate to the back of the head so you can keep it on your head while listening to one cup. Handy for in the studio or having a short conversation.
Both cups can rotate all the way into the headband making them smaller when taking them along. The cups cannot fold flat but as they aren’t very wide this is not a big problem. It looks like some thought went into the design yet the mechanics of it are simple and effective.

specifications:

Type: On-ear, closed
Usage: Home, Studio, Portable
Isolation: lower frequencies are not attenuated much, higher frequencies are.
Driver type: dynamic
Pads: replaceable, pleather with soft memory foam
Inner pad dimensions: height: 60mm, width: 40mm, depth: 20mm
Collapsible: the cups can swivel backward (handy or DJ/musician) but not fold flat
Headphone connector: 3-pin mini XLR
Cable entry: left sided.
Cable: 3m coiled + 3m + 1.2m with 3.5mm TRS and a 6.3mm screw-on adapter
Driver size: ø 50mm (membrane = ø 41mm)
Nom. power rating: not specified, assumed 200mW
Max. voltage: 2.6V
Max. current: 75mA
Max. S.P.L.: 122dB
Impedance: 34Ω (measured)
Efficiency: 99dB @ 1mW
Sensitivity: 114dB @ 1V
Weight: 255 g.
Colour: black
Clamping force: medium/low
Accessories: screw-on 6.3mm adapter, 3 cables, pouch

Sound description:

The AKG K371 has a neutral tonal balance. This means all instruments sound ‘natural’ or at least are at the right level. This does not mean all people will perceive it this way.
Those that want to use this headphone as a monitor or even to check their mix on a headphone can use this headphone and get good results.
Is this the perfect headphone ? No it also lacks here and there a bit and while tonally correct it isn’t a real high flyer when it comes to finer details and smoothness in the treble.
Bass is on the correct level (so NOT for bassheads !) and sounds quite good providing you have a good seal. Mids sound ‘open’ and ‘forward’. There is no cuppy sound and the bass and mids are integrated nicely. Those preferring ‘laid-back’ sound should look elsewhere.
The upper treble and highs is not the K371’s strong point. It certainly isn’t performing poorly though. There is good clarity and treble is not over the top and there is enough ‘air’ and ‘detail’. The K371 doesn’t do things wrong here, it just doesn’t excel.
Those looking for a ‘soft’ and ‘smooth’ audiophile type of treble may be disappointed. Treble is slightly on the coarser side but certainly not ‘grainy’. No sibilance nor shouty sound and treble is at the correct level. Just not super refined.
Those looking for a closed headphone with a neutral sound should definitely audition the K371.

measurements:

Below the frequency response of the K371 (Left, Right)From 50Hz to 2kHz the response is very even and flat. The sub-bass is very well extended to even below 10Hz.
There is a slight dip (ignore the sharp peak around 4kHz) that is ‘filled in’ a bit by the pinna. This (as good as inaudible) dip is followed by small emphasis.
This is caused by small resonances at 5.5khz and 8kHz. It is not extreme so barely audible but lowering it by a few dB makes the sound a bit more ‘relaxed/laid-back’.
Nitpicking, but audible for the critical listener. Around 5kHz the distortion reaches an audible 1%.
The treble response (as opposed to the smooth bass/mids) with its dips and (fairly well damped) peaks is responsible for the very slight ‘coarseness’ in the treble.
Treble extension is excellent and the driver reaches at least 24kHz which is higher than most other headphones in this price class.

Below some comparisons with other headphones.

Below the K371 vs the K550It looks like the K371 was certainly not the first headphone that had ‘Harman’ frequency response. The K550 already had a very similar, but not exactly, frequency response.

Below the slightly cheaper K361 vs the K371

Below a comparison of the K371 with the similar priced DT770 Pro (250 Ohm)

Below the K371 vs the HD600 which is often hailed as being one of the most neutral but bass-light and a bit ‘too hot’ in the 3kHz area.Here we can see that the K371 has better bass extension and the ‘hot’ area around 3kHz is not present in the K371. However, the treble response of the HD600 is much smoother.
Indeed the HD600 lacks in sub-bass but the treble quality of the HD600 is measurably and audible ‘better’.

Below the K371 vs the similar priced Beyerdynamic Custom One Pro in ‘bass setting 2’
These perform similar in several aspects. The COP lacks the 5kHz emphasis and instead has a dip there. The COP extends to 20kHz. Upper mids on the COP are a bit more ‘laid-back’, the K371 has a bit more ‘detail’ due to the slight peak.

below a comparison of the, more expensive, Shure SRH1540 and K371The SRH1540 is a bit ‘fuller’ in the lows, the K371 a bit more ‘neutral’. These also differ in the upper-mids and treble where the sound quality of the SRH1540 is a bit better (more refined).

Another comparison with a cheaper (newer black version) Takstar Pro-82
(in bass setting ‘1’) vs K371
While the bass response of the Pro82 is a bit more neutral in the lowest bass setting it has it is equally well extended. Above 1kHz the K371 measures better and sounds better as well.

Another closed headphone in a similar price class is the NAD VISO HP50 vs K371
Here the HP50 also shows good bass extension. The small bump between 100Hz and 300Hz , however, makes the bass of the HP50 a bit ‘woolly’. The K371 clearly performs better.  Mids of the HP50 have a warm ’tilt’ and lack in clarity where the K371 is more neutral. Treble of the HP50 sounds a bit ‘smoother’ even though the measurements do not seem to support this. Treble extension/’air’ of the K371 is better.

seal

As the seal of a headphone is of importance some experiments are done to see the effect.
(lots of) hair between the headphone and ear or an ear shape that does not allow a proper seal will affect the tonal balance.
Below: Perfect seal, a small seal breach by thin armed (reading) glasses just above the skin, thick armed glasses and a big seal breach.A small seal breach isn’t really problematic at all. The tonal balance is hardly changed when listening to these headphones with or without glasses. Only when there is a more substantial seal breach one will notice lower bass notes are subdued.
It is thus important to get a good seal with these headphones. The very soft and supple pads will ensure a good seal in most circumstances. One should not be moving around too much as that can intermittently break the seal.

Linearity

A test that I perform now and then is the linearity test.
This test measures the frequency response at various SPL (65, 75, 85 and 95dB) and then overlay the plots to see if the driver shows non linear behavior due to damping or other problems. Usually I don’t post them unless something is wrong but in this case I will post the plot anyway. It shows no linearity issues at all, they all overlay which means there is no compression even at 100dB (30Hz)

output resistance / damping-factor

As this is a dynamic headphone the frequency response might be amplifier output resistance dependent when certain higher output resistance amplifiers are used.
To test this the headphone is measured via a low impedance amplifier (0.2Ω) and a high impedance amplifier (120Ω).
On a higher output resistance amplifier the output level will be considerably lower. To compensate for this the amplifier is cranked up to the same level (12.3dB at 1kHz) as the low impedance amplifier. This way the plots are overlay-ed and it is easier to show the tonal balance differences.The headphone does not react much to higher output resistances other than in level (12.3dB). There is hardly any difference.
The output resistance of the used source/amplifier has NO practical influence on the sound/frequency response/tonal balance.

Below the distortion measurements of the K371 (Right channel)

Below the same distortion plot as above but with the vertical scale in percentages instead of level differences.

To get a sense of what level of distortion is audible below a plot showing audibility thresholds when listening to music or 2nd and 3rd harmonic distortion

The distortion in the bass is quite low, 1% is very good for a dynamic driver. Above 150Hz the distortion remains below 0.5%. The elephant in the room is visible around 4-5kHz where there are some resonance issues. This is also seen in the CSD below.
The 2nd harmonic distortion is probably lower than 0.3% around 1kHz as limits of the test rig are around that level.

Below the CSD of the K371. (Left and Right are superimposed) The narrow treble peaks (5kHz, 6.5kHz 10khz, 15kHz and 22kHz) are resonances. This also points toward some less refined treble.

Below the spectrum plot of the K371 (Right channel).
This plot looks quite good. The lower frequencies are fairly well damped.
Around 1.5kHz there is a little lingering. Also see the small ripple in the FR plot, it seems happy to vibrate at that frequency at very low levels. I don’t think this is detrimental to the sound.

The step response plot below  (Left, Right)The excellent bass extension and tonal accuracy is clearly shown in this plot. The horizontal line is barely sloping indicating excellent bass extension. No large resonances.
The membrane isn’t reaching 0dB so impulse response is slightly lower than desirable.

Lowering the slight treble peak

The slightly elevated 5-8kHz area is responsible for a very slight accentuation and can be lowered to normal levels using a passive electronic filter.
The schematic for it is shown below.

The effect of this filter is shown below.
The slight emphasis is lowered and more in line with the mids and treble This ‘relaxes’ the sound a bit making the sound slightly more ‘laid-back’.

drivers

Below pictures of the front and rear of the K371 driver (right side)
Notice how AKG added 3 little weights on the spot where the cable entry is on the left side. I assume this is to balance the weight.

OLYMPUS DIGITAL CAMERA

The front of the driver has some large open holes. Acoustic damping paper ring covering most of the center holes. When you measure in front of the paper the frequency response becomes quite different. The trace above is taken with the center of the driver positioned at the position of the middle of the driver. The trace is taken with the cups shifted on the test rig so the mic is opposite the acoustic paper ring (quite some offset).
Most likely covering the middle of the driver with the paper as well may have took away too much treble. The perceived (as well as measured) amount of treble thus depends on the position on the head. Personally I ind the red trace to have a high relation with the perceived sound.
Notice the 2 feedback ports (also has acoustic paper).

The outer plastic edge of the driver is 50mm, the membrane itself is 43mm in diameter.

summary

The K371 is a very neutral/realistic and dynamic sounding headphone. In its price class (street prices from €110.- to €180.-) this may well be the best sounding closed headphone.
This headphone is excellent for monitoring and even checking a final mix in a studio.
The cups that can swivel backwards can be handy in a studio.
It is also very well suited for portable usage but misses a mic/remote so more suited for personal music enjoyment than phone usage.
While this is a closed headphone there is little isolation from low frequency noises.
On a plain/bus the isolation will not be enough. Higher frequencies are well attenuated though. Also this headphone doesn’t fold into a tiny or flat package so takes up quite some space when not in use.
For lower budget ‘hi-fi’ music enjoyment this headphone sounds quite good. No it doesn’t sound really refined and ‘relaxed’ but sounds good for ‘active’ listening even at higher SPL. This is NOT a basshead headphone for those that are wondering.

Comfort is quite good. It is fairly light weight and clamping force is low.
The pads are replaceable which may well be needed after a while as the pads are pleather. It feels somewhat different from most other pleather pads, these have a rubbery feel to them. Maybe longevity is better than the typical pleather. One should not move around too much as that can momentarily break the seal a bit.
When one has a wider than average head it may be difficult to get a good seal.

The 3 supplied cables is a nice touch and makes this headphone quite universal without having to buy (sometimes expensive) aftermarket cables.
The cables are microphonic, so when the cables rub against your clothes you can hear this contact sound in the left cup.

For €110.- this headphone is amazing value. For €180.- this headphone is still affordable and a good reference headphone, certainly considering this is a closed headphone without any hint of typical ‘closed headphone’ coloration in the sound.
Highly recommended for sure.

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