HD 4.40 BT

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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.


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Sennheiser HD4.40 BT

HD4.40BT pict

The Sennheiser HD4.40 BT  is a closed headphone that can be used wired and via Bluetooth.  All BT formats  up to 4.0 are supported. AptX is present. The HD4.40 BT sells between € 90.- and  € 150.-.  The build quality appears nice. Most parts are plastic but seems sturdy enough. The looks are plain/minimalistic yet do not scream ‘cheap’.
The matt black finish and dark grey accents don’t have the cheap plastic look one can find on similar priced alternatives.

The comfort is quite good. The headphone is pleasantly low in weight. Weighing 225 gram means it can be worn for longer periods.
The headband is covered with rubber but very narrow at the point where it touches the head. After a while this may become less comfortable.

The pads feel soft and will accommodate small to medium ear sizes and shapes. They feel comfortable but can feel a bit warm and sweaty after a while. The ‘pleather’ pads (vinyl/PVC covered cloth)  may start to flake after a few years but are replaceable.
The thickness of the pads and memory foam together ensures a good seal and wearing comfort. The hole for the ears is unusually narrow. The height is good but you need to kind of ‘tuck’ your ears in there. Since I don’t have large ears I could fit them in there but as the ears are ‘bent’ forward a bit inside I can’t say it is very comfortable.

This headphone can be used wired as well wireless via a BlueTooth connection.
When the wire is used the Bluetooth connection cannot be switched on. For this the cable must be removed. It connects via Bluetooth (up to 4.0) and uses AptX. The HD4.40 is compatible with the following profiles: HFP (Hands-Free Profile), HSP (Headset Profile), AVRCP (Audio Video Remote Control Profile) en A2DP (Advanced Audio Distribution Profile).
It has NFC for more convenience when pairing the headphone with BT.
On a full charge the 600mAh LiPo battery it operates approx. 25 hours with Bluetooth.
It takes about 2 hours to charge it fully from a micro-USB port. This port is located on the right earcup.
It does not have noise cancelling. This is available in the more expensive HD4.50.

Isolation from outside noises is decent but not good enough to be used on planes etc.

The headphone has a few buttons. On the right cup there is a button for taking calls.
and other the buttons that are related to playing music. Volume up and down buttons, pause/play skip forward/backward and fast forward and reverse during play.

The cable is from silicone and a bit ‘springy’. In the headphone it connects with a twist-lock mechanism. This ensures it stays put but as an disadvantage it won’t come out when one yanks on the cable. The cable is microphonic but when rubbing against clothes it isn’t that audible. only when touching it close to the ear cup. The cable entry is on the right cup. In most cases the cable entry is on the left cup.
L and R markings are present on the inside of the headband above the yokes.

There is more than enough adjustability in height and width so it will suit most people.
The turning of the cups is a bit stiff and creaks when adjusting it.
Once you have a good fit it stays in place.


 Type: almost over ear, closed, Bluetooth
 Usage: Home, portable
Driver type: dynamic
Pads: replaceable, pleather, memory foam
Internal pad dimensions: height: 60mm, width: 28mm, depth: 21mm
Collapsible: yes but does not fold flat.
Headphone connector: locking 2.5mm TRRS
Cable entry: single sided (right side)
Cable: replaceable, 1.4m with gold plated angled 3.5 mm TRS plug
Driver size: 36mm Ø
Nom. power rating: 0.2W (estimated)
Max. voltage: 1.9V
Max. current: 100mA
Max. S.P.L.:  (passive mode) 119dB
Impedance: 20Ω (measured), spec says 18Ω
Efficiency:  96dB/1mW (113dB/1V)
profiles: A2DP/AVRCP/HSP/HFP
BT version: up to 4.0
Codecs: aptX
Weight: 225 gram
Colour: black with dark grey accents
Clamping force: medium
Accessories: 1.4m angled TRS cable,  0.5m micro USB charging cable, pouch, manuals.

sound description (wired)

The sound is dynamic, full and clear sounding and of good quality.
The bass goes very deep and is boosted. I would say pleasantly boosted. On most pop music and on the go some extra bass is welcome. The bass is ‘punchy’ and ‘full’. Those that like to listen to well recorded music will find the bass a bit too much, too boomy. The bass does not bleed in the mids and is perhaps a tiny bit ‘dis-attached’. I prefer this over bass bleeding in the mids.
The mids (voices and instruments) are clear and ‘forward’ sounding.  These headphones don’t have the typical ‘cuppy’ closed sound in the mids. The mids sound quite dynamic.
The mids aren’t ‘warm’ sounding. There can sometimes be a shrill ‘edge’ to some recordings. Male vocals sound a bit thin, as in not ‘full bodied’.
The treble sounds well extended and has ‘air’ and is of above average quality and is just a tad emphasized but not in an obvious amount. This headphone sounds detailed.
The sound doesn’t break up at higher listening levels and can play quite loud from a decent source. The HD4.40 kind of invites you to play a bit louder.
The stereo image is ‘normal’ as in not narrow and not super wide.


Below the frequency response of the HD4.40 (Left, Right) using a wired connection.FR HD4-40Channel matching above 150Hz is very good. Below 150Hz there is a 1.5dB mismatch.
The left channel is slightly higher in amplitude. This is audible, the bass seems to be more prominent on the left. Measurements at Rtings also show a louder left channel.
I suspect that the left cup has a bit less leakage or more room inside. This may explain the difference.
The dip at 250Hz is the reason why the lower mids don’t sound ‘full’ and voices sound a bit ‘thin’. When the dip weren’t there the boosted bass would bleed into the mids.
The gradual upwards slope between 200Hz and 1kHz points towards the ‘thinner’ mids. The treble is at the correct level. There is a slight emphasis around 7-8kHz.
Above 14kHz the response drops off yet does not seem to affect the ‘air’ as much.
There is a small dip around 4kHz but feel this isn’t very obvious due to the ‘thinner’ mids and the small treble peak which mask the effect.

Using the Bluetooth connection doesn’t change much in the tonal balance.
Below the wired connection vs the Bluetooth connection.
HD4.40 L
It should be noted that the wired plot is made using a sweep and the bluetooth plot is made using a 1minute recording of white noise. Also the headphone may not be placed exactly at the same position.


Closed headphones usually depend on a good seal (pads sealing off properly all around the ear) to get a good subbass response. This headphone is not different.
Below perfect seal, with thin arm reading glasses, with thicker arm (sun) glasses and a seal breach (6.3mm TRS plug between the pad and testfixture)seal
It is evident a good sela is important for the HD4.40 to have a deep bass response.
The sound is quite neutral when a slight seal breach is there. Too much loss of seal results in a very audible drop of lows.
Getting a good seal is quite important for this headphone.

damping factor/output resistance (passive mode only)

A higher output resistance from an amplifier can affect the tonal balance of a headphone. This is dependant on the impedance of the driver as that can vary depending on frequency.
Of course the plots abelow are only valid when connected directly to an amplifier using the 3.5mm TRS plug. When using a BT connection this isn’t applicable.

Below the effect of a 120Ω output resistor is shown versus a (close to) amplifier.
Do note that the traces are overlayed to better show tonal differences in reality the output will be 16.4dB lower due to the higher output Resistance.
R 120 -16.4dBThe tonal balance does not change much. Driving this headphone from a higher output resistance source makes the bass a bit more bloated than it already is.

Below the distortion plot of the HD4.40Dist L

Below the same plot but shown in percentage scale.Dist L percentSmall drivers usually show an increase in distortion at lower frequencies. The HD4.40 is no exception. Below 100Hz the 2nd harmonic distortion increases to 2%. This is not exceptionally high though at almost 100dB SPL (is not the same as 100 Phon). The higher 3rd harmonic distortion points towards ‘compression’ which is a kind of ‘soft clipping’.
Overall the distortion is pretty low.
The distortion in the mids will most likely be below the shown 0.2% as this is the measurement limit of my measurement system.

Below the CSD (Waterfall plot) of the HD4.40. Right and Left channel are superimposed.CSD HD4.40Around 7kHz there is a rather wide band and short lived resonance showing. Aside from the small bump in the frequency response this isn’t very audible nor sound degrading.

Another form of looking in the time domain is the spectrum plot. The amplitude is color coded and both the time scale and frequency scale differ as well.spectr RThe boosted bass below 200Hz is visible. At 1.6kHz and 2.5kHz there us some ‘lingering’.
This doesn’t show up in the CSD because this happens at a lower level than the CSD shows.

Below the step response with a dB scale (so not similar to an oscilloscope plot). (Left, Rightstep HD4.40
The broad rise to 3ms shows the bassy character. The traces remaining horizontal shows bass extension is excellent. There are no big dips and peaks after the initial rise for about 1ms and the rise is gradual. This means impulse response is quite good albeit a bit on the bassy side.


The Sennheiser HD4.40 BT is a decent headphone. Not everyone will like the narrow earpads and the headband which has a small contact point.
The pads themselves are comfortable and replaceable.
Together with Sennheisers app you get more options for tuning the sound to your liking.
Isolation is decent but not good.
The BT connection works well and over quite a big distance. The operation of the buttons takes some getting used to.

With headphones it is all about the sound, comfort and usability.
The sound is decent to good. Bass is emphasized but does not bleed into the mids.
The sound is quite ‘open’ and ‘forward’ with good details and treble.
It is not a neutral high quality headphone but this cannot be expected for the price either. Audiophiles should look elsewhere. Very well suited for casual listening and enjoying music.
The boosted bass and ability to play it loud (wired) make the headphone quite suited for outdoors, travel, at the office and at home.
The comfort can be a hit and miss depending on ear and headsize.
This headphone can be used wired and wireless. The sound quality is the same in these cases.

A decent sounding wireless headphone. For the price it will be difficult to find a bluetooth headphone with similar sound quality.

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