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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.

sound descriptions mine

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Bose Quiet Comfort 25
(noise cancelling)

BOSE QC25

The Bose QC25 (Quiet Comfort) is a noise cancelling headphone which is particularly suited for travelling by plains, trains and automobiles. The noise cancelling function works well for blocking out low frequency noises such as hum from engines etc.

It needs a single 1.5V AAA battery to operate. Without the battery it can also be used but doesn’t sound nearly as good as when the headphone is switched ‘on’.

A bit about Noise Cancelling

The way Noise Cancelling works is by using a microphone inside each cup.
That (small electret) microphone signal is amplified. This is the reason/cause for the audible noise that can be heard during silence or very soft passages in most NC headphones.

The better (read lowest noise) the microphone and microphone pre-amp are the less audible noise there will be.
Note: The Bose QC25 is the best performing NC headphone in this specific area (lowest noise).

The microphone inside the cup ‘listens’ to ALL sounds in the ‘ear chamber’ and thus picks up the music playing AND the, attenuated by the pads and cups, outside noises as well.

The Noise Cancelling trick works as follows.
The ‘inside microphone’ listens to what is ‘heard’ inside the cups and compares this to ‘silence’.
It then applies the ‘opposite’ signal to the driver in order to make the final result ‘silence’.
This is as good as ‘instant’. Audio is a very ‘slow’ signal for electronics.
This works the same as negative feedback in amplifiers. So if the microphone starts to ‘hear’ something the opposite signal is (instantaniously for audio) applied till the microphone ‘hears’ nothing again. That’s what the Noise Cancelling circuitery inside ‘aims’ for.
of course this isn’t ‘perfect’ as only sounds near the microphone are compensated (for a big part).
This only works for lower frequencies because phase differences become a problem due to the speed of sound and the wavelengths of these frequencies.

The driver is thus constantly driven by the microphone signal to be ‘silent’.

When one applies music to the same driver than the NC circuit would ‘hear’ that and apply a counter signal because it wants to keep the microphone signal ‘quiet’.
Thereby removing the wanted audio signal again, at least the lowest part of the audio spectrum.
So the trick is to ‘tell’ the microphone amplifier that the music signal is a wanted signal.
This is extremely easy to do. Replace the ‘0V (silence)’ with the audio signal.
This way the music signal is now the reference and the sound inside the cups thus is not ‘referenced’ to silence any more but to the actual electrical signal.

So the microphone ‘measures’ the acoustical signal inside the cups and compares that to the applied signal. When the headphone driver/acoustics itself ‘colours  the sound’ (which could be dsitortion, resonances or uneven frequency response) it ALSO (thus as a very desired side effect) corrects the ‘errors’ the mic hears to the actual signal. The heard audio signal is thus a LOT closer to the actual applied signal when the NC is switched on than if it were when switched off.
This can easily be seen in the graphs below.

Therefore, when you switch NC on the frequency response ALSO becomes as ‘flat’ as the microphone/amplifier is, at least for frequencies below say 2kHz.

The better (the more linear in FR and less distortion) there is from the driver, the less ‘compensation’ is needed as well.
In the end, it means outside noises are attenuated, at least as well as the mic can do, and the acoustical music signal is a closer ‘copy’ to the applied electrical music signal.
When you switch it on the FR thus becomes much closer to ‘ideal’ (some manufacturers apply a small tilt or bass boost b.t.w.).

measurements

Below the frequency response of this headphone with the power switch ‘OFF

Bose QC25 'off' FR

Yes, that’s an ugly bump between 60Hz and 200Hz reaching a good +10dB. This is the range of ‘bloated/muddy’ bass.
It sounds dark and muffled when switched off, voices are still ‘clear’ though. The treble is too soft but still present and a bit rolled-off.

When the headphone’s noise cancelling circuit is switched on the headphone ‘transforms’.

Bose QC25 'on' FR

Reproduction from the deepest lows to upper mids now becomes ‘balanced’ tilted slightly towards the warmer side of things (approximates ‘room’ curves). Clarity improves and also the treble doesn’t sound rolled-off any more. A quite balanced sound, certainly at lower listening levels. Because of the noise cancelling properties it doesn’t have to play loud anyway.
The treble, while tonally quite pleasant, and hifi-ish sounding still ‘lacks’ compared to more hi-fi oriented headphones. It does sound ‘smooth’ yet lacks the finer nuances top headphones are able to produce.Still, the quality is high enough to enjoy music while travelling.

Below the difference one gets when the QC25 is switched ‘off‘ and switched ‘on‘.

BOSE QC25 on versus off

As can be seen, everything up to 3kHz improves dramatically and you gain some overall volume as well as a few dB in the treble. The jagged treble response is an indicator of ‘lesser quality treble’.

Below the distortion plot of the QC25 with the power switch in the off position.

DIST BOSE QC25 off (Left)

The amount of distortion isn’t really bad, some third harmonic (clipping type) for the lowest frequencies.

Below the distortion plot of the QC25 with the power switch in the on position.

DIST BOSE QC25 on (Left)

Distortion in the bass area is impressively low, this is due to the ‘feedback’ Bass is good and clean sounding.
The distortion above 200Hz is slightly increased till about 3kHz.

Below the CSD of the QC25 with the power switch in the off position.

CSD Bose QC25 off

At 500Hz there is a substantial amount of energy ‘lingering’ on which results in less defined and poor midrange. Above 2kHz some short lived resonances are visible. Nothing to worry about though.

Below the CSD of the QC25 with the power switch in the on position.

CSD Bose QC25 on

The mids are much better controlled now and clarity has improved.

Finally the squarewave response plots and impulse response plot.
On the left side with switch in the ‘off’ position, on the right the ‘on’ position.

Bose QC25 SQR

The low frequency response plot goes from very poor to exemplary.

The 440Hz squarewave shows steeper rising and falling edges which indicate improved ‘attack’ and clarity.

The 100us impulse response plot shows the treble to be somewhat (pleasantly) reduced. slightly improved ringing but still not as fast as some of the better headphones.

At around € 250- € 300 there are better sounding headphones around but the noise cancelling feature and the increase in sound quality the headphone shows still make it a good headphone for travel purposes. For enjoying hifi at home there are better options.

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