This is something that gets asked often, and when it's asked on anything but an iPod specific board, it gets lost in 'X is better than iPod' arguments. So let's clear it up here.
The iPod is said to have a bass fall-off (i.e. decrease) with low impedance phones. This is most probably due to the component chosen as the DC blocking capacitor in the headphone output stage of the iPod. It's said to be particularly noticeable on the 3rd Gen iPod. How actually noticeable is the falloff? With very low impedance phones like many portable-orientated earphones, very noticeable is the answer. However, the higher up the impedance range you go the less noticeable it is.
So let's take said 3rd Gen iPod and hook it up to a no-budget soundcard like everyone else does to run their RMAA tests. I've chosen the Edirol UA-25. Since this question is asked often with canalphones, I've taken the Shure E2c, an increasingly popular choice and justifiably so. Here's how I connected everything up:
The iPod is plugged into a headphone splitter which allows me to connecto both the soundcard and the earphones. By doing this, the iPod 'sees' the headphone as an impedance load and allows me to measure the effect that has on the iPod with the soundcard. For each test, I set the iPod to the same comfortable listening volume using a piece of music for the most realistic possible results. The methodology used for the test is not rock-solid and I would say that it is only a relative guide.
The test doesn't end there though. I'm going to add one more item to the chain:
Look at where the E2c is plugged in. See the extension cable? That contains four resistors, two each for left and right channels, which adds 92 ohms of additional resistance. The E2c has as standard an impedance of 16 ohms. With the 92 ohm extension cable, what you end up with is a 108 ohm impedance phone. Why 92 ohms? Well I just needed to take total impedance to beyond 100 ohms for the test and I just matched the most suitable resistors I had :)
Now I'm going to show you the RMAA result for the frequency response between the 'unloaded' (i.e. the E2c just by itself) and the 'loaded' (the extension cable installed) set-ups.
Right. First you'll be saying 'man... that's a severe falloff even for the loaded E2c'. The key here is interpreting graphs properly and correlating it to real world experience. Note the numbers on the side and the frequency at the bottom. Take iTunes or Foobar on your PC, play your favourite tune and use the EQ to replicate the above. For example, to hear the effect of the bass falloff on the 'loaded' E2c, dial in 1db less on the EQ at 30hz. Do you hear different? Is it a big deal? I'll leave that up to you to decide.
Note that the above graph only shows the response of the iPod, not the phones being used. We're only looking at the effect that the phone has on the iPod. What you hear with the E2c will be different, because to you the effective response will be the response of the E2c (and it isn't flat) plus the above graph.
How do the iPod buds fare? Well, they have a higher nominal impedance (32 ohms) than the Shure E2c so the onset of the bass falloff is delayed. 'Loading' the iBuds with the same extension cable, you can see that the higher cumulative impedance (now 124 ohms) also further reduces the bass falloff.
Of course, if you choose a high impedance phone in the first place like the Koss KSC-75 and the Portapro (both 60 ohms nominal impedance), the Sennheiser HD25-1 (70 ohms), etc then you've gone a long way towards solving the issue. All phones up to a couple of hundred ohms will register some degree of bass falloff, but at around 70-80 ohms the falloff is negligible enough. Here's the iPod's behaviour with the 70 ohm Sennheiser HD25-1 compared with the iBuds and the E2c:
Once again, you're not seeing what you'll hear, because the above response gets coloured further by the response of the headphones. The HD25-1 has a fairly prominent treble and bass so you can bend the green line further with that in mind to arrive at an idea of how it might sound like to you.
There's also another way of looking at this issue. Headphones like the Sennheiser HD212 Pros have excessive bass response. The Sony MDR-V700DJ is another one with a hiked up mid-bass. The MDR-EX71SL earphone is also pretty bloated in the lows. All of these are low impedance phones and are affected by the iPod bass falloff. But the net result of the falloff is to control the 'bloat' response of these headphones and effectively tighten up the bass, so you might not notice anything particularly amiss. So it's sort of a workaround.
So:
Does the iPod have less bass than many other players when used with low impedance phones?
Most certainly.
Is it like a 'totally no bass' problem?
No. You can certainly still hear it. It's just not as present.
Are Apple eejits for not taking care of this issue?
Yes. Especially as it's the only thing which contributes to the 'poor sound quality' that many who don't like the iPod complain about.
So why haven't they fixed it?
You got me there. The 'word on the street' is that they might fix it in the next version. Who knows, they might.
Does the iPod have poorer sound quality than other DAPs?
No - it actually has better sound quality than some DAPs out there. But the determination of 'sound quality' is different among people, and not just to do with the subjectivity of these things. Some people just have the interpretation wrong. For example, the overall sound quality of the new Sony NW-HD5 is not appreciably better than the iPod. However, Sony have knowingly (or cynically?) dialled in a mid-bass boost even without EQ. Therefore, to many people it 'has higher sound quality'.
Is the bass falloff curable?
I think we established in the above test that it more or less is possible. You'll notice though that in addition to the bass recovery, with the E2c other changes have taken place further up the frequency range. So adding additional impedance to an existing low-impedance phone will cure the bass issue, but it might also change the sound of the phone in other ways. The ideal way to cure it will be with a pair of higher-impedance phones. However, if you actually want a large amount of boosted bass it's fair to say the iPod ain't the player for you. You're better off looking at the iRiver players.
So does this mean the iPod is no good and I shouldn't get it?
The differences in the bass delivery might put you off, but the iPod has decent core sound quality, the best playlisting facility in the business which really does change the way you listen to music, a very easy to use and pretty powerful management software which takes a sensible approach to anti-copying, and the best user interface. There's also the dock port, which allows much easier and better connection with home speakers and car audio. Some of the things which Apple haven't taken care in avoiding are strange, but on the whole it's a more mature, better thought out product than others on the market. I wouldn't dismiss it.
I'm also adding an additional requested point...
Is the bass falloff present if I amp the iPod through the Sik Din / Pocketdock and something like a CMOY, PIMETA, Porta Corda, Airhead amps, etc?
No. First of all the Line Out provided by the Din and Pocketdock (and the Apple dock) is designed to drive a Line Out load, so it all works fine. Actually, you won't get the bass falloff even if you plugged the amp into the headphone socket. It's an interesting thing, amping the iPod. Because the iPod has this bass falloff, and the falloff is fully recovered on connecting an amp, it causes a lot of people to say "OMG!!! It sounds so good through an amp! Amps are the best!!!" The truth is that the amps are being helped by the iPod in this sense, and the actual sonic improvements offered by a portable amp if there was no bass falloff are considerably less than what you may hear on an iPod. That's something that you don't get the amp manufacturers talking about on a regular basis.
Note: RMAA results are not necessarily absolute. These graphs serve to show you a relative measurement of the iPod's behaviour with the headphones. If you link to the graphs, you MUST post the adjacent explanatory text.
