Sennheiser HD 555 Headphones Review - Performance

The dotted lines indicate the limits we look for; if anything is to veer too widely above or below these limits, it could indicate an issue that might affect sound quality. The Sennhesier HD 555 has a fairly smooth frequency response, which is what we look for in a pair of headphones; big peaks or troughs in the frequency response would mean certain frequencies are being overly exaggerated or suppressed. The HD 555 has a few peaks and troughs in the mid to high range, though, which is a little less than perfect. But these quirks are nothing major, so the sound they produce should be accurate and clear. Overall, the HD 555 performed well in this test.

Distortion (13.33)
Distortion is the bane of the music lover's life; music sounds bad when the sound gets distorted because the headphones aren't up to the task. It doesn't seem to be with the HD 555's, though; we saw low levels of distortion across the board in our tests.

Our system examines the distortion headphones introduce by playing back a series of very tightly controlled sounds at the typical listening level of around 78 dBSPL and examining the results for any differences from the original waveform. The graph below shows what's called the Total Harmonic Distortion (THD), which is the amount of distortion (as a percentage) of a frequency and the higher frequency harmonics that give a sound its texture. The graph starts with low, bassy frequencies at the left and goes up to higher-pitched frequencies at the right.

Across this wide range, the HD 555 has low distortion, only peaking lightly at the very low end, where we typically see some distortion with even the best headphones. What this means is your music will be cleanly reproduced at all frequencies; there are no big peaks in this graph that would indicate it has problems with particular frequencies. And the distortion at the low end isn't a big problem; if you're a fan of drum and bass, the HD 555 will reproduce the big bass sound you love cleanly.

Tracking (5.59)
Because most people have two ears, headphones have two channels; the left and right channel. In this test, we see if there are any major differences between the sound that comes over the two channels; if there is, the music would sound unbalanced. Again, the HD 555s do well here: we only saw very, very minor differences between the two sides. The maximum difference between the two is just 1.27 percent, which is pretty much inaudible.

Our test system also produces the graph below, which shows the tracking across the frequency range of 80 to 10,000Hz. If the line is at 0 percent, that indicates the sound at that frequency from the two channels is identical. If it goes above that line, the left channel is stronger, and if it goes below, the right channel is stronger.

The HD 555 did well in this test as well; although the tracking goes off slightly at higher frequencies, the great majority of the graph is close to the 0 percent line, which means the HD 555s produces well balanced sound for all but the most finicky of audiophiles if properly placed on the head.

Maximum Usable Volume (9.02)
The distortion test we do above is run at a pretty normal listening level of 78 dBSPL, but some people like it loud, so we also test how high we can crank up the volume. We test this by how high we can take the volume until the distortion in the sound reaches an audible (and annoying) level of 3 percent.

The HD 555s manages to reach an impressively loud 115.4 dBSPL before our testing system cries foul, which should be loud enough for even the most hardcore loud music fans; that's about the equivalent of a jet airplane taking off, and is getting close to the level that will cause permanent hearing damage. We don't recommend you do this test yourself (we use a testing system with replaceable ears; yours are not), but it's good to know you can crank up the volume if you want to without the sound becoming distorted. 

Isolation (1.18)
The flip side of the volume you listen to music at is isolation; how well do the headphones block outside sound? The HD 555s do a poor job of this; their open design and lack of any active noise canceling circuitry means it only blocks an average of 3.2 dB of outside sound. In other words, they don't block much. Contrast that with the typical in-ear ear canal headphones (such as the Etymotic ER6i and the Shure SE210), which block an average of 30 dB of sound.

The graph below shows the same frequency range of 80 to 10,000Hz, with the line indicating how much sound is blocked at that frequency; higher is better.

As you can see, for most frequencies, it doesn't block much at all; pretty much nothing at the low and mid frequencies (such as airplane noise), but a little at the high frequencies (such as music and speech). If you're looking for a pair of headphones to help you hide from the world, the HD 555s are not the right ones for you; look at a pair of in-ear headphones like the ER 6i or a pair with active noise canceling, such as the Bose QuietComfort 2 or 3.

Leakage (2.49)
Give that the HD 555s don't block much outside sound, it's perhaps not surprising that they let a lot of their own sound leak out. We found in our tests that a significant amount of sound leaked out from the headphones; you could easily hear and identify what was being played over them from a few feet away even at moderate volume. The HD 555 are not a good pick if you would rather the person on the bus next to you not know about your unfortunate fondness for New Kids On The Block.