Sound declassified

I am not a music enthusiast. Sure, I listen to music more than six hours a day; sure, I look for my mp3 player before looking for my wallet, cell phone or car keys; sure, I avoid getting into a music store if I recently got my pay check… Sure, but I am not a music enthusiast.

Twelve year olds know more about pop music than I do (literally), so I am by no means a walking music encyclopaedia. I just listen to music for the pure enjoyment of it, without getting too much into the details. That’s why I found it a bit overwhelming when I decided to buy my first pair of “real” headphones, but I also learned a great deal while researching, so I’d like to share some of it with you.

Amplifiers

First, it might be relevant to talk about how amplifiers work. They do pretty much exactly what you’d think they do—they amplify. Commonly, this is achieved using transistors. The basic idea is rather simple: you hook up a relatively high DC source to the transistor on either end, while the signal you want to amplify is connected to the base. Because the transistor usually acts like an open circuit, there will be no current flowing; however, when you supply the base of the transistor with even a small voltage, you allow more current to flow from the DC source. This is a gross oversimplification, but you get the idea.

Amplifiers have several specs of interest: power rating, impedance and fidelity. The output impedance of the amplifier is its impedance perceived at the output—we generally want this to be as low as possible. The power rating is the amount of power the amplifier can supply—we want this to be high. Finally, fidelity is how accurately the signal is amplified—with zero distortion if possible. Naturally, we don’t want our amplifier to add noise or make our music sound like me singing.

I mention this because when it comes to sound, you find amplifiers everywhere. Even if you don’t have a stereo system, you still use them —a simple radio receiver, your computer’s sound card, possibly even your speakers. Ultimately, you should pick your headphones based on how you are going to be using them. It might not be worth it spending hundreds of dollars on headphones that you will end up plugging into a cheap sound card.

Digital audio
A second step to our quest of finding the right headphones is understanding how digital audio works. Most of you probably don’t listen to analog audio: vinyl records, tape recordings, etc. Yet few actually understand how digital audio works (myself included, to be honest). Let us examine what analog and digital mean, exactly.

Sound propagates through air as a wave of varying amplitude and frequency. So, to reproduce sound, you need to make the speakers’ drivers move at exactly that frequency, and adjust the pressure they exert according to the sound intensity. Since the speakers operate using magnets, all you really need to do is feed the speakers a voltage with amplitude and frequency corresponding to that of the desired sound wave (with enough power to make them produce audible sound, of course).

Although analog sound recording came quite a bit before digital recording, it is actually better in terms of accurately describing an audio signal. It poses some problems, though: it is more difficult to store, more difficult to transmit and generally more difficult to work with than with a digital signal. Also, the circuits needed to work with an analog signal need to be a lot more sensitive and well tuned than those used for digital. To put it simply, digital is much cheaper. But how exactly do you represent an analog wave of such high complexity simply in terms of ones and zeroes?

The analog wave is stored as a point of amplitude and position in time. There are two main aspects you need to be aware of: the sampling rate and the bits per sample. A higher sampling rate is better, since it means you have points closer to each other and are throwing out less of the analog signal. A Compact Disc has a sampling rate of 44,100 Hz, meaning there are 44,100 points of amplitude measured inside of one second. Bits per sample, on the other hand, is how many bits you use to store the information about each point. Since we use ones and zeros, we can’t really store every possible value of amplitude. Instead, we break it up in several groups. A Compact Disc recording uses 16 bits per sample, or 65,536 possible amplitude values. Some higher quality recordings have 24 bits per sample, or 16,777,216 possible amplitude values. Wow! Of course, it’d be kind of useless to output a stream of ones and zeroes to your speakers… sound is still analog, so we need analog-to-digital and digital-to-analog converters to make sense of our digital signals.

But what about all those mp3′s you have on your computer or your mp3 player? Are they any different from the digital signal we discussed? Yes and no… Ultimately, they’re still based on the same principles, but with one important distinction: they use various techniques to reduce the size of the digital signal. With a CD, a song of 4 minutes takes up roughly 40.4 MB. We want to try making this 5 MB instead. We achieve it by reducing the bits per sample and by disregarding parts of the signal that are in the higher or lower frequency ranges (that the human ear can’t detect anyway). This is the case for all the “lossy” formats (such as mp3), but lossless formats exist too. Those only compress the original wave to make it take up less space, without throwing out any of the information.

OK, that’s enough technical gibberish. All you need to understand from all this is that digital wave signals are dumbed down version of analog signals, and that lossy encoding is a dumbed down version of digital signals. If you’re only going to listen to 128 kbps mp3, it might not be worth it spending a lot on sound equipment.

And sound equipment can be quite expensive. There are headphones out there in the thousands of dollars range! Do you need something like that? Probably not… but it’s always nice to get something with at least half-decent specs.

Let’s just discuss some headphone specs before we finish:

Frequency Response. The human ear can hear sound roughly in the 15 to 20,000 Hz range. You might see a value such as 8-25000 Hz put as the frequency response for some headphones, and that’s the most info you will usually get. Some headphones will have a flat response (will produce comparable output at different frequencies in the range), while others will deliberately output different frequencies differently (e.g. enhance bass). Even looking at frequency response graphs, you can’t really say much about how good the headphones are. It’s best to just listen to the headphones before buying, if possible.

Impedance. Most will give you a value such as 32 ohms. It varies with frequency, some manufacturers provide something like Impedance (1kHz): 32 Ω, where 1kHz would be the frequency it was measured at. You generally want this to be lower, but it hardly matters, as the amplifier’s power output is of much greater importance when it comes to volume levels. Some of the higher grade headphones have even higher input impedances, such as 300 ohms, while a typical speaker system has an impedance of about 8 ohms.

Sensitivity. This is sometimes referred to as Sound Pressure Level, and is given in dB (decibels). In simple terms, it’s how strong of a sound the headphones produce when supplied with one mW (milliwatt) of power. Typically this is about 100 dB.

Total Harmonic Distortion. This is related to the fidelity, and is considered inaudible if less than 1%.

Load Rating. Probably not of a huge concern, this is the maximum power the headphones can handle over an extended period of time. A common value is about 500 mW.

Transducer Principle, Driver Size, etc. These relate to the driver, the part of the headphones that vibrates and creates sound. You’ll often see something like Dynamic neodymium magnet; it just means the headphones produce sound using magnets that cause a membrane to vibrate.

Cable. I personally find this quite important. Cables can be of varying lengths and thicknesses. If you are going to listen to your headphones at home, a longer cable (or maybe even wireless…) would be better, but if you’re going to be listening to an mp3 player on the go, you actually might want the length to be shorter. The headphones I got had a thick and detachable cable, so I can eventually replace it if it breaks.

In the end, which pair of headphones should you choose? Get ones that suit your taste and meet your needs. I think it’s more important to choose them based on physical characteristics (such as the cable, the size and comfort of the headphones, their appearance, their durability, etc.) than to look for frequency response in the 50 kHz range. I hope this can be of some use to the ones who find themselves in a similar position as me, and that I haven’t bored the music enthusiasts out there

REFERENCES
http://sound.westhost.com/amp-basics.htm
http://sound.westhost.com/impedanc.htm
http://communication.howstuffworks.com/analog-digital3.htm
http://www.sennheiser.com/sennheiser/home_en.nsf/root/spec_hd800
http://www.headphone.com/learning-center/build-a-graph.php