Chapter 4. From Digital Audio to Analog Sound
Now that the signal is back in the green analog domain of in the form of electrical voltages, we can get some air molecules to move by placing another set of electro-acoustic transducers in the signal path: speakers.
Speakers are essentially plastic cones moving back and forth at the frequency of the sounds present in the signal. This is how it works: the plastic cone moves under the influence of a mobile electromagnet attached to it; that mobile electromagnet moves under the influence of a fixed permanent magnet attached to the speaker box. Whenever an electrical signal gets into the speaker, the permanent magnet generates a magnetic field which causes the mobile electromagnet to move; whenever the electrical signal’s sign is changed (it does since the electrical tension behaves like a sine wave), the direction of the magnetic field is reversed, causing the electromagnet to move in the opposite direction (there is a very nice model of this here); as a result, the electromagnet (and the cone) move back and forth, creating compression and rarefaction zones at the wavelength of the electrical signal.
The amplitude of the cone movement determines the volume of the sound produced. Low frequencies are reproduced with fairly heavy driver material like plastic, while higher frequencies need lighter material like silk to move a lot faster under the influence of the moving electromagnet. Speakers, headphones and ear buds all work in the manner described above; the only difference between them is the size of the components (see the pictures here).
Contrary to audiophiles or the everyday music listener, audio engineers want the sound to be reproduced with the highest possible fidelity to the signal as it enters the transducing system. I just said “highest fidelity” which reads HiFi for most people. The irony is that most HiFi systems are not designed with the highest fidelity in mind; instead, they aim at giving the listener the best possible experience by modifying the sound before it comes out of the speakers; it is not uncommon to place amplifiers and other devices like equalizers between the DA converter and the transducer . Even if there is no device physically placed in the chain, speaker manufacturers will include signal processing inside the speakers to give you a “warm and inviting sound”, or “the punchiest mid-range you have ever heard”.
Of course, when you are trying to mix a song with your DAW, you want your sound to be portable to any kind of listening environment: a nice HiFi system, a car audio system, headphones, one ear bud, a club dance floor, crappy PC speakers, etc. If you mix your song on a speaker system that does not reproduce the signal that is in fact coming in, you will add features to your sound which will not translate well to other systems; a typical example of this is mixing on bad headphones: because these tend to lack bass frequencies, you add more into your mix; you then listen in your car (car systems typically overemphasize bass frequencies) and what comes out of your car stereo is a herd of rumbling elephants; yes, that is personal experience right there.
Audio engineers want to use a transducing system which very reliably gives back whatever comes out of the DA converter with as little distortion as possible: this is called having a flat frequency response. This flat response feature is desirable in both speakers and headphones for audio engineering. The process of listening to audio sounds as you are mixing them is called monitoring, so monitoring speakers are called studio monitors, or monitors for short.
Something else which audio engineers like to know before buying their monitors is the range of frequencies that the speaker system can reproduce. Even if the human hearing range is known to be approximately 20 Hz to 20 kHz, certain manufacturers insist on producing speakers whose range extend (far) beyond 20 kHz. Most good monitoring systems are built to reproduce frequency ranges from about 40 Hz to right above 20 kHz. Most speakers have either female XLR or TRS inputs, or both. They sometimes include trim switches to take away some volume, either on the low end or the high end or both; they sometimes also include EQ shelf switches.
The power delivered by monitoring speakers is usually lower than that of loudspeakers: the goal is not to deafen you, but to give you the most accurate sonic representation of the outgoing signal. In fact, audio engineers can sometimes suffer from hearing fatigue or even hearing loss if they work at sound pressure levels which are too high. A good starting point is to listen at 80-85 dB SPL; some engineers say that you should be able to speak over music being played by studio monitors without raising your voice. Either way, listening at quiet levels is beneficial both short and long term.
We have talked a lot about flat frequency response. If you ever do a comparison between speaker models, or go to monitor manufacturer websites, you will not always find frequency response data. The reason for this is simple: the flattest response is not always very flat, which means that the standard user going to buy monitors looking at a wiggly frequency response graph will think the manufacturer is lying to them. Before buying speakers based on manufacturer data, check out tests and frequency response graphs from independent testers if you can find them. There is more frequency response magic here.
In the next table, I have listed famous monitors, or at least monitors people talk about. They are all decent studio monitors, some very good; I have tried to list flagship-style monitors from each company, but of course, all the manufacturers cited produce other models, available on their respective websites. These monitors are studio monitors, not Hi-Fi monitors: the goal is to reproduce, not to dazzle.
The price column indicates the price for a pair of monitors since most of us have two ears. The frequency response column gives the lowest possible frequency produced by the subwoofer (the first number, in Hz) and the highest possible frequency produced by the tweeter (the second number, in kHz). The “drivers” column gives driver diameters in inches: LF for Low Frequency, HF for High Frequency. The power column gives the maximum power available at low (LF) and high (HF) frequencies.
|Vendor||Model||Price||Frequency response||Crossover frequency||Drivers
|Power LF/HF [W]||Output
|Adam||A7x||$1000||42 – 50||2500 Hz||7”/1.5”||100/50||114|
|Dynaudio||BM5 mkIII||$1200||42 – 24||1900 Hz||7”/1.1”||50/50||118|
|Equator Audio||D5||$400||53 – 20||—||5.25”/1”||50/50||103|
|Event||Opal||$3000||35 – 22||1600 Hz||7.1”/1”||270/50||114|
|Focal||Twin6 Be||$3700||40 – 40||—||6.5”/1”||300/100||115|
|Genelec||M040||$1600||48 – 20||2500 Hz||6.5”/1”||80/50||107|
|JBL||LSR305||$300||43 – 24||1725 Hz||5”/1”||41/41||108|
|KRK||Rokit 5 G3||$300||45 – 35||—||5”/1”||30/20||106|
|M-Audio||BX5 D2||$180||56 – 22||3000 Hz||5”/1”||40/30||100|
|Mackie||HR824mk2||$1400||37 – 20||1900 Hz||8.75”/1”||150/100||111|
|Neumann||KH120||$1500||52 – 21||2000 Hz||5.25”/1”||50/50||111|
|Presonus||Eris E5||$300||53 – 22||3000 Hz||5.25”/1”||45/35||102|
|Yamaha||HS8||$700||38 – 30||2000 Hz||8”/1”||75/45||—|
All the monitors listed above are of the same type: 2-way near-field monitors. 2-way means that the speaker has two drivers; if you remember the last time you saw studio pictures, you will notice that most monitors have two sets of cones – two sets of drivers. Why? Because remember, you need different materials to create air waves with different frequency ranges; some speakers even have three drivers to use the best possible materials. The low frequency driver is called the subwoofer and is typically 5 to 8 inches in size; the high frequency driver is called the tweeter and is typically about 1 inch in size.
The issue with the multiple driver system is that at some point in the frequency range, both drivers would move air, which is not desirable because it would introduce interferences and artifacts in the produced sound. A crossover frequency is defined : just below that frequency, the subwoofer starts to fade out and the tweeter takes over. Of course, the way the system handles that crossover is critical for a truly flat frequency response. Near-field means that the monitors are meant to be used for listening at close ranges and are not meant for concerts.
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