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Sorceress

My 5 minute tutorial on audio amplifiers and speakers

Sorceress
Jan 31st, 2014
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  1. My 5 minute tutorial on audio amplifiers and speakers
  2. =====================================================
  3. This is not meant to be expert advice, but to give a general idea of what the ratings of audio things mean.
  4.  
  5. Watts
  6. -----
  7. The Watt rating of your speakers and amplifiers is (roughly speaking) how loud the sound can go before you get distortion.
  8.  
  9. There isn't a fixed point at which this happens as distortion appears gradually as volume is increased. You should choose an amplifier and speakers that are capable of going louder than you intend to listen. Even if you don't want to listen to loud sounds, you will enjoy less distortion if you choose more watts.
  10.  
  11. Some examples (per channel), at an average listening volume:
  12.  
  13. 1 Watt or less : Typical in battery powered devices. Lots of distortion. Poor sound.
  14. 3-6 Watts : Typical in budget range hi-fi systems. Some distortion. Poor bass + treble response.
  15. 10-25 Watts : Typical in middle range hi-fi systems. Low distortion. Quality sound.
  16. 50W+ : Typical in upper range hi-fi systems. Not noticeable distortion. Quality sound.
  17.  
  18. There is much confusion about Watt ratings because it can be unclear whether manufactures have added together the ratings for each channel, or not.
  19.  
  20. Not only this but manufacturers have devised a variety of ways of measuring power. The mathematically pure way of describing the power in a waveform is the "Watts RMS" rating. Other ratings may include "Peak Watts" or "Watts PMPO", which looks only at the momentary and unsustainable waveform peaks, ignoring the larger body of the wave. This can be much higher than the RMS rating (sometimes over 10 times higher), giving a false impression of sound quality.
  21.  
  22.  
  23. Amplifier
  24. ---------
  25. An amplifier has to drive several speakers (channels), so it must be capable of delivering adequate power to each.
  26.  
  27. The amplifier power rating should not be higher than your speakers rated power, otherwise it may overdrive them and cause damage.
  28.  
  29. If the amplifier power rating is lower than that of your speakers, then your speakers will be safer from damage, but too low and the volume will become limited. It really depends how loud you want the sound to be. 10 watts RMS from the amplifier can be quite loud indoors in a typical living room. 100 watts and the sound is likely to travel well beyond your home.
  30.  
  31. It is also important to consider both channels when choosing an amplifier, by looking at the power the amplifier can output to each channel, rather than the power total.
  32.  
  33.  
  34. Impedence
  35. ---------
  36. The impedance rating (Ohms) of your speaker determines whether it needs more volts or more amps than average, to output at a specific volume. (8 ohms being average in the world of speakers)
  37.  
  38. The power output is I^2*R, so if impedance R is reduced factor 4, the current must be doubled to get the same output volume.
  39.  
  40. But since V=I*R = doubled * quatered = halved. Voltage is halved.
  41.  
  42. So,
  43. Less ohms = Less voltage, more current needed.
  44. More ohms = More voltage, less current needed.
  45.  
  46. As you increase volume, there will be greater demands for power, which will in turn require both higher voltage and higher current from your amplifier.
  47.  
  48. If your amplifier is incapable of outputting the higher voltage, the sound will experience clipping. This is one type of distortion, which causes the audio to be noisy and crunchy, and lacking punch. You may experience this with small battery powered devices, whose output is limited to the 3 Volt supply.
  49.  
  50. If your amplifier is incapable of outputting the higher current, (such as impedence mismatch or an inadequate power supply) the speaker will have poor response. eg, where you cannot hear the treble when there is lots of bass.
  51.  
  52. These two phenomena will often occur together, although one may be more prominent than the other depending on the specific choice of amplifier and speaker.
  53.  
  54. Voltage is not a problem in mains powered devices, and indeed many mains powered amplifiers operate internally at 12-30 volts or more. Higher current is universally a challenge however. Higher currents require thicker wires and heavy duty electronic components. These are expensive, heavy and bulky, so amplifiers tend to be designed to deliver voltage over current where possible. This means that 8 ohm (and higher) speakers tend to be most used, as these prefer voltage over current.
  55.  
  56. At the other end of the scale, a battery powered device is not going to be capable of delivering many volts. If it is powered by a 3 volt battery, then that is all it is capable of outputting. So battery powered devices often contain 8 ohm (and lower) speakers, which demand less voltage.
  57.  
  58.  
  59. Speaker Diameter
  60. ----------------
  61. All speakers have a resonant frequency, which is the frequency at which the speaker cone vibrates easiest, and passes current easiest. This correlates strongly with the speaker's diameter. Smaller speakers (5cm across) will have higher resonant frequencies around 1-3kHz, while larger speakers (over 15cm across) may have resonant frequencies below 1kHz.
  62.  
  63. Speakers can struggle to output sound far away from their resonant frequencies. Especially if the amplifier is struggling to supply the voltage and current it needs. A large speaker may struggle to output high pitched frequencies (treble), while small speakers may struggle to output low pitched frequencies (bass). This is often rectified by having two or more differently sized speakers in each speaker case, connected in parallel. The range of the combination is greater than the range of them individually.
  64.  
  65. 1 per case : Budget speakers
  66. 2 per case : Mid range speakers
  67. 3+ per case : Upper range speakers
  68.  
  69. The main problem with connecting speakers in parallel is that the impedance (Ohms) of the combination is reduced, by half or worse. If done thoughtlessly, this can put an amplifier under stress as the speakers may demand more current from it than it is capable of delivering.
  70.  
  71.  
  72. Crossovers
  73. ----------
  74. To address these impedance issues, "crossover" is often employed to connect the speakers together. This is a simple electrical circuit that separates the waveform into high and low frequencies, so that they can be delivered to the corresponding speaker. Thus the amplifier only feels the demands of one speaker.
  75.  
  76. A crossover has another benefit, as it ensures that a speaker isn't given frequencies that it struggles to output, as those frequencies might cause distortion.
  77.  
  78. They are not always perfect however. The different speakers may have slightly different volumes, causing some frequencies to feel stronger than they should. A crossover may also be too exact in how it separates frequencies, causing sound/music to feel strangely disjointed. The design of a crossover should take these things into consideration, but not all do. These problems tends to be exclusive to mid-range systems that have been poorly designed.
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