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    What is Good Quality Audio: Part 1

    September 5, 2017 by Dolby Developer

    Fundamental Concepts: Frequency Response and Dynamic Range

    Dancing About Architecture

    What does it really mean when someone says a new speaker system "sounds good?" They might describe such a system as full, detailed, natural, clear, etc. Those are all the best words but to paraphrase the saying, talking about audio is like dancing about architecture. It can be difficult to communicate what you hear and judge whether it qualifies as "good". This is compounded by our individual listening preferences and familiarity with high quality audio reproduction.

    Nevertheless, there are a number of characteristics that a well-regarded playback system should exhibit. In this three-part series, we will establish some basic audio terminology, discuss what audio attributes to listen for, and provide tips on improving your critical listening skills.

    Today, let's focus on some fundamental audio properties and their relevance to audio playback systems.

    Frequency Response

    Because we can only hear frequencies within a certain range, it follows that any good playback system should be able to reproduce the full range of frequencies that we are able to hear. The technical term for the range of frequencies that a system can reproduce is "frequency response."

    Not only will a good system be capable of reproducing the full audible frequency range, it will do so without under or overemphasizing any part of that range. This is called a flat frequency response and ensures that the tonal balance isn't altered by the system. Frequency response will often be quoted with a tolerance (e.g., 20Hz-20kHz +/- 3dB), meaning that the system can reliably reproduce that range of frequencies to the specified degree of flatness. The system will likely be compromised in its ability to reproduce frequencies outside this quoted range but since we can't hear those frequencies, this is generally not a concern.

    A high quality system's amplifier should have a frequency response of at least 20Hz-20kHz with a tight tolerance (1dB or less). A capable set of speakers and a subwoofer should also cover most or all of that frequency range. Most cheaper and/or smaller systems will likely have a more limited frequency response, but this is expected and not worth fussing over that much.

    As with all the specs and subjective assessments discussed in this article, what's important is how the system performs compared to similar devices in its class and at a similar price point. As an aside, the frequency response you hear in the room is likely not flat and will depend on a few factors, but we'll touch on that a bit later.

    Pressure and Amplitude

    Just as we can hear a range of audio frequencies (pressure waves vibrating faster or slower), we can hear sounds at a range of amplitudes (pressure varying widely or not so much). In other words, we can hear very quiet sounds like whispering up to extremely loud sounds like the roar of a jet engine.

    If a sound wave's amplitude is too low, you won't hear anything; too high, and you'll be in pain and potentially damage your hearing permanently. The lower limit of audibility is generally quoted as 20 micro Pascals (the Pascal is a unit of pressure). If you see amplitude measured in dB SPL (Sound Pressure Level), this is often calibrated such that 0 decibels (dB) SPL is equal to the 20 micro Pascal threshold of hearing.

    As a reference, movies in the cinema are usually played back with a max level of 105dB SPL (or 115dB SPL for the subwoofer). In terms of speaker systems, we are primarily interested in reproducing audio within this amplitude range.

    The Signal to Noise Ratio

    A good playback system will be able to play loudly without distorting or unduly affecting the content being played. The difference between the lowest and highest cleanly reproducible levels is called "dynamic range." For home use cases, it is likely you'll be playing audio at a maximum level 10-20dB quieter than the cinematic standard quoted above, so don't be too concerned if your system doesn't go all the way up to 105dB SPL without audible distortion.

    You'll often find a measurement called signal to noise ratio (S/N ratio) quoted on sound system spec lists. For the purposes of this discussion, consider it an indication of dynamic range (even though this is not technically accurate). A system with a S/N ratio of 80dB or greater is plenty for most consumer use cases. Don't worry about this too much, though; most living rooms have a background noise level of 30-40dB SPL, so you likely won't be able to take advantage of the full dynamic range of your system.

    Next up: Subjective Qualities

    A flat frequency response and adequate dynamic range is the bare minimum standard a high quality system should meet. These figures are often listed on spec sheets and while they are important to note, they alone do not give an adequate picture of the overall sound quality. For that, we'll have to get away from more technical terms, do some listening, and talk about more subjective qualities.

    We'll delve into that in our next installment.

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