What does it really mean for an audio playback system to be accurate?
One common answer is a flat frequency response. Another common answer is short reverb times. But did you really think it was quite this simple? Unfortunately, no.
Do Audio Measurements Correlate to Sound Quality?
Even the most refined audio equipment must send the sound waves through the air to reach your ears, and in doing so it invariably interacts with the room and is typically degraded in some way.
When you are the end user listening to a finely sculpted song through your meticulously arranged HiFi setup, you want to be confident that you are hearing the song as intended by the engineer. So, if frequency response and reverberation times are not the best metric for gauging these things, then what?
Clarity, articulation, intelligibility…speed. You need a fast room that possesses dynamic linearity. This week we’ll share a few of the ways we measure “room speed.”
Users of Room Eq Wizard may already be familiar with this metric. In simple terms, it is the ratio between the sound energy that arrives very quickly after a sonic event (0-50 milliseconds) and the energy that arrives late. It compares the “helpful” reflections to the “bad” reflections in terms of assisting understanding of words or musical notes. Recall: the classic “Haas window” that loosely defines when we begin to hear a distinct echo is about 50 milliseconds. The complete formula is shown below.
#2: Intelligibility or STI (speech transmission index)
The simple description: a modulated signal is played and the recording is analyzed to determine how quiet the room gets. The transmission index (TI) is basically the signal to noise ratio (S/N) measured in dB and expressed in percent. Subjectively determined constants are used that relate the perceived threshold of modulation to the maximum value of modulation–the human ear can only perceive a certain amount of dynamic shift in a short period of time. The STI is a sum of the weighted TI across the audio spectrum.
The Modulation Index Correlates to the S/N
The S/N is used to Determine the TI, which determines the STI
Hopefully you all immediately associate this term with Art Noxon’s MATT signal. The test, a version of the Modulation Transfer Function, was developed as a rapid method of characterizing many different features of a playback system in a matter of only 78 seconds. A very brief description of “articulation” would be: the dynamic range exhibited by a playback system within 60 milliseconds. The test signal ranges from sub-bass to G5 with a 50% duty cycle and dynamic range of over 40 dB between bursts. The difference between the system response level of each burst and the following silent section, measured in dB, is the Articulation.
The image below shows a range of increasingly poor articulation. At middle C, the articulation is about 13 dB (good) but by C#, it is dreadfully low.
The image below shows a phenomenally good range, with 15 dB or more of articulation shown throughout the upper bass. This is comfortably in the “excellent” category.
This week we have shared a few new room measurement tools to help you gain maximum musicality from your playback system. We feel strongly that good “room scores” by these measures translate to the most enjoyable listening experience, and isn’t that what it’s all about? Who needs the hokey pokey!