This article covers the methodology of diagnosing your listening environment – if you have not read the two articles below, this article might be difficult to follow.
1. Theoretical discussion on room tuning: How the listening environment affects the sound
2. Background on room tuning: Investing in audio efficiently
You should have the following materials ready to diagnose your listening environment. There are ways of doing it for free, but accuracy only improves with cost (I did come up with a few ways of doing it without spending money, but they all came with their limitations as you’ll see below).
1. A measurement microphone + a calibration chart
2. An audio interface with a preamp for the microphone
3. Real time analyzer (spectrum analyzer) software
1. Measurement Microphone
So, before you can begin measuring your room’s characteristics, you need a measurement microphone. A measurement microphone is a specific type of microphone that is used for measurements.
Your standard karaoke mic or internal microphones in laptops, or even a studio recording microphone are largely suboptimal for making measurements.
The requirements for a measurement mic are essentially identical to the requirements for any hi-fi device – it needs to be able to register a wide range of frequencies and must have a flat response.
This means that you should not make the measurement using a vocal microphone, for instance, because microphones for vocals cannot pick up sounds outside the vocal range accurately.
SHURE SM58 (for vocals) Microphone’s Frequency Response
The figure above shows the frequency response characteristics of a recording microphone intended for vocalists, and as you can see, it neither has a flat nor wide frequency response. If you were to make measurements using this particular product, the sounds below 100Hz will be deemphasized, while sounds below 50Hz and above 15KHz will be lost altogether (the treble boost should make a human voice sound brighter).
Vocal microphones only need to cover the vocal range, so the response above is sufficient for the intended purpose. However, whereas recording belongs to the domain of sound production, measurements belong to the realm of sound reproduction – in producing music, a mic with a specific qualities might be desirable, but when this mic is used for measurements, the results become unreliable due to the microphone’s idiosyncrasies. Thus, a measurement microphone must have a flat response over a wide range of frequencies, as shown in the chart below:
Frequency response characteristics of an Earthworks M30S Measurement Microphone
The calibration chart above is from the measurement microphone used at Orfeo SoundWorks, and exhibits a desirable, flat response within 1dB up to 30KHz. Performance comes with a cost, however (a good measurement microphone can cost well in the $1000~ range).
Of course, there are many models available, some more expensive, and others cheaper. The cheaper models include:
0. Orfeo SoundWorks Measurement Microphone: ~$150 (Calibration Chart provided, 9V battery)
1. Behringer ECM8000: ~$110 (Calibration Chart not provided, 48V Phantom power)
2. Audix TR40: ~$190 (Calibration Chart not provided, 48V Phantom power)
3. beyerdynamic MM1: ~$200 (Calibration Chart not provided, 48V Phantom power)
While these models have relatively flat and wide frequency responses as per the monicker ‘measurement microphone’, the price reflects on their quality – for instance, the Behringer ECM8000, which is the cheapest among the mix, shows a wide variance in response between individual products.
Frequency response distribution for the Behringer ECM8000, measured by Cross-Spectrum Labs
Thus, a calibration chart, which details the characteristic response of the individual product, is crucial for an accurate measurement, and is included with high-end microphones along with its serial number.
While it is possible to measure using a smartphone, the measurements must take the characteristics of the specific mic equipped into account. Thus, payware RTA programs running on smartphones have an option to adjust for the smartphone and the microphone model (not the individual product).
If you look at this the other way, this also means that freeware programs are limited in their accuracy – if you choose to use smartphones, the recommendation is that you use payware programs for the minimum bit of calibration.
For an even more accurate measurement, the data have to be adjusted as with the Behringer ECM8000. To make an analogy, if the calibration curve provided by the payware applications is a ready-made set of suit, high-end measurement microphones come with a tailored calibration chart specific to the model and the product.
2. Audio Interface
In order to measure using a measurement mic, a power source to provide the microphone with phantom power (48V) is necessary in addition to a microphone amp to amplify the signal from the microphone.
E-MU1616m Audio Interface
(If you look inside the green box, you’ll see the specifications relating to the preamp and phantom power.)
The reason why the microphone requires a separate power source (phantom power) is because there is an amplifier (it’s not exactly an amplifier) within the microphone to increase the signal level and provide power for operation, and the power for this amplifier is provided through the signal cable. Sometimes, the power is provided by AAA batteries or a 9V battery, as is the case with the Orfeo SoundWorks Measurement Microphone.
The mic amplifier is also necessary because the signal level from microphones is far smaller than what CDPs or other source devices provide. Thus, an audio interface with microphones requires a separate microphone amp.
In terms of measuring using an RTA, the relative levels of each frequency is what matters, which means that a mic amplifier is unnecessary so long as the vertical scales can be enlarged. However, audio interfaces that are on the market are targeted for general use, and thus include both the phantom power and the mic amplifier.
3. Real Time Analyzer
Once you have a measurement mic, the corresponding calibration chart, and an audio interface, you need a software that will allow you to see the signals from your hardware. These softwares are known as RTA(Real Time Analyzer)s, and provide details on the frequency response.
A screenshot from TrueRTA
RTAs exist for both PCs and smartphones – of course, the PC versions are far more powerful and easy to use, but the smartphone app might be enough if you are looking for a rough idea.
Orfeo SoundWorks is employing both of TrueRTA and ARTA, which are a little intricate to use, but more accurate in measurement.