From the Desk of Art Noxon

Art Noxon is a fully accredited Acoustical Engineer with Master of Science degrees in Mechanical Engineering/Acoustics and Physics. A Professional Engineer since 1982, he is licensed in Oregon to practice engineering in the public domain with the specialty area of acoustics. A prolific inventor, he developed and patented the iconic TubeTrap, the original corner-loaded bass trap/treble diffuser, 150 other acoustic devices, and counting. Lecturer, writer, and teacher of acoustics, he has presented 7 AES papers, numerous magazine articles, white papers and blogs. He is president of Acoustic Sciences Corporation, the company he founded in 1984.


What is MTF for sound?

What is the concept of MTF (Modulation Transfer Function) for sound signal?

This question was recently posted by ask.com. An answer was given that talked about video pixels, which was not an answer to the question. And so I decided to try to give a short but sensible answer. Here it is…...

The sonic MTF is how the clarity of sound or intelligibility of speech is physically measured. A similar system exists for measuring the clarity of lenses in optics.

It appears as a 3-D plot with the Modulation Level (dB) on the vertical axis, the Modulated Tone (Hz) on a horizontal axis and the Modulation Rate (Hz) on the other horizontal axis. A typical data point would be a Modulation Level of 15 dB for a Modulated Tone of 250 Hz at a tone burst or Modulation Rate of 8 Hz.

Sonic MTF is the sound level difference measured when a rapid tone burst is played. The sound level difference between when the tone is on and off is the Modulation Level. The speed of the tone burst, bursts per second, is the Modulation Rate. The electronic Modulation Level may be 60 dB but when played through loudspeakers into a room, the measured Modulation Level drops to as little as 10 dB at 8 Hz, reduced because of the lingering ambiance between tone bursts. The maximum MTF level a human can perceive is about 20 dB at burst rate of 8 Hz.

Speech and music have an average sound burst rate of 8 per second. B&K measures speech MTF with a RASTI sound meter. ETF analyzers derive MTF by taking the early to late energy, the S/N signal to noise ratio in dB at about 70ms following the direct signal. The MATT Music Articulation Test Tones is a gated tone sweep on a sound test CD by ASC, which can be run into a dB strip chart recorder to create the MTF at 8 Hz.

At this point my answer was cut short because I had used up my allotted 1200 characters. Included below is the rest of my answer…..

In sound the MTF is not well known. The frequency response curve for loudspeakers is a well known sound level vs frequency audio test. The RT60 decay time for sound in a room is another. If these are combined together, we have the ETF waterfall response curves. Make an improvement in the acoustic condition of the room and the room response curve or the ETF barely indicate any change has taken place, even though the difference in sound quality can be very audible.

However, make the MTF test on a room before and after a noticeable acoustic improvement is made and the difference in the MTF curves is very noticeable, typically in the range of 3 to 5 dB improvements, which corresponds to the improvement noticed by listeners in the quality of the performance in the room. MTF measurements correspond to the human perception of musical performance much more than the traditional sound level measurements.

Unlike with sound, in optics the MTF is very well known. It is used in lens design, cameras, photography, videography, printing and ophthalmology because it measures the clarity of the image. Optical MTF physically measures the brightness variation or Modulation Level in dB as a light sensor is moved over a set of closely spaced lines. The color of the lines is the Modulated Frequency and the closeness or number of lines per inch is the Modulation Frequency. Optical MTF answers the basic question which asks how close together can the lines be and still be seen as separate lines.