Headphone Measurement Proceedures - Introduction and Equipment
There are two camps in the world of audio: objectivists and subjectivists. In an oversimplified summary, objectivists believe audio performance is measurable; subjectivists believe audio performance can only be evaluated as an experience. There are, of course, those with a more sophisticated and balanced perspective who believe both have merit; I fall into that camp. I can look at a set of measurements and can tell you roughly what to expect when you put the cans on, but there’s no way to really tell what the experience is going to be like until you listen. None the less, having a good database of measurements will let you know if one headphone has more bass than another; will allow you to rapidly find headphones of similar characteristics; and will let you rule out poorly performing headphones without having to listen.
Unlike speakers, which are more complicated with multiple drivers and crossovers, and get put in rooms with reflections from walls, headphones are usually a single driver directly coupled to your ears. As a result, my experience leads me to believe the relationship is closer between headphone measurements and what you hear, than speaker measurements and what you hear. Similarly, audio electronics gear these day is quite good, and measures of their performance is rather more like splitting hairs --- the difference between 0.001% and 0.0001% distortion is not easily audible. The imperfections of headphones are orders of magnitude larger than electronics gear, so, while I believe it is important to measure electronic gear, I also feel it tends to be less informative regarding the experienced sound quality than headphone measurements.
What I’m trying to get at here is that I’ve found the correlation between headphone measurements and the listening experience quite a bit stronger than with speaker or electronics measurements. Though this may simply be a result of my intimate familiarity with headphone measurements. At any rate, I think you’ll find this journey into objective headphone evaluation interesting and illuminating.
Headphone Measurement Equipment Overview
The Head Acoustics HMS II.3 Head Simulator
The heart of a headphone measurement system is a microphone in the shape of a head. I use a Head Acoustics HMSII.3 head acoustics simulator. At around $20,000, this noggin ain’t cheap, but it’s got a lot of very specialized characteristics and careful engineering that make it a very important tool.
The Head Acoustics HMSII.3 head acoustics simulator.
The head is designed to act exactly like the average human listening system. Inside the head at very much the same position of your eardrums at the end of your ear canal, are two very special microphones (one in each ear) that mimic the exact acoustic impedance characteristics of your eardrum. The shape of the ears is also very precisely defined (IEC specification 60268-7) so as to act just like the average human ear. The goal of this device is to provide a measurement of exactly what is heard at the eardrum for sound arriving at the head. Because headphones are coupled directly to the head, it is only by using a device such as this that headphone measurements can be made that relate strongly to what we hear. There are other headphone measurement couplers, which are not head shaped, that can achieve similar results, but the head allows me to measure both ears at the same time, and allows me to measure the amount of isolation from outside noise accurately.
The Test Chamber
In order to achieve repeatable measurements, I need to have a way to isolate the head from random outside noise that might appear in the measurements. Also, because one of the measurements needed is the amount of isolation from outside noise, I need an environment in which I can repeatably create a controlled amount of noise. To do this I’ve built a custom chamber in which to place the head during tests. It’s about 4’x2’x2’ and is constructed of 1” furniture grade plywood with a layer of drywall very thoroughly glued to the inside walls. The very heavy and stiff walls provide significant acoustic resistance; I currently get about 15dB of isolation from outside noise. This chamber is mounted on air bladders (wheel barrow innertubes) in order to isolate it from mechanically born noise from the rest of my home. Cables are routed through a hole in the chamber that is filled in with modeling clay.
My home-brewed acoustic isolation chamber for testing headphones.
The chamber has a speaker in it that is used to generate pink noise for measuring isolation. The interior of the chamber has some noise damping foam, but also has reflective surfaces designed to evenly distribute noise around the chamber interior so that the sound approaches the headphones from many directions when measuring isolation.
The Audio Precision System 2 Cascade
The Audio Precision System Two Cascade, model SYS2522 dual-domain audio tester.
Once we have a way to accurately simulate the acoustics of headphone listening, we need a piece of gear that is able to generate and analyze the audio signals needed to tease out meaningful measurements. I use an Audio Precision System 2 Cascade dual-domain audio tester. This piece of gear can generate and analyze both analog and digital audio signals with state-of-the-art precision and can be programmed to automatically run the device under test through a broad battery of standard and customized tests. I think if the AP had a soul, I’d marry it. Unfortunately it doesn’t, so I settle for a healthy dose of gear lust whenever I get to play with it.
A fairly old PC controls the Audio Precision (AP); nothing special here. The PC is loaded with a software application that controls the AP, and receives measured data, which it inserts into preconfigured Excel spreadsheet templates. The AP software has control panels, which can be configured into the various tests (frequency response, %THD+noise, impulse response, etc), and then called by a script written in an AP extended basic programming language. It is possible to “drive” the AP manually from these pre-configure panel set-ups to do custom experiments.
The Tektronics model TDS 210 digital oscilloscope used to monitor signals while positioning headphones.
A small Tektronix TDS 210 oscilloscope is mounted above the door of the chamber and is fed left and right channel signals from the monitor output of the AP. I use this o’scope to monitor the signal from the head in order to properly position the headphones on the head prior to test. Positioning of the headphones is critical, and the o’scope provides important feedback on whether or not the headphones are positioned and sealing properly.
The microphones in the head have phantom power voltage and pre-amplification provided by a G.R.A.S. Type 12AA low-noise pre-amplifier mounted next to the chamber. The speaker in the chamber is driven by an NHT C20 power amplifier.
Continue on with Measuring Headphone Frequency Response.