Headphone selection

[K O'Connor]

Hi Guys

Headphones can be a bit tricky to select as wearing them is still a bit unusual despite being ubiquitous and despite how much many individuals use them on a daily basis. Compared to listening to music through loudspeakers distant from us, the headphone experience can be "tinny" and "tiny".

A room mix is exactly that: each ear receives sound from both of the stereo speakers and there is a blend of sounds. The electrical signals to each speaker may be highly distinct, but our brain perceives the musical picture with both directionality and uniformity. This means that headphones are already at a disadvantage compared to room presentation, in that hard-left and hard-right sounds will be separated to an extreme.

The ambience of the room is added to the ambience of the recording, and this too is lost with headphones.

The room imposes no aural or spatial restriction on our physiology, where a headphone is extremely close to our ear canal, or inside it in the case of "ear buds" or "in-ear monitors". Both of the latter are very imperfect and must be customised to the individual. Typical headphones cup the outer ear and can be ventilated or not. Too tight a fit can give a claustrophobic feeling. Too open or loose may disturb others when the music level is too high. The coupling of the cup to the ear is preferred to be tighter if a good bass response is to be attained. It is the absence of bass that dominates the feeling of a "thin sound".

High frequency sounds can impact as harsh very easily with many headphones. This is partly due to the direct coupling of the transducer with the ear canal, compared to the transit through a larger space for room sound, which softens the attack. The proximity to the transducer allows the listener to hear the normal speaker breakup modes as distortinos that cause intermodulation effects, with IM being much harsher to our ear than THD.

In our modern time, the use of small ear-buds and headphones was popularised by the Sony Walkman, then the Sony Discman, then all the copies. This portable sound deteriorated with the advent of digital sources, such as the ipod, where data reduction compounded the fidelity loss imposed by data compression. Unfortunately, there is a generation of music listeners who have never experienced proper fidelity. Live sound is too loud to be decipherable. Background music in bars and restaurants is too loud and is also distorted by the poor acoustic environments. Music mixing and production values are ever-changing and not always for the better. Environmental noise impacts our hearing and is almost entirely responsible for hearing impairment as we age.

With all of this, we try to listen carefully in the music store as we audition headphones.

Headphones are extremely sensitive, with a reference power of 1mW used. Most headphones produce 100dB of sound at 1mW input, which is enough to cause serious hearing damage very quickly. So, having more reasonable sound pressure levels does not take much power at all.

[K O'Connor]

Hi Guys

Headphones are primarily of one type using small transducers that are electromagnetic motors; tiny versions of the dynamic drivers fitted in our stereo speakers, in our cars and for computer speakers. They do not have to handle much power but they do have to be full-range.

The dynamic driver is cone shaped, with a voice coil at the apex working against a small magnet. The surround of the cone is flexible so the entire cone can move as a unit. In general, the cone should be able to retain its stiffness over the entire Human audio range of 20Hz to 20kHz, and this will provide the least distortion. Some transducers may experience cone break-up modes, where the outer part of the cone decouples from the central portion at higher frequencies. This may appear as a peak or dip in the frequency response. Dynamic drivers are very inexpensive to manufacture and may use a variety of materials for the cone, with paper being the most common.

Planar drivers are flat and can produce lower distortion than dynamic drivers. This is not a general statement, simply a reflection of the fact that the lowest-THD measurements of any acoustic driver have been with planar types. Of the available planar headphones, there are two types" electrostatic and planar magnetic.

Electrostatic headphones use a metalised film as the acoustic transducer, with stationary very fine wire coils on either side. A high-voltage is applied to the film and symmetric audio signals are applied to the coils. The film can move all as one in the varying electrostatic field between it and each coil - one coil 'pulls' while the other 'pushes'. The film moves uniformly and THD is extremely low. The coils here are incomplete inasmuch as only one wire end has applied voltage and only relays electrostatic potential. The coils are referred to as 'stators' and appear capacitive to the amplifier.

Planar magnetics use a flat film as the transducer, which has laid upon it the voice coil. In front of and to the rear of the transducer are narrow magnets that the coil works against magnetically. The force over the area of the film is slightly less uniform than for the electrostatic driver as force is only applied where the coil trace is laid. The coil presents a resistive load to the amplifier and is often differentially driven.

Planar headphones have larger transducers than most dynamic headphones, and thus, tend to produce better bass response. They can also play quite loud - far louder than any Human should subject themselves to.

Dynamic headphones have nominal speaker impedances values from 8-ohms to 600-ohms. With typical 100dB/1mW sensitivity, these headphones can often be driven by opamps or simple circuits. Planar magnetics are between 20 and 130 ohms. Electrostatics look like 100pF per stator and require a high bias voltage of 200-500V with signals of up to 70Vrms/stator.

[K O'Connor]

Hi Guys

In my personal experience, I always use "full cup" style headphones, ones that surround the ear rather than insert into the ear. I find this to be a more comfortable arrangement.

Some of the planar speakers use quite large cups and there was a model in the 1980s that angled the driver inward toward the front of the cup to give a feeling more like listening to loudspeakers in free air.

I use some Stax ear-speakers with a custom amplifier. Playing at Human Scale loudness there is no lack of bass or any other part of the experience, and I portend that bass "slam" does not require sheer loudness at all, as others do; rather, that the whole note be reproduced as much as if coming from a point-source as possible - something that a full-range driver can achieve. Ironically, when the sound is very low in distortion, we want to push the loudness higher, but for myself I tend to play it modestly because it just sounds better.

Whether the cup completely blocks outside noise or not can effect the comfort. Complete isolation can be claustrophobic. On the other hand, such isolation can allow the music to be played quieter to save your hearing and allow you to hear it more accurately.

Open-back headphones allow you to still be involved with your living space, which is handy for remaining aware of what is going on around yourself; what your pets or children are doing; if there are emergencies; if aliens have landed on the roof...

It is very easy to play headphones too loud and one has to be a bit determined not to.

[K O'Connor]

Hi Guys

With ear buds and in-ear (IE) monitoring systems one has to be very careful regarding depth of insertion and cleanliness.

Insertion depth particularly affects bass response, which can be weak or over-emphasized. I cannot say for sure, but I have the sense that selecting in-ear phones might be trickier than selecting over-the-ear headphones, and if IE is to be your main method of monitoring sound on stage or in a studio, that a custom fit might be necessary.

Our ears need to breathe, and plugging them traps moisture, fungus, micro-organisms, wax, dirt, sweat, and likely more. So, we have to take great care to clean and keep clean any device we routinely insert in the ear canal. We could end up with a painful infection otherwise. Most IE systems allow for some bypass air so the pressure within the canal is at a natural level, and this contributes to tonal accuracy.