A Quick Guide To Headphones.

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Types of Headphones

  • Transducer (Earspeaker) Technologies
  • Earpiece Construction

Evaluating Headphones

  • Sound Quality
  • Feel and Fit
  • Durability

Headphone Specifications

  • Frequency Response
  • Diffuse-Field Equalization
  • Distortion
  • Sensitivity
  • Impedance

Headphones for Hard-of-Hearing People

TYPES OF HEADPHONES

Headphones are categorized first by their type of transducer (earspeaker) technology and then by the style of wear. Selecting headphones is an individual process. One size does not fit all. The best headphone choice may depend on the shape and size of one’s ears, when and where they will be used, and of course sound quality. Sound professionals and hard-of-hearing people are heavy users of headphones, but have different criteria for evaluating phones. This article tries to examine the selection process from several points of view.

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Transducer (Earspeaker) Technologies

Dynamic headphones are the most common and are available in every form from lightweight foldables to heavy-duty studio monitors. Also called moving coil types, they have transducers that are basically miniature loudspeakers, with diaphragms connected to a central voice coil that moves within a magnetic field generated by permanent magnets. This type can be very efficient and is easily powered from a standard headphone jack on a receiver or other component. Dynamic headphones are also very dependable and are the most common type of headphone in recording studios and the field than other types.

The transducers in isodynamic headphones are miniature versions of magnetic planar loudspeakers. They have thin diaphragms into which a flat voice coil is embedded – like a printed circuit – for even distribution of the drive force. Magnetic assemblies on both sides of the diaphragm supply the magnetic field in which the diaphragm vibrates. Isodynamic diaphragms are not quite as lightweight as electrostatics, but have the same large radiating area and much of the same sonic fidelity. They are not as efficient as regular dynamic phones, and cannot play as loudly.

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Electrostatic headphones have thin and lightweight diaphragms that vibrate inside in an electrostatic field. They can reproduce music with great detail and low distortion, because the response time is very fast. A separate high-voltage DC power supply (“energizer”) polarizes the diaphragm, which is suspended between two metal plates called stators. The polarizing (or bias ) voltage supply is most often AC-based, but there are battery-powered energizers.

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When an audio signal is applied to the stators, it modulates the electrostatic field and moves the diaphragm. A single stator transducer is possible, but the push-pull operation of dual stators gives the lowest distortion. These headphones must be driven by special amplifiers that amplify the audio signal to hundreds of volts. As an alternative to expensive electronics, a step-up transformer (housed in an “adapter box”) connected to the outputs of a regular power amplifier will also work. Electrostatics tend to be expensive and do not play as loudly as dynamic phones can. They are popular in audiophile systems.

Electret headphones (also called fixed electrostatic) have permanently polarized diaphragms (or are backed by a polarizing material that emits an electrostatic field), so a biasing power supply is not necessary. However, they must still be driven with the same high-voltage audio signals from a high-voltage amplifier or a step-up transformer connected to a power amplifier. Electret headphones have many of the same sonic characteristics of electrostatics (fast, detailed sound), but the electret elements may be prone to depolarization as they age, at which point they must be replaced. Check them every 5 to 10 years.

Wireless and cordless headphones operate without a cord. A transmitter (base) plugs into the sound source (the stereo), and the headphones (usually dynamic-type) have a built-in receiver and amplifier. “Wireless” sometimes refers to infrared-based systems (infrared is used in television remote controls) and “cordless” to radio frequency (RF) transmission systems. Infrared systems have a range of 10M or so from the base unit and must be in “line of sight” of the base for clear reception. RF systems can transmit up 100M from the base unit – even through walls.

Both infrared and RF systems are subject to background hiss, but systems that transmit by first digitally encoding the signal have the lowest noise. For example, the Sennheiser IS850 uses digitally-encoded infrared transmission. Infrared headphones have carrier frequencies in the kHz to low MHz range. RF systems may work over FM, VHF or UHF band. FM systems are not common because the band (88MHz to 108MHz) is already very crowded. VHF systems operate between 130MHz and 250MHz. UHF systems are the most popular. They work from 450MHz to over 900MHz and are less susceptible to interference than VHF.

Having two or more transmitters in close proximity can cause interference unless they operate over different frequencies. Look for infrared or RF systems that have more than one channel either to operate more than one transmitter with different audio sources or to be able to choose the frequency that has the clearest reception. Be sure to check the battery life for the headphones. They should last at least 8 hours of continuous play – preferably more. For information about professional wireless headphones, see the section on In-Ear Monitoring Systems. HeadWize also has project plans for making wireless headphones.

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The two types of noise reduction headphones are active and passive. The active form is also called noise cancellation. Noise cancellation headphones have signal processing electronics that sample ambient sounds with miniature microphones and then generate an inverse signal inside the headphones to cancel the noise (up to 70% or 10dB). The active technology is most effective with low frequency noise, such as that from airplane engines, and when the headphones are closed-ear types to provide passive attenuation of ambient high frequencies. Users can experience a change in air pressure in the ears like “popping”. Sound quality is a mixed bag, and poorly designed systems reportedly impart a muffled quality to music. HeadWize also has project plans for making noise cancellation headphones.

Passive noise reduction headphones are closed-ear types that are specially constructed to maximize noise filtering properties. They are much heavier than regular closed-ear headphones, and can be uncomfortable to wear for long periods. The earcups are packed with layers of high-density foam or other sound-absorbing material. The headband applies substantial pressure to press the earcups against the listener’s head, and the earpads may be fluid-filled to ensure a tight seal. Passive noise reduction headphones are used in factories, at race tracks and other places where broadband environmental noise is very high. The sound-absorbing materials can affect sound quality, such as a weak bass response.

Spatial Simulation headphones present a sense of acoustic space in the soundfield. None of the current designs fully externalize the soundfield without the help of signal processing, but they are more pleasant and less fatiguing to listen to for long periods. The three basic types of spatial simulation headphones are crossfeed, in-front localization and surround. Crossfeed and in-front localization headphones enhance the sonic presentation of stereo playback. Surround headphones are for multi-channel audio sources.

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Crossfeed headphones, such as the AKG K1000 and the Precide Ergo and Jecklin Float, have a floating earpiece (inter-aural) design. Small blocks of foam hold the earpieces slightly off the ears, so that each ear can hear a bit of sound from the transducer on the other side. The crossfeed combined with the effect of the sound impacting on the folds of the outer ear replicates some of the localization cues heard in normal hearing. The resulting soundfield is more centered and realistic with a sense of depth. The earpieces on the AKG K1000 can be adjusted to different playback angles to fine tune the spatial effect. Neither of these headphones provides acoustic isolation. For more information about crossfeed headphones, see A Quick Guide To Headphone Accessories.

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In-front localization (IFL) headphones, such as the Vivanco SR2000 IFL and Ultrasone HFI-50, appear to image the soundfield in front of the listener’s head. In regular stereo headphones, the transducers are positioned to radiate directly into the listener’s ears. The transducers in the Vivanco SR2000 IFL headphones are mounted off-center and in front of the ears to simulate the acoustic path followed by soundwaves in normal hearing. (The AKG K1000 can project in-front imaging by allowing the listener to adjust the angle of the transducers away from the ears.) While the image has a distinctly forward characteristic, IFL headphones do not fully externalize the soundfield without additional electronic processing. For more information about IFL headphones, see Technologies for Surround Sound Presentation in Headphones.

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Surround sound headphones have been around since the days of quadraphonics and appear to be making a comeback due to the popularity of home theater systems. There are two types: 4-channel headphones and personal surround systems. Four-channel phones, such as the AKG K290 or those from American Pro, have 4 transducers (2 per side) and are powered from a 4-channel amplifier. When fed with standard 4-channel surround signals, the sonic presentation is reportedly more spacious than regular stereo and, depending on the design, may be able to resolve front/rear spatial cues. The Vivanco SR 3000 S headphones employ in-front localization construction to aid forward imaging. However, the soundfields of all of these phones remain inside (or very close to) the listener’s head, unless a spatial processor further enhances the audio. These phones have plugs that can be adapted for compatibility with regular stereo headphone jacks. (See Technologies For Surround-Sound Presentation in Headphones for more information about 4-channel designs.)

Sennheiser’s “Surrounder” system (product and prototype shown above) generates a 3-D acoustic image by engaging the transfer function of the listener’s head. Unlike 4-channel phones which rest on the ears, the Surrounder sits on the shoulders and let’s the listener’s head move freely within the soundfield for better imaging. The device is ringed with transducers that project the multi-channel soundfield around the head, with a separate low frequency transducer backed by dual transmission lines. This type of system offers no acoustic isolation. Sennheiser has released a version of the Surrounder for the PC gaming market.

Again, surround sound headphones should not be confused with electronic acoustic simulators (includes crossfeed and auralization processors) that simulate 3-D sound with standard stereo headphones. The AKG Hearo surround system comes with 2-channel headphones, a 4-channel acoustic simulator, a Dolby ProLogic decoder AND is wireless as well. For more information about 3D hearing, see the 3-D Audio Primer. For more information about surround phones and acoustic simulators, see Technologies For Surround-Sound Presentation in Headphones and A Quick Guide to Headphone Accessories. HeadWize also has project plans for making surround headphones.

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Vibration or “Force-Feedback” headphones try to recreate the physical sensation of bass frequencies with the help of vibration transducers mounted on the headphone. The transducers vibrate in sympathy with the low frequencies in the audio signal, so that listeners both hear and feel the bass. These headphones are popular for music and gaming. For more information about vibration transducers, see A Quick Guide To Headphone Accessories.

Earpiece Construction

The designs of headphone earcups (open-back or closed-back) and the earpads (circumaural or supra-aural) are often related, but modern headphones like to break the rules. For example, closed-back headphones usually have circumaural (around the ear) earpads, and open-back headphones most commonly have supra-aural (on the ear) earpads.

Yet today, it is possible to find closed-back headphones with supra-aural earpads and open-back headphones with circumaural earpads. And there are now earcups that are half open-back and half closed-back. The portable audio revolution has introduced a new type. In-ear headphones can be “bud” or “canal” type, the latter having an ear cushion or mold for high acoustic isolation. Deciding on a type of earpiece is a matter of sound and personal preference for “style of wear.”

Open-back (open-air) headphones sit lightly on or over the ear, are typically lightweight and very comfortable. The earcups are acoustically transparent, which results in bass leakage and spill-in of ambient noise (hence, the “airy” quality of the sound). Nevertheless, the bass response is usually satisfactory – see Judging Headphones For Accuracy for more information about low frequency reproduction in lightweight phones. When played loudly, they can leak enough sound to be easily heard by others or, during recording sessions, can bleed sound into microphones. The lightweight headphones for portables and most headphones with foam earpads are open-back types. Also called supra-aural because the earpads are usually sitting on the ears.

  • Note 1: Some circumaural headphones (which have earpads that fit around the ears) have open-back earcups. They look like closed-back headphones, but operate like open-backs. These may be described as open-back/circumaural headphones.
  • Note 2: The open-back style is taken to extremes in a type of headphone which HeadWize terms “spatial.” Spatial headphones are so acoustically transparent that each ear can hear sounds coming from the speaker on the other side of the listener’s head. This effect is intended as a form of acoustic simulation to restore the aural crosstalk which is normally missing in headphone listening. See A Quick Guide to Headphone Accessories for more information on crossfeed processors for headphones.
  • Note 3: Sony and Vivanco make a line of hybrid closed-front/open-back headphones, which are supposed to produce a wide and open soundstage. Unlike the earcups of standard open-back headphones, these are vented only in the rear. Unlike true closed-backs, these headphones provide no acoustic isolation.

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  • Note 4: The headbands of most open-back and closed-back headphones fit over the head. Two new style-of-wear designs have become popular: behind-the-neck and over-the-ear (ear-hanging). The behind-the-neck headband fits around the back of the head or neck and must be carefully evaluated for comfort as the headband is usually too small to be worn the other way. The ear-hanging style is identified by the frame attached to the earpiece that “hooks” over the ears and has dispensed with the headband entirely.

Closed-back (closed ear) headphones with circumaural earpads are categorized as “sealed,” because the listener’s ears completely enclosed in earcups with almost air-tight seal to block out external noise. Thus, the sound quality is usually very clear and detailed, and there is little or no program “leakage” to the outside world. They also have excellent bass response, but are heavier (and, for many listeners, less comfortable) than the open-back types. These types of phones are generally preferred for studio monitoring and noise reduction.

  • Note 1: Lightweight headphones can be open-back or closed-back, but if they have supra-aural earpads, they function and sound more like open-backs due to the acoustic leakage.
  • Note 2: There are headphones with circumaural earpads but have open-back earcups that are acoustically transparent. In this case, the circumaural earpads are more of a comfort feature because they provide no acoustic isolation.
  • Note 3: Sony makes a line of hybrid closed-front/open-back headphones. Unlike the earcups of standard open-back headphones, these are vented only in the rear. This design is supposed to produce a wide and open soundstage.

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In-the-ear headphones (also called intra-aural) have tiny transducers that are worn in the ear and are available as earbuds and canal headphones. Sound quality can be excellent with the better units, but is often dependent on how well they fit the listener’s ears. Because low frequencies are both felt and heard, the direct coupling to the ear may seem to slightly reduce bass response. Canal headphones are especially popular with musicians for their superior acoustic isolation.

Earbuds are worn in the opening of the ear (vertical headphones) are earbuds mounted on a headband and worn sideways; clip-on styles have an attached earpiece that fits around the ear to hold the earbud securely in place – communications models may have a mini-mike boom). They are comfortable (after a period of acclimation), but may not fit all ear shapes well. Earbuds can be converted into canal headphones (see below) with custom earmolds from audiology specialists like Radio Partner and Westone Labs.

Earbud fans are known to coax better sound by burning in new earbuds (playing pink noise or loud music through them for a whole day) to “loosen” up the transducers. When evaluating earbuds in the store, a demo model that has been around a while may be more representative of sound quality than one fresh out of the package. To see if an earbud can benefit from custom earmolds, try listening while holding the earbuds against the ears for tighter coupling.

Canal headphones are seated in the ear canal itself, forming an air-tight seal. Similar in feel to ear plugs, the high acoustic isolation of these phones greatly attenuates external noise, but emphasizes internal sounds, such as breathing – also called the “occulsion effect.” Custom molded canal stems (available from audiology specialists like Radio Partner and Westone Labs) that fit deep into (around the second bend of) the ear canal can reduce this effect and provide superior acoustic isolation. The cushions of canal headphones tend to attract ear dirt and need frequent cleaning. These qualms aside, good canal headphones are known for their crystal clarity and detailed presentation. They are excellent for outdoors listening, because they can be played at safer volume levels.

In-ear monitors, worn by musicians and audio engineers, are canal headphones tethered to a belt-pack receiver that can be hard-wired or wireless. Unlike consumer canalphones, the professional models may have frequency responses designed to compensate for hearing loss that musicians typically experience. Thus, a hearing test is an important first step in evaluating in-ear monitors. Custom-molded canal stems are also recommended, as they reduce the “occulsion effect,” especially important for vocalists, who will otherwise hear their own voices over-emphasized. The receivers have their own volume controls and limiter circuitry. Because they are positioned so close to the eardrums, a limiter is recommended in live performance situations where static and other spurious noises could generate loud bursts that may damage hearing.

Wireless in-ear monitor systems can transmit on two carrier bands: VHF or UHF. VHF systems operate between 130Mhz and 250MHz, while UHF systems operate from 450MHz to over 900MHz. UHF systems are more reliable because they are less prone to interference and generally offer more channels (carrier frequencies) than VHF systems. For more information about in-ear monitors, see The Art of Monitoring and Mixing with Headphones and the In-Ear Monitoring White Paper.

EVALUATING HEADPHONES

The two basic criteria for evaluating headphones are sound quality and comfort.

Sound Quality: Headphones should be evaluated the same way that speakers are. Listen to them. How headphones sound is MUCH more important than what the technical specifications claim. Unlike loudspeakers, headphones feed sound directly into the ears – without the acoustic shaping of normal hearing that occurs when sound waves interact with the listener’s head before reaching the eardrums. Because the shape of the listener’s head and ears has an effect on perceived sound, a pair of headphones may sound different to different people. Buyers should take recommedations for specific brands or models only as a starting point. Bring a portable stereo (and maybe a headphone amp for driving inefficient phones) to the store and try a number of brands and models for sound quality. Look for:

  • clear sound, no distortion
  • deep, clear, controlled bass
    • “Mega” bass headphones generally have midbass boost to simulate low bass and can sound “tubby-ish”.
    • Headphones with “shaker” vibration transducers mounted on the sides (such as Panasonic’s Virtual Motion System) will convey a physical sensation of bass.
  • smooth, even frequency response – no tinny or bright highs
  • if wireless or cordless, there should be little or no hiss.
  • if surround-type, check localization and sharpness of sound image.

For more information about evaluating sound quality, see Judging Headphones For Accuracy.

Feel and Fit: Sound is important, but so is fit. The headphones have to be comfortable. They should neither pinch nor be so loose that they slip off easily. Consider the activity you’ll be doing while wearing the headphones. Look for:

  • comfort – any discomfort will feel worse the longer the headphones are worn. Disc jockeys will appreciate built-in shoulder rests.
  • with in-ear types, how snug is the fit? (Earbuds tend to fall out; canal headphones like a good acoustic seal, so consider a custom ear mold for best fit.)
  • adjustability – can the headband and earcups be positioned to suit the wearer. For example, sound professionals sometimes wear headphones with one earcup off during mixing or monitoring sessions. Pivoted earcups are especially easy to position this way.
  • cord style and length – cords that are split wired into each earcup (Y-shaped) are slightly less convenient than cords that enter the headphone from one side. The average length is 6-10 feet. Sound professionals will want long cords for good mobility between instruments and the mixing board.

Durability: This factor is most important when the phones are for portable stereos or for professional use. With consumer stereos, the ultra-lightweight phones for portable stereos tend to be fragile, so they often come with a case for protection from crushing and other damage. In the studio or in the field, headphones that are touched by many hands or tossed around in transit are also likely to be damaged. Cords (especially thin cords) that are stepped on or yanked too often become intermittent. Headbands break and earcups come off. Professional equipment must be solidly built to withstand such abuse. Look for headphones with modular construction for easy repair and replacement of parts.

Related to the durability issue is power-handling. A few milliwatts are enough to drive headphones to high volume, but audio engineers and performers tend to push the power limits and risk destroying them. Consumer phones are generally rated to handle 100mW or less. Professional models range from 100mW up to 1000mW (1W).

HEADPHONE SPECIFICATIONS

Most headphone packages will list the “specifications,” which attempt to describe in technical terms how a pair of headphones will sound. Equipment specifications are sometimes overstated, but in the case of headphones, they may be entirely misleading. It bears repeating that because headphones operate so close to the ears, the sound of a pair of headphones can depend on the shape of the listener’s head and ears. Headphones with “good” specs may not sound good, and those with “bad” specs may not sound bad. That said, let’s proceed:

Frequency Response: The range of frequencies that the headphones can reproduce effectively. The audible bandwidth is 20 Hz – 20,000 Hz (or 20kHz). Outside that range, sounds are not audible to most human ears (except the occasional sound professional and of course, “golden ear” audiophiles – who can often hear into the Megahertz range). Ultra-low frequencies (less than 20Hz) are more felt than heard. Beware claims of a measured “flat” frequency response (sometimes listed as 20Hz to 20kHz +/- 3dB). A headphone with a true flat response will sound terrible, because what the ears perceive as a “flat” response actually has many peaks and valleys due to interaction of the sound with the listener’s head before it reaches the ears. Instead, headphones are often equalized to sound flat. See diffuse-field equalization below.

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Diffuse Field Equalization: There are two types of built-in headphone equalization that attempt to “flatten” the perceived frequency response: free-field and diffuse-field. Free-field EQ assumes that the listener is in front of a sound source in a listening environment without echos, such as the wide outdoors. Diffuse-field EQ substitutes a room with reflecting walls and can be more natural sounding than free-field EQ. The method for measuring DF flatness is defined under the IEC 60268-7:1996 standard. Because diffuse-field EQ is based on an “average” head/ear shape and room model, it may not appeal to all listeners or sound natural with all recordings. A binaural recording may or may not benefit from diffuse-field EQ, depending on how the recording was made (e.g., whether the microphone was inside the dummy ear), so be sure compare models with and without DF EQ. Headphone manufacturers are increasingly standardizing on DF equalization.

Distortion: How accurately the headphones reproduce sounds and given in a percentage of signal distorted. Lower is better – 1% distortion or less (at maximum power). Tests have shown that 1% distortion is at the threshold of audibility. Headphones have less distortion at loud levels than speakers.

Sensitivity (loudness): A measure of headphone efficiency in dBs SPL per milliwatt of input. A low number means that the headphones need more power to sound as loud as those which have a higher sensitivity. Headphones for portables need to be fairly sensitive because of the lower power output of portable stereos. Modern dynamic headphones have sensitivity ratings of 90 dB or more. When shopping for portable headphones, look for a sensitivity rating of 100 dB or greater.

Impedance: A measure of headphone load on an amplifier and stated in ohms. This factor is less important with solid state amplifiers, which can drive most headphone impedances, but can be significant with tube amplifiers, which are more sensitive to load impedances. Both consumer and professional headphones generally have impedances of less than 100 ohms. There are professional models rated at 200 ohms or more to minimize loading effects on distribution amplifiers which are often drive a whole bank of headphones at one time. Be aware that very high impedance phones may require more power – on the order of Watts instead of milliWatts.

HEADPHONES FOR HARD-OF-HEARING PEOPLE

Headphone systems for hard-of-hearing people must be able to reproduce sound at very high volume with low distortion (such as those used by musicians and audio engineers). Where appropriate, evaluate headphones with and without hearing aids. Headphones can be worn over a hearing aids, if the aids have T-coils that can receive signals from the magnetic assemblies in headphones. If the aids have compatible T-coils, then wearing headphones over aids allows the listener to simultaneously monitor over a loop system external sounds, such as telephone rings. A loop system can also transmit the audio signal itself, but is limited to monaural reproduction.

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Unlike loop systems, wireless and cordless headphones and in-ear monitor systems not only have their own built-in amplifiers, but also transmit and receive in true stereo. They can be plugged into audio systems to provide additional volume when listening to programs on loudspeakers with people who are not hard of hearing. Several headphone manufacturers sell portable assisted-listening devices. For example, the Sennheiser Audioport series are wireless headphones in a stethoscope shape that is easy to put on and take off. For portable all-in-one-unit convenience, Radio Shack sells a stereo listener with built-in stereo microphones and 3-band equalizer.

A recent hearing test can serve as a guide for improving fidelity. Canal headphones for musicians often have frequency responses tailored to compensate for some forms of hearing loss. A headphone amplifier and possibly an equalizer to boost select frequencies can further help achieve acceptable volume and sound quality.

Hard-of-hearing people may also experience reduced dynamic range – the range of loudness between the lowest audible sound and the loudest tolerable sound. An amplifier may boost volume so that lowest passages can be heard, but at the same time, make the loudest passages too loud. An audio compressor or limiter will narrow the dynamic range in program material. See Signal Processing Fundamentals for a discussion about audio compressors.

Addendum

7/31/99: Added section on in-front localization headphones. Also updated section on surround headphones.

9/28/00: Added section on passive noise reduction headphones. Revised section on earpiece design. Replaced some images with higher resolution versions.

References:
A Buyer’s Guide To Headphones, c. CBC Halifax.
Borwick, John, The Loudspeaker and Headphone Handbook, 1988.
Frink, Mark, “Choosing Custom In-Ear Monitors,” Mix, July 1997.
Herold, Edward, “Audio for the Elderly,” J. Audio Eng. Soc., October 1988
Hoff, Michael, Earbuds, Equalization and Headphones, c. 1998.
Labriola, Don, “Review: Etymotics 4 Earspeakers,” Electronic Musician, November 1993.
Larcher, Veronique, “Equalization Methods in Binaural Technology,” AES Preprint #4858, 1998.
Masters, Ian G., “Headphones On The Go,” Stereo Review, August 1990
Molenda, Michael, “Intimate References. The EM Guide to Headphones,” Electronic Musician, February 1992.
Moller, H., “Design Criteria For Headphones,” J. Audio Eng. Soc., April 1995
Santucci, Michael, “Musicians Can Protect Their Hearing,” c. 1997,
Sensaphonics Hearing Conservation

c. 1998, 1999, 2000, 2001 Chu Moy.

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