Review: Fostex AR-4i iPhone 4/4S Stereo Audio Interface

Jeff's Rating: 4/5

tl;dr: Five stars for the capabilities, four for the fit and finish. It's a great tool, but not without a few rough edges. (See note about iPhone 4S compatibility).

[UPDATE on iPhone 4S compatibility: I've heard many reports of people having trouble with the AR-4i and the iPhone 4S; my own usage indicates that there is one quirk with this combo: the wireless signals on my phone go away while plugged into the AR-4i. Example recordings with various firmwares below:

Fostex has fixed a few issues (like a strange clicking noise) with firmware updates, but I don't know when they'll get the WiFi/3G connectivity issue fixed—hopefully soon.]

I received a demo Fostex AR-4i unit for review, and was excited to finally record two channels at once (stereo) on the iPhone 4/4S. If you've read my guide to external microphones on iPad, iPhone, and iPod touch, you'll note that (until the AR-4i) there was no way to record stereo inputs on the iPhone 4/4S. This is no longer the case.

Fostex AR-4i

I'll be updating this review over time as I get more time in putting this unit through its paces, especially on more projects, but I'll start with some initial impressions of the device—the audio quality, build quality, and features.

Video Review (and test)

I've recorded a few clips with the unit and Apple's built-in Camera app, and I've also included some footage of the device itself, where I show some of the relevant details of the hardware itself:

Audio Quality

No external audio interface would be worth anything if it didn't do a good job getting a good, clean audio signal from a microphone. The AR-4i comes with two little cardioid mics that will definitely work in a pinch, but won't give a clean signal like a handheld mic or a lavaliere.

You can plug in any mic that gives a normal mic level signal, as long as you adapt the cable into a 1/8" mono plug. I highly recommend the $20 Audio Technica ATR-3350 Lavaliere mic for interview situations (get two of them - they're cheap and they work great in a mobile situation!), but most mics will do just fine.

Since you only have level control for both inputs together (there's only one input level adjustment knob), you have to make sure your two mics have similar levels coming into the unit, otherwise one channel will be louder/softer than the other.

You can hear a couple different recording scenarios in my video embedded above, and I'll try to add a few more audio-only sound samples to this post as I get time. Suffice it to say, though, this device has a low noise floor, and could definitely be used for broadcast recording. It's definitely not studio-quality, but will work for live gigs, podcasting recordings, radio and TV, etc.

Apps Supporting Stereo Sound

I've only used the device with a few apps so far, and I'll continue to update this section as I get time:

  • FiRe 2 (Field Audio Recorder 2) - works beautifully and allows for recording two-channel, uncompressed audio, then exporting via multiple methods as AIFF, WAV, AAC, Ogg, Lossless, etc. (FiRe is also tested and working).
  • Camera (built-in) - works fine for stereo recording.
  • FiLMiC Pro - Untested.
  • Voice Memos (built-in) - Untested.

Build Quality

For a prosumer product, the build quality is pretty good; the plastic case feels pretty solid and could withstand some knocking around inside a hard case. All the mic jacks and knobs feel sturdy and solid, and I don't feel like I need to be gentle with any of the controls on the device.

As a strictly professional unit, I would say the product is good, but with its flaws; the battery holding area is a little cavernous, and hard to use for people with anything but the smallest of hands, and the fabric strip that is meant to help pull out the batteries wasn't long enough to actually fit behind both batteries, so I have to use a long flat tool to pop the batteries out.

The battery door doesn't feel like it will pop off too easily, but with an all plastic, non-hinged design, I do think it will reach the point after a few openings and closings where it will pop off after a slight knock. However, considering the fact that I'll be buying an iPhone 5 in less than a year, and it won't work with this device, is that really a bad thing?

I only have three complaints about the overall hardware design:

  1. The metal (but mounted-in-plastic) tripod mounts - the two tripod sockets fit any standard size tripod or tripod mount, but one on my unit was slightly out of alignment, so I have to be careful when tightening a tripod screw to not snug it up too tight. I like tight tripod connections, so that's a point against this case; I feel that if I keep tightening, it might break the mount free of the casing.
  2. The device is slightly awkward to hold—even with the included metal grip. It doesn't conform to a handheld situation in either orientation, and seems built only to be optimal for tripod-mounted or grip-mounted situations. In the interests of keeping my on-the-go bag light, I don't like having to bring the heavy metal grip with the AR-4i.
  3. It's hard to pull out the iPhone (but it's easy to get it in). The dock connector is a bit tight, and it requires two hands and a little more pressure than I'd like to pop the iPhone free of the dock connector. Care needs to be taken in this process.

Even with these gripes, though, I think it's not a bad deal; the case can stand upright in the vertial orientation pretty well, and lets me view the iPhone screen easily while I'm doing an audio recording. And it doesn't feel like it's going to fall apart. All the internal connections seem to be solid.


Some of the neat little things I've found about the AR-4i:

  • Built-in preamp for cheaper mics means I don't need to post-process the audio as much (for amplification).
  • Built-in LED level is actually a very good measure for when clipping will occur (though I wish it had one or two more LEDs for better granularity).
  • The design allows for easy use in either vertical or horizontal orientations, and video shooting seems to be optimal in the horizontal axis (as it should be), with a hotshoe mount included on the top.
  • The AR-4i App (more below) is a great help towards tailoring the device's capabilities toward any situation. Being able to set, in hardware, limiting and channel panning, or stereo/mono capabilities, is extremely helpful. This allows any app to work with most of the AR-4i's features, since they don't have to specifically support the unit's capabilities.

iPhone App

Fostex announced the availability of the free AR-4i App on the iOS App Store in September 2011. The App (in its initial configuration) is a little rough around the edges: you can tell the user interface text was originally written in another language and translated into English:

AR-4i Interface Text

The app allows you to change a good deal of settings for the hardware itself, including the input mode (stereo, mono (left or right, or both together)), which headphone jack to use (either the iPhone's built-in jack, or the AR-4i's headphone jack), and other advanced settings (listed below):

  • Independent control of Inputs 1, 2 and 3 (a little confusing, though).
  • Low cut filter (off, 200Hz, 500Hz, 1kHz)
  • Limiter (off, fast, slow)
  • Headphone jack control
  • Firmware update and hardware settings reset

There is currently no other device on the market which offers all the features of the AR-4i, and coupled with some of the awesome tweakability of this App, the device really makes the iPhone 4 a professional audio recording platform.

Notes and More Info

Notes will go here.

Read more about the AR-4i on Fostex's website »

Buy the AR-4i

The Fostex AR-4i retails for $149, and can be purchased on with free shipping.

Read my review on »


Hi, I also posted on your youtube channel...
what is the quality of the live music concert recording?
can it be compared to bluemikey for iphone 3GS?
I just need it to record live gigs...


To me, you would be better off buying a dedicated recorder like a Zoom or Tascam for a task like this though I know some people just want to use their IPhones. This is especially true if you want to use XLR mics. The H2 is priced for less than this device and the H4N is about $100 more. I think you have talked about the Iphone microphone cables before (from I have not tried it, but you could you a put a splitter into one of those cables, and I'd guess the results would be reasonably close for much less money.

That, and, if you're doing video, this is a mobile workhorse - you can record a clip with the sound already mixed in (if you're doing field work), then upload to FTP or some other service immediately. Makes editing happen a bit quicker :-)

If you're looking for a decent semi pro video set up it seems like you really cant go wrong with this! Of course purists will introduce alternatives but for peole like me who just want a really good plug and play solution coming across this has saved me a great deal of hassle. I've been looking for 2 weeks for my best set up (tripod, holder/mount, mic/s, app, back up battery etc etc etc ) and nothing comes close to this. Yes it looks a little rough around the edges but for a first gen version its pretty close to perfect and a good price. Stop looking get it ordered and start producing! Be interested to hear other opinions and in field usage reports.

Updated review(10/28/2011), I've been having serious issues with glitches every 5-6 seconds on my recordings, I was blaming the fostex ar-4i unit, I do apologize for it, I just found out it was the fire 2 field recording app the responsible for that issue. after switching recording apps, I got surprisingly clean and smooth line source recordings from motif xf6, korg pa800, ipad 2 w/ behringer uca222 and soundcraft m12 using the basic voice memo app from Iphone 4/Ipod touch 4G. this is a great unit worth to buy it despite its 1/8" inputs. very good AD/DA converters. I'm now beautifully recording my band practices in stereo mode . highly recommended if you already have an Iphone 4 or Ipod touch 4G, I tested them both.

I'm not 100% sure about this, because I don't have an iPod Touch to test with it, but the only thing I could find that would even hint at the device being compatible with a non-iPhone is: "Note: AR-4i works with iOS 4.2 or higher" - but all their online documentation mentions iPhones, and not any other devices.

hello, I would like to ask if anyone can tell in more detail what is highest sampling frequency/bit rate of recorded audio samples and also how high/low is a noise floor with built in vs. external mics. And the most important - do you know if this unit will work with Ipod touch 4th generation (released in 2010-09)?.. I can't find any information about this on manufacturer site, ... i thought that perhaps someone here might know. thank u!!!! Vlado

I'm pretty sure the maximum bitrate I've found is uncompressed or Apple Lossless at 44.1 Khz (that's a limitation of the iPhone itself, it looks like). I can't give specifics on noise floors since I don't have any good test equipment on hand, though I may try to borrow something from a local radio station to do more testing (noise floor and freq response are two things I'm interested in too).

Also, I don't think the iPod touch would be supported, as the casing fits perfectly on an iPhone 4/4S, and may not even physically connect to an iPod Touch.

Have you tested this with your iphone4s. There are alot of us who have been having problems with this and the iPhone 4s, namely interference and static sounding audio. Fostex put something on their webpage about this that they are working on it but no updates, interested to hear your thoughts. Thanks!

The iPhone 4S's compatibility issue would be resolved. Could you test with latest firmware ver 1.1.2?

Thanks for the comment! I've updated to 1.1.2, and tried another recording; the faint clicking sounds are gone, but I'm still not able to stay connected to 3G or WiFi while the iPhone is plugged into the AR-4i (and I'm recording).

I need this unit for livestreaming will it stay connected to 3g or Wi-fi while streaming.

If you have an iPhone 4S, it will not work at this time. However, I haven't had the same problem on my older iPhone 4.

Hey Jeff,

I'm enjoying your work dude. I have both an Ar4i & a guitar jack 2, i'm finding the Ar 4i great for recording & videoing rehearsals with my band.

I'm wondering if there's a cheap possible way of introducing a seperate mic mixer with independent mixing levels for the individual mics thus getting a really cool seperated instrument feel to the live recordings. I've used twin stereo jacks & increased the inputage to 4 x mics, but i can't adjust them individually? it's presumably feasible to have 2x 4 mic mixers, one for each mic port to create 8 mixing areas? I was thinking of getting a seperate headphone mixer too, i've seen those too, relatively cheap & compact, or is this getting a bit too complicated?
Basically do you know of a good cheap option for this as my usual recording equipment is bulky & an arseache to set up etc... i like the idea of having it all battery operated & relatively wire free too.

There'll obviously be a few phasing issues, but sometimes that's quite cool in live recordings. Also there's the phantom power issue of powering the condenser mics should i choose to use them.

Firstly does this make sense & secondly is it reasonably achievable, many thanks fella.


Ps, i'm having a few compatibility issues with my guitar jack 2 & fire, but one question at a time eh??!!

For that much complexity, I would highly recommend using one dedicated mixer (for many years, I've used the Behringer 802 mixer, which has all the options you'd need for a small band setup and then some... and if you ever go larger, you can use this as a headphone mixer or a drum set mixer. It adds in one power connection, but it's worth it to not have a bunch of little battery-powered preamps hanging off your iPhone :)

Thanks Jeff, i've invested in the behringer on your quite literal sound advice.
I'll let you know how i get along.

cheers once again.



Thank you for you honest review of the product.

I am looking at using it for quick testimonials in a doctor's office. The patient giving the testimonial will be 2-4 feet from the iPhone 4s. Would the mics that come with it be clear enough or would you recommend another mic to get as well. The doctor will be using the device so I would prefer not to use a Lav (to try and keep it simple and quick to do), but not opposed to a mic on a table stand.

Also, what is the latest on the AR-4i and iPhone 4s issues.


I think the mics with the AR-4i would work well from 2-4 ft. away. Also, in my own use of the device with my 4s, all the problems I experienced have been resolved with the latest firmware updates, except for the dropping of the 3G signal (which is not a huge deal for me, since I don't want to receive phone calls while recording...).

Hello Jeff, great information, you seem to know what works and what doesn't. I need some specific advice, so sorry for the long post, I figure the more you know the better you can advise.

I am in Costa Rica, and I have a DJ promotional gig where I go to concerts, clubs, etc and record the party, 80% up on the DJ platform recording what the DJ is doing and panning the crowd, and then 20% down with the crowd getting the party mood and people as well as shots of the DJ from below.

I must be as non obtrusive as possible and very agile and 100% one man show run n gun mobile. Right now I am using a Canon SX40 because of the all-in one with the very long zoom and fairly good (faster-better low light then SX30) processor.
FYI in probably 6 months I am going to upgrade substantially to a Pro DSLR, probably the new Nikon D800, but till then I have to make everything work with the SX40 and (I think) an iPhone 4s for external sound recording.

The Photos and Videos come out OK SX40 , but the sound from the is useless, bad clipping and medium high to high volume, and normally the clubs / concerts and my location on the DJ platform or in front of it with the crowd is Very high volume, and a lot of Bass (House Music as example) So did my googling and can see I need a separate sound recorder and then sync in post. And then I saw all the Camera and Video and sound recording apps for iPhone, and found you.

By coincidence, I just got a iPhone 4s yesterday. Then I saw this setup, which mounts the iPhone to the hotshot. I guess you could also just mount a high quality mike to the camera hotshot and have the sound signal ported to the iPhone that would be in your pocket. and that page shows the blue mic microphone.

So any help/advice on the BEST sound recording app for the iPhone 4s to sync the sound in post, your recommendation on the best microphone(s) with very high DB tolerance for that environment, and if you would go with the iPhone mounted on that contraption and an iPhone mic (which is the very best?) or a normal mic on the hotshot and run a line into the iPhone in my pocket, and if so the best mic for that.

Again, hope that is not too much.
Thank you, Raymond

I would go with the Blue Mikey, and I love the FiRe recording app for almost everything; if has great quality settings, level settings, and export options/methods.

I've heard from a few people that the Blue Mikey works particularly well for high SPL/dB applications like concerts and clubs. The only other thing I might try is a plain old Shure SM58 (it'd be mono though) with the appropriate adapter.

Hey Jeff, thanks for all the great info.
I'm trying to capture my band live for promo video, nothing major Youtube or sim. I have a few spare Sm58s. Would these with adapted 3.5mm inputs in an X/Y pattern improve on Fostex mics in an AR4i with relatively high SPLs into an iphone? - Kids are also keen amateur filmmakers... They could also presumably with suitable adapters give stereo mic input to a decent camcorder? Geoff.

Yes, that's a better solution than the mics included with the AR-4i, and would let you tolerate a much higher SPL. I typically use Shure PG81s in an XY pattern when I do choir or small group recordings, though I don't know how they'd tolerate the volume of a rock/pop band!

Is it possible to record stereo in to the AR-4i? direct from the mixer stereo out put to the iPhone? Or do you know a lighter device that could fit the job?
thank you!

Hey Jeff thanks for your quick response!
Do you know what kind of electronic it contains?
I would like to build one with an RCA cable.

That looks like a very good option to me; it's a little less secure (since it doesn't have any other connection to the iPhone besides the dock connector), and is a little less robust, but would serve you well as a simple stereo input (or guitar input... but there are much cheaper options for that, like the Tascam iXZ).