Review of the Audix HT5 omnidirectional headset microphone (for linguists)
Introduction: About the Audix microphone system
Audix is an American company founded in 1984 with its manufacturing facility located near Portland, Oregon. Over the years, Audix microphones have become increasingly more popular, especially for live vocal performance and drum amplification. Audix microphones are known as much for their excellent sound as for their durability and roadworthiness. Most Audix microphones are based on no-nonsense, simple, but elegant designs, and the HT5 is no exception.
The Audix HT5 is designed to be used either in a wireless or a wired recording chain. Since most field workers use wired microphones, I am going to review the microphone in a wired configuration (Model HT5-P), as shown in the photo below (Figure 1).
Figure 1. The Audix HT5 microphone; the headband fits comfortably and keeps the microphone at a constant distance from the talker's mouth to ensure consistent quality across recording sessions
Two important competitive advantages for field recordists
Before I go into the details of the HT5's design and performance, I would like to mention what I consider to be two important competitive advantages over most other head-worn microphones.
The headband
The Audix HT5 uses a unique headband design (Figure 2). It is actually my favorite headband of all the head-worn omnidirectional microphones I have reviewed so far. In my opinion, it can be used in fieldwork, as is, without any modification. I consider this a very important competitive advantage of the HT5, at least for field recording purposes. The headband is very easy to put on, it is very lightweight and flexible; it works well with most types of hair styles and facial hair. It seems surprisingly durable. Most importantly, it places the microphone close enough to the talker's lips to ensure high spectral detail in the recordings. It also keeps the microphone at a constant distance from the talker's mouth, off to the side of the mouth to minimize popping (signal overload due to increased, rapid airflow). I think it is a very clever design and I wish other manufacturers would offer a similar option.
Figure 2. The Audix HT5 omnidirectional headset microphone and the APS911 power supply unit
The APS911 battery adapter
The other important advantage for field recordists is that the HT5 microphone can be powered by an inexpensive, yet very functional battery-powered belt pack adapter (APS-911). The ability to power a condenser microphone with a single AA battery will save you a lot of your recorder's battery power and will supply the microphone with just the right amount of voltage for proper function. It may also help prevent noise caused by poorly designed or implemented the field recorder's onboard phantom power (see more in this post).
What is in the box?
Design and specifications
The HT5 capsule
The Audix HT5 is a miniature pre-polarized omnidirectional headset condenser microphone (Figure 3) designed primarily for stage and broadcast applications. Audix describes the microphone as "lightweight and roadworthy," and I find this claim to be accurate. I haven't had a chance to test the microphone on the road but, having spent some time working with it, I can attest to its durability. There's very little that can go wrong with this microphone from the design point of view. The rubber coating on the steel alloy frame provides a layer of acoustic insulation, which helps reduce handling noise, at least as much as the competitors. Even the cord and mini-XLR connector (Figure 7) feel well-designed and of high quality. Table 1 contains a summary of the engineering characteristics of the microphone. The HT5 has the "sweet spot" sensitivity of 5 mV / Pa @ 1k, which is just about perfect for normal conversational sound levels, with close microphone placement. In practical terms, your recorder's pre-amplifier gain will not have to be turned up too high, which will help reduce pre-amplifier noise and distortion. Most quality field recorders (e.g., Fostex FR-2LE) should be able to work with the Audix HT5 properly (for sound files recorded with the Zoom H4N click here).
Figure 3. The Audix HT5 microphone capsule
Table 1. Audix HT5 specifications
Transducer Type
Pre-Polarized Condenser
Frequency Response
20 Hz - 20 kHz
Polar Pattern
Omnidirectional
Output Impedance
1000 Ohms balanced
Sensitivity
5 mV / Pa @ 1k
Equivalent Noise Level
26 dB (A weighted)
Signal to Noise Ratio
68 dB
Maximum SPL
140 dB
Cable/Connector
Miniature 3 pin or
4 pin (HT5-4PIN)
XLRf connector
Polarity
Positive pressure on
diaphragm produces
positive voltage on pin
2 relative to pin 3
Housing/Finish
Flexible steel alloy /
Black or Beige
Weight
1.1 oz / 31 grams
The APS-911 power supply
The Audix HT5 microphone reviewed here is the version "HT5-P." It comes with the APS-911 power supply unit, which enables the HT5 to be used in a wired configuration. The APS-911 unit is a belt-pack: a small, black box with a belt clip (Figure 5), an on-off switch, and a high-pass (a.k.a., "low-cut") filter selector (Figure 6). The unit performs double duty: (1) it allows the HT5 to be interfaced with standard 48 V phantom power, or (2) it allows the HT5 to be powered by two AA batteries (Figure 4).
Figure 4. The Audix APS911 power supply unit that operates either on a AA battery or standard 48 V phantom power. Note the mini-XLR input (to microphone) and XLR output (to pre-amplifier).
The ability of the HT5 to be used with the APS-911 unit is an important advantage (see Section 2 of this article). In part, it is about saving power. You can switch your recorder's phantom power supply off and thus save a significant amount of power. But the APS-911 performs an even more important function: it ensures that precisely the right amount of voltage is supplied to the HT5 at all times.
It is rare for a field recorder to supply full 48 V of phantom power. If fact, the exact amount of supplied voltage varies from recorder to recorder, and it is typically unstable. Even if your microphone's specification allows for less than 48 V, the electrical and acoustic characteristics of the microphones are likely to change with varying voltage. The sensitivity, frequency response, and maximum SPL values are among the most affected. There's almost no way to predict how the microphone's performance will change under the circumstances. The APS-911 makes sure that the microphone always gets the right, and predictable, amount of voltage, so you can be sure that the microphone's performance is always up to spec.
Figure 5. The back of the APS911 unit showing a belt clip
Figure 6. The APS911 on-off switch with a low-cut filter
Figure 7. The Audix HT5 TAF3 connector to connect to the APS911 unit or a wireless transmitter
Acoustic characteristics
Self-noise
Self-noise (a.k.a., inherent noise) is generated by the recording chain due to electrostatic activity. Self-noise can be theoretically derived from the manufacturer provided specifications, but because its net value results from specific microphone-recording pairings, it is important to analyze real-world self-noise performance in a realistic field recording scenario. I perform a self-noise test with all the microphones I review; you can read more about self-noise testing in this article.
The theoretical output noise of the Audix HT5 is estimated to be -111.81 dBu. In order to not appreciably degrade the microphone's noise performance, the recorder's (or mic pre-amp's, to be exact) A-weighted Equivalent Input Noise (EIN) value should be approximately -122 dBu or lower. For example, if you use the Fostex FR-2LE (EIN = -129 dBu, A-weighted) with the Audix HT5, you should get very good self-noise performance, at least in theory. Knowing that, we now need to test the HT5 with our favorite recorder to measure real-world noise performance with sound levels corresponding to those of normal conversational speech.
I always use USBPre (EIN = -124 dBu) as my field recorder of choice for microphone testing. You can find the details of my test setup here. Figure 8 shows a spectrum of the Audix HT5 microphone, with the Sound Devices USBPre field recorder, at normal conversational levels (normalized to RMS = 70 dB). Noise performance is excellent and compares very favorably with other microphones tested at this website. This is partly due to the self-noise characteristics of the microphone, and partly due to its medium sensitivity (5 mV / Pa), which requires pre-amplifier gain levels that are low enough to not cause too much noise or distortion. In short, the Audix HT5 will give you excellent noise performance, and you can be sure that if you match it with an appropriate field recorder, the only significant source of noise will result from loud environmental ambient noise or poor recording technique. According to my tests, the Audix HT5 is among the lowest self-noise microphones I have tested. If you look closely at Figure 8, you will notice a peak around 60 Hz. This is a typical example of the so-called "rumble" (see later in this article). In any industrialized urban recording environment, it is virtually impossible to avoid eliminate low-frequency rumble, unless a high-pass filter is used. Fortunately, the Audix APS911 power unit, supplied with the microphone, has a high-pass filter available. I typically turn it off, but it is there, if you need it.
Figure 8. Spectrum of the Audix HT5 noise print, calibrated
The influence of phantom power quality
When using condenser microphones, we often forget that phantom power adds another potential source of noise and/or distortion. It is relatively easy to design and build a phantom power supply unit operating on 110 V AC, but it is significantly more difficult to build one operating on two AA batteries and achieve the same level of quality. This is why, we should always make sure that the phantom power supply unit is of high quality, especially when it is built into the recorder itself. Figure 9 shows a comparison of two spectra of the Audix HT5 self-noise with battery power (left panel) and the Zoom H4N's onboard phantom power (right-panel) the RMS is left at its original settings (H4N's recording level of 80). The onboard phantom power supply adds a significant amount of broadband noise to the recording. I am very glad that the Audix HT5 is available with the APS911 unit, which not only is going to save your recorder's battery power, but is going to provide the microphone with clean voltage.
Figure 9. A comparison of noise print spectra with battery power (left panel) and oboard phantom power (right panel) with the Zoom H4N field recorder
Frequency response
The frequency response of the Audix HT5 is not as flat as some of the competitor microphones (e.g., Beyerdynamic Opus 55 MkII), at least on paper (Figure 10). I am not sure whether this is more of a function of testing and reporting or a real design decision on the part of Audix. Either way, the curve is so close to being flat that I wouldn't worry about it.
Figure 10. Frequency response of the Audix HT5 microphone, according to the manufacturer
Low frequency performance
For me, the low-end is what makes or breaks a speech recording microphone. A lot of my work in acoustic phonetics and hearing science focuses on the low end (F0, spectral tilt, nasalization, breathy voice, creaky voice, etc.). I often recommend that researchers take special care to obtain speech signals with neutral low end.
How to achieve neutral low end?
Every element of the recording chain must have a flat low-end response. Achieving a flat low-end is neither easy, nor desirable in mainstream recording technologies. In fact, having a bass roll-ff (a high-pass filter) is often marketed as an advantage of a microphone, power supply pack, pre-amplifier, or recorder. The main reason being that the high-pass filter removes the so-called "rumble" from the recording. Rumble is a collective impressionistic term to describe various types of low-frequency noise that are present in almost any industrialized urban environment. The sound of traffic, aircraft, heavy machinery, as well as common household appliances occupies the low end of the frequency spectrum, and it is often easily removed by applying a high-pass filter. Once the filter is applied, the rumble is gone, but so is in important portion of the speech signal. Buying a flat-response microphone does not, by itself, guarantee flat-response recordings. You need to pay close attention to the specifications sheet and the user manual of every piece of the recording equipment you plan to buy. I would recommend calling the manufacturer's technical support and asking them directly. You can read about other sources of low-frequency noise here.
Low frequency response of the Audix HT5 microphone
I generated a waveform with peaks of equal amplitude at the frequencies of 50, 100, 200, 300, 400, and 500 Hz. I then played the signal out of a flat-response loudspeaker and recorded it with the Audix HT5 microphone. The spectrum in Figure 11 shows the low frequency response of the Audix HT5 microphone. It reproduces each center frequency really well, with no appreciable attenuation or amplification throughout the range. There is a slight dip in the spectrum around the very low end (50 Hz), but it is negligible, as far as I am concerned, and can be predicted from the manufacturer's supplied specification sheet. I am confident that speech signals captured with the Audix HT5 microphone will give me reliable low-frequency data. This neutrality of the low-end is extremely important, and I am very happy to report that the Audix HT5 passes the low-end the test.
Figure 11. The spectrum of a low-frequency response of the Audix HT5 microphone showing no appreciable bias
Audio tests
I have tested the Audix HT5 microphone in a variety of situations with a number of recorders. Each time, the microphone performed admirably. To give you an example, Figure 12 shows a spectrogram of the phrase "buy a large barrel of good beer" with a state-of-the-art Tucker-Davis microphone pre-amplifier. The spectral detail is outstanding, as is the frequency response and transient response of the microphone. Figure 13 shows the same phrase recorded with a $300 Zoom H4N field recorder. The spectral characteristics of the recording seem very good as well, with the exception of a slightly higher level of noise. Still, the ability of the Audix HT5 to be properly interfaced with different types of recording devices is exemplary.
Listen to MP3 at 128 kbps:
Figure 12. A spectrogram of the phrase "buy a large barrel of good beer" recorded with a state-of-the-art Tucker-Davis microphone pre-amplifier
Figure 13. A spectrogram of the phrase "buy a large barrel of good beer" recorded with a $300 Zoom H4N field recorder
Figure 14 shows a spectrogram of my favorite test phrase "czarna krowa" to allow a direct comparison with all other devices tested in this website. The spectrogram looks really good, showing a great deal of detail. Also, you can easily tell the difference between the voice in Figures 12 and 13 and that in Figure 14. My voice (Figure 14) appears much less stable in terms of phonation, has broader formant bandwdiths, a more breathy quality, and traces of creak. The Audix HT5 can capture enough detail to make this type of assessment. The microphone would make an excellent diagnostic tool in a clinical setting.
Figure 14. A spectrogram of the test phrase "czarna krowa"
The FFT (Figure 15) and LPC (Figure 16) graphs confirm the spectrographic findings. The detail is excellent, the spectral envelope properly rendered. The formant peaks appear sharp, with narrow bandwidths, showing the ability of the microphone to capture speech-relevant spectral detail in an unbiased manner.
Figure 15. FFT of the vowel /a/ in "trawe"
Figure 16. LPC of the vowel /o/ in "kropki"
Figure 17. FFT and LPC graphs of the vowel /ai/ in "while" superimposed onto each other. Again, the spectral detail is superb.
Conclusion
The Audix HT5 microphone performed admirably in all of my tests. I found the microphone to be among the best microphones I have ever tested, on par with my favorite Beyerdynamic Opus 55 Mk II. However, the Audix HT5 has two important advantages over the Opus 55 Mk II: (1) the comfortable, sturdy headband, and (2) the APS911 battery-powered belt-pack. The price is very competitive (around $350) and the Audix technical support is first-rate. If you think you need a new field recorder of superb quality at a good price, look no further.
