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I've been seeing many relatively affordable USB audio interfaces (even multichannel ones, in fact) with >= 96 kHz sampling rates. These seem like a cost-effective alternative to some of the more expensive solutions out there.
Are these okay to use for bioacoustic recordings? Specifically, do they have a flat frequency response, and what do I need to consider before using them?
Edit: I'd been mistakenly using soundcard and audio interface interchangeably. Now clarified
The tech-specs of the sound card might help here. If you're lucky the device will come with a frequency response curve or at least something as broad as frequency response '0-96 kHz +/- X dB'. The market is quite exciting and probably driven by HiFi audio consumers (I've even seen 500 kHz soundcards...).
In my experience the soundcards can have a pretty flat frequency responses. Of course, the customer support may not be all that helpful in answering further questions in more detail. Here your friendly neighborhood oscilloscope can be very useful - feed in synthetic signals whose structure is known ( sweeps, white noise, sine waves) and check if the recorded signal's power spectrum and levels are as expected.
One main issue is really the continuous gain knobs that many consumer grade soundcards have. For example I've been using the RME Fireface series and they have markings only at 6, 30 and 60 dB at 7, 12 and 5 o'clock. The progression is non-linear and gain values at intermediate positions need to be measured manually! This is where 'scientific grade' equipment can be useful.
Consider Buying a USB DAC
Sound cards mounted in the computer case, even expensive ones, are subject to electronic noise generated by the motherboard and other expansion cards. This is most noticeable when playing back faint signals, which are the bread and butter of passive acoustic monitoring.
One way to minimize the problem is to use a USB DAC instead of an internal sound card. These devices effectively move the noise-vulnerable analog circuit outside the computer case. Many models have integrated amplifiers, which make it easier to hear faint signals.
Much of the online advice on this topic is oriented toward audiophiles and gamers, but these may be useful:
