Feedback and Room Resonance
There is too much SCHREECH in my life

When there is a sound engineer in situ he will most likely have a graphic equalizer (EQ) in his system. He is able to sculpt or adjust the volumes in small defined frequency ranges up or down. When a room over-resonates with an audio frequency he literally cancels/ reduces that frequency out of his system's output.

Most restaurant/ ballroom systems are used for recorded music and speech making and the sound is run by whoever is at hand. The systems are probably very basic affairs with little option to adjust other than volume. Especially not a graphic EQ. Live Celtic harp music will almost never sound the best on these systems.

The harp will have a dominant resonant frequency where the whole instrument will resonate in the presence of the same frequency. Celtic harps are natural microphones. Rooms also have resonant frequencies and will amplify those resonances when present as well. If the resonance of the room and the harp match then you may have problems.

When amplified, a harp may put a frequency (say 400 Hz) thru the sound system out in to the room, which resonates at an even more amplified/ louder 400hz, which makes the harp resonates more strongly as well, which puts an even stronger 400hz from the harp back into the sound system and around and around the signal goes until the system becomes unstable and you have feedback.

The resonant frequency of a room changes slightly with temperature and humidity but the easiest way to dampen resonances is to fill the room with people. People's clothes and bodies absorb reflected sound quite nicely.

Placing carpet under the harp reduces sound reflections around the harp which is great for reducing resonance. Ditto curtains behind the harpist.

The type of transducer/ microphone and its placement has a big effect on the feedback as well. Have the microphones facing away from the speakers (and glass walls) and as close to the harp as possible.

Pickups are the most stable. Get a piezo preamp with a small 5-7 band graphic EQ built in for personal frequency control.

"Inside the harp" microphones are the least stable. They suffer greatly in noisy reflective environments thru the harp over resonating. The space inside the sound box is a very acoustically complex area with lots of internal resonance nodes. Plugged sound holes do not look professional nor give the best harp sound. Specialty microphones like the Dusty Strings model will suffer less feedback as they have a very tight pick up pattern for absorbing sound.

Microphones placed farther away from the harp (and hence running a little hotter to get a large enough signal) are feedback prone.

Microphones with a broad pickup pattern with poor rejection of frequencies coming from off axis are more prone to feedback as well. (Cheaper mics, expensive larger condensers without a tight cardoid pattern)

Work out your dominant resonant frequencies in a room and help your sound guy at the same time. Once every thing is set up and the sound system is on, make sure every thing is quiet . Then one by one pluck the strings thru the octaves from the bottom to the top of your harp. Certain strings will jump out at you as being louder. Get your sound techie to reduce that specific frequency until the offending string sounds only as loud as its neighbouring strings. When your string volumes all sound in sync - Hey presto! no feedback for the night! you may wish to try this again at the loudest volume you would expect for the event. Life can be very sweet when you know a few tricks of the trade!

So when the screeches and rumbles occur swath your area in carpet and curtains. {Wear your fluffiest frills <grin>}. Get the mics in close and keep the volumes down. EQ if you can.

I will add your knowledge and experience to the “mix” if it can add to the effectiveness of these articles. The rest of the Celtic Harp Amplification Series is available here.

Stephen Vardy
Harpsound Audio
[email protected]